Interpersonal Synchrony Affects the Full-Body Illusion

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This study explores the mechanisms of body perception and may contribute to enhancing how individuals perceive virtual bodies, leading to a more immersive experience in social VR/AR environments.

Abstract

The full-body illusion (FBI) is a phenomenon where individuals experience body perception not in their physical body but in an external virtual body. Previous studies have shown that the relationship between the self and the virtual body influences the occurrence and intensity of the FBI. However, the influence of interpersonal factors on the FBI has not been explored. This study investigated the effect of interpersonal synchrony on body perception through an evaluation experiment involving the FBI. Specifically, the participant and an experimenter clapped together while their movements were recorded by a video camera placed behind the participant and displayed to them via a head-mounted display (HMD). This setup presented synchronous visuotactile stimuli, aligning the visual feedback with the tactile sensations in the participant’s hands, to induce the FBI. The experimenter’s clapping rhythm was manipulated to either be synchronous or asynchronous with the participant’s rhythm, thus controlling the state of movement synchronization between the participant and the experimenter. The impact on the participant’s body perception was then assessed through subjective reports. The results indicated that when the clapping rhythm was synchronized with the other person, there was a significant reduction in touch referral to the participant’s virtual body. Additionally, there was a trend toward a reduction in ownership. This study demonstrated for the first time that interpersonal synchrony affects body perception.

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 under certain 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 (1PP-FBI) (e.g., [10]) and when it is viewed from a third-person perspective (3PP-FBI) (e.g., [3]). This paper focused on 3PP-FBI, and hereafter, we refer to 3PP-FBI simply as FBI. Lenggenhager 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 Lenggenhager 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 Hara 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]. Additionally, in the experiment by Swinkels 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 observed in 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 entitativity (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.

2. Materials and Methods

2.1. Participants

A total of 23 participants took part in the experiment. The participants were recruited from the Tokyo Institute of Technology and through an experimental participation recruitment website (jikken-baito.com), comprising both students and working adults. Their ages ranged from 20 to 37 years old (mean = 25.39, SD = 4.84). The sample included 5 female and 18 male participants. All participants had no history of mental illness and had normal or corrected-to-normal vision. The participants were not informed of the experiment’s purpose beforehand. The experimental procedures were approved by the “Human Subjects Research Ethics Review Committee” at the Tokyo Institute of Technology.

Following previous research [14], a pre-screening procedure was conducted to select participants who were more likely to experience the FBI (see Appendix A and Appendix B for details). Based on the results of the pre-screen, 11 out of 23 participants were identified as experiencing changes in bodily sensation during the hand clapping task. These 11 participants proceeded to the main phase of the experiment, in which the effect of interpersonal synchrony on the FBI was systematically investigated. This procedure was adopted to increase the sensitivity of the experiment by excluding individuals who were unlikely to experience the illusion, following the rationale proposed by [14], to maximize the chance of detecting the effect of experimental manipulations on the FBI.

2.2. System

The experiment was conducted in a laboratory located within the Suzukakedai Campus of the Tokyo Institute of Technology. Figure 1 provides an overview of the experimental setup. Participants wore an HMD (Meta Quest 2, Meta Platforms, Inc., Menlo Park, CA, USA, resolution: 1832 × 1920 pixels) and stood upright in the center of the room. A stereo camera (OVRvision Pro, WIZAPPLY Co., Ltd., Osaka, Japan) was positioned 1.5 m behind the participants, capturing their image from the rear and streaming the footage to the HMD in real-time. The height of the stereo camera was adjusted for each participant to match their eye level. To ensure that the participants could not hear the sound of their clapping or any environmental noise, white noise was played through the speakers of the HMD.

2.3. Experimental Task

The participants were instructed to hold both hands above their right shoulder and clap at a pace of approximately three times per second while watching the video displayed through the HMD. The video depicted their own body as captured by the camera, which served as the “virtual body”. The participants were told to observe the clapping hands of their virtual body. Additionally, they were asked to maintain their hand height so that the clapping motion remained visible to the camera. The video was displayed without delay, meaning that the participant’s video feed was synchronous with their own clapping movements. Before starting the experiment, we confirmed that the participants could not hear their own clapping sounds due to the masking effect of the white noise played through the HMD speakers. The task in all experiments continued until a beep sound, signaling the end, was played through the HMD speakers three minutes after the clapping began. Upon hearing the beep, the participants were instructed to stop clapping and lower their hands to their sides.

