Trends Neurosci May 1986;9:186-192

Microcircuitry and functional architecture of the cat retina.

Sterling P, Freed M, Smith RG

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Neurons in cat retina belong to specific types. Each type is characterized by a specific correspondence between morphology and physiology and forms a regular array that connects lawfully to the arrays of certain other types. Two circuits have been traced quantitatively through these arrays from photoreceptors to alpha- and beta- ganglion cells. The 'cone-bipolar circuit' appears to convey the center-surround receptive field to ganglion cells, using cones in daylight and rods (via gap junctions to cones) in twilight. A 'rod-bipolar circuit' appears to convey the quantal signal and the pure center receptive field to the ganglion cells in starlight.

Eur J Neurosci Feb 1992;4:506-520

Parallel Circuits from Cones to the On-Beta Ganglion Cell.

Cohen E, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Neural integration depends critically upon circuit architecture; yet the architecture has never been established quantitatively (numbers of cells and synapses) for any vertibrate local circuit. Here we describe circuits in the cat retina that connect cones to the on-beta ganglion cell. This cell type is important because on- and off-beta cells contribute about 50% of the optic nerve fibers and the major retinal input to the striate cortex. Three adjacent on-beta cells in the area centralis and their bipolar connections to cones were reconstructed from electron micrographs of 279 serial section. The beta dendritic field is 34±2 micrometers in diameter and encompasses 35 cones. All of these cones connect to the beta cell via 14-17 bipolar cells. These bipolar cells were shown previously by cluster analysis to be of four types (b1-b4); three of these types (b1, b2 and b3) provided 97% of the bipolar contacts to the beta cell, in the ratio 4:2:1. On average, bipolar cells nearest the centre of the beta dendritic field contribute more synapses than those towards the edge, but the peaked distribution of bipolar synapses across the dendritic field is only slightly broader than the optical pointspread function of the cat's eye, and is narrower by half than the centre of the ganglion cell receptive field. This implies that the distribution of bipolar synapses across the beta cell dendritic field contributes little to the extent or shape of the receptive field. Since all three bipolar circuits connect to the same set of cones, they must carry the same spatial and chromatic information; they might convey different temporal frequencies. The numbers of bipolar synapses (mean±SD = 154±8) and amacrine synapses (59±5) converging on three adjacent beta cells are remarkably constant (SD approx. equals 5% of the mean). Thus, as the circuits repeat locally, the fundamental design is accurately reproduced.

Vis Neurosci 1994 Mar;11(2):261-269

Conductances evoked by light in the ON-beta ganglion cell of cat retina.

Freed MA, Nelson R

Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, Bethesda, MD 20892.

When a bar of light (215 x 5000 microns) illuminates the receptive field of an ON-beta ganglion cell of cat retina, the cell depolarizes. Intracellular recording from the cat eyecup preparation shows that this depolarization is due to an increase in conductance (2.4 +/- 0.6 nS). Different phases of this depolarization have different reversal potentials, but all of these reversal potentials are more positive than the cell's resting potential in the dark. When the light is turned on, there is an initial transient depolarization; the reversal potential measured for this transient is positive (23 +/- 11 mV). As the light is left on, the cell partially repolarizes to a sustained depolarization; the reversal potential measured for this sustained depolarization is close to zero (-1 +/- 5 mV). When the light is turned off, the cell repolarizes further; the reversal potential measured for this repolarization is negative (-18 +/- 7 mV), but still above the resting potential in the dark (-50 mV). To explain this variety of reversal potentials, at least two different synaptic conductances are required: one to ions which have a positive reversal potential and another to ions which have a negative reversal potential. Comparing the responses to broad and narrow bars suggests that these two conductances are associated with the center and surround, respectively. Finally, since an ON-beta cell in the area centralis receives about 200 synapses, these results indicate that a single synapse produces an average conductance increase of about 15 pS during a near-maximal depolarization.

J Comp Neurol 1993 Mar 1;329(1):68-84

OFF-alpha and OFF-beta ganglion cells in cat retina. I: Intracellular electrophysiology and HRP stains.

Nelson R, Kolb H, Freed MA

Laboratory of Neurophysiology, National Institute of Neurological Diseases and Stroke, National Institutes of Health, Bethesda, Maryland 20892.

Six OFF-alpha ganglion cells and a single OFF-beta ganglion cell were penetrated with intracellular microelectrodes and marked with horseradish peroxidase (HRP) in a perfused cat eyecup. Gaussian center radii (Rc) ranging from 40 to 217 microns were measured for receptive fields mapped with slits, values in agreement with previous extracellular reports. ON and OFF response components revealed nearly identical Rc's and center locations. Although Gaussian diameters (2Rc) were about 80% of dendritic field diameters overall, in this sample dendritic and receptive fields were not well correlated. Spatial tuning of ganglion cells was evidenced in peaked amplitude-vs.-width functions, fit by difference-of-Gaussians models. Such plots yielded Rc values about 40% less than position-vs amplitude plots. Rs values for surrounds ranged from 200 to 1,700 microns. Rod and cone signals were investigated with flicker. Rod flicker signals in OFF-alpha cells were larger and of shorter latency than in either horizontal or AII amacrine cells. Cone flicker signals were also short in latency, with an ON response time constant of 9 msec, and an OFF response time constant of 3 msec. The OFF-alpha rod-cone transition involved a latency increase of 20-30 msec. The spontaneous and light-evoked impulse rates of OFF-alpha responses varied linearly with extrinsic current, but the amplitude of ON hyperpolarization was little affected. After injection of staining current, the OFF-beta cell transiently depolarized at ON, suggestive of ON inhibition with reversed chloride gradient, a result not seen in OFF-alpha responses. Events (peaked, depolarizing voltage fluctuations) of high, low, and intermediate amplitudes were studied in OFF-alpha responses. High amplitude events (impulses), were OFF-correlated with the stimulus, and exhibited mean rise times (transit time from 25 to 75% of peak amplitude) from 255 to 392 microseconds. Intermediate level events (presumed synaptic origin) were also OFF correlated and had longer rise times (325 microseconds to 1.56 microseconds). Low level events (234-685 microseconds) revealed either ON, ON/OFF, or not stimulus correlation.

Nature 1996 Jun 13;381(6583):613-615

Absence of spectrally specific lateral inputs to midget ganglion cells in primate retina.

Calkins DJ, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Visual information is conveyed to the brain by the retinal ganglion cells. Midget ganglion cells serve fine spatial vision by summing excitation from a receptive field 'centre', receiving input from a single cone in the central retina, with lateral inhibition from a receptive field 'surround', receiving input from many surrounding cones. Midget ganglion cells are also thought to serve colour opponent vision because the centre excitation is from a cone of one spectral type, while the surround inhibition is from cones of the other type. The two major cone types, middle(M)- and long-(L)wavelength sensitive, are equally numerous and randomly distributed in the primate central retina, so a spectrally homogeneous surround requires that the cells mediating la teral interactions (horizontal or amacrine cells) receive selective input from only one cone type. Horizontal cells cannot do this because they receive input indiscriminately from M and L cones. Here we report that the amacrine cells connected to midget g anglion cells are similarly indiscriminate. The absence of spectral specificity in the inhibitory wiring raises doubt about the involvement of midget ganglion cells in colour vision and suggest that colour opponency may instead be conveyed by a different type of ganglion cell.

Vision Res 1996 Nov;36(21):3373-3381

Foveal cones form basal as well as invaginating junctions with diffuse ON bipolar cells.

Calkins DJ, Tsukamoto Y, Sterling P

Mahoney Institute for Neurological Sciences, University of Pennsylvania, Philadelphia 19104-6058, USA.

The response of a mammalian bipolar cell is generally thought to be determined by the location and morphology of synapses from the cone terminal: ON bipolar cells are believed to be depolarized strictly at invaginating contacts and OFF bipolar cells hy perpolarized at basal contacts. This hypothesis was re-investigated in the macaque fovea (1 deg nasal) using electron micrographs of serial sections. We determined the number of invaginating sites available and then identified the contacts to bipolar cell s with axons in the ON level of the inner plexiform layer. A cone terminal forms about 20 active zones marked by ribbons. A few active zones house two invaginating dendrites, so there are 22 invaginating sites per cone. A midget ON bipolar cell collects 1 8 invaginating contacts from one cone, thus only about four invaginating sites remain for diffuse ON bipolar cells. Two diffuse ON cells were reconstructed; each collects about 25 contacts from an estimated 10 cones. Only three or four of these contacts a re invaginating; the rest are basal, adjacent to the triad. This suggests that basal contacts can be depolarizing. The distance from the vesicle release site at active zones to an invaginating contact is 140 +/- 40 nm; to a basal contact adjacent to the t riad is 500 +/- 160 nm, and to the next nearest basal contact is 950 +/- 370 nm.

Nature 1994 Sep 1;371(6492):70-72

M and L cones in macaque fovea connect to midget ganglion cells by different numbers of excitatory synapses.

Calkins DJ, Schein SJ, Tsukamoto Y, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

Visual acuity depends on the fine-grained neural image set by the foveal cone mosaic. To preserve this spatial detail, cones transmit through non-divergent pathways: cone-->midget bipolar cell-->midget ganglion cell. Adequate gain must be establi shed along each pathway; crosstalk and sources of variation between pathways must be minimized. These requirements raise fundamental questions regarding the synaptic connections: (1) how many synapses from bipolar to ganglion cell transmit a cone signal a nd with what degree of crosstalk between adjacent pathways; (2) how accurately these connections are reproduced across the mosaic; and (3) whether the midget circuits for middle (M) and long (L) wavelength sensitive cones are the same. We report here that the midget ganglion cell collects without crosstalk either 28 +/- 4 or 47 +/- 3 midget bipolar synapses. Two cone types are defined by this difference; being about equal in number and distributing randomly in small clusters of like type, they are probabl y M and L.

J Comp Neurol 1986 Aug 1;250(1):1-7

Accumulation of (3H)glycine by cone bipolar neurons in the cat retina.

Cohen E, Sterling P

Cone bipolar neurons in the cat retina were studied in serial sections prepared as electron microscope autoradiograms following intravitreal injection of (3H)glycine. The goal was to learn whether the cone bipolar types that accumulate glycine correspo nd to the types thought on other grounds to be inhibitory. About half of the cone bipolars in a given patch of retina showed specific accumulation of silver grains. The specificity of accumulation was similar to that shown by glycine-accumulating amacrine s. All of the cone bipolars arborizing in sublamina b accumulated glycine but none of the cone bipolars arborizing in sublamina a did so. The types of cone bipolars accumulating glycine did not match the types thought to be inhibitory. Cone bipolar types CBb1 and CBb2 both form gap junctions with the glycine-accumulating AII amacrine, thus raising the possibility that glycine might accumulate in these cone bipolars by diffusion from the AII cell or vice versa. Thus it is logically impossible to tell which of these three cells contains a high-affinity uptake mechanism for glycine and consequently which of the three might actually use glycine as a neurotransmitter.

Philos Trans R Soc Lond B Biol Sci 1990 Dec 29;330(1258):305-321

Demonstration of cell types among cone bipolar neurons of cat retina.

Cohen E, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

We identified all the cone bipolar cells (80) in a small patch of one retina and then studied in detail the complete subset (42) that sends axons to sublamina b of the inner plexiform layer. The point was to learn whether the 'types' suggested previous ly, based on a few examples from a large population, could be substantiated or whether there would be intermediate forms. Tissue from the area centralis (1 degree eccentricity), was prepared as a series of 279 ultrathin sections and photographed in the el ectron microscope. Thirteen cells were reconstructed completely and parcelled into five categories (b1-b5) based on external morphology. For nine of these cells (two from categories b1-b4 and one from b5) most of the synaptic inputs and outputs were ident ified. When these nine cells were parcelled according to their synaptic patterns, they sorted into the same five categories. The remaining 29 cells in the population, though not reconstructed, were studied in detail by tracing their processes through the series. Ten of these cells, those near the margin of the series, were incomplete. The other 19 cells had essentially the same distribution of morphologies and synaptic patterns as the subset studied by total reconstruction: when plotted in multiparametric space, they formed distinct clusters corresponding to the five morphological categories. There was no hint of intermediate forms. That all the neurons in the population sort into some cluster (no intermediate forms), and that each neuron sorts into the s ame cluster by different criteria, argues that the clusters represent natural types. Each type forms a regular array in the region studied with an axonal 'coverage factor' that is close to one.

Philos Trans R Soc Lond B Biol Sci 1990 Dec 29;330(1258):323-328

Convergence and divergence of cones onto bipolar cells in the central area of cat retina.

Cohen E, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

In the central area of cat retina the cone bipolar cells that innervate sublamina b of the inner plexiform layer comprise five types, four with narrow dendritic fields and one with a wide dendritic field. This was shown in the preceding paper (Cohen &a mp; Sterling 1990 a) by reconstruction from electron micrographs of serial sections. Here we show by further analysis of the same material that the coverage factor (dendritic spread x cell density) is about one for each of the narrow-field types (b1, b2, and b4). The same is probably true for the other narrow-field type (b3), but this could not be proved because its dendrites were too fine to trace. The dendrites of types b1, b2, and b4 collect from all the cone pedicles within their reach and do not bypa ss local pedicles in favour of more distant ones. The dendrites of type b5, the wide-field cell, bypass many pedicles. On average 5.1 +/- 1.0 pedicles coverage on a b1 bipolar cell; 6.0 +/- 1.2 converge on a b2 cell and 5.7 +/- 1.5 converge on a b4 cell. Divergence within a type is minimal: one pedicle contacts only 1.2 b1 cells, 1.0 b2 cells, and 1.0 b4 cells. Divergence across types is broad: each pedicle apparently contacts all four types of the narrow-field bipolar cells that innervate sublamina b. Ea ch pedicle probably also contacts an additional 4-5 types of narrow-field bipolar cell that innervate sublamina a. There are several possible advantages to encoding the cone signal into multiple, parallel, narrow-field pathways.

J Neurophysiol 1991 Feb;65(2):352-359

Microcircuitry related to the receptive field center of the on-beta ganglion cell.

Cohen E, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104.

1. We have investigated the anatomic basis for the Gaussian-like receptive field center of the on-beta ("X") ganglion cell in the area centralis of cat retina. Three adjacent on-beta cells were reconstructed from electron micrographs of 279 s erial sections cut vertically through a patch of retina at approximately 1 degree eccentricity. 2. All the bipolar synapses on these cells were identified, and about one-half of these were traced to type b1 bipolar cells, which formed a regular array in t he plane of the retina. 3. On average, seven b1 cells contributed to a beta cell: bipolar axons near the middle of the beta dendritic field tended to give many contacts (12-33 contacts); axons near the edge of the field tended to give few contacts (3-4 co ntacts). 4. Each b1 cell collected from four to seven cones, and the mean number of cones converging through the b1 array to a beta cell was 30. 5. Assuming equal effectiveness for all b1->beta cell synapses, a spatial weighting function was derived fro m these results. The mean radius of this function at 1/e amplitude for three beta cells was 18.0 +/- 1.1 (SD) microns. This is considerably narrower than the corresponding measurements of the beta cell receptive field center (28 +/- 3 microns) at this ecc entricity. 6. It is concluded, in agreement with previous work, that all cones encompassed by the beta cell's dendritic field and those slightly beyond it connect directly to the beta cell via the b1 bipolar cell array. However, the center of the beta cel l receptive field is still broader by approximately 60%. This suggests that pooling of cone signals may occur at the level of the outer plexiform layer.

J Comp Neurol 1979 Dec 15;188(4):599-627

Microcircuitry of cat visual cortex: classification of neurons in layer IV of area 17, and identification of the patterns of lateral geniculate input.

Davis TL, Sterling P

Neurons in the cerebral cortex have been classified primarily by their differences in axonal and dendritic branching patterns observed in material impregnated by the Golgi method. Although these morphological differences are widely believed to reflect differences in connectivity, very little is actually known about the patterns of synaptic input to different cell types. We have obtained such information for 32 adjacent neurons in layer IVab of cat cortical area 17 by reconstructing them from electron m icrographs of 150 serial sections. Synaptic terminals from the lateral geniculate nucleus were labeled in this material by anterograde degeneration and their distribution, as well as that of normal terminals containing flat or round vesicles, was recorded . The neurons were divided into seven classes based on differences in size, shape, dendritic branching pattern and synaptic input. Class I cells were pyramidal with apical and basilar dendrites, dendritic spines, exclusively flat-vesicle terminals on the somas (11/100 micron2), and geniculate terminals on the basilar dendrites. Class II cells were large stellates (20 micron diameter) with dark cytoplasm and numerous flat-vesicle and round-vesicle terminals on the somas (48/100 micron2). Geniculate termin als contacted the cell bodies and primary, secondary, and tertiary dendrites. The Class III cell was stellate with varicose dendrites, a sparse distribution of flat-vesicle terminals (8/100 micron2) on the soma, and both geniculate and round-vesicle termi nals on the dendrites. Class IV cells had radially elongated somas with sharply tapered apical and basilar dendrites bearing spines. There was a medium distribution of flat-vesicles terminals (17/100 mu2), to the somas while geniculate terminals were rest ricted to the secondary dendrites. Class V cells were multipolar with flat-vesicle terminals on the somas (11/100 micron2) and a few geniculate terminals on the dendrites. Class VI cells were mostly small (as small as 7 micron diameter), with a sparse dis tribution on the somas of both flat-vesicle terminals (7/100 micron2). Two cells had geniculate terminals on their somas. Class VII cells had sharply tapered apical and basilar dendrites, both flat-vesicle and round-vesicle terminals on the somas (14/100 micron2), and no geniculate input. The results make clear that the neurons in layer IVab are quite heterogeneous, not merely in their intrinsic morphology, but also in their patterns of connectivity. The geniculate input is not funneled to a single type o f neuron but diverges widely, contacting at least six different cell types, and may form on each a pattern that is characteristic for the type. The reconstruction approach, in providing a detailed identification of the synaptic patterns on substantial num bers of adjacent cells, should make it possible to address directly certain unanswered questions about cortical circuitry...

J Comp Neurol 1987 Jun 1;260(1):76-86

Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex.

Einstein G, Davis TL, Sterling P

The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity i njected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of th e other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neuron s and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer I Vab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicle s and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive fie ld properties of this neuron, and the possible effects are discussed.

J Comp Neurol 1987 Jun 1;260(1):76-86

Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex.

Einstein G, Davis TL, Sterling P

The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity i njected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of th e other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neuron s and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer I Vab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicle s and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive fie ld properties of this neuron, and the possible effects are discussed.

J Comp Neurol 1996 Jan 15;364(3):556-566

ON-OFF amacrine cells in cat retina.

Freed MA, Pflug R, Kolb H, Nelson R

National Institute of Neurological Disorders and Stroke, Maryland 20892, USA.

We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microsco pe. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b) but extends some dendrites to sublamina a. Both of these cell types have axon-like processes that extend > 800 microns from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 +/- 0.3 mm diameter) that were broader than their dendritic arbors (425 +/- 35 microns diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-alpha cell.

J Comp Neurol 1983 Sep 20;219(3):295-304

Four types of amacrine in the cat retina that accumulate GABA.

Freed MA, Nakamura Y, Sterling P

Roughly one-quarter of neurons in the amacrine cell layer accumulate exogenous gamma-aminobutyric acid (GABA). Some of these (8%) are interplexiform cells; the remainder are true amacrine cells. We partially reconstructed, from serial electron microsco py autoradiograms, 25 GABA-accumulating amacrines and distinguished four types based on cytoplasmic appearance, soma size and shape, and the form of primary and secondary processes. Type 1 had a large (609 +/- 60 microns3), dark soma, and multiple, medium -diameter (0.6 microns) processes splayed from the soma margins like the appendages from a crab. Type 2 had a medium (360 +/- 40 microns3), helmet-shaped, pale soma, and medium-diameter (0.8 microns) processes that branched in sublamina alpha. Type 3 had a small (267 +/- 44 microns3), dark, pyriform soma. The latter formed a single stout (3.0 microns) process that bifurcated in the middle of sublamina alpha. Type 4 had a very large, pale soma (860 microns3). This was pyriform, tapering into a stout (2.0 m icrons) process that descended into the middle of sublamina alpha where it emitted smaller tangential processes. It is to be expected that each of these amacrine cell types will have distinct functions in neurotransmitter retinal circuitry.

