Synesthesia Smells Like Evolution: The Benefit of Tasting Colors

It sounds like bad dialogue from a teen drug-abuse PSA, but two to four percent of the world’s population can hear colors, taste words and the like. On the surface, synesthesia sounds alternately annoying and psychedelic, just another curiosity. But it’s a pretty common occurrence; more common, even, than real redheads. Why have humans developed the ability? And why has something so seemingly useless not been eliminated by the paring process of evolution?

Synesthesia is essentially a perceptual quirk in which one type of stimulus (sound, taste, vision, etc.) also sparks sensations of another type. So a synesthete may look at letters printed in black ink and see them in different colors, or be able to taste a song. The different types of synesthesia have been well-studied, but one question remains: How has something so strange, and yet relatively, persisted within the human population, and what purpose does it serve?

In a new paper published in PLoS Biology, David Brang and V.S. Ramachandran of UC San Diego explore how synesthesia may be conserved in the human population. One key would be for synesthesia to be a hereditary genetic trait, rather than an abnormality or mutation. While the actual genetic basis for the phenomenon has yet to be proven, it does typically run in families. According to the paper, 40 percent of synesthetes have a direct relative with the same condition. If synesthesia is a genetic condition, and not a random mutation, then it would be passed on and persist through generations.

There have been at least 60 different forms of synesthesia documented. Yet, even though synesthesia tends to occur within families, the forms in which it is expressed varies. This suggests that a single gene doesn’t exist for synesthesia. Instead, it’s likely a set of genes that induce synesthesia. The set would then be hereditary, but because they have more variables, wouldn’t necessarily mean that the expression of the phenomenon would be the same from a mother to a daughter.

But if synesthesia has a genetic basis, and it seems likely that is does have a complicated one, it would then be susceptible to the proverbial scalpel of natural selection. For something to persist in our genome, it would have to have a beneficial effect, however elusive it may be (like the sickle cell anemia gene’s correlating resistance to malaria). Everything else gets axed. What, then is the positive value of synesthesia?

One theory posed is that synesthesia has a beneficial effect on individual’s creativity and ability to understand metaphor. Brang, however, argues that synesthesia and creativity seem to be different things. Synesthesia is a condition of arbitrarily linking unrelated things while creativity (and in particularly metaphor) tends to have some sort of demonstrable causality. As he writes, "While the link between synesthesia and creativity has received remarkable interest over the last decade, research has not yet directly demonstrated any causal relationship between the two and so the argument, at this point, remains seductive and compelling but not conclusive."

Instead, Brang points to more recent research that suggests that the psychedelic effects of synesthesia actually have a benefit in terms of sensory processing and, in particular, memory. As examples, he notes a pair of remarkable synesthetic savants, including Daniel Tammet who has memorized pi to 22,514 digits. Additionally, it seems that some synesthetes are superior than regular folk at distinguishing between colors, both in low-light and low-contrast situations. Individuals who experience auditory synesthesia are also more able to process rhythmic visual stimuli.

Unlike the vagaries of defining creativity, having increased cognitive abilities does has a direct biological benefit, whether it’s being able to better identify the difference between edible and poisonous berries in the wild or having an increased ability to notice trends in the stock market. A number of questions remain (including the effect of LSD on synesthetes’ cognitive abilities) that Brang thoughtfully notes. But the research stands to highlight the fact that even supposedly “fringe” phenomena like synesthesia offer valuable information about the pressures that shaped the development of our minds.