Owning the body in the mirror: The effect of visual perspective and mirror view on the full-body illusion

Trust in virtual ingroup or outgroup members relies on perceived self-other overlap

Why do we trust each other? We carried out three experiments to test whether interpersonal trust depends on perceived self-other overlap. As previous studies suggest that enfacing (feeling ownership for, and include more into oneself of the face of) an avatar might make one trust this avatar more, we exposed participants to faces of ingroup and outgroup avatars that moved in synchrony or out of synchrony with the participant's own facial movements, and assessed the impact of synchrony on self-other overlap and trust measures. Experiment 1 used ingroup faces and successfully showed that synchrony (manipulated within-participants) increased self-other overlap and trust, which we assessed by means of the Trust Game and the Implicit Association Test (IAT). In Experiment 2, which used outgroup faces and a within-participants design, synchrony still increased scores in the Trust Game but the IAT was no longer affected. Experiment 3 replicated Experiment 2 but with synchrony varying between participants, which eliminated the synchrony effect in both trust measures. Importantly, Inclusion of Other in the Self (IOS) ratings were found to predict the IAT effect in synchronous conditions. Taken altogether, our findings suggest that interpersonal trust is mainly driven by perceived self-other overlap. Besides group identification, appearance, and voluntary movement, synchrony is just one of several sources contributing to perceived self-other overlap.

Gender differences in bodily experience: Insights from virtual reality body illusion

Virtual Reality has significantly improved the understanding of body experience, through techniques such as Body Illusion. Body Illusion allows individuals to perceive an artificial body as their own, changing body perceptual and affective components. Prior research has predominantly focused on female participants, leaving the impact of Body Illusion on males less understood. This study seeks to fill this gap by examining the nuanced bodily experiences of men in comparison to women. 40 participants (20 females and 20 males) were proposed with visuo-tactile synchronous and asynchronous Body Illusion to explore changes in body satisfaction and body size estimation across three critical areas: shoulders, hips, and waist. Results revealed significant initial disparities, with females displaying greater body dissatisfaction and a tendency to overestimate body size. After Body Illusion, females adjusted the hips perceived body size closer to that of the virtual body and reported increased body satisfaction independent of the condition. Conversely, males showed changes only in waist size estimation only after synchronous stimulation without significant shifts in body satisfaction. These results suggest a higher sensitivity of women to embodied experiences, potentially due to societal influences and a greater inclination towards self-objectification. These insights pave the way for creating more refined and effective interventions for body image issues, highlighting the importance of incorporating gender-specific considerations in VR-based prevention and therapeutical programs.

Hierarchical and dynamic relationships between body part ownership and full-body ownership

What is the relationship between experiencing individual body parts and the whole body as one's own? We theorised that body part ownership is driven primarily by the perceptual binding of visual and somatosensory signals from specific body parts, whereas full-body ownership depends on a more global binding process based on multisensory information from several body segments. To examine this hypothesis, we used a bodily illusion and asked participants to rate illusory changes in ownership over five different parts of a mannequin's body and the mannequin as a whole, while we manipulated the synchrony or asynchrony of visual and tactile stimuli delivered to three different body parts. We found that body part ownership was driven primarily by local visuotactile synchrony and could be experienced relatively independently of full-body ownership. Full-body ownership depended on the number of synchronously stimulated parts in a nonlinear manner, with the strongest full-body ownership illusion occurring when all parts received synchronous stimulation. Additionally, full-body ownership influenced body part ownership for nonstimulated body parts, and skin conductance responses provided physiological evidence supporting an interaction between body part and full-body ownership. We conclude that body part and full-body ownership correspond to different processes and propose a hierarchical probabilistic model to explain the relationship between part and whole in the context of multisensory awareness of one's own body.

Beyond peripersonal boundaries: insights from crossmodal interactions

We experience our self as a body located in space. However, how information about self-location is integrated into multisensory processes underlying the representation of the peripersonal space (PPS), is still unclear. Prior studies showed that the presence of visual information related to oneself modulates the multisensory processes underlying PPS. Here, we used the crossmodal congruency effect (CCE) to test whether this top-down modulation depends on the spatial location of the body-related visual information. Participants responded to tactile events on their bodies while trying to ignore a visual distractor presented on the mirror reflection of their body (Self) either in the peripersonal space (Near) or in the extrapersonal space (Far). We found larger CCE when visual events were presented on the mirror reflection in the peripersonal space, as compared to the extrapersonal space. These results suggest that top-down modulation of the multisensory bodily self is only possible within the PPS.

