Impairment of the rubber hand illusion in focal hand dystonia

From rubber hands to neuroprosthetics: Neural correlates of embodiment

Our interaction with the world rests on the knowledge that we are a body in space and time, which can interact with the environment. This awareness is usually referred to as sense of embodiment. For the good part of the past 30 years, the rubber hand illusion (RHI) has been a prime tool to study embodiment in healthy and people with a variety of clinical conditions. In this paper, we provide a critical overview of this research with a focus on the RHI paradigm as a tool to study prothesis embodiment in individuals with amputation. The RHI relies on well-documented multisensory integration mechanisms based on sensory precision, where parietal areas are involved in resolving the visuo-tactile conflict, and premotor areas in updating the conscious bodily representation. This mechanism may be transferable to prosthesis ownership in amputees. We discuss how these results might transfer to technological development of sensorised prostheses, which in turn might progress the acceptability by users.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

Interoceptive accuracy and bias in somatic symptom disorder, illness anxiety disorder, and functional syndromes: A systematic review and meta-analysis

Somatic symptom disorder, illness anxiety disorder, and functional syndromes are characterized by burdensome preoccupation with somatic symptoms. Etiological models propose either increased interoceptive accuracy through hypervigilance to the body, or decreased and biased interoception through top-down predictions about sensory events. This systematic review and meta-analysis summarizes findings of 68 studies examining interoceptive accuracy and 8 studies examining response biases in clinical or non-clinical groups. Analyses yielded a medium population effect size for decreased interoceptive accuracy in functional syndromes, but no observable effect in somatic symptom disorder and illness anxiety disorder. The overall effect size was highly heterogeneous. Regarding response bias, there was a small significant effect in somatic symptom disorder and illness anxiety disorder. Our findings strengthen the notion of top-down factors that result in biased rather than accurate perception of body signals in somatic symptom disorder and illness anxiety disorder.

The sensorimotor theory of pathological pain revisited

Harris (1999) proposed that pain can arise in the absence of tissue damage because changes in the cortical representation of the painful body part lead to incongruences between motor intention and sensory feedback. This idea, subsequently termed the sensorimotor theory of pain, has formed the basis for novel treatments for pathological pain. Here we review the evidence that people with pathological pain have changes to processes contributing to sensorimotor function: motor function, sensory feedback, cognitive representations of the body and its surrounding space, multisensory processing, and sensorimotor integration. Changes to sensorimotor processing are most evident in the form of motor deficits, sensory changes, and body representations distortions, and for Complex Regional Pain Syndrome (CRPS), fibromyalgia, and low back pain. Many sensorimotor changes are related to cortical processing, pain, and other clinical characteristics. However, there is very limited evidence that changes in sensorimotor processing actually lead to pain. We therefore propose that the theory is more appropriate for understanding why pain persists rather than how it arises.

Verbal Suggestion Modulates the Sense of Ownership and Heat Pain Threshold During the "Injured" Rubber Hand Illusion

The appearance of the self-body influences the feeling that one's body belongs to oneself, that is, a sense of ownership (SoO) and pain perception. This can be identified by measuring the SoO and pain thresholds after performing the rubber hand illusion (RHI) with an injured rubber hand. The generation of SoO is thought to be caused by multisensory integration of bottom-up factors (vision, proprioceptive, and touch), and by top-down factors, such as the context effect. The appearance is one of the context effects which may become more effective when used simultaneously with other context effects (e.g., verbal suggestion). However, in the RHI, when appearance and other context effects are used simultaneously, the effect is unclear. In this study, we attempted to identify the influence of verbal suggestion on the SoO and heat pain threshold (HPT). As a preliminary step, in Experiment 1, the "normal" rubber hand and "penetrated nail" as injured rubber hand were used to clarify the context effect with appearance alone during RHI (synchronous/asynchronous), which was conducted within-subjects. In Experiment 2, we only used the "penetrated nail" rubber hand to clarify the context effect with verbal suggestion and appearance during RHI. We randomly classified participants into two suggestion groups ("fear" and "no-fear"). The RHI (synchronous/asynchronous) was conducted for each group. In each experiment, the effect of each condition was assessed by subjective measures of SoO, such as questionnaire, and objective measures of SoO, such as proprioceptive drift and electrodermal activity. Following RHI in each condition, HPT was measured. The main finding was that, in the synchronous condition, the "penetrated nail" appearance with "fear" verbal suggestion modulated questionnaire and HPT, but not electrodermal activity. We conclude that the context-included multisensory integration affected the subjective factors because it contains a higher cognitive process by verbal suggestion.

Cortical and Subcortical Structural Abnormalities in Patients With Idiopathic Cervical and Generalized Dystonia

OBJECTIVES

In this study, we sought to investigate structural imaging alterations of patients with idiopathic dystonia at the cortical and subcortical levels. The common and specific changes in two subtypes of dystonia, cervical dystonia (CD) and generalized dystonia (GD), were intended to be explored. Additionally, we sought to identify the morphometric measurements which might be related to patients' clinical characteristics, thus providing more clues of specific brain regions involved in the mechanism of idiopathic dystonia.

METHODS

3D T1-weighted MRI scans were acquired from 56 patients with idiopathic dystonia and 30 healthy controls (HC). Patients were classified as CD or GD, according to the distinct symptom distributions. Cortical thickness (CT) of 30 CD and 26 GD were estimated and compared to HCs using Computational Anatomy Toolbox (CAT12), while volumes of subcortical structures and their shape alterations (29 CD, 25 GD, and 27 HCs) were analyzed via FSL software. Further, we applied correlation analyses between the above imaging measurements with significant differences and patients' clinical characteristics.

RESULTS

The results of comparisons between the two patient groups and HCs were highly consistent, demonstrating increased CT of bilateral postcentral, superiorparietal, superiorfrontal/rostralmiddlefrontal, occipital gyrus, etc., and decreased CT of bilateral cingulate, insula, entorhinal, and fusiform gyrus (PFWE < 0.005 at the cluster level). In CD, trends of negative correlations were found between disease severity and CT alterations mostly located in pre/postcentral, rostralmiddlefrontal, superiorparietal, and supramarginal regions. Besides, volumes of bilateral putamen, caudate, and thalamus were significantly reduced in both patient groups, while pallidum volume reduction was also presented in GD compared to HCs. Caudate volume reduction had a trend of correlation to increasing disease severity in GD. Last, shape analysis directly demonstrated regional surface alterations in bilateral thalamus and caudate, where the atrophy located in the head of caudate had a trend of correlation to earlier ages of onset in GD.

