Distinct social dynamics of joint action represented by interpersonal neural coupling in congruent and incongruent joint musical performances.

Collective human behavior plays a crucial role in the development of culture. However, whether and how different forms of collective behavior contain different social dynamics remains a cross-disciplinary debate regarding the mentalization during joint action in psychology as well as the sociality of music in ethnomusicology. This study delves into the comparison between congruent and incongruent joint actions from an interpersonal neural standpoint within the context of a joint musical performance. Simultaneously recording the neural activities of fifty pairs of string players during performance, we identified distinct regions within the mentalizing network, specifically the prefrontal cortex (PFC) and the left temporoparietal junction (TPJ), that support congruent (unison) and incongruent (melody-accompaniment) musical performances, respectively. During incongruent performances, higher levels of interpersonal neural coupling (INC) were observed in the left TPJ, an area responsible for adjusting the differences between self and others. In contrast, during congruent performances, higher INC was seen in the PFC, an area associated with monitoring and predicting the actions of others. Quantitative and qualitative data showed converging evidence that incongruent performances were more demanding, requiring more attention to the partner and precise coordination of intonation and rhythm. Moreover, the melody player led the accompanist in terms of INC during incongruent performances, which also revealed greater consensus in ratings between players and the audience. Our study highlighted the social significance of incongruent joint actions.