Ah, the Blue Smell of It!

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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.

Recent studies of synesthetes who see colors in response to numbers and letters provide conflicting answers. Last year Mike Dixon of the University of Waterloo reported that merely imagining a number was enough to provoke synesthesia. And in March Australian researcher Jason Mattingley reported in Nature that the conscious recognition of a number is crucial to the generation of color. "You have to be aware of the meaning of what you see to experience synesthesia," says Dixon.

Not so, say Vilayanur Ramachandran and Edward Hubbard of the University of California at San Diego. Their studies take advantage of a perceptual quirk: when an image in the periphery of our visual field is surrounded by similarly shaped and colored images, the brain has trouble registering its presence--even though the eye picks it up. They reported at a meeting of the Vision Sciences Society in Sarasota, Fla., last week that even when synesthetes can't "see" a peripheral image--say a 5 that's "crowded" by 3s--they see the color associated with the digit in question. That suggests that synesthesia occurs in the earliest stages of perception--before the brain ascribes meaning to what the eye reports.

What does all this tell us about the mind? "What you're seeing here is a window into thought itself," says Ramachandran, who is slated to speak at the Princeton meeting. "It also gives us an experimental handle to investigate the neural basis of more elusive phenomena like metaphor." It's a fair bet, he argues, that synesthesia is caused by genetic mutations that create dense neural connections between areas of the brain that process sensory information. Ramachandran hypothesizes that in normal brains, a handful of these links might play a role in the formulation of metaphors, which often blend sensory elements of language (consider "sharp cheese" or "bitter cold"). That, he says, may explain why synesthesia is far more common among novelists, painters and poets than in the general population. And, perhaps, why the rest of us, who don't experience the world as synesthetes do, can still take pleasure in their visions.