Mice, which normally have only two cone pigments (and therefore two opponent channels), have been engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, possibly indicating trichromacy and suggesting they were able to create or re-enable a third opponent channel. Whether a fourth opponent channel is available to facilitate tetrachromacy is unclear. According to the opponent process theory, humans have three opponent channels, which grant trichromacy. However, there must also be the appropriate post-receptoral mechanism to compare the signals from the four classes of receptors. Tetrachromacy requires that there be 4 independent photoreceptor cell classes with different spectral sensitivity. Normal trichromats would have only three cone types (red, green, blue) active in the short-, medium- and long-wave part of the spectrum, but one subject was found to have a well-separated fourth cone type in a short-wave part assuming the extra cone type adds one more independent color dimension for her, that makes her a tetrachromat overall. However, human tetrachromacy is suspected to exist in a small percentage of the population. Humans Īpes (including humans) and Old World monkeys normally have three types of cone cell and are therefore trichromats. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species. This means that the organism may see wavelengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a normal human, appear to be identical. 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 spectral sensitivity.
The common ancestor of all vertebrates was a tetrachromat, but mammals evolved dichromacy, due to the nocturnal bottleneck, losing two of their four cones. Tetrachromacy is demonstrated among several species of birds, fishes, amphibians, and reptiles.
In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that matching the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors. Organisms with tetrachromacy are called tetrachromats. Tetrachromacy (from Greek tetra, meaning "four" and chromo, meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. The four pigments in a bird's cone cells (in this example, estrildid finches) extend the range of color vision into the ultraviolet.
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