We devised the novel combination of hyperscanning EEG and motion tracking with the implicit body movement synchronization paradigm31. The advantage of this experimental paradigm is twofold. First, we were able to detect an implicit-level interaction that is interpersonal and real time in nature. Previous studies have mainly concentrated on explicit social interactions, i.e., with mutual explicit intention to communicate, such as observing conversation10 and playing economic games16,19, where tasks are possibly complicated by both implicit and explicit aspects of the interaction. However, we aimed to identify an implicit process by minimizing explicit interaction. We achieved this by using simple tasks such as looking at the other participant's finger while holding one's own finger stationary (in the pre- and post-training sessions). Participants had to intentionally neglect the movement of the partner and stay stationary, as the task demanded. Nonetheless, the movement could not be neglected and the participants tended to unconsciously synchronize each other. In this specific sense, the task and the results are social, yet implicit. The fingertip movement synchronization could be potentially applied as a somatic measure of implicit interpersonal interaction. The second advantage of the paradigm rests on the fact that since the instructions in our experimental paradigm were to stay stationary, movement artifacts were minimized in the EEG data. Robustness to noise during face-to-face interaction makes our experimental paradigm optimally sensitive to the underlying EEG dynamics and the functional connectivity of implicit interpersonal interaction.

The increase of fingertip synchrony after the training session indicates that the large, voluntary and intentional mimicry affects the small, involuntary and unintentional body movement synchronization afterwards. Two participants seem to build their own rhythmic structure during the intentional mimicry training, resulting in increased unintentional synchronization. It is consistent with and extending the previous studies showing that motor mimicry increased implicit social interaction between two interacting participants25 and that the spontaneous bi-directional improvisation (i.e. implicit synchronization) increased motor synchrony compared with the uni-directional imitation (i.e. explicit following)32. Correlations between the fingertip synchrony increase and the social anxiety scales further support that the increased fingertip synchrony could be a marker of implicit social interaction. In addition, the drastic decrease of variation of synchrony should be noted. We only suspect the possibility of the ceiling effect of correlation coefficient, but it was also small at larger absolute values of time delay. This might possibly be interpreted as a part of the effect of the cooperative training and further examinations may be needed.

One may still expect that the leader-follower asymmetry in the training would create a lag in the post-training test, even though the instructions for both participants remained the same as the pre-training test, just to stay stationary. According to previous studies as described in the introduction, however, unintentional body movement synchrony with zero-lag is widely observed, such as walking together and clapping. When two people are walking together and they do not pay attention, sometimes one person leads and sometimes the other does alternately and irregularly, but overall, they are more or less in sync due to a natural tendency, the expected outcome would be as correlation peak at lag zero with broad skirts. What we found in our study is qualitatively the same. The only difference was that the correlation was observed without and against the participant's intention.

We found the positive correlation between the VMPFC activity and fingertip synchrony changes from post- to pre-training sessions (Fig. 2C). Meanwhile, the fingertip synchrony increase was negatively associated with individual social anxiety level. Taken together, the VMPFC may be involved in implicit social interaction. However, it remains to be a speculation, which is consistent with the previous findings that the VMPFC has been associated with not only explicit33,34, but also implicit social and emotional attitudes35, including implicit gender bias36, automatic activation of political attitudes37 and implicit preference38.

We also found that theta and beta phase synchronization occurred between two brains due to the training. Long-range theta phase synchronization has been known for its role in implicit social and emotional processing11,39. A previous hyperscanning-EEG study reported beta inter-brain synchronization between the central and right parieto-occipital regions and interpreted it as a brain-to-brain top-down modulation in social interaction14. Furthermore, neural activities at beta frequency can synchronize over long distances, which makes it a likely candidate for neural correlates of higher-level interactions40. Thus, the synchronization between the parietal (PoCG) and central regions (AC) in theta and beta frequency range may be considered as such, for implicit social processing and its top-down modulation. The fact that the neuroelectric signals are synchronized at the millisecond timescale, which is much faster than the finger movements, also supports that the neural synchrony we found was not a mere consequence of motor synchrony, but more dynamically linked with the behavioral synchrony.

We found no alpha activity in our study. Alpha rhythm has been known to be involved in the mirror neuron system, integrating and processing motor and social information41,42. Previous hyperscanning-EEG studies showed inter-brain alpha synchronization between the right centro-parietal regions, indicating socially coordinated dynamics of intentional body movements13,14. However, the current experiment did not involve explicit and intentional motor movements, thus no significant activation should be expected in the motor regions. It could be a part of the reason why we found no significant alpha activity.

Our current findings suggest that IFG, AC, PHG and PoCG may be the neurophysiological substrates of inter-brain synchronization. Frontal (IFG)-parietal (PoCG) interaction as a frontoparietal network has been implicated in social cognitive processes35,43,44, especially playing a key role in interpersonal awareness45. Anticipation of somatosensory events, which may be related to the current experimental setup (where two fingers of two participants stayed close though not touched), is associated with the central (AC and PHG) and parietal (PoCG) coupling46. This centroparietal coupling is also involved in top-down modulatory controls14. Along these lines, in our experiments, the frontoparietal and centroparietal networks may have been crucially involved in body and neural synchronization. It may appear peculiar to some that the inter-brain neurophysiological substrates were temporally synchronized, but spatially asymmetrical. As we mentioned earlier in the discussion, the neural synchrony was dynamically linked with the behavioral synchrony and so the behavior and brain do not have to be one-to-one correspondence. Also, note that the time scales for synchronization are different in the factor of at least 10; the neural synchrony being at 100 milliseconds whereas behavioral synchrony being extended to several seconds. We would rather argue that the two brains worked as one neurophysiological network to dynamically process implicit social functioning and modulatory sensory-motor control.

Our novel experimental paradigm serves as a simple, accurate and efficient implicit measure of interpersonal interaction. While behavioral synchrony in the daily activity such as walking or clapping has been reported, the present effects are more implicit and at microscopic temporal and spatial scales. The simplicity of the experiment can minimize the movement artifact in the EEG data so that the task is optimally sensitive for hyperscanning-EEG study to identify underlying EEG dynamics of implicit interpersonal interaction. The negative correlations of the fingertip synchrony changes with the scales of social anxiety verify that our task can be a marker of implicit social interaction. To the best of our knowledge, this is the first study to demonstrate that not only the fingertip movement but also the neural activity between the two participants synchronized after cooperative interaction. Moreover, inter- and intra-brain connectivity results in the EEG analysis may have both theoretical and methodological implications towards understanding the neural mechanisms underlying impaired social interaction.