Colour blindness

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

Colour blindness, or colour 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", after the realization of his own colour blindness; because of Dalton's work, the condition is sometimes called Daltonism, although this term is now used for a type of colour blindness called deuteranopia.

Colour blindness is usually classed as a disability; however, in select situations color blind people may have advantages over people with normal colour vision. There is anecdotal evidence that color blind individuals are better at penetrating colour camouflage and at least one scientific study confirms this under controlled conditions. Monochromats may have a minor advantage in dark vision, but only in the first five minutes of dark adaptation.

This is a sample image. The pictures below should look similar to people with normal vision (containing numbers, in this case 83), but some of them will not be visible to people with a colour vision deficiency. The contrast in these tests is much subtler than commonly seen in other similar tests.

This image contains the number 37, although someone who is protanopic might not be able to see it.

Someone who is tritanopic might not see this number (56).

Background

The normal human retina contains two kinds of light sensitive 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. The cones are activated when the pigments absorb light. The absorption spectra of the pigments differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths (their peak sensitivities are in the blue, yellowish-green, and yellow regions of the spectrum, respectively). The absorption spectra of all three systems cover much of the visible spectrum, so it is not entirely accurate to refer to them as " blue", " green" and " red" receptors, especially because the "red" receptor actually has its peak sensitivity in the yellow. The sensitivity of normal colour 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 extents. For example, red light stimulates the long wavelength cones much more than either of the others, but the gradual change in hue seen, as wavelength reduces, is the result of the other two cone systems being increasingly stimulated as well.

Causes of colour 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 colour 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 colour blindness also occurs (very rarely) in the aura of some migraine sufferers.

The different kinds of inherited colour 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 rarer. They include problems in discriminating blues from yellows, and the rarest forms of all, complete colour blindness or monochromacy, where one cannot distinguish any colour from grey, as in a black-and-white movie or photograph.

Classification of colour deficiencies

By etiology

Colour vision deficiencies can be classified as acquired or inherited/congenital.

• Acquired
• Inherited/congenital. There are three types of inherited or congenital colour vision deficiencies: monochromacy, dichromacy, and anomalous trichromacy.

• Monochromacy, also known as "total colour blindness", is the lack of ability to distinguish colors; caused by cone defect or absence. Monochromacy occurs when two or all three of the cone pigments are missing and colour and lightness vision is reduced to one dimension.

• 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.
• Cone monochromacy is a rare, total colour blindness that is accompanied by relatively normal vision, electoretinogram, and electrooculogram.

• Dichromacy is a moderately severe color vision defect in which one of the three basic colour mechanisms is absent or not functioning. It is hereditary and sex-linked, affecting predominantly males. Dichromacy occurs when one of the cone pigments is missing and colour is reduced to two dimensions.

• Protanopia is a severe type of colour 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.
• Deuteranopia is a colour 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.
• Tritanopia is an exceedingly rare colour vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors.

• Anomalous trichromacy is a common type of congenital colour vision deficiency caused by the reduced amount (not absence) of one of the 3 types of cone photopigments. Anomalous trichromacy occurs when one of the three cone pigments is altered in its spectral sensitivity, but trichromacy or normal three-dimensional colour vision is not fully impaired.

• Protanomaly is a mild colour 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.
• Deuteranomaly is the most common type of colour vision deficiency, mildly affecting red-green hue discrimination in 5% of all males. It is hereditary and sex-linked.
• Tritanomaly is a rare, hereditary colour vision deficiency affecting blue-yellow hue discrimination.

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 colour blindness. There are two major types of colour blindness: those who have difficulty distinguishing between red and green, and those who have difficulty distinguishing between blue and yellow.

• Total colour blindness
• Partial colour blindness

• Red-green

• Dichromacy (protanopia and deuteranopia)
• Anomalous trichromacy (protanomaly and deuteranomaly)

• Blue-yellow

• Dichromacy (tritanopia)
• Anomalous trichromacy (tritanomaly)

Congenital colour vision deficiencies

Congenital colour vision deficiencies are subdivided based on the number of primary hues needed to match a given sample in the visible spectrum.

