Specificity and Coherence of Body Representations

"As if it were my own hand": inducing the rubber hand illusion through virtual reality for motor imagery enhancement

Brain-computer interfaces (BCI) are widely used in the field of disability assistance and rehabilitation, and virtual reality (VR) is increasingly used for visual guidance of BCI-MI (motor imagery). Therefore, how to improve the quality of electroencephalogram (EEG) signals for MI in VR has emerged as a critical issue. People can perform MI more easily when they visualize the hand used for visual guidance as their own, and the Rubber Hand Illusion (RHI) can increase people's ownership of the prosthetic hand. We proposed to induce RHI in VR to enhance participants' MI ability and designed five methods of inducing RHI, namely active movement, haptic stimulation, passive movement, active movement mixed with haptic stimulation, and passive movement mixed with haptic stimulation, respectively. We constructed a first-person training scenario to train participants' MI ability through the five induction methods. The experimental results showed that through the training, the participants' feeling of ownership of the virtual hand in VR was enhanced, and the MI ability was improved. Among them, the method of mixing active movement and tactile stimulation proved to have a good effect on enhancing MI. Finally, we developed a BCI system in VR utilizing the above training method, and the performance of the participants improved after the training. This also suggests that our proposed method is promising for future application in BCI rehabilitation systems.

The effects of right temporoparietal junction stimulation on embodiment, presence, and performance in teleoperation

Embodiment (the sensation that arises when the properties of an external instrument are processed as if they are the attributes of one's own biological body) and (tele)presence (the sensation of being fully engaged and immersed in a location other than the physical space occupied by one's body) sustain the perception of the physical self and potentially improve performance in teleoperations (a system that enables human intelligence to control robots and requires implementing an effective human-machine interface). Embodiment and presence may be interdependent and influenced by right temporo-parietal junction (rTPJ) activity. We investigated the interplay between embodiment, (tele)presence, and performance in teleoperation, focusing on the role of the rTPJ. Participants underwent a virtual reality task with transcranial direct current stimulation (tDCS) twice, receiving either active or sham stimulation. Behavioral measures (driving inaccuracy, elapsed time in the lap, time spent in attentional lapses, short-term self-similarity, and long-term self-similarity), perceived workload (mental demand, physical demand, temporal demand, own performance, effort, and frustration), embodiment's components (ownership, agency, tactile sensations, location, and external appearance), and presence's components (realism, possibility to act, quality of interface, possibility to examine, self-evaluation of performance, haptic, and sounds) were assessed. The results showed that rTPJ stimulation decreased perceived ownership but enhanced presence with changes in the complexity of visuomotor adjustments (long and short-term self-similarity indices). Structural equation modeling revealed that embodiment increased visuomotor inaccuracy (a composite variable of overall performance, including deviations from the optimal trajectory and the time taken to complete the task), presence reduced workload, and workload increased inaccuracy. These results suggested a dissociation between embodiment and presence, with embodiment hindering performance. Prioritizing virtual integration may lower human performance, while reduced workload from presence could aid engagement. These findings emphasize the intricate interplay between rTPJ, subjective experiences, and performance in teleoperation.

No reduction in motor-evoked potential amplitude during the rubber hand illusion

INTRODUCTION

In the rubber hand illusion (RHI), touches are applied to a fake hand at the same time as touches are applied to a participant's real hand that is hidden in a congruent position. Synchronous (but not asynchronous) tactile stimulation of the two hands may induce the sensation that the fake hand is the participant's own. As such, the illusion is commonly used to examine the sense of body ownership. Some studies indicate that in addition to the subjective experience of limb ownership reported by participants, the RHI can also reduce corticospinal excitability (e.g., as reflected in motor-evoked potential [MEP] amplitude) and alter parietal-motor cortical connectivity in passive participants. These findings have been taken to support a link between motor cortical processing and the subjective experience of body ownership.

METHODS

In this study, we tried to replicate the reduction in MEP amplitude associated with the RHI and uncover the components of the illusion that might explain these changes. As such, we used single-pulse transcranial magnetic stimulation to probe the excitability of the corticospinal motor system as participants experienced the RHI.

RESULTS

Despite participants reporting the presence of the illusion and showing shifts in perceived real hand position towards the fake limb supporting its elicitation, we did not observe any associated reduction in MEP amplitude.

CONCLUSION

We conclude that a reduction in MEP amplitude is not a reliable outcome of the RHI and argue that if such effects do occur, they are unlikely to be large or functionally relevant.

Vicarious touch: Overlapping neural patterns between seeing and feeling touch

Simulation theories propose that vicarious touch arises when seeing someone else being touched triggers corresponding representations of being touched. Prior electroencephalography (EEG) findings show that seeing touch modulates both early and late somatosensory responses (measured with or without direct tactile stimulation). Functional Magnetic Resonance Imaging (fMRI) studies have shown that seeing touch increases somatosensory cortical activation. These findings have been taken to suggest that when we see someone being touched, we simulate that touch in our sensory systems. The somatosensory overlap when seeing and feeling touch differs between individuals, potentially underpinning variation in vicarious touch experiences. Increases in amplitude (EEG) or cerebral blood flow response (fMRI), however, are limited in that they cannot test for the information contained in the neural signal: seeing touch may not activate the same information as feeling touch. Here, we use time-resolved multivariate pattern analysis on whole-brain EEG data from people with and without vicarious touch experiences to test whether seen touch evokes overlapping neural representations with the first-hand experience of touch. Participants felt touch to the fingers (tactile trials) or watched carefully matched videos of touch to another person's fingers (visual trials). In both groups, EEG was sufficiently sensitive to allow decoding of touch location (little finger vs. thumb) on tactile trials. However, only in individuals who reported feeling touch when watching videos of touch could a classifier trained on tactile trials distinguish touch location on visual trials. This demonstrates that, for people who experience vicarious touch, there is overlap in the information about touch location held in the neural patterns when seeing and feeling touch. The timecourse of this overlap implies that seeing touch evokes similar representations to later stages of tactile processing. Therefore, while simulation may underlie vicarious tactile sensations, our findings suggest this involves an abstracted representation of directly felt touch.

