Trichromat-like representation of colors in dichromats: A hypothesis on the evolution of trichromacy

Abstract

Color vision in trichromats is based on three types of photoreceptors with different spectral sensitivities. This trichromatic retinal substrate is often thought to underlie the three-dimensional structure of our perceptual color space, e.g. the perceptual color axes of “light”-“dark”, “red”-“green”, and “blue”-“yellow”. Dichromats are missing one of these three photoreceptor types. In this context, it is usually assumed that their color percept lacks certain qualities, e.g. ‘red-blind’ protanopes lack the ability to perceive “red” (Vienot et al 1995). However, many studies have shown that dichromats use all of the basic color terms “red”, “green”, “blue”, and “yellow”, to describe their color percepts. This indicates that the number of perceptual color categories is not tightly coupled to the spectral dimensionality of the receptoral substrate. We investigated the color vision of protanopes and deuteranopes with the method of hue scaling, using monochromatic stimuli at different intensity levels. Our results confirm earlier findings and demonstrate the dependence of “red” and “green” percepts on intensity (Boynton & Scheibner 1967); for light above 560 nm, at low intensities dichromats describe their color percept as “red”, at higher intensities however as “yellow”. We present a model for the processing of the two photoreceptor signals in dichromats that, under physiologically plausible assumptions, yields a consistent labeling of chromatic stimuli as mixtures of “blue”, “yellow”, “green”, and “red”.We hypothesize that dichromatic ancestors of humans may have taken advantage of such a pseudo-trichromacy, and that the perceptual color axes of red-green and blue-yellow may have evolved even before retinal trichromacy. The hypothesis provides a unifying explanation for molecular, evolutionary, and perceptual features of color vision.

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