Tetrachromacy

Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different cones, one other than RGB. Organisms with tetrachromacy are called tetrachromats. For these organisms, the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no less than four different pure spectral lights.

The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells which are lower intensity light receptors) with different absorption spectra. This means the animal can see colors we cannot even imagine. In practice the number of such receptor types may be greater than four, since different types may be active at different light intensities.

Tetrachromacy has not yet been discovered in any mammals, though it is likely that it occurs in some birds, fish, amphibians, reptiles, arachnids and insects. Humans and closely related primates normally have three types of cone cells and are therefore trichromats (animals with three different cones). However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space.

It has been suggested that women who are carriers for variant cone pigments may be born as full tetrachromats, having four different simultaneously functioning kinds of cones to pick up different colors. One study suggested that 2-3% of the world's women may have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in colour differentiation. Although further studies will need to be conducted to verify tetrachromacy in humans, at least one tetrachromat has been identified - "Mrs. M," an English social worker, was discovered in a study conducted in 1993. Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red-green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation.