Tritanopia

The motoring journalist, Quentin Wilson, once advised anyone wanting to chose the colour of a car to take a woman along as "men have the colour vision of vampire bats." He may well have a point as colour blindness is far more common in men than women. Certainly, I have always struggled with colours but, with me, it doesn't seem to be the standard red-green phenomenon that causes the colour deficient to become flummoxed by Ishihara tests.

The problem I have is generally concerned with telling blue and green apart. I've always been aware of this. It caused me problems when studying inorganic chemistry at school but it rather came to a head when I was at university and couldn't, for the life of me, tell the colours on electronic resistors apart. Green and blue looked the same, as did red and violet. Since then I've been bothered with colour coded charts and, from a distance, a green traffic light looks like a police or ambulance light to me (except it isn't flashing) but I've never actually been tested for this - the Ishihara tests at the opticians only test for red-green colour blindness. However, the condition does actually have a name: tritanopia; but I've always questioned whether I actually have this or whether I just have typically male "vampire bat" vision.

A few months ago, I was talking to my uncle about his time in the Royal Air Force. He served during the Malayan Emergency of the 1950's and I knew he suffered from colour blindness which does limit the roles available in the Air Force. However, when I asked him about this he said that he was fine with red-green but always confused blue and green - the same as me. This does indicate to me that there could be a hereditary role at play and, in fact, it is linked to a Chromosome 7 abnormality. Interestingly, I also have a Kell positive blood group which is also Chromosome 7 linked but I suspect this is merely coincidence.

I have asked myself whether I do see the full spectrum and whether anything is missing. This does rather invoke the question of qualia: the way things appear to us. The problem with this is that I don't go around thinking "ooh, everything looks weird" as everything looks exactly like it always has. Just occasionally, I do notice that people can see clearly things that I can't (or at least struggle with) like my university lecturer who thought I was being a bit vacant with the resistors. Recently, I have been trying some experiments on myself and some of the results are a bit odd.

The first thing I tried out was seeing if I could make out all the colours of the spectrum through a prism. I certainly can't make out all the colours of the rainbow but I do think this is a rather artificial concept. When Isaac Newton originally came up with his spectrum he chose five colours: red, yellow, green, blue and violet. He added orange and indigo to make it match the seven notes of a musical scale although there is no scientific equivalence between music and colour. Personally, I have no idea what indigo is meant to be, but there is some sense to having seven spectral colours. The eye has three colour receptors, or cones, peaking at wavelengths roughly equating to blue, green and red (although actually peaking around orange). This would make a more scientifically sensible rainbow as: red, orange, yellow, green, cyan, blue, violet. In fact it was this realisation that led James Clerk Maxwell (he of the Maxwell's Daemon) to develop the first colour photograph. As for me, I can see clearly red, orange, yellow and green and then I get a bit lost. I can certainly see something, faintly, after that; which I suppose must be blue or violet. But does this mean I am actually tritanopic?

The next effect I have noticed is with rainbows. If I glance at one I can see clearly red through green but if I look more closely I can definitely make out the same faint glow beyond the green as I experience with the prism. However, what I have noticed is that if I cover one eye, this glow disappears. It doesn't matter which eye either. For some reason, I appear to have marginally better colour perception with binocular vision than with either eye separately. I'm struggling to think of a rational reason for this - the only thing I can think of is that I am making use of peripheral vision.

The cones, which help us perceive colour, are concentrated in the centre of the eye whereas the rods, which are more sensitive and help us see at night and detect motion, are very much reduced in the centre of the eye but more concentrated elsewhere - this is why it helps to look out of the side of the eye when trying to locate star clusters like the Pleiades. The rods are also most sensitive to light at a spectrum somewhere between the green and blue cones. If I am actually lacking in the blue cones, I am wondering if my brain has adapted to use information from the rods to increase the perception of colour? The problem with that is the medical consensus is that rods play no role in colour vision. For a normal trichromat there is no need for rods to be used, but for the tritanopic it could be a useful coping strategy and it does seem to be noted that tritanopia causes less day to day problems than other forms of colour blindness.

Out of interest, someone has altered the Sony advert where they blow up a Glasgow tower block with paint (it was blown up with dynamite a few weeks later) The first is with normal colour, the second is meant to reproduce tritanopic vision:





The odd thing is that the second video does look somewhat different - mainly it doesn't look as bright to me - but the colours I think I can see are actually red and blue. I don't know how this actually appears to someone with normal vision. I owned a TV years ago which had faulty colour in a similar manner but I didn't notice this. A girl-friend pointed it out to me and I could tell that the replacement TV set had a better picture. Irritatingly, I had been paying for a colour TV licence.

I suppose the answer would be to get some sort of test but the high street opticians generally only administer Ishihara tests if anything at all. However, I would be interested to know if anyone else experiences better colour perception with binocular vision.