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

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.

Stability of Action and Kinesthetic Perception in Parkinson's Disease

We present a review of action and perception stability within the theoretical framework based on the idea of control with spatial referent coordinates for the effectors at a number of hierarchical levels. Stability of salient variables is ensured by synergies, neurophysiological structures that act in multi-dimensional spaces of elemental variables and limit variance to the uncontrolled manifold during action and iso-perceptual manifold during perception. Patients with Parkinson’s disease show impaired synergic control reflected in poor stability (low synergy indices) and poor agility (low indices of anticipatory synergy adjustments prior to planned quick actions). They also show impaired perception across modalities, including kinesthetic perception. We suggest that poor stability at the level of referent coordinates can be the dominant factor leading to poor stability of percepts.

Motor and Sensory Features of Cervical Dystonia Subtypes: Data From the Italian Dystonia Registry

Introduction: Cervical dystonia (CD) is one of the most common forms of adult-onset isolated dystonia. Recently, CD has been classified according to the site of onset and spread, in different clinical subgroups, that may represent different clinical entities or pathophysiologic subtypes. In order to support this hypothesis, in this study we have evaluated whether different subgroups of CD, that clinically differ for site of onset and spread, also imply different sensorimotor features.

Methods: Clinical and demographic data from 842 patients with CD from the Italian Dystonia Registry were examined. Motor features (head tremor and tremor elsewhere) and sensory features (sensory trick and neck pain) were investigated. We analyzed possible associations between motor and sensory features in CD subgroups [focal neck onset, no spread (FNO-NS); focal neck onset, segmental spread (FNO-SS); focal onset elsewhere with segmental spread to neck (FOE-SS); segmental neck involvement without spread (SNI)].

Results: In FNO-NS, FOE-SS, and SNI subgroups, head tremor was associated with the presence of tremor elsewhere. Sensory trick was associated with pain in patients with FNO-NS and with head tremor in patients with FNO-SS.

Conclusion: The frequent association between head tremor and tremor elsewhere may suggest a common pathophysiological mechanism. Two mechanisms may be hypothesized for sensory trick: a gating mechanism attempting to reduce pain and a sensorimotor mechanism attempting to control tremor.

LSVT-BIG therapy in Parkinson's disease: physiological evidence for proprioceptive recalibration

BACKGROUND

There is growing evidence for proprioceptive dysfunction in patients with Parkinson's disease (PD). The Lee Silvermann Voice Treatment-BIG therapy (LSVT-BIG), a special training program aiming at an increase of movement amplitudes in persons with PD (PwPD), has shown to be effective on motor symptoms. LSVT-BIG is conceptionally based on improving bradykinesia, in particular the decrement of repetitive movements, by proprioceptive recalibration.

OBJECTIVE

To assess proprioceptive impairment in PwPD as compared to matched controls and to probe potential recalibration effects of the LSVT-BIG therapy on proprioception.

METHODS

Proprioceptive performance and fine motor skills were assessed in 30 PwPD and 15 matched controls. Measurements with significant impairment in PwPD were chosen as outcome parameters for a standardized 4 weeks amplitude-based training intervention (LSVT-BIG) in 11 PwPD. Proprioceptive performance served as primary outcome measure. Secondary outcome measures included the motor part of the MDS-UPDRS, the nine-hole-peg test, and a questionnaire on quality of life. Post-interventional assessments were conducted at weeks 4 and 8.

RESULTS

Compared to the control group, PwPD showed significantly larger pointing errors. After 4 weeks of LSVT-BIG therapy and even more so after an additional 4 weeks of continued training, proprioceptive performance improved significantly. In addition, quality of life improved as indicated by a questionnaire.

CONCLUSION

LSVT-BIG training may achieve a recalibration of proprioceptive processing in PwPD. Our data indicates a probable physiological mechanism of a symptom-specific, amplitude-based behavioral intervention in PwPD.

Acquisition of Ownership Illusion with Self-Disownership in Neurological Patients

The multisensory regions in frontoparietal cortices play a crucial role in the sense of body and self. Disrupting this sense may lead to a feeling of disembodiment, or more generally, a sense of disownership. Experimentally, this altered consciousness disappears during illusory own-body perceptions, increasing the intensity of perceived ownership for an external virtual limb. In many clinical conditions, particularly in individuals with a discontinuous or absent sense of bodily awareness, the brain may effortlessly create a convincing feeling of body ownership over a surrogate body or body part. The immediate visual input dominates the current bodily state and induces rapid plastic adaptation that reconfigures the dynamics of bodily representation, allowing the brain to acquire an alternative sense of body and self. Investigating strategies to deconstruct the lack of a normal sense of bodily ownership, especially after a neurological injury, may aid the selection of appropriate clinical treatment.

Tactile and proprioceptive dysfunction differentiates cervical dystonia with and without tremor

Objective

To determine whether different phenotypes of cervical dystonia (CD) express different types and levels of somatosensory impairment.

Methods

We assessed somatosensory function in patients with CD with and without tremor (n = 12 each) and in healthy age-matched controls (n = 22) by measuring tactile temporal discrimination thresholds of the nondystonic forearm and proprioceptive acuity in both the dystonic (head/neck) and nondystonic body segments (forearm/hand) using a joint position‐matching task. The head or the wrist was passively displaced along different axes to distinct joint positions by the experimenter or through a robotic exoskeleton. Participants actively reproduced the experienced joint position, and the absolute joint position‐matching error between the target and the reproduced positions served as a marker of proprioceptive acuity.