J Comp Neurol 1987 Dec 15;266(3):445-455

Rod bipolar array in the cat retina: pattern of input from rods and GABA-accumulating amacrine cells.

Freed MA, Smith RG, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

The potential and actual connections between rod and rod bipolar arrays in the area centralis of the cat retina were studied by electron microscopy of serial ultrathin sections. In the region studied there were about 378,000 rods/mm2 and 36,000-47,000 rod bipolars/mm2. The tangential spread of rod bipolar dendrites was 11.2 microns in diameter, and the "coverage factor" for the rod bipolar cell was 3.5-4.6. We estimate that about 37 rods potentially converge on a rod bipolar cell and that one rod potentially diverges to about four rod bipolar cells. The actual connections, however, are less than this by about half: 16-20 rods actually converge on a bipolar cell and one rod actually diverges to slightly less than two rod bipolar cells. The deg ree of convergence appears to reflect a compromise between the need to signal graded stimulus intensities (requiring wide convergence) and the need to maintain a good signal/noise ratio (requiring narrow convergence). Amacrine varicosities that provide re ciprocal contact at the rod bipolar dyad were studied in serial electron microscopic autoradiograms following intraocular administration of 3H-GABA or 3H-glycine. More that 90% of the reciprocal amacrine processes accumulated GABA in a specific fashion. T his information, in conjunction with Nelson's recordings from the rod bipolar and amacrine cells postsynaptic at the dyad (Nelson et al: Invest. Ophthalmol. 15:946-953, '76; Kolb and Nelson: Vision Res. 23:301-312, '83), suggests that feedback at the rod bipolar output might be positive.

Proc Natl Acad Sci U S A 1992 Jan 1;89(1):236-240

Computational model of the on-alpha ganglion cell receptive field based on bipolar cell circuitry.

Freed MA, Smith RG, Sterling P

Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892.

The on-alpha ganglion cell in the area centralis of the cat retina receives approximately 450 synapses from type b1 cone bipolar cells. This bipolar type forms a closely spaced array (9 microns), which contributes from 1 to 7 synapses per b1 cell throu ghout the on-alpha dendritic field. Here we use a compartmental model of an on-alpha cell, based on a reconstruction from electron micrographs of serial sections, to compute the contribution of the b1 array to the on-alpha receptive field. The computation shows that, for a physiologic range of specific membrane resistance (9500-68,000 omega.cm2) and a linear synapse, inputs are equally effective at all points on the on-alpha dendritic tree. This implies that the electrotonic properties of the dendritic tree contribute very little to the domed shapes of the receptive field center and surround. Rather, these shapes arise from the domed distribution of synapses across the on-alpha dendritic field. Various sources of "jitter" in the anatomical circu it, such as variation in bipolar cell spacing and fluctuations in the number of synapses per bipolar cell, are smoothed by the overall circuit design. However, the computed center retains some minor asymmetries and lumps, due to anatomical jitter, as found in actual alpha-cell receptive fields.

J Neurosci 1988 Jul;8(7):2303-2320

The ON-alpha ganglion cell of the cat retina and its presynaptic cell types.

Freed MA, Sterling P

University of Pennsylvania Medical School, Department of Anatomy, Philadelphia 19104.

Anatomical circuits converging onto the ON-alpha (Y) ganglion cell were studied by computer-assisted reconstruction of substantial portions of 2 alpha cells from electron micrographs of serial sections. The alpha cells in the area centralis were labele d by a Golgi-like retrograde filling with horseradish peroxidase, and certain presynaptic amacrine processes were labeled by uptake of 3H-glycine. About 4400 synapses contacted the alpha cell. Eighty-six percent were from amacrine cells; the rest were fro m bipolar cells. About one-quarter of the amacrine synapses were specifically labeled by 3H-glycine and probably belong to the A4 amacrine. The bipolar inputs were provided by several types: cone bipolar CBb1 (85%), cone bipolar CBb5 (2%), the rod bipolar (5%), and some unidentified cone bipolars (11%). Contacts from each type occurred in specific strata, with the consequence that they tended to form spots or annulli over the alpha dendritic field. The CBb1 bipolars formed a moderately dense array (8000/m m2), with a nearest-neighbor distance of 8.6 +/- 1.3 microns. Most members of the array (84%) contacted the alpha cell, providing 1-7 synapses (average, 2.7 +/- 1.6). The placement of contacts from an individual CBb1 followed certain rules: they were rest ricted to a parent branch of the alpha arbor or to 2 daughter branches, but almost never crossed a branch of the alpha arbor. The synaptic territory of an individual CBb1 was not shared with other b1s (or cone bipolars of any sort), although it was shared with amacrine contacts. Rod bipolar cells also formed a very dense array (54,500/mm2) in the alpha dendritic field, but only a few of these (3%) contacted the alpha cell. The concentric receptive field of the CBb1, combined with the spatial organization of its array, is used to predict the CBb1 contribution to the alpha cell receptive field; this contribution resembles the spatial and temporal organization of the alpha receptive field itself.

Proc Natl Acad Sci U S A 1996 Dec 10;93(25):14598-14601

Phospholipase C beta 4 is involved in modulating the visual response in mice.

Jiang H, Lyubarsky A, Dodd R, Vardi N, Pugh E, Baylor D, Simon MI, Wu D

Department of Pharmacology and Physiology, University of Rochester, NY 14642, USA.

Expression of G protein-regulated phospholipase C (PLC) beta 4 in the retina, lateral geniculate nucleus, and superior colliculus implies that PLC beta 4 may play a role in the mammalian visual process. A mouse line that lacks PLC beta 4 was generated and the physiological significance of PLC beta 4 in murine visual function was investigated. Behavioral tests using a shuttle box demonstrated that the mice lacking PLC beta 4 were impaired in their visual processing abilities, whereas they showed no defi cit in their auditory abilities. In addition, the PLC beta 4-null mice showed 4-fold reduction in the maximal amplitude of the rod a- and b-wave components of their electroretinograms relative to their littermate controls. However, recording from single r od photoreceptors did not reveal any significant differences between the PLC beta 4-null and wild-type littermates, nor were there any apparent differences in retinas examined with light microscopy. While the behavioral and electroretinographic results in dicate that PLC beta 4 plays a significant role in mammalian visual signal processing, isolated rod recording shows little or no apparent deficit, suggesting that the effect of PLC beta 4 deficiency on the rod signaling pathway occurs at some stage after the initial phototransduction cascade and may require cell-cell interactions between rods and other retinal cells.

J Neurosci 1995 Nov;15(11):7673-7683

How retinal microcircuits scale for ganglion cells of different size.

Kier CK, Buchsbaum G, Sterling P

Department of Bioengineering, University of Pennsylvania, School of Medicine, Philadelphia 19104, USA.

Ganglion cell receptive field centers are small in central retina and larger toward periphery. Accompanying this expansion, the distribution of sensitivity across the centers remain Gaussian, but peak sensitivities decline. To identify circuitry that might explain this physiology, we measured the density of bipolar cell synapses on the dendritic membrane of beta (X) and alpha (Y) ganglion cells and the distribution of dendritic membrane across their dendritic fields. Both central and peripheral beta cells receive bipolar cell synapses at a density of approximately 28/100 microns2 of dendritic membrane; central and peripheral alpha cells receive approximately 13/100 microns2. The distribution of dendritic membrane across the dendritic field is dome-like; therefore, the distribution of bipolar cell synapses is also dome-like. As the dendritic field enlarges, total postsynaptic membrane increases with field radius, but only linearly. Consequently, density of postsynaptic membrane in the dendritic field declines, and so does density of synapses within the field. The results suggest a simple model in which the receptive field center's Gaussian profile and peak sensitivity are both set by the density of bipolar cell synapses across the dendritic field.

J Comp Neurol 1983 Apr 20;215(4):465-471

Granule cells in the rat olfactory tubercle accumulate 3H-gamma-aminobutyric acid.

Krieger NR, Megill JR, Sterling P

The rat olfactory tubercle contains high concentrations of gamma-aminobutyric acid (GABA) and its synthetic enzyme, glutamic acid decarboxylase (GAD). We previously demonstrated that GABA and GAD are most concentrated in the polymorphic layer of the tubercle and relatively absent from the plexiform and pyramidal layers. Here we report that the granule cells (the islands of Calleja) in the polymorphic layer accumulate 3H-GABA. 3H-GABA (34.5 Ci/mmole; 1.5 microliter) was injected into the tubercle and an hour later the rat was perfused with a mixture of paraformaldehyde and glutaraldehyde. The tissue was osmicated, dehydrated, and embedded in epon. Silver grains were sparse over the pyramidal and polymorphic cell bodies but numerous over the granule cell bodies in the islands of Calleja and dendrites in the surrounding neuropil. Grain densities for the granule cells were 41/100 micrometer3 compared to 4.2 for the pyramidal and polymorphic cells. Within the island, all the granule cells appeared to be labeled. These results, combined with previous demonstrations of the presence in this region of endogenous GABA and GAD, suggest that the granule neurons of the rat olfactory tubercle are GABA-ergic. These neurons also appear to receive dopamine input and therefore form part of a circuit that includes targets for both major and minor tranquilizers.

J Neurosci 1984 Dec;4(12):2920-2938

Microcircuitry of bipolar cells in cat retina.

McGuire BA, Stevens JK, Sterling P

We have studied 15 bipolar neurons from a small patch (14 X 120 micron) of adult cat retina located within the area centralis. From electron micrographs of 189 serial ultrathin sections, the axon of each bipolar cell was substantially reconstructed with its synaptic inputs and outputs by means of a computer-controlled reconstruction system. Based on differences in stratification, cytology, and synaptic connections, we identified eight different cell types among the group of 15 neurons: one type of rod bipolar and seven types of cone bipolar neurons. These types correspond to those identified by the Golgi method and by intracellular recording. Those bipolar cell types for which we reconstructed three or four examples were extremely regular in form, size, and cytology, and also in the quantitative details of their synaptic connections. They appeared quite as specific in these respects as invertebrate "identified" neurons. The synaptic patterns observed for each type of bipolar neuron were complex but may be summarized as follows: the rod bipolar axon ended in sublamina b of the inner plexiform layer and provided major input to the AII amacrine cell. The axons of three types of cone bipolar cells also terminated in sublamina b and provided contacts to dendrites of on-beta and other ganglion cells. All three types, but especially the Cb1, received gap junction contacts from the AII amacrine cell. Axons of four types of cone bipolar cells terminated in sublamina a of the inner plexiform layer and contacted dendrites of off-beta and other ganglion cells. One of these cone bipolar cell types, CBa1, made reciprocal chemical contacts with the lobular appendage of the AII amacrine cell. These results show that the pattern of cone bipolar cell input to beta (X) and probably alpha (Y) ganglion cells is substantially more complex than had been suspected. At least two types of cone bipolar contribute to each type of ganglion cell where only a single type had been anticipated. In addition, many of the cone bipolar cell pathways in the inner plexiform layer are available to the rod system, since at least four types of cone bipolar receive electrical or chemical inputs from the AII amacrine cell. This may help to explain why, in a retina where rods far outnumber the cones, there should be so many types of cone bipolar cells.

J Neurosci 1986 Apr;6(4):907-918

Microcircuitry of beta ganglion cells in cat retina.

McGuire BA, Stevens JK, Sterling P

We reconstructed from electron micrographs of 189 serial ultrathin sections a major portion of the dendritic tree of an on-beta ganglion cell through its sixth order of branching. One hundred three contacts from three cone bipolar cells were identified. Forty-seven contacts were from a single CBb1 cone bipolar. These were distributed widely over the dendritic tree but were frequently found on the slender "basal tuft" dendrites. Twenty-two additional contacts from a second CBb1 cell were found but not studied in detail. Thirty-four contacts were from a single CBb2 cone bipolar; these also were distributed widely but were primarily on the branches of the main dendritic arborization. A major portion of the dendritic tree of an off-beta cell was also reconstructed through its seventh order of branching. Thirty-five contacts from two cone bipolar cells were identified. Twenty-three contacts were from a single CBa1 cone bipolar and 12 widely distributed over the off-beta cell dendritic tree. We propose that the photopic receptive field center of a beta cell corresponds to the envelope of the receptive fields of the bipolar cells that connect it to the cones. The center response of a beta cell may be generated by a "push-pull" mechanism. For the on-beta cell there would be excitation at light on from CBb1 and disinhibition from CBb2 and the reverse at light off. For the off-beta cell there would be inhibition at light on from CBa2 and withdrawal of excitation from CBa1. Should the bipolars have antagonistic surrounds (so far reported only for CBb1), the beta cell surrounds as well as their centers might be generated by this push-pull mechanism.

J Comp Neurol 1981 Nov 1;202(3):385-396

Neurons and glia in cat superior colliculus accumulate [3H]gamma-aminobutyric acid (GABA).

Mize RR, Spencer RF, Sterling P

We have examined by autoradiography the labeling pattern in the cat superior colliculus following injection of tritiated gamma-aminobutyric acid (GABA). Silver grains were heavily distributed within the zonal layer and the upper 200 micrometer of the superficial gray. Fewer grains were observed deeper within the superficial gray, and still fewer were found within the optic and intermediate gray layers. The accumulation of label was restricted to certain classes of neuron and glia. Densely labeled neurons were small (8-12 micrometer in diameter) and located primarily within the upper 200 micrometer. Dark oligodendrocytes and astrocytes showed a moderate accumulation of label while pale oligodendrocytes and microglia were unlabeled. Label was also selectively accumulated over several other types of profile within the neuropil, including presynaptic dendrites, axons, and axon terminals.

J Comp Neurol 1982 Apr 1;206(2):180-192

Two types of GABA-accumulating neurons in the superficial gray layer of the cat superior colliculus.

Mize RR, Spencer RF, Sterling P

Two types of neuron in the upper superficial gray layer of the cat superior colliculus accumulated exogenous 3H-gamma-aminobutyric acid intensely. The first type was a horizontal cell with a fusiform cell body, horizontal dendrites, a low synaptic density, but a high percentage of cortical synaptic contacts. This cell had presynaptic dendrites. The second type was a granule cell (type A) with a small round cell body, thin and obliquely oriented dendrites, a moderate synaptic density, and few cortical synaptic contacts. These two types differed in size, shape, dendritic morphology, and patterns of synaptic input. They likely participate in different inhibitory mechanisms. Four types of unlabeled neurons were also identified. Type B granule cells were found only within the upper subdivision of the superficial gray layer. They had moderate-sized cell bodies, a high synaptic density, and numerous somatic spines. A third type of granule cell (type C) was found only in the deep subdivision of the superficial gray. This type had a low synaptic density and spines that contained synaptic vesicles. Vertical fusiform and stellate forms were also found. We conclude that at least six types of neurons populate the upper superficial gray layer of the cat superior colliculus.

Proc Natl Acad Sci U S A 1980 Jan;77(1):658-661

Interplexiform cell in cat retina: identification by uptake of gamma-[3H]aminobutyric acid and serial reconstruction.

Nakamura Y, McGuire BA, Sterling P

After intravitreal injection of gamma-[3H] aminobutyric acid (GAB), 2% of the neurons at the outer margin of the inner plexiform layer were intensely labeled. Reconstructions of these neurons from serial electron microscope autoradiograms showed that they are interplexiform cells, which synapse on bipolar processes in the outer plexiform layer and on amacrine and bipolar processes in the inner plexiform layer.

J Comp Neurol 1995 Oct 23;361(3):479-490

Retinal neurons and vessels are not fractal but space-filling.

Panico J, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

Many branched patterns in nature are hypothesized to be fractal, i.e., statistically self-similar across a range of scales. We tested this hypothesis on the two-dimensional arbors of retinal neurons and blood vessels. First, we measured fractalness on synthetic fractal and nonfractal patterns. The synthetic fractal patterns exhibited self-similarity over a decade of scale, but the nonfractal "controls" showed hardly any self-similarity. Neuronal and vascular patterns showed no greater self-similarity than the controls. Second, we manipulated a synthetic fractal pattern to remove its self-similarity and found this to be reflected in a loss of measured fractalness. The same manipulation of the nonfractal control and also of the neural and vascular patterns did not alter their measured fractalness. Third, we "grew" patterns of branched line segments according to a variety of nonfractal algorithms. These patterns were, if anything slightly more fractal than the neural and vascular patterns. We conclude that the biological patterns studied here are not fractal. Finally, we measured extended versions of these patterns: a contiguous array of homotypic neuron arbors and a vascular pattern with a high degree of total detail. These patterns showed a "fractal dimension" of 2, which implies that down to some cut-off scale they fill space completely. Thus, neural and vascular patterns might best be described as quasi-regular lattices.

Biophys J 1994 Jul;67(1):57-63

Rate of quantal transmitter release at the mammalian rod synapse.

Rao R, Buchsbaum G, Sterling P

Department of Bioengineering, University of Pennsylvania, Philadelphia 19104.

Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presented to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and pauses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar cell to distinguish this pause from an extra-long interval between quanta due to the stochastic timing of release? The answer required a model of the circuit that included the rod convergence onto the bipolar cell and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This tonic rate is much higher than at conventional synapses where reliability is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a premium.

Neuron 1995 Mar;14(3):561-569

Mammalian rod terminal: architecture of a binary synapse.

Rao-Mirotznik R, Harkins AB, Buchsbaum G, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

The mammalian rod synapse transmits a binary signal (one photon or none) using tonic, rapid exocytosis. We constructed a quantitative, physical model of the synapse. Presynaptically, a single, linear active zone provides docking sites for approximately 130 vesicles, and a "ribbon" anchored to the active zone provides a depot for approximately 640 vesicles. Postsynaptically, 4 processes invaginate the terminal: 2 (known to have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130-640 nm). The presynaptic structure seems designed to minimize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-or-none response at all 4 postsynaptic processes.

In: Neurobiology and clinical aspects of the outer retina (Archer S, Djamgoz MBA, Vallerga S eds), pp 325-348. London UK: Chapman & Hall, Ltd.

Functional architecture of mammalian outer retina and bipolar cells.

Sterling P, Smith RG, Rao R, Vardi N (1995)

Department of Neuroscience, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

Function in the outer retina has mainly gbeen studied by recording in situ from single neurons. In lower vertebrates this approach to bipolar cells has been extremely fruitful (e.g. Chapter 12), but in mammals bipolar cell recordings can be counted on the fingers of (at most) two hands (Nelson and Kolb, 1983; Dacheux and Raviola, 1986). And, considering that the recordings include both rod bipolar and multiple types of cone bipolar cell (Chapter 11), the electrophysiological data regarding mammalian bipolar neurons are thinly spread. On the other hand, in lower vertebrates information essential to understanding the contribution of the outer retina to image processing (such as optics, sampling frequencies, and synaptic circuitry) hardly exists. So, in lower vertebrates how single neuron responses in the outer contribute to vision remains unclear.

Yet, single cell recording is not the only possible approach to understanding retinal function. An alternative strategy is to determine complete circuit structure ('wiring diagram') plus the chemical architecture and to incorporate this informatlion, together with the optics and ganglion cell electrophysiology, into various computational models. Then, one might calculate backwards to the properties of the bipolar cells and photoreceptors. Such an effort leads to specific predictions regarding the photoreceptor and bipolar cell function, and with this approach a little electrophysiology goes a surprisingly long way. At least, that is our argument in this review. We emphasize cat retina, which is known in most detail, but also note recent data from reabbit and primate that indicate conservation of certain basic circuits and functions.

In the exact center of the area centralis, cone density reaches 30,000-40,000/mm2 (Wassle and Riemann, 1978; Williams et al., 1993), but the circuitry has been studied slightly off center (1 deg eccentricity) where the cone density is about 24,000/mm2 and rod density is about 350,000/mm2 (Sterling et al., 1988). Here, due to the natural blur of the cat's optics (Wassle, 1971; Robson and Enroth-Cugell, 1978), the minimum number of photoreceptors stimulated from a point source is about 10 cones and 140 rods (Figure 13.1a, 2b). This is many more receptors than converging on a single bipolar cell, so even the finest spatial stimulus falling on either type of receptor will affect many bipolar cells. We review first the circuits for daylight that lead from cones because various portions of this circuit are parasitized by the circuits for twilight and starlight that lead from rods (Figure 13.2).