The flubbed body: Pathological body size representation in personal neglect

Personal Neglect (PN) is a disorder in which patients fail to attend or explore the contralateral side of their body. An increasing number of studies have considered PN as a form of body representation disorder frequently observed following damage to parietal areas. The extent and the direction of the body misrepresentation is still unclear with recent studies suggesting a general reduction of contralesional hand size. However, little is known about the specificity of this representation and whether the misrepresentation also generalises to other body parts. We explored the features of the representation of the hands and face in a group of 9 right brain damaged patients with (PN+) and without PN (PN-), when compared to a healthy control group. For this, we used a body size estimation task with pictures, in which patients were required to choose the one that most closely matched the perceived size of their body part. We found that PN + patients showed a labile body representation for both hands and face, having a larger distorted representational range. Interestingly, in comparison with PN + patients and healthy controls, PN- patients also showed misrepresentation of the left contralesional hand which could be related to impaired motor performance of their upper limb. Our findings are discussed within a theoretical framework suggesting a reliance on multisensory integration (body representation, ownership, and motor influences) for an ordered representation of the size of the body.

Does Nice or Nasty Matter? The Intensity of Touch Modulates the Rubber Hand Illusion

Our sense of body ownership results from the ongoing integration of perceptual information coming from the different senses (i.e., multisensory integration). The Rubber Hand Illusion (RHI) has been extensively studied to investigate the malleability of body ownership through contrasting multisensory information. Indeed, during the RHI, stroking a visible rubber hand synchronously to participants’ hand hidden from sight generates the illusion of ownership of the rubber hand (embodiment) and the mis-location of participants’ hand as closer to the rubber hand (proprioceptive drift). It is well known that the RHI is optimally evoked by a pleasant stroking (affective) touch, but what of an unpleasant (painful) stroking touch – does hedonic valence matter? To this aim, participants repeated the RHI while receiving different types of touch: pleasant, painful, and neutral. Results showed, for the first time, that the subjective intensity of the tactile stimulation experienced across the different conditions modulates the strength of the proprioceptive drift. Notably, participants reported a stronger RHI (mis-placed body ownership) from stimulation rated as more intense and involving an interoceptive activation (pain and pleasantness vs. neutral). We propose that interoceptive information, regardless of the valence of the stimuli (positive or negative), are perceived as more intense and enhance, through the activation of the limbic system, multisensory integration. In the context of the RHI, this translates to a stronger illusion in terms of proprioceptive drift.

Age-related changes in visuo-proprioceptive processing in perceived body position

This study investigated age-related change in visuo-proprioceptive processing in the perceived body position using mirror hand/foot illusions, focusing on its temporal characteristics, its dependency on body parts, and its association with older adults’ fall risk. Either immediately or 15 s after the exposure to the mirror-induced inconsistency of visuo-proprioceptive signals regarding the right hand or foot position, participants performed a reaching task using the unseen, illusion-affected hand or foot. Results showed clear age group differences. Specifically, older adults exhibited larger reaching errors than younger adults in the hand condition, and after the 15 s delay in the foot condition. Further, the reaching errors were constant across time for older adults but decreased after the delay in young adults, regardless of the tested body part. Particularly, older adults’ risk of falling, which was assessed by the timed up-and-go test, was negatively correlated with the reduction of reaching error across time. This suggests that older adults, especially those with a high risk of falling, face difficulties in appropriately processing visual and proprioceptive information for body perception in accordance with their external environment.

Prosthetic embodiment: systematic review on definitions, measures, and experimental paradigms

The term embodiment has become omnipresent within prosthetics research and is often used as a metric of the progress made in prosthetic technologies, as well as a hallmark for user acceptance. However, despite the frequent use of the term, the concept of prosthetic embodiment is often left undefined or described incongruently, sometimes even within the same article. This terminological ambiguity complicates the comparison of studies using embodiment as a metric of success, which in turn hinders the advancement of prosthetics research. To resolve these terminological ambiguities, we systematically reviewed the used definitions of embodiment in the prosthetics literature. We performed a thematic analysis of the definitions and found that embodiment is often conceptualized in either of two frameworks based on body representations or experimental phenomenology. We concluded that treating prosthetic embodiment within an experimental phenomenological framework as the combination of ownership and agency allows for embodiment to be a quantifiable metric for use in translational research. To provide a common reference and guidance on how to best assess ownership and agency, we conducted a second systematic review, analyzing experiments and measures involving ownership and agency. Together, we highlight a pragmatic definition of prosthetic embodiment as the combination of ownership and agency, and in an accompanying article, we provide a perspective on a multi-dimensional framework for prosthetic embodiment. Here, we concluded by providing recommendations on metrics that allow for outcome comparisons between studies, thereby creating a common reference for further discussions within prosthetics research.

