readings> tetrachromats

“Which is your favourite colour, Dad?” cry the kids, waving a pack of felt tips in my face. I point to the turquoise one. “Oh, green,” they chorus. “No, blue,” I correct. “Naaahh! That’s green!”, they shout, giggling at Dad’s dismal eyesight.

Well it sure looked blue to me. Turquoise is one of those hues that exist on a perceptual cusp. A touch more blue or green in the mix and you are driven towards quite different subjective judgements. And the puzzle is not only whether there can be slight differences in the tuning of an individual’s colour pathways, but whether our brains even construct the same general experiences.

Is my blue the same as your blue? Or would I call what’s in your head “orange”, or even feel it was an utterly alien hue?

Colour is unlike other sensations because it bears so little direct relationship to the physical stimulus. Tastes and smells are tied to the binding properties of molecules. Sounds vary smoothly with their frequency. Even other visual properties like shape, depth, motion and luminance, seem directly mappable – the cortex patterns have at least some topographical resemblance to the raw stimulus patterns. But redness and blueness are utterly arbitrary mental constructs.

If colours looked like the wavelengths they represent, then really they ought to appear as some kind of surface vibration or texture. A patch of red would have a gentle long-wave fuzziness about it, while blue should give a more intense visual buzz. But no. Instead, the brain somehow turns the gray world Day-Glo.

Well, a murky philosophical issue just got murkier. What would happen if your eyes had four cone pigments instead of three?

Single cone vision – monochromacy – gives us 200 shades of gray. We can distinguish that many luminance levels. Dichromacy – employing a long wave and short wave cone – gives us a blue-yellow spectrum that swells our visual experience geometrically to about 10,000 distinguishable shades. Trichromacy, which adds a third red-green opponent channel, multiplies the total number of shades to several million.

Recently vision researchers have proved that many women – perhaps 1-in-100 – are in fact tetrachromats. Their retinas have four different cone pigments. Do the maths and you will see they should experience hundreds of millions of hues. Their colour experience ought not be just a little richer, but fantastically richer.

Genetic studies have shown that because our red and green photopigments are carried on the X chromosome, and because these pigments are also highly variable, a woman can often inherit two versions of one of them. She might have two red pigments tuned to wavelengths as much as 10 nanometres apart.

The crunch question is of course whether the developing brain is plastic enough to wire itself up to make use of this potential extra dimension of colour. The flabbergasting answer may be yes.

Vision researchers used to believe that laying down retinal circuitry was a very careful affair. A ganglion cell had to form its “on/off” receptive field by comparing excitatory input from one type of cone cell with the inhibitory input from a surround of opponent cells. So a red cone would be wired to a surround of green cones. This would leave nowhere for an extra pigment to go. It would fit no existing opponent channel set-up.

However that model has been turned on its head by a rash of recent findings. It now seems that during development, ganglions connect rather randomly to a surround of cones and so end up being driven by a mix of cone types. A crisp response emerges only by neural learning. The gain is adjusted on the many rivalous inputs to sort out a stable response. And if opponent channels are indeed self-organising in this way, then there now appears much more scope for an extra pigment to form its own opponent channel.

This possibility is now being tested by injecting extra gene pigments into a monkey’s eye to see if there is a take-up – work that could lead to a cure for colour-blindness in humans, or even offer the sci-fi prospect of turning ordinary folk into four colour super-perceivers. Work is also going on to check if women known to have four distinct cone pigments show signs of perceiving many more colours.

Colour-matching experiments have already shown that some subjects see the same “orange” created by different green-red wavelength mixes as actually being different. One of these ladies, keen on tapestry, complains she can never find a full range of coloured yarns as manufacturers leave great gaps in the spectrum.

Of course, because any extra pigment will be squeezed into the already tight 30 nanometre space between existing red and green photopigments, tetrachromacy might not actually be as colourful as calculations suggest. For that, we might have to inject humans with the extra ultra-violet range pigment enjoyed by fish and birds. But still, what wouldn’t you give for a glimpse through another’s eyes to discover what you might be missing?

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