The contribution of white and grey matter differences to developmental dyslexia – Insights from VBM and DTI at 3.0 Tesla

Developmental dyslexia or specific reading disability is a developmental disorder with a biological origin attributable to specific disorders in brain function. Over the last few years, important insights on the neuroanatomical basis of developmental dyslexia have been gained via new structural brain imaging techniques. In the current study, diffusion tensor imaging (DTI) and voxel-based morphometry (VBM) were used to investigate neuroanatomical correlates of dyslexia in German adults. Eight dyslexic adults and healthy controls participated in the study. DTI revealed decreased fractional anisotropy in bilateral fronto-temporal as well as left temporo-parietal white matter in dyslexic participants. In both dyslexics and controls, white matter fibers were predominantly oriented in the anterior-posterior direction, but this orientation was less distinct in dyslexics. Correlational analyses revealed associations between white matter anisotropy and speed of pseudoword reading in both groups. Grey matter density (as measured by VBM) was reduced bilaterally in the superior temporal gyrus in dyslexics.

So far, our results support the notion that dyslexia is characterized by perturbed connectivity between anterior and posterior brain areas. These brain areas are involved in phonological and orthographic aspects of the reading process, as well as in the integration of phonemes and graphemes (transformation process from graphemes into a phonological code). Therefore, our results based on a combined analysis of white and gray matter abnormalities provide exceedingly strong evidence for a disconnection syndrome or dysfunction of cortical areas relevant for reading and spelling. We suggest that this imbalance of neuronal communication between the respective brain areas might be the crucial point for the progression of dyslexia.