Color Vision

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Abstract
Receptors contain one of 4 photopigments whose spectral sensitivities overlap. Photopigment breakdown, initiated by light, results in an early receptor potential whose amplitude is linearly proportional to the number of molecules broken down. However, by the time the bipolars are activated, this linearity is lost because of response attenuation by a feedback gain control; this operates as long as the receptors respond or their photo-pigment is unregenerated, and involves also neighboring receptors of the same type (red cones inhibit red cones, etc.). Adaptation phenomena probably reflect interconnections between like receptors (through horizontal cells, and possibly direct contacts). In contrast to the interactions at the earliest level among like receptors, the later information processing involves interactions between different types of receptors. Two main types of information channel result from the interactions. In the spectrally nonopponent channel, different types of receptors from a given area all have similar effects on later neurons; either all have excitatory or all have inhibitory effects. Brightness is directly proportional to the extent to which these cells are activated. (This spectral organization is overlaid by a spatial organization in which different areas within each cell''s receptive field are arranged in a spatially opponent although spectrally nonopponent manner.) In the other main type of organization, the different recpetors feed in in opposite directions, one type having an excitatory and another an inhibitory effect, so that the response of the later neuron is an increase or a decrease in firing depending on the wavelength of the light. Various cone types are interconnected in different combinations forming at least 4 varieties of such opponent cells. Hue is directly related to which one(s) of these spectrally opponent cells is activated; the color of different spectral regions corresponds not to the absorption characteristics of the cone pigments, but rather to the response characteristics of the spectrally opponent cells. Saturation is determined by the relative extent to which opponent and nonopponent cells are activated.

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