Catherine Preston

Fake hands in action: embodiment and control of supernumerary limbs

Demonstrations that the brain can incorporate a fake limb into our bodily representations when stroked in synchrony with our unseen real hand [(the rubber hand illusion (RHI)] are now commonplace. Such demonstrations highlight the dynamic flexibility of the perceptual body image, but evidence for comparable RHI-sensitive changes in the body schema used for action is less common. Recent evidence from the RHI supports a distinction between bodily representations for perception (body image) and for action (body schema) (Kammers et al. in Neuropsychologia 44:2430–2436, 2006). The current study challenges and extends these findings by demonstrating that active synchronous stroking of a brush not only elicits perceptual embodiment of a fake limb (body image) but also affects subsequent reaching error (body schema). Participants were presented with two moving fake left hands. When only one was synchronous during active touch, ownership was claimed for the synchronous hand only and the accuracy of reaching was consistent with control of the synchronous hand. When both fake hands were synchronous, ownership was claimed over both, but only one was controlled. Thus, it would appear that fake limbs can be incorporated into the body schema as well as the body image, but while multiple limbs can be incorporated into the body image, the body schema can accommodate only one.

Analgesic effects of multisensory illusions in osteoarthritis

Sir, There is increasing evidence that drug-free illusion therapies can be beneficial for the amelioration of chronic pain, particularly so for conditions in which some of the pain is thought to have a cortical origin. For example, mirror therapy and size reduction illusions can reduce pain in complex regional pain syndrome type 1 (CRPS1) patients, the majority of whom have disturbed body representations with some reporting their hand as larger than in reality [1] and others describing parts of their hand as foreshortened [2]. If cortical misrepresentation of body parts contributes to pain, then manipulating the appearance of those body parts might be a useful tool in the reduction of pain. This letter describes an exploratory experiment using unique visuo-proprioceptive illusions that manipulated the perceived size of painful and non-painful parts of the hand in an attempt to modulate pain experienced in OA. Illusions were applied using a MIRAGE system [3] that presents real-time video capture of the actual hand from the same position and perspective as if viewing the real hand directly (Fig. 1a). Visual manipulations can be applied to the image such that they appear to happen to the real hand. The experimental manipulations were stretching (Fig. 1b) and shrinking (Fig. 1c): gently pulling or pushing on part of the hand while simultaneously elongating or shortening the image gives a strong illusion that the hand is being stretched or shortened (see supplementary material for video). Control illusions included stretching and shrinking a non-painful part of the hand and visually enlarging/reducing the entire hand. Fig. 1 (a) A healthy participant having her finger stretched using MIRAGE; examples of arthritic hands after being stretched (b) and shrunk (c) and (d) the percentage pain reduction after manipulation for painful and non-painful body parts. A direct comparison ... A total of 20 OA sufferers [2 males; mean (s.d.) age: 70.5 (6.5) years] with clinically diagnosed arthritic pain in the hands and/or fingers for >12 months were tested. Participants gave written consent in accordance with the Declaration of Helsinki. The study met with ethical approval from the University of Nottingham School of Psychology Ethics Committee (code: 20072010). None of the participants were pain free on the day of testing and none had used pain medication other than paracetamol. Participants verbally reported their pain levels on a 21-point numerical rating scale (NRS) with 0 indicating no pain at all and 20 indicating the most unbearable pain imaginable. The scores were adjusted relative to a baseline condition of gently pulling or pushing the hand without visual manipulation. NRS ratings of pain for the painful part of the hand were taken before and after stretching or shrinking the painful and non-painful parts of the hand as well as before and after enlarging or reducing the entire hand. Overall, illusory manipulation was extremely beneficial, on average halving the reported pain in 85% of participants. Some reported greater reduction in pain for stretching, some for shrinking and some for both. Crucially, pain reduction was only observed when the painful part of the hand was manipulated and not when manipulating the non-painful part (Fig. 1d) or when enlarging or reducing the whole hand (although the latter was only tested in a minority of participants). For those in whom stretching was beneficial, the mean reduction in pain was ∼50% (pre = 6.42 on the NRS; post = 3.33), while beneficial shrinking produced a ∼45% reduction (pre = 5.83; post = 3.15). Remarkably, illusory stretching or shrinking of the painful body part temporarily eliminated all pain in one-third of all participants and in 41% of those for whom the illusion was effective. Although not formally assessed, many participants spontaneously reported an increased range of movement following manipulation and most reported pain reduction before active movement was permitted. Many participants gave spontaneous verbal reports such as: ‘I can feel my whole body relaxing.’; ‘My pain has completely gone.’; and ‘I wasn’t able to move like that before.’ This experiment demonstrates a strong analgesic effect of multisensory illusions for OA, which (though not tested directly) is consistent with the idea that some of the pain experienced in arthritic conditions may result from dysfunctional representations of the body [4]. Furthermore, the methods described may provide a mechanism for promoting therapeutic exercise normally prevented by painful movement. The effects of mini-magnification appear to be specific to the part of the body in which the pain is experienced and do not require active movement of the hand. Intriguingly, both shrinking and stretching were found to be beneficial in different, and sometimes the same, participants. This may be due to two distinct processes: the matching of cortical representations and the reduction of apparent swelling, which may vary between participants according to different physical and cortical representations. Although the current experiment describes striking analgesic effects, it does not completely rule out placebo effects and more comprehensive experimental research is necessary to investigate the underlying mechanisms and validate the beneficial effects. In particular, the effects of having attention drawn to the painful part of the hand while being manipulated must be adequately controlled for. However, these initial results suggest that multisensory illusions are a useful avenue of research for the treatment of pain in OA. Disclosure statement: The authors have declared no conflicts of interest.

