Online HD-tRNS over the right temporoparietal junction modulates social inference but not motor coordination.

Social interactions are fundamental to human cognition, with the right temporoparietal junction (rTPJ) playing a key role in integrating motor coordination and social inference. While transcranial random noise stimulation (tRNS) is a promising technique for modulating cortical excitability in real time, its effect on dynamic social processes remains largely unexplored. This study applied high-definition tRNS (HD-tRNS) over the rTPJ during interaction with an adaptive virtual partner to modulate motor coordination and social inference. Eighty neurotypical adults (49 female) were assigned to one of two experiments: (Exp1) a block design with randomized active and sham stimulation blocks, or (Exp2) a trial-by-trial design with intermixed stimulation protocols. Participants performed a coordination task with a covert virtual partner programmed to behave cooperatively or competitively. Kinematic data and self-reported attributions of humanness and cooperativeness were analyzed. The results showed that HD-tRNS over the rTPJ did not affect motor coordination or overall task performance in either experiment. However, in Exp1, active stimulation progressively reduced attributed humanness and cooperativeness towards the competitive virtual partner, suggesting enhanced detection of antagonistic intent. This gradual modulation of social inference was absent in Exp2, where frequent protocol switching likely disrupted the buildup of stimulation effects. Together, these findings highlight the rTPJ's causal role in self-other distinction, underscore the importance of stimulation protocol design in shaping social cognition, and support the exploration of targeted neuromodulation in clinical and developmental populations with atypical social cognition.Significance statement Social interactions rely on our ability to infer others' intentions, including distinguishing between cooperative and competitive behavior: a process involving the right temporoparietal junction (rTPJ). Here, we used high-definition transcranial random noise stimulation (HD-tRNS) to test the rTPJ's causal role during live social interactions with an adaptive virtual partner. While stimulation did not affect motor coordination, repeated application led participants to gradually attribute less humanness and cooperativeness to a covertly competitive partner, suggesting enhanced sensitivity to competitive intent. These findings provide new insights into the rTPJ's contribution to self-other distinction, demonstrate the potential of HD-tRNS to investigate and modulate social inference, and have implications for understanding and potentially addressing social difficulties in conditions such as autism and schizophrenia.