COLOUR VISION and THEORIES

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COLOUR VISION and THEORIES: 

COLOUR VISION and THEORIES Dr.SUMANGALA M.PATIL Associate professor Department of Physiology BLDEA’s Sri B.M.Patil MC, Bijapur

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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, emit, transmit . 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 740 nm, and there are normally three types of cones. The visible range and number of cone types differ between species.

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A 'red' apple does not emit red light. [1] 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. 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.

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Issac Newton-discovered that white light is split into its component colors when passed through a prism, but that if those bands of colored light are passed through another and rejoined, they make a white beam. The characteristic colors are, in order from short to long wavelength: violet, blue, green, yellow, orange, red. Sufficient differences in wavelength give rise to a difference in perceived hue ; the just noticeable difference in wavelength varies from about 1 nm in the blue-green and yellow wavelengths, to 10 nm and more in the red and blue.

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In very low light levels, vision is scotopic , meaning mediated by rod cells , and not detecting color differences; the rods are maximally sensitive to wavelengths near 500 nm. In brighter light, such as daylight, vision is photopic , in which case the cone cells of the retina mediate color perception, and the rods are essentially saturated; in this region, the eye is most sensitive to wavelengths near 555 nm. Between these regions is known as mesopic vision , in which case both rods and cones are providing meaningful signal to the retinal ganglion cells .

Wavelength and Hue detection: 

Wavelength and Hue detection The perception of "white" is formed by the entire spectrum of visible light, or by mixing colors of just a few wavelengths, such as red, green, and blue, or even by mixing just a pair of complementary colors such as blue and yellow The shift in color perception across these light levels gives rise to differences known as the Purkinje effect .

Physiology of colour perception: 

Physiology of colour 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.

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The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities : short (S), medium (M), and long (L) cone types, also referred to as blue, green, and red cones. While the L cones are often referred to as the red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green ,

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It is important to note that the RGB color model is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye. The peak response of human color receptors varies, even amongst individuals with 'normal' color vision; [4 ] in non-human species this polymorphic variation is even greater, and it may well be adaptive. [ 5]

Theories of colour vision: 

Theories of colour vision Two complementary theories of color vision are the trichromatic theory and the opponent process theory. The trichromatic theory, or Young–Helmholtz theory , proposed in the 19th century by Thomas Young and Hermann von Helmholtz , as mentioned above, states that the retina's three types of cones are preferentially sensitive to blue, green, and red.

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Ewald Hering proposed the opponent process theory in 1872. [6] It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. We now know both theories to be correct, describing different stages in visual physiology.

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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 blue light stimulates almost exclusively S-cones.

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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 found 85 variants in a sample of 236 men [9] ),

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so up to ten percent of women have an extra type of color receptor, and thus a degree of tetrachromatic color vision. Variations in OPN1MW, which codes for the bluish-green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity .

Colour in the Human Brain: 

Colour 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 chiasma : a point where the two optic nerves meet and information from the temporal ( contralateral ) visual field crosses to the other side of the brain.

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After the optic chiasma 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.

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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

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In V1 the simple three-color segregation begins to break down. 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 .

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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 H. 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.

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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. [1

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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 the extended V4.

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This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO. (Area V4 was identitied as by Semir Zeki to be exclusively dedicated to color, but this has since been shown not to be the case. Color processing in the extended V4 occurs in millimeter-sized color modules called globs . This is the first part of the brain in which colour is processed in terms of the full range of hues found in color space .

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Anatomical studies have shown that neurons in extended 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.

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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.

Colour in other Animals: 

Colour in other Animals Other animals, such as tropical fish and birds , have more complex color vision systems than humans. In the latter example, tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift or narrow the spectral sensitivity of the cell. It has been suggested that pigeons are pentachromats .

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. Marine mammals have only a single cone type and are thus monochromats . Several marsupials such as the fat-tailed dunnart ( Sminthopsis crassicaudata ) have been shown to have trichromatic color vision Many invertebrates have color vision. Honey- and bumblebees have trichromatic color vision, which is insensitive to red but sensitive in ultraviolet. Papilio butterflies apparently have tetrachromatic color vision despite possessing six photoreceptor types.

