Achromatopsia: A hereditary disorder of sight due to a lack of cone vision - that type of vision provided by the cone photoreceptors in the retina. In the normal eye, there are some 6 million cone photoreceptors; they are located largely in the center of the retina. Lacking cones, persons with achromatopsia have to rely on their rod photoreceptors. There are about 100 million rod photoreceptors which are located mainly around the periphery of the retina. Rods saturate at higher levels of illumination and do not provide color vision or good detail vision.
Achromats (people with achromatopsia) are the most severely colorblind and have very poor visual acuity. Their eyes do not adapt normally to higher levels of illumination and are very light sensitive (photophobic). There are many degrees of severity of symptoms among achromats. Of all achromats, those who are complete rod monochromats have the most severely impaired vision. There are also incomplete rod monochromats and blue cone monochromats who are less severely affected.
At high levels of illumination, the vision of achromats decreases unless they make use of tinted lenses. In moderately bright indoor spaces or outdoors just after dawn or just before dusk some achromats adapt to their reduced level of visual functioning without resorting to tinted lenses by using visual strategies such as blinking, squinting, or positioning themselves in relation to the light source. Others routinely wear medium tinted lenses in such settings. In full sunlight outdoors or in very bright indoor spaces, almost all achromats need to use very dark tinted lenses to have a reasonable amount of vision, since their retinas do not possess the photoreceptors needed for seeing well in such settings.
Achromatopsia is the subject of a brilliant book "The Island of the Colorblind" by the neurologist/writer Oliver Sacks (Alfred Knopf/Vintage Press). Dr. Sacks did a TV documentary film with the same title about the Pingelapese people of the eastern Caroline Islands in the Pacific who have achromatopsia type 3 (ACHM3), which is also called Pingalese blindness, total colorblindness with myopia and achromatopsia with myopia. From 4 to 10% of Pingelapese people have a severe ocular abnormality manifested by horizontal nystagmus (recurrent flickering back-and-forth eye movements), photophobia, amaurosis (decreased vision), colorblindness, severe myopia (nearsightedness) and gradually developing cataract. This condition is due to recessive inheritance of a gene (that has been mapped to chromosome 8q21-q22). Exactly the same gene has been identified in the Irish which has led to the suggestion that sailors of English/Irish descent may have introduced the ACHM3 gene into the South Pacific Island of Pingelap. The high frequency of the ACHM3 gene among the Pingalese people has been attributed to drastic reduction of the Pingalese population by a typhoon about 1780. Only 9 males are thought to have survived the great storm. From them are descended the Pingalese people. This phenomenon is called founder effect, the growth of a population from a relative tiny number of founding fathers (and mothers).