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A tetrachromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than four different pure spectral lights. The condition of being a tetrachromat is called tetrachromacy.
The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cones in vertebrates as opposed to rods which are lower intensity light receptors) with different absorption spectra. 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 demonstrated as a characteristic property of any mammalian species, though it is likely that it occurs in some birds. Humans and other Old World primates normally have three types of cone cells and are therefore trichromats. However, at low light intensities the rod cells may contribute to colour vision, giving a small region of tetrachromacy in the colour space. In addition, it has been suggested that some female humans, if they carry a gene for the light receptors found in a specific type of colour-blindness (deuteranomaly or protanomaly) as well as genes for the normal receptors, can be born as full tetrachromats, having four different simultaneously functioning kinds of cones.
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