Achromatopsia is a medical condition (also called maskun or rod monochromatism) characterized by a low cone cell count or lack of function in cone cells; these are the light receptors responsible for color perception. It is endemic on the atoll of Pingelap and was described by Oliver Sacks in Island of the Colorblind. Sacks went there with a Norwegian, Knut Nordby who had maskun, and the book narrates his experiences on the island.

People with maskun have difficulty seeing in bright daylight because their rod cells (the receptors responsible for detecting brightness) are saturated. People with normal color vision do not perceive things in the same way as those with maskun, because they depend on color more than on luminosity to identify objects and patterns, whereas achromatopics depend almost entirely on luminosity to identify patterns. The closest that normal-sighted persons can come to experiencing maskun-type vision is in the dark, when the rod cells become the predominant receptors for vision due to their sensitivity to variations in brightness. Achromatopsia can vary in its severity from being mild enough that it is not diagnosed to causing near blindness. It is a relatively rare condition requiring two recessive genes (CNGA3 and CNGB3). In the United States, it affects approximately 1 in 33,000 people. The condition is generally stable over the course of one's life. Many achromats function normally with the aid of darkened lenses, while others use guide dogs, canes, and are considered legally blind.

See also

• Color agnosia