In all, 24 participants (age: mean 23.29, SD 1.97 years; female: n=12, 50%) were recruited for the study. A power analysis using PASS power analysis and sample size software (version 2019; NCSS) was run to determine sample size. All participants provided written informed consent and were compensated with US $40 for their cooperation.

All experimental protocols were approved by the Hanyang University Institutional Review Board (HYUIRB-202107-014-1). Photographed individuals in the figures provided written informed consent to publish the case details.

To generate synchrony between the participant and avatar, a motion-capture system (Motive; version 2.0.2; Natural Point) and 18 Flex 13 cameras (Natural Point) were used. Participants wore a full-body motion-capture suit with 37 reflective markers. To generate a 3D-scanned face, images of the participant’s face were captured using a mobile device (iPhone SE2; Apple Inc). With the captured images, the participant’s 3D face model was generated through the Metashape (Agisoft LLC) and Blender (Blender Institute) software. The virtual environment of the experiment was implemented using Unity software (version 2018.3.0f2; Unity Technologies). The software was run on a desktop PC (Windows 10 OS; Microsoft) with Intel Core i7-6700 (Intel) CPU, 16GB RAM (Samsung), and NVIDIA GeForce GTX 3070 (Nvidia) GPU.

Participants wore a motion-capture suit and HMD (HTC Vive Pro eye; HTC) to experience the virtual environment. The virtual environment was a small room, about 4 m (width) × 4 m (length) × 2.5 m (height), and a virtual mirror was set in front of the participant’s location. A participant could observe the virtual avatar’s body either directly through first-person perspective or by looking in the mirror. The virtual environment and experimental settings are presented in Figure 1.

All data were analyzed using SPSS statistical software (version 27.0; IBM Corp). The evaluations of normality were performed using the skewness, kurtosis, and Kolmogorov-Smirnov tests. A 2 (sync vs async) × 2 (personalized vs generic) repeated measures ANOVA was conducted on all dependent measures to examine the effects of motion synchrony, appearance similarity, and interaction between the 2 variables. Post hoc analyses then followed. The level of statistically significant P value was set to .05.

The average similarity score from the SQ did not show a significant main effect for motion (F_1,23=1.448; P=.24; sync mean .14; async mean –.01), whereas a significant main effect was demonstrated for appearance (F_1,23=32.920; P<.001; η²=.589; personalized mean .92; generic mean –.79). The interaction effect between the 2 factors was not significant (F_1,23=1.178; P=.29).

The result of embodiment is illustrated in Table 3.

The result of virtual experience from the Virtual Experience Questionnaire is illustrated in Table 4.

This study investigated the effects of motion synchronization and appearance similarity on participants’ embodiment, perceived similarity, and subjective experience in VR. The results showed that participants experienced a higher level of agency and body ownership when body motion was synchronized and appearance was similar to theirs. Surprisingly, under the motion and appearance synchronizations, participants reported that the body motion of the virtual avatar was driven by them even when the virtual avatar moved independently, suggesting that the personalized appearance of the virtual avatar’s body can create an illusory agency toward the avatar’s movement that was not performed by the participant. Furthermore, our results indicated that the synchronization of motion could contribute to higher presence and induce more positive emotion.

Two novel findings on embodiment can be listed from the results of this study. First, there was a statistically significant interaction between motion and appearance on agency (significant difference between async-personalized and async-generic conditions in agency). Prior studies have reported that synchronizing motion [1,5-7] could contribute to form a higher level of agency. Results from this study support such previous findings, proving that the integration of visual-motion synchronization, which fulfills proprioception [18], forms a feeling that “I am the only cause of the avatar’s body motion” [1]. In addition to this finding, this study demonstrated that agency was associated with body motion that was not caused by the participant; therefore, the body motion could be felt as the participant’s movement when the virtual avatar resembled the participant’s appearance. This kind of attribution of behavior toward oneself has been shown in several previous studies. For example, Aymerich-Franch et al [19] reported that participants felt shame and guilt for the misbehavior of a humanoid robot, which they did not perform. Likewise, Jun et al [5] reported that participants’ emotions changed to happy, neutral, and sad according to a virtual avatar’s emotional status. Furthermore, this study extends previous findings that body movement can be attributed to oneself with a personalized avatar. In the motion sync condition, however, the agency difference was not significant between the personalized and generic avatars. We speculate that the reason behind this finding is a higher agency score (average score of 2.31 out 3), which might have caused a ceiling effect. The current result can be used in the field of cognitive behavioral therapy, such as behavior modeling, which involves observing others to learn desirable behaviors. Using a personalized avatar, which facilitates more agency toward the avatar’s action, can be more effective compared to using general avatars in behavior modeling.