J Neurosci Methods 1987 Sep;21(1):55-69

Montage: a system for three-dimensional reconstruction by personal computer.

Smith RG

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

This paper describes a simplified system for serial section three-dimensional (3-D) reconstruction. A set of 9 software programs runs on a standard personal computer and produces camera-ready illustrations suitable for publication. The user enters trace points on a digitizing tablet from sections that have been already aligned. A 3-D view of the reconstructed object is generated which can be displayed with hidden lines removed. Analysis of volume, surface area and autoradiographic grain density are performed automatically. A relational database query language allows display and analysis of a selected subset of the data. The system runs under the UNIX operating system which allows the programs to be easily transported to new hardware or modified for other purposes.

J Neurosci Methods 1992 Jul;43(2-3):83-108

NeuronC: a computational language for investigating functional architecture of neural circuits.

Smith RG

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

A computational language was developed to simulate neural circuits. A model of a neural circuit with up to 50,000 compartments is constructed from predefined parts of neurons, called "neural elements". A 2-dimensional (2-D) light stimulus and a photoreceptor model allow simulating a visual physiology experiment. Circuit function is computed by integrating difference equations according to standard methods. Large-scale structure in the neural circuit, such as whole neurons, their synaptic connections, and arrays of neurons, are constructed with procedural rules. The language was evaluated with a simulation of the receptive field of a single cone in cat retina, which required a model of cone-horizontal cell network on the order of 1000 neurons. The model was calibrated by adjusting biophysical parameters to match known physiological data. Eliminating specific synaptic connections from the circuit suggested the influence of individual neuron types on the receptive field of a single cone. An advantage of using neural elements in such a model is to simplify the description of a neuron's structure. An advantage of using procedural rules to define connections between neurons is to simplify the network definition.

Vis Neurosci 1995 May;12(3):545-561

Simulation of an anatomically defined local circuit: the cone-horizontal cell network in cat retina.

Smith RG

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

The outer plexiform layer of the retina contains a neural circuit in which cone synaptic terminals are electrically coupled and release glutamate onto wide-field and narrow-field horizontal cells. These are also electrically coupled and feed back through a GABAergic synapse to cones. In cat this circuit's structure is known in some detail, and much of the chemical architecture and neural responses are also known, yet there has been no attempt to synthesize this knowledge. We constructed a large-scale compartmental model (up to 50,000 compartments) to incorporate the known anatomical and biophysical facts. The goal was to discover how the various circuit components interact to form the cone receptive field, and thereby what possible function is implied. The simulation reproduced many features known from intracellular recordings: (1) linear response of cone and horizontal cell to intensity, (2) some aspects of temporal responses of cone and horizontal cell, (3) broad receptive field of the wide-field horizontal cell, and (4) center-surround cone receptive field (derived from a "deconvolution model"). With the network calibrated in this manner, we determined which of its features are necessary to give the cone receptive field a Gaussian center-surround shape. A Gaussian-like center that matches the center derived from the ganglion cell requires both optical blur and cone coupling: blur alone is too narrow, coupling alone gives an exponential shape without a central dome-shaped peak. A Gaussian-like surround requires both types of horizontal cell: the narrow-field type for the deep, proximal region and the wide-field type for the shallow, distal region. These results suggest that the function of the cone-horizontal cell circuit is to reduce the influence of noise by spatio-temporally filtering the cone signal before it passes through the first chemical synapse on the pathway to the brain.

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J Neurosci 1986 Dec;6(12):3505-3517

Microcircuitry of the dark-adapted cat retina: functional architecture of the rod-cone network.

Smith RG, Freed MA, Sterling P

The structure of the rod-cone network in the area centralis of cat retina was studied by reconstruction from serial electron micrographs. About 48 rods converge on each cone via gap junctions between the rod spherules and the basal processes of the cone pedicle. One rod diverges to 2.4 cones through these gap junctions, and each cone connects to 8 other cones, also through gap junctions. A static cable model of this network showed that at mesopic intensities, when all rods converging on a cone pedicle are continuously active, the collective rod signal would be efficiently conveyed to the pedicle. At scotopic intensities sufficiently low for only one of the converging rods to receive a single photon within its integration time, the quantal rod signal would be poorly transmitted to the cone pedicle. This is because the tiny signal would be dissipated by the large network into which the individual rod diverges. Under this condition, the rod signal would also be poorly conveyed to the rod spherule. If, however, the rods are electrically disconnected from the network, the quantal signal would be efficiently conveyed to the rod spherule. This analysis suggests that the rod signal is conveyed at mesopic intensities by the cone bipolar pathway and, at scotopic intensities, by the rod bipolar pathway, in accordance with the results of Nelson (1977, 1982; Nelson and Kolb, 1985).

Vis Neurosci 1990 Nov;5(5):453-461

Cone receptive field in cat retina computed from microcircuitry.

Smith RG, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia.

The receptive-field profile of the cone in cat-retina was computed. The computation was based on (1) the known anatomical circuit connecting cones via narrow-field bipolar cells to the on-beta ganglion cell; (2) the known physiological receptive-field profile of the on-beta (X) cell at the corresponding eccentricity; and (3) a model in which the beta receptive field arises by linear superposition of cone receptive fields. The computed cone receptive field has a center/surround organization with a center almost as broad as that of the beta cell center. The cone surround is comparably broad to that of the beta cell but somewhat lower in peak amplitude. The problems to which the center/surround receptive field are the solution, namely, signal compression and noise reduction, apparently must be solved before the first synapse of the visual pathway.

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Vis Neurosci 1995 Sep;12(5):851-860

Simulation of the AII amacrine cell of mammalian retina: functional consequences of electrical coupling and regenerative membrane properties.

Smith RG, Vardi N

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

The AII amacrine cell of mammalian retina collects signals from several hundred rods and is hypothesized to transmit quantal "single-photon" signals at scotopic (starlight) intensities. One problem for this theory is that the quantal signal from one rod when summed with noise from neighboring rods would be lost if some mechanism did not exist for removing the noise. Several features of the AII might together accomplish such a noise removal operation: The AII is interconnected into a syncytial network by gap junctions, suggesting a noise-averaging function, and a quantal signal from one rod appears in five AII cells due to anatomical divergence. Furthermore, the AII contains voltage-gated Na+ and K+ channels and fires slow action potentials in vitro, suggesting that it could selectively amplify quantal photon signals embedded in uncorrelated noise. To test this hypothesis, we simulated a square array of AII somas (Rm = 25,000 Ohm-cm2) interconnected by gap junctions using a compartmental model. Simulated noisy inputs to the AII produced noise (3.5 mV) uncorrelated between adjacent cells, and a gap junction conductance of 200 pS reduced the noise by a factor of 2.5, consistent with theory. Voltage-gated Na+ and K+ channels (Na+: 4 nS, K+: 0.4 nS) produced slow action potentials similar to those found in vitro in the presence of noise. For a narrow range of Na+ and coupling conductance, quantal photon events (approximately 5-10 mV) were amplified nonlinearly by subthreshold regenerative events in the presence of noise. A lower coupling conductance produced spurious action potentials, and a greater conductance reduced amplification. Since the presence of noise in the weakly coupled circuit readily initiates action potentials that tend to spread throughout the AII network, we speculate that this tendency might be controlled in a negative feedback loop by up-modulating coupling or other synaptic conductances in response to spiking activity.

Proc Natl Acad Sci U S A 1984 Jun;81(12):3898-3900

Numbers of specific types of neuron in layer IVab of cat striate cortex.

Solnick B, Davis TL, Sterling P

Layer IVab of the visual cortex (area 17) of the cat contains about 51,400 neurons per mm3, including about 400-1200 per mm3 of each of three categories of neuron believed from previous work to represent discrete types. Each type forms about 0.5-1.5% of all the IVab neurons, which suggests that the total number of types in this layer might be much greater than previously supposed, perhaps as many as 50 or more. From their densities and estimates of their dendritic fields, we calculate that each type completely "covers" layer IVab in the tangential plane but only by a small factor (1.3-4.2).

Ann. Ref. Neurosci. 1983; 6:149-185

Peter Sterling

Department of Neuroscience, University of Pennsylvania Medical School, Philadelphia 19104-6058.

As a device for extracting information from a visual image, the vertebrate retina is unparalleled in its range, reliability, and compactness. Signaling in the retina is slower by six orders of magnitude than in an integrated digital circuit. The advantage of the biological structure must therefore derive from the variety of its fundamental elements and from the subtlety of their connections. Each of the five major classes of retinal neuron, whose synaptic contacts were first described systematically by Dowling & Boycott (1966) is now known to have multiple types, totaling in the cat about 60. Specific local circuits involving about one-third of these neurons have been recognized in the electron microscope. Physiological responses have also been documented for about one-third of the types, and evidence regarding the neural transmitter, or at least the sign of the synapse, has accumulated also for about one-third.

These discoveries have abundantly supported certain concepts of retina function developed in the 1960s by Lettvin & Maturana. The function of the retina, they proposed, "is not to transmit information about the point-to-point distribution of light and dark in the image, but to analyze this image at every point in terms of ... arbitrary contexts ..." (Maturana et al., 1960). Each of these "contexts," they suggested, corresonds to some operation on the local image performed by a ganglion cell of particular size and shape (Lettvin et al., 1961). This idea, based on studies of the frog, seemed for a time inapplicable to the cat, which was thought to have a "simple" retina with only center-surround type ganglion cells. Subsequent studies to be reviewed here have firmly established for the cat the validity of this idea.

Lettvin & Maturana also paid special attention to the stratification of processes in the frog's inner plexiform layer, believing tthat the operation performed by a ganglion cell is determined by specific bipolar inputs delivered to the strata of its dendritic arbor. This idea, too, was thought to be inapplicable to the cat, whose inner plexiform layer is less obvioiusly stratified than the frog's. Sutdies to be reviewed here now strongly support this concept for the cat.

Nothing of the actual circuits between particular neuron types was known to Lettvin & Maturana, but fragments of such knowledge accumulated for the cat during the 1970s. Some of the first observations were extremely puzzling. It turned out, for example, that rods and ccones have separate bipolars and thus apparently separate pathways to ganglion cells. But rod signals are also transmitted directly to cones, so why the separate bipolars? Further, the rod bipolar does not contact most ganglion cells, as one might have anticipated, but contacts an amacrine, which in turn contacts, not ganglion cells, but cone bipolar axons! What could be the meaning of this second convergence of rod and cone pathways, and why send the rod signal through such a tortuous route?

The functions of such apparently bizarre paths have been difficult to comprehend for the same reasons that fragments of an integrated circuit cannot be grasped except in the context of its larger diagram. Now, however, with links established between about one-third of the neuron types, broad pathways can be identified and specific hypotheses can be suggested regarding their function. The outline of a detailed mechanistic account of retinal function emerges, and many of the necessary strategies and techniques for achieving it seem at hand. In reviewing this subject now, when many puzzling findings of the past 15 years begin to fit, one is deeply impressed with the intelligence and care of the many individual studies from laboratories that literally girdle the globe.

In this article the first section reviews our knowledge of particular cell types comprising each major class of retinal neuron. Information is presented, where available, for each type regarding morphology, circuitry, distribution, transmitter, and physiology. Such details constitute the primary evidence that the retina is composed of many discrete cell types arranged in a regular mosaic. This section also serves in effect as a "parts list" for the second section, which describes a complex circuit involving thirteen types of neuron and suggests how the circuit might function.

Trends Neurosci. 1986 9: 186-192.

Sterling P, Freed M, Smith RG

Department of Neuroscience, University of Pennsylvania Medical School, Philadelphia 19104-6058.

Neurons in the cat retina belong to specific types. Each type is characterized by a specific corresponddence between morphology and physiology and forms a regular array that connects lawfully to the arrays of certain other types. Two circuits have been traced quantitatively through these arrays from photoreceptors to alpha- and beta-ganglion cells. The 'cone-bipolar circuit' appears to convery the centre-surround receptive field to gangion cells, using cones in daylight and rods (via gap junctions to cones) in twilight. A 'rod-bipolar circuit' appears to convey the quantal signal and the pure center receptive field to the ganglion cells in starlight.

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Neurosci Res Suppl 1987;6:S269-S285

Microcircuitry of the on-beta ganglion cell in daylight, twilight, and starlight.

Sterling P, Cohen E, Freed MA, Smith RG

Department of Anatomy, University of Pennsylvania Medical School, Philadelphia 19104-6058.

Between noon and the end of nightfall, the intensity of light in the environment declines by about ten billion-fold. MOst of the drama in the human experience of this change occurs during the hours that we call "twilight". Colors gradually shift in hue and then desaturate, but spatial resolution is preserved for a while longer. Thus, in a garden the red roses turn purple and then black, but the structure of the bush remains distinct. Only later, as the stars appear, do the details of the foliage dissolve into shadow.

Our experience of these transitions is paralleled to some extent by the behavior of individual ganglion cells in cat retina. So remarkable is their capacity to adapt that they remain responsive to visual stimuli over the full ten log unit range of envioronmental light intensity [1]. In this essay, we review some salient features of this adaptation process. We then summarize recent anatomical studies of the circuits connecting photoreceptors to the ganglion cells and speculate upon the relation of the neural architecture to the function. Only one type of ganglion cell is considered: the ON-center cell known to physiologists as "X" or "brisk-sustained" and to morphologists as "beta" [2-5]. All of the measurements considered here, both physiological and anatomical, refer to neurons in the area centralis.

J Comp Neurol 1980 Aug 15;192(4):737-749

Neurons in cat lateral geniculate nucleus that concentrate exogenous [3H]-gamma-aminobutyric acid (GABA).

Sterling P, Davis TL

About one-quarter of the neurons in the A-laminae of the cat lateral geniculate selectively accumulate exogenous [3H]-gamma-aminobutyric acid (GABA), its analog, [3H]-2,4-diaminobutyric acid (DABA), and the GABA agonist, [3H] muscimol. These neurons are small (12-18 micrometers diameter) and lack a laminar body, which suggests that they correspond to the class III cell identified in Golgi material. GABA and DABA are also accumulated by F-terminals which are post-synaptic to retinal terminals and presynaptic to relay cell dendrites. It is suggested that GABA may be the transmitter for these small neurons which appear to mediate by means of local circuits a feed-forward inhibition onto the relay cells.

J Neurosci 1988 Feb;8(2):623-642

Functional architecture of rod and cone circuits to the on-beta ganglion cell.

Sterling P, Freed MA, Smith RG

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

Photoreceptors connect to the on-beta ganglion cell through parallel circuits involving rod bipolar (RB) and cone bipolar (CB) neurons. We estimated for a small patch in the area centralis of one retina the 3-dimensional architecture of both circuits. This was accomplished by reconstructing neurons and synapses from electron micrographs of 189 serial sections. There were (per mm2) 27,000 cones, 450,000 rods, 6500 CBb1, 30,300 RB, 4100 All amacrines, and 2000 on-beta ganglion cells. The tangential spread of processes was determined for each cell type, and, with the densities, this allowed us to calculate the potential convergence and divergence of each array upon the next. The actual numbers of cells converging and diverging were estimated from serial sections, as were the approximate numbers of chemical synapses involved. The cone bipolar circuit showed narrow convergence and divergence: 16 cones->4 CBb1->1 on-beta 1 cone->1 CBb1->1.2 on-beta This circuit is thought to contribute significantly to the on-beta cell's photopic receptive field because the CBb1 has a center-surround receptive field whose center diameter is greater than the spacing between adjacent CBb1s. Consequently, the receptive fields of the CBb1s converging on a beta cell are probably largely concentric and thus mutually reinforcing in their contributions to the on-beta. The rod bipolar circuit showed a wider convergence and divergence: 1500 rods->100 RB->5 AII->4 CBb1->1 on-beta 1 rod->2 RB->5 AII->8 CBb1->2 on-beta The 1500 rods converging via this circuit account for the spatial extent of the beta cell's dark-adapted receptive field. This convergence also accounts for the ganglion cell's maintained discharge, which is thought to arise from about 6 quantal "dark events" per second. This many dark events would appear in the ganglion cell if each rod in the circuit contributed 0.004 dark events per second, and this is close to what has been measured in monkey rods (Baylor et al., 1984). Divergence in this circuit serves to expand the number of copies of the quantal signal (1 rod->8 CBb1) and so to engage large numbers of chemical synapses that provide amplification. Reconvergence at the last stage (8 CBb1->2 on-beta) may reduce (by signal averaging) the synaptic noise that would otherwise accumulate along the pathway.

J Neurosci 1986 May;6(5):1314-1324

Molecular specificity of defined types of amacrine synapse in cat retina.

Sterling P, Lampson LA

The inner plexiform layer of cat retina contains synaptic structures belonging to 50 or more types of "identified" neurons. To learn whether there are antigens confined to subsets of these synaptic structures, we raised monoclonal antibodies to homogenates of neural retina. Binding patterns of these antibodies were visualized by the peroxidase-antiperoxidase method and studied in serial, ultrathin sections by electron microscopy. Four antibodies stained the synaptic varicosities of certain amacrine cells. Many of the stained varicosities formed reciprocal synapses with a rod bipolar axon terminal, but only about half of the reciprocal synapses associated with a rod bipolar were stained. Other stained varicosities formed synapses with cone bipolar axons, ganglion cell dendrites, and unstained amacrine processes. The patterns were essentially the same for each antibody and were not altered by staining with the antibodies two at a time; therefore, it is likely that all four antibodies stain the same subset of synaptic structures. These patterns would be accounted for if there were staining of all the synaptic varicosities of three of the four types of identified amacrine reciprocally connected to the rod bipolar (A6, A8, A13). This localization suggests that the antigen responsible for the binding pattern is not associated with synaptic transmission. Staining is present in the inner plexiform layer during the period of synaptogenesis and consequently the antibodies are serving as markers for following the development of identified synapses in an identified neural circuit.

Brain Res 1980 Dec;2(3):265-293

A systematic approach to reconstructing microcircuitry by electron microscopy of serial sections.

Stevens JK, Davis TL, Friedman N, Sterling P

To observe certain quantitative features of neuronal geometry and microcircuitry, it is necessary to reconstruct neurons from electron micrographs of serial, ultra-thin sections. We describe here an approach to preparing, photographing, and analyzing moderately long series (100-500 sections). A series is prepared using an assembly line approach: one operator cuts while a second mounts ribbons of sections using various mechanical aids. Photographs are taken in the electron microscope at low magnification and high accelerating voltage. Sequential negatives are aligned using an image combiner and copied, using quasi-coherent illumination, onto 35 mm film. The resulting "movie' is mounted on a precision film transport mounted on an X-Y stage controlled by stepping motors. The movie is viewed through a high resolution video system while a video storage device and switching system permit rapid alternation between frames for comparisons. The profiles of a process in successive frames are "microaligned' by small adjustments of the transport's X-Y position. The absolute X-Y biological coordinates for each frame and the correction necessary to bring it into alignment are stored in a Z80 microprocessor as a process vector. When the movie is re-examined with the stepping motors under control of the computer, the microaligned process shows almost no frame-to-frame jitter. The process vector may be used to generate a "branch schematic' of the neuron. The microaligned profiles can also be digitized and displayed as a reconstruction using a PDP 11/34 computer. Uses of the approach are presented with examples from the cat retina and visual cortex.

Science 1980 Jan 18;207(4428):317-319

Toward a functional architecture of the retina: serial reconstruction of adjacent ganglion cells.

Stevens JK, McGuire BA, Sterling P

Twenty adjacent ganglion cells in cat retina were partially reconstructed from electron micrographs of serial thin sections. Cells were classified by size and by dendritic branching patterns as alpha, beta, or gamma cells. The alpha and beta cells were further subdivided by differences in the laminar distribution of their dendrites in the inner plexiform layer. The distribution of synaptic contacts on the cells was distinctive for each of the five major classes. Contacts on the alpha and beta cells were mainly on the dendrites in the sublamina in which a cell's major dendritic arborization was contained.

