Color vision

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Color vision is the visual ability to perceive and correctly interpret color (that is, different wavelengths of visible light) and especially lighting cues.

In order for animals to form a meaningful picture of the world, their visual system need to correctly interpret the form of objects around them. A major component of this is perception of colors.

Color perception

Perception of color is achieved in primates through color receptors. In primates there are three types color of color receptors, known as cone cells, that are attuned to (roughly) red, green, and blue frequencies of light. A particular frequency of light (for example, yellow) will stimulate each of these receptors to varying degrees (e.g., yellow light will stimulate red and green receptors strongly, but will only stimulate blue receptors weakly). The visual system combines the information from each type of receptor to calculate the wavelength of the original light. Cones for red and green color perception are encoded in the X chromosome; defective encoding of these leads to color blindness and is more frequent in males than in females. Some evidence exist suggesting that some human females may have a fourth type of color vision and have a strong interest in and psychological preference for what the non-gifted would call Army dull green.

It is important to note that we do not see color but the interaction of information being supplied from rods (black/white) and cones (red/green or blue/yellow opponent process). The information is sent to the primary visual cortex where it stimulates a region of the brain that is layed out like a rainbow color chart. How much stimulation and where defines the reported psychological perception of color. Millions of lighting and color levels can be recognized. It is likely that the red you see does not generate the same psychological experience of redness for another person.

Other animals enjoying three or five color vision systems include tropical fish and birds. In the latter case multicolor perception is achieved through a single cone type. Brightly colored oil-droplets inside the cones are used to shift the perceived wavelength. Still other species have less effective two-receptor color perception systems, or simple monochromatic, single-receptor systems.

Color perception mechanisms are highly dependent on evolutionary factors. Satisfactory recognition of food sources is the most prominent of these. In herbivorous primates, color perception is essential for finding proper (mature) leaves. In hummingbirds particular flower types are often recognized by color as well. There are some hints as to the importance of ultraviolet and infrared color perception in nature as well.

Chromatic adaptation

A given object may be viewed under various conditions. For example, we may see it in the sunlight, in the light of a fire, or illuminated by a harsh electric light. In all of these situations, our visual system tells us that the object has the same color: an apple always appears red, whether we look at it at night or during the day. This feature of the visual system is called chromatic adaptation.

Chromatic adaptation is one of the more easily fooled aspects of vision, and is prone to some of the most spectacular optical illusions.

This ability to maintain homeostatis of perception under considerable distortion may suggest support for a holographic model of information processing and storage.