Color vision

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Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect or emit. In animals, the nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are sensitive to different portions of the visible spectrum. For humans, the visible spectrum ranges approximately from 380 to 750 nm, and there are normally three types of cones. The visible range and number of cone types differ between species.

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 the human eye can distinguish between different wavelengths. Three things are needed to see color: a light source, a detector (e.g. the eye) and a sample to view.

The advantage of color, which is a quality constructed by the visual brain and not a property of objects as such, is the better discrimination of surfaces allowed by this aspect of visual processing.

In order for animals to respond accurately to their environments, their visual systems need to correctly interpret the form of objects around them. A major component of this is perception of colors.

1 Color perception
- 1.1 Cone cells in the human eye
2 Chromatic adaptation
3 References
4 See also
5 External links

[edit] Color perception

Normalized typical human cone responses (and the rod response) to monochromatic spectral stimuli

Image:Rodcolours.jpg

Perception of color is achieved in mammals through color receptors containing pigments with different spectral sensitivities. In most Old World monkeys there are three types of color receptors (known as cone cells). This confers trichromatic color vision, so these primates, like humans, are known as trichromats. Many other primates and other mammals are dichromats, and many mammals have little or no color vision.

In the human eye, the cones are maximally receptive to short, medium, and long wavelengths of light and are therefore usually called S-, M-, and L-cones. L-cones are often referred to as the red receptor, but while the perception of red depends on this receptor, microspectrophotometry has shown that its peak sensitivity is in the yellow region of the spectrum.

[edit] Cone cells in the human eye

Cone type	Name	Range	Peak sensitivity
S	β (Blue)	400..500 nm	420 nm
M	γ (Bluish-Green)	450..630 nm	534 nm
L	ρ (Yellowish-Green)	500..700 nm	564 nm

A particular frequency of light stimulates each of these receptor types to varying degrees. Yellow light, for example, stimulates L-cones strongly and M-cones to a moderate extent, but only stimulates S-cones weakly. Red light, on the other hand, stimulates almost exclusively L-cones, and blue light almost exclusively S-cones. The visual system combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.

The pigments present in the L- and M-cones are encoded on the X chromosome; defective encoding of these leads to the two most common forms of color blindness. The OPN1LW gene, which codes for the pigment that responds to red light, is highly polymorphic (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 an extra type of color receptor, and thus a degree of tetrachromatic color vision. Variations in OPN1MW, which codes for the green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity.

Color processing begins at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Opponent mechanisms refer to the opposing color effect of red-green, blue-yellow, and light-dark. Visual information is then sent back via the optic nerve to the optic chiasm: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasm the visual fiber tracts are referred to as the optic tracts, which enter the thalamus to synapse at the lateral geniculate nucleus (LGN). The LGN is segregated into six layers: two magnocellular (large cell) achromatic layers (M cells) and four parvocellular (small cell) chromatic layers (P cells). Within the LGN P-cell layers there are two chromatic opponent types: red vs. green and blue vs. green/red.

After synapsing at the LGN, the visual tract continues on back toward the primary visual cortex (V1) located at the back of the brain within the occipital lobe. Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex".It is at this stage that color processing becomes much more complicated.

In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such"double-opponent" cells were initially described in the goldfish retina by Nigel Daw; their existence in primates was suggested by David Hubel and Torsten Wiesel and subsequently proven by Bevil Conway. As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called blobs, and are thought to come in two flavors, red-green and blue-yellow. Red-green cells compare the relative amounts of red-green in one part of a scene with the amount of red-green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of color constancy.

From V1, color information is sent to V2 in rough stripes that arise particular collections of neurons in Layer III of striate cortex called "blobs." Neurons in V2 then synapse onto V4 cells, the first color visual analysis region. From V4 color information is then sent to the inferior temporal lobe which is thought to integrate color information with shape and form. This pathway (V1 > V2 > V4 > inferior temporal) is known as the ventral stream or the "what pathway". This is separate from the dorsal stream ("where pathway") that is thought to analyze motion, among many other features.

Other animals enjoying three, four or even five color vision systems include tropical fish and birds. In the latter case tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift the spectral sensitivity of the cell. (Some species of bird such as the pigeon in fact possess five distinct types of droplet and may thus be pentachromats). Mammals other than primates generally 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, of which the most prominent is thought to be satisfactory recognition of food sources. 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 less-developed color vision, since adequate light is needed for cones to function properly. There is evidence that ultraviolet light plays a part in color perception in many branches of the animal kingdom.

[edit] Chromatic adaptation

An object may be viewed under various conditions. For example, it may be illuminated by the sunlight, the light of a fire, or a harsh electric light. In all of these situations, the visual system indicates that the object has the same color: an apple always appears red, whether viewed at night or during the day. This feature of the visual system is called chromatic adaptation, or can be referred to as white balance. Though this is generally true there are situations where the apparent brightness of a stimulus will appear reversed relative to its "background" when viewed at night. The petals of yellow flowers will appear dim compared to the green leaves. The opposite is true during the day. This is known as the Purkinje effect.

Chromatic adaptation is one aspect of vision that may fool someone into observing an optical illusion. This ability to maintain homeostasis of perception under considerable distortion may suggest support for a holographic model of information processing and storage.

[edit] References

Wandell, B. "Foundations of Vision". Sinauer Press, MA.
Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. ISBN 0-8385-7701-6
Martin, Paul R (1998). "Colour processing in the primate retina: recent progress." Journal of Physiology. 513 (3), 631-638.
Nolte J. The Human Brain: An Introduction to Its Functional Anatomy. 5th ed. St. Louis: Mosby, Inc.; 2002. ISBN 0-323-01320-1* Rowe, Michael H (2002).
Michael H. Rowe "Trichromatic color vision in primates." News in Physiological Sciences. 17 (3), 93-98.
Verrelli, BC; Tishkoff, S (2004). "Color vision molecular variation." American Journal of Human Genetics. 75 (3), 363-375.
Conway, BR (2001). "Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1)" Journal of Neuroscience. 21 (8), 2768-2783.