Explaining Away the Body: Experiences of Supernaturally Caused Touch and Touch on Non-Hand Objects within the Rubber Hand Illusion

Background

In rubber hand illusions and full body illusions, touch sensations are projected to non-body objects such as rubber hands, dolls or virtual bodies. The robustness, limits and further perceptual consequences of such illusions are not yet fully explored or understood. A number of experiments are reported that test the limits of a variant of the rubber hand illusion.

Methodology/Principal Findings

A variant of the rubber hand illusion is explored, in which the real and foreign hands are aligned in personal space. The presence of the illusion is ascertained with participants'' scores and temperature changes of the real arm. This generates a basic illusion of touch projected to a foreign arm. Participants are presented with further, unusual visuotactile stimuli subsequent to onset of the basic illusion. Such further visuotactile stimulation is found to generate very unusual experiences of supernatural touch and touch on a non-hand object. The finding of touch on a non-hand object conflicts with prior findings, and to resolve this conflict a further hypothesis is successfully tested: that without prior onset of the basic illusion this unusual experience does not occur.

Conclusions/Significance

A rubber hand illusion is found that can arise when the real and the foreign arm are aligned in personal space. This illusion persists through periods of no tactile stimulation and is strong enough to allow very unusual experiences of touch felt on a cardboard box and experiences of touch produced at a distance, as if by supernatural causation. These findings suggest that one''s visual body image is explained away during experience of the illusion and they may be of further importance to understanding the role of experience in delusion formation. The findings of touch on non-hand objects may help reconcile conflicting results in this area of research. In addition, new evidence is provided that relates to the recently discovered psychologically induced temperature changes that occur during the illusion.     

When Right Feels Left: Referral of Touch and Ownership between the Hands

Feeling touch on a body part is paradigmatically considered to require stimulation of tactile afferents from the body part in question, at least in healthy non-synaesthetic individuals. In contrast to this view, we report a perceptual illusion where people experience “phantom touches” on a right rubber hand when they see it brushed simultaneously with brushes applied to their left hand. Such illusory duplication and transfer of touch from the left to the right hand was only elicited when a homologous (i.e., left and right) pair of hands was brushed in synchrony for an extended period of time. This stimulation caused the majority of our participants to perceive the right rubber hand as their own and to sense two distinct touches – one located on the right rubber hand and the other on their left (stimulated) hand. This effect was supported by quantitative subjective reports in the form of questionnaires, behavioral data from a task in which participants pointed to the felt location of their right hand, and physiological evidence obtained by skin conductance responses when threatening the model hand. Our findings suggest that visual information augments subthreshold somatosensory responses in the ipsilateral hemisphere, thus producing a tactile experience from the non-stimulated body part. This finding is important because it reveals a new bilateral multisensory mechanism for tactile perception and limb ownership.     

“I Feel Contaminated in My Fake Hand”: Obsessive-Compulsive-Disorder like Disgust Sensations Arise from Dummy during Rubber Hand Illusion

Despite its theoretical and clinical interest, there are no experimental studies exploring obsessive-compulsive disorder (OCD)-like disgust sensations through using somatosensory illusions. Such illusions provide important clues to the nature and limits of multisensory integration and how the brain constructs body image; and may potentially inform novel therapies. One such effect is the rubber hand illusion (RHI) in which tactile sensations are referred to a rubber hand; if the experimenter simultaneously strokes a subject’s occluded hand together with a visible fake hand, the subject starts experiencing the touch sensations as arising from the dummy. In this study, we explore whether OCD-like disgust may result from contamination of a dummy hand during the RHI; suggesting a possible integration of somatosensory and limbic inputs in the construction of body image. We predicted that participants would experience sensations of disgust, when placing a disgust stimulus (fake feces, vomit or blood) on the dummy hand after establishing the RHI. We found that 9 out of 11 participants experienced greater disgust during the synchronous condition (real hidden hand and fake hand are stroked in synchrony) compared to the asynchronous control condition (real hidden hand and fake hand are stroked in asynchrony); and on average such disgust was significantly greater during the synchronous condition compared to the asynchronous control condition, Z = 2.7, p = .008. These results argue against a strictly hierarchical modular approach to brain function and suggest that a four-way multisensory interaction occurs between vision, touch, proprioception on the one hand and primal emotions like disgust on the other. These findings may inform novel clinical approaches for OCD; that is, contaminating a dummy during the RHI could possibly be used as part of an in-vivo exposure-intervention for OCD.     