Feeling of hand deformation as a monkey's hand: an experiment on a visual body with discomfort and its algebraic analysis

While there are many studies in which body ownership can be transferred to a virtual body, there are few experimental studies of how subjects feel about their own bodies being deformed since a real body cannot be deformed. Here, we propose such an experimental setup, in which a twisted hand is diagonally viewed from behind, which is called a "monkey's hand." Although the subject cannot see the thumb hidden behind his or her arm, he or she feels that the monkey's hand has an ambiguous thumb that functionally never exists but structurally exists. This ambiguity is consistent with experimental results on proprioceptive drift, by which the deformation of the hand is measured. The ambiguity of the presence and absence of the thumb is finally analyzed with a specific algebraic structure called a lattice. This can help us understand disownership as being different from the absence of ownership.

I can see my virtual body in a mirror: The role of visual perspective in changing implicit racial attitudes using virtual reality

INTRODUCTION

Recent studies showed that VR is a valid tool to change implicit attitudes toward outgroup members. Here, we extended this work by investigating conditions under which virtual reality (VR) is effective in changing implicit racial attitudes.

METHODS

To this end, participants were embodied in a Black or White avatar and we manipulated the perspective through which they could see their virtual body. Participants in one condition, could see their virtual body both from a first-person perspective (i.e., by looking down toward themselves) and reflected in a mirror placed in front of them in the VR environment. Participants in another condition could instead see their virtual body only from a first-person perspective (i.e., by looking down toward themselves) as no mirror was placed in the VR environment. Implicit racial attitudes were assessed using the Implicit Association Test (IAT) before and immediately after the VR intervention.

RESULTS

Results showed that when White participants were embodied in a Black avatar compared to a White avatar, they showed a decrease in their implicit pro-White attitudes but only when they could see their virtual body both from a first-person perspective and in a mirror.

DISCUSSION

These results suggest that, in immersive virtual reality interventions, the possibility for participants to see their body also reflected in a mirror, might be a critical factor in changing their implicit racial attitudes.

Combination of visuo-tactile and visuo-vestibular correlations in illusory body ownership and self-motion sensations

Previous studies have shown that illusory ownership over a mannequin's body can be induced through synchronous visuo-tactile stimulation as well as through synchronous visuo-vestibular stimulation. The current study aimed to elucidate how three-way combinations of correlated visual, tactile and vestibular signals contribute to the senses of body ownership and self-motion. Visuo-tactile temporal congruence was manipulated by touching the mannequin's body and the participant's unseen real body on the trunk with a small object either synchronously or asynchronously. Visuo-vestibular temporal congruence was manipulated by synchronous or asynchronous presentation of a visual motion cue (the background rotating around the mannequin in one direction) and galvanic stimulation of the vestibular nerve generating a rotation sensation (in the same direction). The illusory experiences were quantified using a questionnaire; threat-evoked skin-conductance responses (SCRs) provided complementary indirect physiological evidence for the illusion. Ratings on the illusion questionnaire statement showed significant main effects of synchronous visuo-vestibular and synchronous visuo-tactile stimulations, suggesting that both of these pairs of bimodal correlations contribute to the ownership illusion. Interestingly, visuo-tactile synchrony dominated because synchronous visuo-tactile stimulation combined with asynchronous visuo-vestibular stimulation elicited a body ownership illusion of similar strength as when both bimodal combinations were synchronous. Moreover, both visuo-tactile and visuo-vestibular synchrony were associated with enhanced self-motion perception; self-motion sensations were even triggered when visuo-tactile synchrony was combined with visuo-vestibular asynchrony, suggesting that ownership enhanced the relevance of visual information as a self-motion cue. Finally, the SCR results suggest that synchronous stimulation of either modality pair led to a stronger illusion compared to the asynchronous conditions. Collectively, the results suggest that visuo-tactile temporal correlations have a stronger influence on body ownership than visuo-vestibular correlations and that ownership boosts self-motion perception. We present a Bayesian causal inference model that can explain how visuo-vestibular and visuo-tactile information are combined in multisensory own-body perception.

Effects of virtual hands and feet on the onset time and duration of illusory body ownership

In the illusory body ownership, humans feel as if a rubber hand or an avatar in a virtual environment is their own body through visual-tactile synchronization or visual-motor synchronization. Despite the onset time and duration of illusory body ownership has been investigated, it is not clear how the onset time and duration change when a part of the body is missing from the full-body. In this study, we investigated the completeness of the full-body for the illusion onset and duration by comparing the following conditions: complete avatar, avatar missing hands and feet, and avatar with hands and feet only. Our results suggest that avatar hands and feet only shorten the duration of the illusion, and missing body parts, such as only hands and feet or no hands and feet, reduce the sense of body ownership and of agency. However, the effects of avatar completeness on the onset time are unclear, and no conclusions can be made in either direction based on the current findings.

