Developmental dyslexia: atypical cortical asymmetries and functional significance

Using brain magnetic resonance imaging, we measured in 16 young development al dyslexic adults and 14 age-matched controls cortical asymmetries of post erior language-related areas, including Plenum temporale and parietal operc ulum cortical ribbon, and of the inferior frontal region related in the lef t hemisphere to speech processing. In addition, we assessed the sulcal morp hology of the inferior frontal gyrus in both groups according to a qualitat ive method. The dyslexic subjects also performed specific tasks exploring d ifferent aspects of phonological and lexical-semantic processes. Results sh owed that: (1) contrary to most results reported in the literature, there i s a lack of any morphological difference relative to Planum temporale asymm etry between the two groups; (2) there are significant differences between dyslexic and control subjects relative to frontal areas; (3) concerning the parietal region, then is a mon asymmetrical pattern towards the left side in dyslexic subjects than in controls; and (4) relations in dyslexic subjec ts 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 t hese results it is suggested that phonological segmentation skills may rela te to frontal lobe morphology, while phonological memory-based impairment i n people with dyslexia may rather relate to parietal lobe morphology.