EXPRESS: The Influence of Three-Dimensional Cues on Body Size Judgements

Research has shown that body size judgements are frequently biased, or inaccurate. Critically, judgement biases are further exaggerated for individuals with eating disorders, a finding that has been attributed to difficulties integrating body features into a perceptual whole. However, current understanding of which body features are integrated when judging body size is lacking. In this study, we examine whether individuals integrate three-dimensional (3D) cues to body volume when making body size judgements. Computer-generated body stimuli were presented in a 3D Virtual Reality (VR) environment. Participants (N = 412) were randomly assigned to one of two conditions: in one condition the to-be-judged body was displayed binocularly (containing 3D cues to body volume), in the other, bodies were presented monocularly (2D cues only). Across 150 trials, participants were required to make a body size judgement of a target female body from a third-person point of view using an unmarked visual analogue scale (VAS). It was found that 3D cues significantly influenced body size judgements. Namely, thin 3D bodies were judged smaller, and overweight 3D bodies were judged larger, than their 2D counterpart. Furthermore, to reconcile these effects, we present evidence that the two perceptual biases, regression to the mean and serial dependence, were reduced by the additional 3D feature information. Our findings increase our understanding of how body size is perceptually encoded and creates testable predictions for clinical populations exhibiting integration difficulties.

The Development of a Flexible Bodily Representation: Behavioral Outcomes and Brain Oscillatory Activity During the Rubber Hand Illusion in Preterm and Full-Term School-Age Children

During childhood, the body undergoes rapid changes suggesting the need to constantly update body representation based on the integration of multisensory signals. Sensory experiences in critical periods of early development may have a significant impact on the neurobiological mechanisms underpinning the development of the sense of one’s own body. Specifically, preterm children are at risk for sensory processing difficulties, which may lead to specific vulnerability in binding together sensory information in order to modulate the representation of the bodily self. The present study aims to investigate the malleability of body ownership in preterm (N = 21) and full-term (N = 19) school-age children, as reflected by sensitivity to the Rubber Hand Illusion. The results revealed that multisensory processes underlying the ability to identify a rubber hand as being part of one’s own body are already established in childhood, as indicated by a higher subjective feeling of embodiment over the rubber hand during synchronous visual-tactile stimulation. Notably, the effect of visual-tactile synchrony was related to the suppression of the alpha band oscillations over frontal, central, and parietal scalp regions, possibly indicating a greater activation of somatosensory and associative areas underpinning the illusory body ownership. Moreover, an interaction effect between visual-tactile condition and group emerged, suggesting that preterm children showed a greater suppression of alpha oscillatory activity during the illusion. This result together with lower scores of subjective embodiment over the rubber hand reported by preterm children indicate that preterm birth may affect the development of the flexible representation of the body. These findings provide an essential contribution to better understand the processes of identification and differentiation of the bodily self from the external environment, in both full-term and preterm children, paving the way for a multisensory and embodied approach to the investigation of social and cognitive development.

Little evidence for an effect of the rubber hand illusion on basic movement

Body ownership refers to the distinct sensation that our observed body belongs to us, which is believed to stem from multisensory integration. This is commonly shown through the rubber hand illusion (RHI), which induces a sense of ownership over a false limb. Whilst the RHI may interfere with object-directed action and alter motor cortical activity, it is not yet clear whether a sense of ownership over an artificial hand has functional consequences for movement production per se. As such, we performed two motion-tracking experiments (n = 117) to examine the effects of the RHI on the reaction time, acceleration, and velocity of rapid index finger abduction. We observed little convincing evidence that the induction of the RHI altered these kinematic variables. Moreover, the subjective sensations of rubber hand ownership, referral of touch, and agency did not convincingly correlate with kinematic variables, and nor did proprioceptive drift, suggesting that changes in body representation elicited by the RHI may not influence basic movement. Whilst experiment 1 suggested that individuals reporting a greater sensation of the real hand disappearing performed movements with smaller acceleration and velocity following illusion induction, we did not replicate this effect in a second experiment, suggesting that these effects may be small or not particularly robust. Overall, these results indicate that manipulating the conscious experience of body ownership has little impact on basic motor control, at least in the RHI with healthy participants.

The rubber hand illusion in children: What are we measuring?

The rubber hand illusion (RHI) is a much-studied bodily illusion that has been used in a wide number of populations to investigate the plasticity of the mental body representation. In adult participants, the wide adoption of the illusion has led to a proliferation of experimental variations of the illusion, and with that, considerable apparent inconsistencies in both empirical results and conceptual interpretations. In turn, this makes it challenging to integrate empirical findings and to identify what those findings together can tell us about the representation of the body in the brain. More recently, scientists have started applying the illusion to populations of children, in order to better understand how body representations develop in both typically developing children and in clinical populations. With this field now starting to expand, we believe it is both urgent and important to prevent unintended methodological variability from hindering the consistency of the paediatric literature as it has the adult literature. With this aim in mind, we review the 12 currently available paediatric RHI studies, and summarise their key methodological choices and conceptual definitions. We highlight a number of important discrepancies, particularly where seemingly equivalent analysis choices might significantly affect the interpretation of results, and make recommendations for future studies. We hope this will allow this important and emerging field to benefit from the synergy that results from multiple studies using convergent and consistent empirical methods.