The illusion of having a large virtual body biases action-specific perception in patients with mild cognitive impairment

The illusion of having a large body makes us perceive objects as smaller than they really are. This action-specific perception effect occurs because we perceive the property of an object (i.e., size) differently according to our unique action capability (i.e., the affordance of body size). Although the body-ownership illusion contributing to this action-specific perception has been studied, its effects remain unclear in neurological patients. We examined the action-specific perception impairments of MCI patients by means of body-ownership illusion in a non-immersive virtual reality environment. Twenty healthy young adults, 21 healthy older adults, and 15 MCI patients were recruited. We assessed their “original-body action-specific perception” and “enlarged-body action-specific perception” using the original and enlarged sizes of their virtual bodies, respectively. The MCI patients’ original-body action-specific perception was no different than that of the healthy controls (p = 0.679). However, the enlarged-body action-specific perception of the MCI patients was significantly biased (p < 0.001). The inclusion of the enlarged-body action-specific perception provides additional discriminative power for early diagnosis of MCI (89.3% accuracy, 75.0% sensitivity, 100.0% specificity, and 87.5% balanced accuracy).

The effect of visually manipulating back size and morphology on back perception, body ownership, and attitudes towards self-capacity during a lifting task

Body re-sizing illusions can profoundly alter perception of our own body. We investigated whether creating the illusion of a muscled and fit-looking back (Strong) influenced perceived back size, body ownership, and attitudes towards self-capacity during a lifting task. Twenty-four healthy male volunteers performed a standardised lifting task while viewing real-time (delay < 20 ms) video of their own back through a head-mounted display under four different conditions (Normal size, Strong, Reshaped, Large; order randomised). The MIRAGE-mediated reality system was used to modify the shape, size, and morphology of the back. Participants were poor at recognizing the correct appearance of their back, for both implicit (perceived width of shoulders and hips) and explicit (questionnaire) measures of back size. Visual distortions of body shape (Reshaped condition) altered implicit back size measures. However, viewing a muscled back (Strong condition) did not result in a sense of agency or ownership and did not update implicit perception of the back. No conditions improved perceptions/attitudes of self-capacity (perceived back strength, perceived lifting confidence, and perceived back fitness). The results lend support for the importance of the embodiment of bodily changes to induce changes in perception. Further work is warranted to determine whether increased exposure to illusory changes would alter perceptions and attitudes towards self-capacity or whether different mechanisms are involved.

Head-Mounted Display with Increased Downward Field of View Improves Presence and Sense of Self-Location

Common existing head-mounted displays (HMDs) for virtual reality (VR) provide users with a high presence and embodiment. However, the field of view (FoV) of a typical HMD for VR is about 90 to 110 [deg] in the diagonal direction and about 70 to 90 [deg] in the vertical direction, which is narrower than that of humans. Specifically, the downward FoV of conventional HMDs is too narrow to present the user avatar's body and feet. To address this problem, we have developed a novel HMD with a pair of additional display units to increase the downward FoV by approximately 60 ( 10+50) [deg]. We comprehensively investigated the effects of the increased downward FoV on the sense of immersion that includes presence, sense of self-location (SoSL), sense of agency (SoA), and sense of body ownership (SoBO) during VR experience and on patterns of head movements and cybersickness as its secondary effects. As a result, it was clarified that the HMD with an increased FoV improved presence and SoSL. Also, it was confirmed that the user could see the object below with a head movement pattern close to the real behavior, and did not suffer from cybersickness. Moreover, the effect of the increased downward FoV on SoBO and SoA was limited since it was easier to perceive the misalignment between the real and virtual bodies.

Virtual Mirror and Beyond: The Psychological Basis for Avatar Embodiment via a Mirror

In virtual reality (VR), a virtual mirror is often used to display the VR avatar to the user for enhancing the embodiment. The reflected image of the synchronization of the virtual body with the user’s movement is expected to be recognized as the user’s own reflection. In addition to the visuo-motor synchrony, there are some mirror reflection factors that are probably involved in avatar embodiment. This paper reviews literature on the psychological studies that involve mirror-specific self-identification and embodied perception to clarify how the reflected image of the virtual body is embodied. Furthermore, subjective misconceptions about mirror reflections reported in naïve optics have also been reviewed to discuss the potential of virtual mirror displays to modulate avatar embodiment.

Changing your body changes your eating attitudes: embodiment of a slim virtual avatar induces avoidance of high-calorie food

The virtual-reality full-body illusion paradigm has been suggested to not only trigger the illusory ownership of the avatar's body but also the attitudinal and behavioral components stereotypically associated to that kind of virtual body. In the present study, we investigated whether this was true for stereotypes related to body size: body satisfaction and eating control behavior. Healthy participants underwent the full-body illusion paradigm with an avatar having either a larger or a slimmer body than their own, and were assessed for implicit attitudes towards body image and food calorie content at baseline and after each full-body illusion session. Results showed that the illusion emerged regardless of the avatar's body size, whereas the perceived dimension of the own body size changed according to the avatar's body size (i.e., participants felt to be slimmer after embodying their slim avatar and larger after embodying their large avatar). Crucially, we found that implicit attitudes towards food, but not those towards one's own body, were modulated by the size of the virtual body. Compared to baseline, ownership of a slimmer avatar increased the avoidance of high-calorie food, whereas ownership of a larger avatar did not induce changes. Our findings suggest that the illusory feeling of being slimmer drives also the food-related stereotypes associated with that body size, increasing the regulation of eating behaviors.

