White matter microstructure throughout the brain correlates with visual imagery in grapheme-color synesthesia

In this study we show, for the first time, a correlation between the neuroanatomy of the synesthetic brain and a metric that measures behavior not exclusive to the synesthetic experience. Grapheme-color synesthetes (n=20), who experience colors triggered by viewing or thinking of specific letters or numbers, showed altered white matter microstructure, as measured using diffusion tensor imaging, compared with carefully matched non-synesthetic controls. Synesthetes had lower fractional anisotropy and higher perpendicular diffusivity when compared to non-synesthetic controls. An analysis of the mode of anisotropy suggested that these differences were likely due to the presence of more crossing pathways in the brains of synesthetes. Additionally, these differences in white matter microstructure correlated negatively, and only for synesthetes, with a measure of the vividness of their visual imagery. Synesthetes who reported the most vivid visual imagery had the lowest fractional anisotropy and highest perpendicular diffusivity. We conclude that synesthetes as a population vary along a continuum while showing categorical differences in neuroanatomy and behavior compared to non-synesthetes.