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The most complex color vision system in animal kingdom has been found in stomatopods with up to 12 different spectral receptor types which are thought to work as multiple dichromatic units. Some nocturnal geckos have the capability of seeing color in dim light [25] .

Evolution: 

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 (immature) 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.

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There is evidence that ultraviolet light plays a part in color perception in many branches of the animal kingdom , especially insects . In general, the optical spectrum encompasses the most common electronic transitions in matter and is therefore the most useful for collecting information about the environment. The evolution of trichromatic color vision in primates occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal (daytime) activity and began consuming fruits and leaves from flowering plants

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Some animals can distinguish colors in the ultraviolet spectrum. The UV spectrum falls below the human visible range. Birds, turtles, lizards, and fish have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye. So far, there has not been enough evidence to show that any mammals are capable of UV vision .

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UV and multi-dimensional vision [ clarification needed ] is an especially important adaptation in birds. It allows birds to spot small prey from a distance, navigate, avoid predators, and forage while flying at high speeds. Birds also utilize their broad spectrum vision to recognize other birds, and in sexual selection.

Chromatic adaptations: 

Chromatic adaptations An object may be viewed under various conditions. For example, it may be illuminated by 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.

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On the other hand, a camera with no adjustment for light may register the apple as having varying color. 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 .

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Chromatic adaptation is one aspect of vision that may fool someone into observing a color-based optical illusion , such as the same color illusion . Though the human visual system generally does maintain constant perceived color under different lighting, there are situations where the relative brightness of two different stimuli will appear reversed at different illuminance levels.

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For example, the bright yellow petals of flowers will appear dark compared to the green leaves in dim light while the opposite is true during the day. This is known as the Purkinje effect , and arises because the peak sensitivity of the human eye shifts toward the blue end of the spectrum at lower light levels.

Von Kries transform: 

Von Kries transform The von Kries chromatic adaptation method is a technique that is sometimes used in camera image processing. The method is to apply a gain to each of the human cone cell spectral sensitivity responses so as to keep the adapted appearance of the reference white constant. The application of Johannes von Kries 's idea of adaptive gains on the three cone cell types was first explicitly applied to the problem of color constancy by Herbert E. Ives , [30][31] and the method is sometimes referred to as the Ives transform [32] or the von Kries –Ives adaptation. [33]

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The von Kries coefficient rule rests on the assumption that color constancy is achieved by individually adapting the gains of the three cone responses, the gains depending on the sensory context, that is, the color history and surround. Thus, the cone responses c ' from two radiant spectra can be matched by appropriate choice of diagonal adaptation matrices D 1 and D 2 [34] : where S is the cone sensitivity matrix and f is the spectrum of the conditioning stimulus. This leads to the von Kries transform for chromatic adaptation in LMS color space (responses of long-, medium-, and short-wavelength cone response space):

Color blindness: 

Color blindness color vision deficiency is the inability to perceive differences between some of the colors that others can distinguish. It is most oft The English chemist John Dalton published the first scientific paper on the subject in 1798, "Extraordinary facts relating to the vision of colours ", [1] after the realization of his own color blindness. en of genetic nature, but may also occur because of eye , nerve , or brain damage, or exposure to certain chemicals . Because of Dalton's work, the condition was often called daltonism , although this term is now used for a type of color blindness called deuteranopia

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Color blindness is sometimes classed as a disability , however, color blind individuals have an advantage over those with normal color vision. There are some studies which conclude that color blind individuals are better at penetrating certain color camouflages and it has been suggested that this may be the evolutionary explanation for the surprisingly high frequency of congenital red-green colour blindness. [2]

Background: 

Background The normal human retina contains two kinds of light cells: the rod cells ( active in low light ) and the cone cells ( active in normal daylight ). Normally, there are three kinds of cones, each containing a different pigment, which are activated when the pigments absorb light. The technical names for these receptors are S-cones, M-cones, and L-cones, but they are also often referred to as blue cones, green cones, and red cones, respectively.

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The absorption spectra of the cones differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths, with their peak sensitivities in the blue, yellowish-green, and yellow regions of the spectrum, respectively.

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as the "red" receptor actually has its peak sensitivity in the yellow region. The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different degrees . Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue.