Inter-lateral Referral of Sensation in Health and Disease Using a Mirror Illusion-A Scoping Review

OBJECTIVE

Perception of touch is expected at the location where it is applied. However, there are indications that being touched may be perceived on the contralateral side when seen as a reflection in a mirror at midline. Such inter-lateral referral of sensation (RS) lacks evidence, as mirror therapy research usually focusses on movement-based techniques. This study aimed to map out existing research across disciplines regarding the effect of RS in health and disease, and to understand whether there is rehabilitation potential in RS.

METHOD

A scoping review was conducted to map out concepts and keywords across disciplines interested in this topic, using keywords in several languages, and a wide range of databases and additional sources.

RESULTS

The review revealed mostly cross-sectional experiments and included over 486 participants: healthy, or with stroke, complex regional pain syndrome, amputation, nerve graft surgery or radial fracture. Procedures varied regarding stimulation tool, time and location, with two stimulating replacements, one the face and one a variety of areas. Response rates ranged from 0 to 100%.In general, RS was regarded as a phenomenon or even as a predictor of maladaptive neuroplasticity. There was little research into using RS stimulation as a modulatory tool to improve sensory perception.

CONCLUSIONS

RS challenges the understanding of touch perception and elicits a range of questions regarding neuro-processing. A modulatory approach using RS has not been described, requires investigation and, if promising, development as an intervention.

Multisensory integration of visual cues from first- to third-person perspective avatars in the perception of self-motion

In the perception of self-motion, visual cues originating from an embodied humanoid avatar seen from a first-person perspective (1^st-PP) are processed in the same way as those originating from a person's own body. Here, we sought to determine whether the user's and avatar's bodies in virtual reality have to be colocalized for this visual integration. In Experiment 1, participants saw a whole-body avatar in a virtual mirror facing them. The mirror perspective could be supplemented with a fully visible 1^st-PP avatar or a suggested one (with the arms hidden by a virtual board). In Experiment 2, the avatar was viewed from the mirror perspective or a third-person perspective (3^rd-PP) rotated 90° left or right. During an initial embodiment phase in both experiments, the avatar's forearms faithfully reproduced the participant's real movements. Next, kinaesthetic illusions were induced on the static right arm from the vision of passive displacements of the avatar's arms enhanced by passive displacement of the participant's left arm. Results showed that this manipulation elicited kinaesthetic illusions regardless of the avatar's perspective in Experiments 1 and 2. However, illusions were more likely to occur when the mirror perspective was supplemented with the view of the 1st-PP avatar's body than with the mirror perspective only (Experiment 1), just as they are more likely to occur in the latter condition than with the 3^rd-PP (Experiment 2). Our results show that colocalization of the user's and avatar's bodies is an important, but not essential, factor in visual integration for self-motion perception.

The contribution of stimulating multiple body parts simultaneously to the illusion of owning an entire artificial body

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.

Perception of Our Own Body Influences Self-Concept and Self-Incoherence Impairs Episodic Memory

How does our body affect the way we think about our personality? We addressed this question by eliciting the perceptual illusion that pairs of friends swapped bodies with each other. We found that during the illusion, the participants rated their own personality characteristics more similarly to the way they previously rated their friend's personality, and this flexible adjustment of self-concept to the "new" bodily self was related to the strength of illusory ownership of the friend's body. Moreover, a subsequent memory test showed that personality traits rated during the friend-body-swap illusion were generally remembered worse than traits rated during the control conditions; importantly, however, this impairment of episodic recognition memory was reduced for the participants who considerably adjusted their self-concept during the illusory body swapping. These findings demonstrate that our beliefs about own personality are dynamically shaped by the perception of our body and that coherence between the bodily and conceptual self-representations is important for the normal encoding of episodic memories.

Seeing your own or someone else's hand moving in accordance with your action: The neural interaction of agency and hand identity

Forward models can predict sensory consequences of self-action, which is reflected by less neural processing for actively than passively generated sensory inputs (BOLD suppression effect). However, it remains open whether forward models take the identity of a moving body part into account when predicting the sensory consequences of an action. In the current study, fMRI was used to investigate the neural correlates of active and passive hand movements during which participants saw either an on-line display of their own hand or someone else's hand moving in accordance with their movement. Participants had to detect delays (0-417 ms) between their movement and the displays. Analyses revealed reduced activation in sensory areas and higher delay detection thresholds for active versus passive movements. Furthermore, there was increased activation in the hippocampus, the amygdala, and the middle temporal gyrus when someone else's hand was seen. Most importantly, in posterior parietal (angular gyrus and precuneus), frontal (middle, superior, and medial frontal gyrus), and temporal (middle temporal gyrus) regions, suppression for actively versus passively generated feedback was stronger when participants were viewing their own compared to someone else's hand. Our results suggest that forward models can take hand identity into account when predicting sensory action consequences.