Vision Res 1992 Oct;32(10):1809-1815

Gap junctions between the pedicles of macaque foveal cones.

Tsukamoto Y, Masarachia P, Schein SJ, Sterling P

Department of Anatomy, Hyogo College of Medicine, Japan.

Cone terminals ("pedicles") in the fovea of macaque retina were studied in electron micrographs of serial sections. Pedicles were sheathed in glia except for small (0.2 microns 2) fenestrations, 4.8 +/- 1.7 per pedicle. At each fenestration the membranes of adjacent pedicles were contiguous and marked by an adherent junction, which in turn was invariably associated with gap junctions. There were 3.2 +/- 1.4 gap junctions per adherent junction and thus, about fifteen gap junctions per pedicle. The gap junctions were small, 1.6 x 10(-3) +/- 1.8 x 10(-3) microns 2 (mean +/- SD) and were formed indiscriminately with all neighboring pedicles. An upper bound was estimated of 170 connexons per pedicle and thus a coupling conductance of 1.7 x 10(4) pS. Available psychophysical data suggest that the junctions are uncoupled at high luminance. They may couple at lower luminance where spatial averaging would improve contrast sensitivity without cost to spatial acuity.

Proc Natl Acad Sci U S A 1990 Mar;87(5):1860-1864

"Collective coding" of correlated cone signals in the retinal ganglion cell.

Tsukamoto Y, Smith RG, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104.

The signals in neighboring cones are partially correlated due to local correlations of luminance in the visual scene. By summing these partially correlated signals, the retinal ganglion cell improves its signal/noise ratio (compared to the signal/noise ratio in a cone) and expands the variance of its response to fill its dynamic range. Our computations prove that the optimal weighting function for this summation is dome-shaped. The computations also show that (assuming a particular space constant for the correlation function) ganglion cell collecting area and cone density are matched at all eccentricities such that the signal/noise ratio improves by a constant factor. The signal/noise improvement factor for beta ganglion cells in cat retina is about 4.

J Comp Neurol 1995 Jan 16;351(3):374-384

Specific cell types in cat retina express different forms of glutamic acid decarboxylase.

Vardi N, Auerbach P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

We studied the expression of glutamate decarboxylase (GAD), GAD65 and GAD67, in cat retina by immunocytochemistry. About 10% of GABAergic amacrine cells express only GAD65 and 30% express only GAD67. Roughly 60% contain both forms of the enzyme, but GAD67 is present only at low levels in the majority of these double-labeled amacrine cells. The staining pattern in the inner plexiform layer (IPL) for the two GAD forms was also different. GAD65 was restricted to strata 1-4, and GAD67 was apparent throughout the IPL but was strongest in strata 1 and 5. This indicates that somas, as well as their processes, are differentially stained for the two forms of GAD. Cell types expressing only GAD65 include interplexiform cells, one type of cone bipolar cell, and at least one type of serotonin-accumulating amacrine cell. Cell types expressing only GAD67 include amacrine cells synthesizing dopamine, amacrine cells synthesizing nitric oxide (NO), and amacrine cells accumulating serotonin. Cholinergic amacrine cells express a low level of both GAD forms. Our findings in the retina are consistent with previous observations in the brain that GAD65 expression is greater in terminals than in somas. In addition, in retina most neurons expressing GAD67 also contain a second neurotransmitter as well as GABA, and they tend to be larger than neurons expressing GAD65. We propose that large cells have a greater demand for GABA than small cells, and thus require the constant, relatively unmodulated level of GABA that is provided by GAD67.

Vis Neurosci 1994 Jan;11(1):135-142

Horizontal cells in cat and monkey retina express different isoforms of glutamic acid decarboxylase.

Vardi N, Kaufman DL, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

The neurotransmitter used by horizontal cells in mammals has not been identified. GABA has been the leading candidate, but doubt has remained because of failure to clearly demonstrate the GABA synthetic enzyme, glutamic acid decarboxylase (GAD) in these cells. Because GAD was recently shown to exist as two isoforms, 65 kDa and 67 kDa, we considered whether there might be a mismatch between the forms of GAD expressed in horizontal cells and the probes used to detect it. Accordingly, we stained sections of mammalian retina with antibodies specific for each isoform. Cat horizontal cells of both types (A and B) were immunoreactive for GAD67 but negative for GAD65; monkey horizontal cells of both types (H(I) and HII) were positive for GAD65 and negative for GAD67. The findings reconcile previous, apparently conflicting, observations and strengthen considerably the hypothesis that mammalian horizontal cells are GABAergic.

J Comp Neurol 1989 Oct 22;288(4):601-611

Structure of the starburst amacrine network in the cat retina and its association with alpha ganglion cells.

Vardi N, Masarachia PJ, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

To investigate indirect pathways to ganglion cells we studied the starburst amacrine cell network and its relationship to the alpha ganglion cell. Starburst cells were identified by an antiserum to choline acetyltransferase and alpha cells by injection of Lucifer yellow. The density of on and off starburst cells peaks at the area centralis and decreases with eccentricity by a factor of seven. The fine amacrine processes, interrupted by distinct varicosities, arborize in a planar fashion in the inner plexiform layer. The on network, at the junction of strata 3 and 4, and the off network, in stratum 2, have a similar appearance. Neighboring starburst processes run in intimate association to form a network of bundles. As bundles cross each other, loops of irregular size and shape are formed. The loops are smallest in the area centralis and increase by a factor of three towards the periphery; correspondingly, bundle length per unit area decreases with eccentricity. However, the number of varicosities/bundle length stays constant with eccentricity as does the number of processes per bundle. Segments of the starburst network associate over fairly long distances with dendrites of alpha ganglion cells. About 26% of the alpha ganglion dendritic tree shows such association, and this is significantly greater than would be expected if the alpha and starburst processes were independent. We conclude that the functional unit of the starburst cell is a linear bundle of processes and that the starburst network may connect synaptically to the alpha cell.

J Comp Neurol 1992 Jun 15;320(3):394-397

Immunoreactivity to GABAA receptor in the outer plexiform layer of the cat retina.

Vardi N, Masarachia P, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

The distribution of GABAA receptor in the outer plexiform layer of cat retina was studied by immunocytochemistry with monoclonal antibodies. Staining was observed at the base of the cone pedicle, extracellularly, in association with the "triad" synaptic complex. Some bipolar dendrites and the basal processes that interconnect the cone pedicles were also stained. Rod spherules and horizontal cells were negative. The findings support the idea that the cone horizontal cells are GABAergic.

Vis Neurosci 1993 May;10(3):473-478

Identification of a G-protein in depolarizing rod bipolar cells.

Vardi N, Matesic DF, Manning DR, Liebman PA, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

Synaptic transmission from photoreceptors to depolarizing bipolar cells is mediated by the APB glutamate receptor. This receptor apparently is coupled to a G-protein which activates cGMP-phosphodiesterase to modulate cGMP levels and thus a cGMP-gated cation channel. We attempted to localize this system immunocytochemically using antibodies to various components of the rod phototransduction cascade, including Gt (transducin), phosphodiesterase, the cGMP-gated channel, and arrestin. All of these antibodies reacted strongly with rods, but none reacted with bipolar cells. Antibodies to a different G-protein, G(o), reacted strongly with rod bipolar cells of three mammalian species (which are depolarizing and APB-sensitive). Also stained were subpopulations of cone bipolar cells but not the major depolarizing type in cat (b1). G(o) antibody also stained certain salamander bipolar cells. Thus, across a wide range of species, G(o) is present in retinal bipolar cells, and at least some of these are depolarizing and APB-sensitive.

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Vision Res 1996 Dec;36(23):3743-3757

The AII amacrine network: coupling can increase correlated activity.

Vardi N, Smith RG

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Retinal ganglion cells in the cat respond to single rhodopsin isomerizations with one to three spikes. This quantal signal is transmitted in the retina by the rod bipolar pathway: rod-->rod bipolar-->AII-->cone bipolar-->ganglion cell. The two-dimensional circuit underlying this pathway includes extensive convergence from rods to an AII amacrine cell, divergence from a rod to several AII and ganglion cells, and coupling between the AII amacrine cells. In this study we explored the function of coupling by reconstructing several AII amacrine cells and the gap junctions between them from electron micrographs; and simulating the AII network with and without coupling. The simulation showed that coupling in the AII network can: (1) improve the signal/noise ratio in the AII network; (2) improve the signal/noise ratio for a single rhodopsin isomerization striking in the periphery of the ganglion cell receptive field center, and therefore in most ganglion cells responding to a single isomerization; (3) expand the AII and ganglion cells' receptive field center; and (4) expand the "correlation field". All of these effects have one major outcome: an increase in correlation between ganglion cell activity. Well correlated activity between the ganglion cells could improve the brain's ability to discriminate few absorbed external photons from the high background of spontaneous thermal isomerizations. Based on the possible benefits of coupling in the AII network, we suggest that coupling occurs at low scotopic luminances.

Vision Res 1994 May;34(10):1235-1246

Subcellular localization of GABAA receptor on bipolar cells in macaque and human retina.

Vardi N, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058.

The subcellular distribution of GABAA receptor in the macaque and human retina was studied by immunocytochemistry with monoclonal antibodies for the alpha and beta subunits with a particular focus on bipolar cells. Immunoreactivity to GABAA receptor was present on dendritic tips of all bipolar cells. The stain was strongest on bipolar membranes in apposition to horizontal cell processes. Stain was concentrated on the tips of flat and invaginating cone bipolar cells at the base of the cone pedicle and on the invaginating tips of rod bipolar cells. Stain on the cone pedicle membrane was restricted to sites of apposition to stained bipolar dendrites; pedicle membrane in apposition to horizontal cell processes was unstained. Stain was also present on bipolar axon terminals in both on and off strata of the inner plexiform layer. All bipolar cell somas stained faintly; horizontal and Muller cell somas were unstained. The alpha and beta subunits distributed similarly in monkey and human retina. Presence of GABAA receptor on the bipolar dendritic tips suggests that horizontal cells directly affect bipolar cells. Thus, GABAA receptor might mediate the receptive field surround of both off and on bipolar cells. Presence of GABAA receptor on bipolar axon terminals suggests that much of the inhibition feeding back from GABAergic amacrine to bipolar cells is GABAA-mediated.

Neuron 1996 Jun;16(6):1221-1227

Evidence that vesicles on the synaptic ribbon of retinal bipolar neurons can be rapidly released.

von Gersdorff H, Vardi E, Matthews G, Sterling P

Department of Neurobiology and Behavior, State University of New York, Stony Brook, 11794-5230, USA.

We relate the ultrastructure of the giant bipolar synapse in goldfish retina to the jump in capacitance that accompanies depolarization-evoked exocytosis. Mean vesicle diameter is 29 +/- 4 nm, giving 26.4 aF/vesicle, so the maximum evoked capacitance (150 fF within 200 ms) represents fusion of about 5700 vesicles. Two terminals contained, respectively, 45 and 65 ribbon-type synaptic outputs, and a fully loaded ribbon tethers about 110 vesicles. Thus, the tethered pool, about 6000 vesicles, corresponds to the rapidly released pool. Further, the difference between small and large terminals in number of tethered vesicles matches their difference in capacitance jump. This suggests, within a "fire and reload" model of exocytosis, that the ribbon translocates synaptic vesicles very rapidly to membrane docking sites, supporting a maximum release rate of 500 vesicles/active zone/s, until the population of tethered vesicles is exhausted.

Visual Neurosci. 1997 Jul-Aug.14(4):789-94.

Vardi N. Morigiwa K.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

ON cone bipolar cells in rat express the metabotropic receptor mGluR6.

The rod bipolar cell and about five types of ON cone bipolar cells depolarize to light by employing a sign-reversing metabotropic glutamate receptor. Glutamate responses are similar in both rod bipolar and cone bipolar cells, but the receptor mediating this response (mGluR6) was so far demonstrated only in rod bipolar cells. To test if ON cone bipolar cells also express mGluR6, we immunoreacted rat retina with an antibody specific for mGluR6, and studied the staining from serial ultrathin sections. We demonstrate that mGluR6 is indeed expressed in the dendritic tips of cone bipolar cells, the majority of which receive a ribbon synapse, and thus probably are ON cone bipolar cells. We further show that half of the dendritic tips contacting the cones stain for mGluR6, thus implying that all ON cone bipolar cell types express mGluR6.

In: The Synaptic Organization of the Brain, 4th Edit., Gordon Shepherd, (Ed.), Oxford University Press.


Peter Sterling

The retina is a thin sheet of neural tissue lining the posterior hemisphere of the eyeball. It is actually part of the brain itself (~0.5%), evaginating from the lateral wall of the neural tube during embryonic development. The optic stalk grows out from the brain toward the ectoderm, inducing it to form an optical system (cornea, pupil, lens), which projects a physical image of the world onto the retina. The retina's task is to convert this optical image into a "neural image" for transmission down the optic nerve to a multitude of centers for further analysis. The task is complex - which is reflected in the synaptic organization.

The transformation from optical to neural image involves three stages: (1) transduction of the image by photoreceptors; (2) transmission of these signals by excitatory chemical synapses to bipolar cells; and (3) further transmission by excitatory chemical synapses to ganglion cells. Axons from the latter collect in the optic nerve and project forward to the brain. At each synaptic stage there are specialized laterally connecting neurons called, respectively, horizontal and amacrine cells. These modify (largely by inhibitory chemical synapses) forward transmission across the synaptic layers. These elements are shown schematically in Fig. 6.1.

A closer look at this apparently simple design (three interconnected layers and five broad classes of neuron) reveals additional complexity (Figs. 6.2, 6.3). Each neuron class is represented by several or many specific types. Each cell type is distinguished from others in its class by its characteristic morphology, connections, neurochemistry, and function (Rodieck and Brening, 1983; Sterling, 1983). This diversity, amounting to some 80 cellular types (Kolb et al., 1981; Sterling, 1983; Vaney, 1990), was puzzling at first, but a broad explanation has gradually emerged: it is impossible to encode all the information in an optical image using a single neural image. Therefore, the retina uses different cell types to create parallel circuits for different light levels - daylight, twilight, and starlight - but these share certain circuit compaonents and use the same final pathways to the brain (Smith et al., 1986).

This chapter describes key cell types and their interconnection in parallel circuits. It also discusses how the functional architecture of a circuit depends on the functional architecture of its synapses. Finally, it suggests how the flow of visual information shifts between circuits that are specialized for different light levels and how the circuits are switched. The chapter focuses on mammalian retina because that is where the combined knowledge of circuitry and cell physiology is best known. Early efforts centered on cat, so specific measurements, counts, etc., cited here refer to cat central retina. But recent efforts have broadened to include rabbit, rat, monkey, and human. These demonstrate strongly conserved patterns in the circuitry, as well as special adaptations, and some of both will be mentioned.

J. Comp. Neurol. , 1998 May 25. 395(1):43-52

Alpha subunit of Go localizes in the dendritic tips of ON bipolar cells.

Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

The metabotropic glutamate receptor (mGluR6), expressed by rod bipolar cells and ON cone bipolar cells, activates a trimeric guanine nucleotide-binding protein (G-protein) that ultimately closes a cation channel. The G-protein remains unidentified, but the alpha subunit of Go (Go(alpha)) has been suggested as a candidate because it is present in rod bipolar cells. However, the precise subcellular distribution of Go within the rod bipolar cell, and its distribution among cone bipolar cells was not determined. This information is important in assessing the hypothesis that Go couple mGluR6 to its effector. Here I report the distribution of Go (alpha subunit) by immunostaining in several mammalian retinas. The overall distribution is conserved across mammalian species: strongest in the dendrites of ON bipolar cells, moderate in their somas, weak in their axons, and absent from their terminals. Go(alpha) is also present in some amacrine somas and processes. In monkey fovea, where rods and rod bipolar cells are absent, Go(alpha) is present in about half of the bipolar somas which occupy the upper tiers of the bipolar layer, and are therefore identified as ON cone bipolar cells. Ultrastructurally, in monkey and cat, Go(alpha) is present in the dendritic tips of rod bipolar cells and ON cone bipolar cells, which are identified by their invaginating contacts. It is absent from OFF cone bipolar dendrites, which are identified by their flat contacts. It is also absent from axons entering the inner plexiform layer, and their terminals. In the primary dendrites, stain for Go(alpha) mainly associates with the plasma membrane, but in the dendritic tips it is also present in the cytosol. Apparently, Go(alpha) is expressed by the same bipolar cells that also express mGluR6, and is concentrated at the same subcellular location. Thus, Go(alpha) could serve to couple mGluR6 to later stages of its signaling cascade.

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Visual Neurosci. (1998) Sep-Oct 15(5):809-21

Noise removal at the rod synapse of mammalian retina.

van Rossum MC. and Smith RG.

Dept. Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

Mammalian rods respond to single photons with a hyperpolarization of about 1 mV which is accompanied by continuous noise. Since the mammalian rod bipolar cell collects signals from 20-100 rods, the noise from the converging rods would overwhelm the single-photon signal from one rod at scotopic intensities (starlight) if the bipolar cell summed signals linearly (Baylor et al., 1984). However, it is known that at scotopic intensities the retina preserves single-photon responses (Barlow et al., 1971; Mastronarde, 1983). To explore noise summation in the rod bipolar pathway, we simulated an array of rods synaptically connected to a rod bipolar cell using a compartmental model. The performance of the circuit was evaluated with a discriminator measuring errors in photon detection as false positives and false negatives, which were compared to physiologically and psychophysically measured error rates. When only one rod was connected to the rod bipolar, a Poisson rate of 80 vesicles/s was necessary for reliable transmission of the single-photon signal. When 25 rods converged through a linear synapse the noise caused an unacceptably high false positive rate, even when either dark continuous noise or synaptic noise where completely removed. We propose that a threshold nonlinearity is provided by the mGluR6 receptor in the rod bipolar dendrite (Shiells & Falk, 1994) to yield a synapse with a noise removing mechanism. With the threshold nonlinearity the synapse removed most of the noise. These results suggest that a threshold provided by the mGluR6 receptor in the rod bipolar cell is necessary for proper functioning of the retina at scotopic intensities and that the metabotropic domains in the rod bipolar are distinct. Such a nonlinear threshold could also reduce synaptic noise for cortical circuits in which sparse signals converge.

Visual Neurosci. 1998 Jul-Aug; 15(4):743-53,

Regional differences in GABA and GAD immunoreactivity in rabbit horizontal cells.

Johnson MA. Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

Mammalian horizontal cells are believed to be GABAergic because, in most species, they contain both GABA and glutamic acid decarboxylase (GAD), and their terminals are presynaptic to GABA receptors. In adult rabbit, however, GABA and GAD immunoreactivity have not been consistently demonstrated in horizontal cells, casting doubts on the assumption that they too are GABAergic. Here we demonstrate that all rabbit horizontal cell terminals--dendritic terminals of type A, and both dendritic and axonal terminals of type B--immunostain for one isoform of GAD, GAD67, In addition, we show that type A horizontal cell somas and primary dendrites in the visual streak (identified by their immunoreactivity to calbindin) are immunoreactive for the other GAD isoform, GAD65. Double-labeling experiments for GAD65 and GABA reveal that every cell that stains for GAD65 also stains for GABA. The presence of GAD67 in horizontal cell terminals suggests that rabbit horizontal cells are GABAergic. The segregation of the two GAD isoforms to different cell compartments suggests that GABA is released at different sites, possibly by two different mechanisms.

J Neurosci. 1998 May 1 18(9):3373-85,

Microcircuitry and mosaic of a blue-yellow ganglion cell in the primate retina.

Calkins DJ. Tsukamoto Y. Sterling P.

