Pain relief provided by an outgroup member enhances analgesia

Abstract

Pain feels different in different social contexts, yet the mechanisms behind social pain modulation remain poorly understood. To elucidate the impact of social context on pain processing, we investigated how group membership, one of the most important social context factors, shapes pain relief behaviourally and neurally in humans undergoing functional neuroimaging. Participants repeatedly received pain relief from a member of their own group (ingroup treatment) or a member of a disliked outgroup (outgroup treatment). We observed a decrease in pain ratings and anterior insula (AI) pain responses after outgroup treatment, but not after ingroup treatment. Moreover, path analyses revealed that the outgroup treatment induced a stronger relief learning in the AI, which in turn altered pain processing, in particular if the participant entered the treatment with a negative impression toward the outgroup individual. The finding of enhanced analgesia after outgroup treatment is relevant for intergroup clinical settings. More generally, we found that group membership affects pain responses through neural learning and we thus elucidate one possible mechanism through which social context impacts pain processing.

Keywords: anterior insula; classical conditioning; expectation; ingroup bias; outgroup support; reinforcement.

Figure 1.

Experimental design. Except for the group membership of the treatment provider, the outgroup treatment group and the ingroup treatment group received identical pain relief treatment.

Figure 2.

Pain ratings and interaction between social context manipulation (ingroup versus outgroup treatment) and learning during the treatment. (a) Average ratings of subjective pain experience (the scale ranged from −4 to +4 with more negative numbers corresponding to higher pain) before and after ingroup and outgroup treatment. Participants' pain ratings were reduced after they received pain relief from an outgroup treatment provider, but not after they received pain relief from an ingroup treatment provider. (b) Interaction between social context manipulation (ingroup versus outgroup treatment) and individual learning signals from the right AI. After outgroup treatment, the pre- versus post-treatment difference in pain ratings was predicted by learning, while there was no such relationship after ingroup treatment (linear regression; see electronic supplementary material, table S2, right panel, c). (Online version in colour.)

Figure 3.

Path analysis shows that group membership moderates analgesia through learning. We used participants’ impression ratings for the ingroup and outgroup treatment provider, which reflect individualized social priors, as a predictor variable (a). The individual learning signals from the right AI served as a mediator variable (b) and the individual pre- versus post-treatment differences in pain ratings entered as a dependent variable (c). In addition, we included treatment group (ingroup versus outgroup treatment) as a moderator variable to assess between-group model differences. The direct Path (c) from social context evaluation to pre- versus post-treatment changes in pain ratings was not significant. In the outgroup treatment group, social context evaluation affected the pain ratings indirectly via its impact on learning, as reflected by significant indirect path coefficients (Paths a and b). * p < 0.05; *** p < 0.001.

Figure 4.

Individual learning signals from the right AI during outgroup treatment predict neural analgesia. (a) Second-level regression between the individual magnitude of neural learning during the treatment and the pre- versus post-treatment changes in pain-related activation, FWE-corrected (whole brain at cluster level) = 0.02, visualized at p < 0.001 (uncorrected). (b) Illustration of the results in (a). Extraction of contrast estimates from a 6 mm sphere around the SV FWE-corrected activation peak in AI shows a clear relationship between neural learning and pain-related signals for the experimental group. Specifically, a stronger learning signal in the insula during outgroup treatment predicts a larger pre-to-post-treatment reduction in pain-related activation.