The rubber hand illusion: sensitivity and reference frame for body ownership

When subjects view stimulation of a rubber hand while feeling congruent stimulation of their own hand, they may come to feel that the rubber hand is part of their own body. This illusion of body ownership is termed 'Rubber Hand Illusion' (RHI). We investigated sensitivity of RHI to spatial mismatches between visual and somatic experience. We compared the effects of spatial mismatch between the stimulation of the two hands, and equivalent mismatches between the postures of the two hands. We created the mismatch either by adjusting stimulation or posture of the subject's hand, or, in a separate group of subjects, by adjusting stimulation or posture of the rubber hand. The matching processes underlying body ownership were asymmetrical. The illusion survived small changes in the subject's hand posture, but disappeared when the same posture transformations were applied to the rubber hand. Mismatch between the stimulation delivered to the subject's hand and the rubber hand abolished the illusion. The combination of these two situations is of particular interest. When the subject's hand posture was slightly different from the rubber hand posture, the RHI remained as long as stimulation of the two hands was congruent in a hand-centred spatial reference frame, even though the altered posture of the subject's hand meant that stimulation was incongruent in external space. Conversely, the RHI was reduced when the stimulation was incongruent in hand-centred space but congruent in external space. We conclude that the visual-tactile correlation that causes the RHI is computed within a hand-centred frame of reference, which is updated with changes in body posture. Current sensory evidence about what is 'me' is interpreted with respect to a prior mental body representation.     

Beyond the colour of my skin: How skin colour affects the sense of body-ownership

Multisensory stimulation has been shown to alter the sense of body-ownership. Given that perceived similarity between one's own body and those of others is crucial for social cognition, we investigated whether multisensory stimulation can lead participants to experience ownership over a hand of different skin colour. Results from two studies using introspective, behavioural and physiological methods show that, following synchronous visuotactile (VT) stimulation, participants can experience body-ownership over hands that seem to belong to a different racial group. Interestingly, a baseline measure of implicit racial bias did not predict whether participants would experience the RHI, but the overall strength of experienced body-ownership seemed to predict the participants' post-illusion implicit racial bias with those who experienced a stronger RHI showing a lower bias. These findings suggest that multisensory experiences can override strict ingroup/outgroup distinctions based on skin colour and point to a key role for sensory processing in social cognition.     

The Rubber Hand Illusion: two's a company, but three's a crowd

On the one hand, it is often assumed that the Rubber Hand Illusion (RHI) is constrained by a structural body model so that one cannot implement supernumerary limbs. On the other hand, several recent studies reported illusory duplication of the right hand in subjects exposed to two adjacent rubber hands. The present study tested whether spatial constraints may affect the possibility of inducing the sense of ownership to two rubber hands located side by side to the left of the subject's hand. We found that only the closest rubber hand appeared both objectively (proprioceptive drift) and subjectively (ownership rating) embodied. Crucially, synchronous touch of a second, but farther, rubber hand disrupted the objective measure of the RHI, but not the subjective one. We concluded that, in order to elicit a genuine RHI for multiple rubber hands, the two rubber hands must be at the same distance from the subject's hand/body.     

Roughness perception during the rubber hand illusion

Watching a rubber hand being stroked by a paintbrush while feeling identical stroking of one’s own occluded hand can create a compelling illusion that the seen hand becomes part of one’s own body. It has been suggested that this so-called rubber hand illusion (RHI) does not simply reflect a bottom–up multisensory integration process but that the illusion is also modulated by top–down, cognitive factors. Here we investigated for the first time whether the conceptual interpretation of the sensory quality of the visuotactile stimulation in terms of roughness can influence the occurrence of the illusion and vice versa, whether the presence of the RHI can modulate the perceived sensory quality of a given tactile stimulus (i.e., in terms of roughness). We used a classical RHI paradigm in which participants watched a rubber hand being stroked by either a piece of soft or rough fabric while they received synchronous or asynchronous tactile stimulation that was either congruent or incongruent with respect to the sensory quality of the material touching the rubber hand. (In)congruencies between the visual and tactile stimulation did neither affect the RHI on an implicit level nor on an explicit level, and the experience of the RHI in turn did not cause any modulations of the felt sensory quality of touch on participant’s own hand. These findings first suggest that the RHI seems to be resistant to top–down knowledge in terms of a conceptual interpretation of tactile sensations. Second, they argue against the hypothesis that participants own hand tends to disappear during the illusion and that the rubber hand actively replaces it.     