Unawareness after stroke: A review and practical guide to understanding, assessing, and managing anosognosia for hemiplegia

How should stroke patients with poor motor awareness be managed? This question is important because unawareness (or anosognosia) is related to poor rehabilitation and prognosis. This narrative review provides a guide for clinicians and (applied) academics to understanding, assessing and managing anosognosia. Questions addressed are: What is anosognosia? What causes anosognosia? How can anosognosia be assessed? And how can anosognosia be managed? We suggest that anosognosia is a multifaceted disorder, with diverse neuroanatomical and psychopathological origins. Assessment should measure various aspects of awareness, and management should be multidimensional to address problems with motor function, awareness, and emotional/motivational disturbance.

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.

Illusory changes in body size modulate body satisfaction in a way that is related to non-clinical eating disorder psychopathology.

Historically, body size overestimation has been linked to abnormal levels of body dissatisfaction found in eating disorders. However, recently this relationship has been called into question. Indeed, despite a link between how we perceive and how we feel about our body seeming intuitive, until now lack of an experimental method to manipulate body size has meant that a causal link, even in healthy participants, has remained elusive. Recent developments in body perception research demonstrate that the perceptual experience of the body can be readily manipulated using multisensory illusions. The current study exploits such illusions to modulate perceived body size in an attempt to influence body satisfaction. Participants were presented with stereoscopic video images of slimmer and wider mannequin bodies viewed through head-mounted displays from first person perspective. Illusory ownership was induced by synchronously stroking the seen mannequin body with the unseen real body. Pre and post-illusion affective and perceptual measures captured changes in perceived body size and body satisfaction. Illusory ownership of a slimmer body resulted in participants perceiving their actual body as slimmer and giving higher ratings of body satisfaction demonstrating a direct link between perceptual and affective body representations. Change in body satisfaction following illusory ownership of a wider body, however, was related to degree of (non-clinical) eating disorder psychopathology, which can be linked to fluctuating body representations found in clinical samples. The results suggest that body perception is linked to body satisfaction and may be of importance for eating disorder symptomology.

