|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 (from Greek mono, meaning "one" and chromo, meaning "color") is the ability of organisms or machines to perceive only light intensity, without respect to spectral composition (color). Organisms with monochromacy are called monochromats.
For example, about 1 in 30,000 people have monochromatic vision because the color-sensitive cone cells in their eyes do not function. Affected people can distinguish light, dark, and shades of gray but not color.
Many species, such as marine mammals, the owl monkey and the Australian sea lion (pictured at right) are monochromats under normal conditions. In humans, absence of color discrimination or poor color discrimination is one among several other symptoms of severe inherited or acquired diseases, as for example inherited achromatopsia, acquired achromatopsia or inherited blue cone monochromacy.
Vision in humans is due to a system that includes rod and cone photoreceptors, retinal ganglion cells and the visual cortex in the brain. Color vision is primarily achieved through cone cells. Cone cells are more concentrated in the fovea centralis, which is the central portion of the retina. This allows greater spatial resolution and color discrimination.
Rod cells are concentrated in the periphery of the human retina. Rod cells are more light sensitive than cone cells, and are mainly responsible for scotopic (night) vision. Cones in most humans have three types of opsins with different spectral sensitivities which allow for trichromatic color discrimination, whereas rods all have a similar, broad spectral response which does not allow for color discrimination. Because of the distribution of rods and cones in the human eye, people have good color vision near the fovea (where cones are) but not in the periphery (where the rods are).
These types of color blindness can be inherited, resulting from alterations in cone pigments or in other proteins needed for the process of phototransduction:
Monochromacy is one of the symptoms of diseases that occur when only one kind of light receptor in the human retina is functional at a particular level of illumination. It is one of the symptoms of either acquired or inherited disease as for example acquired achromatopsia, inherited autosomal recessive achromatopsia and recessive X-linked blue cone monochromacy.
There are two basic 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."
In humans, who have three types of cones, the short (S, or blue) wavelength sensitive, middle (M, or green) wavelength sensitive and long (L, or red) wavelength sensitive cones have three differing forms of cone monochromacy, named according to the single functioning cone class:
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. While typical mammals are dichromats, with S and L cones, two of the orders of marine 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.[dubious ] The same is true of the owl monkeys, genus Aotus.
A recent study using through PCR analysis of genes OPN1SW, OPN1LW, and PDE6C determined that all mammals in the order Xenarthra (representing sloths, anteaters and armadillos) developed rod monochromacy through a stem ancestor.
Researchers Leo Peichl, Guenther Behrmann and Ronald H. H. Kroeger report that of the many animal species studied, there are three carnivores that are cone monochromats: raccoon, crab-eating raccoon and kinkajou and a few rodents are cone monochromats because they are lacking the S-cone. These researchers also report that the animal's living environment also plays a significant role in the animals' eyesight. They use the example of water depth and the smaller amount of sunlight that is visible as one continues to go down. They explain it as follows, "Depending on the type of water, the wavelengths penetrating deepest may be short (clear, blue ocean water) or long (turbid, brownish coastal or estuarine water.)"  Therefore, the variety of visible availability in some animals resulted in them losing their S-cone opsins.
According to Jay Neitz, a color vision researcher at the University of Washington, each of the three standard color-detecting cones in the retina of trichromats can detect approximately 100 gradations of color. The brain can process the combinations of these three values so that the average human can distinguish about one million colors. Therefore, a monochromat would be able to distinguish about 100 colors.