The role of gamma band oscillations and synchrony on rubber hand illusion and crossmodal integration

The rubber hand illusion represents an illusory experience during the mislocalization of own hand when correlated visuotactile stimuli are presented to the actual and fake hands. The visuotactile integration process appears to cause this illusion; the corresponding brain activity was revealed in many studies. In this study, we investigated the effect of the rubber hand illusion on the crossmodal integration process by measuring EEG. The participants who experienced less intensive illusion showed greater congruency effect on reaction time (RT), greater power increase at the parietal zero electrode (Pz) and smaller interelectrode synchrony of the gamma band activity. On the other hand, the participants who experienced more intense illusion showed greater interelectrode synchrony. The results suggested that the gamma band activity in the parietal area reflects the visuotactile integration process and that its synchrony causes the illusory intensity.     

How vestibular stimulation interacts with illusory hand ownership

Artificial stimulation of the peripheral vestibular system has been shown to improve ownership of body parts in neurological patients, suggesting vestibular contributions to bodily self-consciousness. Here, we investigated whether galvanic vestibular stimulation (GVS) interferes with the mechanisms underlying ownership, touch, and the localization of one’s own hand in healthy participants by using the “rubber hand illusion” paradigm. Our results show that left anodal GVS increases illusory ownership of the fake hand and illusory location of touch. We propose that these changes are due to vestibular interference with spatial and/or temporal mechanisms of visual-tactile integration leading to an enhancement of visual capture. As only left anodal GVS lead to such changes, and based on neurological data on body part ownership, we suggest that this vestibular interference is mediated by the right temporo-parietal junction and the posterior insula.     

The role of distance from the body and distance from the real hand in ownership and disownership during the rubber hand illusion

A key tool for investigating body ownership is the rubber hand illusion, in which synchronous multisensory feedback can induce feelings of ownership over a fake hand. Much research in the field aims to tease apart the mechanisms that underlie this phenomenon. Currently there is conflicting evidence as to whether increasing the distance between the real and fake hands (within reaching space) can reduce the illusion. The current study examines this further by modulating, not only the absolute distance between the real and fake hands but also their relative distance from body midline. It is found that the strength of the illusion is reduced only when the fake hand is both far from the real hand and far from the trunk; illusion scores over a fake hand in the same position can then be increased by moving the real hand nearer. This is related to peripersonal space surrounding the trunk and the hand. Subjective disownership of the real hand, and proprioceptive drift measures were also taken and may be driven by different mechanisms.     

The rubber hand illusion revisited: visuotactile integration and self-attribution

Watching a rubber hand being stroked, while one's own unseen hand is synchronously stroked, may cause the rubber hand to be attributed to one's own body, to "feel like it's my hand." A behavioral measure of the rubber hand illusion (RHI) is a drift of the perceived position of one's own hand toward the rubber hand. The authors investigated (a) the influence of general body scheme representations on the RHI in Experiments 1 and 2 and (b) the necessary conditions of visuotactile stimulation underlying the RHI in Experiments 3 and 4. Overall, the results suggest that at the level of the process underlying the build up of the RHI, bottom-up processes of visuotactile correlation drive the illusion as a necessary, but not sufficient, condition. Conversely, at the level of the phenomenological content, the illusion is modulated by top-down influences originating from the representation of one's own body.     

Having a body versus moving your body: How agency structures body-ownership

We investigated how motor agency in the voluntary control of body movement influences body awareness. In the Rubber Hand Illusion (RHI), synchronous tactile stimulation of a rubber hand and the participant's hand leads to a feeling of the rubber hand being incorporated in the participant's own body. One quantifiable behavioural correlate of the illusion is an induced shift in the perceived location of the participant's hand towards the rubber hand. Previous studies showed that the induced changes in body awareness are local and fragmented: the proprioceptive drift is largely restricted to the stimulated finger. In the present study, we investigated whether active and passive movements, rather than tactile stimulation, would lead to similarly fragmented body awareness. Participants watched a projected image of their hand under three conditions: active finger movement, passive finger movement, and tactile stimulation. Visual feedback was either synchronous or asynchronous with respect to stimulation of the hand. A significant overall RHI, defined as greater drifts following synchronous than asynchronous stimulation, was found in all cases. However, the distribution of the RHI across stimulated and non-stimulated fingers depended on the kind of stimulation. Localised proprioceptive drifts, specific to the stimulated finger, were found for tactile and passive stimulation. Conversely, during active movement of a single digit, the proprioceptive drifts were not localised to that digit, but were spread across the whole hand. Whereas a purely proprioceptive sense of body-ownership is local and fragmented, the motor sense of agency integrates distinct body-parts into a coherent, unified awareness of the body.     