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Many of the genes involved in color vision are on the X chromosome , making color blindness more common in males than in females. Any recessive genetic characteristic that persists at a level as high as 5% is generally regarded as having some advantage over the long term. At one time the U.S. Army found that color blind persons could spot "camouflage" colors that fooled those with normal color vision . Humans are the only trichromatic primates with such a high percentage of color blindness.

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Another possible advantage might result from the presence of a tetrachromic female. Owing to X-chromosome inactivation, women who are heterozygous for anomalous trichromacy ought to have at least four types of cone in their retinae . It is possible that this affords them an extra dimension of color vision

Genetic modes of inheritance: 

Genetic modes of inheritance Color blindness can be inherited genetically. It is most commonly inherited from mutations on the X chromosome but the mapping of the human genome has shown there are many causative mutations – mutations capable of causing color blindness originate from at least 19 different chromosomes and many different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM) database at Johns Hopkins University ).

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Other causes of color blindness include brain or retinal damage caused by shaken baby syndrome , accidents and other trauma which produce swelling of the brain in the occipital lobe, and damage to the retina caused by exposure to ultraviolet light . Most ultraviolet light damage is caused during childhood and this form of retinal degeneration is the leading cause of blindness in the world macular degeneration , and as part of the retinal damage caused by diabetes .

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Some of the inherited diseases known to cause color blindness are: cone dystrophy cone-rod dystrophy achromatopsia (aka rod monochromatism , aka stationary cone dystrophy, aka cone dysfunction syndrome) blue cone monochromatism , Leber's congenital amaurosis . retinitis pigmentosa (initially affects rods but can later progress to cones and therefore color blindness) Inherited color blindness can be congenital (from birth), or it can

Types: 

Types The most common are red-green hereditary photoreceptor disorders, damage to the retina, optic nerve, or higher brain areas. Higher brain areas implicated in color processing include the parvocellular pathway of the lateral geniculate nucleus of the thalamus , and visual area V4 of the visual cortex it is possible to acquire color blindness only in a portion of the visual field but maintain normal color vision elsewhere. Some forms of acquired color blindness are reversible. Transient color blindness also occurs (very rarely) in the aura of some migraine sufferers

Treatment and management: 

Treatment and management There is generally no treatment to cure color deficiencies. However, certain types of tinted filters and contact lenses may help an individual to better distinguish different colors. Optometrists can supply a singular red-tint contact lens to wear on the non-dominant eye. [29] This may enable the wearer to pass some color blindness tests, but they have little practical use.

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The effect of wearing such a device is akin to wearing red/blue 3D glasses and can take some time getting used to as certain wavelengths can "jump" out and be overly represented [ citation needed ] . Additionally, computer software and cybernetic devices have been developed to assist those with visual color difficulties such as an eyeborg , a "cybernetic eye" that allows individuals with color blindness to hear sounds representing colors. [30]

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The GNOME desktop environment provides colorblind accessibility using the gnome- mag and the libcolorblind software. Using a gnome applet, the user may switch a color filter on and off choosing from a set of possible color transformations that will displace the colors in order to disambiguate them. The software enables, for instance, a color blind person to see the numbers in the ishihara test.

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In September 2009, the journal Nature reported that researchers at the University of Washington and University of Florida were able to give trichromatic vision to squirrel monkeys , which normally have only dichromatic vision, using gene therapy .

Design implications of colourblind: 

Design implications of colourblind a red-green colorblind person who is incapable of distinguishing colors on a map printed on paper may have no such difficulty when viewing the map on a computer screen or television. In addition, some color blind people find it easier to distinguish problem colors on artificial materials, such as plastic or in acrylic paints, than on natural materials, such as paper or wood.

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Third, for some color blind people, color can only be distinguished if there is a sufficient "mass" of color: thin lines might appear black while a thicker line of the same color can be perceived as having color.

Compensations: 

Compensations Anomalous Trichromats tend to learn to use texture and shape clues and so are often able to spot camouflage clothing, netting, and paint that has been designed to deceive individuals with color-normal vision. [1]

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British Rail color lamp signals use more easily identifiable colors: the red is really blood red, the amber is quite yellow and the green is a bluish color. Most British road traffic lights are mounted vertically on a black rectangle with a white border (forming a "sighting board") and so dichromats simply look for the position of the light within the rectangle — top, middle or bottom.