Motor resonance and inhibitory mechanisms in action observation as revealed by corticospinal excitability.

Motor resonance (MoR) refers to the automatic activation of motor circuits during action observation, reflecting an internal simulation of the observed movement. This phenomenon is thought to arise from the activity of mirror neuron regions, which modulate primary motor cortex (M1) excitability via cortico-cortical pathways. MoR, which is believed to be involved in the mechanism underlying action understanding and motor learning, has been widely studied using visual stimuli and transcranial magnetic stimulation (TMS). However, the optimal form of movement presentation and TMS timing remains unclear. This study compared the effects of static photographs and videos on the MoR activation and explored the ideal timing for TMS. Participants observed abduction movements of the index finger (controlled by the first dorsal interosseous muscle, FDI) and the little finger (controlled by the abductor digiti minimi muscle, ADM) presented as photographs or videos. The task included three conditions: Photo (static images), Video (full movement videos), and Postvideo (post-movement period). TMS was applied over the primary motor cortex at 0, 320, or 640 ms from movement onset (Photo, Video) and at the same intervals from movement offset (Postvideo). Motor evoked potentials (MEPs) were recorded from FDI and ADM. The Postvideo condition yielded the strongest MEP modulations, with inhibition in the non-matching muscle and excitation in the muscle corresponding to the observed movement. In contrast, Photo and Video conditions showed time-dependent reductions in cortical excitability, especially in non-matching muscles. These findings suggest that applying TMS after movement observation provides a more accurate approach to studying MoR and highlights the role of motor surround inhibition in motor control.