Color blindness, or
colour blindness, a
color visionColor vision is the capacity of an organism or machine to distinguish objects based on the wavelengths 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...
deficiency, is the inability to perceive differences between some of the
colorColor or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s that others can distinguish. It is most often of
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
tic nature, but may also occur because of
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
,
nerveA nerve is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
, or
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
damage, or due to exposure to certain chemicals. The English chemist
John DaltonJohn Dalton FRS was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness .-Early life:John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth in Cumberland,...
published the first
scientificScience is in its broadest sense to any systematic knowledge-base or prescriptive practice that is capable of resulting in a prediction or predictable type of outcome...
paper on the subject in 1798, "Extraordinary facts relating to the vision of colours", after the realization of his own color blindness; because of Dalton's work, the condition is often called
daltonism, although this term is now used for a type of color blindness called deuteranopia.
Color blindness is sometimes classed as a
disabilityDisability is defined by the Americans with Disabilities Act of 1990 as "a physical or mental impairment that substantially limits one or more major life activities."...
; in certain situations, however, color blind people have an advantage over people with normal color vision. There are some studies which conclude that color blind individuals are better at penetrating certain camouflages.
Background
The normal human
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
contains two kinds of light cells: the
rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
s (
active in low lightScotopic vision is the vision of the eye under low light conditions. The term comes from Greek skotos meaning darkness and -opia meaning a condition of sight. In the human eye cone cells are nonfunctional in low light - scotopic vision is produced exclusively through rod cells so there is no color...
) and the
cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s (
active in normal daylightPhotopic vision is the vision of the eye under well-lit conditions. In humans and many animals, photopic vision allows color perception, mediated by cone cells....
). Normally, there are three kinds of cones, each containing a different pigment. The cones are activated when the pigments absorb light. The
absorption spectraA material's absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. Atoms, for example, have absorption lines at wavelengths corresponding to the differences between the energy levels of its atomic orbitals. Each chemical...
of the cones 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 "
blueBlue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
", "
greenGreen is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
" and "
redRed is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked human eye...
" receptors, especially because the "red" receptor actually has its peak sensitivity in the
yellowYellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–580 nm is yellow, as is light with a suitable mixture of somewhat longer and shorter wavelengths...
. 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 extents. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue. Many of the genes involved in color vision are on the
X chromosomeThe X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system...
, making color blindness more common in males than in females.
Causes
Any recessive genetic characteristic that persists at a level as high as 5% is generally regarded as possibly having some advantage over the long term. In
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
it was discovered that analysis of color aerial photos yielded more information if at least one team member was color blind. Humans are the only trichromatic primates with such a high percentage of color blindness.
Another possible advantage might result from the presence of a tetrachromic female. Owing to X-chromosome inactivation, women who are
heterozygousZygosity is the similarity of genes for a trait in an organism. If both genes are the same, the organism is homozygous for the trait. If both genes are different, the organism is heterozygous for that trait...
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, by analogy to New World monkeys where heterozygous females gain trichromacy in a basically dichromatic species.
Genetic modes of inheritance
Color blindness can be inherited genetically. It is most commonly inherited from mutations on the
X chromosomeThe X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system...
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 UniversityThe Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university located in Baltimore, Maryland, United States. Johns Hopkins also maintains full-time campuses elsewhere in Maryland, Washington, D.C., Italy, China, and Singapore...
). These are some of the inherited diseases known to cause color blindness:
Cone dystrophyA cone dystrophy is an inherited ocular disorder characterized by the loss of cone cells, the photoreceptors responsible for both central and color vision....
, Cone-rod dystrophy,
AchromatopsiaAchromatopsia , is a medical syndrome that exhibits symptoms relating to at least five separate individual diseases. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder,...
(aka Rod Monochromatism, aka Stationery Cone Dystrophy, aka Cone Dysfunction Syndrome), Blue cone monochromatism,
Retinitis pigmentosaRetinitis pigmentosa is a group of genetic eye conditions. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some sight all their lives . Others go...
(initially affects rods but can later progress to cones and therefore color blindness), Diabetes, Age-Related
Macular degenerationMacular degeneration is a medical condition usually of older adults that results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of visual impairment in the elderly...
,
RetinoblastomaRetinoblastoma is a rapidly developing cancer which develops in the cells of the retina, the light detecting tissue of the eye. In the developed world, Rb has one of the best cure rates of all childhood cancers , with more than nine out of every ten sufferers surviving into adulthood...