Department of Ophthalmology, University of Rochester Medical Center, New York

Perception of hue is opponent, involving the antagonistic comparison of signals from different cone types. For blue versus yellow opponency, the antagonism is first evident at a ganglion cell with firing that increases to stimulation of short wavelength-sensitive (S) cones and decreases to stimulation of middle wavelength-sensitive (M) and long wavelength-sensitive (L) cones. This ganglion cell, termed blue-yellow (B-Y), has a distinctive morphology with dendrites in both ON and OFF strata of the inner plexiform layer (Dacey and Lee, 1994). Here we report the synaptic circuitry of the cell and its spatial density. Reconstructing neurons in macaque fovea from electron micrographs of serial sections, we identified six ganglion cells that branch in both strata and have similar circuitry. In the ON stratum each cell collects approximately 33 synapses from bipolar cells traced back exclusively to invaginating contacts from S cones, and in the OFF stratum each cell collects approximately 14 synapses from bipolar cells (types DB2 and DB3) traced to basal synapses from approximately 20 M and L cones. This circuitry predicts that spatially coincident blue-yellow opponency arises at the level of the cone output via expression of different glutamate receptors. S cone stimuli suppress glutamate release onto metabotropic receptors of the S cone bipolar cell dendrite, thereby opening cation channels, whereas M and L cone stimuli suppress glutamate release onto ionotropic glutamate receptors of DB2 and DB3 cell dendrites, thereby closing cation channels. Although the B-Y cell is relatively rare (3% of foveal ganglion cells), its spatial density equals that of the S cone; thus it could support psychophysical discrimination of a blue-yellow grating down to the spatial cutoff of the S cone mosaic.

Vision Res. 1998 May 38(10):1359-69, 1998 May.

Neurochemistry of the mammalian cone 'synaptic complex'.

Vardi N. Morigiwa K. Wang TL. Shi YJ. Sterling P.

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, 19104-6058, USA.

The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors. Conclusions: (i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.

Neuron. 1998 21(4):643-4.

"Knocking out" a neural circuit.

Sterling P.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Although the visual system occupies nearly half of the mammalian brain, we still do not completely understand its first synaptic stage. One reason is that the dendrites postsynaptic to photoreceptors comprise such a maze of fine processes that doubt remains whether all the second oreder circuits have been identified - even after 4 decades of electron microscopy. Now advanced functional methods applied to a mammalian rod pathway suggest a circuit previously unsuspected from anatomy.

J. Neurophysiol. 1998 Dec; 80(6):3163-72.

Transmitter concentration at a three-dimensional synapse.

Rao-Mirotznik R. Buchsbaum G. Sterling P.

Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

intensities from starlight to 1000-fold brighter, the mammalian rod synapse transmits a binary signal, the capture of 0 or 1 photon. Zero is signified by tonic exocytosis, and 1 is signified by a brief pause. The synapse is three dimensional: vesicles discharge at the apex of a deep cleft created by the invagination of four postsynaptic processes. Two horizontal cell spines bearing alpha-amino-3-hydroxy-5- methyl- 4-isoxazolepropionic acid (AMPA) receptors reach near to the release sites (16 nm), and two bipolar dendrites bearing mGluR6 receptors end far from the release sites (up to 640 nm). We considered two hypotheses for signal transfer: transmitter quanta might be integrated in the cleft and sensed as a steady concentration (high for 0 and low for 1); or quanta might be sensed at the postsynaptic membrane as discrete postsynaptic potentials (PSPs) and integrated within the dendrite. We calculate from a passive diffusion model that the invagination empties rapidly (tau approximately 1.7 ms). Further calculations suggest that a glutamate concentration high enough to hold a bipolar cell in darkness at one end of its response range would require approximately 4,000 vesicles/s. On the other hand, the glutamate pulse from a single vesicle would reach both nearby AMPA receptors (low affinity) and distant mGluR6 receptors (high affinity) at spatiotemporal concentrations matched to their apparent binding affinities. Thus one vesicle could evoke a discrete PSP in all four postsynaptic processes. We calculate from a stochastic model that PSPs could transfer the binary signal at approximately 100 vesicles/s. Thus dendritic integration of unitary PSPs is both plausible and 40-fold more efficient than the alternative mechanism. The rod's deep invagination, rather than serving to pool transmitter, may serve to prevent "spillover" of transmitter to neighboring rods. Spillover, by pooling the noise from neighboring rods, would impair transmission of their binary signals.

J. Comp. Neurol. 1999 Mar 8; 405(2):173-84.

Differential expression of ionotropic glutamate receptor subunits in the outer retina.

Morigiwa K. Vardi N.

Department of Physiology, Osaka University Medical School, Japan.

Ionotropic glutamate receptors (iGluRs) are extremely diverse in their subunit compositions. To understand the functional consequences of this diversity, it is necessary to know the subunits that are expressed by known cell types. By using immunocytochemistry with light and electron microscopy, we localized several subunits (GluR2/3, GluR4, and GluR6/7) in cat retinal neurons, postsynaptic to photoreceptors. Type A horizontal cells express all three subunits strongly, whereas type B horizontal cells express GluR2/3 strongly, GluR6/7 weakly, and do not express GluR4. When they are present, the subunits are expressed strongly throughout the cytoplasm of the somata and primary dendrites; however, in the terminals, they are concentrated at the postsynaptic region, just opposite the presumed site of photoreceptor glutamate release. Surprisingly, all bipolar cell classes (OFF cone bipolar cells, ON cone bipolar cells, and rod bipolar cells) express at least one iGluR subunit at their dendritic tips. Cone bipolar cells forming basal contacts with the cones (presumably OFF cells) express all three subunits in association with the electron-dense postsynaptic membrane. Invaginating dendrites of cone bipolar cells (presumably ON cells) express GluR2/3 and GluR4. Rod bipolar cells (ON cells) express GluR2/3 in their invaginating dendrites. The function of iGluRs in horizontal cells and OFF bipolar cells clearly is to mediate their light responses. GluR6/7 subunit in the receptor of these cells may be responsible for the dopamine-mediated enhancement of glutamate responses that have been observed previously in these cells. The function of iGluRs in ON bipolar cells remains an enigma.

J. Neurosci. 1999 Jun 1 19(11):4221-8.

Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones.

Wang TL. Sterling P. Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.

Nature Neuroscience. 1999 Oct; 2(10):851-3.

Deciphering the retina's wiring diagram.

Sterling P.

Department of Neuroscience, University of Pennsylvania, 123 Anatomy/Chemistry Bldg., Philadelphia, Pennsylvania 19104-6058, USA.

By simultaneously recording from retinal ganglion cells while stimulating a single cone, Chichilnisky and Baylor demonstrate that the strength of physiological connections within a retinal microcircuit is linearly proportional to the number of anatomically defined synapses.

Over the last few decades, considerable effort has been devoted to constructing a detailed 'wiring diagram' for the retina, that is, a quantitative map of all its excitatory and inhibitory synaptic connections1, 2. Propelling this effort has been a faith that the diagram would ultimately make functional sense—that we would be able to 'read' the diagram of a neural circuit just as we do for an electronic circuit. Were this faith to prove unjustified—if, for example, the physiological strength of an input proved to be unrelated to the number of anatomically defined synapses—then the retina's synaptic diagram would be far less informative than we might otherwise hope. It is certainly reasonable to question the usefulness of a purely structural diagram, given that neurons contain nonlinear elements such as voltage-gated channels and second-messenger cascades. However, a report by Chichilnisky and Baylor in this issue of Nature Neuroscience3 provides reassurance. By analyzing how individual cones contribute to the receptive field of a retinal gangion cell, the authors provide support for a key hypothesis derived from the wiring diagram, that the strength of an excitatory input is proportional to the number of chemical synapses that underlie it.

Retinal ganglion cells are the output neurons of the retina. They are separated by two synapses from the photoreceptors, and the intervening wiring suggests that ganglion cells are capable of considerable integration. Cones are connected laterally to each other through electrical synapses, and they project forward via glutamatergic synapses onto bipolar cells. Similarly, the bipolar cells are connected laterally via electrical synapses and also make forward connections via glutamatergic synapses onto the wide-spreading dendrites of ganglion cells. Thus, a ganglion cell receives connections, via bipolar cells, from many cones (convergence), whereas a given cone is connected to several ganglion cells (divergence). Quantitative anatomical studies have revealed that a ganglion cell receives many synapses from a bipolar cell that is aligned with the center of its dendritic tree, but few synapses from bipolar cells at the edge of the dendritic tree. Most of the synapses formed by these bipolar cells are onto neighboring ganglion cells with which they are more closely aligned.

This wiring causes a 'dome-like' distribution of excitatory synapses across a ganglion cell's dendritic tree. The distribution of light sensitivity across the central region of a ganglion cell's receptive field is also dome-shaped, and so it was natural to hypothesize that this might reflect the distribution of synaptic inputs4, 5. Furthermore, the fact that a single bipolar cell can form synapses with more than one ganglion cell suggests an explanation for the overlap between the receptive field centers of neighboring ganglion cells6, 7. These hypotheses could be tested by recording simultaneously from several adjacent ganglion cells while stimulating the overlying cones one at a time. A method for multiple recordings has been available for several years8, but cones are so closely packed in the intact retina that it is difficult to stimulate them one at a time. That is what Chichilnisky and Baylor have now achieved in a technical and analytical tour de force that confirms both hypotheses.

The authors placed a small piece of peripheral retina from a macaque monkey in a perfusion chamber, with the ganglion cell layer contacting a multi-electrode array. This allowed simultaneous recording of the spike trains from many ganglion cells. To stimulate individual cones, they projected the image of a color monitor onto the photoreceptors through reducing optics, so that a single pixel would span about 25 m, comparable to the distance between adjacent cones. Each pixel could be driven by the monitor's red, green and blue guns, and by adjusting their relative intensities, a given pixel could be made to emit light that was optimal for one of the three cone types, which are sensitive to short (S), medium (M) or long (L) wavelengths. By using a variety of flickering patterns, it was possible to calculate the average stimulus displayed in the period just preceding a spike and hence to identify the combination of gun intensities that was most likely to trigger a spike in a given ganglion cell.

This approach allowed the authors to cleanly identify the well known 'blue/yellow' (BY) ganglion cells9. These cells fire selectively in response to an increase in the intensity of blue light and a decrease in the intensity of yellow light (that is, a mixture of red and green light; Fig. 1a). The cell responds oppositely to excitation of S and M+L cones, despite the fact that all bipolar synapses excite the BY cell. The reason is that bipolar types selective for S or M+L cones respond oppositely to the glutamate released from cones. S bipolar dendrites express a metabotropic receptor (mGluR6) that closes a cation channel, whereas M+L bipolar dendrites express an ionotropic receptor that opens a cation channel. This molecular difference between the two classes of bipolar cells is largely responsible for the spectral opponency of the BY ganglion cells, and thus represents a key mechanism for blue/yellow perception10.

The BY ganglion cell was ideal for the authors' purpose. S cones comprise only ten percent of all cones, and in the coarse mosaic of peripheral retina (Fig. 1b), they are spaced widely enough to be stimulated individually by the flash of a single pixel. By stimulating individual S cones and recording from BY ganglion cells, the authors could probe for divergence and convergence of the inputs to the ganglion cells, and they could directly test for linearity of S-cone summation.

A single S cone signal did indeed diverge to provide input to adjacent BY ganglion cells, contributing strongly to one BY cell and weakly to another. Because the two BY cells were recorded simultaneously, their different responses to stimulation of the same cone could not be attributed to a change in cone photosensitivity; rather, it must have been caused by a difference in the cone's synaptic connections. Furthermore, two S cones did indeed converge onto a given BY cell, with one cone always dominating and the other contributing only weakly. Finally, a plot of the BY cell's average spike rate versus stimulus intensity for illumination of both S cones neatly superimposed onto plots of rate versus stimulus intensity for illumination of either S cone separately. The finding that all three curves superimpose proves that the signals from both S cones combine linearly. Thus, despite the existence of nonlinear mechanisms, such as sodium action potentials in ganglion cell dendrites11, the overall circuit provides a linear summation of S cone signals.

These functional results clearly match the structural microcircuit (Figs. 1c and 2). An S cone provides most of its synapses to one S bipolar cell and a few synapses to the neighboring bipolar cells10. In turn, an S bipolar cell provides most of its synapses to one BY ganglion cell and a few synapses to the neighbors10. Thus, the synaptic connections from one S cone diverge to several BY cells, and connections from several S cones converge upon one BY cell, with one cone predominating. The structural circuit also shows the BY cell receiving twofold more synapses from S bipolar cells than from M+L bipolar cells10. This might explain why a BY cell responds faster to S cones than to M and L cones3 ( Fig. 1a), because speed increases with amplification.

The BY cell's microcircuit, now based on functional as well as structural evidence, suggests an explanation for a striking result from human psychophysics13. When M and L cones are desensitized by exposure to yellow light, it is possible to perceive a tiny blue flash that is designed to excite a single S cone. Sensitivity to this flash shows a punctate distribution, corresponding to the distribution of S cones14. This punctate perceptual sensitivity might now be explained by two simple facts: the density of the BY mosaic is the same as the S cone mosaic10, 14, and each BY cell is dominated by one of the several converging S cones3, 10. Thus, perception of the colored flash seems attributable to sharply increased firing by a single BY cell, plus a smaller increase by its nearest neighbors. If this explanation is correct (others are possible but more complicated), it implies that this color percept results from activity in a 'labeled line'. An important goal of neuroscience is to correlate neural structure and function with behavior, so this result is deeply satisfying.

Why should ganglion cell circuits have evolved to create dome-like receptive fields with partial overlap? For example, what is gained by having several S cones converge onto one BY cell, with one S cone predominant, and by having each cone provide divergent inputs to several ganglion cells? Signals in adjacent photoreceptors tend to be correlated because of correlations in the natural scene. Calculations suggest that, when partially correlated signals sum in a ganglion cell, a dome-like weighting of the inputs leads to an optimal signal-to-noise ratio15. The same might be true for more central mechanisms that sum the partially correlated signals of ganglion cells. This hypothesis might be tested directly using the efficient experimental system that Chichilnisky and Baylor have provided.

The structure–function relationship shown here is unlikely to prove completely general. For example, ganglion cells are known to show nonlinear responses to certain stimuli, and it would be surprising if these were to be predicted simply from reading the anatomical circuit. Similarly, it is unlikely that the functions of cortical circuits will be obvious from their wiring, given that cortical synapses can undergo plastic changes in their physiological strength. Despite this, the present results encourage one to press on with the effort to correlate structure and function.

J. Neurosci. 1999 Apr 15; 19(8):2954-9.

AMPA receptor activates a G-protein that suppresses a cGMP-gated current.

Kawai F. Sterling P.

Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6058, USA.

The AMPA receptor, ubiquitous in brain, is termed "ionotropic" because it gates an ion channel directly. We found that an AMPA receptor can also modulate a G-protein to gate an ion channel indirectly. Glutamate applied to a retinal ganglion cell briefly suppresses the inward current through a cGMP-gated channel. AMPA and kainate also suppress the current, an effect that is blocked both by their general antagonist CNQX and also by the relatively specific AMPA receptor antagonist GYKI-52466. Neither NMDA nor agonists of metabotropic glutamate receptors are effective. The AMPA-induced suppression of the cGMP-gated current is blocked when the patch pipette includes GDP-beta-S, whereas the suppression is irreversible when the pipette contains GTP-gamma-S. This suggests a G-protein mediator, and, consistent with this, pertussis toxin blocks the current suppression. Nitric oxide (NO) donors induce the current suppressed by AMPA, and phosphodiesterase inhibitors prevent the suppression. Apparently, the AMPA receptor can exhibit a "metabotropic" activity that allows it to antagonize excitation evoked by NO.

Neuron 1999 Oct;24(2):313-21.

Evidence that circuits for spatial and color vision segregate at the first retinal synapse

Calkins DJ. Sterling P.

Department of Ophthalmology, University of Rochester Medical Center, New York

Different psychophysical "channels mediate our perception of fine spatial patterns and color. The spatial channel compares the photon catch betwen adjacent cones, regardless of type, and thus supports perceptions as fine as the cone mosaic (Williams, 1986). Color channels compare the photon catch between diffeent cone types (Hurvich and Jameson, 1957; Krauskopf et al., 1982). The "blue/yellow" channel compares the catch in cones sensitive to short (S) wavelengths with the catch in neighboring cones sensitive to middle (M) and long (L) wavelengths (Pugh and Mollon, 1979). The "red/green" channel compares the photon catch in L and S cones with that in neighboring M cones (Thornton and Pugh, 1983; Calkins et al., 1992). Each comparison is equivalent to a subtraction: for blue/yellow, S-(M+L); for red/green, L-M+(S). The color channels are rather coarse, with an acuity of about one-third that of the spatial channel (Anderson et al., 1991; Sekiguchi et al., 1993).

What neural circuits servie the psychophysical channels for spatial acuity and color? The standard view is that messages for both channels are conveyed by one type of ganglion cell, termed "P", because it projects to a parvocellular neuron in the lateral geniculate nucleus. The parvocellular neuron would relay both messages to primary visual cortex where, finally, spatial and color information would begin to segregate (Ingling and Martinez-Uriegas, 1983; Lennie and D'Zmura, 1988; Hubel and Livingstone, 1990; De Valois and De Valois, 1993). This circuit design, by compressing two messages on one axon, would minimize the number of axons in the optic nerve and geniculo-striate radiation. But it would also compromise the filtering of one or both messages and would reduce their shares of bandwidth (Atick, 1992).

Although this "double duty" hypothesis for the P cell still predominates (e.g., Boycott and Wassle, 1999), its anatomical and physiological basis has recently been eroded. Meanwhile new evidence has accumulated for an alternative view, that the densely distributed P cell serves only the fine spatial channel and that sparsely distributed ganglion cells serve the color channels (Rodieck, 1991). This circuit design would optimize filtering and bandwidth at the cost of additional axons. Here, we summarize the current evidence for both views, starting with the origin of the P cell hypothesis.

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J. Neurosci. 1999 Nov 15;19(22):9756-9767

Functional circuitry of the retinal ganglion cell's nonlinear receptive field.

Demb JB. Haarsma L. Freed MA. Sterling P.

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

A retinal ganglion cell commonly expresses two spatially overlapping receptive field mechanisms. One is the familiar "center/surround," which sums excitation and inhibition across a region somewhat broader than the ganglion cell's dendritic field. This mechanism responds to a drifting grating by modulating firing at the drift frequency (linear response). Less familiar is the "nonlinear" mechanism, which sums the rectified output of many small subunits that extend for millimeters beyond the dendritic field. This mechanism responds to a contrast-reversing grating by modulating firing at twice the reversal frequency (nonlinear response). We investigated this nonlinear mechanism by presenting visual stimuli to the intact guinea pig retina in vitro while recording intracellularly from large brisk and sluggish ganglion cells. A contrast-reversing grating modulated the membrane potential (in addition to the firing rate) at twice the reversal frequency. This response was initially hyperpolarizing for some cells (either ON or OFF center) and initially depolarizing for others. Experiments in which responses to bars were summed in-phase or out-of-phase suggested that the single class of bipolar cells (either ON or OFF) that drives the center/surround response also drives the nonlinear response. Consistent with this, nonlinear responses persisted in OFF ganglion cells when ON bipolar cell responses were blocked by L-AP-4. Nonlinear responses evoked from millimeters beyond the ganglion cell were eliminated by tetrodotoxin. Thus, to relay the response from distant regions of the receptive field requires a spiking interneuron. Nonlinear responses from different regions of the receptive field added linearly.

Neurosci. Res. Suppl (1991) strong>15: S185-S198

Spatial summation by ganglion cells: Some consequences for the efficient encoding of natural scenes.

