Humans have five distinct senses: sight, sound, touch, taste, and smell. For decades, researchers have thought that each sense is processed separately in the cortex, or outer layers of the brain, and then later integrated by separate brain areas. Cortical tissue near the back of the brain holds cells sensitive to vision, for example, while the area above the ear reacts to sounds. But recently, researchers have begun to question this so-called “uni-modal” model of sensory processing, suggesting instead that cortical regions respond to and integrate information from several senses at once.

In Roush, Ro saw an opportunity to study an extreme case of sensual mixing. Synesthesia is not well understood, but the condition has long been reported by creative types. The abstract painter Wassily Kandinsky, for example, saw “wild, almost crazy lines” sketched in his mind while listening to an opera, and claimed that as a child, he heard a hissing noise when mixing paint colors. Novelist Vladimir Nabokov saw the V in his name as “pale, transparent pink” and the N as a “greyish-yellowish oatmeal color.” Physicist Richard Feynman described the hues of equations. Psychologists dismissed these claims as hallucinations until the 1990s, when brain-scanning experiments on synesthetes confirmed that their sensory brain circuits take inputs from one sense and interpret them as coming from another.