Red-letter day for brain connectivity

People with grapheme-colour synesthesia - who see a cascade of colours associated with individual letters when looking at a page of text - appear to have more neural connections in areas of the brain involved in word processing and binding perceptions together, a new study shows.

The study, 'Increased structural connectivity in grapheme-color synesthesia', was published in the June issue of Nature Neuroscience.

Romke Rouw and Steven Scholte from the University of Amsterdam in the Netherlands used diffusion tensor imaging (DTI) to look at brain differences between grapheme-colour synesthetes and healthy controls without the condition.

DTI allows non-invasive visualisation of the white matter tracts, or axons, connecting neural cell bodies. The researchers found that synesthetes had more axons connecting three brain areas: the right fusiform gyrus, near regions involved in word and colour 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 colours. Some synesthetes, known as projectors, report stronger experiences that are projected into the external world, while others, known as associators, report weaker experiences that appear in their 'mind's eye'.

The degree of structural connectivity in the right temporal cortex was correlated with the strength of the synesthetic experience.

These results suggest a two-stage model of synesthesia, with increased connectivity in some areas being important for generating perceptual binding, and connectivity in other areas determining the intensity of the resultant perceptions.

Nature Neuroscience, DOI: 10.1038/nn1906

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