Neural Basis Of Color Vision A The Photoreceptors

Vision starts in the photoreceptor cells of the retina and their outermost organelle that contains specialized protein molecules, called opsins (from the Greek word "to see''), which absorb and amplify light energy. In the human retina, there are four types of photorecep-tors, defined primarily by the opsin they contain. Usually one type of cone contains only one type of opsin. Each type of opsin absorbs a particular waveband of light in the visible spectrum. The wavelength selectivity, defined by an absorption spectrum, is related to the opsin's amino acid sequence interacting with a specific isomer of vitamin A.

One set of photoreceptors, called rods because of the shape of their long outer segments, function only at dim light levels. Their extraordinarily high sensitivity to light drives them into saturation at daylight levels. Rods are very sensitive and numerous in our retina but only work well in moonlight and play little to no role in

Chromatic Color Achromatic wavelength both energy

Figure 1 A segment of a painting by Jan van Eyck (1434) in which the luminance information has been removed (left) rendering an image based on chromatic contrasts alone. (Right) The chromatic contrast has been removed, rendering an achromatic black-and-white image of the same scene; the latter has more fine detail. In the middle and fusion of these two different forms of contrast produces color vision. (See color insert in Volume 1).

Chromatic Color Achromatic wavelength both energy

Figure 1 A segment of a painting by Jan van Eyck (1434) in which the luminance information has been removed (left) rendering an image based on chromatic contrasts alone. (Right) The chromatic contrast has been removed, rendering an achromatic black-and-white image of the same scene; the latter has more fine detail. In the middle and fusion of these two different forms of contrast produces color vision. (See color insert in Volume 1).

color vision. Rod vision is without color and has become vestigial in modern society.

Color vision is mediated by the other three photoreceptors called cones, which require broad daylight to work well. Figure 2 shows the absorption spectra of the three cones of human vision. Cones are much more important to human vision than rods because if they are lost or fail to function, one is legally blind. The absence of rod function, on the other hand, is only a minor inconvenience. Cones' tolerance of high levels of illumination depends in great part on their ability to adapt to light. As light levels increase, cones reduce their sensitivity and speed up their responses, making them almost impossible to saturate. For color vision, the responses of the three types of cones are compared. Figure 2 shows the spectral colors that result from these comparisons. The colors we see in the spectrum are not near the peak absorption of the cone opsins because they depend on sophisticated neural comparisons.

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