An investigation into the mechanisms of inter-brain synchrony during early social interactions

Over the last 20 years there has been a growing increase in the amount of research investigating how and why two or more individual’s brain activity can synchronise during social interaction. What we know so far from this research is that inter-brain synchrony (defined through temporally coordinated patterns of brain activity between two interacting individuals, Holroyd 2022) tends to associate with moments of behavioural coordination (i.e., when two individuals are doing or attending to the same thing at the same time) and task cooperation (i.e., the action or process of two individuals working together to the same end). These observations have led many researchers to theorise over whether and how behavioural coordination mechanistically drives inter-brain synchrony (Wass et al., 2020; Hamilton, 2021). There is also some very recent evidence to suggest that increased inter-brain synchrony actually facilitates/ supports aspects of social interaction. For example, inter-brain synchrony has been shown to predict team performance (Reinero et al., 2021), although this research is primarily based on correlational study designs.

Taken together however the field of inter-brain synchrony shares one fundamental limitation; that is that it does not account (although see recent animal research e.g., Kingsbury et al., 2019; Zhang et al., 2019), empirically for the mechanisms that give rise to inter-brain synchrony, which would help to falsify claims that inter-brain synchrony is a core mechanism facilitating social interaction. This is because of two main reasons; Firstly, the study of inter-brain synchrony has primarily been investigated as a time-invariant property, almost no studies have explored how inter-brain synchrony varies over time relative to individual moments of behavioural coordination. Secondly, little attention has been paid to the changes in the underlying signal properties (i.e., increases in power, changes in frequency) that must take place for two unsynchronised signals to become synchronised (e.g., Haresign et al., 2022).

Using two-person naturalistic biobehavioural recording techniques, coupled with state of the art, EEG pre-processing and analyses procedures (see chapters 5 and 6), the present thesis examines the mechanisms that give rise to inter-brain synchrony during parent-infant social interactions.

Evidence is presented showing how inter-brain synchrony does not arise around individual moments of gaze coordination. This is despite previous investigations suggesting that increased inter-brain synchrony (averaged over all moments of eye contact) associates with gaze synchrony. Evidence also shows the contribution of behavioural coordination across multiple modalities to inter-brain synchrony during parent-infant social interaction.

Discussion is focused on the contribution of these findings to our understanding of the mechanisms that give rise to inter-brain synchrony.