Monochromacy

Monochromacy, also called achromatopsia or total color blindness, is the rarest form of color vision deficiency. People with monochromacy see the world almost entirely in shades of gray, black, and white. Most people with this condition have severely reduced visual acuity and extreme sensitivity to light (photophobia).

Rod monochromacy (typical achromatopsia) is the most common form. People with this condition have no functioning cone cells at all — only rod cells. This results in complete color blindness, very poor visual acuity, nystagmus (involuntary eye movements), and extreme light sensitivity. It is autosomal recessive and affects roughly 1 in 30,000 people.

Cone monochromacy is a less severe but also significantly less common condition, with only a handful of cases ever identified. A person with cone monochromacy has only a single type of functioning cone in the retina. Unlike rod monochromacy, visual acuity remains normal. However, the person still cannot distinguish colors because color perception requires comparison between at least two cone types.