Color vision is a psychophysical phenomenon that exists only in our minds. A 'red' apple does not emit red light. Rather, it simply absorbs all the frequencies of light shining on it except the frequencies we call red, which are reflected. An apple is perceived to be red only because human can distinguish between different frequencies—and we have language to describe that difference. Three things are needed to see colour: a light source, a detector (which might be your eye) and a sample to view.
In order for animals to respond accurately to their environments, their visual system need to correctly interpret the form of objects around them. A major component of this is perception of colors.
Perception of color is achieved in mammals through color receptors containing pigments with different spectral sensitivities. In most primates there are three types of color receptors, known as cone cells, that are maximally receptive to yellow, green, and blue frequencies of light, and this allows for trichromatic color vision. The cone type that is most sensitive to long-wavelength light is often referred to as the red receptor, but though the perception of red depends on this receptor, microspectrophotometry has shown that its peak sensitivity is in the yellow region of the spectrum. A particular frequency of light (for example, yellow) will stimulate each of these receptor types to varying degrees (e.g., yellow light will stimulate the yellow receptors strongly and the green receptors to a moderate extent, but will only stimulate blue receptors weakly, while red light will stimulate only the yellow receptors, and violet light will stimulate only the blue receptors). The visual system combines the information from each type of receptor to give rise to different percepts for different wavelengths of light. The pigments present in the yellow- and green-senstive cones are encoded in the X chromosome; defective encoding of these leads to the most usual forms of color blindness, which are more frequent in males than in females. The OPN1LW gene, which codes for the yellow pigment, exists in many different variants (a recent study by Verrelli and Tishkoff, 2004, found 85 variants in a sample of 236 men), so it is possible for a woman to have more than one variant, and thus a degree of tetrachromacy in her color vision.
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 different cells respond to inputs of different color. 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 spectral sensitivity of the cell. Mammals other than primates mostly have less effective two-receptor color perception systems, allowing only dichromatic color vision; marine mammals have only a single cone type and are thus monochromats.
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. On the other hand, nocturnal mammals have a less-developed color vision, since adequate light is needed for cones to function properly. In insects and birds there is evidence that ultraviolet light plays a part in color perception.
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.
- News story on genetic variation of the yellow pigment (http://www.psycport.com/stories/ascribe_2004_07_14_eng-ascribe_eng-ascribe_014026_988726893508805748.xml.html)
- Verrelli, B. C., & Tishkoff, S. (2004). American Journal of Human Genetics