2.4. Conditions

A few seconds after the participant began clapping, the experimenter appeared within the camera’s field of view and stood approximately 50 cm to the right of the participant (see Figure 2). The experimenter positioned their hands above their left shoulder and began clapping. Participants were instructed to observe their own and the experimenter’s clapping hands at a pace of three times per second. The experimenter adjusted their clapping rhythm while visually monitoring the participant’s clapping to control the synchronization state between their movements. The task was performed under two conditions, where the synchronization with the experimenter varied. In the other in sync (OS) condition, the experimenter clapped in synchrony with the participant, ensuring that their hand movements were aligned in timing. In the other in async (OA) condition, the experimenter did not synchronize their clapping with the participants. Instead, the experimenter continuously varied the clapping tempo between a fast rate (approximately five claps per second) and a slow rate (approximately one clap per second) to ensure that their clapping was not aligned with that of the participant.

2.5. Procedure

The participants first received a full explanation of the experimental procedure. They then practiced clapping at a pace of three times per second while watching a timer. Following this practice, the pre-screen task was conducted, and then only the participants who met the criteria proceeded to the main experiment. In the experiment, they performed the clapping task under either the OS or OA condition. After completing the clapping task, they moved to a computer and filled out a subjective report, as detailed in Section 2.6. Next, the participants repeated the clapping task and the subjective report under the alternate condition (either OA or OS condition). The order of the OS/OA conditions was randomized and counterbalanced across the participants. Finally, an unstructured interview was conducted to verify that the experiment had been carried out successfully and to gather any additional observations from the participants. In this interview, each participant reported that they felt that the experimenter’s clapping was synchronized with their own in the OS condition, and not synchronized in the OA condition, confirming that the manipulation of interpersonal synchrony was subjectively perceived as intended.

2.6. Evaluation

To assess body perception during the clapping task, the participants completed a subjective report after each clapping task.

Table 1. Questionnaire.

Question Number	Question Text
Q1	It seemed as if I was feeling the touch of my hand in the location where my hands that I saw clapped.
Q2	It seemed as though the touch I felt was caused by the hands I saw hitting each other.
Q3	I felt as if the body that I saw was my body.
Q4	It seemed as if I might have more than one body.
Q5	It seemed as if the touch I was feeling came from somewhere between my own body and the body that I saw.
Q6	It seemed as if I could control the hand that I saw as my own hand.
Q7	It felt as if my (real) body was drifting toward the front (toward the body that I saw).
Q8	It appeared (visually) as if the body that I saw was drifting backward (toward my body).
Q9	It seemed as though I was in two places at the same time.

presents the questions used in this experiment. There were nine questions in total (Q1: “It seemed as if I was feeling the touch of my hand in the location where my hands that I saw clapped”; Q2: “It seemed as though the touch I felt was caused by the hands I saw hitting each other”; Q3: “I felt as if the body that I saw was my body”; Q4: “It seemed as if I might have more than one body”; Q5: “It seemed as if the touch I was feeling came from somewhere between my own body and the body that I saw”; Q6: “It seemed as if I could control the hand that I saw as my own hand”; Q7: “It felt as if my (real) body was drifting toward the front (toward the body that I saw)”; Q8: “It appeared (visually) as if the body that I saw was drifting backward (toward my body)”; Q9: “It seemed as though I was in two places at the same time”). The questions were adapted from previous studies investigating the FBI [5,13,14,23]. However, Q1, Q2, and Q9 were slightly modified to align with the clapping task used in this experiment. Additionally, Q6 was added to assess the sense of agency. In the experiment where the participants observed both their own virtual body and the experimenter’s body, the phrase “the body that I saw” in Q5, Q7, and Q8 referred specifically to the participant’s virtual body. Responses to each question were recorded on a 7-point Likert scale ranging from −3 to +3, where −3 indicated “Strongly Disagree”, 0 indicated “Yes or No”, and +3 indicated “Strongly Agree”. The questions were displayed on a laptop screen, and participants used the keyboard and touchpad to record their responses. The order of the questions was randomized for each trial. Depending on participant preference, the questions were displayed in either Japanese or English.

Previous studies have reported that in the condition where the FBI occurs compared with the condition where it does not, (1) the scores for questions related to touch referral (Q1, Q2) and ownership (Q3) were significantly higher, and (2) there were no significant differences for the other questions (Q4, Q5, Q7, Q8, Q9) [3,5,13]. For the experiment, however, since it was unknown whether interpersonal synchrony would influence body perception, we examined the potential impact across all items.

In the experiment, we examined whether the synchronization of bodily movements with another person affected the responses. Since the questionnaire used a Likert scale, which consists of ordinal and discrete data, we employed the Wilcoxon signed rank test, a non-parametric analysis method. Additionally, we calculated Cliff’s delta to assess the effect size. In the experiment, we compared the response values between condition OA and condition OS for each question. All data points were included in the statistical calculations without excluding any outliers. A significance level of p < 0.05 was considered statistically significant.

