Color vision in two observers with highly biased LWS/MWS cone ratios

Abstract

Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HEP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09/1 and 0.03/1, compared with ratios in the range of 0.6/1 to 10/1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on a white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.

Organization of the Human Trichromatic Cone Mosaic

“…Unique yellow, the wavelength that appears neither reddish nor greenish and represents the neutral point of the red-green color mechanism, is thought to be driven mainly by differences in L and M cone excitation. Several investigators have noted that whereas estimates of L:M cone ratio vary widely, the wavelength that subjects judge uniquely yellow is nearly constant, varying with a SD of only 2-5 nm (Pokorny et al, 1991;Jordan and Mollon, 1997;Miyahara et al, 1998;Brainard et al, 2000;Neitz et al, 2002). In agreement with these studies, measures of unique yellow did not correlate with direct measurements of L:M cone ratio in six of our subjects (HS, YY, MD, JP, JC, and BS; data not shown).…”

“…In addition, we obtained the first direct estimate of L:M ratio in a carrier of a protan color-vision defect. We observed a very low L:M cone ratio (0.37:1; 27% L), which was expected based on her phenotype and genotype (Miyahara et al, 1998). [In females, X inactivation produces, on average, expression of pigment genes from each chromosome in one-half of the cones.…”

“…Subjects BS and HS were chosen for this study as representing the upper and lower limits of normal L:M cone ratios, respectively. Subject HS is a genetically confirmed carrier of a protan color vision defect and thus was expected to have a low L:M cone ratio (Miyahara et al, 1998). Previous results from the flicker photometric ERG indicate that subject BS had an L:M cone ratio near the high end of the normal distribution ) (see Fig.…”

L and M cone proportions in polymorphic New World monkeys

“…In such individuals the relative representation of the two gene products will presumably be subject to the same influences as those determining L:M proportions in platyrrhine monkeys. There have been many studies of vision in carriers of color vision defects~Jorgen-son et al, 1992;Jordan & Mollon, 1993;Miyahara et al, 1998; and the references therein!. Among other things, these studies reveal a considerable degree of individual variation in the effects of the presence of an altered gene array with some women having effectively normal color vision while others show significant change.…”

Erratum

“…32 Even with the extreme L/M bias found in carriers of dichromacy,°y is within the normal range. 33 Further evidence can be gleaned from the constancy of unique hues and of the subjective experience of color across the life span. 34 This is despite the yellowing of the lens in later life (its absorption of shorter-wavelength light is enhanced by increasing pupillary constriction with age, meaning that light cannot enter the eye through the opticallythinner outer zones of the eye).…”