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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.