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Causes of colorblindness
An unusual type of colorblindness occurs in the mind, not the eye. This is a drawing of a patient who was unable to see the world in color after a cerebral accident. The patient was an artist, and the drawings were made from memory. The drawings represent (clockwise): a banana, a tomato, a cantaloupe and leaves. Interestingly, this is analogous to the luminance-based paintings of some Picasso paintings.
In most colorblindness, the person has a different mixture of cones from normal. The cone cells on the retina are the crucial physical components in seeing color. People with normal color perception have three different cones, each cone is tuned to perceive mostly either Long wavelengths (reddish), Middle wavelengths (greenish), or Short wavelengths (bluish), referred to as L-, M-, and S- cones. Our minds determine what "color" we are seeing by determining the ratio between signals from different types of cones. In people with colorblindness, either a type of cone is missing, or the cone has a different peak absorption from normal.
Congenital color vision deficiencies overwhelmingly affect the L-cones or the M-cones. Full color blindness and insensitivity to blue light are relatively rare. L-cone and M-cone deficiencies are collectively known as red-green colorblindness, because they reduce the ability to distinguish those two colors. Yellow-blue colorblindness stems from the much rarer deficiencies involving the S-cones.
Colorblindness is primarily a congenital defect. However, colorblindness can also be acquired, as a result of disease or accident (as in the painting at right). In this case, the color vision deficiency is caused by disruption to the neural pathways between the eye and the vision centers of the brain, rather than by loss of cone function in the eye. For example, achromatopsia, the loss of all color vision, can be acquired as a result of brain damage, and Parkinson's disease commonly induces symptoms that are similar to tritanopia.
Types of colorblindness
Researchers have classified the different forms of colorblindness according to the affected cone type, and the degree of deficiency. Complete loss of one cone type is known as dichromacy. It is the more severe color deficiency.
Cone type
Generic
defect
nameAnomalous
trichromacy
nameDichromacy
nameL-cone
protan
protanomaly
protanopia
M-cone
deutan
deutanomaly
deutanopia
S-cone
tritan
-
tritanopia
Many forms of so-called colorblindness are really natural variations in the proteins. Just as some people have brown or blue eyes, different people have different pigments in their L- and M- cones. People's ability to discern colors varies with difference in the absorption maxima of the cones. Generally, the ability to discern reds and greens decreases when the absoprtion maxima of the cones is more similar. Altered sensitivity in one cone type is known as an anomalous trichromacy. This occurs when there is variation in the cone pigments, caused by a difference in their amino acid sequence. The human red and green color vision pigments are identical at all but 15 of their 364 amino acids, and yet their absorption maxima differ by 31 nm. The spectral difference between these 2 pigments is determined by 7 and only 7 amino acid residues.
Protans and deutans can be anomalous trichromats or dichromats. There are no S-cone anomalous trichromats, so tritans are always dichromats.