Monochromacy

Monochromacy is the condition of possessing only a single channel for conveying information about colour. Monochromats possess a complete inability to distinguish any colors and perceive only variations in brightness. It occurs in two primary forms:

1. Rod monochromacy, frequently called achromatopsia, where the retina contains no cone cells, so that in addition to the absence of colour 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.
2. 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 colour 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). Visual acuity usually falls to the 20/50 to 20/400 range

Dichromacy

Protanopes, deuteranopes, and tritanopes are dichromats; that is, they can match any colour 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 colour 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 colour 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 colour 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

Anomalous trichromacy

Those with protanomaly, deuteranomaly, or tritanomaly are trichromats, but the color matches they make differ from the normal. They are called anomalous trichromats. In order to match a given spectral yellow light, protanomalous observers need more red light in a red/green mixture than a normal observer, and deuteranomalous observers need more green. From a practical stand point though, many protanomalous and deuteranomalous people breeze through life with very little difficulty doing tasks that require normal colour vision. Some may not even be aware that their color perception is in any way different from normal. The only problem they have is passing that "Blank Blank" colour vision test.

Protanomaly and deuteranomaly can be readily observed using an instrument called an anomaloscope, which mixes spectral red and green lights in variable proportions, for comparison with a fixed spectral yellow. If this is done in front of a large audience of men, as the proportion of red is increased from a low value, first a small proportion of people will declare a match, while most of the audience sees the mixed light as greenish. These are the deuteranomalous observers. Next, as more red is added the majority will say that a match has been achieved. Finally, as yet more red is added, the remaining, protanomalous, observers will declare a match at a point where everyone else is seeing the mixed light as definitely reddish.

• Protanomaly(1% of males): Having a mutated form of the long-wavelength pigment, whose peak sensitivity is at a shorter wavelength than in the normal retina, protanomalous individuals are less sensitive to red light than normal. This means that they are less able to discriminate colors, and they do not see mixed lights as having the same colors as normal observers. They also suffer from a darkening of the red end of the spectrum. This causes reds to reduce in intensity to the point where they can be mistaken for black. Protanomaly is a fairly rare form of colour blindness, making up about 1% of the male population.
• Deuteranomaly(most common - 6% of males): Having a mutated form of the medium-wavelength pigment. The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. Unlike protanomaly the intensity of colors is unchanged. This is the most common form of colour blindness, making up about 6% of the male population. The deuteranomalous person is considered "green weak". Similar to the protanomalous person, he is poor at discriminating small differences in hues in the red, orange, yellow, green region of the spectrum. He makes errors in the naming of hues in this region because they appear somewhat shifted towards red for him. One very important difference between deuteranomalous individuals and protanomalous individuals is deuteranomalous individuals do not have the loss of "brightness" problem.
• Tritanomaly

Clinical forms of colour blindness

Total colour blindness

Achromatopsia is strictly defined as the inability to see colour. Although the term may refer to acquired disorders such as colour agnosia and cerebral achromatopsia, it typically refers to congenital colour vision disorders (i.e. more frequently rod monochromacy and less frequently cone monochromacy).

In color agnosia and cerebral achromatopsia, a person cannot perceive colors even though the eyes are capable of distinguishing them. Some sources do not consider these to be true colour blindness, because the failure is of perception, not of vision. They are forms of visual agnosia.

Red-green colour blindness

Those with protanopia, deuteranopia, protanomaly, and deuteranomaly have difficulty with discriminating red and green hues.

Genetic red-green colour blindness affects men much more often than women, because the genes for the red and green colour receptors are located on the X chromosome, of which men have only one and women have two. Such a trait is called sex-linked. Genetic females (46, XX) are red-green colour blind only if both their X chromosomes are defective with a similar deficiency, whereas genetic males (46, XY) are colour blind if their only X chromosome is defective.

The gene for red-green color blindness is transmitted from a colour blind male to all his daughters who are heterozygote carriers and are perceptually unaffected. In turn, a carrier woman passes on a mutated X chromosome region to only half her male offspring. The sons of an affected male will not inherit the trait, since they receive his Y chromosome and not his (defective) X chromosome.