Perception of body shape and size without touch or proprioception: evidence from individuals with congenital and acquired neuropathy

The degree to which mental representations of the body can be established and maintained without somatosensory input remains unclear. We contrast two "deafferented" adults, one who acquired large fibre sensory loss as an adult (IW) and another who was born without somatosensation (KS). We compared their responses to those of matched controls in three perceptual tasks: first accuracy of their mental image of their hands (assessed by testing recognition of correct hand length/width ratio in distorted photographs and by locating landmarks on the unseen hand); then accuracy of arm length judgements (assessed by judgement of reaching distance), and finally, we tested for an attentional bias towards peri-personal space (assessed by reaction times to visual target presentation). We hypothesised that IW would demonstrate responses consistent with him accessing conscious knowledge, whereas KS might show evidence of responses dependent on non-conscious mechanisms. In the first two experiments, both participants were able to give consistent responses about hand shape and arm length, but IW displayed a better awareness of hand shape than KS (and controls). KS demonstrated poorer spatial accuracy in reporting hand landmarks than both IW and controls, and appears to have less awareness of her hands. Reach distance was overestimated by both IW and KS, as it was for controls; the precision of their judgements was slightly lower than that of the controls. In the attentional task, IW showed no reaction time differences across conditions in the visual detection task, unlike controls, suggesting that he has no peri-personal bias of attention. In contrast, KS did show target location-dependent modulation of reaction times, when her hands were visible. We suggest that both IW and KS can access a conscious body image, although its accuracy may reflect their different experience of hand action. Acquired sensory loss has deprived IW of any subconscious body awareness, but the congenital absence of somatosensation may have led to its partial replacement by a form of visual proprioception in KS.

Re-association of Body Parts: Illusory Ownership of a Virtual Arm Associated With the Contralateral Real Finger by Visuo-Motor Synchrony

Illusory ownership can be induced in a virtual body by visuo-motor synchrony. Our aim was to test the possibility of a re-association of the right thumb with a virtual left arm and express the illusory body ownership of the re-associated arm through a synchronous or asynchronous movement of the body parts through action and vision. Participants felt that their right thumb was the virtual left arm more strongly in the synchronous condition than in the asynchronous one, and the feeling of ownership of the virtual arm was also stronger in the synchronous condition. We did not find a significant difference in the startle responses to a sudden knife appearance to the virtual arm between the two synchrony conditions, as there was no proprioceptive drift of the thumb. These results suggest that a re-association of the right thumb with the virtual left arm could be induced by visuo-motor synchronization; however, it may be weaker than the natural association.

Awareness of voluntary action, rather than body ownership, improves motor control

Awareness of the body is essential for accurate motor control. However, how this awareness influences motor control is poorly understood. The awareness of the body includes awareness of visible body parts as one’s own (sense of body ownership) and awareness of voluntary actions over that visible body part (sense of agency). Here, I show that sense of agency over a visible hand improves the initiation of movement, regardless of sense of body ownership. The present study combined the moving rubber hand illusion, which allows experimental manipulation of agency and body ownership, and the finger-tracking paradigm, which allows behavioral quantification of motor control by the ability to coordinate eye with hand movements. This eye–hand coordination requires awareness of the hand to track the hand with the eye. I found that eye–hand coordination is improved when participants experience a sense of agency over a tracked artificial hand, regardless of their sense of body ownership. This improvement was selective for the initiation, but not maintenance, of eye–hand coordination. These results reveal that the prospective experience of explicit sense of agency improves motor control, suggesting that artificial manipulation of prospective agency may be beneficial to rehabilitation and sports training techniques.

Sensorimotor signals underlying space perception: An investigation based on self-touch

Perception of space has puzzled scientists since antiquity, and is among the foundational questions of scientific psychology. Classical "local sign" theories assert that perception of spatial extent ultimately derives from efferent signals specifying the intensity of motor commands. Everyday cases of self-touch, such as stroking the left forearm with the right index fingertip, provide an important platform for studying spatial perception, because of the tight correlation between motor and tactile extents. Nevertheless, if the motor and sensory information in self-touch were artificially decoupled, these classical theories would clearly predict that motor signals - especially if self-generated rather than passive - should influence spatial perceptual judgements, but not vice versa. We tested this hypothesis by quantifying the contribution of tactile, kinaesthetic, and motor information to judgements of spatial extent. In a self-touch paradigm involving two coupled robots in master-slave configuration, voluntary movements of the right-hand produced simultaneous tactile stroking on the left forearm. Crucially, the coupling between robots was manipulated so that tactile stimulation could be shorter, equal, or longer in extent than the movement that caused it. Participants judged either the extent of the movement, or the extent of the tactile stroke. By controlling sensorimotor gains in this way, we quantified how motor signals influence tactile spatial perception, and vice versa. Perception of tactile extent was strongly biased by the amplitude of the movement performed. Importantly, touch also affected the perceived extent of movement. Finally, the effect of movement on touch was significantly stronger when movements were actively-generated compared to when the participant's right hand was passively moved by the experimenter. Overall, these results suggest that motor signals indeed dominate the construction of spatial percepts, at least when the normal tight correlation between motor and sensory signals is broken. Importantly, however, this dominance is not total, as classical theory might suggest.