, and
Leber's congenital amaurosisLeber's congenital amaurosis is a rare inherited eye disease that appears at birth or in the first few months of life, and affects around 1 in 80,000 of the population.It was first described by Theodore Leber in the 19th century...
.
Inherited color blindness can be congenital (from birth), or it can commence in childhood or adulthood. Depending on the mutation, it can be stationary, that is, remain the same throughout a person's lifetime, or progressive. As progressive phenotypes involve deterioration of the retina and other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e., an acuity of 6/60 or worse, and often leave a person with complete blindness.
Color blindness always pertains to the cone photoreceptors in retinas as the cones are capable of detecting the color frequencies of light.
About 5–8 percent of males, but less than 1 percent of females, are color blind in some way or another, whether it be one color, a color combination, or another mutation. The reason males are at a greater risk of inheriting an X linked mutation is because males only have one X chromosome (XY, with the Y chromosome being significantly shorter than the X chromosome), and females have two (XX); if a woman inherits a normal X chromosome in addition to the one which carries the mutation, she will not display the mutation. Men do not have a second X chromosome to override the chromosome which carries the mutation. If 5% of variants of a given gene are defective, the probability of a single copy being defective is 5%, but the probability that two copies are both defective is 0.05 × 0.05 = 0.0025, or just 0.25%.
Other causes
Other causes of color blindness include
Shaken Baby SyndromeShaken baby syndrome or Battered Baby Syndrome is a form of child abuse that occurs when an abuser violently shakes an infant or small child, creating a whiplash-type motion that causes acceleration-deceleration injuries. The injury is estimated to affect between 1,200 and 1,600 children every...
(this can cause retina and brain damage and therefore can cause color blindness), accidents and other trauma (swelling of the brain in the occipital lobe), and UV damage to the retina (from not wearing appropriate protection). Most UV damage is caused during childhood and this form of retinal degeneration is the leading cause of blindness in the world. Damage often presents itself later on in life.
Types
There are many types of color blindness. The most common are red-green 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 nucleusThe lateral geniculate nucleus is the primary processing center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain, and is thus part of the central nervous system....
of the
thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
, and visual area V4 of the
visual cortexThe term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17...
. 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
auraAn aura is the perceptual disturbance experienced by some migraine sufferers before a migraine headache, and the telltale sensation experienced by some people with epilepsy before a seizure. It often manifests as the perception of a strange light, an unpleasant smell or confusing thoughts or...
of some
migraineMigraine is a neurological syndrome characterized by altered bodily perceptions, severe headaches, and nausea. Physiologically, the migraine headache is a neurological condition more common to women than to men. The word migraine was borrowed from Old French migraigne...
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,
dichromacyDichromacy in humans 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, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two...
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 is 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-whiteBlack-and-white is a number of monochrome forms in visual arts. Most forms of visual technology start out in black and white, then slowly evolve into color as technology progresses....
movie or photograph.
By cause
Color vision deficiencies can be classified as acquired or inherited.
- Inherited: There are three types of inherited or congenital color vision deficiencies: monochromacy, dichromacy, and anomalous trichromacy.
- Monochromacy, also known as "total color 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 color and lightness vision is reduced to one dimension.
-
-
- Rod monochromacy
Achromatopsia , is a medical syndrome that exhibits symptoms relating to at least five separate individual diseases. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder,...
(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 (photophobiaPhotophobia is a symptom of excessive sensitivity to light and the aversion to sunlight or well-lit places. In ordinary medical terms photophobia is not a morbid fear, but an experience of discomfort or pain to the eyes due to light exposure....
), involuntary eye oscillations (nystagmus), and poor vision.
- Cone monochromacy is a rare, total color 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 color mechanisms is absent or not functioning. It is hereditary and, in the case of Protanopia or Deuteranopia, sex-linked, affecting predominantly males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.
- 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 hereditary, sex-linked, and present in 1% of all males.
- Deuteranopia is a color vision deficiency in which the green retinal photoreceptors are absent, moderately affecting red-green hue discrimination. It is a form of dichromatism in which there are only two cone pigments present. It is likewise hereditary, sex-linked, and present in 1% of all males.
- 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.
- Anomalous trichromacy is a common type of inherited color vision deficiency, occurring when one of the three cone pigments is altered in its spectral sensitivity. This results in an impairment, rather than loss, of trichromacy (normal three-dimensional color vision).
