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TetrachromatA 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 cone cells in vertebrates as opposed to rod cells 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, fish, amphibians, reptiles and insects. 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. It has also been suggested that women who are carriers for certain kinds of color blindness (protanomaly or deuteranomaly) may be born as full tetrachromats, having four different simultaneously functioning kinds of cones. External links
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