Hand size representation in healthy children and young adults

Whereas we experience our body as a coherent volumetric object, the brain appears to maintain highly fragmented representations of individual body parts. Little is known about how body representations of hand size and shape are built and evolve during infancy and young adulthood. This study aimed to investigate the effect of hand side, handedness, and age on the development of central hand size representation. The observational study with comparison groups was conducted with 90 typically developing Belgian school children and young adults (48 male and 42 female; age range = 5.0-23.0 years; 49 left-handed and 41 right-handed). Participants estimated their hand size and shape using two different tasks. In the localization task, participants were verbally cued to judge the locations of 10 anatomical landmarks of an occluded hand. An implicit hand size map was constructed and compared with actual hand dimensions. In the template selection task, the explicit hand shape was measured with a depictive method. Hand shape indexes were calculated and compared for the actual, implicit, and explicit conditions. Participants were divided into four age groups (5-8 years, 9-10 years, 11-16 years, and 17-23 years). Implicit hand maps featured underestimation of finger length and overestimation of hand width, which is already present in the youngest children. Linear mixed modeling revealed no influence of hand side on finger length underestimation; nonetheless, a significant main effect of age (p = .001) was exposed. Sinistrals aged 11 to 16 years showed significantly less underestimation (p = .03) than dextrals of the same age. As for the hand shape, the implicit condition differed significantly with the actual and explicit conditions (p < .001). Again, the implicit shape index was subjected to handedness and age effects, with significant differences being found between sinistrals and dextrals in the age groups of 9 and 10 years (p = .029) and 11 to 16 years (p < .001). In conclusion, the implicit metric component of the hand representation in children and young adults is misperceived, featuring shortened fingers and broadened hands since a very young age. Crucially, the finger length underestimation increases with age and shows a different developmental trajectory for sinistrals and dextrals. In contrast, the explicit hand shape is approximately veridical and seems immune from age and handedness effects. This study confirms the dual character of somatoperception and establishes a point of reference for children and young adults.

Virtual image of a hand displaced in space influences action performance of the real hand

The rubber hand illusion (RHI) demonstrates that under some circumstances a fake hand can be regarded as part of one’s body; the RHI and related phenomena have been used to explore the flexibility of the body schema. Recent work has shown that a sense of embodiment may be generated by virtual reality (VR). In a series of experiments, we used VR to assess the effects of the displacement of the virtual image of subjects’ hands on action. Specifically, we tested whether spatial and temporal parameters of action change when participants perform a reaching movement towards the location of their virtual hand, the position of which was distorted on some trials. In different experiments, participants were sometimes provided with incorrect visual feedback regarding the position of the to-be-touched hand (Experiment 1), were deprived of visual feedback regarding the position of the reaching hand when acting (Experiment 2) or reached with the hand, the apparent position of which had been manipulated (Experiment 3). The effect was greatest when participants reached towards (Experiment 1) or with (Experiment 3) the displaced hand when the hand was visible during the reaching, but not when the vision of the hand was removed during the action (Experiment 2). Taken together, these data suggest that visual images of one’s hand presented in VR influence the body schema and action performance.

Which Body Would You Like to Have? The Impact of Embodied Perspective on Body Perception and Body Evaluation in Immersive Virtual Reality

In this experiment, we aimed to measure the conscious internal representation of one's body appearance and allow the participants to compare this to their ideal body appearance and to their real body appearance. We created a virtual representation of the internal image participants had of their own body shape. We also created a virtual body corresponding to the internal representation they had of their ideal body shape, and we built another virtual body based on their real body measures. Participants saw the three different virtual bodies from an embodied first-person perspective and from a third-person perspective and had to evaluate the appearance of those virtual bodies. We observed that female participants evaluated their real body as more attractive when they saw it from a third-person perspective, and that their level of body dissatisfaction was lower after the experimental procedure. We believe that third-person perspective allowed female participants to perceive their real body shape without applying the negative prior beliefs usually associated to the "self", and that this resulted in a more positive evaluation of their body shape. We speculate that this method could be applied with patients suffering from eating disorders, by making their body perception more realistic and therefore improve their body satisfaction.

The Representation of Body Size: Variations With Viewpoint and Sex

Perceived body size is a fundamental construct that reflects our knowledge of self and is important for all aspects of perception, yet how we perceive our bodies and how the body is represented in the brain is not yet fully understood. In order to understand how the brain perceives and represents the body, we need an objective method that is not vulnerable to affective or cognitive influences. Here, we achieve this by assessing the accuracy of full-body size perception using a novel psychophysical method that taps into the implicit body representation for determining perceived size. Participants were tested with life-size images of their body as seen from different viewpoints with the expectation that greater distortions would occur for unfamiliar views. The Body Shape Questionnaire was also administered. Using a two-alternative forced choice design, participants were sequentially shown two life-size images of their whole body dressed in a standardized tight-fitting outfit seen from the front, side, or back. In one image, the aspect ratio (with the horizontal or vertical dimension fixed) was varied using an adaptive staircase, while the other was undistorted. Participants reported which image most closely matched their own body size. The staircase honed in on the distorted image that was equally likely as the undistorted photo to be judged as matching their perception of themselves. From this, the perceived size of their internal body representation could be calculated. Underestimation of body width was found when the body was viewed from the front or back in both sexes. However, females, but not males, overestimated their width when the body was viewed from the side. Height was perceived accurately in all views. These findings reveal distortions in perceived size for healthy populations and show that both viewpoint and sex matter for the implicit body representation. Though the back view of one’s body is rarely–if ever–seen, perceptual distortions were the same as for the front view. This provides insight into how the brain might construct its representation of three-dimensional body shape.