Scrambled body differentiates body part ownership from the full body illusion

Illusory body ownership can be induced in a body part or a full body by visual-motor synchronisation. A previous study indicated that an invisible full body illusion can be induced by the synchronous movement of only the hands and feet. The difference between body part ownership and the full body illusion has not been explained in detail because there is no method for separating these two illusions. To develop a method to do so, we scrambled or randomised the positions of the hands and feet and compared it with the normal layout stimulus by manipulating visual-motor synchronisation. In Experiment 1, participants observed the stimuli from a third-person perspective, and the questionnaire results showed that the scrambled body stimulus induced only body part ownership, while the normal layout stimulus induced both body part ownership and full body ownership when the stimuli were synchronous with participants' actions. In Experiment 2, we found similar results as with the first-person perspective stimuli in a questionnaire. We did not find significant skin conductance response difference between any conditions in either Experiment 2 or 3. These results suggest that a spatial relationship is necessary for the full body illusion, but not for body part ownership.

Distinct neural response to visual perspective and body size in the extrastriate body area

Neuroimaging research has independently implicated the extrastriate body area (EBA) in distinguishing between different visual perspectives and morphologies of bodies within visual processing. However, the combined processing of these physical attributes towards neural EBA response remains unclear, and may be crucial in influencing higher-order, aesthetic evaluation of bodies. Indeed, EBA alterations amongst eating disorder patients have been associated with disturbances in body image, and disruption to EBA activity amongst healthy individuals has been shown to influence aesthetic evaluations made towards bodies. Therefore, the present study used images of slim and large female bodies viewed from egocentric and allocentric perspectives, to investigate neural EBA response amongst healthy females (N = 30). In addition, participants provided behavioural aesthetic and weight evaluations of all model stimuli. Results revealed an interaction, bilaterally, between visual perspective and body size towards EBA activity, with multi-voxel pattern analysis revealing distinct neural patterns between the four conditions. However, EBA activity did not relate to non-clinical eating disorder psychopathology. No direct relationship was found between EBA activity and behavioural evaluations of model stimuli; however, a whole brain analysis revealed that higher-order, prefrontal regions were associated with cognitive evaluations of large bodies. Taken together, our results suggest that the EBA is an integral core region in discriminating between multiple physical attributes of the body, which is likely to provide important information to higher-order brain regions which make aesthetic evaluations towards bodies.

Self-Concept Clarity and the Bodily Self: Malleability Across Modalities

The self has fascinated scholars for centuries. Although theory suggests that the self-concept (cognitive self-understanding) and bodily self (pre-reflective awareness of one’s body) are related, little work has examined this notion. To this end, in Study 1, participants reported on self-concept clarity (SCC) and completed the rubber hand illusion (RHI), a paradigm in which synchronous (vs. asynchronous) stimulation between a prosthetic hand and one’s own hand leads one to “embody” the prosthetic hand. Whereas participants were equally susceptible to the RHI during synchronous stroking, low-SCC individuals were more vulnerable to the illusion during asynchronous stroking, when the effect is unwarranted. Conceptually replicating and extending this finding, in Study 2, low-SCC individuals were more susceptible to the body-swap illusion—the impression that another person’s body is one’s own. These findings suggest that a clear sense of self implies clarity and stability of both the self-concept and the bodily self.

Women With Obesity Are Not as Curvy as They Think: Consequences on Their Everyday Life Behavior

Two studies explore the impact of body size on daily life activities of women with obesity. In the first study, ethnographic techniques (first-person perspective video recordings) and subsequent interviews based on the video recordings were used. Results showed atypical behavior of women with obesity and ex-obese women related to memories of embarrassing experiences regarding personal body size (sitting, passing doors sideways, over-careful navigation in public space, and choosing clothes sizes too large.) Women with obesity seem to behave as if they thought they had a larger body than it actually was. These atypical behaviors are related to memories of embarrassing experiences regarding personal body size and stigma. Overweight women exhibit the same behavior but to a lesser and less systematic degree. In the second study, the represented (imagined) body size was compared to the perceived (in a mirror) body size with digital morphing techniques. In the mirror condition, the perceived image is accurate, while in the absence of a mirror women with obesity overestimate their body size by about 30%. Moreover, overestimation of imagined body size increased according to the weight status. Finally, women who had bariatric surgery had poorer estimates than women who had not. This would result of being continuously reminded of obesity and its stigma by daily embarrassing experiences, by being confronted with an environment designed for normal weight (e.g., narrow seats, turnstiles etc.) that makes obesity salient. We suggest that body size overestimation is a case of accentuation where things that matter are perceived bigger. These results could also been explained by the allocentric lock theory.

