Some important constraints on color spatial vision

There are many constraints on color vision. Two of these are especially significant in considering the role that color vision might play in spatial processing and will be briefly described here. First, color vision can operate only at relatively high light levels. There are three classes of cone photoreceptors, each with its own photopigment. The S cones are maximally sensitive in the short visible wavelengths; the M cones in the medium wavelengths; and the L cones in the longer wavelengths. The cones are responsible for transduction at moderate to high light levels and thus for color vision. The great majority of all cones are either L or M cones. Both absorb light over the entire visible spectrum, and their spectral absorption functions differ only slightly. A different class of photoreceptors, rods, is responsible for vision at low light levels. They are not active at the high light levels at which color vision is prominent, and the cones are not responsive at very low light levels.

To estimate the amount of light present at a given position (more properly, the light coming from a given direction), the visual system need only count the number of photons it absorbs in the corresponding retinal region. (Here we ignore the added complexities resulting from differential sensitivity to different wavelengths and from nonlinear intensity-response functions.) This estimate can be based on the output of a single receptor type (the rods at low, or scotopic, light levels, for example) or on a combination of multiple receptor types in a specific region. To judge the chromaticity at a single point, however, the system must compare the number of photons absorbed in each of at least two photoreceptors that contain photopigments that differ in spectral sensitivity. To estimate luminance, thus, the critical information lies in the sum of the responses of the various photoreceptors present, but to judge color, it is the difference between receptors' outputs that is critical. Insofar as the receptor spectral sensitivities overlap, the signal from the sum will be larger than that from the difference. The L and M cones, which constitute some 90% or more of the cone population, have closely spaced spectral sensitivity peaks, only about 30 nm apart, and thus give a much larger sum than the difference signal. In order to have a difference signal large enough to work with, the light level must be high and there must be at least two photoreceptor types with different spectral sensitivities. Thus, color vision cannot operate at all at scotopic levels and only poorly at low photopic light levels. Consequently, a spatial vision system that depended solely on color discriminations would be a major handicap for any species that is active in low light levels.

Spatial discriminations based on color differences have other constraints as well. A local luminance judgment can be based on the output of a single L or M cone. Detecting a variation in luminance across space can be accomplished by comparing the outputs of two such receptors at different retinal positions. Since the two receptors can be adjacent (representing adjacent visual directions), spatial resolution can in principle be as fine as the receptor spacing. To judge the color at any single point, however, requires a comparison between the outputs of two or more different photoreceptor types near one another in retinal location. To detect a variation in color across space, it is necessary to make at least two local color judgments, each of which requires a comparison of the outputs of at least two different cone types. This perforce limits spatial acuity for patterns that vary only in color.