Active control as evidence in favor of sense of ownership in the moving Virtual Hand Illusion

The sense of ownership, the feeling that our body belongs to ourselves, relies on multiple sources of sensory information. Among these sources, the contribution of visuomotor information is still debated. We tested the effect of active control in the sense of ownership in the moving Virtual Hand Illusion. Participants reported sense of ownership and sense of agency over a virtual arm in which we manipulated the morphological congruence of the hand and the visuomotor information. We found that congruent active control enhanced and maintained the reported sense of ownership over a hand that appeared detached from the body, but not in a morphological congruent limb. Also, incongruent active control, achieved by adding noise to the trajectory of the movement, decreased both reported sense of agency and ownership. Overall, our results are consistent with a framework in which active control acts as evidence for eliciting a sense of ownership.

Changes in Body Representation of the Human Upper Limb as a Function of Movement and Visual Hand Position

Several disease presentations are linked to a mismatch between the real body and the body’s internal representation of itself. In order to develop effective rehabilitation therapies, it is necessary to understand the mechanisms underlying changes in body representation. In this study, we focused on changes in body representation of the upper limb as a large part of the body and investigated the conditions under which such changes occur. Participants were presented four conditions which differentially affected their sense of ownership and agency, including a movement condition related to their sense of agency, and a visual hand information condition related to the sense of ownership. In the experiment, participants were asked to move their upper limb forward and backward on a manipulandum. Results of the study showed that visual hand position affected changes in body representation relevant to both conscious and unconscious body parts. In addition, changes in the representation of the unconscious body part occurred with, and were dependent on, active movement.

Self Beyond the Body: Action-Driven and Task-Relevant Purely Distal Cues Modulate Performance and Body Ownership

Our understanding of body ownership largely relies on the so-called Rubber Hand Illusion (RHI). In this paradigm, synchronous stroking of the real and the rubber hands leads to an illusion of ownership of the rubber hand provided that it is physically, anatomically, and spatially plausible. Self-attribution of an artificial hand also occurs during visuomotor synchrony. In particular, participants experience ownership over a virtual or a rubber hand when the visual feedback of self-initiated movements follows the trajectory of the instantiated motor commands, such as in the Virtual Hand Illusion (VHI) or the moving Rubber Hand Illusion (mRHI). Evidence yields that both when the cues are triggered externally (RHI) and when they result from voluntary actions (VHI and mRHI), the experience of ownership is established through bottom-up integration and top-down prediction of proximodistal cues (visuotactile or visuomotor) within the peripersonal space. It seems, however, that depending on whether the sensory signals are externally (RHI) or self-generated (VHI and mRHI), the top-down expectation signals are qualitatively different. On the one hand, in the RHI the sensory correlations are modulated by top-down influences which constitute empirically induced priors related to the internal (generative) model of the body. On the other hand, in the VHI and mRHI body ownership is actively shaped by processes which allow for continuous comparison between the expected and the actual sensory consequences of the actions. Ample research demonstrates that the differential processing of the predicted and the reafferent information is addressed by the central nervous system via an internal (forward) model or corollary discharge. Indeed, results from the VHI and mRHI suggest that, in action-contexts, the mechanism underlying body ownership could be similar to the forward model. Crucially, forward models integrate across all self-generated sensory signals including not only proximodistal (i.e., visuotactile or visuomotor) but also purely distal sensory cues (i.e., visuoauditory). Thus, if body ownership results from a consistency of a forward model, it will be affected by the (in)congruency of purely distal cues provided that they inform about action-consequences and are relevant to a goal-oriented task. Specifically, they constitute a corrective error signal. Here, we explicitly addressed this question. To test our hypothesis, we devised an embodied virtual reality-based motor task where action outcomes were signaled by distinct auditory cues. By manipulating the cues with respect to their spatial, temporal and semantic congruency, we show that purely distal (visuoauditory) feedback which violates predictions about action outcomes compromises both performance and body ownership. These results demonstrate, for the first time, that body ownership is influenced by not only externally and self-generated cues which pertain to the body within the peripersonal space but also those arising outside of the body. Hence, during goal-oriented tasks body ownership may result from the consistency of forward models.

Effects of transcutaneous electrical nerve stimulation and visuotactile synchrony on the embodiment of an artificial hand

The rubber hand illusion (RHI) is an experimental paradigm known to produce a bodily illusion. Transcutaneous electrical nerve stimulation (TENS) combined with the RHI induces a stronger illusion than the RHI alone. Visuotactile stimulus synchrony is an important aspect of the RHI. However, the effect of TENS and visuotactile stimulus synchrony in TENS combined with the RHI remains unknown. The purpose of this study was to investigate the effects of TENS and visuotactile stimulus synchrony on the embodiment of an artificial hand when using TENS combined with the RHI. The participants underwent four experimental conditions in random order: TENS/noTENS × Synchronous/Asynchronous. TENS was set at an intensity such that it generated a feeling of electrical paresthesia in the radial nerve area of the hand but did not cause pain, i.e., 100-Hz pulse frequency, 80-µs pulse duration, and a constant pulse pattern. A visuotactile stimulus, either temporally synchronous or asynchronous, was generated using paintbrush strokes. To evaluate the outcome measures, the participants completed a questionnaire report and proprioceptive drift assessments (motor response and perceptual response). There were significant main effects of TENS and visuotactile synchrony, but no interaction between these factors, on the results of the questionnaire and the perceptual response. In contrast, there was no significant effect on the result of the motor response. These findings indicate that TENS and visuotactile synchrony might affect differently the embodiment of an artificial hand when using TENS combined with the RHI.

