Corticospinal suppression in response to pics with implied hand actions: A follow up TMS study

Presentation of bodily actions is known to affect motor system activity in perceivers' brain. A previous study (Gianelli, Kuehne, Lo Presti, Mencaraglia & Dalla Volta, 2020) employing hand-tool interaction with apparent motion showed early suppression of corticospinal excitability in hand muscles. To control for the role of apparent motion and to investigate the suppression duration, in the present follow up study participants observed pics displaying hand-tool actions, with no apparent motion but only implied motion. Single pulse TMS was delivered on the hand sector of the left motor cortex at 1 s after fixation cross (baseline), at 150, 350, 500 and 700 ms from stimulus onset, while motor evoked potentials (MEPs) were recorded from the contralateral first dorsal interosseus muscle. Results showed a difference in MEP amplitude between hand action-related and control pics where hand action observation suppressed corticospinal excitability, suggesting early and enduring motor inhibition. In addition, MEP amplitude decreased over time. These findings rule out a necessary role of apparent motion, indicating that the simple presentation of hand actions with implied motion effectively induced motor inhibition. Corticospinal suppression may act to prevent the motor system from automatically transforming observed actions into overt movements whenever an action is observed.