COLOR VISION

'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. 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 visible light shining on it except for a group of frequencies that is perceived as 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.

Contents

Physiology of color perception

Cone cells in the human eye

Color in the human brain

In other animals

Evolution

Mathematics of color perception

Chromatic adaptation

References

See also

External links

Physiology of color perception

Perception of color is achieved in mammals through color receptors containing pigments with different spectral sensitivities. In most primates closely related to humans 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 greenish-yellow region of the spectrum.
The peak response of human color receptors varies, even amongst individuals with 'normal' color vision;^[1]
in non-human species this polymorphic variation is even greater, and it may well be adaptive.^[2]

Cone cells in the human eye

Cone type	Name	Range	Peak wavelength[3][4]
S	β	400–500 nm	420–440 nm
M	γ	450–630 nm	534–545 nm
L	ρ	500–700 nm	564–580 nm

A range of wavelengths of light stimulates each of these receptor types to varying degrees. Yellowish-green light, for example, stimulates both L and M cones equally strongly, but only stimulates S-cones weakly. Red light, on the other hand, stimulates L cones much more than M cones, and S cones hardly at all; blue-green light stimulates M cones more than L cones, and S cones a bit more strongly, and is also the peak stimulant for rod cells;
and violet light stimulates almost exclusively S-cones. The brain 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 yellowish light, is highly polymorphic (a recent study by Verrelli and Tishkoff, 2004, found 85 variants in a sample of 236 men), so up to ten percent of women [1] have an extra type of color receptor, and thus a degree of tetrachromatic color vision.^[5] Variations in OPN1MW, which codes for the bluish-green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity.

Color in the human brain

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 explained by Edwin H. Land in his retinex theory.
From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form). Neurons in V2 then synapse onto cells in area V4. Area V4 is a relatively large visual area, the largest by far cortical area outside V1, encompassing almost as much cortex as V1. Neurons in V4 were originally proposed by Semir Zeki to be exclusively dedicated to color, but this has since been shown not to be the case. Quantitative studies have argued that there is no higher concentration of color cells in V4 than in primary visual cortex, although this remains controversial. Independent of color sensitivity, V4 neurons have been shown to be very sensitive to the shape of stimuli, curvature, and stereo-scopic depth. V4 neurons have also been shown to be modulated by attention. The role of V4 neurons in color vision remains to be better characterized: indeed the vast majority of scientific papers examining the function of V4 do not concern color processing.
Anatomical studies have shown that neurons in V4 provide input to the inferior temporal lobe . "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the ventral stream or the "what pathway", distinguished from the dorsal stream ("where pathway") that is thought to analyze motion, among many other features.

In other animals

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.

Evolution

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, especially insects.
Color vision evolved in the ancestors of modern monkeys, apes, and humans as they switched to diurnal (daytime) activity and consumption of fruits from flowering plants.^[6]

Mathematics of color perception

A "physical color" is a combination of pure spectral colors (in the visible range). Since there are, in principle, infinitely many distinct spectral colors, the set of all physical colors may be thought of as an infinite-dimensional vector space, in fact a Hilbert space. We call this space H_color. More technically, the space of physical colors may be considered to be the (mathematical) cone over the simplex whose vertices are the spectral colors.
An element C of H_color is a function from the range of visible wavelengths—considered as an interval of real numbers [W_min,W_max]—to the real numbers, assigning to each wavelength w in [W_min,W_max] its intensity C(w).
A humanly perceived color may be modeled as three numbers: the extents to which each of the 3 types of cones is stimulated. Thus a humanly perceived color may be thought of as a point in 3-dimensional Euclidean space. We call this space R³_color.
Since each wavelength w stimulates each of the 3 types of cone cells to a known extent, these extents may be represented by 3 functions r(w), g(w), b(w) corresponding to the response of the so-called "red", "green", and "blue" cone cells, respectively.
Finally, since a beam of colored light can be composed of many different wavelengths, to determine the extent to which a physical color C in H_color stimulates each cone cell, we must calculate the integral (with respect to w), over the interval [W_min,W_max], of C(w)
★ r(w) (for red), of C(w)
★ g(w) (for green), and of C(w)
★ b(w) (for blue). The triple of resulting numbers associates to each physical color C (which is a region in H_color) to a particular perceived color (which is a single point in R³_color). This association is easily seen to be linear. It may also easily be seen that many different regions in the "physical" space H_color can all result in the same single perceived color in R³_color, so a perceived color is not unique to one physical color.
Thus human color perception is determined by a specific, non-unique linear mapping from the infinite-dimensional Hilbert space H_color to the 3-dimensional Euclidean space R³_color.
Technically, the image of the (mathematical) cone over the simplex whose vertices are the spectral colors, by this linear mapping, is also a (mathematical) cone in R³_color. Moving directly away from the vertex of this cone represents maintaining the same chromaticity while increasing its intensity. Taking a cross-section of this cone yields a 2D chromaticity space. Both the 3D cone and its projection or cross-section are convex sets; that is, any mixture of spectral colors is also a color.