Embodiment into a robot increases its acceptability

Recent studies have shown how embodiment induced by multisensory bodily interactions between individuals can positively change social attitudes (closeness, empathy, racial biases). Here we use a simple neuroscience-inspired procedure to beam our human subjects into one of two distinct robots and demonstrate how this can readily increase acceptability and social closeness to that robot. Participants wore a Head Mounted Display tracking their head movements and displaying the 3D visual scene taken from the eyes of a robot which was positioned in front of a mirror and piloted by the subjects’ head movements. As a result, participants saw themselves as a robot. When participant’ and robot’s head movements were correlated, participants felt that they were incorporated into the robot with a sense of agency. Critically, the robot they embodied was judged more likeable and socially closer. Remarkably, we found that the beaming experience with correlated head movements and corresponding sensation of embodiment and social proximity, was independent of robots’ humanoid’s appearance. These findings not only reveal the ease of body-swapping, via visual-motor synchrony, into robots that do not share any clear human resemblance, but they may also pave a new way to make our future robotic helpers socially acceptable.

Is This My Own Body? Changing the Perceptual and Affective Body Image Experience among College Students Using a New Virtual Reality Embodiment-Based Technique

Body image disturbances (BIDs) have been widely studied using virtual reality (VR) devices that induce a full body illusion (FBI) and allow manipulation of the individual’s perceptual and affective experiences of the body. This study aimed to assess whether the induction of the FBI over a virtual body would produce changes in body-related anxiety and BIDs using a new whole-body visuo-tactile stimulation procedure. Fifty non-clinical participants were randomly assigned to synchronous or asynchronous visuo-tactile groups. During the pre-assessment, all participants filled in BIDs and body-anxiety questionnaires. Then, they were embodied into two virtual bodies (VBs): firstly, with their real measurements, and secondly, with a larger-size body. Body image disturbances, body anxiety, fear of gaining weight, and FBI levels were assessed after exposure to each avatar. All participants in both conditions showed higher levels of BIDs and body anxiety after owning the larger-size VB than after owning the real-size VB (p < 0.05). The synchronous visuo-tactile group had higher scores, although the differences did not reach statistical significance. This study provides evidence of the usefulness of this new embodiment-based technique to induce changes in BIDs or body anxiety in a non-clinical sample, being suitable for use in future body image interventions.

The Effect of Visual Capture Towards Subjective Embodiment Within the Full Body Illusion

Typically, multisensory illusion paradigms emphasise the importance of synchronous visuotactile integration to induce subjective embodiment towards another body. However, the extent to which embodiment is due to the 'visual capture' of congruent visuoproprioceptive information alone remains unclear. Thus, across two experiments (total N = 80), we investigated how mere visual observation of a mannequin body, viewed from a first-person perspective, influenced subjective embodiment independently from concomitant visuotactile integration. Moreover, we investigated whether slow, affective touch on participants' own, unseen body (without concomitant touch on the seen mannequin) disrupted visual capture effects to a greater degree than fast, non-affective touch. In total, 40% of participants experienced subjective embodiment towards the mannequin body following mere visual observation, and this effect was significantly higher than conditions which included touch to participants own, unseen body. The velocity of the touch that participants received (affective/non-affective) did not differ in modulating visual capture effects. Furthermore, the effects of visual capture and perceived pleasantness of touch was not modulated by subthreshold eating disorder psychopathology. Overall, this study suggests that congruent visuoproprioceptive cues can be sufficient to induce subjective embodiment of a whole body, in the absence of visuotactile integration and beyond mere confabulatory responses.

Modulation of vection latencies in the full-body illusion

Current neuroscientific models of bodily self-consciousness (BSC) argue that inaccurate integration of sensory signals leads to altered states of BSC. Indeed, using virtual reality technology, observers viewing a fake or virtual body while being exposed to tactile stimulation of the real body, can experience illusory ownership over-and mislocalization towards-the virtual body (Full-Body Illusion, FBI). Among the sensory inputs contributing to BSC, the vestibular system is believed to play a central role due to its importance in estimating self-motion and orientation. This theory is supported by clinical evidence that vestibular loss patients are more prone to altered BSC states, and by recent experimental evidence that visuo-vestibular conflicts can disrupt BSC in healthy individuals. Nevertheless, the contribution of vestibular information and self-motion perception to BSC remains largely unexplored. Here, we investigate the relationship between alterations of BSC and self-motion sensitivity in healthy individuals. Fifteen participants were exposed to visuo-vibrotactile conflicts designed to induce an FBI, and subsequently to visual rotations that evoked illusory self-motion (vection). We found that synchronous visuo-vibrotactile stimulation successfully induced the FBI, and further observed a relationship between the strength of the FBI and the time necessary for complete vection to arise. Specifically, higher self-reported FBI scores across synchronous and asynchronous conditions were associated to shorter vection latencies. Our findings are in agreement with clinical observations that vestibular loss patients have higher FBI susceptibility and lower vection latencies, and argue for increased visual over vestibular dependency during altered states of BSC.

