Blue cone monochromatism

Monochromacy (from Greek mono, meaning 'one 'and chromo, meaning 'color') is the ability of organisms or machines to distinguish only one single frequency of the electromagnetic light spectrum. In the physical sense, no source of electromagnetic radiation is purely monochromatic but can be considered as a gaussian distribution of frequencies shaped around a peak. In the same way, a visual system of an organism or a machine cannot be monochromat but will distinguish a continuous set of frequencies around a peak, depending by the intensity of the light. Organisms with monochromacy are called monochromats. Monochromacy (from Greek mono, meaning 'one 'and chromo, meaning 'color') is the ability of organisms or machines to distinguish only one single frequency of the electromagnetic light spectrum. In the physical sense, no source of electromagnetic radiation is purely monochromatic but can be considered as a gaussian distribution of frequencies shaped around a peak. In the same way, a visual system of an organism or a machine cannot be monochromat but will distinguish a continuous set of frequencies around a peak, depending by the intensity of the light. Organisms with monochromacy are called monochromats. Many species, such as all 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 starts with rods and cones photoreceptors, passes through retinal ganglion cells and arrives in the brain visual cortex. Color vision is achieved through cone cells, each one able to distinguish between a continuous band of frequencies, retinal ganglion cells and the visual cortex. Rods, which are extremely abundant (about 120 million), are in the periphery of the human retina. Rods respond only to faint levels of light and are very light sensitive, therefore, completely useless in daylight. Cones have three types of pigments with different color sensitivities, whereas rods only have one and so are achromatic (colorless). 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. The same is true of the owl monkeys, genus Aotus.