Interpersonal alignment in infra-slow EEG rhythms anticipates mutual recognition
Across multiple timescales, neural rhythms play a crucial role in coordinating cognitive and physiological processes, with slower frequencies potentially relevant during social interaction. While electroencephalography (EEG) has been widely used to study such processes, examining these slower aspects during social interaction presents unique methodological challenges. Here we address this issue by investigating infra-slow EEG, respiratory, and electrodermal signals recorded simultaneously during a perceptual crossing experiment (PCE)--a recently established paradigm in social cognition and EEG hyperscanning. Through innovative spectral-characterization methods and a novel dyadic regression approach, we establish a methodological framework for examining infra-slow dynamics that coordinate brain and body during social interaction. Our analysis revealed that, when time-locked to task responses, the spectral power of infra-slow oscillations (ISOs) at 0.05 Hz and 0.1 Hz showed enhanced inter-participant similarity in dyads achieving mutual recognition relative to those that did not. Although not strictly simultaneous, this inter-personal alignment in the spectral power of ISOs was accompanied by concurrent dynamics both in respiratory pressure and electrodermal activity, suggesting that ISOs reflect the integration of autonomic, cognitive, and social processes, with their alignment facilitating shared understanding and joint action. Crucially, we find that task-responses in participants who successfully engaged in the PCE task were coupled to the phase of the 0.1 Hz rhythm. Additionally, we show that ISOs and aperiodic activity tracked behaviorally relevant transitions, notably prior to and during the perceptual crossing task. Taken together, our findings highlight an important role for ISOs in mediating the temporal dynamics of social cognition and show that inter-participant alignment in ISOs and physiological signals can serve as a proxy for the complex interplay between body physiology, cognition, and social behavior.