Some previous studies have demonstrated atypical brain lateralization in autism spectrum disorder (ASD). However, most of these reports have focused on language-related asymmetries in adults, and the developmental trajectory of hemispheric asymmetries in the important phase that occurs at 2-5 years of age remains unclear. Thus, we used structural magnetic resonance imaging and diffusion tensor imaging in a longitudinal study of grey matter asymmetries across all cortical parcellation units and white matter lateralization across the white matter skeleton using voxel-based morphometry and tract-based spatial statistics in 34 toddlers with ASD and a matched group of 26 toddlers with developmental delay (DD) at 2-3 years old and with follow-up at 4-5 years of age. We found the total brain volume and fractional anisotropy (FA) of white matter was higher in the ASD group than in the DD group at baseline and 2 years later. The ASD and DD groups showed a rightward asymmetry in a large number of cortical parcellation units and in the white matter skeleton at both time points. Grey matter lateralization was associated with the social and communicative disturbances observed in ASD at baseline, while white matter asymmetry was significantly related to social disturbances and repetitive behaviours seen at 4-5 years of age. In conclusion, both ASD and DD toddlers had widespread rightward asymmetry, and the patterns of lateralization were similar across the groups. GM and WM showed asynchronous development of hemispheric asymmetries at 2-5 years of age, and this lateralization was associated with ASD symptoms.
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