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S. Plainis, N.R. Parry, D.J. Mc Keefry, I.J. Murray; Reaction Times (RTs) to Chromatic Stimuli as a Function of Eccentricity in the Macula Region . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1914.
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Purpose: To use reaction time (RT) vs cone contrast functions to measure sensitivity to cone-opponent stimuli as a function of eccentricity within the macular region of the human retina. Methods: Simple RTs were measured to the onset of a coloured gaussian spot (s.d. = 0.2°) on a white isoluminant background (x = 0.310, y = 0.316) with a mean luminance of 12.5 cd.m-2. The stimuli were modulated in chromaticity along red-green and blue-yellow cardinal axes in MBDKL colour space: 0° (red), 90° (blue), 180° (green) and 270° (yellow). Four colour-normal subjects were tested at six eccentricities: 0, 1, 2, 3, 4 and 6.3°. At each eccentricity, isoluminance and macular pigment optical density were determined before measuring the RT vs cone contrast function. Results: For 3 chromatic axes (0, 180 and 270°), sensitivity was maximum at the centre, and reduced with eccentricity by between 0.3 and 1 log units. Sensitivity to the 90° (blue) stimulus reached a maximum at 2° eccentricity. Overall, sensitivity to red and green stimuli was substantially greater than to the blue and yellow stimuli, and this relationship was maintained at all eccentricities. Conclusions: It seems likely that these data can be accounted for, at least in part, by the retinal distribution of the different cone types. We estimate that approximately 77% of the energy of the blue centrally-presented stimulus falls within the S-cone free zone, assuming this has a radius of 0.15° or 0.75 stimulus Z-units. However, at 1° eccentricity, the stimulus falls outside this area and thus other factors, including macular pigment, may be contributing to the central depression.
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