Enhancing motor imagery in the third-person perspective by manipulating sense of body ownership with virtual reality.

Virtual reality (VR)-guided motor imagery (MI) is a widely used approach for motor rehabilitation, especially for patients with severe motor impairments. Most approaches provide visual guidance from the first-person perspective (1PP). MI training with visual guidance from the third-person perspective (3PP) remains largely unexplored. We argue that 3PP MI training has its own advantages and can supplement 1PP MI. For some movements beyond the view of 1PP, such as shoulder shrugging and other axial movements, MI are suitable performed under 3PP. However, the efficiency of existing paradigms for 3PP MI is unsatisfactory. We speculate that the absence of sense of body ownership (SOO) from 3PP could be one possible factor and hypothesize that 3PP MI could be enhanced by eliciting SOO over a 3PP avatar. Based on our hypothesis, a novel paradigm was proposed to enhance 3PP MI by inducing full-body illusion (FBI) from 3PP, which is similar to the so-called out-of-body experience (OBE), using synchronous visuo-tactile stimulus with VR. The event-related Electroencephalograph (EEG) desynchronization (ERD) at motor-related regions from 31 healthy participants were calculated and compared with a control paradigm without "OBE" FBI induction. This study attempts to enhance 3PP MI with FBI induction. It offers an opportunity to perform MI guided by action observation from 3PP with elicited SOO to the observed avatar. We believe that 3PP MI could provide more possibilities for effective rehabilitation training, when SOO could be elicited to a virtual avatar and the present work demonstrates its viability and effectiveness.

Predicting the bodily self in space and time.

To understand how the human brain distinguishes itself from external stimulation, it was examined if motor predictions enable healthy adult volunteers to infer self-location and to distinguish their body from the environment (and other agents). By uniquely combining a VR-setup with full-body motion capture, a full-body illusion paradigm (FBI) was developed with different levels of motion control: (A) a standard, passive FBI in which they had no motion control; (B) an active FBI in which they made simple, voluntary movements; and (C) an immersive game in which they real-time controlled a human-sized avatar in third person. Systematic comparisons between measures revealed a causal relationship between (i) motion control (prospective agency), (ii) self-other identification, and (iii) the ability to locate oneself. Healthy adults could recognise their movements in a third-person avatar and psychologically align with it (action observation); but did not lose a sense of place (self-location), time (temporal binding), nor who they are (self/other). Instead, motor predictions enabled them to localise their body and to distinguish self from other. In the future, embodied games could target and strengthen the brain's control networks in psychosis and neurodegeneration; real-time motion simulations could help advance neurorehabilitation techniques by fine-tuning and personalising therapeutic settings.

The third-person perspective full-body illusion induced by visual-tactile stimulation in virtual reality for stroke patients.

This paper attempts to induce the third-person perspective full body illusion (3PP-FBI) with virtual reality (VR) in stroke patients. Nineteen individuals with stroke were recruited. The 3PP-FBI induction method, which was well-established in healthy individuals, using synchronous visual-tactile stimulation on one body part was used. Questionnaire scores and proprioceptive drift values were collected under different conditions for characterizing the induced 3PP-FBI. Results showed that synchronous visual-tactile stimulation of a single body part (back or upper limb) was sufficient to elicit 3PP-FBI in stroke patients, forming a sense of ownership (SOO) over the entire virtual body. Moreover, the intensity of 3PP-FBI was stronger when the back was stimulated, compared to stimulating the impaired upper limb. This study demonstrated the viability of visual-guided rehabilitation training while having a SOO to a virtual body from the third-person perspective, in anticipation of achieving better rehabilitation outcome for movements beyond the first-person perspective.

Full body illusion and cognition: A systematic review of the literature.

