Aperiodic activity shapes inter-brain synchrony during cooperative puzzle solving: dissociable task-enhanced and partner-specific spectral signatures

Inter-brain synchrony (IBS) is increasingly used as a neural marker of social coordination. Yet standard hyperscanning pipelines rarely distinguish partner-specific coupling from parallel task engagement, and scarcely account for aperiodic (1/f) contributions to phase-based metrics. Here, in 10 dyads (N = 20) of close friends from Latin America recorded with dual mobile EEG during cooperative Jenga puzzle-solving, video co-watching, and eyes-closed resting, we combined cluster-permutation statistics, dyad-shuffled surrogate controls, spectral parameterization, and edge-level mixed-effects modeling to dissociate task-enhanced from partner-specific inter-brain synchrony, and oscillatory from aperiodic contributions. Puzzle-solving elicited robust theta band IBS enhancement (p ' 0.002), yet surrogate analyses revealed negligible partner-specific theta effects. Alpha IBS showed the opposite pattern, with no interactive condition modulation but exceeding surrogates during cooperation in a right centroparietal cluster (p = 0.024, d = 0.65) revealing partner-specific coupling. Critically, detectable oscillatory theta peaks were rare ('6%); alpha peaks sparse during cooperation (10%) but ubiquitous at rest (98%). Exploratory edge-level mixed-effects modeling identified aperiodic (and not oscillatory) predictors of IBS: theta tracked dyad-averaged slope (β = +0.053, p = 0.003), alpha tracked slope similarity between partners (β = -0.039, p = 0.005). These results dissociate two IBS signatures: task-enhanced theta associated with shared aperiodic features, and partner-specific alpha consistent with aperiodic alignment rather than oscillatory coordination. Our findings underscore the necessity of incorporating spectral parameterization and surrogate controls in EEG hyperscanning.