Illusory body ownership1,2,3 can be induced to a virtual body by visual-tactile contingent stimulation4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22 or visual-motor congruent actions23,24,25,26. The Rubber Hand Illusion is representative of visual-tactile-stimulation induced illusory body ownership. Stroking a participant’s hand and a rubber hand with paintbrushes at the same time causes illusory body ownership of the rubber hand if the participant sees only the rubber hand and paintbrush4,6. Virtual Reality systems have often been used for induction of visual-motor-contingent body ownership. When visual body movements are presented using a head-mounted display (HMD) and are synchronized with a participant’s actual body movements, he/she feels the virtual body as his/her own body23,24. The methods to cause illusory body ownership can be categorized into passive contingent visual-tactile stimulation and active synchronicity of visual body stimuli and motor actions. The active method induces a sense of agency in addition to body ownership and generally induces stronger body ownership than the passive method27.

The conscious experience of ownership of body parts such as the Rubber Hand Illusion4,5,6,12,13,15,16,17,18,19,22,24,28,29,30,31 and the experience of global ownership such as Full-Body Ownership7,10,14,20,21,23,25,26,27,32,33,34,35 should be considered separately to understand self-consciousness1. The studies of Full-Body Ownership contribute to investigate the idea of “minimal phenomenal selfhood”, that is, the conscious experience of being a self, and relate to the embodiment and the simplest form of self-consciousness1. The out-of-body experience has been investigated in neurological and clinical studies36,37,38. During an out-of-body experience, a person has the feeling of seeing their own body and the environment from a viewpoint that is distant from the physical body. It has been observed that brain damage and stimulus to the temporo-parietal junction can induce the out-of-body experience. Thus, the temporo-parietal junction is a critical region for the conscious experience of the normal self and its embodiment39.

Out-of-body experiences can be linked to illusory body ownership using passive visual-tactile stimulation7,8,9,10. Lenggenhager et al.7 presented a virtual body in front of the participant, and visually synchronized tactile sensation to his/her back to induce the full-body ownership illusion. The illusory body ownership of the virtual body caused the participant’s proprioceptive self-localization to drift toward the virtual body7,11. Pomés and Slater10 replicated the study of Lenggenhager et al.7 by measuring behavioral responses to a threat to a virtual body and included a questionnaire on the proprioceptive drift. They found significant perceptions of both a participant’s own body drifting toward the virtual body placed in front, and the virtual body moving backward in the synchronous condition. A significant positive correlation was observed between the feeling of illusory-body drift forward and responses to the threat, although the feeling of illusory body ownership and response to the threat were not significantly different between the synchronous and asynchronous conditions. Thus, the proprioceptive drift forward is associated with a greater response to the threat, while the feeling that the virtual body is moving backward decreases the response to the threat.

The proprioceptive drift of own body-part location was originally reported in the Rubber-Hand-Illusion studies4,6. Thus, the drift of proprioceptive self-body or body-part location has been considered as one of the behavioral measurements of illusory body ownership. However, it is reported that proprioceptive drift depends on the duration of visual-tactile sensations; the drift occurs with synchronous, asynchronous, or no tactile stimulation using short and frequent stimulations, and is prevented only by continuous exposure to asynchronous stimulation12. Thus, the feeling of ownership cannot be measured by the proprioceptive drift alone.

Body ownership can be induced in a wide variety of bodies16,28,32,33,34 or still objects5. Various studies have investigated how the experience of body ownership to different bodies changes human behavior and implicit social attitudes18,25,26,35,40. Illusory body ownership in different skin colors decreases implicit racial bias18,25. Adults’ illusory body ownership to a child body avatar modulates child-like implicit attitudes as well as object-size perception26. Thus, illusory body ownership can be induced to various bodies in different shapes, colors, and ages. In passive visual-tactile contingent stimulations, the synchronicity of visual and tactile stimuli is critical to induce such illusions, while in active visual-motor stimulations, the synchronicity of visual stimuli and motor action is critical.

Recently, it has been reported that body ownership can be induced to an empty space by presenting visual-tactile stimuli17,20,21. An entire invisible body ownership is induced when participants observe a paintbrush moving in an empty space and by defining the contours of an invisible body through an HMD from a first-person perspective while receiving simultaneous touches on the corresponding parts of their real body. The illusory ownership of an entire invisible body reduces autonomic and subjective social anxiety responses caused by standing in front of an audience20. In contrast, an illusion of missing body parts through illusory ownership of an amputated virtual body can be induced by eliminating a virtual (visual) body part and not applying physical touches to the body part corresponding to the missing part22. This illusory experience of amputation decreases corticospinal excitability of the illusory amputated body part.

The purpose of our study was to test whether the illusory ownership of an invisible body could be induced by the active method of visual-motor synchronicity, and if the illusory invisible body could be experienced in front of the observer similar to the full-body ownership illusion.

In Experiment 1, we tested whether illusory body ownership can be induced by presenting only visual gloves and socks in synchrony and consistent with the observer’s own movements. The gloves and socks were presented in front of and facing away from the observers, in third-person perspective. We compared the synchronous condition, i.e. the virtual gloves and socks moved synchronously with the observer’s action, with the asynchronous condition, i.e. the gloves and socks moved independently of the observer’s action. In Experiment 2, we compared the invisible condition, i.e. where only gloves and socks were presented, with the visible body condition so that a whole-body avatar was presented. The whole-body avatar was also presented in front of and facing away from the observers. In these experiments, after participants moved their own body by observing the avatar stimuli for 5 min, a threat stimulus appeared suddenly (see the Methods section for details). Then, participants answered a questionnaire (see Figs 1 and 2). Finally, in Experiment 3, we tested whether self-localization drift could occur with illusory body ownership induced by only visual gloves and socks. When illusory body ownership occurs with the virtual invisible body in front of the participant, self-location will drift toward the virtual invisible body similar to the full-body ownership illusion7. All experiments were conducted in within-group designs where all subjects (20, 20, and 10 naïve participants for Experiment 1, 2, and 3, respectively) performed all conditions (synchronous vs asynchronous conditions in Experiment 1 and 3, and visible and invisible bodies in Experiment 2).

Figure 1 Results of Experiment 1. Subjective ratings of questionnaires. The error bars indicate SE. Full size image