Brain Synchronization During Social Interaction Is Higher Among Strangers

It is well known that when two people engage in a joint activity, their brain activity becomes synchronized, and that social interaction strategies differ between strangers and people who know each other. However, the differences in neural processes during social interaction are still not fully understood. Researchers from Waseda University in Japan have shown that interactions between strangers lead to higher levels of both inter- and intra-brain synchronization compared to close friends or acquaintances. The study was published in the journal Scientific Reports.

In everyday life, we constantly engage in social interactions and communicate with acquaintances, friends, romantic partners, and often with strangers. Previous research has shown that interacting with people with whom we have stronger social ties—such as romantic partners, family members, and friends—can be beneficial, as people tend to feel less lonely after close communication. Since the intensity and quality of friendships positively correlate with life satisfaction, it is expected that strong social relationships contribute to well-being. However, in our daily lives, we also have many opportunities for social interaction outside our close social circles, such as with strangers or acquaintances with whom we have weak social ties. Interestingly, research suggests that interacting with people with weak social ties can also enhance well-being. For example, Sandstrom and Dunn (2014) demonstrated that a simple social interaction with a coffee shop barista (a stranger) can increase people’s sense of belonging and well-being.

Study Design and Methods

In the new study, the team from Waseda University examined the geometric properties of interpersonal neural networks in pairs of strangers and acquaintances during an anti-phase joint tapping task. The study involved 14 pairs of strangers (meeting for the first time during the experiment) and 13 pairs of acquaintances (one participant brought their partner). The results were obtained using dual electroencephalography (EEG) with 29 channels.

Brain activity was recorded using EEG electrodes placed on the scalp under four tapping conditions: slow tapping at 0.5-second intervals, fast tapping at 0.25-second intervals, free tapping at a preferred frequency, and tapping coordinated with a metronome at 0.50-second intervals (a pseudo-condition). The study examined how brain signals synchronized in the theta (4–7 Hz), alpha (8–12 Hz), and beta (13–30 Hz) frequency bands.

Key Findings

EEG analysis showed that pairs of strangers exhibited greater intra-brain synchronization in the theta range compared to pairs of acquaintances. Stranger pairs had significantly higher local efficiency in intra-/inter-brain neural networks, and there was a stronger positive correlation between local and global efficiency in the theta range, as well as in other frequency bands.

The results indicate that pairs of strangers demonstrate a higher level of social interaction during tasks that require mutual prediction than pairs of acquaintances. The researchers suggest that the lack of prior communication between strangers requires a more complex process of predicting each other’s actions or behavior in a joint task. As a result, this increased interaction leads to more efficient information transfer between closely connected nodes within the neural network.

Context and Implications

Previous studies have shown that pairs with strong social ties (such as romantic partners) exhibit greater synchronization in the gamma EEG range (30–60 Hz) than pairs with weaker social ties (strangers), but not in the theta, alpha, or beta ranges. Gamma oscillations are associated with social cognition, such as empathy, mentalization, and emotions. Therefore, inter-brain gamma EEG synchronization may reflect social cognition between pairs rather than the behavioral rhythms of joint actions. However, when participants perform tasks involving joint actions, their EEGs tend to converge in lower frequency ranges. For example, inter-brain EEG synchronization is stronger in the theta and alpha ranges during joint tasks than when performing tasks alone (without joint action). Additionally, theta inter-brain synchronization is linked to the process of movement coordination. Thus, the results from the Japanese researchers may indicate that theta synchronization of brain activity depends on participation in joint actions rather than on social or emotional cognition.

“Our results challenge the common understanding that stronger social ties predict greater brain synchronization and offer new insights into neural networks during social interactions,” says lead researcher Dr. Yuto Kurihara, a research fellow at the Faculty of Humanities, Waseda University.

This is the first study to compare the geometric structures of neural networks in interpersonal relationships and coordination. The findings show that weak social ties lead to higher brain synchronization in the theta EEG range and better information integration during joint tasks.