Results

Tactile temporal discrimination thresholds were significantly elevated in both CD subgroups compared to controls. Proprioceptive acuity of both the dystonic and nondystonic body segments was elevated in patients with CD and tremor with respect to both healthy controls and patients with CD without tremor. That is, tactile abnormalities were a shared dysfunction of both CD phenotypes, while proprioceptive dysfunction was observed in patients with CD with tremor.

Conclusions

Our findings suggest that the pathophysiology in CD can be characterized by 2 abnormal neural processes: a dysfunctional somatosensory gating mechanism involving the basal ganglia that triggers involuntary muscle spasms and abnormal processing of proprioceptive information within a defective corticocerebellar loop, likely affecting the feedback and feedforward control of head positioning. This dysfunction is expressed mainly in CD with tremor.

A Case of Abnormal Postures in the Left Extremities after Pontine Hemorrhage: Dystonia or Pseudodystonia?

It is difficult to determine the pathoanatomical correlates of dystonia because of its complex pathophysiology, and most cases with secondary dystonia are associated with basal ganglia lesions. Moreover, it is a challenging issue that patients with abnormal postures accompanied by other neurological findings in the affected body part (e.g., sensory loss) can be diagnosed with true dystonia or pseudodystonia. Here, we report a case of abnormal postures with loss of proprioception in the left extremities after right dorsal pontine hemorrhage.

Sensorimotor Control in Dystonia

This is an overview of the sensorimotor impairments in dystonia, a syndrome characterized by sustained or intermittent aberrant movement patterns leading to abnormal movements and/or postures with or without a tremulous component. Dystonia can affect the entire body or specific body regions and results from a plethora of etiologies, including subtle changes in gray and white matter in several brain regions. Research over the last 25 years addressing topics of sensorimotor control has shown functional sensorimotor impairments related to sensorimotor integration, timing, oculomotor and head control, as well as upper and lower limb control. In the context of efforts to update the classification of dystonia, sensorimotor research is highly relevant for a better understanding of the underlying pathology, and potential mechanisms contributing to global and regional dysfunction within the central nervous system. This overview of relevant research regarding sensorimotor control in humans with idiopathic dystonia attempts to frame the dysfunction with respect to what is known regarding motor control in patients and healthy individuals. We also highlight promising avenues for the future study of neuromotor control that may help to further elucidate dystonia etiology, pathology, and functional characteristics.

Motor Learning Deficits in Parkinson's Disease (PD) and Their Effect on Training Response in Gait and Balance: A Narrative Review

Parkinson's disease (PD) is a neurological disorder traditionally associated with degeneration of the dopaminergic neurons within the substantia nigra, which results in bradykinesia, rigidity, tremor, and postural instability and gait disability (PIGD). The disorder has also been implicated in degradation of motor learning. While individuals with PD are able to learn, certain aspects of learning, especially automatic responses to feedback, are faulty, resulting in a reliance on feedforward systems of movement learning and control. Because of this, patients with PD may require more training to achieve and retain motor learning and may require additional sensory information or motor guidance in order to facilitate this learning. Furthermore, they may be unable to maintain these gains in environments and situations in which conscious effort is divided (such as dual-tasking). These shortcomings in motor learning could play a large part in degenerative gait and balance symptoms often seen in the disease, as patients are unable to adapt to gradual sensory and motor degradation. Research has shown that physical and exercise therapy can help patients with PD to adapt new feedforward strategies to partially counteract these symptoms. In particular, balance, treadmill, resistance, and repeated perturbation training therapies have been shown to improve motor patterns in PD. However, much research is still needed to determine which of these therapies best alleviates which symptoms of PIGD, the needed dose and intensity of these therapies, and long-term retention effects. The benefits of such technologies as augmented feedback, motorized perturbations, virtual reality, and weight-bearing assistance are also of interest. This narrative review will evaluate the effect of PD on motor learning and the effect of motor learning deficits on response to physical therapy and training programs, focusing specifically on features related to PIGD. Potential methods to strengthen therapeutic effects will be discussed.

Defective Tool Embodiment in Body Representation of Individuals Affected by Parkinson's Disease: A Preliminary Study

When efficiently used for action, tools become part of the body, with effect on the spatial-temporal movement parameters and body size perception. Until now, no previous investigation has been reported about tool embodiment in Parkinson’s disease (PD), which is a neurological disease characterized by several sensory and motor symptoms affecting body and action. We enrolled 14 individuals affected by PD and 18 healthy individuals as controls. We studied the spatial-temporal parameters on self-paced free pointing movement task, via an optoelectronic system, before and after a short training in which a 27-cm long rod was used to point toward a far target. Moreover, we investigated changes in estimation of arm length through the Tactile Estimation Task. After the tool-use training, controls showed changes in spatial-temporal parameters: they were slower to perform movements and reported a higher value of deceleration than the baseline. However, such a difference did not emerge in the PD individuals. In the Tactile Discrimination Task, no difference emerged before and after the tool-use training in both groups. Our results were suggestive of possible difficulties of the tool embodiment process in PD. We discussed our results in relation to aberrant multisensory integration as well as in terms of the effect of PD sensory and motor symptoms on body schema plasticity. The present study points at a novel way to conceive PD sensory motor signs and symptoms in terms of their effect on individuals’ body representation.