DISCUSSION
intentional synchronization, FM
With the relatively recent application of brain coherence tools to data collected simultaneously from dyads, there is an emerging interest in understanding how the balance between between‐ and within‐brain networks contribute to effective social interactions. Despite recognition that both within‐ and between brains coupling are critical for social function (Balconi & Fronda, Next, we examined changes in between and within‐brain coupling between participants while performing the task. As with the behavioral analysis mentioned above, we also compared the neural activation of real dyads, who performed the task together, with “pseudo dyads,” to rule out the possibility that the between‐brain synchrony reflected nonspecific brain activation related to the fact that all participants were performing a similar task, irrespective of interpersonal engagement. Results indicate greater between‐brain coupling in the L.IFG–L.IFG and dmPFC–dmPFC of “real dyads” compared with “pseudo dyads,” in both the FM and IS conditions, confirming that between‐brain coupling depends on real social engagement between participants. Between‐brain coupling in the baseline BB condition did not differ from the between‐brain coupling found in pseudo dyads, further confirming that the between‐brain coupling found in the IS and FM conditions relates to mutual attempts to coordinate behavior in these conditions. It seems that participants were attempting to synchronize spontaneously in the FM condition, as evidenced by a higher than baseline between‐brain coupling in that condition, but given the complexity of the 3D movements they did not achieve behavioral synchronization.Moreover, we found greater between‐brain coupling in the R.IFG–R.IFG in the IS condition compared with the BB condition (in Real groups), indicating that between‐brain coupling in the IS condition is high and different from a condition with no face‐to‐face interaction. In line with this, greater activity in the right IFG was found during mutual execution of a motor act compared with performing the same motor act alone (Bhat et al., We also found greater between‐brain coupling between participants during the IS condition compared with the BB condition in the dmPFC–dmPFC ROI. Given the role of the dmPFC in monitoring and error detection, it is likely that the participants were coordinating their predictions of the other's movement. This suggests that intentional synchronization requires enhanced coordinated monitoring of the other's actions. It should be noted that the observed greater between‐brain coupling between participants in the “real dyads” may reflect the common cognitive processing, which participants share while engaging in an act of motor movement during the IS condition (Hamilton, Increased between‐brain coupling emerging even in the absence of behavioral synchrony may indicate that between‐brain coupling is not an epiphenomena of doing the same thing at the same time and may represent preparation for synchronization. That is, between‐brain synchrony may support preparedness for behavioral synchrony.However, since participants in the FM and IS conditions shared sensory input and motor output, but only during the IS condition were highly synchronized, it is more likely that the greater between‐brain synchrony in the IS condition represents joint activation related to the ongoing mutual adaptation within dyads, which is specific to synchrony. This assumption is further supported by the correlation between between‐brain coupling and task performance in this study. Interestingly, we also found increase in between‐brain coupling in one ROI (dmPFC–dmPFC) in the back‐to‐back condition in real versus pseudo‐pairs. Between‐brain coupling in the absence of real face‐to‐face interaction could be explained by the shared environment that includes odors and audio signals as well as preparation for synchronization. This suggests that between‐brain networks contribute to effective performance in the IS condition such that coordinated predictions regarding the other action is critical for synchronization.Notably, analysis of within‐brain coupling indicated greater within‐brain coupling in both the FM and IS conditions, indicating that within‐brain coupling between the observation‐execution and monitoring system is increased during social interactions. Nonetheless, while we found that between‐brain coupling is critical for intentional synchronization, within‐brain coupling predicted synchronization during free movement. It appears that when participants were explicitly instructed to synchronize their behavior, they generally succeeded in doing so, and this synchrony is increased when between‐brain in general increases—that is, when gaps are detected and corrected in a coupled manner. However, during free movement when there is high self‐focus on one's own movement, large gaps may develop in behavior, leading to high within‐brain coupling. Intriguingly, within‐brain coupling negatively predicted intentional synchronization, suggesting that reliance on within‐brain communication between the error monitoring and observation–execution systems actually weakens behavioral synchronization. It is possible that during the initial phase of attempting to synchronize in the FM condition, when there is a large gap between the participants, coordination between these networks within one's brain is critical. However, when synchronization is high in the IS condition, it becomes more effective to rely on the others' predictions and movement direction as opposed to one's own representation of action and prediction. Notably, Koban et al. (