Like victims of catastrophic head injuries, people with synesthesia often appear in neuroscience papers identified only by their initials to illustrate the mysteries of the brain. But synesthesia's not a freak occurrence. It's estimated that 2-4% of people have abnormal connections between their senses. The condition may not be an accident at all, but a trait that evolution has retained for a reason.
The authors of a new review paper, David Brang and V. S. Ramachandran, ask why synesthesia has survived. Since it runs in families, synesthesia seems to be partially genetic. But it appears in many different forms--more than 60 have been documented. Garden-variety synesthetes see numbers, letters, or sounds in specific colors. Less commonly, synesthetes may experience each day of the week as a certain point in space, or feel touches on their own bodies when seeing another person touched. Many genes may be involved, and the interaction between synesthesia genes and a person's environment might lead to all kinds of outcomes.
It's possible that the genes promoting synesthesia have been kept around by evolution because they have a "hidden agenda." Another such trait, Brang and Ramachandran say, is sickle-cell anemia, which in addition to its unhelpful medical effects grants protection against malaria. Aside from the obvious sensory quirks, do synesthetes have a sneaky superpower?