Bodily illusions modulate tactile perception

Touch differs from other exteroceptive senses in that the body itself forms part of the tactile percept. Interactions between proprioception and touch provide a powerful way to investigate the implicit body representation underlying touch. Here, we demonstrate that an intrinsic primary quality of a tactile object, for example its size, is directly affected by the perceived size of the body part touching it. We elicited proprioceptive illusions that the left index finger was either elongating or shrinking by vibrating the biceps or triceps tendon of the right arm while subjects grasped the tip of their left index finger. Subjects estimated the distance between two simultaneous tactile contacts on the left finger during tendon vibration. We found that tactile distances feel bigger when the touched body part feels elongated. Control tests showed that the modulation of touch was linked to the perceived index-finger size induced by tendon vibration. Vibrations that did not produce proprioceptive illusion had no effect on touch. Our results show that the perception of tactile objects is referenced to an implicit body representation and that proprioception contributes to this body representation. We also provide, for the first time, a quantitative, implicit measure of distortions of body size.     

The Crossmodal Congruency Task as a Means to Obtain an Objective Behavioral Measure in the Rubber Hand Illusion Paradigm

The rubber hand illusion (RHI) is a popular experimental paradigm. Participants view touch on an artificial rubber hand while the participants'' own hidden hand is touched. If the viewed and felt touches are given at the same time then this is sufficient to induce the compelling experience that the rubber hand is one''s own hand. The RHI can be used to investigate exactly how the brain constructs distinct body representations for one''s own body. Such representations are crucial for successful interactions with the external world. To obtain a subjective measure of the RHI, researchers typically ask participants to rate statements such as "I felt as if the rubber hand were my hand". Here we demonstrate how the crossmodal congruency task can be used to obtain an objective behavioral measure within this paradigm.The variant of the crossmodal congruency task we employ involves the presentation of tactile targets and visual distractors. Targets and distractors are spatially congruent (i.e. same finger) on some trials and incongruent (i.e. different finger) on others. The difference in performance between incongruent and congruent trials - the crossmodal congruency effect (CCE) - indexes multisensory interactions. Importantly, the CCE is modulated both by viewing a hand as well as the synchrony of viewed and felt touch which are both crucial factors for the RHI.The use of the crossmodal congruency task within the RHI paradigm has several advantages. It is a simple behavioral measure which can be repeated many times and which can be obtained during the illusion while participants view the artificial hand. Furthermore, this measure is not susceptible to observer and experimenter biases. The combination of the RHI paradigm with the crossmodal congruency task allows in particular for the investigation of multisensory processes which are critical for modulations of body representations as in the RHI.     

Perception of Body Ownership Is Driven by Bayesian Sensory Inference

Recent studies have shown that human perception of body ownership is highly malleable. A well-known example is the rubber hand illusion (RHI) wherein ownership over a dummy hand is experienced, and is generally believed to require synchronized stroking of real and dummy hands. Our goal was to elucidate the computational principles governing this phenomenon. We adopted the Bayesian causal inference model of multisensory perception and applied it to visual, proprioceptive, and tactile stimuli. The model reproduced the RHI, predicted that it can occur without tactile stimulation, and that synchronous stroking would enhance it. Various measures of ownership across two experiments confirmed the predictions: a large percentage of individuals experienced the illusion in the absence of any tactile stimulation, and synchronous stroking strengthened the illusion. Altogether, these findings suggest that perception of body ownership is governed by Bayesian causal inference—i.e., the same rule that appears to govern the perception of outside world.     

No Pain Relief with the Rubber Hand Illusion

The sense of body ownership can be easily disrupted during illusions and the most common illusion is the rubber hand illusion. An idea that is rapidly gaining popularity in clinical pain medicine is that body ownership illusions can be used to modify pathological pain sensations and induce analgesia. However, this idea has not been empirically evaluated. Two separate research laboratories undertook independent randomized repeated measures experiments, both designed to detect an effect of the rubber hand illusion on experimentally induced hand pain. In Experiment 1, 16 healthy volunteers rated the pain evoked by noxious heat stimuli (5 s duration; interstimulus interval 25 s) of set temperatures (47°, 48° and 49°C) during the rubber hand illusion or during a control condition. There was a main effect of stimulus temperature on pain ratings, but no main effect of condition (p = 0.32), nor a condition x temperature interaction (p = 0.31). In Experiment 2, 20 healthy volunteers underwent quantitative sensory testing to determine heat and cold pain thresholds during the rubber hand illusion or during a control condition. Secondary analyses involved heat and cold detection thresholds and paradoxical heat sensations. Again, there was no main effect of condition on heat pain threshold (p = 0.17), nor on cold pain threshold (p = 0.65), nor on any of the secondary measures (p<0.56 for all). We conclude that the rubber hand illusion does not induce analgesia.     

