Subtle Variations in Human Color Vision

The differences in color vision between normal trichromats and various color defectives are major and dramatic, but there are also reliable differences in color vision among individuals within each of these groups and study of these small variations can provide a useful tool for understanding the biological basis of color vision. One particular example comes from the study of variations in the color matching among normal trichromats. In a standard version of the color-matching task, people are asked to adjust the proportions of red and green primaries to match a yellow test light (with lights drawn from this part of the spectrum, trichromats make dichromatic matches). Color discrimination is particularly acute for lights from this part of the spectrum and, consequently, subjects make extremely reliable matches. It has long been known that individual matches may differ in that some people consistently require relatively more red light to complete the match, and others need relatively more green light. As previously explained, differences in color matches mostly arise from differences in the spectral positioning of the retinal photopigments, so an implication is that individuals who make different color matches must have different photopigments.

In recent years, the biological basis of this small variation in color matching has been uncovered. As predicted, it reflects the fact that normal human trichromats show small variations in the spectral positioning of their photopigments. Unexpectedly however, the variation is not continuously distributed in the population but rather appears to have a discrete character. This variation has been shown to result principally from a polymorphism in the gene specifying the L cone opsin. This polymorphism, involving a difference of only a single nucleotide, in turn causes a variation in a single amino acid in the photopigment molecule and thus there are two forms of the L cone pigment. This variation allows the L cone pigment to occupy either one of two spectral positions, with the two varying in their 1max values by about 4 nm. A male who has the longer of the two pigment versions will require systematically less red light in the color match than will a male who has the shorter of the two pigments. These two polymorphic variants of the L cone opsin gene are nearly equally represented in the population. It is remarkable that this very small genetic variation sorts individuals of normal color vision into groups that go through life experiencing the colors of the world as biased to appear either slightly redder or slightly greener.

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