# Interpersonal Synchrony Affects the Full-Body Illusion
## 1 Introduction  
The ability to perceive the existence, position, movement, and shape of one's own body is referred to as body perception. Body perception is a crucial ability for us to engage in social life, for example, it is necessary in situations where we perform coordinated actions with others. Even in a simple task such as clapping hands in sync with another person, it is essential to accurately understand the state of one's own body in order to accomplish this. Bodily sensations are formed by the brain's integration of multiple body signals (e.g., visual and tactile information) [1, 2]. These bodily sensations contribute to the formation of the sense of ownership (the feeling that an object is perceived as a part or whole of one's own body) and the sense of self-location (the feeling of existing in a certain position in space). People usually experience bodily sensations with respect to their own bodies. However, body perception has plasticity, and it has been experimentally confirmed that undercertain conditions, people may perceive an external virtual body as if it were their own body. These body illusions have been extensively studied to explore the nature of body perception (e.g., [3; 4; 5; 6; 7]); for example, the rubber hand illusion (RHI) is a well-known phenomenon where a rubber arm feels as if it were the participant's own arm [8]. In this experiment, a participant's arm and a rubber arm are placed side by side, with the participant's real arm hidden from view by a screen, allowing only the rubber arm to be seen. The experimenter then simultaneously applies tactile stimuli to both the participant's hidden arm and the visible rubber arm using a brush. This procedure causes the participant to feel as though the tactile sensation is occurring on the rubber arm, and they perceive the rubber arm as their own. While RHI involves a change in body perception concerning a body part, the full-body illusion (FBI) is a phenomenon where a virtual full-body is perceived as if it were one's own (see [9] for review). The FBI can be categorized into two types based on the spatial relationship between the virtual body and the perspective: when the virtual body is viewed from a first-person perspective (1 PP-FBI) (e.g., [10]) and when it is viewed from a third-person perspective (3 PP-FBI) (e.g., [3]). This paper focused on 3 PP-FBI, and hereafter, we refer to 3 PP-FBI simply as FBI. [PERSON] and colleagues confirmed the occurrence of the FBI through the following experiment [3]: participants were equipped with a head-mounted display (HMD) and stood upright. The experimenter traced the participants' backs with a pen in an up-and-down motion, which was filmed by a video camera placed 2 m behind them. The footage was then replayed in real-time on the HMD worn by the participants. As a result, the participants felt as though the sensation of being stroked on the back was occurring on the virtual body seen in the video. Additionally, they perceived the virtual body in the video as if it were their own. These illusions did not occur under conditions where pre-recorded video footage was played on the HMD, causing a mismatch between the visual and tactile stimuli. Thus, the FBI can be induced by synchronous stimuli across multiple sensory modalities such as vision and touch. It is also associated with the referral of touch [11], the emergence of a sense of ownership toward the virtual body [3; 5], and a drift in self-location toward the virtual body [12]. In the experiment conducted by [PERSON] et al., the tactile stimuli were applied by the experimenter [3]. However, there are also other methods to induce the FBI, where the participant themselves provides the tactile stimuli to their own body. For instance, in the experiment by [PERSON] et al., the participants laid supine and induced the FBI by operating a pulley device that stimulated their back in sync with the movements of their own hand [13].