Synesthesia tied to brain connections

People who experience specific colors when looking at particular letters, such as seeing sky blue when shown an R, possess an unusual abundance of connections in brain areas involved in word and color perception, a new brain-imaging investigation finds.

The condition, one of several forms of sensory mixing collectively referred to as synesthesia, remains poorly understood. Romke Rouw and H. Steven Scholte of the University of Amsterdam studied 18 women who reported color-letter synesthesia and 18 women who cited no synesthesia.

One imaging technique measured the number of nerve fibers that link cells in various brain areas. Another technique assessed blood-flow changes, a marker of neural activity, as participants with synesthesia viewed letters that evoked colors and as women without synesthesia looked at the same letters.

The volunteers with synesthesia displayed an excess number of connections and showed unusually pronounced activity in three brain areas, the researchers report in the June Nature Neuroscience. These regions are known to influence word and color perception as well as sensory integration and conscious thought.