Theta band behavioral fluctuations synchronized interpersonally during cooperation.

Human behavior fluctuates. A growing body of evidence has demonstrated that behavioral performance in perception fluctuates rhythmically, with dynamics closely resembling spectral features of neural oscillations. However, it is unclear whether the behavioral fluctuations in a complex cooperation context can also express similar rhythmic features, and, more importantly, whether these behavioral rhythms are synchronized among co-actors in a neurophysiologically relevant manner. To answer these questions, we applied a time-resolved approach, previously used for probing individual-level behavioral oscillations in perception, in a complex social interaction context, and further probed dyad-level behavioral synchrony. Twenty pairs of male participants completed, in dyad, joint-action tasks with cooperation or competition demand. We extracted behavioral rhythms from ongoing cooperative performance and measured behavioral synchrony by computing the phase coherence of these behavioral rhythms between dyad members. Despite the absence of significant behavioral oscillations in individuals' amplitude spectrum, we observed enhanced theta-band phase coherence between co-actors' behavioral rhythms during cooperation compared to competition conditions. These results indicate that cooperative behaviors of co-actors fluctuated synchronously within the theta band, providing a behavioral counterpart of theta-band interbrain synchrony in cooperation reported in previous hyperscanning studies. Furthermore, the observed behavioral synchrony could be used as a sensitive predictor of cooperation pattern, as evidenced by its significant correlation with leader-follower relationship during cooperation.