Are Movements Necessary for the Sense of Body Ownership? Evidence from the Rubber Hand Illusion in Pure Hemiplegic Patients

A question still debated within cognitive neuroscience is whether signals present during actions significantly contribute to the emergence of human’s body ownership. In the present study, we aimed at answer this question by means of a neuropsychological approach. We administered the classical rubber hand illusion paradigm to a group of healthy participants and to a group of neurological patients affected by a complete left upper limb hemiplegia, but without any propriceptive/tactile deficits. The illusion strength was measured both subjectively (i.e., by a self-report questionnaire) and behaviorally (i.e., the location of one’s own hand is shifted towards the rubber hand). We aimed at examining whether, and to which extent, an enduring absence of movements related signals affects body ownership. Our results showed that patients displayed, respect to healthy participants, stronger illusory effects when the left (affected) hand was stimulated and no effects when the right (unaffected) hand was stimulated. In other words, hemiplegics had a weaker/more flexible sense of body ownership for the affected hand, but an enhanced/more rigid one for the healthy hand. Possible interpretations of such asymmetrical distribution of body ownership, as well as limits of our results, are discussed. Broadly speaking, our findings suggest that the alteration of the normal flow of signals present during movements impacts on human’s body ownership. This in turn, means that movements have a role per se in developing and maintaining a coherent body ownership.     

Crossing the Hands Increases Illusory Self-Touch

Manipulation of hand posture, such as crossing the hands, has been frequently used to study how the body and its immediately surrounding space are represented in the brain. Abundant data show that crossed arms posture impairs remapping of tactile stimuli from somatotopic to external space reference frame and deteriorates performance on several tactile processing tasks. Here we investigated how impaired tactile remapping affects the illusory self-touch, induced by the non-visual variant of the rubber hand illusion (RHI) paradigm. In this paradigm blindfolded participants (Experiment 1) had their hands either uncrossed or crossed over the body midline. The strength of illusory self-touch was measured with questionnaire ratings and proprioceptive drift. Our results showed that, during synchronous tactile stimulation, the strength of illusory self-touch increased when hands were crossed compared to the uncrossed posture. Follow-up experiments showed that the increase in illusion strength was not related to unfamiliar hand position (Experiment 2) and that it was equally strengthened regardless of where in the peripersonal space the hands were crossed (Experiment 3). However, while the boosting effect of crossing the hands was evident from subjective ratings, the proprioceptive drift was not modulated by crossed posture. Finally, in contrast to the illusion increase in the non-visual RHI, the crossed hand postures did not alter illusory ownership or proprioceptive drift in the classical, visuo-tactile version of RHI (Experiment 4). We argue that the increase in illusory self-touch is related to misalignment of somatotopic and external reference frames and consequently inadequate tactile-proprioceptive integration, leading to re-weighting of the tactile and proprioceptive signals.The present study not only shows that illusory self-touch can be induced by crossing the hands, but importantly, that this posture is associated with a stronger illusion.     

Rubber hands feel touch, but not in blind individuals

Psychology and neuroscience have a long-standing tradition of studying blind individuals to investigate how visual experience shapes perception of the external world. Here, we study how blind people experience their own body by exposing them to a multisensory body illusion: the somatic rubber hand illusion. In this illusion, healthy blindfolded participants experience that they are touching their own right hand with their left index finger, when in fact they are touching a rubber hand with their left index finger while the experimenter touches their right hand in a synchronized manner (Ehrsson et al. 2005). We compared the strength of this illusion in a group of blind individuals (n = 10), all of whom had experienced severe visual impairment or complete blindness from birth, and a group of age-matched blindfolded sighted participants (n = 12). The illusion was quantified subjectively using questionnaires and behaviorally by asking participants to point to the felt location of the right hand. The results showed that the sighted participants experienced a strong illusion, whereas the blind participants experienced no illusion at all, a difference that was evident in both tests employed. A further experiment testing the participants' basic ability to localize the right hand in space without vision (proprioception) revealed no difference between the two groups. Taken together, these results suggest that blind individuals with impaired visual development have a more veridical percept of self-touch and a less flexible and dynamic representation of their own body in space compared to sighted individuals. We speculate that the multisensory brain systems that re-map somatosensory signals onto external reference frames are less developed in blind individuals and therefore do not allow efficient fusion of tactile and proprioceptive signals from the two upper limbs into a single illusory experience of self-touch as in sighted individuals.     

