No atypical white-matter structures in grapheme- or color-sensitive areas in synesthetes

Grapheme-color synesthetes experience colors when presented with written language characters. In this study diffusion-weighted imaging was used to investigate white matter alterations in color-and grapheme-processing brain areas in synesthetes as a possible factor for the color sensations. Regions of interest were defined by means of neuroanatomical atlantes, functional localizer tasks and retinotopic mapping. None of the regions showed differences in white matter structure between synesthetes and a control population, as revealed by fractional anisotropy and mean diffusivity measures. Quite the contrary, the data broadly supported the null hypothesis of no group differences in white matter microstructure. This finding is in line with recent studies suggesting no atypical neuroanatomy in grapheme-color synesthetes.