Because one X chromosome is inactivated at random in each cell during a woman's development, it is possible for her to have four different cone types, as when a carrier of protanomaly has a child with a deuteranomalic man. Denoting the normal vision alleles by P and D and the anomalous by p and d, the carrier is PD pD and the man is Pd. The daughter is either PD Pd or pD Pd. Suppose she is pD Pd. Each cell in her body expresses either her mother's chromosome pD or her father's Pd. Thus her red-green sensing will involve both the normal and the anomalous pigments for both colors. Such women are tetrachromats, since they require a mixture of four spectral lights to match an arbitrary light.

Blue-yellow colour blindness

Those with tritanopia and tritanomaly have difficulty with discriminating blue and yellow hues.

Colour blindness involving the inactivation of the short-wavelength sensitive cone system (whose absorption spectrum peaks in the bluish-violet) is called tritanopia or, loosely, blue-yellow colour blindness. The tritanopes neutral point occurs at 570 nm; where green is perceived at shorter wavelengths and red at longer wavelengths. Mutation of the short-wavelength sensitive cones is called tritanomaly. Tritanopia is equally distributed among males and females. Jeremy H. Nathans (with the Howard Hughes Medical Institute) proved that the gene coding for the blue receptor lies on chromosome 7, which is shared equally by men and women. Therefore it is not sex-linked. This gene does not have any neighbor whose DNA sequence is similar. Blue colour blindness is caused by a simple mutation in this gene (2006, Howard Hughes Medical Institute).

Epidemiology

Colour blindness affects a significant number of people, although exact proportions vary among groups. In Australia, for example, it occurs in about 8 percent of males and only about 0.4 percent of females. Isolated communities with a restricted gene pool sometimes produce high proportions of colour blindness, including the less usual types. Examples include rural Finland, Hungary, and some of the Scottish islands. In the United States, about 7 percent of the male population - or 10 million men - and 0.4 percent of the female population either cannot distinguish red from green, or see red and green differently (Howard Hughes Medical Institute, 2006). It has been found that more than 95 percent of all variations in human colour vision involve the red and green receptors in male eyes. It is very rare for males or females to be "blind" to the blue end of the spectrum.

Diagnosis

The Ishihara colour test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red-green colour deficiencies. A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background colour combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.

However, the Ishihara color test is criticized for containing only numerals and thus not being useful for young children, who have not yet learned to use numerals. It is often stated that it is important to identify these problems as soon as possible and explain them to the children to prevent possible problems and psychological traumas. For this reason, alternative colour vision tests were developed using only symbols (square, circle, car).

Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of color blindness. In academic studies of colour blindness, on the other hand, there is more interest in developing flexible tests ( , for example) to collect thorough datasets, identify copunctal points, and measure just noticeable differences.

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 distinguish different colors better. Additionally, software has been developed to assist those with visual colour difficulties.

Design implications of colour blindness

Colour codes present particular problems for color blind people as they are often difficult or impossible for colour blind people to understand.

Good graphic design avoids using color coding or color contrasts alone to express information, as this not only helps colour blind people, but also aids understanding by normally sighted people. The use of Cascading Style Sheets on the world wide web allows pages to be given an alternative color scheme for colour-blind readers. This colour scheme generator helps a graphic designer see color schemes as seen by eight types of color blindness. When the need to process visual information as rapidly as possible arises, for example in a train or aircraft crash, the visual system may operate only in shades of grey, with the extra information load in adding colour being dropped. This is an important possibility to consider when designing, for example, emergency brake handles or emergency phones.

Misconceptions and compensations

Color blindness is not the swapping of colors in the observer's eyes. Grass is never red, stop signs are never green. The colour impaired do not learn to call red "green" and vice versa. However, dichromats often confuse red and green items. For example, they find it difficult to distinguish a Granny Smith from a Braeburn or the red and green of a traffic light without other cues (for example, shape or location). This is demonstrated nicely in this simulation of the two types of apple as viewed by a trichromat or by a dichromat.

Color blindness almost never means complete monochromatism. In almost all cases, color blind people retain blue-yellow discrimination, and most color blind individuals are anomalous trichromats rather than complete dichromats. In practice this means that they often retain a limited discrimination along the red-green axis of colour space although their ability to separate colors in this dimension is severely reduced.