Enduring suppression of corticospinal excitability when observing pictures of hand actions

The presentation of bodily actions is known to affect the motor system activity in the observer’s brain. Recently, we showed corticospinal inhibition of First Dorsal Interosseus (FDI) muscle when observers were presented with pictures displaying the hand in the act of using common tools. In right FDI, Motor Evoked Potentials (MEPs) elicited by single pulse Transcranial Magnetic Stimulation (TMS) on left motor cortex were progressively reduced from the first investigated timing (150ms after pic onset) until the last one (700ms). Nevertheless, it remains to be established the precise onset of corticospinal suppression and whether the suppression persists until the stimulus is on or it fades away with time. Here, we presented the same set of pics while TMS pulses were delivered at both early (100, 350ms) and later (1200, 2000ms) timings. Results showed no early differences in MEP amplitude between hand action pics and their scrambled versions, used as controls. Moreover, at 100ms, MEPs did not differ from baseline in any conditions. In addition, at later timings hand action-related MEPs were significantly smaller than at early timings and also smaller than all scrambled-related MEPs. In conclusion, the present study reveals that motor inhibition arises between 100 and 150ms after the onset of hand action and suggests that this effect persists as long as pics are presented.