Congenital achromatopsia, or total color blindness, occurs in individuals whose eyes have no functional cones, the cells which fill the centre of the retina and allow the perception of colour as well as detail. Achromatopes rely on rods for visual input; these are distributed around the periphery of the retina and are very light sensitive. Normally sighted people use rods for low-light or scotopic vision (at night, say). Thus achromatopes have great difficulty with reading, requiring the use of a magnifier, and are very uncomfortable in bright sunlight, though they see well in situations that to the normally sighted would seem very dark indeed.

It is impossible for me to imagine what it must be like to have never had the ability to see colours, just as it is for achromatopes to imagine what I see. They are generally more sensitive than I to form, texture, outlines, boundaries, perspective, depth and movement, all of which will provide them with the information they need to make sense of the world as they perceive it.

Achromatopsia can be the result of trauma, but is most usually caused by recessive genes, and thus the incidence is less than 1 in 30,000 in the normal population. However, in 2 isolated island environments, 1 on Fuur in Norway, 1 on Pingelap in Micronesia, small in-breeding populations have given rise to much higher incidence of this pathology. Though the Norwegian population has mostly died out, on Pingelap the incidence of achromatopsia is 1 in 2.

I learned everything I know about achromatopsia from Oliver Sacks' interesting book The Island of the Colour-Blind.

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