Consistent inter-individual differences in susceptibility to bodily illusions

Illusory senses of ownership and agency (that the hand or effector that we see belongs to us and moves at our will, respectively) support the embodiment of prosthetic limbs, tele-operated surgical devices, and human-machine interfaces. We exposed forty-eight individuals to four different procedures known to elicit illusory ownership or agency over a fake visible rubber hand or finger. The illusory ownership or agency arising from the hand correlated with that of the finger. For both body parts, sensory stimulation across different modalities (visual with tactile or visual with kinesthetic) produced illusions of similar strength. However, the strengths of the illusions of ownership and agency were unrelated within individuals, supporting the proposal that distinct neuropsychological processes underlie these two senses. Developing training programs to enhance susceptibility to illusions of agency or ownership for people with lower natural susceptibility could broaden the usefulness of the above technologies.     

Illusory hand ownership in a patient with personal neglect for the upper limb,but no somatoparaphenia

The symptoms of patients with left personal neglect are characterized by inattention towards contralesional (left) body parts while at the same time explicitly ascertaining ownership for the neglected hemibody. It is currently unknown if personal neglect is associated with more subtle or implicit disturbances of own body perception and body ownership as measured with the rubber hand illusion. In this study, we report data from a patient with a right hemispheric lesion and personal neglect, without associated somatosensory deficits. We administered to the patient (and to 12 age‐matched controls) the rubber hand illusion paradigm to the right and left hands, to elicit illusory ownership for a fake hand, before and after recovery from personal neglect for the left arm. In a first session, run when the patient showed personal neglect affecting the left arm, he experienced a significantly enhanced subjective illusion of embodiment for the left fake hand as compared to the right hand (as assessed through a standard questionnaire). After recovery from personal neglect for the left arm (second session), the results of the left and right rubber hand illusion experiments were comparable, with no modulation of hand ownership. We argue that personal neglect may consist not only in an inattentional disorder, but also in a deficit of multisensory body representation characterized by a high sensitivity to experimental manipulations of subjective aspects of body ownership.     

Fake hand in movement: Visual motion cues from the rubber hand are processed for kinesthesia

The feeling that a fake (e.g. rubber) hand belongs to a person's own body can be elicited by synchronously stroking the fake hand and the real hand, with the latter hidden from view. Here, we sought to determine whether visual motion signals from that incorporated rubber hand would provide relevant cues for sensing movement (i.e. kinesthesia). After 180 s of visuo-tactile synchronous or asynchronous stroking, the fake hand was moved along the lateral or the sagittal axis. After synchronous stroking, movement of the rubber hand induced illusory movement of the static (real) hand in the same direction; the illusion was slightly more frequent and more intense when the fake hand was moved along the sagittal axis. We therefore conclude that visual signals of motion originating from the rubber hand are integrated for kinesthesia by the central nervous system just as visual signals from the real hand are.     

The Empathy Quotient (EQ) predicts perceived strength of bodily illusions and illusion-related sensations of pain

In this study we test the possible implications of high empathising skills on bodily self–other distinction by measuring the strength of a body ownership illusion and a related experience of illusory pain. One-hundred adult participants completed the Empathy Quotient (EQ) questionnaire. Twenty participants from the top quintile and 20 participants from the bottom quintile of the EQ distribution took part in a laboratory experiment. In the experiment, a classical Rubber Hand Illusion (RHI) was induced followed by the presentation of a series of painful stimuli to the rubber hand. Participants were asked to self-rate the strength of their subjective experience of the RHI and of the illusory pain. A proprioceptive location judgment on the position of the hidden hand was also required before and after RHI induction, to record drifts towards the rubber hand. We found a significant difference between high- and low-empathy participants in RHI and pain score. The EQ was not related with the proprioceptive location judgement drift. It thus appears to be a better predictor of subjective ownership feelings and phenomenological self–other merging than of the behavioural components of bodily illusions.     

The rubber hand illusion in a mirror

In the rubber hand illusion (RHI) one’s hand is hidden, and a fake hand is visible. We explored the situation in which visual information was available indirectly in a mirror. In the mirror condition, compared to the standard condition (fake hand visible directly), we found no reduction of the RHI following synchronised stimulation, as measured by crossmanual pointing and by a questionnaire. We replicated the finding with a smaller mirror that prevented visibility of the face. The RHI was eliminated when a wooden block replaced the fake hand, or when the hand belonged to another person or mannequin. We conclude that awareness of the reflection is the critical variable, despite the distant visual localisation of the hand in a mirror and the third-person perspective. Stimuli seen in a mirror activate the same response as stimuli seen in peripersonal space, through knowledge that they are near one’s body.     