Tsukamoto Y, Sterling P

A basic feature of retinal structure is the anatomical convergence of cones onto ganglion cells. This was appreciated by 19th Century anatomists, Cajal formost among them, and it was demonstrated functionally as "spatial summation" by physiologists starting about 50 years ago with Hartline (14) and later Barlow (3) and Kuffler (17). The processes of anatomical convergence and phnhysiological summation are accomplished through architecturally precise circuits. That is, for a particular type of ganglion cell at a given retinal locus, the dendritic spread, number of cones, and receptive field dimensions are invariant (7-11, 13, 23, 25). Each locus has several types of ganglion cell that form parallel systems, summming signals from the same cone array through equally precise, but quantitatively different ar4chitectures. Across the retina, on the other hand, the details of each circuit vary markedly and systematically (7, 20, 25, 37). We hypothesize that the local precision of the ganglion cell circuit and its variation with eccentricity reflect tunint (through evolution) fo efficiency in the enciding of natural scenes. The pressure to achieve a certain level of efficiency arises from the need to achieve a certain standard of performance on the one hand and physical constraints on the encoding processes on the other. In this essay we review both the standards of performance and the constraints and summarize the circkuit architecture used by the on-beta ganglion cell in cat area centralis for spatially summing cone signals. We compare this architecture to that of beta cells across the retina and to that of two other types of ganglion cell: the on-alpha cell (cat) and the parasol cell (monkey). The goal is to discover some consequences of these architectures for the efficient coding of natural scenes.

Prog. Brain Res 1992 90: 107-131.

GABAergic circuits in the mammalian retina

Freed MA

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

This chapter's theme is that GABAergic circuits modify signals running through both rod and cone bipolar pathways. These modifiying circuits, in keeping with their name, are often circular chains of neurons, as distinct from the linear bipolar pathways, and therefore seem to mediate feedback. Another function of these modifying circuits can be understood in the context that, since each neuronal type is repeated across the retina, rod and cone bipolar pathways represent many parallel routes: the modifying circuits convey information laterally between these parallel routes. One modifying circuit cell is the horizontal cell, which interacts with rods, cones and bipolar cells in some manner not fully understood. In mammals, there are at least 2 types of horizontal cell (Ramon Y Cajal, 1972). There is equivocal evidence that both types of horizontal cell are GABAergic. Another modifying circuit cell is the amacrine cell, which interacts with bipolar cells, ganglion cells and other amacrine cells. There is good evidence that specific types of amacrine cells are GABAergic. A third kind of modifying neuron, the interplexiform cell, receives synapses from amacrine cells in the inner plexiform layer and synapses upon bipolar cell dendrites in the outer plexiform layer, constituting a longer feedback loop. In mammals, there is good evidence that 2 types of interplexiform cell are GABAergic.

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J Optical Soc. Am. A 2000 Mar 17:(3) 635-640.

Cost of cone coupling to trichromacy in primate fovea.

Hsu A, Smith RG, Buchsbaum G, Sterling P

Cone synaptic terminals couple electrically to their neighbors. This reduces the amplitude of temporally uncorrelated voltage differences between neighbors. For an achromatic stimulus coarser than the cone mosaic, the uncorrelated voltage difference between neighbors represents mostly noise; so noise is reduced more than the signal. Here coupling improves signal-to-noise ratio and enhances contrast sensitivity. But for a chromatic stimulus the uncorrelated voltage difference between neighbors of different spectral type represents mostly signal; so signal would be reduced more than the noise. This cost of cone coupling to encoding chromatic signals was evaluated using a compartmental model of the foveal cone array. When cones sensitive to middle (M) and long (L) wavelengths alternated regularly, and the conductance between a cone and all of its immediate neighbors was 1000 pS (similar to 2 connexons/cone pair), coupling reduced the difference between the L and M action spectra by nearly fivefold, from about 38% to 8%. However, L and M cones distribute randomly in the mosaic, forming small patches of like type, and within a patch the responses to a chromatic stimulus are correlated. In such a mosaic, coupling still reduced the difference between the L and M action spectra, but only by 2.4-fold, to about 18%. This result is independent of the L/M ratio. Thus "patchiness" of the L/M mosaic allows cone coupling to improve achromatic contrast sensitivity while minimizing the cost to chromatic sensitivity.

J Neurophysiol. 2000 In Press

Rate of quantal excitation to a retinal ganglion cell evoked by sensory input.

Freed MA

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

To determine the rate and statistics of light-evoked transmitter release from bipolar synapses, intracellular recordings were made from On-alpha ganglion cells in the periphery of the intact, superfused, cat retina. Sodium channels were blocked with tetrodotoxin to prevent action potentials. A light bar covering the receptive field center excited the bipolar cells that contact the alpha cell and evoked a transient then a sustained depolarization. The sustained depolarization was quantified as change in mean voltage (), and the increase in voltage noise that accompanied it was quantified as change in voltage variance (2). As light intensity increased, and 2 both increased, but their ratio held constant. This behavior is consistent with Poisson arrival of transmitter quanta at the ganglion cell. The response component attributable to glutamate quanta from bipolar synapses was isolated by application of CNQX (6-cyano-7-nitroquinoxaline). As CNQX concentration increased, the signal/noise ratio of this response component (CNQX/CNQX) held constant. This is also consistent with Poisson arrival and justified the application of fluctuation analysis. Two different methods of fluctuation analysis applied to CNQX and CNQX produced similar results, leading to an estimate that a just-maximal sustained response was caused by about 3700 quanta s-1. The transient response was caused by a rate which was no more than 10-fold greater. Since the On-alpha cell at this retinal locus has about 2200 bipolar synapses, one synapse released about 1.7 quanta s-1 for the sustained response and no more than 17 quanta s-1 for the transient. Consequently, within the ganglion cell's integration interval, here calculated to be about 16 ms, a bipolar synapse rarely releases more than one quantum. Thus for just-maximal sustained and transient depolarizations, the conductance modulated by a single bipolar cell synapse is limited to the quantal conductance (about 100 pS at its peak). This helps preserve linear summation of quanta. The 2/ ratio remained constant even as the ganglion cell's response saturated, which suggested that even at the peak of sensory input, summation remains linear, and that saturation occurs before the bipolar synapse.

J Neurosci. (2000) In Press. Parallel cone bipolar pathways to a ganglion cell use different rates and amplitudes of quantal excitation.

Freed MA (2000)

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

The cone signal reaches the cat's On-beta (X) ganglion cell via several parallel circuits (bipolar cell types b1, b2, and b3). These circuits might convey different regions of the cone's temporal bandwidth. To test this, I presented a step of light which elicited a transient depolarization followed by a sustained depolarization. The contribution of bipolar cells to these response components was isolated by blocking action potentials with tetrodotoxin and by blocking inhibitory synaptic potentials with bicuculline and strychnine. Stationary fluctuation analysis of the sustained depolarization gave the rate of quantal bombardment: about 5,100 quanta s-1 for small central cells and about 45,000 quanta s-1 for large peripheral cells. Normalizing these rates for the vastly different numbers of bipolar synapses (150-370 per small cell vs. 2000 per large cell), quantal rate was constant across the retina, about 22 quanta synapse-1 s-1. Non-stationary fluctuation analysis gave the mean quantal EPSP amplitude: about 240 microvolt for the transient depolarization and 30 microVolt for the sustained depolarization. The b1 bipolar cell is known from noise analysis of the On-alpha ganglion cell to have a near-maximal sustained release of only about 2 quanta synapse-1 s-1. This implies that the other bipolar types (b2 and b3) contribute many more quanta to the sustained depolarization (more than 46 synapse-1 s-1). Type b1 probably contributes large quanta to the transient depolarization. Thus, bipolar cell types b1 and b2/b3 apparently constitute parallel circuits that convey, respectively, high and low frequencies.

In: Analysis and Modeling of Neural Systems, Ed. by Frank H. Eeckman. Kluwer Academic Publishers.

Retinal circuits for daylight: why ballplayers don't wear shades.

Sterling, P., Cohen, E., Smith, R.G., and Tsukamoto, Y. (1992)

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

A natural scene contains fine spatial detail at low contrast (Srinivasan et al., 1982), and to represent it as an optical image on the retina requires quite a lot of light. This is because the number of photons arriving at a given locus fluctuates according to Poisson statistics. For an image to emerge above this fluctuating background of "photon noise" an object must be brighter than the mean luminance by at least one standard deviation (corresponding to the square root of the mean luminance) (Rose, 1973). Consider an example from baseball, a high fly ball just barely visible against the bright sky. At 100 meters from the outfielder's eye the ball subtends only 12 cones and is brighter than the sky by about 0.3%. To represent this low contrast spot requires that the ball reflect onto the retina at least 12,000 photons per cone integration time (sqrt(12000) / 12000 = 0.3%). The number of extra photons per cone above the mean is only 30.

Were an outfielder to wear sunglasses (which they do not, except to follow the ball directly across the sun), the mean luminance would be reduced by, say, ten-fold. The least detectable contrast at 100 meters would then be 0.9% (sqrt(1200)/1200=0.9%). Since the contrast of the ball against the sky is independent of mean luminance (and thus still only 0.3%) the ball would be invisible. To provide the minimum number of photons requisite for detection (12,000) the ball's image on the retina must be larger. Now it must subtend 120 cones but this occurs only when the ball closes the distance to 32 meters. Thus, for the optical image on the outfielder's retina, every photon is precious, even in broad daylight (see Pelli, 1990; Banks et al., 1987).

Noise in the optical image from photon fluctuations is only the first problem, however, because creating the neural image involves additional noise. Each step in visual transduction and synaptic transmission involves Poisson statistics whose noise levels also follow the "square root law" (Attwell, 1986). Thus, how soon the outfielder sees the ball will depend on the efficiency of transduction and also on efficient encoding by the neural circuits leading from the cones. One anticipates that the urgent need to preserve the signal/noise (S/N) ratio of the neural image would be expressed in the design of these circuits.

Here we review the functional archkitecture of the circuit in cat retina that connects cones to one type of ganglion cell. This is the "on-beta" ("X") cell (Boycott and Wassle, 1974), which responds linearly (Enroth-Cugell and Robson, 1966), and whose receptive field is fitted by a difference-of-Gaussians function (Linsenmeier et al., 1982). The on-beta cell and its complement, the "off-beta", have the highest sampling densities and the narrowest sampling apertures in the cat retina (Wassle et al., 1981; Cleland et al., 1979). The beta cell arrays apparently relay to the higher visual areas the finest grained neural image. We describe the structure of the cone circuit to the on-beta and suggest how the architecture might contribute to efficient coding of a fine, low contrast, neural image.

Vision Res. (1998)38: 2539-2549

Functional architecture of primate cone and rod axons

Hsu, A., Tsukamoto, Y., Smith, R.G., and Sterling, P.

The cone axon is nearly four times thicker than the rod axon (1.6 vs. 0.45 um diameter). To assess how signal transfer and integration at the terminal depend on cable dimensions, a transducer (cone = ohmic conductance, rod = current source) coupled via passive cable to a sphere with a chloride conductance (representing GABAa receptor) was modelled. For a small signal in peripheral cone with a short axon, a steady photosignal transfers independently of axon diameter despite a significant chloride conductance at the cone terminal. A temporally varying photosignal also transfers independently of axon diameter up to 20 Hz and is attenuated only 20% at 50 Hz. Thus, to accomplish the basic electrical functions of a peripheral cone, a thin axon would suffice. For a foveal cone with a long axon, a steady photosignal transfers independently of axon diameter, but a temporally varying photosignal is attenuated 5-fold at 50 Hz for a thick axon and 10-fold for a thin axon. This might contribute to the lower sensitivity of central retina to high temporal frequencies. The cone axon contains 14-fold more microtubules than the rod axon, and its terminal contains at least 20-fold more ribbon synapses than the rod's. Since ribbon synapses sustain high rates of exocytosis, the additional microtubules (which require a thicker axon) may be needed to support a greater flux of synaptic vesicle components.

Nature 1995 377:676-677.

Tuning retinal circuits.

Sterling P

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.

WHILE studying the retina more than 100 years ago, Santiago Ramon y Cajal noted that deposits of silver dichromate completely filled the arborization of a single neuron but stopped at the cell boundary. He concluded that contiguous neurons are discrete and that signals between them must cross an extracellular gap, now known as a chemical synapse. He vigorously defended this idea against 'reticularism', the view that neurons form continuous networks, with his penetrating observations and towering polemics, and by 1935 reticularism was apparently crushed [1].

But we now know that the reticularists were also right: retinal neurons can couple with one another by means of clusters of fine intercellular channels called gap junctions [2] and it seems that this coupling is crucial for retinal function. New results described on page 734 of this issue by Mills and Massey [3] offer a deeper insight into reticularism with the demonstration that these gap junctions differ in pore size at different points in a circuit and that they are regulated by different second messengers.

The authors injected tracers of different molecular weights into an AII amacrine cell of the rabbit retina (maintained in vitro) and followed their spread into adjacent neurons to which the AII is known to be coupled. As expected [4], the smaller tracer spread into the neighbouring AII cells and cone bipolar cells. A larger tracer with the same charge also spread readily into AII cells, but penetrated poorly into cone bipolars (see Figure 2 and Figure 3 of Mills and Massey's paper [3]). High concentrations of cyclic AMP were already known to curtail tracer spread to AII cells [4], and the authors found that cyclic GMP had a similar effect on tracer spread to cone bipolar cells (see their Figure 4).

This discovery of a difference in pore size and in second messengers raises a host of questions that may eventually link the architecture of microcircuits to that of their computationally important molecules. To grasp the key issues requires some knowledge of the overall circuitry in which the AII cell is a critical link (see Figure 1).

Figure 1. To convey the full range of environmental intensities requires three different but partially overlapping neural circuits. The mammalian retina accomplishes this at no extra cost in space or noise by using four sets of gap junctions (red), at least three of which are regulated. In daylight, cone terminals excite (depolarize) cone bipolar cells that chemically excite the ganglion cell. This last excitatory stage occupies most of the ganglion cell's dendritic surface (some space is reserved for inhibitory synapses). Gap junctions couple the cone terminals strongly enough to improve signal-to-noise ratio, but not so much as to degrade acuity. In twilight, cones are less active, but the 50 rods surrounding each cone couple to it via their terminals. In effect, rods 'parasitize' the cone terminals and thus the rest of the cone bipolar circuit. In starlight, only one rod per thousand transduces a photon over one second, so the rod-cone junction conveys mostly noise. Consequently, this junction uncouples, and rod terminals excite rod bipolar cells to excite All amacrine cells chemically. The All cell couples to the cone bipolar axon, in effect parasitizing the cone bipolar-to-ganglion cell synapses. Gap junctions couple All cells strongly enough to spread current widely and thus enlarge the ganglion cell's summation area well beyond its dendritic tree. This would improve signal-to-noise ratio in the dimmest light, but degrade acuity in brighter light, so this junction is also regulated.

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Effects of Noise on the Spike Timing Precision of Retinal Ganglion Cells.

van Rossum, MCW., O'Brien BJ. and Smith, RG. J. Neurophysiol. 2003 89: 2406-2419;

Information in a spike train is limited by variability in the spike timing. This variability is caused by noise from several sources including synapses and membrane channels, but how deleterious each noise source is and how they affect spike train coding is unknown. Combining physiology and a multi-compartment model we studied the effect of synaptic input noise and voltage-gated channel noise on spike train reliability for a mammalian ganglion cell. For tonic stimuli the standard deviation of the interspike intervals increased supra-linearly with increasing interspike interval. Voltage gated channel noise and synaptic noise caused fluctuations in the interspike interval of comparable amplitude. Spikes initiated on the dendrites caused additional spike timing fluctuations. Ca and KCa channels present in the model reduced spike train variability. For transient stimuli, synaptic noise was dominant. Spontaneous background activity strongly increased fluctuations in spike timing, but decreased the latency before the first spike. These results constrain the neural coding strategy.

In: Computation in Neurons and Neural Systems" (1994) Ed. by Frank H. Eekman. Kluwer Academic Publishers, Boston.

Simulating the foveal cone receptive field

Hsu A and Smith RG

The foveal midget ganglion cell has a receptive field center fed by one cone. The surround might also be fed by the same center cone since a cone terminal laterally connects to neighboring cones through electrical coupling and horizontal cells. To explore the contributions of the cone lateral connections to the receptive field, we constructed a compartmental model of the primate foveal outer plexiform layer based on the known anatomy and physiology. The similarity between the computed cone receptive field and the measured midget cell receptive field suggest that much of the retina's spatial filtering occurs at the very first synapse.

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Electrical coupling between mammalian cones

DeVries S; Qi X; Smith R; Makous W; Sterling P

Current Biology 2002 12:1900-1907.


Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cones noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image.


In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eyes optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eye's optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%.


We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.

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Contrast Threshold of a Brisk-Transient ganglion cell

Dhingra NK, Kao YH, Sterling P, Smith RG

J. Neurophysiol. 2003 89: 2360-2369.


We have measured the contrast threshold for a mammalian brisk-transient ganglion cell. The intact retina (guinea pig) was maintained in vitro, and extracellular spikes were recorded to a spot with sharp onset, flashed for 100 ms over the receptive field center. An "ideal observer" was given the spike responses from 100 trials at each contrast and then asked to predict the stimulus contrast on 100 additional trials in a single-interval, two-alternative forced-choice procedure. The prediction was based on spike count, latency or temporal pattern. Brisk-transient cells near 37(C detected contrasts as low as 0.8% (mean ( SEM = 2.8 ( 0.2%) and discriminated contrast increments with about 40% greater sensitivity. Performance was temperature sensitive, declining with a Q10 ~2, similar to that of retinal metabolism. These measurements of performance provide an important benchmark for comparison to retinal cell types upstream of the ganglion cell and downstream - to behavior. For example, human psychophysical threshold for a stimulus that just covers the dendritic field of one human brisk-transient cell is the same as found here. This suggests that neural processing across many levels of noisy central synapses might be highly efficient.

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Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit

Michael A. Freed, Robert G. Smith, and Peter Sterling

Neuron 2003 38: 89-101.


In isolation, a presynaptic terminal generally releases quanta according to Poisson statistics, but in a circuit its release statistics might be shaped by local feedback. We monitored quantal release of glutamate from retinal bipolar cell terminals (which receive GABA-ergic feedback from amacrine cells) by recording spontaneous EPSCs in their postsynaptic amacrine and ganglion cells. EPSCs were temporally correlated in about one third of these cells, arriving in brief bursts (~20 ms) more often than expected from a Poisson process. Correlations were suppressed by antagonizing the GABAC receptor (expressed on bipolar terminals), and correlations were induced by raising extracellular calcium or osmolarity (which increase release probability). Simulations of the feedback circuit produced ``bursty'' release when the bipolar cell escaped intermittently from inhibition. Correlations of similar strength and duration were also present in light-evoked EPSCs and ganglion cell spikes. These correlations were also suppressed by a GABAC antagonist, indicating that bursts of glutamate from bipolar terminals induce spike bursts in ganglion cells.

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Spike generator limits efficiency of information transfer in a retinal ganglion cell

Dhingra NK and Smith RG

J. Neurosci. 2004 24: 2914-2922.


The quality of the signal a retinal ganglion cell transmits to the brain is important for preception because it sets the minimum detectable stimulus. The ganglion cell converts graded potentials into a spike train with a selective filter but in the process adds noise. To explorehow efficiently information is transferred to spikes, we measured contrast detection threshold and increment threshold from graded potential and spike responses of brisk-transient ganglion cells. Intracellular responses to a spot flashed over the receptive field center of the cell were recorded in an intact mammalian retina maintained in vitro at 37°C. Thresholds were measured in a single-interval forced-choice procedure with an ideal observer. The graded potential gave a detection threshold of 1.5% contrast, whereas spikes gave 3.8%. The graded potential also gave increment thresholds approximately twofold lower and carried 60% more gray levels. Increment threshold dipped below the detection threshold at a low contrast ( < 5%) but increased rapidly at higher contrasts. The magnitude of the dipper for both graded potential and spikes could be predicted from a threshold nonlinearity in the responses. Depolarization of the cell by current injection reduced the detection threshold for spikes but also reduced the range of contrasts they can transmit. This suggests that contrast sensitivity and dynamic range are related in an essential trade-off.

