Color vision is a neural process occurring in our brain that

depends on a comparison of responses of at least two, but normally three, spectrally different cone photoreceptors. The brain uses these cone systems to detect achromatic and chromatic contrasts. Achromatic contrasts depend on light energy gradients across an image; chromatic contrasts depend on wavelength gradients of light across an image. Achromatic contrast is detected by the two longer but not by the shortwave-sensitive cones; in the fovea, achromatic contrast can resolve the dimensions of neighboring cones (about 1 mm on the retina). Chromatic contrasts are detected using all three types of cones and is done by comparing their responses to the same object. From an evolutionary perspective, the first comparison was between the short- and the longer wavelength-sensitive cones creating blue/yellow color vision. This still occurs in most mammals and in about 2% of human males. In this case, if an object affects the short-wave more than long-wave cones, it appears blue. The converse appears yellow. If the object affects both cone systems equally, it appears white, gray, or black. In the Land color vision model this comparison between cone signals occurs after each cone system's response is normalized over visual space. The second comparison evolved in primates when the longer wave cones were split into long- and middle-wave cones; this created red/green color vision. If an object affects the longwave more than the middle wave-sensitive cones, it appears red. The converse appears green. If an object affects both of these cones equally, it appears yellow, white, or bluish depending on the former comparison. Color vision compares responses of groups of cones rather than single neighboring cones; it has a lower spatial resolution than achromatic vision. The neural processing responsible for color perception occurs in visual cortex. The retina and the lateral geniculate nucleus provide the signals from each cone mechanism in a form that allows the comparisons necessary for color vision. Our brain combines cues from both chromatic and achromatic contrast to perceive a particular color. There are several different mechanisms involved. One depends on the blue/yellow and red/green comparison signals and is most related to the hue of a color, reflected by the words red, green, etc. The second is related to the amount of white, gray, or black that is mixed with the previous signal. This confers a quality called saturation of a color; it distinguishes pinks from reds. The third depends on the achromatic system to establish the lightness or darkness of a color. Together these separate operations provide us with about 1 million different colors. All these operations, which involve both achromatic and chromatic contrast detection, provide cues to the form of an object. Although we can mentally separate the form of an object from its color, it is not known where in cerebral cortex this separation of form from color occurs.

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