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.

8The full-body ownership illusion exploits multisensory perception to induce a feeling of ownership for an 9 entire artificial body. Whilst previous research has shown that the synchronous visuotactile stimulation of a single 10 body part is sufficient for illusory ownership over the whole body, the effect of combining multisensory stimulation 11 across multiple body parts remains unknown. Therefore, 48 healthy adults participated in conditions of a full-12 body ownership illusion involving synchronous or asynchronous visuotactile stimulation to one, two or three body 13 parts simultaneously (2 x 3 design). We developed a novel questionnaire to isolate the sense of ownership of 14 five specific body parts (left leg, right leg, left arm, right arm, and trunk) from the full-body ownership experience 15 and sought not only to test for greater (part and whole) body ownership in synchronous versus asynchronous 16 stimulation, but also, potentially varying degrees of illusion intensity related to the number of body parts 17 stimulated. As expected, illusory full-body ownership and all five body-part ownership ratings were significantly 18 higher following synchronous stimulation (all p values ≤ .01). Since non-stimulated body parts also received 19 significantly higher ownership ratings following synchronous stimulation, the results are consistent with an illusion 20 engaging the entire body. We further noted that ownership ratings for the right body parts (often stimulated) were 21 significantly higher than ownership ratings for the left body parts (never stimulated). Regarding explicit feelings 22 of full-body ownership, subjective ratings were not significantly enhanced by increasing the number of 23 synchronously stimulated body parts (synchronicity x number stimulated interaction; p .099). Instead, median 24 ratings indicated a moderate affirmation (+1) of full-body illusory sensation for all three synchronous conditions; 25 a finding mirrored by full-body illusion onset time. The results support the notion that feelings of full-body 26 ownership are mediated by a generalisation from stimulated part(s)-to-whole, supported by processes related to 27 multisensory body perception.