Abstract
Experiencing a body part as one’s own, i.e., body ownership, depends on the integration of multisensory bodily signals (including visual, tactile, and proprioceptive information) with the visual top-down signals from peripersonal space. Although it has been shown that the visuo-spatial viewpoint from where the body is seen is an important visual top-down factor for body ownership, different studies have reported diverging results. Furthermore, the role of visuo-spatial viewpoint (sometime also called first-person perspective) has only been studied for hands or the whole body, but not for the lower limbs. We thus investigated whether and how leg visuo-tactile integration and leg ownership depended on the visuo-spatial viewpoint from which the legs were seen and the anatomical similarity of the visual leg stimuli. Using a virtual leg illusion, we tested the strength of visuo-tactile integration of leg stimuli using the crossmodal congruency effect (CCE) as well as the subjective sense of leg ownership (assessed by a questionnaire). Fifteen participants viewed virtual legs or non-corporeal control objects, presented either from their habitual first-person viewpoint or from a viewpoint that was rotated by 90°(third-person viewpoint), while applying visuo-tactile stroking between the participants legs and the virtual legs shown on a head-mounted display. The data show that the first-person visuo-spatial viewpoint significantly boosts the visuo-tactile integration as well as the sense of leg ownership. Moreover, the viewpoint-dependent increment of the visuo-tactile integration was only found in the conditions when participants viewed the virtual legs (absent for control objects). These results confirm the importance of first person visuo-spatial viewpoint for the integration of visuo-tactile stimuli and extend findings from the upper extremity and the trunk to visuo-tactile integration and ownership for the legs.
Highlights
The experience of the body as one’s own, and its location in space, critically depend on multisensory and sensorimotor integration of bodily signals (Gallagher, 2000; Jeannerod, 2003; Blanke and Metzinger, 2009; Longo et al, 2010; Tsakiris, 2010; Blanke, 2012)
The three-way repeated measures ANOVA on reaction time (RT) congruency effect (CCE) showed a significant main effect of Visuo-spatial viewpoint [F(1,14) = 32.75, p < 0.001, η2p = 0.70]: the CCE magnitude was larger when the viewed legs or control object were seen from the first-person viewpoint (M = 83.7 ms, SEM = 13.0 ms) than seen from the third-person viewpoint (M = 25.6 ms, SEM = 9.4 ms)
The other two pairwise comparisons did not reach the level of significance after correction for multiple comparisons: no significant differences in the CCE were found between legs or objects when they were viewed in the firstperson viewpoint [t(14) = 2.20, p = 0.044, α(corr) = 0.0125] or when they were viewed in the third-person viewpoint [t(14) = −1.34, p = 0.203, α(corr) = 0.0125]
Summary
The experience of the body as one’s own (i.e., the sense of body ownership), and its location in space, critically depend on multisensory and sensorimotor integration of bodily signals (Gallagher, 2000; Jeannerod, 2003; Blanke and Metzinger, 2009; Longo et al, 2010; Tsakiris, 2010; Blanke, 2012). Leg Illusion, CCE, and Ownership based on the integration and weighting of different sensory bodily inputs (proprioceptive, tactile, visual, auditory, vestibular, and visceral) according to spatio-temporal laws of multisensory bodily perception (Macaluso and Maravita, 2010; Blanke, 2012; Apps and Tsakiris, 2014; Samad et al, 2015) These bottom-up multisensory and motor signals are further integrated and compared with more stable, offline body representations, such as perceptual, conceptual and semantic knowledge of the body (Carruthers, 2008; de Vignemont, 2010; Longo et al, 2010; Serino and Haggard, 2010), as well as visual top-down signals about the form and position of the body (Blanke et al, 2015). Successful induction of bodily illusion through multisensory conflicts have been shown at the level of fingers (Dieguez et al, 2009), hands (Rubber hand illusion: Botvinick and Cohen, 1998), feet (Lenggenhager et al, 2015) or an entire body (Full body illusion: Ehrsson, 2007; Lenggenhager et al, 2007)
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