Brain Regions Associated to a Kinesthetic Illusion Evoked by Watching a Video of One's Own Moving Hand

It is well known that kinesthetic illusions can be induced by stimulation of several sensory systems (proprioception, touch, vision…). In this study we investigated the cerebral network underlying a kinesthetic illusion induced by visual stimulation by using functional magnetic resonance imaging (fMRI) in humans. Participants were instructed to keep their hand still while watching the video of their own moving hand (Self Hand) or that of someone else''s moving hand (Other Hand). In the Self Hand condition they experienced an illusory sensation that their hand was moving whereas the Other Hand condition did not induce any kinesthetic illusion. The contrast between the Self Hand and Other Hand conditions showed significant activation in the left dorsal and ventral premotor cortices, in the left Superior and Inferior Parietal lobules, at the right Occipito-Temporal junction as well as in bilateral Insula and Putamen. Most strikingly, there was no activation in the primary motor and somatosensory cortices, whilst previous studies have reported significant activation in these regions for vibration-induced kinesthetic illusions. To our knowledge, this is the first study that indicates that humans can experience kinesthetic perception without activation in the primary motor and somatosensory areas. We conclude that under some conditions watching a video of one''s own moving hand could lead to activation of a network that is usually involved in processing copies of efference, thus leading to the illusory perception that the real hand is indeed moving.     

Virtual Hand Illusion Induced by Visuomotor Correlations

Background

Our body schema gives the subjective impression of being highly stable. However, a number of easily-evoked illusions illustrate its remarkable malleability. In the rubber-hand illusion, illusory ownership of a rubber-hand is evoked by synchronous visual and tactile stimulation on a visible rubber arm and on the hidden real arm. Ownership is concurrent with a proprioceptive illusion of displacement of the arm position towards the fake arm. We have previously shown that this illusion of ownership plus the proprioceptive displacement also occurs towards a virtual 3D projection of an arm when the appropriate synchronous visuotactile stimulation is provided. Our objective here was to explore whether these illusions (ownership and proprioceptive displacement) can be induced by only synchronous visuomotor stimulation, in the absence of tactile stimulation.

Methodology/Principal Findings

To achieve this we used a data-glove that uses sensors transmitting the positions of fingers to a virtually projected hand in the synchronous but not in the asynchronous condition. The illusion of ownership was measured by means of questionnaires. Questions related to ownership gave significantly larger values for the synchronous than for the asynchronous condition. Proprioceptive displacement provided an objective measure of the illusion and had a median value of 3.5 cm difference between the synchronous and asynchronous conditions. In addition, the correlation between the feeling of ownership of the virtual arm and the size of the drift was significant.

Conclusions/Significance

We conclude that synchrony between visual and proprioceptive information along with motor activity is able to induce an illusion of ownership over a virtual arm. This has implications regarding the brain mechanisms underlying body ownership as well as the use of virtual bodies in therapies and rehabilitation.     

The Importance of Synchrony and Temporal Order of Visual and Tactile Input for Illusory Limb Ownership Experiences – An fMRI Study Applying Virtual Reality