In practice, it would be quite difficult to measure an individual's cones' three responses to various physical color stimuli. So instead, three specific benchmark test lights are typically used; let us call them R, G, and B. In order to calibrate human perceptual space, scientists allowed human subjects to try to match any physical color by turning dials to create specific combinations of intensities (I_R, I_G, I_B) for the R, G, and B lights, resp., until a match was found. This needed only to be done for physical colors that are spectral (since a linear combination of spectral colors will be matched by the same linear combination of their (I_R, I_G, I_B) matches). Note that in practice, often at least one of R, G, B would have to be added with some intensity to the ''physical test color'', and that combination matched by a linear combination of the remaining 2 lights. Across different individuals (without color blindness), the matchings turned out to be nearly identical.
By considering all the resulting combinations of intensities (I_R, I_G, I_B) as a subset of 3-space, a model for human perceptual color space is formed. (Note that when one of R, G, B had to be added to the test color, its intensity was counted as negative.) Again, this turns out to be a (mathematical) cone—not a quadric, but rather all rays through the origin in 3-space passing through a certain convex set. Again, this cone has the property that moving directly away from the origin corresponds to increasing the intensity of the R, G, B lights proportionately. Again, a cross-section of this cone is a planar shape that is (by definition) the space of "chromaticities" (informally: distinct colors); one particular such cross section, corresponding to constant X+Y+Z of the CIE 1931 color space, gives the CIE chromaticity diagram.
It should be noted that this system implies that for any hue or non-spectral color, there are infinitely many distinct physical spectra that are all perceived as that hue or color. So, in general there is no such thing as ''the'' combination of spectral colors that we perceive as (say) yellow-green; instead there are infinitely many possibilities.
The CIE chromaticity diagram is horseshoe-shaped, with its curved edge corresponding to all spectral colors (the ''spectral locus''), and the remaining straight edge corresponding to the most saturated purples—mixtures of red and violet.

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, human vision perceives that the object has the same color: an apple always appears red, whether viewed at night or during the day. On the other hand, a camera with no adjustment for light may register the apple as having many different shades. This feature of the visual system is called chromatic adaptation, or color constancy; when the correction occurs in a camera it is referred to as white balance.
Chromatic adaptation is one aspect of vision that may fool someone into observing an color-based optical illusion. Though the human visual system generally does maintain constant perceived color under different lighting, there are situations where the brightness of a stimulus will appear reversed relative to its "background" when viewed at night. For example, the bright yellow petals of flowers will appear dark compared to the green leaves in very dim light. The opposite is true during the day. This is known as the Purkinje effect, and arises because in very low light, human vision is approximately monochromatic and limited to the region near a wavelength of 550nm (green).

References

1. Neitz, Jay & Jacobs, Gerald H. (1986). "Polymorphism of the long-wavelength cone in normal human colour vision." ''Nature''. '323', 623-625.
2. Jacobs, Gerald H. (1996). "Primate photopigments and primate color vision." ''PNAS''. '93' (2), 577–581.
3. Color Science: Concepts and Methods, Quantitative Data and Formulae, , Günther, Wyszecki, Wiley Series in Pure and Applied Optics, 1982, ISBN 0-471-02106-7
4. The Reproduction of Colour, R. W. G. Hunt, , , Wiley–IS&T Series in Imaging Science and Technology, 2004, ISBN 0-470-02425-9
5. Roth, Mark (2006). "Some women may see 100 million colors, thanks to their genes" ''Post-Gazette.com''
6. Steven Pinker. ''How the Mind Works'', 1997. p. 191. ISBN 0-393-04535-8.



★ 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.

★ McCann, M., ed. 1993. ''Edwin H. Land's Essays.'' Springfield, Va.: Society for Imaging Science and Technology.

★ Readings on Color, Volume 2: The Science of Color, , Alex, Byrne, MIT Press, 1997, ISBN 0-262-52231-4

★ Human Color Vision, , Peter K., Kaiser, Optical Society of America, 1996, ISBN 1-55752-461-0

★ Color Science: Concepts and Methods, Quantitative Data and Formulae, , Günther, Wyszecki, Wiley-Interscience, 2000, ISBN 0-471-39918-3

★ Colour Blindness: Causes and Effects, , Donald, McIntyre, Dalton Publishing, 2002, ISBN 0-9541886-0-8

★ The Science of Color, , Steven K., Shevell, Optical Society of America, 2003, ISBN 0-444-512-519

See also

★ Color theory

★ Mantis shrimps are at least octachromats

★ Primary color

★ Trichromacy

★ Tetrachromacy

★ Visual perception

External links

★ "Evidence that men, women literally see the world differently: Study shows color vision may have been adaptive during evolution."

★ Spectral Sensitivity of the Eye.

★ Vision may not be what we thought.

★ Overview of color vision.

★ The decoding model: a symmetrical model of color vision.

★ Working examples of Chromatic Adaptation.

★ ''Dale Purves Lab''[2]

★ Egopont color vision test