Abstract
A normalization process makes the average spectral distribution reaching the eye appear achromatic -- white or gray. There are six possible ways the relative activations of L-, M- and S-cones can differ from white. 1) S activity higher, M and L activity relatively lower compared to white; 2) M higher, S and L lower; 3) L higher, S and M lower; 4) M and L higher, S lower; 5) S and L higher, M lower; 6) S and M higher, L lower. Thus, to fully encode hue require six channels, for each of the ways the relative activities of the cones can differ from white. However, humans experience only four hues—1) red, the hue associated with S+L high, M low; 2) green the hue associated with M high, S+L low; 3) blue, associated with S+M high, L low; 4) yellow, associated with L high, S+M low. Thus only two-thirds of the hue information available is encoded and transmitted to the brain.. For example, the difference between a light at 430 nm and a 470+630 nm mixture is entirely a change between the relative activities of S vs. L+M cones, for which we have no hue encoders. Under carefully controlled conditions, when the relative brightness of the lights is adjusted appropriately, a good match is achieved in hue, saturation and brightness. This is a true failure of color vision and since each hue dimension produces about 100 gradations, we only see about 1% of the hues compared to a perfect encoder.