The Windows to the Soul Part 2: Colour Vision

In 1844, the founder of modern chemistry John Dalton, died. By his own request, his eyes were removed at autopsy and the vitreous humour examined. Fifty years earlier, Dalton had provided one of the first accounts of colour blindness based on his personal experiences of the condition. He theorised that colour blindness is due to discolouration of the vitreous and he hoped to have this proven after his death.

However, when his eyes were examined, no discolouration was found. Dalton’s theory was wrong but his name became so closely associated with the condition that it was given the term Daltonism. One hundred and fifty years later, in 1995, DNA analysis of Dalton’s eye tissue confirmed that he had a type of red-green colour blindness, but not the type that had historically been assumed. Instead of protanopia (red deficient), Dalton suffered from deuteranopia (green deficient).

Colour blindness affects around ten percent of males and less than one percent of females. It is caused by mutations in genes affecting the colour-detecting cells of the retina, the cone cells. Cone cells contain one of three photopigments, each absorbing light at a different wavelenght. The absence or malformation of a cone cell type results in colour blindness. The difference in colour blindness rates between men and women is due to the way in which the genes for the photopigments are inherited (two of the three genes are on the X chromosome).

A quirk of the system is that mothers of colour blind boys have the potential to be tetrachromatic, possessing four different photopigments – the three standard pigments plus a fourth mutated variant on their second copy of the X chromosome. A tetrachromat may have a vastly different experience of colour than the rest of us, perhaps able to distinguish a hundred times more shades than trichromats. One of the few reported human tetrachromats can see ten colours in a rainbow.

Tetrachromacy may be hard to imagine but it pales in comparison to the planet’s most complex eye, that of the dodecachromatic mantis shrimp. The mantis shrimp or stomatopod is an aggresive marine crustacean best known for its powerful claws that are capable of smashing through aquarium glass. The stomatopod eye, however, is even more impressive. Stomatopod eyes contain sixteen photopigments. Twelve of these enable colour vision that extends into the ultraviolet and infrared regions. The other four detect polarised light. The stomatopod is the only known creature whose eyes can detect circularly polarised light.

Each eye is divided into two hemispheres and a central midband made up of six rows of photoreceptor cells. The midband detects colour and polarised light while the two hemispheres potentially allow depth perception in a single eye.

The reason for this extraordinary level of complexity is most likely a combination of the need to find prey and avoid predators as well as a means of communication between individuals. Perhaps, like the human brain and the tail of the bird-of-paradise, the stomatopod eye is an example of runaway evolution.

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