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Maskun is a medical condition (also called achromatopsia) characterized by the inability to perceive any colors, a severe and rare form of color blindness. It is caused by the lack of any functioning cone cells in the retina; these are the light receptors responsible for color perception. It is endemic on Pohnpei and was described by Oliver Sacks in Island of the Colorblind. Sacks went there with a Dane 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.