Audiovisual stimuli based on out-of-body illusions

The aim of this study was to investigate whether synchronized audiovisual stimuli can produce an out-of-body illusion. We hypothesized that significantly more out-of-body illusions would be evoked in the synchronized audiovisual condition than in the asynchronous audiovisual condition. Therefore, to investigate this hypothesis, tasks with tactile-visual and audiovisual stimuli were performed. In each task, the stimuli of the two modalities were either synchronized or asynchronous. Thus, each participant was assigned four conditions that combined the task and the difference in synchronization. Two questionnaires were then completed to assess the induction of an out-of-body illusion.

The results of Questionnaire 1 showed that the OBI index, which indicates whether audiovisual and tactile-visual stimuli triggered an out-of-body illusion, was significantly higher in the synchronized condition than in the asynchronous condition. This was consistent with the results of a previous study^9.10. Furthermore, the OBI index was also significantly higher than the control index in the synchronized audiovisual and tactile-visual conditions, which was also consistent with previous results^9.10. Furthermore, as hypothesized, the control index did not differ significantly between the synchronized and asynchronous conditions for either audiovisual or tactile-visual stimuli. In Questionnaire 2, the level of anxiety for both audiovisual and tactile-visual stimuli was significantly higher in the synchronized conditions than in the asynchronous conditions, similar to a previous study⁹. These results showed that audiovisual stimuli produced out-of-body illusions, which was consistent with results from previous studies. However, this study was the first to show that out-of-body illusions can be induced by synchronized audiovisual stimuli without somatic sensation stimuli. An out-of-body illusion was also induced by synchronized tactile-visual stimuli, similar to the results of a previous study.

The results of Questionnaire 1 showed no significant differences between the modalities of the stimuli (tactile-visual and audiovisual stimuli) in the OBI and control indices for the synchronized and asynchronous conditions. Likewise, in Questionnaire 2, no significant difference was observed in fear responses in the synchronized and asynchronous conditions, regardless of modality. Therefore, the out-of-body illusion was similarly induced by both synchronized tactile-visual and audiovisual stimuli.

Previous studies used tactile stimuli to create an out-of-body illusion^9,11,25. Therefore, stimuli had to be applied directly to the participant’s body to create an out-of-body illusion. Conversely, when an out-of-body illusion was induced by synchronized audiovisual stimuli, it could be induced by providing stimuli that did not need to be delivered directly to the participants’ bodies. These results showed that audiovisual stimuli can create an out-of-body illusion throughout the body. In addition, previous studies also reported that out-of-body illusion brings about changes in physical self-positioning^10,26,27. A positive value of the OBI index in this study implied that the variant of the study based on audiovisual synchronized stimuli also included changes in self-localization.

Based on previous studies, what mechanism triggers an out-of-body illusion? This phenomenon may be related to information processing in all aspects and its multisensory integration. Multisensory integration is the construction of multiple sensory stimuli, such as what we see, hear, and touch, as coherent rather than separate perceptions. Among the numerous regions of the brain where multisensory integration occurs, the temporoparietal junction is significantly involved in an out-of-body illusion. Previous studies showed that out-of-body illusion due to drug abuse, general anesthesia, and sleep is related to the inability to integrate multisensory information from the actual body at the temporoparietal junction^8.28. Guterstam et al.²⁹ reported that people experienced an out-of-body illusion in a functional magnetic resonance imaging study that scanned human brain activity. They proposed that the involvement of two brain networks was associated with changes in body ownership and self-location, respectively. Illusional self-localization was associated with activity in the hippocampus and posterior cingulate, retrosplenial, and intraparietal cortices. Furthermore, body ownership was associated with premotor-intraparietal activity. These brain regions could also be involved in an out-of-body illusion induced by synchronous audiovisual stimulation.

