Monochromacy

Monochromacy
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
Classification and external resources
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Monochromacy (mono meaning one and chromo color) is among organisms or machine the ability 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 (OMIM 216900 262300 139340 613093), acquired achromatopsia or inherited blue cone monochromacy (OMIM 303700).

Vision in humans is due to a system that starts with Rods and Cones photoreceptors, pass through retina ganglion cells and arrives in the brain visual cortex. Color vision is achieved through cones cells, each one able to distinguish between a continuous band of frequencies, retinal ganglion cells and visual cortex. Rods, which are extremely abundant in numbers (about 120 million) are in the periphery of the human retina. Rods only respond to faint levels of light and are very light sensitive, therefore, completely useless in daylight because bright light bleaches them.Cones, which are mostly near the fovea in the eye and are less active in dim light, more useful in bright light, and essential for color vision. There are three types of cones in normal human eyes (short, medium, and long wavelength, sometimes called blue, green, and red); each detects a different range of wavelengths. Rods outnumber cones by about 20 to 1 in the human retina, but cones provide about 90% of the brain's input. Cones respond faster than rods, and have three types of pigments with different color sensitivities, where 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).

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