Individual differences in vicarious pain as a shift in the self-other boundary.

There is inconsistent evidence concerning whether physical pain and vicarious pain share neural resources. This may reflect different methodological approaches (e.g., univariate vs. multivariate fMRI analyses) and/or participant characteristics. Here we contrast people who report experiencing pain when seeing others in pain (vicarious pain responders) with non-responders (who do not report pain). Cues indicated the level and location of an electrical shock delivered to the participant (self) or experimenter (other), with behavioural ratings and neural responses (fMRI) obtained. Non-responders tend to rate their own pain as worse than others given identical cues, whereas responders show greater similarity between self and other ratings. Univariate neuroimaging analyses showed activity in regions of the pain matrix such as insula, mid-cingulate, and somatosensory cortices contrasting physical versus vicarious pain, and when regressing the level of self-pain. But these analyses did not differ by group. Multivariate analyses, by contrast, revealed several group differences. The ability to classify self versus other was less accurate in the vicarious pain responders (in the same regions implicated in the univariate analyses of physical pain). In conclusion, the degree of shared neural responses to physical and vicarious pain is increased in vicarious pain responders consistent with the notion of differences in the self-other boundary.