In the so-called rubber hand illusion, synchronous visuotactile stimulation of a visible rubber hand together with one''s own hidden hand elicits ownership experiences for the artificial limb. Recently, advanced virtual reality setups were developed to induce a virtual hand illusion (VHI). Here, we present functional imaging data from a sample of 25 healthy participants using a new device to induce the VHI in the environment of a magnetic resonance imaging (MRI) system. In order to evaluate the neuronal robustness of the illusion, we varied the degree of synchrony between visual and tactile events in five steps: in two conditions, the tactile stimulation was applied prior to visual stimulation (asynchrony of −300 ms or −600 ms), whereas in another two conditions, the tactile stimulation was applied after visual stimulation (asynchrony of +300 ms or +600 ms). In the fifth condition, tactile and visual stimulation was applied synchronously. On a subjective level, the VHI was successfully induced by synchronous visuotactile stimulation. Asynchronies between visual and tactile input of ±300 ms did not significantly diminish the vividness of illusion, whereas asynchronies of ±600 ms did. The temporal order of visual and tactile stimulation had no effect on VHI vividness. Conjunction analyses of functional MRI data across all conditions revealed significant activation in bilateral ventral premotor cortex (PMv). Further characteristic activation patterns included bilateral activity in the motion-sensitive medial superior temporal area as well as in the bilateral Rolandic operculum, suggesting their involvement in the processing of bodily awareness through the integration of visual and tactile events. A comparison of the VHI-inducing conditions with asynchronous control conditions of ±600 ms yielded significant PMv activity only contralateral to the stimulation site. These results underline the temporal limits of the induction of limb ownership related to multisensory body-related input.     

The body beyond the body: expectation of a sensory event is enough to induce ownership over a fake hand

More than 100 papers have been published on the rubber hand illusion since its discovery 14 years ago. The illusion has been proposed as a demonstration that the body is distinguished from other objects by its participation in specific forms of intermodal perceptual correlation. Here, we radically challenge this view by claiming that perceptual correlation is not necessary to produce the experience of this body as mine. Each of 15 participants was seated with his/her right arm resting upon a table just below another smaller table. Thus, the real hand was hidden from the participant''s view and a life-sized rubber model of a right hand was placed on the small table in front of the participant. The participant observed the experimenter''s hand while approaching—without touching—the rubber hand. Phenomenology of the illusion was measured by means of skin conductance response and questionnaire. Both measures indicated that participants experienced the illusion that the experimenter''s hand was about to touch their hidden hand rather than the rubber hand, as if the latter replaced their own hand. This did not occur when the rubber hand was rotated by 180° or replaced by a piece of wood. This illusion indicates that our brain does not build a sense of self in a merely reactive way, via perceptual correlations; rather it generates predictions on what may or may not belong to itself.     

Effects of fusion between tactile and proprioceptive inputs on tactile perception

Tactile perception is typically considered the result of cortical interpretation of afferent signals from a network of mechanical sensors underneath the skin. Yet, tactile illusion studies suggest that tactile perception can be elicited without afferent signals from mechanoceptors. Therefore, the extent that tactile perception arises from isomorphic mapping of tactile afferents onto the somatosensory cortex remains controversial. We tested whether isomorphic mapping of tactile afferent fibers onto the cortex leads directly to tactile perception by examining whether it is independent from proprioceptive input by evaluating the impact of different hand postures on the perception of a tactile illusion across fingertips. Using the Cutaneous Rabbit Effect, a well studied illusion evoking the perception that a stimulus occurs at a location where none has been delivered, we found that hand posture has a significant effect on the perception of the illusion across the fingertips. This finding emphasizes that tactile perception arises from integration of perceived mechanical and proprioceptive input and not purely from tactile interaction with the external environment.     

The Human Touch: Skin Temperature during the Rubber Hand Illusion in Manual and Automated Stroking Procedures

A difference in skin temperature between the hands has been identified as a physiological correlate of the rubber hand illusion (RHI). The RHI is an illusion of body ownership, where participants perceive body ownership over a rubber hand if they see it being stroked in synchrony with their own occluded hand. The current study set out to replicate this result, i.e., psychologically induced cooling of the stimulated hand using an automated stroking paradigm, where stimulation was delivered by a robot arm (PHANToM^TM force-feedback device). After we found no evidence for hand cooling in two experiments using this automated procedure, we reverted to a manual stroking paradigm, which is closer to the one employed in the study that first produced this effect. With this procedure, we observed a relative cooling of the stimulated hand in both the experimental and the control condition. The subjective experience of ownership, as rated by the participants, by contrast, was strictly linked to synchronous stroking in all three experiments. This implies that hand-cooling is not a strict correlate of the subjective feeling of hand ownership in the RHI. Factors associated with the differences between the two designs (differences in pressure of tactile stimulation, presence of another person) that were thus far considered irrelevant to the RHI appear to play a role in bringing about this temperature effect.     