3. Results

The comparison results for each question are shown in Figure 3 and

Table 2. Comparison of subjective reports for each question under condition OA and condition OS was conducted using the Wilcoxon signed rank test. W represents the test statistic, p denotes the significance probability, and r is the effect size of Cliff’s delta. Double asterisks (**) means p < 0.01. Asterisk (*) means p < 0.05. Dagger (†) means 0.05 < p < 0.1.

Question Number	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Q9
W	16.5	0	0	2.5	2	2	3.5	7	2
p	0.465	0.008 **	0.066 †	0.027 *	0.040 *	0.129	0.577	0.891	0.257
r	0.132	0.562	0.273	0.455	0.413	0.256	0.050	0.033	0.165

. For Q2, Q4, and Q5, the response values under the OA condition were significantly higher than those under the OS condition (Q2: W = 0.0, p = 0.008, r = 0.562; Q4: W = 2.5, p = 0.027, r = 0.455; Q5: W = 2.0, p = 0.040, r = 0.413). On the other hand, no significant differences were found between the OA and OS conditions for Q1, Q3, Q6, Q7, Q8, and Q9 (Q1: W = 16.5, p = 0.465, r = 0.132; Q3: W = 0.0, p = 0.066, r = 0.273; Q6: W = 2.0, p = 0.129, r = 0.256; Q7: W = 3.5, p = 0.577, r = 0.050; Q8: W = 7.0, p = 0.891, r = 0.033; Q9: W = 2.0, p = 0.257, r = 0.165). Therefore, the results of the experiment suggest that interpersonal synchrony, particularly as it relates to the clapping task, affects the FBI, as indicated by the significant changes in Q2, which was associated with the illusion.

4. Discussion

This study investigated whether interpersonal synchrony with others affected the FBI. In the experiment, a significant difference was observed in Q2, and a marginally significant difference was noted in Q3. Since two out of the three FBI-related questions showed effects, it is suggested that interpersonal synchrony affects the 3PP-FBI. These findings represent novel contributions to the understanding of these phenomena.

The clearer difference observed in Q2 compared with Q3 warrants further exploration of the reasons why interpersonal synchrony may have had a more pronounced effect on touch referral than on ownership. Q2 pertained to touch referral, and the significantly lower response values in the OS condition compared with the OA condition suggest that interpersonal synchrony may reduce touch referral. Q3, which concerned ownership, also showed a trend toward lower response values in the OS condition, although the difference was not statistically significant. This trend aligns with the pattern observed for touch referral. Previous research has indicated that touch referral tends to precede the experience of ownership, and that touch referral does not necessarily result in a clear sense of ownership [24], but it might contribute to the emergence of ownership [25]. Therefore, the more pronounced effect on touch referral compared with ownership is consistent with these findings.

The smaller effect observed for Q3 compared with Q2 may also be due to the participants’ awareness that the virtual self was a real-time reflection of their own body captured by the camera. This awareness, known as self-recognition, may have influenced their responses, indicating that Q3 may not purely measure ownership [26,27]. This factor could have affected the results of this study by diminishing the observed differences in ownership.

Q4 and Q5 also showed lower response values in the OS condition compared with the OA condition. Q4 was related to self-identification, and Q5 concerned disembodiment, suggesting that interpersonal synchrony may weaken self-identification with the self-virtual body. However, regarding Q5, the average response value in the OA condition was close to 0, indicating that participants tended to respond ‘neither agree nor disagree’ . Therefore, caution is needed when interpreting whether the participants felt stronger self-identification in the OA condition compared with the OS condition. Additionally, Q4 and Q5 have been criticized for containing multiple elements and having low reliability [28], so caution is required in interpreting these results.

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. These findings confirm that classic FBI mechanisms apply even in an interactive, two-person context, and justify extending the FBI framework to situations involving real-time social coordination.

In this study, we adopted a simple clapping interaction as the experimental task. However, transferring our findings to more complex and realistic VR/AR environments remains hypothetical. Future research should explore cooperative or competitive multiplayer VR settings to examine whether similar effects on body perception emerge under more dynamic and interactive conditions. Previous work has already shown that social interactions in VR can affect cognitive and perceptual processes. For instance, Yakura et al. [35] found that participants experienced higher levels of immersion when ‘audience avatars’ in a VR concert faithfully replicated real-world movements. Although that study did not directly assess body perception, the reported increase in presence implies that avatar behavior may modulate the embodied experience. These findings suggest that even more complex interactions in VR/AR could similarly influence body perception, warranting further investigation.

