Impact of virtual embodiment on motor resonance after a Mirror-PAS protocol

The Mirror Neuron System (MNS) is endowed with an action-observation network whereby the movement observation leads to greater excitability of the primary motor area (M1) (motor resonance). The activity in MNS seems to be modulated by learning new visuo-motor associations, i.e., Hebbian associative plasticity. A method to induce such plasticity and reshape motor resonance is the so-called mirror Paired Associative Stimulation (PAS) protocol, in which a transcranial magnetic stimulation (TMS) pulse over the M1 is paired to a visual stimulus showing ipsilateral hand movements. Further, observing virtual movements while being embodied in a virtual body impacts motor learning. Here, we carried out a mirror-PAS protocol in immersive virtual reality (VR) (i.e., Head Mounted Display) to investigate the impact of body representation on motor resonance when observing a grasping movement from a first-person perspective (1PP) while being embodied in a virtual body. A control group observed the same movement from a 1PP in a non-immersive setting (i.e., computer monitor). During the virtual mirror-PAS, a single-pulse TMS was applied over the right M1 paired to the observation of an ipsilateral grasping movement. Moreover, we recorded Motor Evoked Potentials induced by single-pulse TMS over the right M1 during the observation of grasping movements made with both hands. Following the virtual mirror-PAS protocol, we found differences between groups in motor resonance, sense of ownership, and agency toward the virtual arm performing the grasping movement. These results show the pivotal role of virtual embodiment in modulating the mirror-PAS's effects on corticomotor excitability.