Effect of Visual Information on Active Touch During Mirror Visual Feedback

Several studies have demonstrated that observation of a dummy or mirror-reflected hand being stroked or moving at the same time as the hidden hand evokes a feeling that the dummy hand is one’s own, such as the rubber hand illusion (RHI) and mirror visual feedback (MVF). Under these conditions, participants also report sensing the tactile stimulation applied to the fake hands, suggesting that tactile perception is modulated by visual information during the RHI and MVF. Previous studies have utilized passive stimulation conditions; however, active touch is more common in real-world settings. Therefore, we investigated whether active touch is also modulated by visual information during an MVF scenario. Twenty-three participants (13 men and 10 women; mean age ± SD: 21.6 ± 2.0 years) were required to touch a polyurethane pad with both hands synchronously, and estimate the hardness of the pad while observing the mirror reflection. When participants observed the mirror reflection of the other hand pushing a softer or harder pad, perceived hardness estimates were significantly biased toward softer or harder, respectively, even though the physical hardness of the pad remained constant. Furthermore, perceived hardness exhibited a strong correlation with finger displacement of the mirrored, but not hidden, hand. The modulatory effects on perceived hardness diminished when participants touched the pad with both hands asynchronously or with their eyes closed. Moreover, participants experienced ownership of the mirrored hand when they touched the pad with both hands synchronously but not asynchronously. These results indicate that hardness estimates were modulated by observation of the mirrored hand during synchronous touch conditions. The present study demonstrates that, similar to passive touch, active touch is also modulated by visual input.

Illusory resizing of the painful knee is analgesic in symptomatic knee osteoarthritis

BACKGROUND

Experimental and clinical evidence support a link between body representations and pain. This proof-of-concept study in people with painful knee osteoarthritis (OA) aimed to determine if: (i) visuotactile illusions that manipulate perceived knee size are analgesic; (ii) cumulative analgesic effects occur with sustained or repeated illusions.

METHODS

Participants with knee OA underwent eight conditions (order randomised): stretch and shrink visuotactile (congruent) illusions and corresponding visual, tactile and incongruent control conditions. Knee pain intensity (0-100 numerical rating scale; 0 = no pain at all and 100 = worst pain imaginable) was assessed pre- and post-condition. Condition (visuotactile illusion vs control) × Time (pre-/post-condition) repeated measure ANOVAs evaluated the effect on pain. In each participant, the most beneficial illusion was sustained for 3 min and was repeated 10 times (each during two sessions); paired t-tests compared pain at time 0 and 180s (sustained) and between illusion 1 and illusion 10 (repeated).

RESULTS

Visuotactile illusions decreased pain by an average of 7.8 points (95% CI [2.0-13.5]) which corresponds to a 25% reduction in pain, but the tactile only and visual only control conditions did not (Condition × Time interaction: p = 0.028). Visuotactile illusions did not differ from incongruent control conditions where the same visual manipulation occurred, but did differ when only the same tactile input was applied. Sustained illusions prolonged analgesia, but did not increase it. Repeated illusions increased the analgesic effect with an average pain decrease of 20 points (95% CI [6.9-33.1])-corresponding to a 40% pain reduction.

DISCUSSION

Visuotactile illusions are analgesic in people with knee OA. Our results suggest that visual input plays a critical role in pain relief, but that analgesia requires multisensory input. That visual and tactile input is needed for analgesia, supports multisensory modulation processes as a possible explanatory mechanism. Further research exploring the neural underpinnings of these visuotactile illusions is needed. For potential clinical applications, future research using a greater dosage in larger samples is warranted.

Body ownership and the four-hand illusion

Recent studies of the rubber hand illusion (RHI) have shown that the sense of body ownership is constrained by several factors and yet is still very flexible. However, exactly how flexible is our sense of body ownership? In this study, we address this issue by investigating the following question: is it possible that one may have the illusory experience of owning four hands? Under visual manipulation, the participant adopted the experimenter’s first-person perspective (1PP) as if it was his/her own. Sitting face to face, the participant saw four hands—the experimenter’s two hands from the adopted 1PP together with the subject’s own two hands from the adopted third-person perspective (3PP). We found that: (1) the four-hand illusion did not occur in the passive four-hand condition. (2) In the active four-hand condition, the participants tapped their index fingers, imitated by the experimenter. When tactile stimulations were not provided, the key illusion was not induced, either. (3) Strikingly, once all four hands began to act with the same pattern and received synchronous tactile stimulations at the same time, many participants felt as if they had two more hands. These results show that the sense of body ownership is much more flexible than most researchers have suggested.

