The building blocks of the full body ownership illusion

Antonella Maselli, Mel Slater, Antonella Maselli, Mel Slater

Abstract

Previous work has reported that it is not difficult to give people the illusion of ownership over an artificial body, providing a powerful tool for the investigation of the neural and cognitive mechanisms underlying body perception and self consciousness. We present an experimental study that uses immersive virtual reality (IVR) focused on identifying the perceptual building blocks of this illusion. We systematically manipulated visuotactile and visual sensorimotor contingencies, visual perspective, and the appearance of the virtual body in order to assess their relative role and mutual interaction. Consistent results from subjective reports and physiological measures showed that a first person perspective over a fake humanoid body is essential for eliciting a body ownership illusion. We found that the illusion of ownership can be generated when the virtual body has a realistic skin tone and spatially substitutes the real body seen from a first person perspective. In this case there is no need for an additional contribution of congruent visuotactile or sensorimotor cues. Additionally, we found that the processing of incongruent perceptual cues can be modulated by the level of the illusion: when the illusion is strong, incongruent cues are not experienced as incorrect. Participants exposed to asynchronous visuotactile stimulation can experience the ownership illusion and perceive touch as originating from an object seen to contact the virtual body. Analogously, when the level of realism of the virtual body is not high enough and/or when there is no spatial overlap between the two bodies, then the contribution of congruent multisensory and/or sensorimotor cues is required for evoking the illusion. On the basis of these results and inspired by findings from neurophysiological recordings in the monkey, we propose a model that accounts for many of the results reported in the literature.

Keywords: first person perspective; full body ownership illusion; multisensory integration; out-of-body experiences; perceptual illusions; rubber hand illusion; virtual reality.

Figures

Figure 1
Figure 1
Perspective levels and body appearances levels used for the different conditions of the experiments. Gender-matched avatars were assigned to each participant. [A,B (D,E)] Show the two levels of the factor “perspective” for a female (male) participant: in the 1PP conditions (A,D,C,F) when looking down participants could see a virtual body co-located with their own physical body, while in the 3PP conditions (B,E) participants saw a virtual body in their near extrapersonal space when looking to the left. The two levels of the factor “bodily appearance” are shown in [A,C (D,F)] for a female (male) participant: in the HA conditions participants had a co-located virtual body with a realistic human appearance (A,D), while in the MA condition a co-located virtual body resembling a plastic mannequin (C,F).
Figure 2
Figure 2
The vibrotactile device used in the experiments. The left panel shows the haptic vest used to deliver touch sensations: vibrators were located on the vest, mapping the location of the contact points of the ball's pre-recorded path with the avatar's chest. The right panel shows the paths followed by the virtual ball (blue lines) and its contact points with the avatar chest (yellow dots). In synchronous conditions the vibrators array was synchronized with the pre-recorded path, so that each vibrator was activated when the ball reached the corresponding path-chest contact point.
Figure 3
Figure 3
Snapshots of the legs separation event are shown for the mannequin and human appearance modes in the case of a female participants. The legs separation event lasted 10 s during which the legs moved away from the body reaching a maximum distance of 1 m. The legs returned to their original position immediately after the 10 s, recomposing a whole body.
Figure 4
Figure 4
Questionnaire data from experiment 1, in which we investigate the role of viusotactile and visual sensorimotor contingencies. No difference was found in the mybody item showing that the illusion was not affected by either of the two multisensory modalities (when a 1PP is provided over a realistic virtual body). Significant differences were found for the touch item with respect to the visuotactile mode, for both levels of the head tracking factor. This showed that the vibrotactile device used to deliver touch sensations was effective. *p < 0.05, **p < 0.01, ***p < 0.001 from Mann–Whitney test.
Figure 5
Figure 5
Questionnaire data from experiment 2, in which we compare groups with different perspective levels. 3PP (9 subjects), 1PP9 (9 subjects) and 1PP36 (36 subjects, where all 1PP conditions were grouped together irrespectively of the visuotactile and head tracking modes). The significant difference in the mybody item showed that 1PP is an essential condition for the full body ownership illusion. **p < 0.01, ***p < 0.001 from Mann–Whitney test.
Figure 6
Figure 6
Questionnaire data from experiment 3, in which we compared two groups that had different virtual bodies. Mannequin appearance: MA (9 subjects); human appearance: HA9 (9 subjects) and HA36 (36 subjects, where all the human appearance conditions in the 1PP were grouped together irrespectively of the visuotactile and head tracking mode). Significant differences were found when comparing the MA group with the HA36 group. This showed that a realistic body appearance significantly enhances the illusion experience. **p < 0.01 from Mann–Whitney test.
Figure 7
Figure 7
Path analysis model that fits data from experiment 2. The boxes represent the variables: perspective can be 0 or 1 for 3PP and 1PP, respectively; ownership is the response to the questionnaire item mybody; HRDbase is the heart rate deceleration baseline, and HRDlegs is the heart rate deceleration measured right after the legs start separating. The circles represent random error terms.

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