Brain Mediators of the Vicarious Facilitation of Pain

IntroductionThe response to noxious stimuli can be facilitated by observing another person in pain [۱]. This effect has been suggested to result from a sensorimotor resonance through the mirror neurons, or an increased level of arousal [۲,۳]. In this study, we used the mediation effect parametric mapping (MEPM) to understand which regions of the brain mediate this effect.MethodWe scanned ۲۱ pain-free volunteers (۱۰ females; mean age = ۲۵.۲±۴.۱) using fMRI while performing a vicarious pain facilitation paradigm. In each trial (N=۲۴) participants observed neutral, fearful, or painful facial expressions, before receiving a shock that elicited the nociceptive flexion reflex (NFR), an objective indicative of nociceptive processing in the spinal cord and, recorded their subjective pain ratings. We used multi-level MEPM to test the mediation effect of trial-by-trial voxel-wise brain activity in response to shocks for the relationship between the emotional valence of the observed facial expressions and pain ratings or NFR responses, after FDR correction (p<۰.۰۵, k=۱۰).ResultsThe inferior frontal gyrus, inferior parietal lobe, ventromedial prefrontal cortex, paracentral lobule, cerebellum, and anterior cingulate cortex were significant mediators of vicarious facilitation of pain after observing both fearful and painful facial expressions. With the painful facial expressions, more widespread regions of the brain mediated this effect, additionally including the precentral gyrus, superior frontal gyrus, superior temporal gyrus, the temporoparietal junction, superior parietal lobule, insula, and thalamus. The increase in NFR after observing painful facial expressions was mediated by the putamen and superior temporal gyrus.Conclusions۲The vicarious facilitation of pain is mediated by regions that are part of the human motor neuron system or are involved in affective theory of mind, in addition to the regions involved in the emotional modulation of pain.