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Marina Danilova, Chloe Chan, John Mollon; Individual differences in cone ratio: Measurements by counterphase modulation photometry and by spatial acuity. Journal of Vision 2011;11(15):33. doi: 10.1167/11.15.33.
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Within the normal population, there are believed to be large individual differences in the ratio of long-wave (L) to middle-wave (M) cones. Do these variations reveal themselves in psychophysical measures? We propose a temporal and a spatial measure that both appear to be sensitive to the L:M ratio: (i) Heterochromatic counterphase modulation photometry using a test originally introduced by Estévez et al. (1983, American Journal of Optometry and Physiological Optics, 60, 892) as a test for color deficiency. Red and green lights are flickered in counterphase and the subject adjusts their relative depth of modulation to minimize the apparent flicker. (ii) Spatial acuity for Landolt C targets that isolate individual cone mechanisms. Targets are briefly presented in the parafovea, where acuity is thought to be limited by the sampling density of the cones, rather than in the fovea, where acuity is limited by optical factors. A steady white background is continuously present. Each of these measures shows good test-retest reliability and the settings are readily made by untrained subjects. The two measures correlate well: in one study of 20 young adults, we find a Spearman rank-order correlation of rs = 0.78, p = 0.0006.
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