Word of the Week - Tetrachromat

Tetrachromat - a woman who sees four distinct ranges of colour rather than the normal three.

The retinas of most people's eyes contain three types of cone cells which act as receptors for blue, green, and red wavelengths of light. The brain combines information it receives from each type of cone to process perceptions of colour. Each of these receptors can generally pick up 100 gradations of colour, so the average human is able to distinguish between 1 million distinctive hues. (100 shades of blue x 100 shades of green times 100 shades of red = 1,000,000 possibilities!)

Tetrachromats, however, are rather special. In addition to the standard cone cells -- red, blue, and green -- tetrachromats possess a fourth type of cone cell that reacts to wavelengths between red and green. The addition of the fourth cone permits tetrachromats to distinguish between shades that appear the same to non-tetrachromats. In fact, they may be able to distinguish between up to 100 million distinctive hues. (100 shades of blue x 100 shades of green x 100 shades of red x 100 shades of "tan" = 100,000,000,000 possibilities.)

There seems to be disagreement as to the distribution of tetrachromacy in the human population. Earlier this week, a guest on the local CBC morning show said that tetrachromacy was limited to 3% of women. However, I have also read that some scientists believe that as many as 50% of women and 8% of men carry four types of photoreceptor cells.

The more interesting question is whether or not any humans possess brains (or optic nerves, at least) which are wired to deal with 100 million different colour gradations, but that is opening a can of worms that I don't want to deal with.

Even more interesting is the existence of pentachromats in the natural world -- creatures such as pigeons and butterflies whose retinas contain five different types of colour receptors. However, the word of the week is tetrachromat, so I think I'll end here.