Impact of Parkinson's disease and dopaminergic medication on proprioceptive processing

Subthalamic nucleus neurons encode syllable sequence and phonetic characteristics during speech

Speech is a complex behavior that can be used to study unique contributions of the basal ganglia to motor control in the human brain. Computational models suggest that the basal ganglia encode either the phonetic content or the sequence of speech elements. To explore this question, we investigated the relationship between phoneme and sequence features of a spoken syllable triplet and the firing rate of subthalamic nucleus (STN) neurons recorded during the implantation of deep brain stimulation (DBS) electrodes in individuals with Parkinson's disease. Patients repeated aloud a random sequence of three consonant-vowel (CV) syllables in response to audio cues. Single-unit extracellular potentials were sampled from the sensorimotor STN; a total of 227 unit recordings were obtained from the left STN of 25 subjects (4 females). Of these, 113 (50%) units showed significant task-related increased firing and 53 (23%) showed decreased firing (t test relative to inter-trial period baseline, P < 0.05). Linear regression analysis revealed that both populations of STN neurons encode phoneme and sequence features of produced speech. Maximal phoneme encoding occurred at the time of phoneme production, suggesting efference copy- or sensory-related processing, rather than speech motor planning (-50 ms and +175 ms relative to CV transition for consonant and vowel encoding, respectively). These findings demonstrate that involvement of the basal ganglia in speaking includes separate single unit representations of speech sequencing and phoneme selection in the STN.NEW & NOTEWORTHY Speech is a unique human behavior that requires dynamic execution of precisely timed and coordinated movements, resulting in intelligible vocalizations. Here, we demonstrate that activity of individual neurons in the subthalamic nucleus (STN) of the basal ganglia encode syllable sequence order and phoneme identity during a speech production task. These findings advance our understanding of neural substrates of human speech and shed light on potential involvement of the STN in complex human behaviors.

Proprioceptive and olfactory deficits in individuals with Parkinson disease and mild cognitive impairment

BACKGROUND

Individuals with neurodegenerative diseases such as Parkinson disease (PD) and Alzheimer's (AD) disease often present with perceptual impairments at an early clinical stage. Therefore, early identification and quantification of these impairments could facilitate diagnosis and early intervention.

OBJECTIVES

This study aimed to compare proprioceptive and olfactory sensitivities in individuals diagnosed with PD and mild cognitive impairment (MCI).

METHODS

Proprioception in the forearm and olfactory function were measured in neurotypical older adults, individuals with PD, and individuals with MCI. Position and passive motion senses were assessed using a passive motion apparatus. The traditional Chinese version of the University of Pennsylvania smell identification test (UPSIT-TC) and the smell threshold test (STT) were used to identify and discriminate smell, respectively.

RESULTS

Position sense threshold between the groups differed significantly (p < 0.001), with the PD (p < 0.001) and MCI (p = 0.004) groups showing significantly higher than the control group. The control group had significantly higher mean UPSIT-TC scores than the PD (p < 0.001) and MCI (p = 0.006) groups. The control group had a significantly lower mean STT threshold than the PD and MCI groups (p < 0.001 and p = 0.008, respectively). UPSIT-TC scores significantly correlated with disease progression in PD (r = - 0.50, p = 0.008) and MCI (r = 0.44, p = 0.04).

CONCLUSIONS

Proprioceptive and olfactory sensitivities were reduced in individuals with PD and MCI, and these deficits were related to disease severity. These findings support previous findings indicating that perceptual loss may be a potential biomarker for diagnosing and monitoring disease progression in individuals with neurodegenerative diseases.

Brain Networks Involved in Sensory Perception in Parkinson's Disease: A Scoping Review

Parkinson's Disease (PD) has historically been considered a disorder of motor dysfunction. However, a growing number of studies have demonstrated sensory abnormalities in PD across the modalities of proprioceptive, tactile, visual, auditory and temporal perception. A better understanding of these may inform future drug and neuromodulation therapy. We analysed these studies using a scoping review. In total, 101 studies comprising 2853 human participants (88 studies) and 125 animals (13 studies), published between 1982 and 2022, were included. These highlighted the importance of the basal ganglia in sensory perception across all modalities, with an additional role for the integration of multiple simultaneous sensation types. Numerous studies concluded that sensory abnormalities in PD result from increased noise in the basal ganglia and increased neuronal receptive field size. There is evidence that sensory changes in PD and impaired sensorimotor integration may contribute to motor abnormalities.