Misattribution of movement agency following right parietal TMS

Single pulse transcranial magnetic stimulation (TMS) was used to disrupt the right inferior parietal lobe (rIPL) whilst neurologically intact participants made self/other judgments about whole arm reaching movements. Visual feedback of a physically coincident virtual hand was perturbed or left unperturbed (randomly) while TMS was delivered to either the rIPL or the vertex (blocked). Visual feedback of the virtual hand was veridical until the hand became occluded by a virtual bar approximately half way through the movement. TMS was delivered on 50% of trials at random during occlusion of the hand. The position of the virtual hand relative to the real hand was also perturbed during occlusion of the virtual hand on 50% of trials at random. At the end of the reach participants were required to make a verbal judgment as to whether the movement they had seen was self (unperturbed) or other (perturbed). The results revealed that when TMS was applied over rIPL, participants were more likely to misattribute agency to the computer, making more other responses for both perturbed and unperturbed trials. These findings highlight the role of a parietal neural comparator as a low-level mechanism in the experience of agency.

Anosognosia for hemiplegia as a global deficit in motor awareness: evidence from the non-paralysed limb.

The current study adds to the growing empirical research into the mechanisms underlying unawareness of paralysis following stroke (anosognosia for hemiplegia or AHP) by investigating action awareness for the non-paralysed limb in a single AHP patient. Visual feedback representing patient GGs goal-directed reaching movements was either modified by a computer or left unperturbed. Unlike healthy and brain-damaged controls, GG was unable to detect computer-generated visual perturbations as large as 20°. GG also failed to report awareness of the large on-line corrective movements that he made when compensating (often unsuccessfully) for the visual perturbations. These results suggest that the motor comparators implicated in AHP are functioning, but not at optimum levels. Moreover, because the current findings reveal a deficit in awareness for reaches with the unimpaired limb, it is suggestive of common right hemisphere networks for motor awareness in both limbs and that AHP may be a global deficit in motor awareness as opposed to a specific lack of awareness for a particular motor deficit.

How Long is Your Arm? Using Multisensory Illusions to Modify Body Image from the Third Person Perspective

Updating body representations from the 3rd person perspectives (3PP) seems to require viewing the real body, unlike when viewing from a 1st person perspective. Here, 3PP updating was investigated through induction of a physically impossible multisensory illusion in which participants viewed real-time 3PP video of themselves having their arm pulled until it stretched to twice its normal length. The illusion elicited the subjective experience that the participants own arm had been stretched and caused an overestimation of reaching distance, although actual reaches were unaffected. Multisensory illusions from the 3PP can alter body image when applied to real bodies.

Self-denial and the role of intentions in the attribution of agency

The ability to distinguish between our own actions and those of an external agent is a fundamental component of normal human social interaction. Both low- and high-level mechanisms are thought to contribute to the sense of movement agency, but the contribution of each is yet to be fully understood. By applying small and incremental perturbations to realistic visual feedback of the limb, the influence of high-level action intentions and low-level motor predictive mechanisms were dissociated in two experiments. In the first, participants were induced to claim agency over movements that were subject to large perturbations and to deny agency over self-produced unperturbed movements despite the application of motor corrections by low-level mechanisms. A control experiment confirmed that if reaches met with their intended goal then they were more likely to be attributed to the agent, regardless of the discrepancy between the actual and seen positions of the limb.

Eye rotation does not contribute to shifts in subjective straight ahead: Implications for prism adaptation and neglect

Prism adaptation has received much attention in recent years as a potential method for the rehabilitation of visual neglect. Recent theories as to the underlying mechanisms include oculomotor resetting and pathological realignment of subjective straight ahead (SSA). Typical prism adaptation procedures involve both ocular rotation and manual correction making the precise mechanisms and contribution of these to the amelioration of neglect difficult to determine. This experiment separated the contributions of ocular rotation and manual error reduction to SSA realignment in normal participants by shifting the eye alone, the hand alone or both together. Rotating the eye alone did not contribute to SSA realignment whereas shifting the hand did.