Synesthesia is a remarkable blending of senses that, in most people, are separate and distinct. While those of us born without the trait may say metaphorically, "This wine tastes wonderfully dry" or "I sure feel blue today," the synesthete actually experiences such perceptions. For her or him (more likely the former – synesthesia is much more common among women), a taste can be round or pointy, a word can taste like potatoes, the sound of a violin can be felt on the face, a letter or number or even a smell can have its own vivid and recurring color.
Hypersensitivity is an oft-noted aspect of synesthesia. One woman, interviewed on NPR's Diane Rehm Show, puts it this way: "Like many synesthetes, I have a heightened appreciation for all kinds of sensory phenomena. I tend to get overloaded quickly: like there's just too much sensory perception coming in at one time, and I have a hard time sorting it out and coping with it. Shopping can do it. Being in a store where there's a lot of noise, colors, smells - it's just too much."
What causes synesthesia? Neuroscientists believe it results from extraordinarily dense connections in the brain – and, furthermore, that everyone has the capacity to be synesthetic. This latter view has evolved rather recently. The prevailing view used to be that each sense is processed separately in the cortex (the outer layer of the brain), with sensory information only coming together later. But this “uni-modal” model is now being questioned. It could be that cortical regions respond to and integrate information from several senses at once. Or, that our cortices are primed to process just one sense but information from other senses modulates or adjusts the signal from the primary sense.
As an example, consider what happens when someone suffers a stroke and, while one sense becomes impaired, another type of sensory information comes to the fore. The case of Sherrilyn Roush, a synesthete studied at Rice University, is particularly interesting. After a stroke, her skin became sensitive to sound. “My entire body rebels at certain pitches,” she relates, adding that, even in her quiet apartment, she’ll sometimes have to wear ear plugs in order to concentrate. Since robust connections between brain regions have been found in people who are not synesthetic as well as those who are, our senses appear fundamentally more interconnected that scientists ever knew. Someone like Roush doesn’t have to be born synesthetic (as most synesthetes are) – she can acquire it.
If this is the "how" of synesthesia, the "why" remains a mystery. An intriguing theory has been advanced by Daphne Maurer of McMaster University in Ontario, Canada. She suggests that all of us are born perceiving across sense modalities and that we learn to differentiate senses in reaction to the cognitive logjam we experience as babies. Looking at it the other way ‘round, Simon Baron-Cohen of Cambridge University pictures synesthesia as a breakdown in the normal maturation of the brain. The two views aren’t necessarily incompatible. If all of us are born synesthetes, cognitive maturation would imply that connections between brain regions be "pruned" during infancy. If this process is interrupted or impaired, the person would remain synesthetic.
Here we encounter the first of many intriguing correspondences between synesthesia, autism, savantism, and prodigiousness. Synesthesia has been found to occur in more than twice as many people with autism (18.9 percent) as in the general population (7.2 percent). And sensory hypersensitivity is common to both. I’ll explore the likely implications of this in my next post.