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Colour blindness

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   CAPTION: Colour blindness
   Classifications and external resources

   ICD- 10 H53.5
   ICD- 9  368.5

   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 color vision
   deficiency. The contrast in these tests is much subtler than commonly
   seen in other similar tests.
   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.
   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).
   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.

               o 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.
               o 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.

               o 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.
               o 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.
               o 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.

               o 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.
               o 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.
               o 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

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

          + Blue-yellow

               o Dichromacy (tritanopia)
               o 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.
   Prevalence of colour blindness
   Men Women Total References
   Overall - - -
   Overall (United States) - - 1.30%
   Red-green (Overall) 7 to 10% - -
   Red-green (Caucasians) 8% - -
   Red-green (Asians) 5% - -
   Red-green (Africans) 4% - -
   Monochromacy - - -
   Rod monochromacy (no cones) 0.00001% 0.00001% -
   Dichromacy 2.4% 0.03% -
   Protanopia (L-cone absent) 1% to 1.3% 0.02% -
   Deuteranopia (M-cone absent) 1% to 1.2% 0.01% -
   Tritanopia (S-cone absent) 0.001% 0.03% -
   Anomalous Trichromacy 6.3% 0.37% -
   Protanomaly (L-cone defect) 1.3% 0.02% -
   Deuteranomaly (M-cone defect) 5.0% 0.35% -
   Tritanomoly (S-cone defect) 0.0001% 0.0001% -

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.
   Image:Braeburn_GrannySmith_dichromat_sim.jpg

   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.
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