During social interaction, both participants are continuously active, each modifying their own actions in response to the continuously changing actions of the partner. This continuous mutual adaptation results in interactional synchrony to which both members contribute. Freely exchanging the role of imitator and model is a well-framed example of interactional synchrony resulting from a mutual behavioral negotiation. How the participants' brain activity underlies this process is currently a question that hyperscanning recordings allow us to explore. In particular, it remains largely unknown to what extent oscillatory synchronization could emerge between two brains during social interaction. To explore this issue, 18 participants paired as 9 dyads were recorded with dual-video and dual-EEG setups while they were engaged in spontaneous imitation of hand movements. We measured interactional synchrony and the turn-taking between model and imitator. We discovered by the use of nonlinear techniques that states of interactional synchrony correlate with the emergence of an interbrain synchronizing network in the alpha-mu band between the right centroparietal regions. These regions have been suggested to play a pivotal role in social interaction. Here, they acted symmetrically as key functional hubs in the interindividual brainweb. Additionally, neural synchronization became asymmetrical in the higher frequency bands possibly reflecting a top-down modulation of the roles of model and imitator in the ongoing interaction.

Several neuroimaging studies have consistently shown activations of areas surrounding the temporo-parietal junction (TPJ) during tasks exploring the sense of agency. Beyond TPJ, activations in different structures, such as the dorsolateral prefrontal cortex (dLPFC), the pre-supplementary motor area (pre-SMA), the insula and the precuneus have been reported. Moreover, a possible dissociation between self- and external-agency attribution has been suggested. To test the hypothesis of distinct neural correlates for self- and external-agency attribution a quantitative meta-analysis, based on activation likelihood estimation (ALE) method, across 15 PET and fMRI studies (228 subjects) was conducted. Results show converging activations including the TPJ, pre-SMA, precuneus and dorsomedial prefrontal cortex (dMPFC) in external-agency, while insula activation was related to self-agency. We discuss these findings, highlighting the role of the insula, and calling for the use of alternative paradigms such as intentional binding and interactive imitation to study agency.

In a classic report, Murray and Trevarthen (Emotional regulation of interaction between two-month-olds and their mothers. In T. Field & N. Fox (Eds), Social perception in infants (pp. 101± 125). Norwood, NJ: Ablex, 1985) suggested that 6-to 12-week-olds were sensitive to contingency in maternal stimulation because they became upset during the replay of their mothers' behavior during an initial period of live interaction over closed circuit TV. However, a number of concerns were raised about the procedure and it was argued that infants may simply have become increasingly fussy over time. To clarify this very important issue, we adopted a three-condition design (live ±replay ±live) and modified Murray and Trevarthen's procedure by inserting a seamless shift from the initially live interaction period to a replay period of good maternal behavior. This was followed by a seamless shift back to a subsequent live interaction. Two-month-old smiling and gaze decreased while frowning and mouth closures increased during the replay period relative to the initial live interaction period, replicating Murray and Trevarthen's results. Fussiness over time did not account for these results because seven of the ten babies increased their positive behaviors and decreased negative behaviors during the second live interaction period, and there was a significant increase in their visual attention to their mothers, thus suggesting a renewed interest in the previously noncontingent partner.