Strengthening interhemispheric connectivity increases interregional information transfer between human primary motor areas

Communication between brain regions is thought to be supported by the temporal alignment of their oscillatory activity creating rhythmic windows of opportunity for efficient information transfer. We examined this possibility by manipulating the strength of coupling between the left primary motor cortex (lM1) and the right primary motor cortex (rM1) and examined the impact on activity transfer between the two areas measurable in the electroencephalogram (EEG) in 60 right-handed individuals. In three experiments we manipulated the strength of the pathway connecting the lM1 and rM1 by applying paired associative stimulation (ccPAS) using three different patterns, two of which increase interregional coupling strength. We measured the impact on EEG communication transfer in the alpha, beta, and theta bands at rest. Augmenting the strength of the lM1-to-rM1 pathway enhanced the interregional communication volume in the high-theta/low alpha band, and both volume and speed in the beta band, respectively. These changes, visible at rest, are related to changes in the transcallosal inhibitory influence of the lM1 over rM1 cortical excitability following ccPAS. By contrast, augmenting the rM1-to-lM1 route failed to modulate communication transfer in right-handed individuals. The results provide direct evidence of oscillatory intra-areal information transfer, demonstrating a link between motor network physiology and the resonant frequencies mediating its interactions.