Robotics-Based Characterization of Sensorimotor Integration in Parkinson's Disease and the Effect of Medication

Integration of multi-modal sensory inputs and modulation of motor outputs based on perceptual estimates is called Sensorimotor Integration (SMI). Optimal functioning of SMI is essential for perceiving the environment, modulating the motor outputs, and learning or modifying motor skills to suit the demands of the environment. Growing evidence suggests that patients diagnosed with Parkinson's Disease (PD) may suffer from an impairment in SMI that contributes to perceptual deficits, leading to motor abnormalities. However, the exact nature of the SMI impairment is still unclear. This study uses a robot-assisted assessment tool to quantitatively characterize SMI impairments in PD patients and how they affect voluntary movements. A set of assessment tasks was developed using a robotic manipulandum equipped with a virtual-reality system. The sensory conditions of the virtual environment were varied to facilitate the assessment of SMI. A hundred PD patients (before and after medication) and forty-three control subjects completed the tasks under varying sensory conditions. The kinematic measures obtained from the robotic device were used to evaluate SMI. The findings reveal that across all sensory conditions, PD patients had 36% higher endpoint error, 38% higher direction error in reaching tasks, and 43% higher number of violations in tracing tasks than control subjects due to impairment in integrating sensory inputs. However, they still retained motor learning ability and the ability to modulate motor outputs. The medication worsened the SMI deficits as PD patients, after medication, performed worse than before medication when encountering dynamic sensory environments and exhibited impaired motor learning ability.

Reaching and Grasping Movements in Parkinson's Disease: A Review

Parkinson's disease (PD) is known to affect the brain motor circuits involving the basal ganglia (BG) and to induce, among other signs, general slowness and paucity of movements. In upper limb movements, PD patients show a systematic prolongation of movement duration while maintaining a sufficient level of endpoint accuracy. PD appears to cause impairments not only in movement execution, but also in movement initiation and planning, as revealed by abnormal preparatory activity of motor-related brain areas. Grasping movement is affected as well, particularly in the coordination of the hand aperture with the transport phase. In the last fifty years, numerous behavioral studies attempted to clarify the mechanisms underlying these anomalies, speculating on the plausible role that the BG-thalamo-cortical circuitry may play in normal and pathological motor control. Still, many questions remain open, especially concerning the management of the speed-accuracy tradeoff and the online feedback control. In this review, we summarize the literature results on reaching and grasping in parkinsonian patients. We analyze the relevant hypotheses on the origins of dysfunction, by focusing on the motor control aspects involved in the different movement phases and the corresponding role played by the BG. We conclude with an insight into the innovative stimulation techniques and computational models recently proposed, which might be helpful in further clarifying the mechanisms through which PD affects reaching and grasping movements.

The effect of limb position on a static knee extension task can be explained with a simple spinal cord circuit model

The influence of proprioceptive feedback on muscle activity during isometric tasks is the subject of conflicting studies. We performed an isometric knee extension task experiment based on two common clinical tests for mobility and flexibility. The task was carried out at four preset angles of the knee, and we recorded from five muscles for two different hip positions. We applied muscle synergy analysis using nonnegative matrix factorization on surface electromyograph recordings to identify patterns in the data that changed with internal knee angle, suggesting a link between proprioception and muscle activity. We hypothesized that such patterns arise from the way proprioceptive and cortical signals are integrated in neural circuits of the spinal cord. Using the MIIND neural simulation platform, we developed a computational model based on current understanding of spinal circuits with an adjustable afferent input. The model produces the same synergy trends as observed in the data, driven by changes in the afferent input. To match the activation patterns from each knee angle and position of the experiment, the model predicts the need for three distinct inputs: two to control a nonlinear bias toward the extensors and against the flexors, and a further input to control additional inhibition of rectus femoris. The results show that proprioception may be involved in modulating muscle synergies encoded in cortical or spinal neural circuits.NEW & NOTEWORTHY The role of sensory feedback in motor control when limbs are held in a fixed position is disputed. We performed a novel experiment involving fixed position tasks based on two common clinical tests. We identified patterns of muscle activity during the tasks that changed with different leg positions and then inferred how sensory feedback might influence the observations. We developed a computational model that required three distinct inputs to reproduce the activity patterns observed experimentally. The model provides a neural explanation for how the activity patterns can be changed by sensory feedback.

Proprioceptive recalibration following implicit visuomotor adaptation is preserved in Parkinson's disease

Individuals with Parkinson's disease (PD) and healthy adults demonstrate similar levels of visuomotor adaptation provided that the distortion is small or introduced gradually, and hence, implicit processes are engaged. Recently, implicit processes underlying visuomotor adaptation in healthy individuals have been proposed to include proprioceptive recalibration (i.e., shifts in one's proprioceptive sense of felt hand position to match the visual estimate of their hand experienced during reaches with altered visual feedback of the hand). In the current study, we asked if proprioceptive recalibration is preserved in PD patients. PD patients tested during their "off" and "on" medication states and age-matched healthy controls reached to visual targets, while visual feedback of their unseen hand was gradually rotated 30° clockwise or translated 4 cm rightwards of their actual hand trajectory. As expected, PD patients and controls produced significant reach aftereffects, indicating visuomotor adaptation after reaching with the gradually introduced visuomotor distortions. More importantly, following visuomotor adaptation, both patients and controls showed recalibration in hand position estimates, and the magnitude of this recalibration was comparable between PD patients and controls. No differences for any measures assessed were observed across medication status (i.e., PD off vs PD on). Results reveal that patients are able to adjust their sensorimotor mappings and recalibrate proprioception following adaptation to a gradually introduced visuomotor distortion, and that dopaminergic intervention does not affect this proprioceptive recalibration. These results suggest that proprioceptive recalibration does not involve striatal dopaminergic pathways and may contribute to the preserved visuomotor adaptation that arises implicitly in PD patients.

