Color blindness Color blindness 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 might also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The English chemist John Dalton in 1794 published the first scientific paper on the subject, "Extraordinary facts relating to the vision of colors", after the realization of his own color blindness. Color blindness is usually labeled as a disability; however, in select situations colorblind people have advantages over people with a full color range. Color blind hunters are better at picking out prey against a confusing background, and the military have found that color blind soldiers can sometimes see through camouflage that fools everyone else. Monochromats may have a minor advantage in dark vision, but only in the first five minutes of dark adaptation. Rates of occurrence Although exact numbers vary in various populations, color blindness affects a significant proportion of people. Among Americans, approximately 10% of males suffer from some form of color perception deficiency. Isolated communities with a restricted gene pool sometimes produce high proportions of color blindness, including the less usual types: examples include rural Finland and some of the Scottish islands. Causes of color blindness There are many types of color blindness. The most common varieties are hereditary (genetic) photoreceptor disorders. Higher brain areas implicated in color 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. In order to understand retinal color blindness, it is necessary to know that 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). There are three kinds of cones, each containing a different pigment. The cones are activated when the pigments absorb light. The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of these three pigments: different colors are recognized when the different types of cone are stimulated to different extents. The different kinds of color blindness result from one or more of the different cone systems either not functioning at all, or functioning in an unusual way. When one cone system is compromised, dichromacy results. The most frequent forms of color blindness result from problems with either the middle or long wavelength 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 rare. They include problems in discriminating blues from yellows, and complete color blindness, or monochromacy, where one cannot distinguish any color from gray. Red-green color blindness Types of red-green color blindness There are several types of red-green color blindness: · Protanopia: 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 nanometers that is they cannot discriminate light of this wavelength from white. Their sensitivity to light in the orange and red part of the spectrum is also reduced. 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 colorblindness. · Deuteranopia: 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 nanometers. This is one of the rare forms of colorblindness making up about 1% of the male population. 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. · Protanomaly: 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 well 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. Protoanomaly is a fairly rare form of colorblindness making up about 1% of the male population. · Deuteranomaly: 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 colorblindness making up about 8% of the male population. Genetics of red-green color blindness Genetic red-green color blindness affects men much more often than women, because the genes for the red and green color 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 colorblind, only if both their X chromosomes are defective with a similar deficiency, whereas genetic males (46, XY) are color blind if their only X chromosome is defective. Blue-yellow color blindness 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. It is equally distributed among males and females, because the gene coding for the short-wavelength receptor is not sex-linked (being located on chromosome 7). This is either Autosomal Dominant or X linked Dominant. Monochromacy Complete inability to distinguish any colors is called monochromacy. It occurs in two forms: cone monochromacy, where only a single cone system appears to be functioning, so that no colors can be distinguished, but vision is otherwise more or less normal; and achromatopsia, or maskun, or rod monochromacy 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, complete color blindness (maskun) is very common in Pohnpei: about 1/12 of the population there has maskun.