MODULATING THE ROLE OF INHIBITORY PROCESSES INVOLVED IN MOTOR RESONANCE DURING THE OBSERVATION OF ACTION-RELATED STIMULI: A COMBINED SPTMS AND HD-TDCS STUDY

Although single pulse Transcranial Magnetic Stimulation (spTMS) applied over the Primary Motor Cortex (M1) proved an excellent proxy of motor facilitation during action observation, some evidence showed corticospinal suppression during action processing compared both to baseline and control stimuli (Gianelli et al., 2020), suggesting an involvement of inhibitory processes during action observation. The present study aimed to probe the putatively inhibitory role of dorsal premotor cortex (dPMC) in motor resonance using a perturb-and-measure approach. Sixty-four right-handed volunteers entered a within-subject protocol consisting of one preliminary phase and two experimental sessions in which, after 20 minutes of either cathodal high definition (HD) or sham tDCS applied to the left dPMC, motor evoked potentials (MEPs) elicited by spTMS on left M1 were collected while participants attended tool pictures immediately followed by hand-tool interaction images. For each trial, a single-pulse stimulation was given either on the fixation cross (baseline), the tool (control) or the action (critical) picture. MEP amplitudes elicited by critical stimuli were compared with baseline and control condition for sham and cathodal tDCS separately. In line with Gianelli et al., (2020), results showed MEPs suppression for critical stimuli compared to both baseline and control condition after sham stimulation, and the same pattern was found after cathodal tDCS. These findings suggest that action understanding involves a dynamic interplay between facilitatory and inhibitory processes and, notwithstanding the need for further investigation on the functional role of dPMC, pave the way for clinical applications of novel neuromodulation protocols integrated with traditional action observation treatments.