- Protanomaly is a mild color vision defect in which an altered spectral sensitivity of red retinal receptors (closer to green receptor response) results in poor red-green hue discrimination. It is hereditary, sex-linked, and present in 1% of all males. It is often passed from mother to child.
- Deuteranomaly, caused by a similar shift in the green retinal receptors, is by far 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.
- Tritanomaly is a rare, hereditary color vision deficiency affecting blue-yellow hue discrimination. Unlike most other forms, it is not sex-linked.
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. 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.
- Total color blindness
- Partial color blindness
- Red-green
-
-
- Dichromacy (protanopia and deuteranopia)
- Anomalous trichromacy (protanomaly and deuteranomaly)
- Dichromacy (tritanopia)
- Anomalous trichromacy (tritanomaly)
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 spectrumThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
.
Monochromacy
Monochromacy is the condition of possessing only a single channel for conveying information about color. Monochromats possess a complete inability to distinguish any colors and perceive only variations in brightness. 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
Pingelap is an atoll in the Pacific Ocean, part of Pohnpei state of the Federated States of Micronesia, consisting of three islands: Pingelap Island, Sukoru and Daekae, linked by a reef system and surrounding a central lagoon, although only Pingelap Island is inhabited. The entire system has a land...
, a part of the PohnpeiPohnpei "upon a stone altar " is the name of one of the four states in the Federated States of Micronesia , and among the Senyavin Islands . Palikir, the FSM's capital, is located on Pohnpei...
state, Federated States of MicronesiaThe Federated States of Micronesia is an island nation located in the Pacific Ocean, north of New Guinea. It is a sovereign state in free association with the United States. The Federated States of Micronesia were formerly part of the Trust Territory of the Pacific Islands, a United Nations Trust...
, 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
Visual acuity is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, 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 color they see with some mixture of just two spectral lights (whereas normally humans are
trichromatTrichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...
s 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 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 greenish wavelength around 492 nm
A nanometre is a unit of length in the metric system, equal to one billionth of a meter....
– 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 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
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
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 (less than 1% of males and females): Lacking the short-wavelength cones, those affected are unable to distinguish between the colors in the blue-yellow section of the spectrum. This form of color blindness is not sex-linked.
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 standpoint though, many protanomalous and deuteranomalous people breeze through life with very little difficulty doing tasks that require normal color 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 a color vision test.
Protanomaly and deuteranomaly can be readily observed using an instrument called an
anomaloscopeAn anomaloscope is an instrument used to test for color blindness. It is used to measure quantitative and qualitative anomalies in colour perception. It can be used to determine whether a person is a tritanope, dichromat or achromatop. The apparatus was invented by the German ophthalmologist and...
, 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, 0.01% of females): Having a mutated form of the long-wavelength (red) 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 color blindness, making up about 1% of the male population. Both protanomaly and deuteranomaly are carried on the X chromosome.
- Deuteranomaly (most common - 6% of males, 0.4% of females): Having a mutated form of the medium-wavelength (green) 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 color blindness, making up about 6% of the male population. The deuteranomalous person is considered "green weak". For example, in the evening, dark green cars appear to be black to Deuteranomalous people. Similar to the protanomates, deuteranomates are poor at discriminating small differences in hue
Hue is one of the main properties of a color, defined technically , as “the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,”...
s in the red, orange, yellow, green region of the spectrum. They make errors in the naming of hues in this region because the hues appear somewhat shifted towards red. One very important difference between deuteranomalous individuals and protanomalous individuals is deuteranomalous individuals do not have the loss of "brightness" problem.
- Tritanomaly (equally rare for males and females [0.01% for both]): Having a mutated form of the short-wavelength (blue) pigment. The short-wavelength pigment is shifted towards the green area of the spectrum. This is the rarest form of anomalous trichromacy color blindness. Unlike the other anomalous trichromacy color deficiencies, the mutation for this color blindness is carried on chromosome 7. Therefore it is equally prevalent in both male & female populations. The OMIM gene code for this mutation is 304000 “Colorblindness, Partial Tritanomaly”.
Total color blindness
AchromatopsiaAchromatopsia , is a medical syndrome that exhibits symptoms relating to at least five separate individual diseases. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder,...
is strictly defined as the inability to see color. Although the term may refer to acquired disorders such as
color agnosiaColor agnosia , also known as "cerebral achromatopsia", is a medical or psychological condition that prevents a person from recognizing colors even though the eyes are capable of distinguishing them...
and
cerebral achromatopsiaCerebral achromatopsia is a type of color-blindness that is caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina. It is often confused with congenital achromatopsia. Cerebral achromatopsia differs because it is caused only by trauma or...