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How efficiently a ganglion cell codes the visual signal

Smith RG, Dhingra NK, Kao YH, Sterling P

Proc. IEEE Eng. Med. Biol. Soc. (2001) IEEE, Piscataway, NJ, 1: 663-665


The retina's visual message is transmitted to the brain by ganglion cells that integrate noisy synaptic inputs to create a spike train. We asked how efficiently the retinal ganglion cell spike generator creates the spike train message. Intracellular and extracellular recordings were made from in vitro guinea pig retina, in response to a spot of light flashed over the receptive field center. Responses were analyzed with an "ideal observer," a program that discriminated between two contrasts based on an optimal decision rule. Spike trains from ganglion cells had thresholds as low as 1% contrast, but thresholds for the corresponding graded potentials were lower by a factor of 2. Using a computational model of the ganglion cell, we asked what factors in the spike generator mechanism are responsible for the spike train's loss in performance. The model included dendritic/axonal morphology, noisy synaptic inputs and membrane channels. Adaptation of spike rate was provided by K(Ca) channels which were activated by Ca2+ flux during spikes. When K(Ca) channels were included, they controlled the duration of the inter-spike interval and thus set the level of noise in the spike train. These results imply that the spike generator adds noise to the spike train signal.

Psychophysics to biophysics: how a perception depends on circuits, synapses, and vesicles.

Dhingra NK, Smith RG, and Sterling P

In: A. Kaneko (Ed) The Neural Basis of Early Vision. (2003) Keio University International symposia for Life Sciences and Medicine, Springer-Verlag, Tokyo, Vol. 11.


One stimulus that we detect very efficiently is a small square that just covers the dendritic field of a brisk-transient ganglion cell. Since this cell type is the most sensitive of the geniculo-striate projecting ganglion cells, it may largely mediate this behavior. To compare the neuron's sensitivity to that of psychophysical detection we measured its visual threshold with a method borrowed from psychophysics. Recordings were extracellular from a mammalian brisk-transient ganglion cell (guinea pig), whose impulse response was nearly identical to that of the primate brisk-transient cell. The stimulus, a 100 ms spot covering the receptive field center, was detected by an "ideal observer" with knowledge of the spike patterns from 100 trials at each contrast. Based on this knowledge, the ideal observer used a single-interval, forced-choice procedure to predict the stimulus contrast on 100 additional trials. Brisk-transient cells at 37°C detected contrasts as low as 0.8% (mean ± SD = 2.8 % ± 0.2 ) and discriminated between contrast increments with about 40% greater sensitivity. These thresholds are the same as human psychophysical thresholds for comparable stimuli, suggesting that across many levels of noisy central synapses, little or no information is lost. Recording intracellularly, we found detection threshold of the ganglion cell's graded potential to be about half that of the spike response, implying a considerable loss in converting the signals from analog-to-digital. To reach detection threshold ganglion cell needed ~1000 quanta, and to respond at full contrast it needed ~2000 quanta. Since the ribbon synapses that contact this cell contain an aggregate of ~10,000 releasable vesicles, the safety factor for this circuit seems to be about 5.

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Direction selectivity in a model of the starburst amacrine cell

Tukker JJ Taylor WR, and Smith RG

Visual Neuroscience (2004) 21: 611-625.


The starburst amacrine cell (SBAC), found in all mammalian retinas, is thought to provide the directional inhibitory input recorded in On-Off direction selective ganglion cells (DSGCs). While voltage recordings from the somas of SBACs have not shown robust direction selectivity (DS), the dendritic tips of these cells display direction-selective calcium signals, even when gamma aminobutyric acid (GABAa,c) channels are blocked, implying that inhibition is not necessary to generate DS. This suggested that the distinctive morphology of the starburst could generate a DS signal at the dendritic tips, where most of its synaptic output is located. To explore this possibility, we constructed a compartmental model incorporating realistic morphological structure, passive membrane properties, and excitatory inputs. We found robust direction selectivity at the dendritic tips but not at the soma. Thin bars produced robust DS, but two-spot apparent motion and annulus radial motion gave little DS. For these stimuli, DS was caused by the interaction of a local synaptic input signal with a temporally delayed "global" signal, that is, an excitatory postsynaptic potential (EPSP) that spread from the activated inputs into the soma and throughout the dendritic tree. In the preferred direction the signals in the dendritic tips coincided, allowing summation, whereas in the null direction the local signal preceded the global, preventing summation. Sine-wave gratings gave the greatest amount of DS, especially at high velocities and low spatial frequencies. The sine-wave DS responses could be accounted for by a simple model which summed phase-shifted signals from different parts of the cell. By testing different artificial morphologies, we discovered DS was relatively independent of the detailed morphology, but depended on having a sufficient number of inputs at the distal tips and a limited electrotonic isolation. Adding voltage-gated calcium channels to the model showed that their threshold effect can amplify DS in the intracellular calcium signal.

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Transmission of scotopic signals from the rod to rod-bipolar cell in the mammalian retina.

Taylor WR, and Smith RG

Vision Research (2004) 44: 3269-3276.


Mammals can see at low scotopic light levels where only 1 rod in several thousand transduces a photon. The single photon signal is transmitted to the brain by the ganglion cell, which collects signals from more than 1000 rods to provide enough amplification. If the system were linear, such convergence would increase the neural noise enough to overwhelm the tiny rod signal. Recent studies provide evidence for a threshold nonlinearity in the rod to rod bipolar synapse, which removes much of the background neural noise. We argue that the height of the threshold should be 0.85 times the amplitude of the single photon signal, consistent with the saturation observed for the single photon signal. At this level, the rate of false positive events due to neural noise would be masked by the higher rate of dark thermal events. The evidence presented suggests that this synapse is optimized to transmit the single photon signal at low scotopic light levels.

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Transmission of single photon signals through a binary synapse in the mammalian retina

Berntson A, Smith RG, and Taylor WR

Visual Neurosci. (2004) 21:693-702


At very low light levels the sensitivity of the visual system is determined by the efficiency with which single photons are captured, and the resulting signal transmitted from the rod photoreceptors through the retinal circuitry to the ganglion cells and on to the brain. Although the tiny electrical signals due to single photons have been observed in rod photoreceptors, little is known about how these signals are preserved during subsequent transmission to the optic nerve. We find that the synaptic currents elicited by single photons in mouse rod bipolar cells have a peak amplitude of 5?6 pA, and that about 20 rod photoreceptors converge upon each rod bipolar cell. The data indicates that the first synapse, between rod photoreceptors and rod bipolar cells, signals a binary event: the detection, or not, of a photon or photons in the connected rod photoreceptors. We present a simple model that demonstrates how a threshold nonlinearity during synaptic transfer allows transmission of the single photon signal, while rejecting the convergent neural noise from the 20 other rod photoreceptors feeding into this first synapse.

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Postsynaptic calcium feedback between rods and rod bipolar cells in the mouse retina

Berntson A, Smith RG, and Taylor WR

Visual Neurosci. (2004) 21:913-924


Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward current. Saturating light steps evoked an inward current with an initial peak that inactivated, with a time constant of about 60?70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracellular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashes of light were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time course of the light responses. Recovery from inactivation observed for paired-pulse stimuli occurred with a time constant of about 375 ms. Calcium feedback could act to increase the dynamic range of the bipolar cells, and to reduce variability in the amplitude and duration of the single-photon signal. This may be important for nonlinear processing at downstream sites of convergence from rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.

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Sluggish and brisk ganglion cells detect contrast with similar sensitivity

Xu Y, Dhingra NK, Smith RG, Sterling P

J. Neurophysiol. (2005) 93:2388-2395


Roughly half of all ganglion cells in mammalian retina belong to the broad class, termed "sluggish". Many of these cells have small receptive fields and project via lateral geniculate nuclei to visual cortex. However, their possible contributions to perception have been largely ignored because sluggish cells seem to respond weakly compared to the more easily studied "brisk" cells. By selecting small somas under infrared DIC optics and recording with a loose seal, we could routinely isolate sluggish cells. When a spot was matched spatially and temporally to the receptive field center, most sluggish cells could detect the same low contrasts as brisk cells. Detection thresholds for the two groups determined by an "ideal observer" were similar: threshold contrast for sluggish cells was 4.7 ± 0.5% (mean ± SE), and for brisk cells was 3.4 ± 0.3% (Mann-Whitney test: p>0.05). Signal-to-noise ratios for the two classes were also similar at low contrast. However, sluggish cells saturated at somewhat lower contrasts (contrast for half-maximum response was 14 ± 1% vs. 19 ± 2% for brisk cells) and were less sensitive to higher temporal frequencies (when the stimulus frequency was increased from 2 Hz to 4 Hz, the response rate fell by 1.6-fold). Thus the sluggish cells covered a narrower dynamic range and a narrower temporal bandwidth, consistent with their reported lower information rates. Because information per spike is greater at lower firing rates, sluggish cells may represent "cheaper" channels that convey less urgent visual information at a lower energy cost.

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Voltage-gated sodium channels improve contrast sensitivity of a retinal ganglion cell.

Dhingra NK, Freed, MA, Smith RG

J. Neurosci. (2005) 25:8097-8103


Voltage-gated channels in a retinal ganglion cell are necessary for spike generation. However, they also add noise to the graded potential and spike train of the ganglion cell, which may degrade its contrast sensitivity, and they may also amplify the graded potential signal. We studied the effect of blocking Na+ channels in a ganglion cell on its signal and noise amplitudes and its contrast sensitivity. A spot was flashed at 1- 4 Hz over the receptive field center of a brisk transient ganglion cell in an intact mammalian retina maintained in vitro. We measured signal and noise amplitudes from its intracellularly recorded graded potential light response and measured its contrast detection thresholds with an "ideal observer." When Na+ channels in the ganglion cell were blocked with intracellular lidocaine N-ethyl bromide (QX-314), the signal-to-noise ratio (SNR) decreased (p < 0.05) at all tested contrasts (2-100%). Likewise, bath application of tetrodotoxin (TTX) reduced the SNR and contrast sensitivity but only at lower contrasts (50%), whereas at higher contrasts, it increased the SNR and sensitivity. The opposite effect of TTX at high contrasts suggested involvement of an inhibitory surround mechanism in the inner retina. To test this hypothesis, we blocked glycinergic and GABAergic inputs with strychnine and picrotoxin and found that TTX in this case had the same effect as QX-314: a reduction in the SNR at all contrasts. Noise analysis suggested that blocking Na+ channels with QX-314 or TTX attenuates the amplitude of quantal synaptic voltages. These results demonstrate that Na+ channels in a ganglion cell amplify the synaptic voltage, enhancing the SNR and contrast sensitivity.

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Chromatic properties of horizontal and ganglion cell responses follow a dual gradient in cone opsin expression.

Yin L, Smith RG, Sterling P, Brainard DH

J. Neurosci. (2006) 26:12351-12361


In guinea pig retina, immunostaining reveals a dual gradient of opsins: cones expressing opsin sensitive to medium wavelengths (M) predominate in the upper retina, whereas cones expressing opsin sensitive to shorter wavelengths (S) predominate in the lower retina. Whether these gradients correspond to functional gradients in postreceptoral neurons is essentially unknown. Using monochromatic flashes, we measured the relative weights with which M, S, and rod signals contribute to horizontal cell responses. For a background that produced 4.76 log10 photoisomerizations per rod per second (Rh*/rod/s), mean weights in superior retina were 52% (M), 2% (S), and 46% (rod). Mean weights in inferior retina were 9% (M), 50% (S), and 41% (rod). In superior retina, cone opsin weights agreed quantitatively with relative pigment density estimates from immunostaining. In inferior retina, cone opsin weights agreed qualitatively with relative pigment density estimates, but quantitative comparison was impossible because individual cones coexpress both opsins to varying and unquantifiable degrees. We further characterized the functional gradients in horizontal and brisk-transient ganglion cells using flickering stimuli produced by various mixtures of blue and green primary lights. Cone weights for both cell types resembled those obtained for horizontal cells using monochromatic flashes. Because the brisk-transient ganglion cell is thought to mediate behavioral detection of luminance contrast, our results are consistent with the hypothesis that the dual gradient of cone opsins assists achromatic contrast detection against different spectral backgrounds. In our preparation, rod responses did not completely saturate, even at background light levels typical of outdoor sunlight (5.14 log10 Rh*/rod/s).

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Design of a Neuronal Array

Borghuis BG, Ratliff CP, Smith RG, Sterling P, Balasubramanian V.

J. Neurosci. (2008) 28:3178-3189.


Retinal ganglion cells of a given type overlap their dendritic fields such that every point in space is covered by three to four cells. We investigated what function is served by such extensive overlap. Recording from pairs of ON or OFF brisk-transient ganglion cells at photopic intensities, we confirmed that this overlap causes the Gaussian receptive field centers to be spaced at2 SDs (). This, together with response nonlinearities and variability, was just sufficient to provide an ideal observer with uniform contrast sensitivity across the retina for both threshold and suprathreshold stimuli. We hypothesized that overlap might maximize the information represented from natural images, thereby optimizing retinal performance for many tasks. Indeed, tested with natural images (which contain statistical correlations), a model ganglion cell array maximized information represented in its population responses with 2spacing, i.e., the overlap observed in the retina. Yet, tested with white noise (which lacks statistical correlations), an array maximized its information by minimizing overlap. In both cases, optimal overlap balanced greater signal-to-noise ratio (from larger receptive fields) against greater redundancy (because of larger receptive field overlap). Thus, dendritic overlap improves vision by taking optimal advantage of the statistical correlations of natural scenes.

Contributions of Horizontal Cells

Smith RG

In: Allan I. Basbaum, Akimichi Kaneko, Gordon M. Shepherd, and Gerald Westheimer (Editors) The Senses: A Comprehensive Reference, Vol 1, Vision I, Richard Masland, and Thomas D Albright, Eds. San Diego, Academic Press, p 348-350.


Horizontal cells transmit signals laterally between photoreceptors. In most vertebrates, horizontal cells comprise two or more types that differ in their size and synaptic connectivity. The horizontal cell receives synaptic input signals exclusively from photoreceptors and transmits back to them an inverted signal. This signal, called "negative feedback", modulates the photoreceptors' release of neurotransmitter. The feedback signal is generated by a specialized chemical synapse between the photoreceptor terminal and the horizontal cell's dendritic tip. The horizontal cell is laterally coupled to its neighbors through gap junctions, which enlarge its receptive field in dim illumination, reducing noise. The horizontal cell feedback creates a receptive field surround for bipolar and ganglion cells and contributes to a higher quality contrast signal in the ganglion cell.

Retinal On- bipolar cells express a new PCP-2 splice variant that accelerates the light response.

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Xu, Y, Sulaiman, P, Fedderson, R., Liu, J., Smith, R.G. and Vardi, N.

J. Neurosci. (2008) 28:8873-8884.


PCP2, a member of the GoLoco domain-containing family, is present exclusively in cerebellar Purkinje cells and retinal ON-bipolar cells. Its function in these tissues is unknown. Biochemical and expression system studies suggest that PCP2 is a guanine nucleotide dissociation inhibitor, though a guanine nucleotide exchange factor has also been suggested. Here we studied the function of PCP2 in ON bipolar cells because their light response depends on Gao1, which is known to interact with PCP2. We identified a new splice variant of PCP2 (Ret-PCP2) and localized it to rod bipolar and ON cone bipolar cells. Electroretinogram recordings from PCP2-null mice showed a normal a-wave but a slower falling phase of the b-wave (generated by activity of ON bipolar cells) relative to the wild type. Whole-cell recordings from rod bipolar cells showed, both under Ames solution and after blocking GABAA/C and glycine receptors, that PCP2-null rod bipolar cells were more depolarized than wild type cells with greater inward current when clamped to -60 mV. Also under both conditions, the rise time of the response to intense light was slower by 28% (Ames) and 44% (inhibitory blockers) in the null cells. Under Ames we also observed >30% longer decay time in the PCP2 null rod bipolar cells. We conclude that PCP2 facilitates cation channels' closure in the dark, shortens the rise time of the light response directly, and accelerates the decay time indirectly via the inhibitory network. These data can be most easily explained if Ret-PCP2 serves as a guanine nucleotide exchange factor.

Physiology and morphology of color-opponent ganglion cells in a retina expressing a dual gradient of S and M opsins

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Yin L, Smith, R.G., Sterling, P. and Brainard D.H

J. Neurosci. (2008) 29:2706-2724.


Most mammals are dichromats, having short-wavelength sensitive (S) and middle-wavelength sensitive (M) cones. Smaller terrestrial species commonly express a dual gradient in opsins, with M opsin concentrated superiorly and declining inferiorly, and vice-versa for S opsin. Some ganglion cells in these retinas combine S and M-cone inputs antagonistically, but no direct evidence links this physiological opponency with morphology; nor is it known whether opponency varies with the opsin gradients. By recording from more than 3000 ganglion cells in guinea pig, we identified small numbers of color-opponent cells. Chromatic properties were characterized by responses to monochromatic spots and/or spots produced by mixtures of two primary lights. Superior retina contained cells with strong S+/M- and M+/S- opponency, whereas inferior retina contained cells with weak opponency. In superior retina, the opponent cells had well-balanced M and S weights, while in inferior retina the weights were unbalanced, with the M weights being much weaker. The M and S components of opponent cell receptive fields had approximately the same diameter. Opponent cells injected with Lucifer yellow restricted their dendrites to the ON stratum of the inner plexiform layer and provided sufficient membrane area (~2.1e+4 µm2) to collect ~3.9e+3 bipolar synapses. Two bistratified cells studied were non-opponent. The apparent decline in S/M opponency from superior to inferior retina is consistent with the dual gradient and a model where photoreceptor signals in both superior and inferior retina are processed by the same post-receptoral circuitry.

Loss of sensitivity in an analog neural circuit

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Borghuis, B.G., Sterling, P. and Smith, R.G.

J. Neurosci. (2009) 29:3045-3058.


A low contrast spot that activates just one ganglion cell in the retina is detected in the cell?s spike train with about the same sensitivity as it is detected behaviorally. This is consistent with Barlow?s proposal that the ganglion cell and later stages of spiking neurons transfer information essentially without loss. Yet, when losses of sensitivity by all preneural factors are accounted for, predicted sensitivity near threshold is considerably greater than behavioral sensitivity, implying that somewhere in the brain information is lost. We hypothesized that the losses occur mainly in the retina - where graded signals are processed by analog circuits that transfer information at high rates and low metabolic cost. To test this, we constructed a model that included all preneural losses for an in vitro mammalian retina, and evaluated the model to predict sensitivity at the cone output. Recording graded responses postsynaptic to the cones (from the type A horizontal cell) and comparing to predicted preneural sensitivity, we found substantial loss of sensitivity (4.2-fold) across the first visual synapse. Recording spike responses from brisk-transient ganglion cells stimulated with the same spot, we found a similar loss (3.5-fold) across the second synapse. The total retinal loss approximated the known overall loss, supporting the hypothesis that from stimulus to perception, most loss near threshold is retinal.

Cone photoreceptor cells: soma and synapse

Smith, R.G.

Published online (Elsevier) :In press


Photoreceptors are the vertebrate retina's primary site for transduction of light into a neural signal. The cone photoreceptor plays a crucial role in daylight vision because it transmits fast changes in light contrast. To improve its sensitivity over 5 log units of background illumination, the cone contains several mechanisms for adaptation: the transduction cascade, biophysical properties, and in the ribbon synapse. The ribbon is part of a complex local circuit called the triad that combines adaptation with spatial filtering to maximize the amount of information the cone transmits to second-order neurons.

Rod photoreceptor cells: soma and synapse

Smith, R.G.

Published online (Elsevier) :In press


The rod photoreceptor is responsible for vision at night over the range of starlight through moonlight to twilight. In starlight, the rod receives a photon about once in 20 minutes, requiring spatial summation, but this would amplify the dark noise if the visual pathway were linear. The rod synapse is specialized to transmit single-photon signals by removing the dark continuous noise with a threshold nonlinearity. At twilight, the rod receives more than one photon per integration time (~200 ms in mammals) and thus cannot transmit single-photon signals. Instead its signals at twilight are coupled to cones through gap junctions.

Ideal observer analysis of signal quality in retinal circuits

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Smith, R.G. and Dhingra, N.K.

