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Mixed-up Senses—Synesthesia

"The song was full of glittering orange diamonds." "The paint smelled blue." "The sunset was salty." "The pickle tasted like a rectangle."

To 1 in 500,000 people with a condition called synesthesia, sensation and perception mix, so that the brain perceives a stimulus to one sense as coming from another. Most commonly, letters, numbers, or periods of time evoke specific colors. These associations are involuntary, are very specific, and persist over a lifetime. For example, a person might report that 3 is always mustard yellow, or Thursday brown.

We do not know what causes synesthesia, although it does seem to be inherited and more common in women. One of the authors of this book (R. L.) has it—she has always perceived days of the week and months as specific colors. People have reported the condition to psychologists and physicians for at least 200 years. Synesthe-sia has been attributed to an immature nervous system that cannot sort out sensory stimuli or altered brain circuitry that routes stimuli to the wrong part of the cerebral cortex.

PET (position emission tomography) scanning reveals a physical basis to synesthesia. Brain scans of six nonsynesthetes were compared with those of six synesthetes who reported associating words with colors. Cortical blood flow was monitored while a list of words was read aloud to both groups. Interestingly, cortical blood flow was greatly elevated in the synesthetes compared with the non-synesthetes. Furthermore, while blood flow was increased in word-processing areas for both groups, the scans revealed that areas important in vision and color processing were also lit up in those with synesthesia. ■

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