Figure 2.

Example of data processing streamline for one dyad (and one channel/ROI) – A. participants seat next to each other and watch an episode of the BBC series ‘Dipdap’ (Phase 1). After watching one episode, the two participants engage in a social interaction task, when they chat about unrelated topics (Phase 2). They than watch another – novel - episode of Dipdap (Phase 3). During co-watching (phase 1 and 3), a separator ensures that participants do not engage in any form of communication. The two Dipdap episodes were randomly allocated to phase 1 or 3 (counterbalanced across dyads). They are all non-verbal, self-contained, identical for duration and comparable in terms of audio/visual features. B. Full session Nirs Signal (HbCBSI) plotted for participant A (red) and participant B (blue). Nirs signal during each video cowatching is highlighted. C. Wavelet coherence spectrogram for video 1 and video 2. Bars show the frequency of interest used in analysis. D. Bars plot of the mean for the three frequencies of interest (High: 0.1-0.2 Hz, Medium: 0.03-0.1 Hz, Low: 0.02-0.03 Hz) for video 1 and video 2. Data plotted in B., C. and D. belongs to the same dyad.