Rod-Cone Interaction in the Dark-adapted Fovea
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- Cone second plateau experiment
- 150 degrees x 40 degrees preadapting light
- red test target with 66 minute diameter, 1 second, 644 nm
- pupil: undilated and dilated
- 10 minutes of dark adaptation
- 4 minutes of preadapting light
- 45 minutes of data collection with 1 sec stimulus
- Preadapting luminance = 4.0 log cd/m2 or 4.5-4.8 log photopic trolands for undilated pupil diameters between 2 and 3 mm.
- In dilated condition, the illuminance was passed through a .9 log filter to approximate the illuminance of the dilated condition.
- Results (the same for dilated and undilated)
- Threshold decreases and plateaues at about 10 minutes and is constant until about 20 minutes, but declines ~.25 log units between 20 and 45 minutes.
- Rod background, Rod Plateau and Cone Plateau Experiment
- Targets: 66' white, 10' white, 66' red, 10' red
- Rod Adapting field: 11 degree white (3400 tungsten) -3.1 log cd/m2 (this adapting field is ~ .5 log units below the dark-adapted threshold for the 66' white target
- "the dark-adapted thresholds are significantly below the cone-plateau thresholds"
- Threshold pattern was the same across target types "conclusively showing that only cones detected the test targets."
- Rod background threshold increase relative to 45-dark adapted threshold
Threshold increase in log units
|Subject ||66' W ||10' W||66' R ||10' R
- Discussion (leaves something to be desired)
- These results are described using the idea that parafoveal cones activate lateral inhibitory elements which desensitize the foveal cone pathway: "Specifically, dark-light signals from parafoveal rods could have activated lateral inhibitory elements, which in turn could have densensitized foveal cone pathways." "...the rod background and the rod dark-light signals present during the early cone plateau were about equally effective in activating the hypothesized lateral inhibition"
- The cone plateau = cone sensitivity decreased (relative to 2nd plateau) by inhibition which is fueled by dark noise in the rods.
- Cone sensitivity with rod background = cone sensitivity decreased (relative to 2nd plateau) by inhibition which is fueled by background light in the rods.
- The 2nd cone plateau = cone sensitivity without rod-fed inhibition
- "The rod background was much less effective than the dark-light signal in terms of its ability to raise rod thresholds." explained as indicating a basic difference between dark light and real light
- Westheimer (1968): "dark light (in the rods) is neither susceptible to sensitization by lateral interaction nor capable of exerting it." explained as above
- Change in cone threshold is subtle in the range where rod adaptation is changing dramatically. Explanation: inhibitory influence has a "ceiling at relatively low levels of rod adaptation."
- "I have shown elsewhere...inhibition... is not limited to the fovea but produces delayed components of cone dark adaptation at other eccentricities as well."
- "Other investigators have found similar threshold increase in extrafoveal dark-adaptation curves..." (i.e. Wooten and Butler, 1976)
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© 1995 Lance Hahn (email@example.com)