In recent years, an increasing number of studies employed the full body illusion paradigm (i.e., the experimentally induced illusory ownership over a fake/virtual body) to investigate the role of body ownership in higher-level cognition. However, to date, no systematic review of this topic is present. To fill this gap, here we identified 102 papers classifiable in 4 cognitive domains and 12 subdomains. Overall, they manipulated the identity of the fake body, and/or its spatial perspective, and/or the kind of multisensory stimulation, and/or its spatiotemporal congruency. The effects were measured through implicit and/or explicit indices. In summary, results showed that participants' behavior shifted away from the self and/or towards that of the embodied fake/virtual body. The findings have been interpreted within various exploratory frameworks, with the most common being the Proteus effect, the cortical body matrix and the inverse model of motor control. Future studies should focus on building a unitary theoretical explanation of the effects, gaining solid neural evidence, creating standard experimental procedures, and exploring the practical applications.

Multisensory and Sensorimotor Integration in the Embodied Self: Relationship between Self-Body Recognition and the Mirror Neuron System.

The embodied self is rooted in the self-body in the "here and now". The senses of self-ownership and self-agency have been proposed as the basis of the sense of embodied self, and many experimental studies have been conducted on this subject. This review summarizes the experimental research on the embodied self that has been conducted over the past 20 years, mainly from the perspective of multisensory integration and sensorimotor integration regarding the self-body. Furthermore, the phenomenon of back projection, in which changes in an external object (e.g., a rubber hand) with which one has a sense of ownership have an inverse influence on the sensation and movement of one's own body, is discussed. This postulates that the self-body illusion is not merely an illusion caused by multisensory and/or sensorimotor integration, but is the incorporation of an external object into the self-body representation in the brain. As an extension of this fact, we will also review research on the mirror neuron system, which is considered to be the neural basis of recognition of others, and discuss how the neural basis of self-body recognition and the mirror neuron system can be regarded as essentially the same.

Top-Down Influence Leads to a Reduced Sense of Body Ownership in Individuals With Depersonalization Tendencies: A Focus on Full Body Illusion.

Sense of body ownership, that is, the feeling that "my body belongs to me," has been examined by both the rubber hand illusion (RHI) and full body illusion (FBI). In a study that examined the relationship between RHI and depersonalization, a symptom in which people experience a lower sense of body ownership, people with a high depersonalization tendency experienced RHI through the bottom-up process of visual-tactile integration. Why is it that people with depersonalization feel a lower sense of body ownership over their bodies? Case studies of depersonalization suggest that the top-down cognition in people with depersonalization may make them less likely to feel a sense of body ownership. However, the top-down influence on the sense of body ownership in depersonalization has not yet been experimentally demonstrated. By incorporating top-down manipulation (e.g., instructing participants to regard a fake body as their own) into the FBI procedure, we aimed to clarify the cause of the reduced sense of body ownership in people with a high depersonalization tendency. The FBI procedure was conducted in a virtual reality environment using an avatar as a fake body. The avatar was presented from a third-person perspective, and visual-tactile stimuli were presented to create an illusion. To examine the degree of illusion, we measured the skin conductance responses to the fear stimulus presented after the visual-tactile stimuli presentation. The degree of depersonalization was measured using the Japanese version of the Cambridge Depersonalization Scale. To manipulate the top-down influence, we provided self-association instructions before the presentation of the visual-tactile stimuli. We predicted that the higher the degree of depersonalization, the lower the degree of illusion in the self-association instruction. The results showed that participants with a higher depersonalization tendency had a lower degree of illusion (rho = -0.424, p = 0.035) in the self-association condition. This indicates that in people with a high depersonalization tendency, top-down cognition of the body as their own leads to a decrease in the sense of body ownership.

LIVE-streaming 3D images: A neuroscience approach to full-body illusions.

