Tetrachromatic vision

Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cells in the eye. Organisms with tetrachromacy are called tetrachromats.

In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that to match the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors.

Tetrachromacy is demonstrated among several species of birds, fish, amphibians, reptiles, insects and some mammals. It was the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.

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 that the animal may see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish between colors that, to a normal human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.

The goldfish (Carassius auratus auratus) and zebrafish (Danio rerio) are examples of tetrachromats, containing cone cells sensitive for red, green, blue and ultraviolet light.

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