# Interpersonal Synchrony Affects the Full-Body Illusion
## 1 Introduction
Additionally, in the experiment by [PERSON] et al., the participants induced the FBI by rubbing the back of their neck with their own hand instead of their back [14]. Through experiments on body illusion, it has become evident that the relationship between the self and the virtual body influences the occurrence and intensity of the FBI. For example, the angle from which the virtual body is viewed (with stronger ownership when viewed from behind rather than from the front or at an angle) [15; 16] and the similarity of the virtual body's appearance to the person [1] are influential factors. While the impact of the self-virtual body relationship on body perception has been widely studied, the effect of interpersonal factors, such as the relationship between the self and others, on body perception has been less explored. However, as previously mentioned, body perception forms the foundation for our interactions with others, making it important to investigate how social interactions influence body perception. In this paper, we focused on the potential impact of interpersonal synchrony on body perception within the context of social interaction. Interpersonal coordination is defined as \"the degree to which the behaviors in an interaction are non-random, patterned, or synchronized in both timing and form\" [17], and within this, behaviors that are temporally aligned are referred to as interpersonal synchrony. Interpersonal synchrony is observedin various activities such as music performance, dance, and sports. Clapping in time with others, as above-mentioned, is an example of this. Previous research has shown that synchronizing bodily movements with others can produce various prosocial effects such as increased emitativity (a sense of unity) [18] and enhanced self-other overlap [19]. Furthermore, increased self-other overlap has been reported to promote social bonding and cooperative behavior [20; 21]. These findings suggest that synchrony might not only influence relationships with others, but also impact body perception. However, as far as we know, there has been no experimental investigation into whether interpersonal synchrony affects body perception. Therefore, this study aimed to explore the relationship between interpersonal synchrony and body perception by examining the impact of interpersonal synchrony on the FBI. To achieve the objectives of this paper, it was necessary to present the participants with synchronous multisensory stimuli to induce the FBI. Additionally, we needed to devise an experimental task that facilitated the establishment of interpersonal synchrony between the participants and others. To address both of these requirements simultaneously, we employed clapping as the task. Previous studies have controlled the synchronization state between individuals by adjusting the rhythm of clapping [22]. Moreover, clapping generates tactile stimuli on the palms, which, when combined with synchronous visual stimuli, could induce the FBI. Based on the above considerations, we conducted an experiment in which the participants wore an HMD and clapped their hands while being recorded by a camera positioned behind them. The recorded footage was immediately presented to the participants, and since this setup was similar to the paradigms used in previous studies--where the participants induced the FBI by providing synchronous visuotactile stimuli to themselves--we expected that the participants would experience the FBI with respect to the virtual body, as in earlier research. In this condition, an additional person (the experimenter) clapped beside the participant. The experimenter adjusted the rhythm of their clapping to control the synchronization state between the participant and the other person's body movements. We examined whether this difference in synchronization affected the FBI. The following working hypothesis was tested:  
* H: Differences in the synchronization of bodily movements with another person lead to changes in body perception.