Previous studies have not thoroughly investigated the neuroscientific aspects of an out-of-body illusion induced by audiovisual synchronous stimulation. Therefore, to address this issue, we considered previous research on out-of-body illusion through visual-somatosensory synchronous stimulation. In an out-of-body illusion based on visual-somatosensory synchronous stimulation, different neural information processing systems in the brain managed visual and somatosensory stimuli. The illusion was caused by their multisensory integration^12,14,16. Therefore, when an out-of-body illusion was induced by synchronous visual-somatosensory stimulation, visual and somatosensory information were first processed separately in the nervous system and subsequently integrated. Next, we considered an out-of-body illusion based on audiovisual synchronous stimulation according to previous findings. Specifically, visual information is processed in the visual cortex in the occipital lobe, while auditory information is processed in the auditory cortex in the temporal lobe. In addition, the temporoparietal junction, associated with an out-of-body illusion, integrates visual and auditory inputs³⁰. When visual and auditory stimuli are presented synchronously³¹ and from the same place³²More multisensory neurons that are involved in multisensory integration are activated. This suggests that visual and auditory information may be closely related. Thus, in an out-of-body illusion based on audiovisual synchronous stimulation, the audiovisual synchronous stimulation can be processed separately and subsequently integrated, as in visual-somatosensory synchronous stimulation.

Further research is needed to determine the characteristics of audiovisual stimuli sufficient to produce an out-of-body illusion. Our results suggest that spatially and temporally matched audiovisual stimuli may have sufficient stimulus characteristics to produce an out-of-body illusion. A previous study suggested that people perceive time differently in near and far space and, consequently, the distance to the signal source affects sound³³. Therefore, people can experience a sense of embodiment when both the visual and auditory stimulus sources are spatially and temporally consistent. Specific features of simultaneous audiovisual stimuli that produce an out-of-body illusion will be further clarified in future research based on the results of this study.

This study has some limitations. First, only the questionnaire results were included and no objective indirect tests were conducted. Specifically, we did not measure threat-evoked skin conductance responses (SCR) during the threat procedure and relied solely on fear ratings. However, fear ratings correlated with SCR and brain activity in areas related to fear and pain expectancy^9,29,34,35. Previous studies reported that fear reports, such as when illusory body parts were physically threatened during body possession illusions, correlated with objective physiological measures of fear and pain expectancy responses. The more intensely people experienced an illusion of body ownership, the greater the fear reported when the illusionary body was threatened. Furthermore, the greater the fear, the greater the threat-evoked skin conductance responses (SCR) and threat-evoked BOLD activation in areas associated with fear and pain expectation, such as the anterior cingulate cortex and insular cortex^9,29,34. Therefore, an anxiety report was an index of the emotional defensive responses triggered by threats to one’s own body or what was perceived as one’s own body in body illusions. Anxiety ratings are a valid indicator of illusory changes in body ownership and self-location⁴. Future studies should further verify this phenomenon using questionnaires and physiological indicators such as the SCR. In addition, assessing the sense of self-location using measurement methods other than questionnaires may also be useful to further investigate the phenomenon of out-of-body illusion^10,25,26.

The paradigm of this study was an out-of-body illusion based on the first-person perspective. Other studies examined out-of-body illusions based on a third-person perspective via a VR environment using a VR HMD^36.37. These studies showed that visual and tactile sensations produced significantly more out-of-body illusions when stimuli were delivered synchronously than asynchronously. Another study reported that the sense of embodiment changed when the movements of the virtual body were synchronized with those of the participant’s real body²⁵. Ehrssons⁹ This paradigm involved first-person multisensory integration, resulting in a strong perceptual illusion of body ownership and self-location. Vice versa Lenggenhager³⁶ This paradigm involved visual-tactile stimulation from a third-person perspective, resulting in a weaker cognitive sense of illusory self-identification. Full-body illusions experienced from a first-person perspective were significantly stronger than those experienced from a third-person perspective^{38,39,40,41,42}. Future studies should verify the experimental conditions using the third-person perspective.