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In order for animals to respond accurately to their environments, their visual system need to correctly interpret the form of objects around them. A major component of this is perception of colors.
Perception of color is achieved in mammals through color receptors containing pigments with different spectral sensitivities. In most Old World primates there are three types of color receptors, known as cone cells, that are maximally receptive to short, medium, and long wavelengths of light (known as S-, M-, and L-cones and roughly corresponding to blue, green, and yellow), allowing for trichromatic color vision. L-cones are often referred to as the red receptor, but while the perception of red depends on this receptor, microspectrophotometry has shown that its peak sensitivity is in the yellow region of the spectrum.
A particular frequency of light stimulates each of these receptor types to varying degrees. Yellow light, for example, stimulates L-cones strongly and M-cones to a moderate extent, but only stimulates S-cones weakly. Red light, on the other hand, stimulates only L-cones, and violet light only S-cones. The visual system combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.
The pigments present in the L- and M-cones are encoded on the X chromosome; defective encoding of these leads to the two most common forms of color blindness. The OPN1LW gene, which codes for the yellow pigment, is highly polymorphic (a recent study by Verrelli and Tishkoff, 2004, found 85 variants in a sample of 236 men), so it is possible for a woman to have an extra type of color receptor, and thus a degree of tetrachromatic color vision. Variations in OPN1MW, which codes for the green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity.
It is important to note that we do not see color but the interaction of information being supplied from rod cells (black/white) and cones (via the red-green and blue-yellow opponent process). The information is sent to the primary visual cortex where different cells respond to inputs of different color. How much stimulation and where defines the reported psychological perception of color. Millions of color and brightness levels can be distinguished. The psychological response to color is very complex, and it is thus likely that the experience a particular shade of red generates is not the same as another person's.
Other animals enjoying three, four or even five color vision systems include tropical fish and birds. In the latter case tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift the spectral sensitivity of the cell. (Some species of bird such as the pigeon in fact possess five distinct types of droplet and may thus be pentachromats.) Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromatic color vision; marine mammals have only a single cone type and are thus monochromats.
Color perception mechanisms are highly dependent on evolutionary factors, of which the most prominent is satisfactory recognition of food sources. In herbivorous primates, color perception is essential for finding proper (mature) leaves. In hummingbirds particular flower types are often recognized by color as well. On the other hand, nocturnal mammals have less-developed color vision, since adequate light is needed for cones to function properly. There is evidence that ultraviolet light plays a part in color perception in many branches of the animal kingdom.
A given object may be viewed under various conditions. For example, we may see it in the sunlight, in the light of a fire, or illuminated by a harsh electric light. In all of these situations, our visual system tells us that the object has the same color: an apple always appears red, whether we look at it at night or during the day. This feature of the visual system is called chromatic adaptation.
Chromatic adaptation is one of the more easily fooled aspects of vision, and is prone to some of the most spectacular optical illusions. distortion may suggest support for a holographic model of information processing and storage.
- "Evidence that men, women literally see the world differently: Study shows color vision may have been adaptive during evolution." 
- Verrelli, BC; Tishkoff, S (2004). "Color vision molecular variation." American Journal of Human Genetics. 75 (3), 363-375. 
- Martin, Paul R (1998). "Colour processing in the primate retina: recent progress." Journal of Physiology. 513 (3), 631-638. 
- Rowe, Michael H (2002). "Trichromatic color vision in primates." News in Physiological Sciences. 17 (3), 93-98. 
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