When Sean Day hears the wail of a saxophone, he sees a writhing mass of neon-purple snakes hovering in the air. The hum of a harmonica, on the other hand, has a pleasantly greenish hue, while plunking pianos evoke a fine blue mist. Eating is colorful too. When Day takes a spoonful of mango sorbet, the wall before him turns lime green, rippled with cherry-red stripes.
Day is no human kaleidoscope. Nor is the Taiwan-based linguist afflicted with anything you would call a disorder. He's a synesthete--one of a small group of otherwise ordinary citizens who perceive the world in extraordinary ways. Synesthesia is a kind of crossing of sensory signals in which the stimulation of one sense evokes another; purple may smell like kiwi; the aroma of mint may feel like glass; letters and digits might scroll by in Technicolor. This week a few dozen synesthetes and the psychologists who study them will gather at Princeton University for the first meeting of the American Synesthesia Association--a society devoted to furthering research into the phenomenon in the hopes that it will reveal something about the inner workings of the human mind.
Although the society is new, physicians and scholars have known about the condition for centuries. History, in fact, teems with brilliant synesthetes--including such luminaries as novelist Vladimir Nabokov, composer Franz Liszt and physicist Richard Feynman. Synesthesia enjoyed a certain spiritual currency in the late 19th century, especially among the European avant-garde. Many artists, most notably abstract painter Wassily Kandinsky, were famed for their synesthetic pretensions. "I saw all my colors," wrote Kandinsky, recalling his experience of a Wagner opera. "Wild lines verging on the insane formed drawings before my very eyes."
Scientists have only recently begun to unravel the mysteries of synesthesia. They estimate that roughly 1 in 2,000 people has the condition and that there are nearly as many types of synesthesia as there are permutations of the senses. While synesthetic responses are usually as unique as fingerprints, the condition runs in families. Nabokov, for example, for whom the letter b evoked the color burnt sienna, and t, pistachio green, often argued with his equally synesthetic mother about the true colors of the alphabet.
Such revelations are not surprising. When synesthetes are asked to link letters and words to their corresponding hues, the responses tend to be precise. (The n Feynman read in his physics equations wasn't just violet; it was "mildly violet-bluish.") Tested a year later, synesthetes report the same colors 9 times out of 10; control groups repeat them just a third of the time.
Brain imaging shows that this consistency has a physical basis. Words activate not only the language centers of the synesthete's brain--as they would in anybody else's--but also the vision- and color-processing centers. What remains open to debate, however, is how intimately the synesthetic response is tied to consciousness.