Developmental dyslexia: atypical cortical asymmetries and functional significance

Using brain magnetic resonance imaging, we measured in 16 young developmental dyslexic adults and 14 age-matched controls cortical asymmetries of posterior language-related areas, including Planum temporale and parietal operculum cortical ribbon, and of the inferior frontal region related in the left hemisphere to speech processing. In addition, we assessed the sulcal morphology of the inferior frontal gyrus in both groups according to a qualitative method. The dyslexic subjects also performed specific tasks exploring different aspects of phonological and lexical-semantic processes. Results showed that: (1) contrary to most results reported in the literature, there is a lack of any morphological difference relative to Planum temporale asymmetry between the two groups; (2) there are significant differences between dyslexic and control subjects relative to frontal areas; (3) concerning the parietal region, there is a more asymmetrical pattern towards the left side in dyslexic subjects than in controls; and (4) relations in dyslexic subjects between parietal asymmetry coefficient and the level of performance in a phonological task have a particular reliance on verbal short-term memory, frontal asymmetry and performance in a non-word reading task. Considering these results it is suggested that phonological segmentation skills may relate to frontal lobe morphology, while phonological memory-based impairment in people with dyslexia may rather relate to parietal lobe morphology.