|Classification and external resources|
Monochromacy is a disease state in human vision, but is normal in pinnipeds (such as Neophoca cinerea shown here), cetaceans, Owl Monkeys and some other animals
Monochromacy, also known as "total color blindness", is a complete inability to distinguish colors. This is distinguished from more common forms of color blindness, in which the affected individual can perceive color differences, but cannot make the same distinctions between colors as can an unaffected person.
Organisms with monochromacy are called monochromats. The perceptual effect of any arbitrarily chosen light from the visible spectrum can be matched by any pure spectral light.
In vertebrates there are typically two kinds of receptors: rods, which primarily distinguish between levels of illumination, and cones, which are responsible for perception of color. There are different types of cones; each perceives only a single color. 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. Monochromacy is caused by either a defect or the complete absence of the retinal cones.
Some individuals possess diseases or injuries that lead to nyctalopia, or night blindness, where rod cells stop responding properly to light.
There are two known types of monochromacy. "Animals with monochromatic vision may be either rod monochromats or cone monochromats. These monochromats contain photoreceptors which have a single spectral sensitivity curve." There is also a third, theoretical type that has never been identified.
- Rod monochromacy is the condition of having only rods in the retina. A rod monochromat is truly unable to see any color and can see only shades of grey.
- 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. In humans, who have three different types of cones, there are three differing forms of cone monochromacy. There are three types named according to the single functioning cone class:
- Blue cone monochromacy, also known as S-cone monochromacy
- Green cone monochromacy, also known as M-cone monochromacy
- Red cone monochromacy, also known as L-cone monochromacy
- Cone monochromacy, type II, if its existence were established, would be the case 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, and of course would be unable 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.
Animals that are monochromats
It used to be confidently claimed that most mammals other than primates were monochromats. In the last half-century, however, evidence of at least dichromatic color vision in a number of mammalian orders has accumulated. Two of the orders of sea mammals, the pinnipeds (which includes the seal, sea lion, and walrus) and cetaceans (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 monkeys, 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 blind, however, are either dichromats or anomalous trichromats.
According to Jay Neitz, a renowned color vision researcher at the University of Washington, each of the three standard color-detecting cones in the retina of trichromats — blue, green and red — can pick up about 100 different gradations of color. The brain can process the combinations of these three values so that the average human can distinguish about one million different hues. Therefore, a monochromat would be able to distinguish about 100 colors.
- "Colour Blindness." Tiresias.org. Accessed September 29, 2006.
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- "Color Vision: Almost Reason for Having Eyes" by Jay Neitz, Joseph Carroll, and Maureen Neitz Optics & Photonics News, January 2001 1047-6938/01/01/0026/8- Optical Society of America