Tetrachromacy

Tetrachromacy is color vision based on four independent types of cone photoreceptors or four independent channels of color information. It can arise from retinal biology or from additional neural processing.

Cone cell: A light-sensitive retinal cell that contributes to color vision through its spectral response. Tetrachromacy begins with four distinct cone responses rather than the usual three.
Trichromacy: Color vision based on three independent cone types or three independent channels of color information. It is the common human baseline against which four-channel color vision is defined.
Human color vision: The visual system's detection and interpretation of color in humans, usually based on three cone classes. Claims of human tetrachromacy concern departures from the usual three-cone system.
Functional tetrachromacy: Tetrachromacy in which four independent signals support color distinctions unavailable to trichromatic observers. This is the perceptual capacity that genetic evidence alone cannot establish.
Spectral sensitivity: The relative efficiency with which a photoreceptor or sensor responds to different wavelengths. Four cone classes must have distinguishable spectral sensitivities to provide four inputs.
Photopic vision: Vision under bright-light conditions, mediated mainly by cone photoreceptors. Tetrachromacy concerns cone-mediated color vision in the light-adapted eye.
Mammalian color vision: Color vision across mammals, with cone number and spectral sensitivity varying among species. Most mammals are dichromats, making four-cone color vision a distinct biological arrangement.
Anomalous trichromacy: A form of color vision deficiency in which three cone classes remain but one has an altered spectral sensitivity. Altered cone pigments can mimic a genetic route to four variants without producing four independent channels.
Color opponent process: A neural coding scheme that represents color through differences between signals from photoreceptors. Opponent circuits determine whether an extra cone signal remains distinct in the visual system.
Opsin: A light-sensitive protein that combines with a chromophore to form a photopigment. Differences in cone opsins can produce distinct wavelength sensitivities.

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