Slide Slam K15
Whole brain grey and white matter structural abnormalities in dyslexia
Aleksi Sihvonen1,2, Paula Virtala1, Anja Thiede1, Peter Palo-oja1, Jussi Numminen3, Marja Laasonen1,3,4, Teija Kujala1; 1University of Helsinki, 2The University of Queensland, 3Helsinki University Hospital, 4University of Eastern Finland
Developmental dyslexia is a highly prevalent neurodevelopmental disorder negatively influencing the individual’s learning and career outcomes. Studies on its neuroanatomical origins have continued for half a century without consistent results, the most consistent finding being lowered total brain volume. Here, we adopted a whole brain approach and set out to evaluate both the grey matter and white matter structural aberrations in adult dyslexic participants. We used voxel-based (VBM) and surface-based morphometry (SBM) as well as quantitative isotropy aided differential tractography coupled with comprehensive neuropsychological evaluation in a cross-sectional study of 45 adults (23 dyslexic and 22 typically reading participants). In the VBM and SBM analyses, we found both decreased grey matter volume and cortical thickness in the left insula in participants with dyslexia. Moreover, they had decreased grey matter volume in left superior temporal gyrus, putamen, globus pallidus, and parahippocampal gyrus. Higher grey matter volumes and cortical thickness in these areas correlated with better reading and phonological skills, deficits of which are pivotal to dyslexia. Crucially, the total brain volume did not differ between the dyslexic and typically reading groups, suggesting that dyslexia does not (only) rely on brain volume reduction as a predisposing factor or as a de rigueur developmental consequence. Differential tractography analyses revealed structural white matter anomalies in dyslexics in the left ventral route and bilaterally in the dorsal route compared to the controls. Connectivity deficits were also observed in the corpus callosum, forceps major, vertical occipital fasciculus and corticostriatal and thalamic pathways. Altered structural connectivity in the observed differential tractography results correlated with reading skills and phonological processing. Using a whole brain structural and hodological approach, the current study provides novel evidence for the extent of the grey matter and structural connectome aberrations in dyslexia. The results conform current functional neuroanatomical models of reading and dyslexia, but also provide novel network-level and tract-level evidence on structural connectivity anomalies in dyslexia, including the vertical occipital fasciculus. Our results also support the current theories on dyslexia suggesting that it is primarily based on phonological deficits but also associated with significant implicit learning deficits, by associating structural anomalies in dyslexia with neural architectures subserving phonological processing and implicit learning.