, it typically refers to congenital color 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 color blindness, because the failure is of perception, not of vision. They are forms of
visual agnosiaVisual agnosia is the inability of the brain to make sense of or make use of some part of otherwise normal visual stimulus and is typified by the inability to recognize familiar objects or faces...
.
Red-green color blindness
Those with protanopia, deuteranopia, protanomaly, and deuteranomaly have difficulty with discriminating red and green hues.
It is sex-linked: genetic red-green color blindness affects men much more often than women, because the
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
s for the red and green color receptors are located on the X
chromosomeA chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions...
, of which men have only one and women have two. Females (46, XX) are red-green color blind only if
both their X chromosomes are defective with a similar deficiency, whereas males (46, XY) are color blind if their single X chromosome is defective.
The gene for red-green color blindness is transmitted from a color blind male to all his daughters who are
heterozygoteZygosity is the similarity of genes for a trait in an organism. If both genes are the same, the organism is homozygous for the trait. If both genes are different, the organism is heterozygous for that trait...
carriers and are usually unaffected. In turn, a carrier woman has a fifty percent chance of passing on a mutated X chromosome region to each of her male offspring. The sons of an affected male will not inherit the trait from him, since they receive his Y chromosome and not his (defective) X chromosome. Should an affected male have children with a carrier or colorblind woman, their daughters may be colorblind by inheriting an affected X chromosome from each parent.
Because one X chromosome is
inactivatedIn those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male , rendered inactive in a process called Lyonization...
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 color blindness
Those with tritanopia and tritanomaly have difficulty with discriminating blue and yellow hues.
Color 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 color blindness. The tritanopes neutral point occurs near a yellowish 570 nm; 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 InstituteHoward Hughes Medical Institute is a United States non-profit medical research institute based in Chevy Chase, Maryland. It was founded by the American businessman Howard Hughes in 1953. It is one of the largest private funding organizations for biological and medical research in the United States...
) 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 color blindness is caused by a simple mutation in this gene. (2006, Howard Hughes Medical Institute).
Prevalence
Color 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 color blindness, including the less usual types. Examples include rural
FinlandFinland , officially the Republic of Finland
, is a Nordic country and democracy situated in the Fennoscandian region of northern Europe. It borders Sweden on the west, Russia on the east, and Norway on the north, while Estonia lies to its south across the Gulf of Finland...
,
HungaryHungary , in English officially the Republic of Hungary , is a landlocked country in the Carpathian Basin of Central Europe, bordered by Austria, Slovakia, Ukraine, Romania, Serbia, Croatia, and Slovenia. Its capital is Budapest. Hungary is a member of OECD, NATO, EU, V4 and is a Schengen state...
, and some of the
ScottishScotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
islands. In the United States, about 7 percent of the male population – or about 10.5 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 color 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 color blindness
| | Men | Women | Total | References |
|---|
| Overall |
— |
— |
— |
|
| Overall (United States) |
— |
— |
— |
|
| Red-green (Overall) |
7 to 10% |
— |
— |
|
| Red-green (Caucasians) |
8% |
— |
— |
|
| Red-green (Asians) |
5% |
— |
— |
|
| Red-green (Africans) |
4% |
— |
— |
|
| Monochromacy |
— |
— |
— |
|
| Rod monochromacy (disfunctional, abnormally shaped or no cones) |
0.00001% |
0.00001% |
— |
|
| Dichromacy |
2.4% |
0.03% |
1.30% |
|
| 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% |
— |
|
| Tritanomaly (S-cone defect) |
0.01% |
0.01% |
— |
|
Diagnosis
The
Ishihara color testThe Ishihara Color Test is a test for red-green color deficiencies. It was named after its designer, Dr. Shinobu Ishihara, a professor at the University of Tokyo, who first published his tests in 1917....
, which consists of a series of pictures of colored spots, is the test most often used to diagnose red-green color deficiencies.
A figure (usually one or more
Arabic digitsThe Arabic numerals are the ten digits . They are descended from Indian numerals and the Hindu-Arabic numeral system developed by Indian mathematicians, by which a sequence of digits such as "975" is read as a whole number...
