Avatar embodiment in VR: Are there individual susceptibilities to visuo-tactile or cardio-visual stimulations?

Abstract

Virtual reality has obvious potential to help humans developing/recovering brain functions, which operates through modulation of multisensory inputs. Some interventions using VR rely on the need to embody a virtual avatar, which stimulates cognitive-motor adaptations. Recent research has shown that embodiment can be facilitated by synchronizing natural sensory inputs with their visual redundancy on the avatar, e.g., the user’s heartbeat flashing around its avatar (cardio-visual stimulation) or the user’s body being physically stroked while the avatar is touched in synchronized conditions (visuo-tactile stimulation). While different full-body illusions have proven obvious interest in health and disease, it is unknown to date whether individual susceptibilities to illusion are equivalent with respect to cardio-visual or visuo-tactile stimulations. In fact, a number of factors like interoception, vestibular processing, a pronounced visual dependence, a specific cognitive ability for mental rotations, or user traits and habits like empathy and video games practice may interfere with the multifaceted construct of bodily self-consciousness, the conscious experience of owning a body in space from which the world is perceived. Here, we evaluated a number of dispositions in twenty-nine young and healthy participants submitted alternatively to cardio-visual and visuo-tactile stimulations to induce full-body illusions. Three components of bodily self-consciousness consensually identified in recent research, namely self-location, perspective taking and self-identification were quantified by self-reported feeling (questionnaires), and specific VR tasks used before and after multisensory stimulations. VR tasks allowed measuring self-location in reference to a virtual ball rolling toward the participant, perspective taking through visuomotor response times when mentally rotating an avatar suddenly presented at different angles, and self-identification through heart rate dynamics in response to a threatening stimulus applied to the (embodied) avatar. Full-body illusion was evidenced by self-reported quotations of self-identification to the avatar reaching scores in agreement with the literature, lower reaction times when taking the perspective of the avatar and a marked drop in heart rate showing obvious freezing reaction changes when the user saw the avatar being pierced by a spear. Changes in bodily self-consciousness components are not significantly dependent on the type of multisensory stimulation (visuo-tactile or cardio-visual). A principal component analysis demonstrated the lack of covariation between those components, pointing to the relative independence of self-location, perspective taking and self-identification measurements. Moreover, none of these components showed significant covariations with any of the individual dispositions. These results support the hypothesis that cardio-visual and visuo-tactile stimulations affect the main components of bodily self-consciousness in an extent that, in average, is mostly independent of individual perceptive-cognitive profiles, at least in healthy young people. Although this is an important observation at group level, which indicates a similar probability of inducing embodiment with either cardio-visual or visuo-tactile stimulations in VR, these results do not discard the fact that some individuals might have higher susceptibility to specific sensory inputs, which would represent a target to adapt efficient VR stimulations.