Color blindness, or color vision deficiency, in humans is the inability to perceive differences between some or all colors that other people can distinguish.It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The English chemist John Dalton in 1798 published the first scientific paper on the subject, "Extraordinary facts relating to the vision of colours",[1] after the realization of his own color blindness; because of Dalton's work, the condition is sometimes called Daltonism, although this term is now used for a type of color blindness called deuteranopia.
Color blindness is usually classed as a disability; however, in selected situations color blind people may have advantages over people with normal color vision.There is anecdotal evidence that color blind individuals are better at penetrating color camouflage and at least one scientific study confirms this under controlled conditions.[2] Monochromats may have a minor advantage in dark vision, but only in the first five minutes of dark adaptation.
Causes of color blindness
There are many types of color blindness. The most common are hereditary (genetic) photoreceptor disorders, but it is also possible to acquire color blindness through 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. Acquired color blindness is generally unlike the more typical genetic disorders. For example, 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.
The different kinds of inherited color blindness result from partial or complete loss of function of one or more of the different cone systems. When one cone system is compromised, dichromacy results. The most frequent forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", though the term is an over-simplification and somewhat misleading. Other forms of color blindness are much more rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
[edit] Classification of color deficiencies
[edit] By etiology
Color vision deficiencies can be classified as acquired or inherited/congenital.[3][4]
Acquired
Inherited/congenital. There are three types of inherited or congenital color vision deficiencies: monochromacy, dichromacy, and anomalous trichromacy.[3]
Monochromacy, also known as "total color blindness",[5] is the lack of ability to distinguish colors; caused by cone defect or absence.[6] Monochromacy occurs when two or all three of the cone pigments are missing and color and lightness vision is reduced to one dimension.[5]
Rod monochromacy (achromatopsia) is a rare, nonprogressive inability to distinguish any colors as a result of absent or nonfunctioning retinal cones. It is associated with light sensitivity (photophobia), involuntary eye oscillations (nystagmus), and poor vision.[6]
Cone monochromacy is a rare, total color blindness that is accompanied by relatively normal vision, electoretinogram, and electrooculogram.[6]
Dichromacy is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, affecting predominantly males.[6] Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.[5]
Protanopia is a severe type of color vision deficiency caused by the complete absence of red retinal photoreceptors. It is a form of dichromatism in which red appears dark. It is congential, sex-linked, and present in 1% of all males.[6]
Deuteranopia is a color vision deficiency, moderately affecting red-green hue discrimination in 1% of all males. It is hereditary and sex-linked form of dichromatism in which there are only two cone pigments present.[6]
Tritanopia is an exceedingly rare color vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors.[6]
Anomalous trichromacy is a common type of congenital color vision deficiency caused by the reduced amount (not absence) of one of three cone photopigment types.[6] Anomalous trichromacy occurs when one of the three cone pigments is altered in its spectral sensitivity, but trichromacy or normal three-dimensional color vision is not fully impaired.[5]
Protanomaly is a mild color vision defect in which a deficiency of red retinal receptors results in poor red-green hue discrimination. It is congenital, sex-linked, and present in 1% of all males.[6]
Deuteranomaly is the most common type of color vision deficiency, mildly affecting red-green hue discrimination in 5% of all males. It is hereditary and sex-linked.[6]
Tritanomaly is a rare, hereditary color vision deficiency affecting blue-yellow hue discrimination.[6]
[edit] By clinical appearance
Based on clinical appearance, color blindness may be described as total or partial. Total color blindness is much less common than partial color blindness.[7] There are two major types of color blindness: those who have difficulty distinguishing between red and green, and those who have difficulty distinguishing between blue and yellow.[8][9]
Total color blindness
Partial color blindness
Red-green
Dichromacy (protanopia and deuteranopia)
Anomalous trichromacy (protanomaly and deuteranomaly)
Blue-yellow
Dichromacy (tritanopia)
Anomalous trichromacy (tritanomaly)
[edit] Congenital color vision deficiencies
Congenital color vision deficiencies are subdivided based on the number of primary hues needed to match a given sample in the visible spectrum.
[edit] Monochromacy
Monochromacy is the condition of possessing only a single channel for conveying information about color.[10] Monochromats possess a complete inability to distinguish any colors and perceive only variations in brightness.[10] It occurs in two primary forms:
Rod monochromacy, frequently called achromatopsia, where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult. While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 1/12 of the population there has it. The island was devastated by a storm in the 18th century, and one of the few male survivors carried a gene for achromatopsia; the population is now several thousand, of whom about 30% carry this gene.
Cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. Blue cone monochromacy (X chromosome) is caused by a complete absence of L- and M-cones. It is encoded at the same place as red-green color blindness on the X chromosome. Peak spectral sensitivities are in the blue region of the visible spectrum (near 440 nm). They generally show nystagmus ("jiggling eyes"), photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness).[11] Visual acuity usually falls to the 20/50 to 20/400 range
[edit] Dichromacy
Protanopes, deuteranopes, and tritanopes are dichromats; that is, they can match any color they see with some mixture of just two spectral lights (whereas normally humans are trichromats and require three lights). These individuals normally know they have a color vision problem and it can affect their lives on a daily basis. Protanopes and deuteranopes see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population.
Protanopia (1% of the males): Lacking the long-wavelength sensitive retinal cones, those with this condition are unable to distinguish between colors in the green-yellow-red section of the spectrum. They have a neutral point at a wavelength of 492 nm—that is, they cannot discriminate light of this wavelength from white. For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows and from greens primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible. E.g. Pink flowers, reflecting both red light and blue light, may appear just blue to the protanope. Very few people have been found who have one normal eye and one protanopic eye. These unilateral dichromats report that with only their protanopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow. This is a rare form of color blindness.
Deuteranopia(1% of the males): Lacking the medium-wavelength cones, those affected are again unable to distinguish between colors in the green-yellow-red section of the spectrum. Their neutral point is at a slightly longer wavelength, 498 nm. The deuteranope suffers the same hue discrimination problems as the protanope, but without the abnormal dimming. The names red, orange, yellow, and green really mean very little to him aside from being different names that every one else around him seems to be able to agree on. Similarly, violet, lavender, purple, and blue, seem to be too many names to use logically for hues that all look alike to him. This is one of the rarer forms of colorblindness making up about 1% of the male population, also known as Daltonism after John Dalton. (Dalton's diagnosis was confirmed as deuteranopia in 1995, some 150 years after his death, by DNA analysis of his preserved eyeball.) Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow.
Tritanopia