Color blindness
Color blindness or color vision deficiency is the decreased ability to see color, differences in color, or distinguish shades of color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception.
Color blindness is usually a sex-linked inherited problem or variation in the functionality of one or more of the three classes of cone cells in the retina, which mediate color vision. The most common form is caused by a genetic condition called congenital red–green color blindness, which affects up to 1 in 12 males and 1 in 200 females. The condition is more prevalent in males, because the opsin genes responsible are located on the X chromosome. Rarer genetic conditions causing color blindness include congenital blue–yellow color blindness, blue cone monochromacy, and achromatopsia. Color blindness can also result from physical or chemical damage to the eye, the optic nerve, parts of the brain, or from medication toxicity. Color vision also naturally degrades in old age.
Diagnosis of color blindness is usually done with a color vision test, such as the Ishihara test. There is no cure for most causes of color blindness; however, there is ongoing research into gene therapy for some severe conditions causing color blindness. Minor forms of color blindness do not significantly affect daily life and color blind people automatically develop adaptations and coping mechanisms to compensate for the deficiency. However, diagnosis may allow an individual, or their parents/teachers, to actively accommodate the condition. Color blind glasses may help the red–green color blind at some color tasks, but they do not grant the wearer "normal color vision" or the ability to see "new" colors. Some mobile apps can use a device's camera to identify colors.
Depending on the jurisdiction, color blind people are ineligible for certain careers, such as aircraft pilots, train drivers, police officers, firefighters, and members of the armed forces. The effect of color blindness on artistic ability is controversial, but a number of famous artists are believed to have been color blind.
Classification
Much terminology has existed and does exist for the classification of color blindness, but the typical classification for color blindness follows the von Kries classifications, which uses severity and affected cone for naming.Based on severity
Based on clinical appearance, color blindness may be described as total or partial. Total color blindness is much less common than partial color blindness. Partial color blindness includes dichromacy and anomalous trichromacy, but is often clinically defined as mild, moderate or strong.Monochromacy
Monochromacy is often called total color blindness since there is no ability to see color. Although the term may refer to acquired disorders such as cerebral achromatopsia, it typically refers to congenital color vision disorders, namely rod monochromacy and blue cone monochromacy.In 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 agnosia.
Monochromacy is the condition of possessing only a single channel for conveying information about color. Monochromats are unable to distinguish any colors and perceive only variations in brightness. Congenital monochromacy occurs in two primary forms:
- Rod monochromacy, frequently called complete 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.
- Cone monochromacy is the condition of having only a single class of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. Cone monochromacy is divided into classes defined by the single remaining cone class. However, red and green cone monochromats have not been definitively described in the literature. Blue cone monochromacy is caused by lack of functionality of L and M cones, and is therefore mediated by the same genes as red–green color blindness. Peak spectral sensitivities are in the blue region of the visible spectrum. People with this condition generally show nystagmus, photophobia, reduced visual acuity, and myopia. Visual acuity usually falls to the 20/50 to 20/400 range.
Dichromacy
Anomalous trichromacy
Anomalous trichromacy is the mildest type of color deficiency and is usually characterized by the inability to distinguish between colors which are distinguishable to a person with normal color vision, but the severity ranges from almost dichromacy to almost normal trichromacy. In fact, many mild anomalous trichromats have very little difficulty carrying out tasks that require normal color vision and some may not even be aware that they have a color vision deficiency. The types of anomalous trichromacy include protanomaly, deuteranomaly and tritanomaly. It is approximately three times more common than dichromacy. Anomalous trichromats exhibit trichromacy, but the color matches they make differ from normal 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. This difference can be measured by an instrument called an Anomaloscope, where red and green lights are mixed by a subject to match a yellow light.Based on affected cone
There are two major types of color blindness: difficulty distinguishing between red and green, and difficulty distinguishing between blue and yellow. These definitions are based on the phenotype of the partial color blindness. Clinically, it is more common to use a genotypical definition, which describes which cone/opsin is affected.Red–green color blindness
Red–green color blindness includes protan and deutan CVD. Protan CVD is related to the L-cone and includes protanomaly and protanopia. Deutan CVD is related to the M-cone and includes deuteranomaly and deuteranopia. The phenotype of deutans and protans is quite similar. Common colors of confusion include red/brown/green/yellow as well as blue/purple. Both forms are almost always symptomatic of congenital red–green color blindness, so affects males disproportionately more than females. This form of color blindness is sometimes referred to as daltonism after John Dalton, who had red–green dichromacy. In some languages, daltonism is still used to describe red–green color blindness.- Protan : Lacking, or possessing anomalous L-opsins for long-wavelength sensitive cone cells. Protans have a neutral point at a cyan-like wavelength around 492 nm —that is, they cannot discriminate light of this wavelength from white. For a protanope, the brightness of red 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 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. A 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 shorter than neutral point as blue and those longer than it as yellow.
- Deutan : Lacking, or possessing anomalous M-opsins for medium-wavelength sensitive cone cells. Their neutral point is at a slightly longer wavelength, 498 nm, a more greenish hue of cyan. Deutans have the same hue discrimination problems as protans, but without the dimming of long wavelengths. Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths shorter than neutral point as blue and longer than it as yellow.
Blue–yellow color blindness
- Tritan : Lacking, or possessing anomalous S-opsins or short-wavelength sensitive cone cells. Tritans see short-wavelength colors. The OMIM gene code for this mutation is 304000 "Colorblindness, Partial Tritanomaly".
- Tetartan is a hypothetical "fourth type" of color blindness, and a type of blue–yellow color blindness. Given the molecular basis of human color vision, it is unlikely this type could exist.
Summary of cone complements
Presentation
A color blind person will have decreased color discrimination along the red–green axis, blue–yellow axis, or both. It is a common misconception that color blindness always equals monochromacy. The vast majority of color blind people are only affected on their red–green axis.The first indication of color blindness generally consists of a person using the wrong color for an object, such as when painting, or calling a color by the wrong name. The colors that are confused are very consistent among people with the same type of color blindness.