CONCLUSIONS

Our study demonstrates wide-spread morphometric changes of CT, subcortical volumes, and shapes in idiopathic dystonia. CD and GD presented similar patterns of morphometric abnormalities, indicating shared underlying mechanisms in two different disease forms. Especially, the clinical associations of CT of multiple brain regions with disease severity, and altered volume/shape of caudate with disease severity/age of onset separately in CD and GD might serve as potential biomarkers for further disease exploration.

Contribution of interaction force to the sense of hand ownership and the sense of hand agency

When performing willed actions, we have the unified and coherent experience of owning and controlling our body. Body ownership is believed to emerge from the integration of coherent multisensory signals, while agency is believed to emerge from the coherence between predicted and perceived outcomes of actions. As a consequence, body ownership and agency can both be modulated by multisensory conflicts. The contribution of active movement generation to ownership and agency has not been parametrically explored. Here, we investigated the contribution of interaction force between the agent and the environment to the sense of hand ownership (SO) and the sense of hand agency (SA). By combining robotics and virtual reality, we manipulated the sensorimotor and visual information during immersive scenarios to induce and quantify altered states of SO and SA. First, we demonstrated that SO and SA could be successfully manipulated by our experimental paradigms. Second, we showed that interaction force strongly contributes to SA, but to a lesser extent to SO. Finally, we showed that SO and SA interact beyond their common multisensory basis. Our results, based on two independent studies, provide a direct link between sensorimotor interactions and subjective body experience and demonstrate a new dissociation between SO and SA.

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

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

Neural mechanisms underlying the Rubber Hand Illusion: A systematic review of related neurophysiological studies

INTRODUCTION

Many researchers took advantage of the well-established rubber hand illusion (RHI) paradigm to explore the link between the sense of body ownership and the different brain structures and networks. Here, we aimed to review the studies that have investigated this phenomenon by means of neurophysiological techniques.

METHODS

The MEDLINE, accessed by Pubmed and EMBASE electronic databases, was searched using the medical subject headings: "Rubber hand illusion" AND "Transcranial magnetic stimulation (TMS)" OR "Evoked potentials (EP)" OR "Event related potentials (ERP)" OR "Electroencephalography (EEG)".

RESULTS

Transcranial magnetic stimulation studies revealed a significant excitability drop in primary motor cortex hand circuits accompanying the disembodiment of the real hand during the RHI experience and that the perceived ownership over the rubber hand is associated with normal parietal-motor communication. Moreover, TMS provided causal evidence that the extrastriate body area is involved in the RHI and subsequently in body representation, while neuromodulation of ventral premotor area and the inferior parietal lobe did not result in an enhancement of embodiment. EP and ERP studies suggest that pre-existing body representations may affect larger stages of tactile processing and support predictive coding models of the functional architecture of multisensory integration in bodily perceptual experience. High-frequency oscillations on EEG play a role in the integrative processing of stimuli across modalities, and EEG activity in γ band activity in the parietal area reflects the visuotactile integration process. EEG studies also revealed that RHI is associated with the neural circuits underlying motor control and that premotor areas play a crucial role in mediating illusory body ownership.

CONCLUSION

Neurophysiological studies shed new light on our understanding of the different aspects that contribute to the formation of a coherent self-awareness in humans.

Impact of visuomotor feedback on the embodiment of virtual hands detached from the body

It has been shown that mere observation of body discontinuity leads to diminished body ownership. However, the impact of body discontinuity has mainly been investigated in conditions where participants observe a collocated static virtual body from a first-person perspective. This study explores the influence of body discountinuity on the sense of embodiment, when rich visuomotor correlations between a real and an artificial virtual body are established. In two experiments, we evaluated body ownership and motor performance, when participants interacted in virtual reality either using virtual hands connected or disconnected from a body. We found that even under the presence of congruent visuomotor feedback, mere observation of body discontinuity resulted in diminished embodiment. Contradictory evidence was found in relation to motor performance, where further research is needed to understand the role of visual body discontinuity in motor tasks. Preliminary findings on physiological reactions to a threat were also assessed, indicating that body visual discontinuity does not differently impact threat-related skin conductance responses. The present results are in accordance with past evidence showing that body discontinuity negatively impacts embodiment. However, further research is needed to understand the influence of visuomotor feedback and body morphological congruency on motor performance and threat-related physiological reactions.

Proprioceptive drift is affected by the intermanual distance rather than the distance from the body's midline in the rubber hand illusion

In the rubber hand illusion (RHI), simultaneous brush stroking of a subject’s hidden hand and a visible rubber hand induces a transient illusion of the latter to “feel like it’s my hand” and a proprioceptive drift of the hidden own hand toward the rubber hand. Recent accounts of the RHI have suggested that the illusion would only occur if weighting of conflicting sensory information and their subsequent integration results in a statistically plausible compromise. In three different experiments, we investigated the role of distance between the two hands as well as their proximity to the body’s midline in influencing the occurrence of the illusion. Overall, the results suggest that the illusion is abolished when placing the two hands apart, therefore increasing the mismatch between the visual and proprioceptive modality, whereas the proximity of the two hands to the body’s midline plays only a minor role on the subjective report of the illusion. This might be driven by the response properties of visuotactile bimodal cells encoding the peripersonal space around the hand.

Brain activity associated with the rubber foot illusion

The internal representation of the body is constantly updated by sensory information based on interactions with the environment. The internal representation of the hand can be experimentally manipulated with the rubber hand illusion (RHI) paradigm. Brain activity during the RHI provides insight into the neural mechanisms underpinning the reconstruction of the internal representation of the hand. Recently, the RHI paradigm has been employed for the lower limb, revealing that the illusion is also induced in the lower limb (rubber foot illusion; RFI). However, the neural correlates of the RFI remain unknown. We used functional magnetic resonance imaging (fMRI) to examine brain activity during the RFI. Forty-four healthy volunteers participated in the fMRI experiment. Significant increases in activation were observed in the bilateral medial and middle frontal gyri, left supplemental motor area, bilateral inferior parietal lobuli, precunei, calcarine cortices, and cerebellar hemispheres; and in the vermis and bilateral thalami during the right RFI. During the left RFI, significant increases in activation were observed in the bilateral medial, middle, and superior frontal gyri; left inferior frontal gyrus and supplemental motor area, bilateral inferior parietal lobuli and middle temporal gyri, and in the left cerebellar hemisphere, vermis, and bilateral thalami. Conjunction analysis revealed that the prefrontal cortex including the bilateral medial and middle frontal gyri, parietal cortex including the bilateral inferior parietal lobuli, and cerebellum including the bilateral cerebellar hemispheres and vermis were conjointly activated during the right and left RFIs. The distribution of co-activated brain areas during the RFI was similar to the previously reported distribution of brain areas activated during the RHI. Co-activation of these brain areas may be associated with the reconstruction of the internal representation of the body. The fact that these areas are activated both in the RFI and RHI will have implications for the treatment of patients with disturbed internal bodily representation.