Progress in Retinal and Eye Research

Refereed review article


The function of the retina is crucial, for it must encode visual signals so the brain can detect objects in the visual world. However, the biological mechanisms of the retina add noise to the visual signal and therefore reduce its quality and capacity to inform about the world. Because an organism's survival depends on its ability to unambiguously detect visual stimuli in the presence of noise, its retinal circuits must have evolved to maximize signal quality, suggesting that each retinal circuit has a specific functional role. Here we explain how an ideal observer can measure signal quality to determine the functional roles of retinal circuits. In a visual discrimination task the ideal observer can measure from a neural response the increment threshold, the number of distinguishable response levels, and the neural code, which are fundamental measures of signal quality relevant to behavior. It can compare the signal quality in stimulus and response to determine the optimal stimulus, and can measure the specific loss of signal quality by a neuron's receptive field for non-optimal stimuli. Taking into account noise correlations, the ideal observer can track the signal to noise ratio available from one stage to the next, allowing one to determine each stage's role in preserving signal quality. A comparison between the ideal performance of the photon flux absorbed from the stimulus and actual performance of a retinal ganglion cell shows that in daylight a ganglion cell and its presynaptic circuit loses a factor of ~10-fold in contrast sensitivity, suggesting specific signal-processing roles for synaptic connections and other neural circuit elements. The ideal observer is a powerful tool for characterizing signal processing in single neurons and arrays along a neural pathway.

Maximizing contrast resolution in the outer retina of mammals.

PMID: 20361204]

Lipin MK, Smith RG, Taylor WR (2010)

Biological Cybernetics, Biol Cybern. 103:57-77


The outer retina removes the first-order correlation, the background light level, and thus more efficiently transmits contrast. This removal is accomplished by negative feedback from horizontal cell to photoreceptors. However, the optimal feedback gain to maximize the contrast sensitivity and spatial resolution is not known. The objective of this study was to determine, from the known structure of the outer retina, the synaptic gains that optimize the response to spatial and temporal contrast within natural images. We modeled the outer retina as a continuous 2D extension of the discrete 1D model of Yagi et al. (Proc Int Joint Conf Neural Netw 1: 787-789, 1989). We determined the spatio-temporal impulse response of the model using small-signal analysis, assuming that the stimulus did not perturb the resting state of the feedback system. In order to maximize the efficiency of the feedback system, we derived the relationships between time constants, space constants, and synaptic gains that give the fastest temporal adaptation and the highest spatial resolution of the photoreceptor input to bipolar cells. We found that feedback which directly modulated photoreceptor calcium channel activation, as opposed to changing photoreceptor voltage, provides faster adaptation to light onset and higher spatial resolution. The optimal solution suggests that the feedback gain from horizontal cells to photoreceptors should be approximately 0.5. The model can be extended to retinas that have two or more horizontal cell networks with different space constants. The theoretical predictions closely match experimental observations of outer retinal function.

Dendritic Spikes Amplify the Synaptic Signal to Enhance Detection of Motion in a Simulation of the Direction-Selective Ganglion Cell.

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[PMID: 20808894]

Schachter MJ, Oesch N, Smith RG, Taylor WR

PLoS Comput Biol 6(8): e1000899. doi:10.1371/journal.pcbi.1000899.


The On-Off direction-selective ganglion cell (DSGC) in mammalian retinas responds most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree, which are thought to propagate to the soma with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas somatic PSPs (postsynaptic potentials) are only weakly directional, indicating that spike generation includes marked enhancement of the directional signal. We used a realistic computational model based on anatomical and physiological measurements to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate electrotonic regions, each summing its local excitatory and inhibitory synaptic inputs to initiate spikes. Within each local region the local spike threshold nonlinearly amplifies the preferred response over the null response on the basis of PSP amplitude. Using inhibitory conductances previously measured in DSGCs, the simulation results showed that inhibition is only sufficient to prevent spike initiation and cannot affect spike propagation. Therefore, inhibition will only act locally within the dendritic arbor. We identified the role of three mechanisms that generate directional selectivity (DS) in the local dendritic regions. First, a mechanism for DS intrinsic to the dendritic structure of the DSGC enhances DS on the null side of the cell?s dendritic tree and weakens it on the preferred side. Second, spatially offset postsynaptic inhibition generates robust DS in the isolated dendritic tips but weak DS near the soma. Third, presynaptic DS is apparently necessary because it is more robust across the dendritic tree. The pre- and postsynaptic mechanisms together can overcome the local intrinsic DS. These local dendritic mechanisms can perform independent nonlinear computations to make a decision, and there could be analogous mechanisms within cortical circuitry.

Trigger features and excitation in the retina

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Taylor WR, Smith RG.

Curr Opin Neurobiol, doi:10.1016/j.conb.2011.07.001


This review focuses on recent advances in our understanding of how neural divergence and convergence give rise to complex encoding properties of retinal ganglion cells. We describe the apparent mismatch between the number of cone bipolar cell types, and the diversity of excitatory input to retinal ganglion cells, and outline two possible solutions. One proposal is for diversity in the excitatory pathways to be generated within axon terminals of cone bipolar cells, and the second invokes narrow-field glycinergic amacrine cells that can apparently act like bipolar cells by providing excitatory drive to ganglion cells. Finally we highlight two advances in technique that promise to provide future insights; automation of electron microscope data collection and analysis, and the use of the ideal observer to quantitatively compare neural performance at all levels.

Parallel Mechanisms Encode Direction in the Retina

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[PMID: 21867884]

Trenholm S, Johnson K, Li X,Smith RG, Awatramani GB (2011)

Neuron, doi 10.1016/j.neuron.2011.06.020


In the retina, presynaptic inhibitory mechanisms that shape directionally selective (DS) responses in output ganglion cells are well established. However, the nature of inhibition-independent forms of directional selectivity remains poorly defined. Here, we describe a genetically specified set of ON-OFF DS ganglion cells (DSGCs) that code anterior motion. This entire population of DSGCs exhibits asymmetric dendritic arborizations that orientate toward the preferred direction. We demonstrate that morphological asym- metries along with nonlinear dendritic conductances generate a centrifugal (soma-to-dendrite) preference that does not critically depend upon, but works in parallel with the GABAergic circuitry. We also show that in symmetrical DSGCs, such dendritic DS mech- anisms are aligned with, or are in opposition to, the inhibitory DS circuitry in distinct dendritic subfields where they differentially interact to promote or weaken directional preferences. Thus, pre- and post- synaptic DS mechanisms interact uniquely in distinct ganglion cell populations, enabling efficient DS coding under diverse conditions.

The role of starburst amacrine cells in visual signal processing.

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[PMID: 22310373]

Taylor WR, Smith RG. (2012)

Visual Neuroscience 29:73-81.


Starburst amacrine cells (SBACs) within the adult mammalian retina provide the critical inhibition that underlies the receptive field properties of direction-selective ganglion cells (DSGCs). The SBACs generate direction-selective output of GABA that differentially inhibits the DSGCs. We review the biophysical mechanisms that produce directional GABA release from SBACs and test a network model that predicts the effects of reciprocal inhibition between adjacent SBACs. The results of the model simulations suggest that reciprocal inhibitory connections between closely spaced SBACs should be spatially selective, while connections between more widely spaced cells could be indiscriminate. SBACs were initially identified as cholinergic neurons and were subsequently shown to contain release both acetylcholine and GABA. While the role of the GABAergic transmission is well established, the role of the cholinergic transmission remains unclear.

Cones Respond to Light in the Absence of Beta Transducin Subunit

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[PMID: 23516284]

Nikonov SS, Lyubarsky A, Fina ME, Nikonova ES, Sengupta A, Chinniah C, Ding X-Q, Smith RG, Pugh EN Jr, Vardi N, Dhingra A.

J Neurosci 2013 33:5182-5194


Mammalian cones respond to light by closing a cGMP-gated channel via a cascade that includes a heterotrimeric G-protein, cone transducin, comprising GAt2, GB3 and GGt2 subunits. The function of GBG in this cascade has not been examined. Here, we investigate the role of GB3 by assessing cone structure and function in GB3-null mouse (Gnb3 -/-). We found that GB3 is required for the normal expression of its partners, because in the Gnb3 -/- cone outer segments, the levels of GAt2 and GGt2 are reduced by fourfold to sixfold, whereas other components of the cascade remain unaltered. Surprisingly, Gnb3 cones produce stable responses with normal kinetics and saturating response amplitudes similar to that of the wild-type, suggesting that cone phototransduction can function efficiently without a GB subunit. However, light sensitivity was reduced by approximately fourfold in the knock-out cones. Because the reduction in sensitivity was similar in magnitude to the reduction in G?t2 level in the cone outer segment, we conclude that activation of GAt2 in Gnb3-/- cones proceeds at a rate approximately proportional to its outer segment concentration, and that activation of phosphodiesterase and downstream cascade components is normal. These results suggest that the main role of GB3 in cones is to establish optimal levels of transducin heteromer in the outer segment, thereby indirectly contributing to robust response properties.

Directional Summation in non-Direction Selective Retinal Ganglion Cells.

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[PMID: 23516351]

Abbas SY, Hamade KC, Yang EJ, Nawy S, Smith RG, Pettit DL

PLoS Comput Biol (2013) e1002969 doi:10.1371/journal.pcbi.1002969


Retinal ganglion cells receive inputs from multiple bipolar cells which must be integrated before a decision to fire is made. Theoretical studies have provided clues about how this integration is accomplished but have not directly determined the rules regulating summation of closely timed inputs along single or multiple dendrites. Here we have examined dendritic summation of multiple inputs along On ganglion cell dendrites in whole mount rat retina. We activated inputs at targeted locations by uncaging glutamate sequentially to generate apparent motion along On ganglion cell dendrites in whole mount retina. Summation was directional and dependent13 on input sequence. Input moving away from the soma (centrifugal) resulted in supralinear summation, while activation sequences moving toward the soma (centripetal) were linear. Enhanced summation for centrifugal activation was robust as it was also observed in cultured retinal ganglion cells. This directional summation was dependent on hyperpolarization activated cyclic nucleotide-gated (HCN) channels as blockade with ZD7288 eliminated directionality. A computational model confirms that activation of HCN channels can override a preference for centripetal summation expected from cell anatomy. This type of direction selectivity could play a role in coding movement similar to the axial selectivity seen in locust ganglion cells which detect looming stimuli. More generally, these results suggest that non-directional retinal ganglion cells can discriminate between input sequences independent of the retina network.

NaV1.1 Channels in axon initial segments of retinal bipolar cells augment input to magnocellular visual pathways

PMID: 24107939]

Puthussery T, Venkataramani S, Gayet-Primo J, Smith RG, Taylor WR

J Neurosci (2013) 33(41): 16045-16059, doi: 10.1523/JNEUROSCI.1249-13.2013


In the primate visual system, the ganglion cells of the magnocellular pathway underlie motion and flicker detection and are relatively transient, while the more sustained ganglion cells of the parvocellular pathway have comparatively lower temporal resolution, but encode higher spatial frequencies. Although it is presumed that functional differences in bipolar cells contribute to the tuning of the two pathways, the properties of the relevant bipolar cells have not yet been examined in detail. Here, by making patch-clamp recordings in acute slices of macaque retina, we show that the bipolar cells within the magnocellular pathway, but not the parvocellular pathway, exhibit voltage-gated sodium (NaV), T-type calcium (CaV), and hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents, and can generate action potentials. Using immunohistochemistry in macaque and human retinae, we show that NaV1.1 is concentrated in an axon initial segment (AIS)-like region of magnocellular pathway bipolar cells, a specialization not seen in transient bipolar cells of other vertebrates. In contrast, CaV3.1 channels were localized to the somatodendritic compartment and proximal axon, but were excluded from the AIS, while HCN1 channels were concentrated in the axon terminal boutons. Simulations using a compartmental model reproduced physiological results and indicate that magnocellular pathway bipolar cells initiate spikes in the AIS. Finally, we demonstrate that NaV channels in bipolar cells augment excitatory input to parasol ganglion cells of the magnocellular pathway. Overall, the results demonstrate that selective expression of voltage-gated channels contributes to the establishment of parallel processing in the major visual pathways of the primate retina.

Dendritic Computation of Direction in Retinal Neurons.

Smith RG, Taylor WR

In Eds: Cuntz H, Remme MWH, Torben-Nielsen B, "The Computing Dendrite: From Structure to Function", Springer Series in Computational Neuroscience, ISBN-13: 978-1461480938


The retina utilizes a variety of dendritic mechanisms to compute ­direction from image motion. The computation is accomplished by starburst amacrine cells (SBACs) which are GABAergic neurons presynaptic to direction-selective ganglion cells (DSGCs). SBACs are symmetric neurons with several branched dendrites radi- ating out from the soma. Larger EPSPs are produced in the dendritic tips of SBACs as a stimulus sequentially activates inputs from the base of each dendrite outwards. The directional difference in EPSP amplitude is further amplified near the dendritic tips by voltage-gated channels to produce directional release of GABA. Reciprocal inhibition between adjacent SBACs may also amplify directional release. Directional signals in the independent SBAC branches are preserved because each dendrite makes selective contacts only with DSGCs of the appropriate preferred-­direction. Directional signals are further enhanced within the dendritic arbor of the DSGC, which essentially comprises an array of distinct dendritic compartments. Each of these dendritic compartments locally sum excitatory and inhibitory inputs, ampli- fies them with voltage-gated channels, and generates spikes that propagate to the axon via the soma. Overall, the computation of direction in the retina is performed by several local dendritic mechanisms both presynaptic and postsynaptic, with the result that directional responses are robust over a broad range of stimuli.

Post-receptor adaptation: lighting up the details.

PMID: 25004365]

Smith RG, Delaney KR, Awatramani GB

Curr Biol. (2014) July 7; 24(13):R608-10. doi: 10.1016/j.cub.2014.05.058.


The very first rays of the rising sun enrich our visual world with spectacular detail. A recent study reveals how retinal circuits downstream of photoreceptors 'functionally re-wire' to trade-off sensitivity for high spatial acuity during night-day transitions.

Nonlinear dendritic integration of electrical and chemical synaptic inputs drives fine-scale correlations.

PMID: 25344631]

Trenholm S, McLaughlin AJ, Schwab DJ, Turner MH, Smith RG, Rieke F, Awatramani GB. (2014)

Nat Neurosci. 17:1759-1766. doi: 10.1038/nn.3851


Throughout the CNS, gap junction-mediated electrical signals synchronize neural activity on millisecond timescales via cooperative interactions with chemical synapses. However, gap junction-mediated synchrony has rarely been studied in the context of varying spatiotemporal patterns of electrical and chemical synaptic activity. Thus, the mechanism underlying fine-scale synchrony and its relationship to neural coding remain unclear. We examined spike synchrony in pairs of genetically identified, electrically coupled ganglion cells in mouse retina. We found that coincident electrical and chemical synaptic inputs, but not electrical inputs alone, elicited synchronized dendritic spikes in subregions of coupled dendritic trees. The resulting nonlinear integration produced fine-scale synchrony in the cells' spike output, specifically for light stimuli driving input to the regions of dendritic overlap. In addition, the strength of synchrony varied inversely with spike rate. Together, these features may allow synchronized activity to encode information about the spatial distribution of light that is ambiguous on the basis of spike rate alone.

Inhibitory Input to the Direction Selective Ganglion Cells Is Saturated at Low Contrast

PMID: 26063782]

Lipin, MY, Taylor WR, Smith RG. (2015)

J. Neurophysiology 2015 June DOI: 10.1152/jn.00413.2015


Direction selective ganglion cells (DSGCs) respond selectively to motion towards a "preferred" direction, but much less to motion towards the opposite "null" direction. Directional signals in the DSGC depend on GABAergic inhibition, and are observed over a wide range of speeds, which precludes motion detection based on a fixed temporal correlation. A voltage-clamp analysis, using narrow bar stimuli similar in width to the receptive field center, demonstrated that inhibition to DSGCs saturates rapidly above a threshold contrast. However, for wide bar stimuli that activate both the center and surround, inhibition depends more linearly on contrast. Excitation for both wide and narrow bars was also more linear. We propose that positive feedback, likely within the starburst amacrine cell or its network, produces steep saturation of inhibition at relatively low contrast, which renders GABA-release essentially contrast and speed invariant, and thereby enhances the signal-to-noise ratio for direction selective signals in the spike train over a wide range of stimulus conditions. This mechanism enhances directional signals at the expense of lower sensitivity to other stimulus features such as contrast and speed. This renders GABA-release essentially contrast and speed invariant, which enhances directional signals for small objects, and thereby increases the signal-to-noise ratio for direction selective signals in the spike train over a wide range of stimulus conditions. The steep saturation of inhibition confers to a neuron immunity to noise in its spike train because when inhibition is strong, no spikes are initiated.

Time-course of EPSCs in On-type starburst amacrine cells in mouse retina is independent of dendritic location.

PMID: 27219620]

Stincic T, Smith RG, Taylor WR. (2016)

Journal of Physiology 594:5685-5694 DOI: 10.1113/jp272384


Direction selectivity in the retina relies critically on directionally asymmetric GABA release from the dendritic tips of starburst amacrine cells (SBACs). GABA release from each radially directed dendrite is larger for motion outward from the soma toward the dendritic tips than for motion inwards toward the soma. The biophysical mechanisms generating these directional signals remain controversial. A model based on electron-microscopic reconstructions of the mouse retina proposed that an ordered arrangement of kinetically distinct bipolar cell inputs to ON and OFF type SBACs could produce directional GABA release. We tested this prediction by measuring the time-course of EPSCs in ON type SBACs in the mouse retina, activated by proximal and distal light stimulation. Contrary to the prediction, the kinetics of the excitatory inputs were independent of dendritic location. Computer simulations based on 3D reconstructions of SBAC dendrites demonstrated that the response kinetics of distal inputs were not significantly altered by dendritic filtering. These direct physiological measurements, do not support the hypothesis that directional signals in SBACs arise from the ordered arrangement of kinetically distinct bipolar cell inputs.

Species-specific wiring for direction selectivity in the mammalian retina.

PMID: 27350241]

Ding H, Smith RG, Poleg-Polsky A, Diamond JS, Briggman KL. (2016)

Nature 535:105-110 doi:10.1038/nature18609


Directionally tuned signaling in starburst amacrine cell (SAC) dendrites lies at the heart of the direction selective (DS) circuit in the mammalian retina. The relative contributions of intrinsic cellular properties and network connectivity to SAC DS remain unclear. We present a detailed connectomic reconstruction of SAC circuitry in mouse retina and describe previously unknown features of synapse distributions along SAC dendrites: 1) input and output synapses are segregated, with inputs restricted to proximal dendrites; 2) the distribution of inhibitory inputs is fundamentally different from that observed in rabbit retina. An anatomically constrained SAC network model suggests that SAC-SAC wiring differences between mouse and rabbit retina underlie distinct contributions of synaptic inhibition to velocity and contrast tuning and receptive field structure. In particular, the model indicates that mouse connectivity enables SACs to encode lower linear velocities that account for smaller eye diameter, thereby conserving angular velocity tuning. These predictions are confirmed with calcium imaging of mouse SAC dendrites in response to directional stimuli.

Directional Excitatory Input to Direction-Selective Ganglion Cells in the Rabbit Retina

Percival KA, Venkataramani S, Smith RG, Taylor WR (2017)

J. Comp. Neurol. Accepted Jan 31, 2017.


Directional responses in retinal ganglion cells are generated in large part by direction-selective release of GABA from starburst amacrine cells onto direction-selective ganglion cells (DSGCs). The excitatory inputs to DSGCs are also widely reported to be direction-selective, however, recent evidence suggests that glutamate release from bipolar cells is not directional, and directional excitation seen in patch-clamp analyses may be an artifact resulting from incomplete voltage control. Here we test this voltage-clamp-artifact hypothesis in recordings from 62 On-Off DSGCs in the rabbit retina. The strength of the directional excitatory signal varies considerably across the sample of cells, but is not correlated with the strength of directional inhibition, as required for a voltage-clamp artifact. These results implicate additional mechanisms in generating directional excitatory inputs to DSGCs.

Rod Photoreceptor Cells: Soma and Synapse

Krizaj D, Smith RG (2017)

Reference Module in Neuroscience and Biobehavioral Psychology, Elsevier, 2017. ISBN 9780128093245

End of abstracts

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