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 spatial distance rule in the moving and classical rubber hand illusions

The rubber hand illusion (RHI) is a perceptual illusion in which participants perceive a model hand as part of their own body. Here, through the use of one questionnaire experiment and two proprioceptive drift experiments, we investigated the effect of distance (12, 27.5, and 43 cm) in the vertical plane on both the moving and classical RHI. In both versions of the illusion, we found an effect of distance on ownership of the rubber hand for both measures tested. Our results further suggested that the moving RHI might follow a narrower spatial rule. Finally, whereas ownership of the moving rubber hand was affected by distance, this was not the case for agency, which was present at all distances tested. In sum, the present results generalize the spatial distance rule in terms of ownership to the vertical plane of space and demonstrate that also the moving RHI obeys this rule.     

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。     

Higher-order cognitive factors affect subjective but not proprioceptive aspects of self-representation in the rubber hand illusion

In the current study we look at whether subjective and proprioceptive aspects of selfrepresentation are separable components subserved by distinct systems of multisensory integration. We used the rubber hand illusion (RHI) to draw the location of the ‘self’ away from the body, towards extracorporeal space (Out Condition), thereby violating top-down information about the body location. This was compared with the traditional RHI which drew position of the ‘self’ towards the body (In Condition). We were successfully able to draw proprioceptive position of the limbs in and out from the body suggesting body perception is a purely bottom-up process, resistant to top-down effects. Conversely, we found subjective self-representation was altered by the violation of top-down body information – as the strong association of subjective and proprioceptive factors found in the In Condition became non-significant in the Out Condition. Interestingly, we also found evidence that subjective embodiment can modulate tactile perception.     

Feeling the burn: When it looks like it hurts,and belongs to me,it really does hurt more

We examined changes in pain sensitivity in the rubber hand illusion (RHI). Experiment 1 investigated changes in pain tolerance immediately after a “healthy” and “wounded” RHI when immersing the hand in a cold pressor ice bath. There was 19% increased pain tolerance and increased perception detection threshold after the healthy RHI, but 11% reduction after the wounded RHI. Experiment 2 examined pain experience during the wounded RHI with capsaicin-induced hyperalgesia. Pain intensity and unpleasantness was higher on the illusion arm during the synchronous RHI, compared with asynchronous trials. There was no change in pain experience on the control arm, and both arms had similar pain sensitivity after the experiment. Our results highlight the impact of embodying a substitute limb on pain, with increased tolerance and reduced tactile sensitivity when the fake limb is healthy and apparently pain-free, but increased pain sensitivity when the self-attributed limb appears to be wounded.     

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.     

Rubber hand illusion, empathy, and schizotypal experiences in terms of self-other representations

When participants observed a rubber hand being touched, their sense of touch was activated (rubber hand illusion: RHI). While this illusion might be caused by multi-modal integration, it may also be related to empathic function, which enables us to simulate the observed information. We examined individual differences in the RHI, including empathic and schizotypal personality traits, as previous research had suggested that schizophrenic patients would be more subject to the RHI. The results indicated that people who experience a stronger RHI might have stronger empathic and schizotypal personalites simultaneously. We discussed these relationships in terms of self-other representations.     

Application of the rubber hand illusion paradigm: comparison between upper and lower limbs

The “rubber hand illusion (RHI)” is a perceptual illusion, which allows the integration of artificial limbs into the body representation of a person by means of combined visual and tactile stimulation. The illusion has been frequently replicated but always concerning the upper limbs. The present study verified an analog illusion that can be called the “rubber foot illusion” (RFI). In a conjoint experiment using both a rubber hand and a rubber foot, brushstrokes were applied to the respective real and rubber limb placed alongside the real one. However, only the artificial limb’s handling was visible. The brushstrokes were given either synchronously, with a delay of ±0.5 s, or without tactile stimulation of the real limb. Questionnaire data and the proprioceptive drift towards the rubber limb (determined by calling on the subjects to show where they locate their unseen limb) defined the illusion strength. Results revealed that the illusion was induced in both limbs with comparable strength, but only in the synchronous condition.     

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.     