橡胶手错觉:拥有感研究的实验范式及其应用

橡胶手错觉是一种健康个体将非肉体的假手视为自己真实身体的一部分的体验, 这种错觉可以通过同时轻刷被试面前可见的橡胶手及其不可见的真手而产生.橡胶手错觉已成为一种研究身体拥有感的重要范式, 其产生机制可以分为“自下而上的认知匹配”与“自上而下的认知匹配”两种加工方式.前者涉及视觉与触觉刺激的同步性; 而后者涉及被试心中预存的身体意象与身体图式(包括真假手之间模态特征,位置空间的相似性).综合上述证据, 身体模型假说与个人边缘空间理论进一步为拥有感产生的复杂机制提供了整合两种加工方式的解释.橡胶手错觉范式已经被用于探索精神分裂症患者等特殊被试病理分析,错觉产生和心理特质之间的关系, 以及神经外科和术后恢复上.未来的研究应该更加重视范式本身的拓展, 应用虚拟现实技术来提高身体模态的仿真效果, 利用橡胶手拥有感的易感性作为筛选与预测身体意象障碍疾病的指标.     

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.     

我观故我在？— 从橡胶手错觉对自我身体所有权的探讨

橡胶手错觉是一种能将非自我的肢体感知为自我的肢体的反应。继发现橡胶手错觉现象后,研究者通过操纵自变量以及观测不同的因变量,得到了大量新的研究结果。橡胶手错觉的出现与强度受到时间、空间和手姿势的影响。橡胶手错觉的产生机制为单纯的多感觉整合或多感觉整合与身体表征共同作用的结果。未来研究侧重于被试取样、研究策略以及医学领域中瘫痪患者的认知神经康复和截肢病人的假肢控制方面。     

Multisensory integration and age-dependent sensitivity to body representation modification induced by the rubber hand illusion

The aim of the present study was to compare junior and senior healthy participants for their multimodal integration capability. The instrument used for the investigation was the rubber hand illusion (RHI) where synchronous and asynchronous multimodal stimulation were applied. The study focused on assessing the rate of integration for visuo-tactile and visuo-proprioceptive stimuli. Methods: From a large sample of right-handed volunteers, 50 senior employees (containing younger and middle age adults) and 51 senior retired (senior adults) participants were recruited. Results: The between-subject analyses revealed that individuals both the junior and the senior participants responded to induction of RHI with vivid ownership and disownership experiences and a higher mislocalization error in the synchronous condition. However, the between-group analysis showed that participants in the senior group reported less vivid ownership and less vivid total RHI experiences scores compared to members of the junior group, but no mislocalization error differences were found between the groups. Conclusion: The results indicated that when visuo-tactile stimuli synchronously presented, the gain in multisensory integration decreased in seniors group. In contrast, in the case of visuo-proprioceptive synchronous presentation, the efficiency of multisensory integration remained unchanged across the lifespan.     

手近效应：研究内容、影响因素与神经机制

把手放在刺激旁边,会对知觉、记忆、语义和执行控制等认知加工产生影响,这类现象被称为手近效应。手近效应反映了身体与环境的互动对认知的塑造作用,为具身认知提供了新证据。本文从手近效应的内容,影响因素,及其认知、神经机制等方面对相关研究进行梳理。并从手近效应的神经机制,应用研究,以及动作意图和人际社会因素的调节作用等方面探讨当前手近效应还未解决的问题。     

The influence of embodiment on multisensory integration using the mirror box illusion

We examined the relationship between subcomponents of embodiment and multisensory integration using a mirror box illusion. The participants’ left hand was positioned against the mirror, while their right hidden hand was positioned 12″, 6″, or 0″ from the mirror – creating a conflict between visual and proprioceptive estimates of limb position in some conditions. After synchronous tapping, asynchronous tapping, or no movement of both hands, participants gave position estimates for the hidden limb and filled out a brief embodiment questionnaire. We found a relationship between different subcomponents of embodiment and illusory displacement towards the visual estimate. Illusory visual displacement was positively correlated with feelings of deafference in the asynchronous and no movement conditions, whereas it was positive correlated with ratings of visual capture and limb ownership in the synchronous and no movement conditions. These results provide evidence for dissociable contributions of different aspects of embodiment to multisensory integration.