The existence of synesthesia is at least an interesting bit of trivia, and at most the focus of numerous psychological, neurological, and population-based studies. I think many people have heard of it, if not by name then by its description. The rock/hip-hop group N.E.R.D. brought attention to the condition with their highly successful third album, Seeing Sounds. The album title was in fact inspired by synesthesia, which the band learned about while watching the Discovery Channel.

Letters and numbers as they might be seen by a synesthete.

But the most common type of synesthesia, though, is to see letters and numbers (collectively known as “graphemes”) as having particular colors (see left). A rare group of synesthetes tastes words. I was a sound –> color synesthete, triggered specifically by music (not environmental sounds). Some of the known associated traits of synesthesia — photographic memory, left-right confusion, and very little sense of direction — fit me to a T. Others — difficulty with arithmetic — don’t describe me at all. But, as I said, I hardly if ever experience synesthesia anymore, and the few experiences I do have are tainted by my awareness of the phenomenon (e.g. I’ll find myself thinking, “This song should be blue,” rather than just seeing it for what it is). It’s not unheard of for synesthesia to fade during puberty, probably because of all the changes occurring in the brain.

fMRI of grapheme (blue) and color areas (yellow)

So what is happening in the brains of synesthetes?  It could be as simple as cross-activation of specialized parts of the brain, like the part that recognizes graphemes and the part that processes color (they’re adjacent; see right). Some think that synesthete brains have failed to clear out some of the excess synapses that are formed in infancy, so brain areas that would normally be separated remain connected. Recent research suggests that synesthesia might just be at the tail end of a spectrum of normal perceptual processes.

In any case, about 1 in 2000 people have synesthesia, and the specific synesthetic experience can be extremely unique to each individual. While the condition is intriguing (and some people really wish they had it), I wouldn’t say there is any particular benefit to it, although the combination of senses can sometimes improve recall of images, numbers, words, etc. In fact, synesthetes often go through an awkward or uncomfortable moment of realization that their experience is not universal (imagine the look you’d get after announcing, “That song was so green!”).

Some would say that everyone is already a bit of synesthete. Why? Look at the figure below and, without scrolling any further, ask yourself which of these two things you would call “kiki” and which you would call “bouba.”

Funny enough, just about everyone who tries this calls the orange thing a kiki and the purple thing a bouba, even 2.5-year-olds. Apparently, there is some normal cross-talk between the processing of shapes and word sounds. For more on the “bouba/kiki effect,” click here.

So, are you a synesthete? Do you know one? It would be great to hear about your experiences with synesthesia in the comments.

To do your own research on synesthesia, check out these (hover for info):

Bien N, Ten Oever S, Goebel R, & Sack AT (2011). The sound of size Crossmodal binding in pitch-size synesthesia: A combined TMS, EEG and psychophysics study. NeuroImage PMID: 21787871

van Leeuwen TM, Petersson KM, & Hagoort P (2010). Synaesthetic colour in the brain: beyond colour areas. A functional magnetic resonance imaging study of synaesthetes and matched controls. PloS one, 5 (8) PMID: 20711467

Maurer D, Pathman T, & Mondloch CJ (2006). The shape of boubas: sound-shape correspondences in toddlers and adults. Developmental science, 9 (3), 316-22 PMID: 16669803