Maturation of interhemispheric signal propagation in autism spectrum disorder and typically developing controls: a TMS-EEG study.

Brain maturation from childhood to adulthood is associated with changes in structural and functional connectivity between remote brain regions. Altered connectivity plays an important role in the pathology of autism spectrum disorder (ASD), a severe neurodevelopmental disorder. ASD is associated with abnormal brain development and structurally altered interhemispheric connections. Cortico-cortical connectivity can be studied by a combination of transcranial magnetic stimulation (TMS) with concurrent EEG (TMS-EEG). TMS-evoked Interhemispheric Signal Propagation (ISP) is a correlate of interhemispheric connectivity related to the microstructure of the corpus callosum (CC). We used TMS-EEG to measure ISP in 22 ASD subjects (10-21 years) and 22 typically developing control subjects (9-19 years). We expected (1) maturational changes of ISP from childhood to young adulthood and also (2) reduced interhemispheric signal transfer in ASD. ISP was positively correlated with age in both ASD and typically developing control subjects. No difference in ISP between ASD and typically developing controls was found. Our findings demonstrate maturation of effective interhemispheric connectivity during adolescence. As ISP is related to the microstructure of the CC, the developmental change of ISP likely reflects maturation of the CC during the second life decade. The results support ISP as a valid parameter reflecting functional interhemispheric connectivity. Our results do not support a global deficit of interhemispheric connectivity in ASD.