Inspired by recent technological advances in the gaming industry, we used capture cards to create and LIVE-stream high quality 3D-images. With this novel technique, we developed a real-life stereoscopic 3D full-body illusion paradigm (3D projection). Unlike previous versions of the full-body illusion that rely upon unwieldy head-mounted displays, this paradigm enables the unobstructed investigation of such illusions with neuroscience methods (e.g., transcranial direct current stimulation, transcranial magnetic stimulation, electroencephalography, and near-infrared spectroscopy) and examination of their neural underpinnings. This paper has three aims: (i) to provide a step-by-step guide on how to implement 3D LIVE-streaming, (ii) to explain how this can be used to create a full-body illusion paradigm; and (iii) to present evidence that documents the effectiveness of our methods (de Boer et al., 2020), including suggestions for potential applications. Particularly significant is the fact that 3D LIVE-streaming is not GPU-intensive and can easily be applied to any device or screen that can display 3D images (e.g., TV, tablet, mobile phone). Therefore, these methods also have potential future clinical and commercial benefits. 3D LIVE-streaming could be used to enhance future clinical observations or educational tools, or potentially guide medical interventions with real-time high-quality 3D images. Alternatively, our methods can be used in future rehabilitation programs to aid recovery from nervous system injury (e.g., spinal cord injury, brain damage, limb loss) or in therapies aimed at alleviating psychosis symptoms. Finally, 3D LIVE-streaming could set a new standard for immersive online gaming as well as augmenting online and mobile experiences (e.g., video chat, social sharing/events).

Development of the bodily self: Effects of visuomotor synchrony and visual appearance on virtual embodiment in children and adults.

The sense of a bodily self is thought to depend on adaptive weighting and integration of bodily afferents and prior beliefs. Evidence from studies using paradigms such as the rubber hand illusion and full body illusion suggests changes in the integration of visuotactile bodily signals throughout childhood. Here, we extended this line of research by assessing how bottom-up visuomotor synchrony and expectancy, modulated by visual appearance of virtual avatars, contribute to embodiment in children. We compared responses to a first-person perspective virtual full body illusion from 8- to 12-year-old children and adults while manipulating synchrony of the avatar's movements (synchronous, 0.5-s delay, or 1-s delay compared with the participant's movements) and appearance of the avatar (human or skeleton). We measured embodiment with both subjective questionnaires and objective skin conductance responses to virtual threat. Results showed that children experienced ownership for the virtual avatar in a similar way as adults, which was reduced with increasing asynchrony, and for the skeleton avatar as compared with the human avatar. This modulation of ownership was not reflected in the skin conductance responses, which were equally high in all experimental conditions and only showed a modulation of repetition by age. In contrast, in children the subjective experience of agency was less affected by the dampening effects of visuomotor asynchrony or reduced human likeness and was overall higher. These findings suggest that children can easily embody a virtual avatar but that different aspects of embodiment develop at different rates, which could have important implications for applications of embodied virtual reality.

Influence of vestibular signals on bodily self-consciousness: Different sensory weighting strategies based on visual dependency.

Previous studies showed that the vestibular system is crucial for multisensory integration, however, its contribution to bodily self-consciousness more specifically on full-body illusions is not well understood. Thus, the current study examined the role of visuo-vestibular conflict on a full-body illusion (FBI) experiment that was induced during a supine body position. In a mixed design experiment, 56 participants underwent through a full-body illusion protocol. During the experiment, half of the participants received synchronous visuo-tactile stimulation, and the other half received asynchronous visuo-tactile stimulation, while their physical body was lying in a supine position, but the virtual body was standing. Additionally, the contribution of individual sensory weighting strategies was investigated via the Rod and Frame task (RFT), which was applied both before (pre-FBI standing and pre-FBI supine) and after the full-body illusion (post-FBI supine) protocol. Subjective reports of the participants confirmed previous findings suggesting that there was a significant increase in ownership over a virtual body during synchronous visuo-tactile stimulation. Additionally, further categorization of participants based on their visual dependency (by RFT) showed that those participants who rely more on visual information (visual field dependents) perceived the full-body illusion more strongly than non-visual field dependents during the synchronous visuo-tactile stimulation condition. Further analysis provided not only a quantitative demonstration of full-body illusion but also revealed changes in perceived self-orientation based on their field dependency. Altogether, findings of the current study make further contributions to our understanding of the vestibular system and brought new insight for individual sensory weighting strategies during a full-body illusion.