Close to you: embodied simulation for peripersonal space in primary somatosensory cortex

Background

An increasing body of evidence has demonstrated that in contrast to the classic understanding the primary somatosensory cortex (SI) reflects merely seen touch (in the absence of any real touch on the own body). Based on these results it has been discussed that SI may play a role in understanding touch seen on other bodies. In order to further examine this understanding of observed touch, the current study aimed to test if mirror-like responses in SI are affected by the perspective of the seen touch. Thus, we presented touch on a hand and close to the hand either in first-person-perspective or in third-person-perspective.

Principal Findings

Results of functional magnetic resonance imaging (fMRI) revealed stronger vicarious brain responses in SI/BA2 for touch seen in first-person-perspective. Surprisingly, the third-person viewpoint revealed activation in SI both when subjects viewed a hand being stimulated as well as when the space close to the hand was being touched.

Conclusions/Significance

Based on these results we conclude that vicarious somatosensory responses in SI/BA2 are affected by the viewpoint of the seen hand. Furthermore, we argue that mirror-like responses in SI do not only reflect seen touch, but also the peripersonal space surrounding this body (in third-person-perspective). We discuss these findings with recent studies on mirror responses for action observation in peripersonal space.     

Rubber Hand Illusion Reduces Discomfort Caused by Cold Stimulus

There is a growing interest in body-ownership disruptions and their consequences for subjective experiences such as tactile sensations or pain. Here, we investigated the effect of the rubber hand illusion (RHI) on the perceived discomfort caused by cold stimulus applied to the real hand. The results showed reduced discomfort to cold reflected in behavioural and subjective measures. The stronger the illusion, the later the cold temperature became unpleasant and the less intense the experience was rated. We discuss the link between thermoception and body ownership as well as possible theoretical and methodological implications for studies on pain experience under RHI.     

Vibrotactile Sensory Substitution Elicits Feeling of Ownership of an Alien Hand

Tactile feedback plays a key role in the attribution of a limb to the self and in the motor control of grasping and manipulation. However, due to technological limits, current prosthetic hands do not provide amputees with cutaneous touch feedback. Recent findings showed that amputees can be tricked into experiencing an alien rubber hand as part of their own body, by applying synchronous touches to the stump which is out of view, and to the rubber hand in full view. It was suggested that similar effects could be achieved by using a prosthesis with touch sensors that provides synchronous cutaneous feedback through an array of tactile stimulators on the stump. Such a prosthesis holds the potential to be easily incorporated within one’s body scheme, because it would reproduce the perceptual illusion in everyday usage. We propose to use sensory substitution – specifically vibrotactile – to address this issue, as current haptic technology is still too bulky and inefficient. In this basic study we addressed the fundamental question of whether visuo-tactile modality mismatch promotes self-attribution of a limb, and to what extent compared to a modality-matched paradigm, on normally-limbed subjects. We manipulated visuo-tactile stimulations, comprising combinations of modality matched, modality mismatched, synchronous and asynchronous stimulations, in a set of experiments fashioned after the Rubber Hand Illusion. Modality mismatched stimulation was provided using a keypad-controlled vibrotactile display. Results from three independent measures of embodiment (questionnaires, pointing tests and skin conductance responses) indicate that vibrotactile sensory substitution can be used to induce self-attribution of a rubber hand when synchronous but modality-conflicting visuo-tactile stimulation is delivered to the biological finger pads and to the equivalent rubber hand phalanges.     

The illusion of owning a third arm

Could it be possible that, in the not-so-distant future, we will be able to reshape the human body so as to have extra limbs? A third arm helping us out with the weekly shopping in the local grocery store, or an extra artificial limb assisting a paralysed person? Here we report a perceptual illusion in which a rubber right hand, placed beside the real hand in full view of the participant, is perceived as a supernumerary limb belonging to the participant''s own body. This effect was supported by questionnaire data in conjunction with physiological evidence obtained from skin conductance responses when physically threatening either the rubber hand or the real one. In four well-controlled experiments, we demonstrate the minimal required conditions for the elicitation of this “supernumerary hand illusion”. In the fifth, and final experiment, we show that the illusion reported here is qualitatively different from the traditional rubber hand illusion as it is characterised by less disownership of the real hand and a stronger feeling of having two right hands. These results suggest that the artificial hand ‘borrows’ some of the multisensory processes that represent the real hand, leading to duplication of touch and ownership of two right arms. This work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.