) 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 color 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 color 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 color blindness, on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure
just noticeable differenceIn psychophysics, a just noticeable difference, customarily abbreviated with lowercase letters as jnd, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus...
s.
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. Optometrists can supply a singular red-tint contact lens to wear on the dominant eye. This may enable the wearer to pass some color blindness tests, but they have little practical use. 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. Additionally,
computer softwareComputer software, or just software is a general term used todescribe the role that computer programs, procedures anddocumentation play in a computer system.The term includes:...
has been developed to assist those with visual color difficulties.
The
GNOMEGNOME is a desktop environment—a graphical user interface which runs on top of a computer operating system —composed entirely of free and open source software...
desktop environmentIn graphical computing, a desktop environment commonly refers to a style of graphical user interface that is based on the desktop metaphor which can be seen on most modern personal computers today...
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 the colors. The software enables, for instance, a color blind person to see the numbers in the ishihara test.
In September 2009, the journal
NatureNature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
reported that researchers at the
University of WashingtonUniversity of Washington, founded in 1861, is a public research university in Seattle, Washington, United States. UW is the largest university in the northwestern United States and one of the oldest public universities on the west coast. The university has three campuses, with its flagship campus...
and
University of FloridaThe University of Florida is a public land-grant, sea-grant, space-grant major research university located on a campus located in Gainesville, Florida, in the United States. The university traces its origins to 1853, and has continuously operated on its present Gainesville campus since the fall...
were able to give trichromatic vision to squirrel monkeys using
gene therapyGene therapy is the insertion of genes into an individual's cells and tissues to treat a disease, such as a hereditary disease in which a deleterious mutant allele is replaced with a functional one. Although controversial, Gene Therapy can also be used for human genetic enhancement changing one's...
.
Design implications of color blindness
Color codeColor code may refer to:*Bottled gases*Color codes in three-phase electric power *Electronic color code*Web colors for HTML color codes*25-pair color code...
s present particular problems for color blind people as they are often difficult or impossible for color blind people to perceive.
Good
graphic designThe term graphic design can refer to a number of artistic and professional disciplines which focus on visual communication and presentation. Various methods are used to create and combine symbols, images and/or words to create a visual representation of ideas and messages. A graphic designer may...
avoids using color coding or color contrasts alone to express information, as this not only helps color blind people, but also aids understanding by normally sighted people. The use of
Cascading Style SheetsCascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...
on the
world wide webThe World Wide Web is a system of interlinked hypertext documents accessed via the Internet. With a web browser, one can view Web pages that may contain text, images, videos, and other multimedia and navigate between them using hyperlinks...
allows pages to be given an alternative color scheme for color-blind readers.
This color scheme generator helps a graphic designer see color schemes as seen by eight types of color blindness. For an example of a map that could present a significant problem to a color blind reader, see
this graphic from a New York Times article. The typical red-green color blind reader will find the green sections of the map nearly indistinguishable from the orange, rendering the graphic unreadable.
Designers should take into account that color-blindness is highly sensitive to differences in material. For example, 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. Thirdly, 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.
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 color being dropped. This is an important possibility to consider when designing, for example, emergency brake handles or emergency phones.
Color blindness and occupations
Color blindness may make it difficult or impossible for a person to engage in certain occupations. Persons with color blindness may be legally or practically barred from occupations in which color perception is an essential part of the job (
e.g., mixing paint colors), or in which color perception is important for safety (
e.g., operating vehicles in response to color-coded signals). This originates from the Lagerlunda train crash of 1875 in Sweden. Following the crash, Professor
Alarik Frithiof HolmgrenA Swedish physiologist, Alarik Frithiof Holmgren was a professor at Upsala University and a vocal opponent of vivisection, and particularly the use of curare to immobilize subjects so they appeared peaceful while feeling great pain...
, a physiologist, investigated and concluded that the color blindness of the engineer (who had died) had caused the crash. Professor Holmgren then created the first test using different-colored skeins to exclude people from jobs in the transportation industry on the basis of color blindness.