Embodied Medicine: Mens Sana in Corpore Virtuale Sano

Progress in medical science and technology drastically improved physicians’ ability to interact with patient’s physical body. Nevertheless, medicine still addresses the human body from a Hippocratic point of view, considering the organism and its processes just as a matter of mechanics and fluids. However, the interaction between the cognitive neuroscience of bodily self-consciousness (BSC), fundamentally rooted in the integration of multisensory bodily inputs, with virtual reality (VR), haptic technologies and robotics is giving a new meaning to the classic Juvenal’s latin dictum “Mens sana in corpore sano” (a healthy mind in a healthy body). This vision provides the basis for a new research field, “Embodied Medicine”: the use of advanced technologies for altering the experience of being in a body with the goal of improving health and well-being. Up to now, most of the research efforts in the field have been focused upon how external bodily information is processed and integrated. Despite the important results, we believe that existing bodily illusions still need to be improved to enhance their capability to effectively correct pathological dysfunctions. First, they do not follow the suggestions provided by the free-energy and predictive coding approaches. More, they lacked to consider a peculiar feature of the human body, the multisensory integration of internal inputs (interoceptive, proprioceptive and vestibular) that constitute our inner body dimension. So, a future challenge is the integration of simulation/stimulation technologies also able to measure and modulate this internal/inner experience of the body. Finally, we also proposed the concept of “Sonoception” as an extension of this approach. The core idea is to exploit recent technological advances in the acoustic field to use sound and vibrations to modify the internal/inner body experience.

Identifying Oneself with the Face of Someone Else Impairs the Egocentered Visuo-spatial Mechanisms: A New Double Mirror Paradigm to Study Self-other Distinction and Interaction

Looking at our face in a mirror is one of the strongest phenomenological experiences of the Self in which we need to identify the face as reflected in the mirror as belonging to us. Recent behavioral and neuroimaging studies reported that self-face identification not only relies upon visual-mnemonic representation of one’s own face but also upon continuous updating and integration of visuo-tactile signals. Therefore, bodily self-consciousness plays a major role in self-face identification, with respect to interplay between unisensory and multisensory processing. However, if previous studies demonstrated that the integration of multisensory body-related signals contributes to the visual processing of one’s own face, there is so far no data regarding how self-face identification, inversely, contributes to bodily self-consciousness. In the present study, we tested whether self–other face identification impacts either the egocentered or heterocentered visuo-spatial mechanisms that are core processes of bodily self-consciousness and sustain self–other distinction. For that, we developed a new paradigm, named “Double Mirror.” This paradigm, consisting of a semi-transparent double mirror and computer-controlled Light Emitting Diodes, elicits self–other face merging illusory effect in ecologically more valid conditions, i.e., when participants are physically facing each other and interacting. Self-face identification was manipulated by exposing pairs of participants to an Interpersonal Visual Stimulation in which the reflection of their faces merged in the mirror. Participants simultaneously performed visuo-spatial and mental own-body transformation tasks centered on their own face (egocentered) or the face of their partner (heterocentered) in the pre- and post-stimulation phase. We show that self–other face identification altered the egocentered visuo-spatial mechanisms. Heterocentered coding was preserved. Our data suggest that changes in self-face identification induced a bottom-up conflict between the current visual representation and the stored mnemonic representation of one’s own face which, in turn, top-down impacted bodily self-consciousness.

Inducing ownership over an 'other' perspective with a visuo-tactile manipulation

Seeing our body from a 'self' perspective while performing a movement improves our ability to detect asynchrony between the visual and proprioceptive information concerning that movement: a signature of enhanced body ownership referred to as the 'self-advantage'. We consequently experience no self-advantage when seeing our body from an 'other' perspective. Here we ask whether introducing visuo-tactile stimulation (VTS), similar to that used in the rubber hand illusion to invoke ownership over a dummy hand, would produce a self-advantage when viewing the body from a typically 'other' perspective. Prior to the experiment, participants watched a live video of their own back using a camera mounted behind them while their back was tapped with a rod for 2 min. The video was either synchronous (sVTS) or asynchronous (aVTS) with the tapping. Participants then raised their hands and made a stereotyped finger movement that they watched from the same camera either in the original, natural perspective or upside down. Participants indicated which of two periods (one with minimum delay and one with an added delay of 33-264 ms) appeared delayed. Sensitivity was calculated using psychometric functions. The sVTS group showed a self-advantage of about 45 ms in the natural visual condition compared to the upside down condition, whereas the aVTS group showed no difference between the two conditions. Synchronous visuo-tactile experience increased the feeling of ownership over a typically 'other' perspective in a quantifiable way indicating the multisensory and malleable nature of body representation.