Kinesthetic Senses

The kinesthetic senses are the senses of position and movement of the body, senses we are aware of only on introspection. A method used to study kinesthesia is muscle vibration, which engages afferents of muscle spindles to trigger illusions of movement and changed position. When vibrating elbow flexors, it generates sensations of forearm extension, when vibrating extensors, sensations of forearm flexion. Vibrating the elbow joint produces no illusion. Vibrating flexors and extensors together at the same frequency also produces no illusion, because what is perceived is the signal difference between antagonist muscles of each arm and between arms. The size of the illusion depends on how the muscle has been conditioned beforehand, due to a property of muscle called thixotropy. When measuring the illusion, blindfolded subjects may carry out a matching or pointing task. In pointing, signals from muscle spindles are less important than in matching. Afferent signals from kinesthetic receptors project to areas of somatosensory cortex to generate sensations of detection and location. This is referred to the body model, which provides information about size and shape of body parts. Kinesthesia, together with vision and touch, is associated with the sense of body ownership. All three can combine or each, on its own, can generate ownership. Related is the sense of agency, the sense of being responsible for one's own actions. In recent times, much progress has been made using neuroimaging techniques to identify the various areas of the brain likely to be responsible for generating these sensations. © 2017 American Physiological Society. Compr Physiol 8:1157-1183, 2018.

Body ownership determines the attenuation of self-generated tactile sensations

Self-perception depends on the brain's abilities to differentiate our body from the environment and to distinguish between the sensations generated as a consequence of voluntary movement and those arising from events in the external world. The first process refers to the sense of ownership of our body and relies on the dynamic integration of multisensory (afferent) signals. The second process depends on internal forward models that use (efferent) information from our motor commands to predict and attenuate the sensory consequences of our movements. However, the relationship between body ownership and sensory attenuation driven by the forward models remains unknown. To address this issue, we combined the rubber hand illusion, which allows experimental manipulation of body ownership, and the force-matching paradigm, which allows psychophysical quantification of somatosensory attenuation. We found that a rubber right hand pressing on the left index finger produced somatosensory attenuation but only when the model hand felt like one's own (illusory self-touch); reversely, the attenuation that was expected to occur during actual self-touch with the real hands was reduced when the participants simultaneously experienced ownership of a rubber right hand that was placed at a distance from their left hand. These results demonstrate that the sense of body ownership determines somatosensory attenuation. From a theoretical perspective, our results are important because they suggest that body ownership updates the internal representation of body state that provides the input to the forward model generating sensory predictions during voluntary action.

Body ownership and a new proprioceptive role for muscle spindles

Knowledge of which body parts belong to us is referred to as the sense of body ownership. There is increasing evidence that this important aspect of human proprioception is highly malleable. Research into ownership of individual body parts was stimulated by Botvinick and Cohen's rubber-hand illusion (Nature 391,1998, 756), which demonstrated that an artificial body part can be incorporated in one's body representation and can cause real body parts to be sensed erroneously. Here, we review key studies that have advanced our understanding of the sense of body ownership, including the important role played by multisensory integration and spatiotemporal congruence of sensory signals. We also discuss our recent discovery that body ownership can be induced in response to movement stimuli by signals from a single class of sensory receptor, namely muscle spindles.

The virtual hand illusion is moderated by context-induced spatial reference frames

The tendency to perceive an artificial effector as part of one’s own body is known to depend on temporal criteria, like the synchrony between stimulus events informing about the effector. The role of spatial factors is less well understood. Rather than physical distance, which has been manipulated in previous studies, we investigated the role of relative, context-induced distance between the participant’s real hand and an artificial hand stimulated synchronously or asynchronously with the real hand. We replicated previously reported distance effects in a virtual reality setup: the perception of ownership increased with decreased distance, and the impact of synchrony was stronger for short distances. More importantly, we found that ownership perception and impact of synchrony were affected by previous distance: the same, medium distance between real and artificial hand induced more pronounced ownership after having experienced a far-distance condition than after a near-distance condition. This suggests that subjective, context-induced spatial reference frames contribute to ownership perception, which does not seem to fit with the idea of fixed spatial criteria and/or permanent body representations as the sole determinants of perceived body ownership.

How our body influences our perception of the world

Incorporating the fact that the senses are embodied is necessary for an organism to interpret sensory information. Before a unified perception of the world can be formed, sensory signals must be processed with reference to body representation. The various attributes of the body such as shape, proportion, posture, and movement can be both derived from the various sensory systems and can affect perception of the world (including the body itself). In this review we examine the relationships between sensory and motor information, body representations, and perceptions of the world and the body. We provide several examples of how the body affects perception (including but not limited to body perception). First we show that body orientation effects visual distance perception and object orientation. Also, visual-auditory crossmodal-correspondences depend on the orientation of the body: audio "high" frequencies correspond to a visual "up" defined by both gravity and body coordinates. Next, we show that perceived locations of touch is affected by the orientation of the head and eyes on the body, suggesting a visual component to coding body locations. Additionally, the reference-frame used for coding touch locations seems to depend on whether gaze is static or moved relative to the body during the tactile task. The perceived attributes of the body such as body size, affect tactile perception even at the level of detection thresholds and two-point discrimination. Next, long-range tactile masking provides clues to the posture of the body in a canonical body schema. Finally, ownership of seen body parts depends on the orientation and perspective of the body part in view. Together, all of these findings demonstrate how sensory and motor information, body representations, and perceptions (of the body and the world) are interdependent.

