Social closeness modulates brain dynamics during distrust anticipation

Anticipation of trust from a partner with high social closeness generates the preconditions for cooperative and reciprocal interactions to occur. However, if there is uncertainty in the interaction because the partner is a stranger or because the person has mistrusted us on another occasion, an aversive experience is generated. Hence, low social closeness or mistrust makes people keep track of the others behavior. In case of experiencing deviations from social norms, the person will monitor the intentions of the partner and update their priors regarding their social preferences. The anterior insula (AIns) seems to be sensitive to these social norm violations in functional magnetic resonance imaging (fMRI) experiments, however, the monitoring of partners with different levels of social closeness has not been investigated. In our study we wanted to find the brain regions related to (dis)trust anticipation from partners who differ in their level of social closeness. For this, we designed an experiment in which participants played an economic decision game with three people (trustors): A computer, a stranger, and a real friend. We covertly manipulated their decisions in the game so that they unexpectedly distrusted our participants. Using task-fMRI, our whole-brain analysis found that the AIns was active during the anticipation of the decisions from human partners (humans vs computer), but not during anticipation between high and low social closeness (friend vs stranger). However, using a psychophysiological interaction analysis, we found increases in functional coupling between the AIns and regions in the "mentalizing" network (such as temporal regions and parieto-occipital cortices) during trust anticipation between a high versus low social closeness partner. These results suggest that there may be a modulation of the AIns activity, specifically for high social closeness trustors, by regions that encode the intentions underlying the truster behavior.