Neuropsychological Changes in Complex Regional Pain Syndrome (CRPS)

Complex Regional Pain Syndrome (CRPS) is a poorly understood chronic pain condition of multifactorial origin. CRPS involves sensory, motor, and autonomic symptoms primarily affecting one extremity. Patients can also present with neuropsychological changes such as reduced attention to the CRPS-affected extremity, reminiscent of hemispatial neglect, yet in the absence of any brain lesions. However, this "neglect-like" framework is not sufficient to characterise the range of higher cognitive functions that can be altered in CRPS. This comprehensive literature review synthesises evidence of neuropsychological changes in CRPS in the context of potential central mechanisms of the disorder. The affected neuropsychological functions constitute three distinct but not independent groups: distorted body representation, deficits in lateralised spatial cognition, and impairment of non-spatially-lateralised higher cognitive functions. We suggest that many of these symptoms appear to be consistent with a broader disruption to parietal function beyond merely what could be considered "neglect-like." Moreover, the extent of neuropsychological symptoms might be related to the clinical signs of CRPS, and rehabilitation methods that target the neuropsychological changes can improve clinical outcomes in CRPS and other chronic pain conditions. Based on the limitations and gaps in the reviewed literature, we provide several suggestions to improve further research on neuropsychological changes in chronic pain.

Weakening the subjective sensation of own hand ownership does not interfere with rapid finger movements

When we perform a movement we generally have a clear distinction between which parts of the world constitute our body and which parts do not. However, how the sense of ownership over our body supports movement is not yet fully understood. We aimed to see whether a sense of ownership over the hand supports the performance of rapid hand movements. In three experiments (n = 48, n = 30, n = 24), we presented participants with congruent and incongruent visuotactile and visuoproprioceptive information regarding their own hand. In keeping with previous experiments, multisensory disintegration resulted in a reduction in the subjective sensation of ownership over the hand, as reflected in questionnaire responses. Following sensory stimulation, participants were required to rapidly abduct their index finger whilst the movement was tracked. We examined the hypothesis that, should a sense of ownership over the limb be necessary for generating rapid movements with that limb, reaction time would increase when hand ownership was reduced, whilst the acceleration and velocity of the movement would decrease. We observed that reductions in own hand ownership did not interfere with rapid index finger abduction, suggesting that the motor system may not be reliant on a subjective sense of ownership over the body in order to generate movement.

The Role of Body-Related Afferent Signals in Human Sense of Agency

At present, most of the neurocognitive models of human sense of agency (ie, "this action is due to my own will") have been traditionally rooted in a variety of internal efferent signals arising within the motor system. However, recent neuroscientific evidence has suggested that also the body-related afferent signals that subserve body ownership (ie, "this body is mine") might have a key role in this process. Accordingly, in the present review paper, we briefly examined the literature investigating how and to what extent body ownership contributes to building up human motor consciousness. Evidence suggests that, if required by the context, body ownership per se can act on agency attribution (ie, independently from efferent signals). Hence, a unitary and coherent subjective experience of willed actions (ie, "this willed action is being realized by my own body") requires both awareness of being an agent and of owning the body.

Rubber hand illusion modulates the influences of somatosensory and parietal inputs to the motor cortex

The rubber hand illusion (RHI) paradigm experimentally produces an illusion of rubber hand ownership and arm shift by simultaneously stroking a rubber hand in view and a participant’s visually occluded hand. It involves visual, tactile, and proprioceptive multisensory integration and activates multisensory areas in the brain, including the posterior parietal cortex (PPC). Multisensory inputs are transformed into outputs for motor control in association areas such as PPC. A behavioral study reported decreased motor performance after RHI. However, it remains unclear whether RHI modifies the interactions between sensory and motor systems and between PPC and the primary motor cortex (M1). We used transcranial magnetic stimulation (TMS) and examined the functional connections from the primary somatosensory and association cortices to M1 and from PPC to M1 during RHI. In experiment 1, short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) were measured before and immediately after a synchronous (RHI) or an asynchronous (control) condition. In experiment 2, PPC-M1 interaction was measured using two coils. We found that SAI and LAI were reduced in the synchronous condition compared with baseline, suggesting that RHI decreased somatosensory processing in the primary sensory and the association cortices projecting to M1. We also found that greater inhibitory PPC-M1 interaction was associated with stronger RHI assessed by questionnaire. Our findings suggest that RHI modulates both the early and late stages of processing of tactile afferent, which leads to altered M1 excitability by reducing the gain of somatosensory afferents to resolve conflicts among multisensory inputs.

NEW & NOTEWORTHY Perception of one’s own body parts involves integrating different sensory information and is important for motor control. We found decreased effects of cutaneous stimulation on motor cortical excitability during rubber hand illusion (RHI), which may reflect decreased gain of tactile input to resolve multisensory conflicts. RHI strength correlated with the degree of inhibitory posterior parietal cortex-motor cortex interaction, indicating that parietal-motor connection is involved in resolving sensory conflicts and body ownership during RHI.

Body ownership increases the interference between observed and executed movements

When we successfully achieve willed actions, the feeling that our moving body parts belong to the self (i.e., body ownership) is barely required. However, how and to what extent the awareness of our own body contributes to the neurocognitive processes subserving actions is still debated. Here we capitalized on immersive virtual reality in order to examine whether and how body ownership influences motor performance (and, secondly, if it modulates the feeling of voluntariness). Healthy participants saw a virtual body either from a first or a third person perspective. In both conditions, they had to draw continuously straight vertical lines while seeing the virtual arm doing the same action (i.e., drawing lines) or deviating from them (i.e., drawing ellipses). Results showed that when there was a mismatch between the intended and the seen movements (i.e., participants had to draw lines but the avatar drew ellipses), motor performance was strongly "attracted" towards the seen (rather than the performed) movement when the avatar's body part was perceived as own (i.e., first person perspective). In support of previous studies, here we provide direct behavioral evidence that the feeling of body ownership modulates the interference of seen movements to the performed movements.

