Maskun is a medical condition (also called achromatopsia) characterized by a low cone count or lack of function in cone cells; these are the light receptors responsible for colour perception. It is endemic on Pohnpei and was described by Oliver Sacks in Island of the Colourblind. Sacks went there with a Norwegian 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 colour vision do not perceive things in the same way as those with maskun, because they depend on colour 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. 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.