Spatial limits on the nonvisual self-touch illusion and the visual rubber hand illusion: Subjective experience of the illusion and proprioceptive drift

The nonvisual self-touch rubber hand paradigm elicits the compelling illusion that one is touching one’s own hand even though the two hands are not in contact. In four experiments, we investigated spatial limits of distance (15 cm, 30 cm, 45 cm, 60 cm) and alignment (0°, 90° anti-clockwise) on the nonvisual self-touch illusion and the well-known visual rubber hand illusion. Common procedures (synchronous and asynchronous stimulation administered for 60 s with the prosthetic hand at body midline) and common assessment methods were used. Subjective experience of the illusion was assessed by agreement ratings for statements on a questionnaire and time of illusion onset. The nonvisual self-touch illusion was diminished though never abolished by distance and alignment manipulations, whereas the visual rubber hand illusion was more robust against these manipulations. We assessed proprioceptive drift, and implications of a double dissociation between subjective experience of the illusion and proprioceptive drift are discussed.     

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.     

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).     

Affective touch modulates the rubber hand illusion

Introduction

Humans experience touch as pleasant when this occurs with a certain velocity (1–10cm/s). Affective, pleasant touch is thought to be mediated by a distinct neural pathway consisting of un-myelinated tactile afferents (C tactile fibers) that respond to stroking with a low velocity on the hairy skin. As pleasant touch provides additional information on bodily signals we hypothesized that, compared to regular touch, pleasant touch would have a stronger effect on body ownership as measured through induction of the rubber hand illusion (RHI).

Methods

Two experiments involving the RHI were conducted. In the first experiment, the effects of stroking velocity (3 cm/s and 30 cm/s) and stroking material (soft/rough) on the RHI were tested. In the second experiment, the effect of an additional stroking velocity (0.3 cm/s) and side of stimulation (hairy and glabrous) was examined.

Results

The first experiment showed that low velocity stroking in combination with a soft material was not only regarded as most pleasant but also resulted in an enhanced RHI on proprioceptive drift and temperature measurements. In the second experiment, we confirmed that stroking with a velocity of 3 cm/s resulted in a larger RHI in terms of proprioceptive drift. In addition, compared to regular touch, pleasant touch of the hairy skin resulted in a larger proprioceptive drift, while similar stroking on the glabrous side of the skin did not induce a stronger effect of RHI on proprioceptive drift.

Conclusion

Our data suggest that pleasant touch modulates the body representation which is consistently reflected in a larger proprioceptive drift. Our data also suggest that C tactile fibers are likely to be involved in the modulation of body ownership.     

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.     

The use of realistic and mechanical hands in the rubber hand illusion,and the relationship to hemispheric differences

Embodiment, as measured through the rubber-hand illusion (RHI), depends on the similarity between object to be embodied and part of the body it replaces. We compared a fake hand similar to a real hand, and one matched in size but made of wires (mechanical). Left and right versions were tested to investigate whether the effect of appearance was stronger in the left hand. We found that the mechanical hand induced embodiment, though to a reduced degree relative to the realistic hand (N = 120). Left and right versions of the mechanical hand did not differ in strength of the illusion. However, with the left realistic hand there was a stronger relationship between drift (an objective measure of the illusion) and agreement on the questionnaire (subjective experience). With the mechanical hand, objective and subjective measures were unrelated. We discuss the results in relation to factors that influence the RHI and hemispheric differences.     

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.     

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.     

Visual information and rubber hand embodiment differentially affect reach-to-grasp actions

During the “rubber hand illusion” (RHI) participants feel touch originating from an artificial hand, which is felt to belong to the own body. The perceived location of the real hand is shifted towards the location of the artificial hand. However, evidence as to whether the RHI is accompanied by alterations of hand action is mixed. We found that the perceived size of one's own hand was affected by the size of the artificial hand that was used to elicit the illusion. Moreover, we tested a possible transfer of the RHI to a reach-to-grasp action. We observed that hand transport (i.e., reach) errors after RHI induction were independent of artificial hand size, showing that the parameter which is important for these reaching errors is the hand's perceived location. Results thus show that the RHI affects not only perceptual, but also action processing. In contrast, grip aperture was affected by artificial hand size independent of the RHI, suggesting that visual information about hand size affects grasping independent of embodiment of the artificial hand. Grip size increased with artificial hand size; this effect is explained by a higher reliance on proprioceptive information during blind reaching after receiving distorted visual information.     

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.