Handedness modulates proprioceptive drift in the rubber hand illusion

Preference for use of either the left or right hand (‘handedness’) has been linked with modulations of perception and sensory processing—both of space and the body. Here we ask whether multisensory integration of bodily information also varies as a function of handedness. We created a spatial disparity between visual and somatosensory hand position information using the rubber hand illusion, and use the magnitude of illusory shifts in hand position (proprioceptive ‘drift’) as a tool to probe the weighted integration of multisensory information. First, we found drift was significantly reduced when the illusion was performed on the dominant vs. non-dominant hand. We suggest increased manual dexterity of the dominant hand causes greater representational stability and thus an increased resistance to bias by the illusion induction. Second, drift was generally greatest when the hand was in its habitual action space (i.e., near the shoulder of origin), compared to when it laterally displaced towards, or across the midline. This linear effect, however, was only significant for the dominant hand—in both left- and right-handed groups. Thus, our results reveal patterns of habitual hand action modulate drift both within a hand (drift varies with proximity to action space), and between hands (differences in drift between the dominant and non-dominant hands). In contrast, we were unable to find conclusive evidence to support, or contradict, an overall difference between left- and right-handers in susceptibility to RHI drift (i.e., total drift, collapsed across hand positions). In sum, our results provide evidence that patterns of daily activity—and the subsequent patterns of sensory input—shape multisensory integration across space.

Dissociation of feeling and belief in the rubber hand illusion

The Rubber Hand Illusion (RHI) has been widely used to investigate the perception of the bodily self. Commonly used measures of the illusion are self-report questionnaires and proprioceptive drift of the participants' hands towards the rubber hand. Recent studies have shown that these measures can be dissociated, suggesting they may arise from distinct mechanisms. In previous studies using questionnaires, participants were asked to base responses on their subjective feelings of body ownership, rather than their beliefs. This makes sense given the obvious fact that whereas participants may feel like the rubber hand is part of their body, they do not believe that it is. It is not clear, however, whether a similar dissociation between feelings and beliefs also exists for proprioceptive drift. Here, we investigated the presence of a dissociation between feeling and belief in the context of the RHI. When participants reported their feelings there was an increase both in the sense of body ownership over the fake hand as well as in the proprioceptive drift, compared to when they reported their beliefs. Strikingly, unlike the sense of ownership, proprioceptive drift was unaffected by the synchrony of stimulation. This may be an important way in which the two measures of the RHI differ.

Impaired performance of patients with writer's cramp in complex fine motor tasks

OBJECTIVE

Writer's cramp (WC) is a task-specific focal dystonia. WC is characterized by involuntary contractions of muscles of the hand and arm during handwriting, resulting in impaired writing with exaggerated finger forces. The generalization of symptoms to other fine motor tasks is widely discussed. The aim of the study was to determine affected fine motor aspects with an extensive testing battery.

METHODS

Twelve people with WC and twelve healthy controls were examined. Performance in the Jebsen-Taylor Hand Function Test, Nine-Hole-Peg Test and 2-point discrimination was evaluated. To analyze object manipulation skills, we examined grip forces, temporal measures and other aspects of force control during (1) lifting actions with variations of weight and surface (2) cyclic movements (3) visuomotor tracking (4) fast force changes and (5) grip strength. In addition, correlation between the dependent variables of the fine motor tasks and the handwriting deficits was assessed.

RESULTS

WC patients had impaired performance in the visuomotor tracking task (root mean square error (RMSE), p = 0.03 and time lag, p = 0.05) and the fast force changes (frequency, p = 0.01). There were no statistically significant group differences in the other tasks. We found a correlation between the RMSE of the tracking task and the time needed to write the test sentence (r = 0.643, p = 0.01).

CONCLUSION

WC patients revealed abnormalities in complex fine motor performance in tasks with high demands on coordination and visual components, specifically in tracking and fast force changes.

SIGNIFICANCE

This suggests a deficit in visuomotor integration, coordination and cognitive aspects related to movement processing particularly with respect to low forces. These insights may prove useful in the development of targeted training approaches.

Losing my hand. Body ownership attenuation after virtual lesion of the primary motor cortex

A fundamental component of the self-awareness is the sensation that we are acting with our own body. Thus, a coherent sense of self implies the existence of a tight link between the sense of body ownership and the motor system. Here, we investigated this issue by taking advantage of a well-known experimental manipulation of body ownership, i.e., the rubber hand illusion (RHI), during which the subjects perceive a fake hand as part of their own body. To test the effect of the motor system down-regulation on the RHI susceptibility, we designed a sham-controlled study, where the primary motor cortex (M1) excitability was modulated by off-line low-frequency repetitive transcranial magnetic stimulation (rTMS). After rTMS (real or sham), subjects underwent the RHI either on the right hand, contralateral to the inhibited hemisphere (Experiment 1), or on the left hand, ipsilateral to the inhibited hemisphere (Experiment 2). Only in Experiment 1, the procedure strengthened the illusory experience, as proved by a significant increase, in rTMS compared to Sham, of both subjective (Embodiment/Disembodiment Questionnaires) and objective (Proprioceptive Drift) RHI measures. This evidence demonstrates that, when the M1 activity is down-regulated, the sense of body ownership is attenuated and the subjects become more prone to incorporate an alien body part. This, in turn, supports the existence of a mutual interaction between the sense of body ownership and the motor system, shedding new light on the construction of a coherent sense of self as an acting body.

IPL-M1 interaction shapes pre-reflective social differentiation in the human action system: new insights from TBS and TMS combined

The conscious experience of being the author of our own actions is thought to be grounded in pre-reflective and low-level sensorimotor representations of the self as different from the other. It has been suggested that the inferior parietal lobe (IPL) is generally involved in self-other differentiation processes and in providing an explicit sense of action authorship. However, direct evidence for its causal and functional role in distinguishing self-related and other-related sensorimotor representations is lacking. The current study employed theta-burst stimulation (TBS) to condition left IPL's activity before a social version of the rubber hand illusion led participants to illusorily attribute observed finger movements to their own body. We recorded motor evoked potentials to single-pulse transcranial magnetic stimulation over the primary motor cortex (M1) as proxies of action authorship during action observation. The results showed that in a control condition (intermediate TBS over the left IPL) others' actions facilitated whereas self-attributed movements inhibited the motor system. Critically, continuous TBS disrupted this mismatch between self and other representations. This outcome provides direct evidence for the IPL's role in providing fundamental authorship signals for social differentiation in the human action system.

Actual and Illusory Perception in Parkinson's Disease and Dystonia: A Narrative Review

Sensory information is continuously processed so as to allow behavior to be adjusted according to environmental changes. Before sensory information reaches the cortex, a number of subcortical neural structures select the relevant information to send to be consciously processed. In recent decades, several studies have shown that the pathophysiological mechanisms underlying movement disorders such as Parkinson's disease (PD) and dystonia involve sensory processing abnormalities related to proprioceptive and tactile information. These abnormalities emerge from psychophysical testing, mainly temporal discrimination, as well as from experimental paradigms based on bodily illusions. Although the link between proprioception and movement may be unequivocal, how temporal tactile information abnormalities and bodily illusions relate to motor disturbances in PD and dystonia is still a matter of debate. This review considers the role of altered sensory processing in the pathophysiology of movement disorders, focusing on how sensory alteration patterns differ between PD and dystonia. We also discuss the evidence available and the potential for developing new therapeutic strategies based on the manipulation of multi-sensory information and bodily illusions in patients with these movement disorders.

