Four cone pigments in women heterozygous for color deficiency

Abstract

We find that additivity of trichromatic color matches holds, even in parafoveal vision, for most observers if rods are prevented from seeing the stimuli. Some heterozygous women are exceptions. The failure of the additivity law for these women implies that their eyes contain more than three types of cone with different spectral sensitivities.

Richer color experience in observers with multiple photopigment opsin genes

“…Specifically, perceptual deviations experienced by four-pigment individuals in psychophysically controlled color-matching tasks have been reported as negligible differences and, as such, are generally seen as continued support for the accepted theory of neural trivariance of the color signal (Jordan & Mollon, 1993;MacLeod, 1985;Miyahara, Pokorny, Smith, Baron, & Baron, 1998;Nagy et al, 1981;J. Neitz et al, 1993).…”

“…That is, females who possess two distinct genetic variants at certain codons (with one variant on each of two X chromosomes). 3 Previous research proposed that such genetic heterozygosity may have perceptual consequences in individualswho actually express all four types of photopigment genes, because each gene type produces different retinal photopigmentsensitivities, in effect yielding a four-cone-class retinal phenotype (Deeb & Motulsky, 1996;DeVries, 1948;Jordan & Mollon, 1993;Krill & Beutler, 1965;Mollon, 1992;Nagy, MacLeod, Heyneman, & Eiser, 1981;Schmidt, 1955). 4 A few investigators have conjectured that some individuals who possess four photopigments in their retinas might have a dimension of perceptual experience that is not experienced by trichromat individuals (Deeb & Motulsky, 1996;Mollon, 1992Mollon, , 1995.…”

“…5 As discussed by Cohn, Emmerich, and Carlson (1989) heterozygous females fail to be detected by the use of an anomaloscope, although there are reported shifts in their anomaloscope color matches (Crone, 1959;Feig & Ropers, 1978;Krill & Beutler, 1964;Pickford, 1959;Schmidt, 1955), as well as shifts using flicker photometry (Crone, 1959;Ya-suma, Tokuda, & Ichikawa, 1984). However unlike normal controls, these heterozygotes exhibit failures of additivity of trichromatic color matches after exposure to a light bleaching of the rod system (Nagy et al, 1981). Also, heterozygous females for L-cone polymorphisms were found to exhibit higher absolute thresholds to small spots of red light, relative to normal controls (Krill & Beutler, 1964, 1965.…”

“…The Rayleigh match stimulus configuration involves monocular viewing of a small spot of 2º or 10º (foveal) visual angle presented in a dark surround in which half of the field serves as the stimulus standard composed of a homogeneous yellow light. The observers' task is to adjust the ratio of a mixture of red and green lights to match their sensation in the adjusted-field half of the stimulus to that in the standard-field half of the stimulus (Jordan & Mollon, 1993;Krill & Beutler, 1965;Miyahara et al, 1998;Nagy et al, 1981;Schmidt, 1955). 7 This stimulus configuration is aptly described as a uniform, context-free, noncomplex viewing condition, and it was not designed with the intention to approximate complex real-world viewing situations.…”

“…However, if an observer received useful information from three different classes of photoreceptors (say, "red," "green," and a third called "red-prime") in the redto-green range of the spectrum, then the Rayleigh colormatching procedure may not provide the degrees of adjustment needed to capture the impact of these three different kinds of color detectors. In view of this, it is perhaps not surprising that findings from some Rayleigh matching circumstances show negligibledifferences from trichromacy in the color perception of heterozygote females (Miyahara et al, 1998;Nagy et al, 1981). For these reasons, if deviations from trichromacy exist in female heterozygotes,then anomaloscope color matching may not be the best method for capturing such deviations.…”