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Sky-blue-pink. A colour never before seen?

A recurring conundrum in philosophy is the impossibility of sharing, or describing to a blind person, the subjective sensation of colour. Is my sensation of red the same as yours? Or do you see an entirely different hue that I cannot even dream of?

It seems impossible for me to imagine a colour that I have never seen. I don’t mean some subtle shade in a paint catalogue, intermediate between colours that I know well. I mean a completely new colour, as different from the familiar as red is from blue. Proverbially we call it sky-blue-pink, but of course it would resemble no name-able colour.

I have long argued that subjective hues are constructions manufactured in the brain as convenient internal labels for light of different wavelengths. There is no reason why your brain should use the same label for red as my brain does, just because both are labelling light of the same wavelength. I have even gone so far as to speculate that bats might hear in colour. The bat’s brain constructs a detailed picture of the world using echoes instead of light. A bat, when echolocating an insect, might use the subjective sensation that we call ‘red’ as a convenient label for the furry texture of a moth, and might use ‘blue’ as an internal label for the leathery texture of a locust. These qualia are just conveniences, to be pressed into service in the way that is most useful for the species concerned. Since the mammalian brain has the capacity to construct the qualia that we call hues, and use them as internal labels to facilitate sensory distinctions, why wouldn’t bats, as fully paid-up mammals, press into sonar service the labels that we call red and blue? By the same token, I went on, perhaps rhinoceroses smell in colour.

But always, lurking in the background is the desire to imagine a completely strange and alien colour sensation, a colour never seen. I have no hope of ever enjoying that remarkable experience, not even on an LSD trip. But yesterday I nearly hit another cyclist who shot a red light and pleaded colour-blindness as his excuse, and immediately an intriguing thought occurred to me.

A typical red-green colour-blind man’s deficiency lies purely in his retina (sexism alert: a man is 16 times more likely to be colour-blind than a woman). He lacks the gene for making one of the three opsin molecules (red-green-blue) that normally reside in the cones and mediate our trichromatic system. But if I am right that subjective hues are labels that reside in the brain, there is no reason to suppose that the colour-blind man lacks the normal set of brain labels. His retina is deficient but not his brain. A man who cannot see red (operationally cannot distinguish red from green) has, according to my reasoning, a fully functioning label for red (or green), lurking unused in his brain, unused because the retina never stimulates it. If I am right, it should be possible to stimulate the unused ‘label’ directly in the brain.

The brain surgeon Wilder Penfield (1891-1976) discovered, while performing operations on patients under local anaesthetic, that he could, by electrically stimulating particular regions of the brain, reliably arouse particular sensations as reported by the patient: specific memories, a hitherto forgotten view from a childhood window, a particular song, a shape or a colour.

What I now wonder is whether Penfield, or any other brain surgeon, ever did this on a colour-blind patient. Isn’t it possible that, if the right spot in the brain could be discovered, the patient might suddenly experience an entirely new colour, never before seen by him: as different from anything he had ever encountered as red, for me, is different from blue. This would mean that a colour-blind person might be privileged to know what it is like suddenly to enjoy an entirely new sensation. He therefore might be able to make a unique contribution to the philosophical debate on the impossibility of communicating a subjective experience to somebody else. Of course it would be new only to him, not to the rest of us, but his sensation of utter novelty should be as stunning to him as sky-blue-pink would be to me.

If I were colour-blind, I wonder if I might volunteer for the experiment! Probably I wouldn’t dare, but it’s a fascinating thought – to see a totally new colour, never before even imagined or dreamed of.

Dreamed of? Perhaps I am assuming too much. Do colour-blind people see in dreams colours that they have never met in the real world? Some eight percent of my male readers are colour-blind. Can anybody answer my question?



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