The role of affective touch in whole-body embodiment remains equivocal.

Previous studies have highlighted that affective touch delivered at slow velocities (1-10 cm/s) enhances body-part embodiment during multisensory illusions, yet its role towards whole-body embodiment is less established. Across two experiments, we investigated the role of affective touch towards subjective embodiment of a whole mannequin body within the full body illusion, amongst healthy females. Participants perceived affective touch to be more pleasant than non-affective touch, but this did not enhance subjective embodiment within the illusion and no interaction between synchrony (Experiment 1), or congruency (Experiment 2), and the velocity of touch was observed. Finally, the perceived pleasantness of touch was not modulated by subthreshold eating disorder psychopathology, as measured by means of a self-report questionnaire. Therefore, the present findings suggest that enhancement of embodiment due to affective touch may be body-part specific, and not generalise to greater ownership towards a whole body.

Controlling the Sense of Embodiment for Virtual Avatar Applications: Methods and Empirical Study.

BACKGROUND: The sense of embodiment (SoE) is the feeling of one's own body, and research on the SoE extends from the rubber hand illusion to the full-body ownership illusion with a virtual avatar. OBJECTIVE: The key to utilizing a virtual avatar is understanding and controlling the SoE, and it can be extended to several medical applications. In this study, we aimed to clarify these aspects by considering the following three subcomponents of SoE: sense of agency, ownership, and self-location. METHODS: We defined a human avatar (HA), point light avatar (PLA), and out-of-body point light avatar (OBPLA) and compared them in three user studies. In study 1, 28 participants were recruited and the three avatar conditions (HA, PLA, and OBPLA) were compared. In study 2, 29 new participants were recruited, and there were two avatar conditions (HA ad PLA) and two motion synchrony conditions (synchrony and asynchrony). In study 3, 29 other participants were recruited, and there were two avatar conditions (PLA and OBPLA) and two motion synchrony conditions (synchrony and asynchrony). Dependent measures included sense of agency, ownership, and self-location; emotional response; presence; and simulator sickness. RESULTS: The findings of study 1 showed that the three avatar generation methodologies can control the sense of ownership and self-location in a stepwise manner while maintaining a high sense of agency. In studies 2 and 3, we found dependencies among the three subcomponents of SoE and observed that they affected users' subjective experiences. CONCLUSIONS: Our findings may have implications for boosting the effects of virtual avatar applications in medical areas, by understanding and controlling the SoE with a full-body illusion.

Gulliver's virtual travels: active embodiment in extreme body sizes for modulating our body representations.

It is noted that the perceptual experience of body and space can be modulated by changing the action capabilities or by manipulating the perceived body dimensions through a multisensory stimulation. This study adds to pre-existing literature by investigating the alterations in bodily experience following embodiment to both enlarged and shrunked bodies, while participants actively navigated in a virtual environment. A normal-sized body served as a reference condition. After each embodied navigation, participants estimated the height and width of three different body parts. Results revealed that the embodiment over shrunked body induced a significant reduction in participants' body image, while no changes were reported after the embodiment over the enlarged body. Findings were discussed in terms of previous literature exploring the constraints implicated in the ownership over different bodies.

The self-generated full body illusion is accompanied by impaired detection of somatosensory stimuli.

Research has found that body illusions may be accompanied by consequences for the real body whereby various somatosensory and homeostatic bodily functions may be impaired. These findings stem from research where an experimenter induced the body illusions. In line with advances in the domains of videogames and virtual reality where the real body is used as a controller we investigate if these consequences also accompany self-generated body illusions. In two preregistered experiments we made use of a head-mounted display set-up to induce the full body illusion (FBI) whereby touch is felt to originate from a 3PP body, and examined effects in the simple detection of supra-threshold vibrotactile stimuli presented to the participants' back and head. Results of both experiments indicate that it is possible to induce a FBI through self-stroking of the neck and that the FBI is accompanied by reduced accuracy and delayed reaction times in detection of somatosensory stimuli. In an additional preregistered control experiment the alternative explanation that a difference in motion presented in the conditions was responsible for these findings was ruled out. Our findings corroborate previous studies that have found body illusions to be accompanied by bodily consequences and further extend these findings to the domain of self-induced body illusions. These results are relevant for video games and VR setups that are geared towards virtual embodiment as they advance our understanding of the conditions and mechanisms in which bodily consequences may express themselves.

