To move or not to move: motor cortical output is enhanced during pain observation regardless of motor preparation state

Previous transcranial magnetic stimulation (TMS) studies have reported a decrease in motor cortical output during pain observation. In contrast, recent behavioral studies have shown that response times are faster after pain observation. This suggests that there is a mismatch between motor activity "during" versus "after" pain observation. We propose that these opposing effects of pain observation on motor activity may be explained by task constraints, as participants in TMS studies are instructed to keep still and relax their hands, whereas participants in behavioral studies maintain a state of readiness to respond. Task and methodological differences make it difficult to compare the results from TMS and behavioral paradigms examining the motor consequences of pain observation. As such, the aim of the current study is to directly test whether task instructions affect motor activity in TMS and behavioral measures of motor activity in the context of pain observation, within a single experiment. Participants watched videos of hands in painful versus nonpainful scenarios while TMS-induced motor evoked potentials were recorded. In the "active" block, participants responded to a cue that appeared immediately after each video; in the "passive" block, they relaxed their hand. Contrary to expectations, participants showed enhanced motor cortical output during pain observation (vs. no-pain) in "both" blocks. We discuss these results in relation to the wider literature on the social neuroscience of empathy.NEW & NOTEWORTHY The current study provides novel results showing an increase motor cortical output, measured via transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs), during empathic pain observation regardless of motor preparation state. Interestingly, this finding runs counter to the empathy for pain literature, which often finds a decrease in motor cortical output during empathic pain observation. We discuss potential explanations for this discrepancy and relate these results to the wider empathy for pain literature.