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Marina V. Danilova, John D. Mollon; Relative Numbers Of L And M Cones: Correlation Between Estimates From Acuity For Isolated Cone Types And From A Temporal Measure. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3906.
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To compare relative numbers of long-wave (L) and middle-wave (M) cones estimated from spatial and temporal psychophysical measures.
A group of 10 observers was tested in two tasks. (i) To measure visual acuity for isolated L or M cones Landolt C targets were presented on a white field of 10 cd/m2. Landolt C diameter was varied in different experimental blocks from 0.93 deg to 4.44 deg in order to estimate the subject’s contrast sensitivity and acuity for cone-isolating stimuli. The stimuli were presented in the parafovea; the center of the Landolt C was always at eccentricity of 5 deg. The Landolt C randomly appeared to the right or left of the fixation point for 100 ms, to ensure that observers fixated centrally. Cone contrast was varied to find thresholds for each size of the target. Cone contrast sensitivity functions were fitted with exponential curves; visual acuity for each class of cones was estimated as the spatial frequency corresponding to sensitivity 1.0. (ii) Counterphase modulation photometry (Estévez et al, 1983, The Oscar color vision test, Amer. J. Optom. Physiol. Opt., 60, 892) was used as a temporal measure of L/M cone ratio. The observers’ task was to minimize flicker produced by red and green lights modulated in counterphase. The scale of the device is calibrated so that negative values signal dominance of M-cones and positive values indicate dominance of L-cones.
(i) There were marked individual differences in visual acuities corresponding to L- or M-addressed Landolt Cs: the ratios of acuity ranged from 0.84 to 2.48. (ii) The Oscar test settings varied from -3.2 to +0.8. The correlation between these two measures was R2=0.79, p<0.01.
Visual acuity for cone-isolating stimuli is related to the sampling density of the corresponding photoreceptor array and can be used to estimate the relative number of cones. This spatial measure of L/M cone ratio correlates with the temporal measure incorporated in the Oscar test.
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