What do Vladimir Nabokov, Richard Feynman, Olivier Messiaen, Wassily Kandinsky and Nikola Tesla have in common? They all were, or at least claimed to be, synaesthetes. Each experienced sensory crossovers. Nabokov had "coloured hearing"; the sounds associated with letters of the alphabet evoked for him specific hues (the "b" sound, he wrote in Speak, Memory, was "burnt sienna," whereas "s" was "a curious mixture of azure and mother-of-pearl"). Tesla, a pioneer in the field of electricity, could discern flavour in certain sights; looking at small squares of paper in a dish of liquid would fill his mouth with a horrible taste. And so on.
Synaesthesia, a recognised phenomenon for the last 300 years or so, enjoyed a special vogue in the late 19th century among spiritualistically inclined artists. About one person in 2,000 is gifted-or cursed-with sensory crossover; that, at any rate, is the estimate in the scientific literature. Around 20 forms of synaesthesia have been reported. The most common involve written letters or numbers triggering colours, which are seen in the mind's eye or projected outside the body. In rarer cases, synaesthetes say they can see the fragrance of a flower, taste the note of a trumpet, or even hear the decor of a room. One synaesthete said that the pain she felt when she tore a ligament in her leg turned the whole world orange.
It is tempting to dismiss all this as the product of over-vivid imaginations. "The confessions of a synaesthete must sound tedious and pretentious to those who are protected from such leakings," Nabokov observed. Which of us has not tasted "sharp" cheese or seen a "loud" colour or felt "bitter" cold? Perhaps synaesthetes just need a little ironic distance from their metaphors.
When it comes to deciding whether synaesthesia is for real, traditional philosophy isn't a great deal of help. It is conceptually impossible for the synaesthete to be mistaken about the subjective experiences he reports, since they are directly present to consciousness.
For synaesthesia to be scientifically interesting, it needs to have some measurable consequences. It turns out that it does. To begin with, people who report synaesthetic experiences tend to be remarkably consistent over time. In one 1993 study, synaesthetes were 92 per cent consistent in linking particular colours to sounds after a full year, whereas non-synaesthetes, when asked to come up with such colour associations, could manage only 38 per cent consistency after a mere week. Synaesthesia also seems to affect normal perception, sometimes improving perceptual performance. In one study, those synaesthetes for whom numbers triggered colours were instantly able to detect patterns in a black-and-white chart of numerals, patterns that became apparent to non-synaesthetes only after several seconds. The most concrete evidence for the reality of synaesthesia, however, comes from brain-scanning technologies. Images of the brains of synaesthetes who report coloured hearing, for example, show increased activation of colour-processing areas of the cortex in response to sound.
Do synaesthetes, then, have crossed wires inside their skulls? Some researchers think so. One hypothesis, advanced by the University of Cambridge psychologist Simon Baron-Cohen (a relation of Ali G), is that synaesthesia arises from an overabundance of neural connections in the brain. It is possible that at birth we experience the world synaesthetically with a single undifferentiated sense. Infants less than a year old, for example, show a very similar neural response to a bright light and a loud sound. But as the brain develops with age, many neural connections get pruned away, and the senses begin to operate independently of one another. In synaesthetes, it is thought, this pruning process does not get carried out completely, possibly for genetic reasons (synaesthesia runs in families, and is more common among women and left-handed individuals).
Appealing as this hypothesis may seem, there is one interesting datum with which it does not square: the fact that certain psychedelic drugs can induce temporary synaesthesia. It is hard to believe that a hit of LSD could cause new neural connections to sprout and then vanish within the space of a few hours. Maybe synaesthetes have the same brain architecture as non-synaesthetes, but it simply functions differently. Peter Grossenbacher at the US's National Institute of Mental Health, thinks synaesthesia has something to do with the interface between sensation and perception. Normally, the brain keeps the right sort of sensory signals on the right sensory-processing channels. Auditory information, for example, is inhibited from flowing along the channel to the visual centre. But in synaesthetic brains, Grossenbacher thinks, this inhibition somehow breaks down, allowing the different senses to get jumbled.
Whatever its underlying mechanism, synaesthesia must make reality a tad more interesting, like an everyday fantasia. Some synaesthetes say that giving it up would be like seeing the world in black and white. As for us normal, sense-segregated types, we must take our Nabokovian experiences where we can find them. When I am placed on hold, I have noticed, certain kinds of piped-in music cause me to see red and feel a headache. n