My body until proven otherwise: Exploring the time course of the full body illusion.

Evidence from the Full Body Illusion (FBI) has shown that adults can embody full bodies which are not their own when they move synchronously with their own body or are viewed from a first-person perspective. However, there is currently no consensus regarding the time course of the illusion. Here, for the first time, we examined the effect of visuomotor synchrony (synchronous/asynchronous/no movement) on the FBI over time. Surprisingly, we found evidence of embodiment over a virtual body after five seconds in all conditions. Embodiment decreased with increased exposure to asynchronous movement, but remained high in synchronous and no movement conditions. We suggest that embodiment of a body seen from a first-person perspective is felt by default, and that embodiment can then be lost in the face of contradictory cues. These results have significant implications for our understanding of how multisensory cues contribute to embodiment.

The Effect of a Virtual-Reality Full-Body Illusion on Body Representation in Obesity.

Background. The effective illusory ownership over an artificial body in modulating body representations in healthy and eating disorders population has been repeatedly reported in recent literature. In this study, we extended this research in the field of obesity: specifically, we investigated whether ownership over a virtual body with a skinny abdomen might be successfully experienced by participants affected by obesity. Methods. Fifteen participants with obesity and fifteen healthy-weight participants took part at this study in which the VR-Full-Body Illusion was adopted. The strength of illusion was investigated through the traditional Embodiment Questionnaire, while changes in bodily experience were measured through a body size estimation task. Results. Participants with obesity as well as healthy-weight participants reported to experience the illusion. About the body size estimation task, both groups reported changes only in the estimation of the abdomen's circumference after the experimental condition, in absence of any another difference. Discussion. Participants with obesity reported to experience the illusion over a skinny avatar, but the modulation of the bodily experience seems controversial. Future lines of research exploiting this technique for modulating body representations in obesity, specifically in terms of potential therapeutic use, were discussed.

Putting Oneself in the Body of Others: A Pilot Study on the Efficacy of an Embodied Virtual Reality System to Generate Self-Compassion.

Compassion-based interventions (CBIs) have been shown to be effective for increasing empathy and compassion, and reducing stress, anxiety, and depression. CBIs are based on constructive meditations where imagery abilities are essential. One of the major difficulties that participants report during the training is the difficulty related to imagery abilities. Virtual reality (VR) can be a useful tool to overcome this limitation because it can facilitate the construction and sustainment of mental images. The machine to be another (TMTBA) uses multi-sensory stimulation to induce a body swap illusion. This system allows participants to see themselves from a third perspective and have the illusion of touching themselves from outside. The main objective of the present study was to analyze the efficacy of a self-compassion meditation procedure based on the TMTBA system versus the usual meditation procedure (CAU) in increasing positive affect states, mindful self-care, and adherence to the practice, and explore the influence of imagery abilities as moderators of the effects of the condition on adherence. A sample of 16 participants were randomly assigned to two conditions: TMTBA-VR and CAU. All participants had to listen to an audio meditation about self-compassion and answer questionnaires before and after the training. The TMTBA-VR condition also had a body swap experience at the end of the meditation while listening to self-compassionate messages. Afterward, they were invited to practice this meditation for 2 weeks and then measured again. After the compassion practice, both conditions significantly increased positive qualities toward self/others, decreased negative qualities toward self, and increased awareness and attention to mental events and bodily sensations, with no differences between the conditions. After 2 weeks, both conditions showed a similar frequency of meditation practice and increases in specific types of self-care behaviors, with the frequency of clinical self-care behaviors being significantly higher in TMTBA. Finally, lower imagery ability in the visual and cutaneous modality were moderators of the efficacy of the TMTBA (vs. CAU) condition in increasing adherence to the practice. Embodied VR could be an interesting tool to facilitate and increase the efficacy of CBIs by facilitating the construction of positive and powerful mental images.

