The title of Vladimir Nabokov’s novel Ada carries a secret code. Nabokov was a synaesthete, who experienced colours when looking at plain letters of the alphabet. He saw the letter A as yellow and D as black. Nabokov was also a lepidopterist who wrote about butterflies, and the yellow-black-yellow sequence he saw in the word “ADA” mimicked the appearance of the yellow swallowtail.
“Synaesthesia” means “joined sensation”, in which an item of one kind elicits a sensation of a different kind. Letters are normally black on white, but the letter-colour synesthete nevertheless always attaches the same colour to a particular letter, and this colour can usually be located precisely on a colour chart. Synaesthetes are slower to name the actual colour of a letter if it does not match the elicited colour.
By far the most common forms of synaesthesia are colour sensations elicited by letters and digits, or by days of the week. Although colours seem to be the most frequently reported sensations, other elicited sensations include smells, touches, tastes, sounds or temperatures. Similarly, synaesthesia can be elicited by many different kinds of input. For instance, colours have been linked to smells, sounds, tastes—and even orgasm. Curiously, synaesthesia very seldom works both ways. People who associate colours with letters do not see letters when shown colours, and those who have coloured orgasms mercifully do not, as far as I know, have orgasms induced by colours.
In 1880, the Victorian scientist Sir Francis Galton estimated that about one in 20 people was a synaesthete. A more recent estimate is one in 23 for any type of synaesthesia and one in 90 for those who experience colours induced by letters and digits. One difficulty, however, is to determine exactly who is a synaesthete and who is not. It’s partly a matter of degree. For me, Tuesday does have a yellowish-brown tinge, but I don’t think I am a true synaesthete. Synaesthesia is also distinguished from metaphor, as in purple prose or hot jazz. Nor is it a matter of simple association, as in linking yellow with “banana” or even orange with “orange”. Synaesthesia is not simply the result of a vivid imagination either. Rather, synaesthetic experience is simple, immediate, and automatic, with no pictorial elaboration.
Galton observed that synaesthesia tends to run in families, suggesting a genetic influence. This is confirmed in more recent studies, but there are also cases of synaesthesia in just one member of an identical twin pair, implying that a synaesthesia gene, if such exists, is not always expressed. Nabokov’s mother Vera was also a synaesthete, as is his son Dmitri. In Vera, though, colours were induced not only by words but also by musical sounds, suggesting that inherited synaesthesia can take different forms between generations.
In the past, synaesthesia was not taken seriously, because of the suspicion that it could be easily faked. It gained scientific respectability, though, when brain imaging showed that areas of the brain corresponding to the synaesthetic sensation were activated. For example, in word-colour synaesthetes the area of the brain responsible for colour processing is activated by words alone. The same area is activated by actual colours, but not when people are asked to imagine colours. We therefore now have reason to believe that synaesthesia is indeed real, albeit restricted to a small percentage of the population.
Brain imaging suggests that synaesthesia is due to connections between brain areas, but these connections don’t seem to be the result of learning. (The colours attached to letters are not those of the letters shown on Sesame Street or of those attached to the fridge for young children learning to read.) One suggestion is that synaesthesia is a by-product of the way connections are formed in the developing brain.
Newborn babies actually have too many connections in the brain, and these are pruned back in the course of development. Synaesthesia may therefore be the result of incomplete pruning, like a garden whose shoots were not properly pruned back in the spring. It has even been suggested that all newborns are synaesthetes, but most lose it at around 3 three months. This does not explain why synaesthesia so often involves printed words or letters, which are not acquired until after the age of three. Nevertheless the brain area responsible for processing colour is next to that responsible for identifying words or letters, suggesting the relatively late emergence of reading skills may have invaded an incompletely pruned area already partly dedicated to colour perception.
Synaesthesia, though, is not an infirmity. Besides Nabokov, a number of creative individuals, including Richard Feynman, David Hockney and Jean Sibelius, have been synaesthetes, and one study has shown synaesthetes to have higher average intelligence than non-synaesthetes.