Judging the position of the artificial hand induces a "visual" drift towards the real one during the rubber hand illusion

When subjects look at a rubber hand being brush-stroked synchronously with their own hidden hand, they might feel a sense of ownership over the rubber hand. The perceived mislocalization of the own hand towards the rubber hand (proprioceptive drift) would reflect an implicit marker of this illusion occurring through the dominance of vision over proprioception. This account, however, contrasts with principles of multisensory integration whereby percepts result from a "statistical sum" of different sensory afferents. In this case, the most-known proprioceptive drift should be mirrored by complementary visual drift of the rubber hand in the opposite direction. We investigated this issue by designing two experiments in which subjects were not only requested to localize their own hand but also the rubber hand and further explored the subjective feeling of the illusion. In both experiments, we demonstrated a (visual) drift in the opposite direction of the proprioceptive drift, suggesting that both hands converge toward each other. This might suggest that the spatial representations of the two hands are integrated in a common percept placed in between them, contradicting previous accounts of substitution of the real hand by the rubber hand.

Polygenic Risk of Spasmodic Dysphonia is Associated With Vulnerable Sensorimotor Connectivity

Spasmodic dysphonia (SD), or laryngeal dystonia, is an isolated task-specific dystonia of unknown causes and pathophysiology that selectively affects speech production. Using next-generation whole-exome sequencing in SD patients, we computed polygenic risk score from 1804 genetic markers based on a genome-wide association study in another form of similar task-specific focal dystonia, musician's dystonia. We further examined the associations between the polygenic risk score, resting-state functional connectivity abnormalities within the sensorimotor network, and SD clinical characteristics. We found that the polygenic risk of dystonia was significantly associated with decreased functional connectivity in the left premotor/primary sensorimotor and inferior parietal cortices in SD patients. Reduced connectivity of the inferior parietal cortex was correlated with the age of SD onset. The polygenic risk score contained a significant number of genetic variants lying near genes related to synaptic transmission and neural development. Our study identified a polygenic contribution to the overall genetic risk of dystonia in the cohort of SD patients. Associations between the polygenic risk and reduced functional connectivity of the sensorimotor and inferior parietal cortices likely represent an endophenotypic imaging marker of SD, while genes involved in synaptic transmission and neuron development may be linked to the molecular pathophysiology of this disorder.

No consistent cooling of the real hand in the rubber hand illusion

In the rubber hand illusion (RHI), participants view a rubber hand that is stroked synchronously with their real, hidden hand. This procedure results in experiencing an increased sense of ownership over the rubber hand and demonstrates how multisensory information (vision, touch) can influence the sense of body ownership. However, it has also been suggested that a (lack of) sense of ownership over an own body part may in turn influence bodily processes. This suggestion has previously been supported by the observation that a decrease in skin temperature in the real hand correlated with ownership over the rubber hand. However, this finding has not been consistently replicated. Our lab has conducted several studies in which we recorded temperature of the hands during the RHI using various measures and in different circumstances, including continuous temperature measurements in a temperature-controlled room. An overall analysis of our results, covering five attempts to replicate the traditional RHI experiment and totalling 167 participants, does not show a reliable cooling of the real hand during the RHI. We discuss this failure to replicate and consider several possible explanations for inconsistencies between reports of hand temperature during the RHI.

The Moving Rubber Hand Illusion Reveals that Explicit Sense of Agency for Tapping Movements Is Preserved in Functional Movement Disorders

Functional movement disorders (FMD) are characterized by motor symptoms (e.g., tremor, gait disorder, and dystonia) that are not compatible with movement abnormalities related to a known organic cause. One key clinical feature of FMD is that motor symptoms are similar to voluntary movements but are subjectively experienced as involuntary by patients. This gap might be related to abnormal self-recognition of bodily action, which involves two main components: sense of agency and sense of body ownership. The aim of this study was to systematically investigate whether this function is altered in FMD, specifically focusing on the subjective feeling of agency, body ownership, and their interaction during normal voluntary movements. Patients with FMD (n = 21) and healthy controls (n = 21) underwent the moving Rubber Hand Illusion (mRHI), in which passive and active movements can differentially elicit agency, ownership or both. Explicit measures of agency and ownership were obtained via a questionnaire. Patients and controls showed a similar pattern of response: when the rubber hand was in a plausible posture, active movements elicited strong agency and ownership; implausible posture of the rubber hand abolished ownership but not agency; passive movements suppressed agency but not ownership. These findings suggest that explicit sense of agency and body ownership are preserved in FMD. The latter finding is shared by a previous study in FMD using a static version of the RHI, whereas the former appears to contrast with studies demonstrating altered implicit measures of agency (e.g., sensory attenuation). Our study extends previous findings by suggesting that in FMD: (i) the sense of body ownership is retained also when interacting with the motor system; (ii) the subjective experience of agency for voluntary tapping movements, as measured by means of mRHI, is preserved.

The Cortical Processing of Sensorimotor Sequences is Disrupted in Writer's Cramp

Evidence for pre-existing abnormalities in the sensory and motor systems has been previously reported in writer's cramp (WC). However, the processing of somatosensory information during motor planning has received little attention. We hypothesized that sensorimotor integration processes might be impaired partly due to a disruption in the parieto-premotor network. To test this assumption, we designed 2 nonwriting motor tasks in which subjects had to perform a 4-finger motor sequence either on the basis of sensory stimuli previously memorized (SM task) or freely generated (SG task). Brain activity was measured by combining event-related functional magnetic resonance imaging and coherency electroencephalography in 15 WC patients and 15 normal controls. The bold signal was decreased in patients in both tasks during sensory stimulation but not during movement execution. However, the EEG study showed that coherency was decreased in patients compared with controls, during the delay of the SM task and during the execution of the SG task, on both the whole network and for specific couples of electrodes. Overall, these results demonstrate an endophenotypic impairment in the synchronization of cortical areas within the parieto-premotor network during somatosensory processing and motor planning in WC patients.

