Individual differences in cone photopigments of normal trichromats measured by dual Rayleigh-type color matches

Abstract

Individual differences in color matches of normal trichromats are well documented. Recently, variants of the classical Rayleigh match have been measured to explore the cause(s) of these individual differences. Interobserver differences in the wavelength of peak sensitivity of photopigment (λ max) are of primary interest because they are attributed to an X-chromosome-linked polymorphism. Color-matching equations, however, show the Rayleigh match cannot distinguish between interobserver differences in λ max and interobserver differences in the optical density of photopigment. Further analysis of color-matching equations reveals that the ratio of two particular Rayleigh-type matches amplifies the effect of individual differences in the λ max of L cones relative to the effects of optical density and pre-receptoral spectrally selective filtering. The ratio of these two color matches was measured for 17 color-normal males. The range of the results for the 17 observers is too large to be explained by only individual differences in photopigment optical density and pre-receptoral filtering. This implies there are interobserver differences in λ max. The results are accounted for quantitatively by a small difference (3–5 nm) in the λ max of the L-cone photopigment. The ratio of two Rayleigh-type matches is a rapid and convenient measurement for assessing the L-cone λ max in the eye of an individual observer and therefore may be useful for classifying normal trichromats into phenotypic sub-types.