Perturbing and measuring neural activity in the pain resonance network: TMS studies

Transcranial magnetic stimulation (TMS) studies show that watching painful stimuli on another person’s body decreases the excitability of the correspondent corticospinal body part representation in the observers. This observational pain-related inhibition (OPRI) is similar to that found during actual pain perception, suggesting that seeing pain in others triggers pain sensorimotor resonance in the corticospinal system. Using a perturb-and-measure TMS paradigm we provide causal evidence that the primary somatosensory (S1), motor (M1) and premotor (PMc) cortices play distinct critical roles in OPRI. In two experiments, we combined repetitive TMS (1Hz rTMS, to suppress neural activity in selected cortical regions) and single-pulse TMS (spTMS, to assess OPRI) during pain observation. In keeping with previous studies, a standard OPRI effect was found in a no rTMS condition. The effect was stronger in the subjects who showed greater dispositional empathy (assessed by the Interpersonal Reactivity Index) and provided higher observed-pain scores. OPRI was suppressed and increased by M1 and S1 disruption, respectively and left unaffected by V1 disruption. Importantly, disruption of PMc changed the OPRI into a muscle-specific facilitatory response. Thus, while S1 normal functioning seems to keep under control a potentially excessive embodiment of others’ pain, normal functioning of PMc and M1 exert a same-direction, different strength modulatory effect that allow an optimal tuning of resonant corticospinal mapping of observed pain. These findings highlight the causative connectivity between PMc-M1-S1 regions and the corticospinal system during embodied pain empathy and suggest that TMS can disclose specific inter-regional neural interactions during social perception.