Timing disownership experiences in the rubber hand illusion

Some investigators of the rubber hand illusion (RHI) have suggested that when standard RHI induction procedures are employed, if the rubber hand is experienced by participants as owned, their corresponding biological hands are experienced as disowned. Others have demurred: drawing upon a variety of experimental data and conceptual considerations, they infer that experience of the RHI might include the experience of a supernumerary limb, but that experienced disownership of biological hands does not occur. Indeed, some investigators even categorically deny that any experimental paradigm has been employed or any evidence can be adduced to support the claim that disownership experiences occur during the RHI. It goes without saying that RHI experiences can be elusive, and that there is some evidence to support claims that supernumerary limb experiences can sometimes occur. Here, however, we test the claim that the conscious experience of disownership can occur during the RHI. In order to test this claim, we developed two new online proxies—onset time for the illusion and illusion duration—and combined these with established questionnaires that concern the conscious contents of the RHI, in particular ownership/disownership experiences. Both online proxy data and post hoc questionnaire data converge in supporting the claim that disownership experiences do occur, at least when the left hand is the object of investigation. Our findings that onset time and illusion duration are reliable measures suggest that investigations of the RHI stand to benefit by devoting more attention to data collected while the RHI is being experienced, in particular data concerning temporal dynamics.

Individual Differences in the Rubber Hand Illusion Are Related to Sensory Suggestibility

In the rubber hand illusion (RHI), watching a rubber hand being stroked in synchrony with one’s own hidden hand may induce a sense of ownership over the rubber hand. The illusion relies on bottom-up multisensory integration of visual, tactile, and proprioceptive information, and on top-down processes through which the rubber hand is incorporated into pre-existing representations of the body. Although the degree of illusory experience varies largely across individuals, the factors influencing individual differences are unknown. We investigated whether sensory suggestibility might modulate susceptibility to the RHI. Sensory suggestibility is a personality trait related to how individuals react to sensory information. Because of its sensory nature, this trait could be relevant for studies using the RHI paradigm. Seventy healthy volunteers were classified by Sensory Suggestibility Scale (SSS) scores as having high or low suggestibility and assigned to either a high- (High-SSS) or a low-suggestibility (Low-SSS) group. Two components of the RHI were evaluated in synchronous and asynchronous stroking conditions: subjective experience of sense of ownership over the rubber hand via a 9-statement questionnaire, and proprioceptive drift as measured with a ruler. The High-SSS group was generally more susceptible to the subjective component; in the synchronous condition, they rated the statement assessing the sense of ownership higher than the Low-SSS group. The scores for this statement significantly correlated with the total SSS score, indicating that the higher the sensory suggestibility, the stronger the sense of ownership. No effect of sensory suggestibility on proprioceptive drift was observed, suggesting that the effect is specific for the subjective feeling of ownership. This study demonstrates that sensory suggestibility may contribute to participants’ experience of the illusion and should be considered when using the RHI paradigm.

Changes in resting-state connectivity in musicians with embouchure dystonia

OBJECTIVE

Embouchure dystonia is a highly disabling task-specific dystonia in professional brass musicians leading to spasms of perioral muscles while playing the instrument. As they are asymptomatic at rest, resting-state functional magnetic resonance imaging in these patients can reveal changes in functional connectivity within and between brain networks independent from dystonic symptoms.

METHODS

We therefore compared embouchure dystonia patients to healthy musicians with resting-state functional magnetic resonance imaging in combination with independent component analyses.

RESULTS

Patients showed increased functional connectivity of the bilateral sensorimotor mouth area and right secondary somatosensory cortex, but reduced functional connectivity of the bilateral sensorimotor hand representation, left inferior parietal cortex, and mesial premotor cortex within the lateral motor function network. Within the auditory function network, the functional connectivity of bilateral secondary auditory cortices, right posterior parietal cortex and left sensorimotor hand area was increased, the functional connectivity of right primary auditory cortex, right secondary somatosensory cortex, right sensorimotor mouth representation, bilateral thalamus, and anterior cingulate cortex was reduced. Negative functional connectivity between the cerebellar and lateral motor function network and positive functional connectivity between the cerebellar and primary visual network were reduced.

CONCLUSIONS

Abnormal resting-state functional connectivity of sensorimotor representations of affected and unaffected body parts suggests a pathophysiological predisposition for abnormal sensorimotor and audiomotor integration in embouchure dystonia. Altered connectivity to the cerebellar network highlights the important role of the cerebellum in this disease. © 2016 International Parkinson and Movement Disorder Society.

Decreased motor cortex excitability mirrors own hand disembodiment during the rubber hand illusion

During the rubber hand illusion (RHI), subjects experience an artificial hand as part of their own body, while the real hand is subject to a sort of 'disembodiment'. Can this altered belief about the body also affect physiological mechanisms involved in body-ownership, such as motor control? Here we ask whether the excitability of the motor pathways to the real (disembodied) hand are affected by the illusion. Our results show that the amplitude of the motor-evoked potentials recorded from the real hand is significantly reduced, with respect to baseline, when subjects in the synchronous (but not in the asynchronous) condition experience the fake hand as their own. This finding contributes to the theoretical understanding of the relationship between body-ownership and motor system, and provides the first physiological evidence that a significant drop in motor excitability in M1 hand circuits accompanies the disembodiment of the real hand during the RHI experience.

DOI:http://dx.doi.org/10.7554/eLife.14972.001

The feeling of body ownership — that the various parts of your body are all part of you — is something that we typically take for granted. However, brain damage can disrupt this sensation and leave individuals convinced that an arm or a leg is no longer their own. Even in healthy people, the ‘rubber hand illusion’ can temporarily produce a similar phenomenon. Individuals watch a lifelike rubber hand being touched while their own hand – which is hidden from view – is touched at the same time. This creates the feeling that the artificial hand has become part of their body, while their real hand is left in a ‘disembodied’ state.

How does the brain generate this illusory sense of ownership and accompanying sense of disembodiment? A person’s ability to move their body is thought to contribute to their feeling of body ownership. Therefore, della Gatta, Garbarini et al. asked whether the brain’s ability to move the real hand changes during the rubber hand illusion.

In the experiments, the region of the brain that controls hand movement was artificially stimulated in a number of volunteers. When an individual had been primed by the rubber hand illusion to perceive a fake hand as part of their own body, their brain was temporarily less able to activate the muscles of their real hand. This is as if the brain no longer considered the real hand as part of the body. Thus, the altered sense of body ownership experienced during the rubber hand illusion is not a bizarre fantasy, but corresponds to a physiological reaction that accompanies changes in brain activity.

The next step is to further define and quantify the relationship between the sense of body ownership and the control of body movement. Specifically, how does activity in the brain areas that control movement contribute to the sense of body ownership? And how do these brain regions communicate with one another to generate a sense of self?

