Early suppression of corticospinal excitability during the observation of pictures of hand actions

The presentation of bodily actions is known to affect the motor system activity in the observer’s brain. This motor resonance is thought to assist different motor and even cognitive functions. Nevertheless, the way different indexes of brain motor activity are modulated by presented bodily actions is still debated. Motor evoked potentials (MEPs) are markers of corticospinal drive, and their modulation is better interpreted when baseline activity is available for comparison. Here, we presented participants with pictures of hand actions involving common tools. As control stimuli, scrambled versions of the hand action pics were used. Single pulse TMS was delivered on hand motor cortex at different timings from stimuli onset (150, 350, 500, 700ms) and at the presentation of a baseline (fixation cross) while recording from FDI muscle. Results showed an early (at 150ms) difference in MEP amplitude between hand action and scrambled pics. In addition, at this timing the hand action-related MEPs were significantly smaller than the baseline MEPs, suggesting early motor inhibition. Conversely, at the same timing control stimuli did not differ from baseline. In conclusion, beside a commonly observed corticospinal facilitation, hand action presentation may also rapidly induce corticospinal inhibition. The factors promoting opposite corticospinal modulations still need to be fully elucidated.