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 Tilted Self: Visuo-Graviceptive Mismatch in the Full-Body Illusion.

The bodily self is a fundamental part of human self-consciousness and relies on online multimodal information and prior beliefs about one's own body. While the contribution of the vestibular system in this process remains under-investigated, it has been theorized to be important. The present experiment investigates the influence of conflicting gravity-related visual and bodily information on the sense of a body and, vice versa, the influence of altered embodiment on verticality and own-body orientation perception. In a full-body illusion setup, participants saw in a head-mounted display a projection of their own body 2 m in front of them, on which they saw a tactile stimulation on their back displayed either synchronously or asynchronously. By tilting the seen body to one side, an additional visuo-graviceptive conflict about the body orientation was created. Self-identification with the seen body was measured explicitly with a questionnaire and implicitly with skin temperature. As measures of orientation with respect to gravity, we assessed subjective haptic vertical and the haptic body orientation. Finally, we measured the individual visual field dependence using the rod-and-frame test. The results show a decrease in self-identification during the additional visuo-graviceptive conflict, but no modulation of perceived verticality or subjective body orientation. Furthermore, explorative analyses suggest a stimulation-dependent modulation of the perceived body orientation in individuals with a strong visual field dependence only. The results suggest a mutual interaction of graviceptive and other sensory signals and the individual's weighting style in defining our sense of a bodily self.

Altered bodily self-consciousness and peripersonal space in autism.

There is some evidence that disordered self-processing in autism spectrum disorders is linked to the social impairments characteristic of the condition. To investigate whether bodily self-consciousness is altered in autism spectrum disorders as a result of multisensory processing differences, we tested responses to the full body illusion and measured peripersonal space in 22 adults with autism spectrum disorders and 29 neurotypical adults. In the full body illusion set-up, participants wore a head-mounted display showing a view of their 'virtual body' being stroked synchronously or asynchronously with respect to felt stroking on their back. After stroking, we measured the drift in perceived self-location and self-identification with the virtual body. To assess the peripersonal space boundary we employed an audiotactile reaction time task. The results showed that participants with autism spectrum disorders are markedly less susceptible to the full body illusion, not demonstrating the illusory self-identification and self-location drift. Strength of self-identification was negatively correlated with severity of autistic traits and contributed positively to empathy scores. The results also demonstrated a significantly smaller peripersonal space, with a sharper (steeper) boundary, in autism spectrum disorders participants. These results suggest that bodily self-consciousness is altered in participants with autism spectrum disorders due to differences in multisensory integration, and this may be linked to deficits in social functioning.

From avatars to body swapping: The use of virtual reality for assessing and treating body-size distortion in individuals with anorexia.

In the last 30 years, virtual reality (VR) has offered innovative solutions for assessing and treating body representation disturbances in anorexia nervosa (AN). The most recent and innovative trend is the exploitation of the so-called VR-based body swapping illusion. The aim of this case study was to report the use of this VR protocol within a multidisciplinary treatment of AN. A patient with a Diagnostic and Statistical Manual-5 diagnosis of AN underwent an intensive multidisciplinary outpatient treatment. Three sessions of a VR-based body swapping illusion (i.e., the experimental induction of being the owner of a virtual body as a result of a visuotactile stimulation) were delivered within the treatment protocol (i.e., beginning of the treatment; end of one cycle of the treatment; 1 year of follow-up). We report the results obtained, discussing how the VR full body illusion was both able to effectively monitor changes of multisensory bodily integration and to act as a driver for these changes.