DOI:http://dx.doi.org/10.7554/eLife.14972.002

Explicit Agency in Patients with Cervical Dystonia: Altered Recognition of Temporal Discrepancies between Motor Actions and Their Feedback

BACKGROUND

Abnormalities in the cognitive processing of movement have been demonstrated in patients with dystonia. The sense of agency, which is the experience of initiating and controlling one's own actions, has never before been studied in these patients.

OBJECTIVES

We investigated whether the sense of agency is altered in patients with cervical dystonia.

METHODS

We used an explicit metacognitive agency task in which participants had to catch targets with a cursor by moving a computer's mouse. The task included several conditions in which the control over the cursor could be disrupted by adding a spatial or a temporal discrepancy between the mouse and the cursor's movements. Participants had to acknowledge these discrepancies and reflect them in metacognitive judgements of agency.

RESULTS

Twenty cervical dystonia patients and 20 matched controls were included in the study. Despite performing equally well as the matched controls, cervical dystonia patients did not fully recognize alterations of agency when a temporal lag was added between their movement and the visual feedback. Moreover, they relied predominantly on their perceived performance to provide judgements of agency and less on their objective degree of controls. There was no correlation between agency scores and clinical severity of dystonia measured by the Toronto Western Spasmodic Torticollis Rating Scale.

CONCLUSION

We demonstrated an abnormal processing of agency in cervical dystonia patients, even for motor actions not affected by dystonia. The exact contribution of abnormal agency to dystonia pathophysiology remains to be clarified.

Impairment of a parieto-premotor network specialized for handwriting in writer's cramp

Handwriting with the dominant hand is a highly skilled task singularly acquired in humans. This skill is the isolated deficit in patients with writer's cramp (WC), a form of dystonia with maladaptive plasticity, acquired through intensive and repetitive motor practice. When a skill is highly trained, a motor program is created in the brain to execute the same movement kinematics regardless of the effector used for the task. The task- and effector-specific symptoms in WC suggest that a problem particularly occurs in the brain when the writing motor program is carried out by the dominant hand. In this MRI study involving 12 WC patients (with symptoms only affecting the right dominant hand during writing) and 15 age matched unaffected controls we showed that: (1) the writing program recruited the same network regardless of the effector used to write in both groups; (2) dominant handwriting recruited a segregated parieto-premotor network only in the control group; (3) local structural alteration of the premotor area, the motor component of this network, predicted functional connectivity deficits during dominant handwriting and symptom duration in the patient group. Dysfunctions and structural abnormalities of a segregated parieto-premotor network in WC patients suggest that network specialization in focal brain areas is crucial for well-learned motor skill. Hum Brain Mapp 37:4363-4375, 2016. © 2016 Wiley Periodicals, Inc.

Cortical sensorimotor alterations classify clinical phenotype and putative genotype of spasmodic dysphonia

BACKGROUND AND PURPOSE

Spasmodic dysphonia (SD), or laryngeal dystonia, is a task-specific isolated focal dystonia of unknown causes and pathophysiology. Although functional and structural abnormalities have been described in this disorder, the influence of its different clinical phenotypes and genotypes remains scant, making it difficult to explain SD pathophysiology and to identify potential biomarkers.

METHODS

We used a combination of independent component analysis and linear discriminant analysis of resting-state functional magnetic resonance imaging data to investigate brain organization in different SD phenotypes (abductor versus adductor type) and putative genotypes (familial versus sporadic cases) and to characterize neural markers for genotype/phenotype categorization.

RESULTS

We found abnormal functional connectivity within sensorimotor and frontoparietal networks in patients with SD compared with healthy individuals as well as phenotype- and genotype-distinct alterations of these networks, involving primary somatosensory, premotor and parietal cortices. The linear discriminant analysis achieved 71% accuracy classifying SD and healthy individuals using connectivity measures in the left inferior parietal and sensorimotor cortices. When categorizing between different forms of SD, the combination of measures from the left inferior parietal, premotor and right sensorimotor cortices achieved 81% discriminatory power between familial and sporadic SD cases, whereas the combination of measures from the right superior parietal, primary somatosensory and premotor cortices led to 71% accuracy in the classification of adductor and abductor SD forms.

CONCLUSIONS

Our findings present the first effort to identify and categorize isolated focal dystonia based on its brain functional connectivity profile, which may have a potential impact on the future development of biomarkers for this rare disorder.

Are You Suggesting That's My Hand? The Relation Between Hypnotic Suggestibility and the Rubber Hand Illusion

Hypnotic suggestibility (HS) is the ability to respond automatically to suggestions and to experience alterations in perception and behavior. Hypnotically suggestible participants are also better able to focus and sustain their attention on an experimental stimulus. The present study explores the relation between HS and susceptibility to the rubber hand illusion (RHI). Based on previous research with visual illusions, it was predicted that higher HS would lead to a stronger RHI. Two behavioral output measures of the RHI, an implicit (proprioceptive drift) and an explicit (RHI questionnaire) measure, were correlated against HS scores. Hypnotic suggestibility correlated positively with the implicit RHI measure contributing to 30% of the variation. However, there was no relation between HS and the explicit RHI questionnaire measure, or with compliance control items. High hypnotic suggestibility may facilitate, via attentional mechanisms, the multisensory integration of visuoproprioceptive inputs that leads to greater perceptual mislocalization of a participant's hand. These results may provide insight into the multisensory brain mechanisms involved in our sense of embodiment.

Losing dexterity: patterns of impaired coordination of finger movements in musician's dystonia

Extensive training can bring about highly-skilled action, but may also impair motor dexterity by producing involuntary movements and muscular cramping, as seen in focal dystonia (FD) and tremor. To elucidate the underlying neuroplastic mechanisms of FD, the present study addressed the organization of finger movements during piano performance in pianists suffering from the condition. Principal component (PC) analysis identified three patterns of fundamental joint coordination constituting finger movements in both patients and controls. The first two coordination patterns described less individuated movements between the “dystonic” finger and key-striking fingers for patients compared to controls. The third coordination pattern, representing the individuation of movements between the middle and ring fingers, was evident during a sequence of strikes with these fingers in controls, which was absent in the patients. Consequently, rhythmic variability of keystrokes was more pronounced during this sequence of strikes for the patients. A stepwise multiple-regression analysis further identified greater variability of keystrokes for individuals displaying less individuated movements between the affected and striking fingers. The findings suggest that FD alters dexterous joint coordination so as to lower independent control of finger movements, and thereby degrades fine motor control.