Individual differences such as attentional focus and social sensitivity may modulate the impact of interpersonal synchrony on body perception. For instance, Thériault et al. [36] reported that directing the participants’ attention during an RHI paradigm altered the illusion’s strength, indicating that top-down factors can influence body-ownership. Similarly, Asai et al. [37] showed that higher levels of empathy (a form of social sensitivity) predicted greater susceptibility to the RHI. Although our current proof-of-concept did not control for these variables, future studies should measure or manipulate attention and empathy to clarify how they interact with social coordination in shaping the FBI outcomes.

We selected hand-clapping because it provides a simple, controllable form of interpersonal synchrony compatible with the FBI paradigm’s need for synchronous visuotactile stimulation. Previous works (e.g., [22]) have frequently used clapping to elicit prosocial effects. Although social interactions can differ in temporal structure, modality, and goal-directedness, clapping in unison represents a low-level coordination without semantic or goal content. By showing that even this basic form of synchrony modulates touch referral and ownership in an FBI context, we have established a foundation for future work. Subsequent studies should investigate how more complex or naturalistic interactions—such as synchronous dancing, collaborative tasks, or vocal turn-taking—affect the FBI and related aspects of bodily self-consciousness.

This study experimentally confirmed that interpersonal synchrony affects the FBI. However, the evaluation method relied on subjective reports. Previous research has employed objective evaluation methods, and combining these methods could provide a more comprehensive understanding of the phenomena. Additionally, this experiment only confirmed whether synchrony influenced the FBI. Further research is needed to investigate how synchrony specifically impacts the FBI. While several hypotheses about the mechanisms of synchrony’s influence on the FBI have been proposed, these hypotheses are difficult to test based on the current results alone. Evaluating bodily sensations toward others or groups could contribute to elucidating these mechanisms, which remains a task for future research.

Another limitation of this study is that although the participants were not directly informed of the experiment’s purpose after completing the pre-screen task, they might have inferred it based on the questionnaire they answered. If the participants guessed the study’s purpose, the demand characteristics could have biased the results [38]. This potential awareness may have influenced their responses in the main experiment, which was conducted immediately afterward. In future work, we need to consider methods to avoid or mitigate the influence of demand characteristics on the results. Specifically, possible approaches include using a cover story to conceal the true purpose of the experiment, or employing the Sensory Suggestibility Scale (SSS) to assess the participants’ susceptibility and then incorporating this tendency into the analysis to account for the possibility that participants may unconsciously report the expected experiences. These approaches will help minimize potential bias and provide more reliable insights into the effects of interpersonal synchrony on the FBI.

This study involved 23 participants, and data were collected to investigate the effect of interpersonal synchrony on body perception. The data from 11 participants who met the criteria were used for the analysis. The number of participants included in this analysis was comparable to prior studies, which typically involved 10 to 20 participants (e.g., [3,13,39]). However, by further increasing the sample size, the phenomenon addressed in this study is expected to be further clarified, and a more detailed analysis could be facilitated. This aspect should be considered in future research.

Following Swinkels et al. [14], we pre-screened participants to select those likely to experience the FBI, ensuring the illusion was reliably induced. Because some individuals find it difficult to experience the illusion [14,40], including low responders could obscure the synchrony effects. Future work should develop FBI paradigms that incorporate interpersonal synchrony and reliably induce the illusion across diverse participants. Such paradigms would broaden sampling, improve generalizability, and enable more ecologically valid studies of how social interaction affects body perception.

In the present study, interpersonal synchrony was manipulated by having the experimenter visually monitor the participant’s movements and adjust their own clapping accordingly. Whether synchrony was achieved was assessed based on the participants’ subjective reports. However, maintaining precise synchrony manually is inherently challenging, and the actual level of synchrony may have varied or been subject to bias. Furthermore, no objective measure was employed to quantify the degree of synchrony achieved. Future work should aim to more rigorously control and objectively evaluate the synchronization between individuals.

5. Conclusions

In this study, an evaluation experiment using the FBI was conducted to investigate whether interpersonal synchrony affected body perception. Specifically, the participant and the experimenter stood side by side and clapped, while a video camera positioned behind the participant recorded the scene. The footage was then presented to the participant through an HMD, providing synchronous visuotactile stimuli to induce the FBI. The experimenter adjusted the clapping rhythm to either synchronize with or desynchronize from the participant’s rhythm, thereby controlling the synchronization state between the participant and the experimenter. Body perception was then assessed through subjective reports. The results showed that when the clapping rhythm was synchronized with another person, touch referral to the participant’s virtual body significantly decreased, and there was a trend toward reduced ownership. This study was the first to demonstrate that interpersonal synchrony influences body perception.