Inter-brain coupling during naturalistic collaborative Tangram solving in child dyads: an exploratory fNIRS hyperscanning study

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Collaborative problem solving plays a central role in children's cognitive and social development, yet how neural activity relates to real-time peer coordination remains poorly understood. Using functional near-infrared spectroscopy (fNIRS) hyperscanning, we simultaneously recorded brain activity from 15 dyads of school-aged children during a naturalistic Tangram puzzle task, including rest, solo problem-solving, observation, and collaboration. We examined task-related neural activation and inter-brain connectivity (IBC) to assess how interpersonal neural dynamics relate to task context and behavior. No prespecified region-of-interest activation effect survived correction for multiple comparisons. At the exploratory channel level, a posterior-parietal channel showed a positive HbO response during collaboration relative to baseline. The global IBC did not differ significantly across experimental conditions, indicating that inter-brain synchrony was not determined solely by collaboration or social presence. Nevertheless, an exploratory association between global HbO coupling during Duo and total manual activity showed a relevant magnitude (Spearman rho = 0.581), a bootstrap 95% confidence interval excluding zero [0.038, 0.891], positive coefficients in every leave-one-dyad-out analysis, and a similar magnitude after excluding the first 8 s of each block (rho = .538), although it did not meet the FDR threshold across the broader sensitivity family, q = 0.131. Building on the emerging literature on dual-child portable fNIRS hyperscanning, the study reinforces the viability of this approach in an ecologically valid, hands-on task and suggests that block-wise inter-brain coupling may capture meaningful variation in dyad-level behavioral engagement.