Monochromacy
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'''Monochromacy''', also known as "total color blindness"<ref name="Tiresias">[http://www.tiresias.org/guidelines/colour_blindness.htm "Guidelines: Color Blindness."] Tiresias.org. Accessed September 29, 2006.</ref>, is the lack of ability to distinguish colors, caused by cone defect or absence.<ref name="Cassin">Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainsville, Florida: Triad Publishing Company, 1990.</ref> Monochromacy occurs when two or all three of the cone pigments are missing and color and lightness vision is reduced to one dimension.<ref name="Tiresias"/>
Organisms with monochromacy are called monochromats. Monochromats are truly [[color blindness|color blind]] and can see only shades of black, gray and white. The perceptual effect of any arbitrarily chosen light from its [[visible spectrum]] can be matched by any pure spectral light.
The normal explanation of monochromacy is that the organism's [[retina]] contains only a single kind of light receptor cell, or at least that only one kind is active at any particular level of illumination. In vertebrates, which typically have two kinds of receptors, called [[rod cell|rods]] and [[cone cell|cones]], active at low and higher levels of illumination respectively, there are two main kinds of monochromacy:
#'''[[rod monochromacy]]''' is the condition of having only rods in the retina. A rod monochromat will be unable to see well in normal daylight levels of illumination.
#'''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 [[hue]]s. (see below)
In principle there could also be a second kind of cone monochromacy, in which the retina contains no rods, and only a single type of cone. Such an animal would be unable to see at all at lower levels of illumination, but it would have good pattern vision at normal daylight levels (though it would not be able to distinguish hues). In practice it is hard to produce an example of such a retina, at least as the normal condition for a species; there are animals (for example, many birds) with very cone-rich retinas, but they all tend to have multiple types of cones. Some individuals do possess diseases or injuries that lead to [[nyctalopia]], or night blindness, where rod cells stop responding properly to light.
In cone monochromats, at low light intensities the rods and cones may be active simultaneously, allowing some degree of color discrimination. However it is unlikely that this will be functionally significant since the neural apparatus for hue discrimination would presumably not be present in an animal that was monochromatic most of the time.
It used to be confidently claimed that most [[mammal]]s other than [[human]]s and our fellow [[primate]]s were monochromats. In the last half-century, however, evidence of at least [[dichromat]]ic color vision in a number of mammalian [[order (biology)|orders]] has accumulated. Two of the orders of sea mammals, the [[pinniped]]s (which includes the seal, sea lion, and walrus) and [[cetacean]]s (which includes dolphins and whales) clearly are cone monochromats, since the short-wavelength sensitive cone system is genetically disabled in these animals. The same is true of the [[owl monkey]]s, genus ''Aotus''.
Both rod and cone monochromacy occur as very rare forms of color blindness in humans. Rod monochromacy, or [[maskun]], is the more common of the two. The majority of people described as [[Color blindness|color blind]], however, are either [[dichromat]]s or anomalous [[trichromat]]s.
Monochromacy has been subdivided into typical (rod monochromacy) and atypical forms.<ref name="Alpern">Alpern M. [http://www.iovs.org/cgi/reprint/13/9/648.pdf "What is it that confines in a world without color?"] ''Invest Ophthalmol.'' 1974 Sep;13(9):648-74. PMID 4605446.</ref><ref>Hansen E. "Typical and atypical monochromacy studied by specific quantitative perimetry." ''Acta Ophthalmol (Copenh).'' 1979 Apr;57(2):211-24. PMID 313135.</ref>
Clinically, some monochromats have normal [[visual acuity]] and others have poor visual acuity.<ref>http://www.iovs.org/cgi/reprint/13/9/648.pdf#search=%22%22red%20cone%20monochromacy%22%22</ref>
==Cone monochromacy==
'''Cone monochromacy''' is a rare, total color blindness that is accompanied by relatively normal vision, electoretinogram, and electrooculogram.<ref name="Cassin">Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainsville, Florida: Triad Publishing Company, 1990.</ref> There are three types named according to the single functioning cone class:
#Blue cone monochromacy, also known as S-cone monochromacy<ref name="Tiresias"/>
#Green cone monochromacy, also known as M-cone monochromacy<ref name="Tiresias"/>
#Red cone monochromacy, also known as L-cone monochromacy<ref name="Tiresias"/>
==References==
{{reflist}}
==See also==
*[[Cone dystrophy]]
*[[Achromatopsia]]
{{Color vision}}
[[Category:Vision]]
[[Category:Optometry]]
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