Synesthesia, a condition characterized by one sensory experience generating another so that shapes have tastes, for instance is estimated to affect between 1 in 200 to 1 in 2,000 people. The most common form involves seeing specific letters or numbers (graphemes) in specific colors. For these individuals, known as grapheme-color synesthetes, an ordinary "5," in black ink on a white background, always appears red or a "k," greenish-blue.
According to research published in the March 24 issue of Neuron, not only do these grapheme-color synesthetes really see the colors they report, as measured in behavioral tests, but functional magnetic resonance imaging (fMRI) of their brains also shows activation in the color-selective regions of the cortex when they view black-and-white letters or numbers.
The results, say researchers from the University of California, San Diego and the Salk Institute for Biological Studies, lend support to the hypothesis that cross-activation of adjacent brain regions is the mechanism underlying synesthesia.
"We specifically designed our experiment to test the cross-activation hypothesis we initially advanced in 2001," said V.S. Ramachandran, a coauthor of the study and director of the Center for Brain and Cognition at UC San Diego. "The fMRI findings quite clearly demonstrate cross-activation in this case between the number/letter region and color region of the fusiform gyrus in grapheme-color synesthetes."
When control subjects viewed numbers or letters, fMRI scans showed increased activity (increased blood-flow) only in the grapheme-selective regions of their brains, said Edward Hubbard, former UC San Diego graduate student and first author of the paper. Meanwhile, the hV4 area, a part of the brain network sensitive to and specialized for color perception, did
Contact: Inga Kiderra
University of California - San Diego