# Interpersonal Synchrony Affects the Full-Body Illusion
## 4 Discussion
Q1, while primarily about touch referral, emphasized the location of the tactile sensation due to the inclusion of the term \"location\" in the question, suggesting it may also relate to self-location. The response values for Q1 did not differ significantly between the OA and OS conditions and were positive in both conditions. This may indicate that self-location drift occurred similarly in both conditions, suggesting that interpersonal synchrony did not affect self-location drift. This hypothesis is supported by the lack of significant differences between the OA and OS conditions for Q7, Q8, and Q9, all of which included elements of self-location. These findings suggest that interpersonal synchrony does not impact self-location. Several hypotheses can be considered to explain how interpersonal synchrony affects body perception. One plausible mechanism is that interpersonal synchrony blurs the boundary between the self and others, which may give rise to a sense of joint ownership that influences body perception. In the literature, \"agency\" refers to the sense of controlling one's own actions [29], and it has been reported that during joint actions, a sense of \"we-agency\"--the feeling that \"we\" rather than \"I\" are causing an action--can sometimes emerge [30; 31]. In such cases, self-agency may appear to dissolve into we-agency, leading to a weakening of self-agency. By analogy, if a form of \"we-ownership\" were to arise, it could share a similar relationship with self-ownership: rather than self-ownership vanishing, it might merge into a broader ownership that includes both the self and other. Under this scenario, participants would still retain self-ownership to some degree, but that self-ownership would be partially subsumed within a shared, collective ownership. Such a shift could manifest as a relative reduction in self-ownership measures, which is consistent with our finding of decreased self-related ratings in the OS condition. Importantly, this apparent shift from individual to shared embodiment does not necessarily imply reduced immersion in the VR environment. Instead, synchronous interaction may enhance social cohesion--fostering a collective sense of \"we\"--while immersion itself becomes oriented toward a shared bodily experience rather than a strictly personal one. Thus, interpersonal synchrony might redirect the focus of embodiment and presence from the individual self to a communal self, rather than trading off social cohesion against VR immersion. The second hypothesis is that the synchronization of bodily movements between oneself and others may simultaneously generate body perception for both one's virtual body and the other's virtual body, thereby influencing overall body perception. Previous research has shown that it is possible to experience ownership over multiple bodies simultaneously [32; 33; 34]. Furthermore, it has been shown that even objects that are clearly not one's own body, such as another person's body or a mannequin, can induce a sense of ownership when synchronous visuotactile stimuli are applied [3; 10]. Based on this, it could be argued that the self and the other's bodies were perceived as a single virtual object, with ownership arising over this object. However, since the appearance of this virtual object differs from the body schema of the real body, it was expected that ownership would be reduced. Previous studies have demonstrated that two visually identical bodies can both induce the FBI when stroked in synchrony [32; 33]. In our paradigm, participants viewed a live video of themselves alongside another person (the experimenter), so the self and other were clearly distinguishable. We then manipulated interpersonal synchrony by having the experimenter clap either in time (synchronous) or out of time (asynchronous) with the participant. Despite this clear self-other distinction, synchronous clapping still reduced the participants' touch referral and showed a trend toward decreased ownership of their virtual body.