Driving motor vehicles
Some countries (e.g.,
SingaporeSingapore , officially the Republic of Singapore, is an island city-state located at the southern tip of the Malay Peninsula, lying north of the equator, south of the Malaysian state of Johor and north of Indonesia's Riau Islands. At , Singapore is a microstate and the smallest nation in Southeast...
prior to the 1990s or
RomaniaRomania is a country located in Southeastern and Central Europe, North of the Balkan Peninsula, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea. Almost all of the Danube Delta is located within its territory...
and
TurkeyTurkey , known officially as the Republic of Turkey
, is a Eurasian country that stretches across the Anatolian peninsula in Western Asia and Thrace in the Balkan region of southeastern Europe...
even to the present day) have refused to grant individuals with color blindness driving licenses. In Romania, there is an ongoing effort to remove the legal restrictions that prohibit its colorblind citizens from getting drivers' licenses.
The usual justification for such restrictions is that drivers of motor vehicles must be able to recognize certain color-coded signals, such as traffic lights or warning lights.
Piloting aircraft
While many aspects of aviation depend on color coding, only a few of them are critical enough to be interfered with by some milder types of color blindness. Some examples include color-gun signaling of aircraft that have lost radio communication, color-coded glide-path indications on runways, and the like. Some jurisdictions restrict the issuance of pilot credentials to persons who suffer from color blindness for this reason. Restrictions may be partial, allowing color-blind persons to obtain certification but with restrictions, or total, in which case color-blind persons are not permitted to obtain piloting credentials at all.
In the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the
Federal Aviation AdministrationThe Federal Aviation Administration is an agency of the United States Department of Transportation with authority to regulate and oversee all aspects of civil aviation in the U.S...
requires that pilots be tested for normal color vision as part of the medical certification that is prerequisite to obtaining a pilot's license. If testing reveals color blindness, the applicant may be issued a license with restrictions, such as no night flying and no flying by color signals—such a restriction effectively prevents a pilot from working for an airline. The government allows several types of tests, including medical standard tests (
e.g., the
Ishiharawas a Japanese ophthalmologist who created the Ishihara color test to detect colour blindness.-Early life and career:Shinobu graduated from medicine in 1905 on a military scholarship and immediately joined the army as a doctor, serving mainly as a surgeon...
, Dvorine, and others) and specialized tests oriented specifically to the needs of aviation. If an applicant fails the standard tests, he or she will receive a restriction on their medical certificate that states- "Not valid for night flying or by color signal control." He/she may apply to the FAA to take specialized tests, administered by the FAA Flight Surgeon. If he or she passes those, he or she may be issued a "Statement Of Demonstrated Ability" (SODA). They will then receive a new medical certificate with the restriction removed.
Misconceptions and compensations
Color blindness is not the swapping of colors in the observer's eyes. Grass is never red, and stop signs are never green. The color impaired do not learn to call red "green" and vice versa. However, dichromats often confuse red and green items. For example, they may find it difficult to distinguish a
BraeburnThe Braeburn is a cultivar of apple that is firm to the touch with a red/orange vertical streaky appearance on a yellow/green background. Its color intensity varies with different varieties....
from a
Granny SmithThe Granny Smith green apple is a tip-bearing apple cultivar. It originated in Australia in 1868 from a chance seedling propagated by Maria Ann Smith , from whom comes the name. It is thought to be a seed from Malus sylvestris, the European Wild Apple, with the domestic apple M...
and in some cases, the red and green of a traffic light without other clues (e.g., shape or location). This is demonstrated in this simulation of the two types of apple as viewed by a trichromat or by a dichromat.
Anomalous Trichromats are often able to readily spot camouflage clothing, netting, and paint that has been designed for individuals with color-normal vision. They tend to learn to see texture and shape. This lets them see through some camouflage patterns.
Traffic light colors are confusing to some dichromats: there is insufficient apparent difference between the red/amber traffic lights, and that of sodium street lamps; also the green can be confused with a grubby white lamp. This is a risk factor on a high-speed undulating road where angular cues can't be used. 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 use a white rectangle; dichromats simply look for the position of the light within the rectangle (top, middle or bottom; British traffic lights are never mounted horizontally).
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 color space although their ability to separate colors in this dimension is severely reduced.
See also
- :Category:Articles with images not understandable by color blind users
- Cerebral achromatopsia
Cerebral achromatopsia is a type of color-blindness that is caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina. It is often confused with congenital achromatopsia. Cerebral achromatopsia differs because it is caused only by trauma or...
- Motion blindness
Motion blindness, also known as motion agnosia, is a neurological condition caused by brain damage . Visual perception is intact in other ways and a person with motion blindness is therefore able to perceive color and form, for example, as accurately as a normally sighted person...
- Tetrachromacy
External links