Synesthesia is a neurological condition characterised by the stimulation of one sense causing an involuntary reaction from another sense. Although its study went out of fashion in the 19th century, the last decade or so has seen a resurgence in the study of its genetics, how it works, and how its study might be applied to other areas of science.
Synesthesia is most commonly exemplified by letters perceived as having distinct, inherent colors — grapheme-color synesthesia — although the form it can take is limited, in theory, only by the number of possible combinations of different senses. Vladimir Nabokov, as well as his wife and child, famously “suffered” from this type of synesthesia, which he described in an interview in 1962:
V is a kind of pale, transparent pink: I think it’s called, technically, quartz pink: this is one of the closest colors that I can connect with the V. And the N, on the other hand, is a greyish-yellowish oatmeal color. But a funny thing happens: my wife has this gift of seeing letters in color, too, but her colors are completely different. There are, perhaps, two or three letters where we coincide, but otherwise the colors are quite different.
It turned out, we discovered one day, that my son, who was a little boy at the time — I think he was 10 or 11 — sees letters in colors, too. Quite naturally he would say, “Oh, this isn’t that color, this is this color,” and so on. Then we asked him to list his colors and we discovered that in one case, one letter which he sees as purple, or perhaps mauve, is pink to me and blue to my wife. This is the letter M. So the combination of pink and blue makes lilac in his case. Which is as if genes were painting in aquarelle.
The earliest scientific study found that grapheme-color synesthetes had persistent, stable visualizations of letter colors, but did not share them with other synesthetes, who might see particular letters and symbols in a different color. For a long time, the research stopped here as cognitivism went out of fashion and behaviorism rose in popularity, but more recent research proving it was a genuine phenomenon has resulted in a resurgence in its popularity.
For some time it was believed that synaesthesia was not a “real” psychological condition and that subjects were merely confabulating - making up - the entire experience, or that they were speaking metaphorically (e.g. “She has a very sharp voice.”). Prof. Simon Baron-Cohen’s “gold standard” test changed this markedly. Baron-Cohen’s test was simple: he recorded subjects’ grapheme-colour associations, and then tested them using the same lists several months or years later. Synaesthetes performed significantly better than control subjects, showing that they could not be confabulating.
A later test by Ramachandran further established the legitimacy of synaesthesia. While Baron-Cohen’s test illustrated the stability of grapheme-colour associations, it did not show that they were necessarily perceptual. In other words, Baron-Cohen’s synaesthetes could have had a photographic memory and simply memorised the associations between the graphemes and colours without actually experiencing them perceptually. Ramachandran’s test employed “pop out”, a phenomenon well known to psychologists. This phenomenon can be easily demonstrated by looking at a field of identical characters and asking subjects to pick out the characters that are anomalous, for example, by dint of having a different colour or shape to the rest. The anomalous characters appear to “pop out” to the subject, allowing for near instantaneous identification.
The cause of synesthesia is still subject to research, but it’s generally believed to be the result of a genetic mutation on the X chromosome, explaining its dominance in woman and high heritability. Some researchers think its heritability could suggest an evolutionary benefit. Sickle cell anemia, for example, can be deadly, but also provides malaria immunity. Does synesthesia provide a similar benefit?
It might if you’re a mathmetician or an artist. One of the peculiarities of some forms of synesthesia is that equations are visualised in 3D space, which might help someone like physicist Richard Feynmann, another famous synesthete, with his work. David Hockney, also a synesthete, once told Robert Burton that when he was designing a piece of art intended to accompany a production of a Maurice Ravel piece, he listened to the relevant section of the score and “the tree painted itself.” It’s also been suggested that savants like Daniel Tammett get their incredible skills from a combination of autism and synesthesia, using it to help visualise complex memory recall tasks like recounting pi, or performing absurd mental arithmetic.
This similarity in function is driving much of the current research into synesthesia. Recent neuroimaging advances enable visualization of synesthetes’ brains and are helping scientists see the brain as a bundle interwoven connections, rather than discrete areas serving a single purpose. It’s hoped that this kind of research might be applicable to less understood disorders like schizophrenia and Alzheimer’s, that are also caused by abnormal brain function.
"We’re trying to understand how a different activity pattern in your brain can change the way you perceive reality," says Tomson, pointing out that studying disorders such as depression or schizophrenia in people who already have the disorder can be tricky. Not only are the network properties of these illnesses more complex than the relatively simple circuitry involved with synesthesia, but patients are often on medication, which makes it impossible to tell how their brains would function on their own.
"Synesthesia is a perfect model because we have a healthy brain that has some kind of perceptual tweak that changes the relationship between various regions of the brain," she says
A recent paper by David Eagleman — best known for his work on time perception, but also responsible for a great deal of study into synesthesia — argues synsthesia falls on a spectrum (like autism) as the product of multiple neuronal processes gone awry, causing unusual cognitive pairings. Other studies suggest most people have the capacity for this “sensory crosstalk,” especially when we’re younger and our neural networks have yet to streamline. One study by V.S. Ramachandran helped a young man with Asperger’s communicate his emotions by instructing him to assign a color to them. Eventually, he was able to gauge his feelings about a person by the color of their “halo.”