Watch enough French films and you’ll entertain thoughts of life being more interesting and romantic if it was permanently in black and white. Did you know that that’s actually a lived reality for a minority of the world’s population?
People with complete colour blindness, also known as monochromacy and the rarest form of colour blindness, don’t experience colour at all and the clearness of their vision may also be affected.
The first type of monochromacy, cone monochromacy results from a failure of two of the three cone cell photopigments to work. Within this cateogory, there is also red cone monochromacy, green cone monochromacy, and blue cone monochromacy. People with cone monochromacy have trouble distinguishing colours because the brain needs to compare the signals from different types of cones in order to see colour.
When only one type of cone works, this comparison isn’t possible. People with blue cone monochromacy, may also have reduced clearness of vision, nearsightedness, and uncontrollable eye movements, a condition known as nystagmus.
The second type of monochromacy, rod monochromacy or achromatopsia is the rarest and is the most severe form of colorblindness. Present at birth, none of the cone cells have functional photopigments.
Lacking all cone vision, people with rod monochromacy see the world in black, white, and gray. And since rods respond to dim light, people with rod monochromacy tend to be photophobic which means they would be very uncomfortable in bright environments. They also experience nystagmus similar to those who suffer from blue cone monochromacy.