Abstract
The full-body ownership illusion exploits multisensory perception to induce a feeling of ownership of an entire artificial body. Although previous research has shown that synchronous visuotactile stimulation of a single body part is sufficient for illusory ownership of the whole body, the effect of combining multisensory stimulation across multiple body parts remains unknown. Therefore, 48 healthy adults participated in a full-body ownership illusion with conditions involving synchronous (illusion) or asynchronous (control) visuotactile stimulation to one, two, or three body parts simultaneously (2×3 design). We used questionnaires to isolate illusory ownership of five specific body parts (left arm, right arm, trunk, left leg, right leg) from the full-body ownership experience and sought to test not only for increased ownership in synchronous versus asynchronous conditions but also for potentially varying degrees of full-body ownership illusion intensity related to the number of body parts stimulated. Illusory full-body ownership and all five body-part ownership ratings were significantly higher following synchronous stimulation than asynchronous stimulation (p-values < .01). Since non-stimulated body parts also received significantly increased ownership ratings following synchronous stimulation, the results are consistent with an illusion that engages the entire body. Furthermore, we noted that ownership ratings for right body parts (which were often but not always stimulated in this experiment) were significantly higher than ownership ratings for left body parts (which were never stimulated). Regarding the effect of stimulating multiple body parts simultaneously on explicit full-body ownership ratings, there was no evidence of a significant main effect of the number of stimulations (p = .850) or any significant interaction with stimulation synchronicity (p = .160), as assessed by linear mixed modelling. Instead, median ratings indicated a moderate affirmation (+1) of an illusory full-body sensation in all three synchronous conditions, a finding mirrored by comparable full-body illusion onset times. In sum, illusory full-body ownership appears to be an ‘all-or-nothing’ phenomenon and depends upon the synchronicity of visuotactile stimulation, irrespective of the number of stimulated body parts.
Highlights
All recruits were naïve to the full-body ownership illusion, confirming that they had not participated in a similar study before based upon minimal information, such as viewing a mannequin’s body in a head-mounted display (HMD)
Since all experimental questionnaire data pertaining to synchronous visuotactile stimulation are associated with positive ratings, all data pertaining to asynchronous visuotactile stimulation are associated with negative ratings, and control data are associated with negative ratings regardless of stimulation synchronicity
Consistent with this, our planned comparisons revealed that synchronous visuotactile stimulation resulted in significantly increased illusory full-body ownership ratings compared to asynchronous visuotactile stimulation, while there were no differences between conditions varying by the number of synchronously stimulated body parts (Table 2; Q8)
Summary
How does one come to perceive that one’s body, the single, integrated biological entity in which one senses and acts upon one’s environment, belongs exclusively to oneself? What are the neurocognitive principles governing the perception of one’s own body not as a set of fragmented body parts but as the gestalt that delineates the boundaries between what is the self and what is not? The feeling of ‘body ownership’ [1,2,3,4] attracts attention across diverse academic fields, the distinction between part and whole, referred to as ‘body-part ownership’ and ‘full-body ownership’, respectively [2,3,5,6,7,8,9], has been studied less often. Subjective reports of referral of touch (the illusory experience of directly feeling the touches applied to the artificial body) plus some degree of illusory ownership of the entire mannequin are well supported in the majority of participants [5,6,7,9]. These results support multisensory integration of visual, tactile and proprioceptive input as an essential framework to investigate the feeling of full-body ownership [2,3,14,15]
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