Second, it was evident that the 2 factors (ie, motion and appearance) in this study affected body ownership independently. Prior studies have found that synchronizing motion [11] and personalized avatars [1,10-12] could contribute to form a higher level of body ownership. The current findings support previous studies by showing that the integration of motion synchronization [1] and visual aspects with higher fidelity [10] form a feeling that the body is one’s own. Prior studies have revealed that the level of visual fidelity increased in the order of point-light avatar [1], robot avatar [10], human avatar [5], size matched avatar [11], and 3D-scanned avatar [12]. This study extends these previous findings by revealing that body ownership can be further increased with a avatar that has a 3D-scanned face and size-matched body.

In addition, it was evident that the factors of subjective virtual experience (ie, presence, simulator sickness, and emotion) were affected by motion synchrony. This result supports previous studies that confirmed higher presence [1,11] and positive emotion [1,5] by motion synchronization. Furthermore, a moderate interaction between appearance similarity (personalized vs generic) and motion synchrony (sync vs async) in emotional valence (significant difference between sync and async in personalized avatar, whereas no difference in generic avatar condition in valence; Table 4) showed that motion synchronization with a personalized avatar could induce more positive emotions than a generic avatar. On the contrary, the effect on arousal and simulator sickness was not significant. Low arousal scores across the conditions suggest that the participants were in a calm state during the experimental tasks. We speculate that low arousal might have affected simulator sickness, resulting in a floor effect across conditions.

In addition to the main findings addressed above, it is worth noting the methodology we used for generating personalized avatars in this study. In fact, many prior studies have attempted to create personalized avatars by 3D scanning a participant’s face and body [2,10,12,14]. However, due to complex settings such as red-green-blue-depth camera [14] or multiple time-synchronized red-green-blue cameras [12,13], it is challenging to use this methodology to construct a personalized avatar; thus, it is not appropriate for general use. Inspired by Gorisse et al [10] and Jung et al [8], our study proposed a more convenient method for creating a personalized avatar using a single smartphone and body-size measuring. The result from the SQ showed that our method could enhance a feeling of appearance similarity between the person and avatar. Thus, in future metaverse applications, each user can easily generate their personalized avatar using their smartphone to dive into VR.

There are a few limitations in this study. First, the participants were restricted to healthy young adults. Future studies should consider recruiting participants across diverse age groups and health statuses. Second, as the prerecorded animation of the async condition consisted of simple body movements that can be performed easily by the participants, it would be stimulating to use a body motion that is “impossible” to perform to generalize the effect of appearance similarity on agency in the follow-up studies. Third, this study asked participants to move freely in the main task and measured arousal and valence as emotional variables. Although appearance similarity did not affect participants’ emotions, future studies should consider participants who have negative feelings toward their body, such as individuals with eating disorders [20]. An attitude toward one’s body should be considered for future medical applications.

This study proposed a method for creating personalized avatars using a single smartphone camera and an avatar’s body size manipulation. Furthermore, this study’s result indicated that participants perceived that the virtual avatar’s appearance was similar to them. Furthermore, participants established a higher sense of embodiment toward the virtual avatar’s body that was similar to theirs in body motion and appearance. Moreover, we discovered that the synchronization of appearance could result in a sense of agency toward an avatar’s movement. We hope that the findings in the current study can contribute to the fields of physical activity promotion [21], social cognition training [22], and pain intervention [23], suggesting that matching body motion and appearance can enhance the VR experience.