Three-dimensional coherence of the conscious body image

We experience our body as a coherent object in the three-dimensional (3-D) world. In contrast, the body is represented in somatosensory cortex as a fragmented collection of two-dimensional (2-D) maps. Recent results have suggested that some forms of higher level body representations maintain this fragmentation, for example by showing different patterns of distortion for two surfaces of a single body part, such as the palmar and dorsal hand surfaces. This study investigated the 3-D coherence of the conscious body image of the hand by comparing perceptual biases of perceived hand shape on the dorsal and palmar surfaces. Participants made forced-choice judgements of whether observed hand images were thinner or wider than their own left or right hand, and perceptual distortions of the hand image were assessed by fitting psychometric functions. The results suggested that the hand is consciously represented as a fully coherent, 3-D object. Specifically: (a) Similar overall levels of distortion were found on the palmar and dorsal hand surfaces, (b) comparable laterality effects were found on both surfaces (left hand represented as wider than right hand), and (c) the magnitude of distortions were strongly correlated across the two surfaces. Whereas other recent results have suggested that perceptual abilities such as position sense, tactile size perception, and tactile localization may rely on fragmented, 2-D representations of individual skin surfaces, the present results suggest that, in striking contrast, the conscious body image represents the body (or, at least the hand) as a coherent, 3-D object.

Over my fake body: body ownership illusions for studying the multisensory basis of own-body perception

Which is my body and how do I distinguish it from the bodies of others, or from objects in the surrounding environment? The perception of our own body and more particularly our sense of body ownership is taken for granted. Nevertheless, experimental findings from body ownership illusions (BOIs), show that under specific multisensory conditions, we can experience artificial body parts or fake bodies as our own body parts or body, respectively. The aim of the present paper is to discuss how and why BOIs are induced. We review several experimental findings concerning the spatial, temporal, and semantic principles of crossmodal stimuli that have been applied to induce BOIs. On the basis of these principles, we discuss theoretical approaches concerning the underlying mechanism of BOIs. We propose a conceptualization based on Bayesian causal inference for addressing how our nervous system could infer whether an object belongs to our own body, using multisensory, sensorimotor, and semantic information, and we discuss how this can account for several experimental findings. Finally, we point to neural network models as an implementational framework within which the computational problem behind BOIs could be addressed in the future.

The rubber hand illusion depends on a congruent mapping between real and artificial fingers

The rubber hand illusion (RHI), in which a visible artificial hand is touched (or moves) synchronously with the participant's unseen own hand, indicates that body representations can undergo rapid changes. While several constraints for this illusion have been described, some reports highlight a remarkable flexibility of body representations, even contradicting a priori assumptions regarding body appearance and anatomy (e.g., the subjective embodiment of a third arm). Here we examine the impact of congruence between touches at (or movements of) the real and the artificial hand, as well as the role of predictability of touches (or movements). We implemented two versions of the RHI paradigm, based on passive tactile stimulation and active voluntary movements. The results show that (a) predictability does not modulate perceived embodiment, and that (b) congruent mapping between real and artificial fingers is a necessary condition for both the tactile and the motor RHI. Together with previously reported constraints for bodily illusions, these results are reduced to four principles, which determine subjective embodiment: temporal synchrony, congruence of mapping between real and artificial body parts, body unity and body shape.

Dorsal and ventral streams across sensory modalities

In this review, we describe the current models of dorsal and ventral streams in vision, audition and touch. Available theories take their first steps from the model of Milner and Goodale, which was developed to explain how human actions can be efficiently carried out using visual information. Since then, similar concepts have also been applied to other sensory modalities. We propose that advances in the knowledge of brain functioning can be achieved through models explaining action and perception patterns independently from sensory modalities.

The moving rubber hand illusion revisited: comparing movements and visuotactile stimulation to induce illusory ownership

The rubber hand illusion is a perceptual illusion in which a model hand is experienced as part of one's own body. In the present study we directly compared the classical illusion, based on visuotactile stimulation, with a rubber hand illusion based on active and passive movements. We examined the question of which combinations of sensory and motor cues are the most potent in inducing the illusion by subjective ratings and an objective measure (proprioceptive drift). In particular, we were interested in whether the combination of afferent and efferent signals in active movements results in the same illusion as in the purely passive modes. Our results show that the illusion is equally strong in all three cases. This demonstrates that different combinations of sensory input can lead to a very similar phenomenological experience and indicates that the illusion can be induced by any combination of multisensory information.

Merging second-person and first-person neuroscience

Schilbach et al. contrast second-person and third-person approaches to social neuroscience. We discuss relations between second-person and first-person approaches, arguing that they cannot be studied in isolation. Contingency is central for converging first- and second-person approaches. Studies of embodiment show how contingencies scaffold first-person perspective and how the transition from a third- to a second-person perspective fundamentally involves first-person contributions.

The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Action and perception in the rubber hand illusion

Voluntary motor control over artificial hands has been shown to provoke a subjective incorporation of the artificial limb into body representations. However, in most studies projected or mirrored images of own hands were presented as 'artificial' body parts. Using the paradigm of the rubber hand illusion (RHI), we assessed the impact of tactile sensations and voluntary movements with respect to an unambiguously body-extraneous, artificial hand. In addition to phenomenal self-reports and pointing movements towards the own hand, we introduced a new procedure for perceptual judgements enabling the assessment of proprioceptive drift and judgement reliability regarding perceived hand location. RHI effects were comparable for tactile sensations and voluntary movements, but characteristic discrepancies were found for pointing movements. They were differently affected by the induction methods, and RHI effects were uncorrelated between both methods. These observations shed new light on inconsistent results concerning RHI effects on motor responses.