Insights into grapheme-colour synesthesia are presented in a paper in the June 2007 issue of Nature Neuroscience. People with this condition - who see a cascade of colours associated with individual letters when looking at a page of text - appear to have more neural connections at several locations in the brain.
Romke Rouw and Steven Scholte used a technique called diffusion tensor imaging (DTI) to look at brain differences between grapheme-colour synesthetes and healthy controls without this condition. DTI allows non-invasive visualisation of the white matter tracts, or axons, connecting neural cell bodies. The researchers found that synesthetes had increased connectivity at three locations in the brain: the right fusiform gyrus, near regions involved in word and color processing, and the left intraparietal sulcus and frontal cortex, both part of a network of regions involved in binding and consciousness.
The study also found differences among the synesthetes, according to how they perceived the association between words and colors. Some synesthetes, known as projectors, report experiences that are projected into the external world, while others, known as associators, report experiences that appear in their 'mind's eye'. The degree of structural connectivity in a region known as the right temporal cortex was correlated with the subjective location of the synesthetic experience.
Romke Rouw (University of Amsterdam, The Netherlands)
Additional contact for comment on paper:
Edward Hubbard (INSERM, Gif-Sur-Yvette, France)
Abstract available online.
(C) Nature Neuroscience press release.
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