Sensory-motor integration in focal dystonia

Traditional definitions of focal dystonia point to its motor component, mainly affecting planning and execution of voluntary movements. However, focal dystonia is tightly linked also to sensory dysfunction. Accurate motor control requires an optimal processing of afferent inputs from different sensory systems, in particular visual and somatosensory (e.g., touch and proprioception). Several experimental studies indicate that sensory-motor integration - the process through which sensory information is used to plan, execute, and monitor movements - is impaired in focal dystonia. The neural degenerations associated with these alterations affect not only the basal ganglia-thalamic-frontal cortex loop, but also the parietal cortex and cerebellum. The present review outlines the experimental studies describing impaired sensory-motor integration in focal dystonia, establishes their relationship with changes in specific neural mechanisms, and provides new insight towards the implementation of novel intervention protocols. Based on the reviewed state-of-the-art evidence, the theoretical framework summarized in the present article will not only result in a better understanding of the pathophysiology of dystonia, but it will also lead to the development of new rehabilitation strategies.

Increased cortico-striatal connectivity during motor practice contributes to the consolidation of motor memory in writer's cramp patients

Sensorimotor representations of movements are created in the sensorimotor network through repeated practice to support successful and effortless performance. Writer's cramp (WC) is a disorder acquired through extensive practice of finger movements, and it is likely associated with the abnormal acquisition of sensorimotor representations. We investigated (i) the activation and connectivity changes in the brain network supporting the acquisition of sensorimotor representations of finger sequences in patients with WC and (ii) the link between these changes and consolidation of motor performance 24 h after the initial practice. Twenty-two patients with WC and 22 age-matched healthy volunteers practiced a complex sequence with the right (pathological) hand during functional MRI recording. Speed and accuracy were measured immediately before and after practice (day 1) and 24 h after practice (day 2). The two groups reached equivalent motor performance on day 1 and day 2. During motor practice, patients with WC had (i) reduced hippocampal activation and hippocampal-striatal functional connectivity; and (ii) overactivation of premotor-striatal areas, whose connectivity correlated with motor performance after consolidation. These results suggest that patients with WC use alternative networks to reach equiperformance in the acquisition of new motor memories.

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.

Impaired Limb Proprioception in Adults With Spasmodic Dysphonia

OBJECTIVES

Focal dystonia of the head and neck are associated with a loss of kinesthetic acuity at muscles distant from the dystonic sites. That is, while the motor deficits in focal dystonia are confined, the associated somatosensory deficits are generalized. This is the first systematic study to examine, if patients diagnosed with spasmodic dystonia (SD) show somatosensory impairments similar in scope to other forms of focal dystonia.

METHODS

Proprioceptive acuity (ability to discriminate between two stimuli) for forearm position and motion sense was assessed in 14 spasmodic dystonia subjects and 28 age-matched controls using a passive motion apparatus. Psychophysical thresholds, uncertainty area (UA), and a proprioceptive acuity index (AI) were computed based on the subjects' verbal responses.

RESULTS

The main findings are as follows: first, the SD group showed significantly elevated thresholds and UAs for forearm position sense compared with the control group. Second, 9 of 14 dystonia subjects (64%) exhibited an AI for position sense above the control group maximum. Three SD subjects had a motion sense AI above the control group maximum.

CONCLUSIONS

The results indicate that impaired limb proprioception is a common feature of SD. Like other forms of focal dystonia, spasmodic dystonia does affect the somatosensation of nondystonic muscle systems. That is, SD is associated with a generalized somatosensory deficit.

Proprioceptive dysfunction in focal dystonia: from experimental evidence to rehabilitation strategies

Dystonia has historically been considered a disorder of the basal ganglia, mainly affecting planning and execution of voluntary movements. This notion comes from the observation that most lesions responsible for secondary dystonia involve the basal ganglia. However, what emerges from recent research is that dystonia is linked to the dysfunction of a complex neural network that comprises basal ganglia–thalamic–frontal cortex, but also the inferior parietal cortex and the cerebellum. While dystonia is clearly a motor problem, it turned out that sensory aspects are also fundamental, especially those related to proprioception. We outline experimental evidence for proprioceptive dysfunction in focal dystonia from intrinsic sensory abnormalities to impaired sensorimotor integration, which is the process by which sensory information is used to plan and execute volitional movements. Particularly, we will focus on proprioceptive aspects of dystonia, including: (i) processing of vibratory input, (ii) temporal discrimination of two passive movements, (iii) multimodal integration of visual-tactile and proprioceptive inputs, and (iv) motor control in the absence of visual feedback. We suggest that these investigations contribute not only to a better understanding of dystonia pathophysiology, but also to develop rehabilitation strategies aimed at facilitating the processing of proprioceptive input.

Imaging insights into basal ganglia function, Parkinson's disease, and dystonia

Recent advances in structural and functional imaging have greatly improved our ability to assess normal functions of the basal ganglia, diagnose parkinsonian syndromes, understand the pathophysiology of parkinsonism and other movement disorders, and detect and monitor disease progression. Radionuclide imaging is the best way to detect and monitor dopamine deficiency, and will probably continue to be the best biomarker for assessment of the effects of disease-modifying therapies. However, advances in magnetic resonance enable the separation of patients with Parkinson's disease from healthy controls, and show great promise for differentiation between Parkinson's disease and other akinetic-rigid syndromes. Radionuclide imaging is useful to show the dopaminergic basis for both motor and behavioural complications of Parkinson's disease and its treatment, and alterations in non-dopaminergic systems. Both PET and MRI can be used to study patterns of functional connectivity in the brain, which is disrupted in Parkinson's disease and in association with its complications, and in other basal-ganglia disorders such as dystonia, in which an anatomical substrate is not otherwise apparent. Functional imaging is increasingly used to assess underlying pathological processes such as neuroinflammation and abnormal protein deposition. This imaging is another promising approach to assess the effects of treatments designed to slow disease progression.

Alleviating manoeuvres (sensory tricks) in cervical dystonia

BACKGROUND

There is limited information on the phenomenology, clinical characteristics and pathophysiology of alleviating manoeuvres (AM), also called 'sensory tricks' in cervical dystonia (CD).

METHODS

Individual data, collected from 10 sites participating in the Dystonia Coalition (http://clinicaltrials.gov/show/NCT01373424), included description of localisation and phenomenology of AM collected by systematic review of standardised video examinations. Analyses correlated demographic, neurologic, and psychiatric features of CD patients with or without effective AM.

RESULTS

Of 154 people studied, 138 (89.6%) used AM, of which 60 (43.4%) reported partial improvement, 55 (39.8%) marked improvement, and 4 (0.03%) no effect on dystonic posture. Light touch, usually to the lower face or neck, was used by >90%. The presence or location of AM did not correlate with the severity of the dystonia.

CONCLUSIONS

In this large and comprehensive study of CD, we found no clinical predictors of effective AM. Further studies of sensorimotor integration in dystonia are needed to better understand the pathophysiology of AM.