Are you on my wavelength? Interpersonal coordination in dyadic conversations

Conversation between two people involves subtle non-verbal coordination in addition to speech. However, the precise parameters and timing of this coordination remain unclear, which limits our ability to theorise about the neural and cognitive mechanisms of social coordination. In particular, it is unclear if conversation is dominated by synchronisation (with no time lag), rapid and reactive mimicry (with lags under 1 second) or traditionally observed mimicry (with several seconds lag), each of which demands a different neural mechanism. Here we describe data from high-resolution motion capture of the head movements of pairs of participants (n=31 dyads) engaged in structured conversations. In a pre-registered analysis pathway, we calculated the wavelet coherence of head motion within dyads as a measure of their non-verbal coordination and report two novel results. First, low frequency coherence (0.2-1.1Hz) is consistent with traditional observations of mimicry, and modelling shows this behaviour is generated by a mechanism with a constant 600msec lag between leader and follower. This is in line with rapid reactive (rather than predictive or memory-driven) models of mimicry behaviour, and could be implemented in mirror neuron systems. Second, we find an unexpected pattern of lower-than-chance coherence between participants, or hypo-coherence, at high frequencies (2.6-6.5Hz). Exploratory analyses show that this systematic decoupling is driven by fast nodding from the listening member of the dyad, and may be a newly identified social signal. These results provide a step towards the quantification of real-world human behaviour in high resolution and provide new insights into the mechanisms of social coordination.