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May 9, 2011 | by  | in Features |
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Why Vision is the Bomb-Diggity, & How Others See

Most of us will have done those tests for colourblindness, where there’s a number made of dots surrounded by other dots, and occasionally, you can’t see a number, and you freak out. (And it just turns out to be a control…) But have you ever wondered how they work?

It’s very easy to take our sight for granted. Yes, you might whinge when sitting too far back in a lecture, or when the font in a book is stupidly small, but even imperfect vision is an amazing thing. Colours, words, faces—everything around you can be categorised and understood within an incredibly short amount of time.

Achromatism and colour blindness

Colour plays a huge part in our perception of the world, and this is how it works. The middle chunk of your eye has cells, called ‘cones’, that are specialised to detect colours. The colour we see is reflected wavelengths of light, and humans can perceive waves between 400nm (red) and 700nm (violent). When you see a banana, that banana is absorbing the entire visible spectrum except yellow, and so you see it as yellow. (If you don’t get that, no worries. It’s an aside.)

Some people are born with a rare form of colourblindness, called achromatopsia, where no colour is perceived at all, and everything looks like a black-and-white photograph. According to my fairly old and probably out-of-date textbook, its relative occurrence is about 1:40,000. When congenital, this is most often due to a malfunction in the cones, but acquired achromatopsia tends to be a result of damage to the region of the brain that recognises colour. In this case, your memory of colour is also obliterated. People who are born with it are also highly sensitive to light and are unable to focus normally, and therefore cannot see fine detail.

The colourblindness we are most used to is not this extreme, though—in fact, people can be colourblind for most of their lives without realising it. This is caused by the loss of one of the three types of cones. Incidentally, men are much more likely to be colourblind, because the X chromosome carries most of the genes for cones, and men are XY while women are XX. The Y chromosome in men is basically a short, crap version of the X, so they have fewer genes. That way, men only have one copy of the gene for cones, whereas women have two. Guts, yo.

Red-green colourblindness is the most common variety; about 8 per cent of men have it, but only 0.03 per cent of women. A cool theory about how colourblindness came to be common has arisen from the study of marmosets (and who wouldn’t want to study marmosets. They’re so tiny and cuddly, and probably vicious). About two-thirds of marmosets are missing one type of cone, and they stand guard while the others, with three cones and full colour vision, gather food. The idea is that the monkeys with three cones can tell the difference between berries, fruit, etc., the monkeys with two cones are more sensitive to movement, and therefore less likely to be fooled by camouflaged predators.

Other ways of seeing

Bees are one up on humans, though. They can see UV light as a colour, which completely changes their view of the world, and especially flowers. Here’s an approximation of how bees might see a yellow flower.

It’s a bit off, as bees can’t see red (which is the lowest wavelength we can see). Obviously, no-one can know exactly what it looks like until someone invents a way of enabling humans to see UV light. That seems unnecessary though. Synesthesia is a cool but weird disorder that essentially blends two or more senses. It happens when two normally separate areas of the brain activate simultaneously. The most common is that numbers and letters of the alphabet are perceived as colours—for instance, three is ruby red and the letter ‘D’ is the sky-at-night blue, or something. Other instances include the association of sound with colour, of ordered sequences with personalities, and individual words with taste.

The closest most of us will get to this sensation is through illusions. Despite what your brain knows, those two lines totally don’t look the same length, or the leaning tower of Pisa on the left doesn’t look as leaning as the exact same picture on the right. It’s odd that your brain compensates for this. It really shows that the world we see isn’t the world that is necessarily there, which is kind of awesome. What you see, or smell, or hear, may not be same as the person standing next you (Perhaps this explains some people’s horrendous taste in music. Or so you’d like to hope.)

All in all, vision is pretty awesome. The fact that we can see an amazing spectrum of colours, and that that some people see numbers as colours and whatnot, is an incredible feat of engineering. The next time you can’t read the board in a lecture, just think how much more confusing it would look if you could see UV light—and then enjoy the fuzziness of the mostly readable lecture slides!

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