Tetrachromat

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 color receptors (called cones in vertebrates) 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 mammal 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 possess a rare genetic combination from two parents with different kinds of color blindness, can be born as full tetrachromats, having four different simultaneously functioning kinds of cones.

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