Interhemispheric inhibition is dynamically regulated during action observation.

It is now well established that the motor system plays a pivotal role in action observation and that the neurophysiological processes underlying perception and action overlaps. However, while various experiments have shown a specific facilitation of the contralateral motor cortex during action observation, no information is available concerning the dynamics of interhemispheric interactions. The aim of the present study was, therefore, to assess interhemispheric inhibition during the observation of others' actions. We designed a transcranial magnetic stimulation (TMS) experiment in which we measured both corticospinal excitability and interhemispheric inhibition, this latter by means of the ipsilateral silent period (iSP), while participants observed two motor tasks (tapping or grasping). We show that the iSP is enhanced during movement observation and that this modulation is tuned to the kinematics of the observed movements. An additional experiment was performed in which the TMS intensity was adjusted to match corticospinal excitability between rest and action observation. This resulted in a relative decrease of iSP. Overall, our data strongly suggest that action observation, as action execution, involves interhemispheric inhibitory mechanisms between the two motor cortices, and that this neural activity appears to be firmly shaped by the ongoing observed movement and its intrinsic dynamics.