Quantitative Sensory Testing (QST) in Drug-Naïve Patients with Parkinson's Disease

BACKGROUND

Pain is highly prevalent in patients with Parkinson's disease (PD), but underlying pathophysiological mechanisms are largely unclear. Alterations in somatosensory processing might contribute to sensory abnormalities in PD.

OBJECTIVE

This study investigated sensory processing in PD patients.

METHODS

We used the standardized "Quantitative Sensory Testing" (QST) protocol (German Research Network on Neuropathic Pain) to investigate 13 somatosensory parameters in 19 PD patients naïve to dopaminergic medication and 19 healthy controls matched for age, gender, and handedness. We tested for differences in sensory parameters between i) drug-naïve PD patients and healthy controls, ii) patients' more and less affected body side, and iii) for an association of somatosensory parameters with disease-specific factors.

RESULTS

We did not observe any significant group differences in somatosensory parameters between PD patients and healthy subjects. In PD patients, QST mean z-scores did not differ between the predominantly and the less affected body side, PD patients with and without PD-specific chronic pain or between different PD subtypes. Age, but not PD disease severity, was associated with a greater loss of function in thermal and mechanical detection thresholds.

CONCLUSIONS

Somatosensory processing, as assessed with the well-established QST protocol, was normal in drug-naïve PD patients. Thus, somatosensory abnormalities previously reported in medicated PD patients might rather be a result of dopaminergic medication, or may occur later in the course of the disease or with increasing age.

Mental representation of the body in action in Parkinson's disease

Mixed findings characterize studies in Parkinson's disease (PD): some studies indicate a relationship between physical impairments and the ability to mentally represent the body, while others suggest spared abilities for this cognitive function. To clarify the matter, in the present study we explored the mental representations of the body in action in the same PD patients, taking also into account lateralization of symptoms and visual imagery skills. 10 PD patients with left- (lPD), 10 with right (rPD) lateralized symptoms (lPD), and 20 matched healthy controls have been recruited for the study. All patients were screened for neuropsychological impairments. To explore a more implicit component we used the hand laterality task (HLT), while the mental motor chronometry (MMC) was used to explore a more explicit one. Two control tasks, with objects instead of body parts, were administered to control for visual imagery skills. In the HLT, we detected the effects of biomechanical constraints effects in both controls and PD patients. In the latter group, importantly, this was true independently from lateralization of symptoms. In the MMC, we found the expected positive correlation between executed and imagined movements for both hands in controls only, while all PD patients, again independently form lateralization, only showed this effect for the left hand. In terms of visual imagery, only rPD patients differed from controls when asked to implicitly rotate letters, and in terms of accuracy only. However, this difference is explained by executive functions measured through the neuropsychological assessment rather than by a "pure" visual imagery impairment. In summary, our findings suggest that two different aspects of the mental representations of the body in action, one more implicit and the other more explicit, can be differently affected by PD. These impairments are unlikely explained by a basic visual imagery deficit. When present, impairments concern a higher dimension, related to motor functions and awareness, and not driven by sensory impairments, as shown by the independence of effects from physical laterality of symptoms.

Sensory Re-weighting for Postural Control in Parkinson's Disease

Postural instability in Parkinson's disease (PD) is characterized by impaired postural responses to transient perturbations, increased postural sway in stance and difficulty transitioning between tasks. In addition, some studies suggest that loss of dopamine in the basal ganglia due to PD results in difficulty in using proprioceptive information for motor control. Here, we quantify the ability of subjects with PD and age-matched control subjects to use and re-weight sensory information for postural control during steady-state conditions of continuous rotations of the stance surface or visual surround. We measure the postural sway of subjects in response to a pseudorandom, surface-tilt stimulus with eyes closed, and in response to a pseudorandom, visual-tilt stimulus. We use a feedback control model of the postural control system to interpret our results, focusing on sensory weighting as a function of stimulus amplitude. We find that subjects with PD can re-weight their dependence upon sensory information in response to changes in surface- or visual-stimulus amplitude. Specifically, subjects with PD behaved like age-matched control subjects by decreasing proprioceptive contribution to stance control with increasing surface-tilt amplitude and decreasing visual contribution with increasing visual-tilt amplitude. However, subjects with PD do not decrease their reliance on proprioception as much as age-matched controls for small increases in surface-stimulus amplitudes. Levodopa medication did not affect sensory re-weighting behaviors for postural control. The impairment in PD subject's ability to respond differently to small changes in surface rotation amplitudes is consistent with an increased threshold for perceiving proprioceptive signals, which may result from decreased signal-to-noise in the dopaminergic pathways associated with sensory processing and/or sensory integration.

Attenuated beta rebound to proprioceptive afferent feedback in Parkinson's disease

Motor symptoms are defining traits in the diagnosis of Parkinson’s disease (PD). A crucial component in motor function is the integration of afferent proprioceptive sensory feedback. Previous studies have indicated abnormal movement-related cortical oscillatory activity in PD, but the role of the proprioceptive afference on abnormal oscillatory activity in PD has not been elucidated. We examine the cortical oscillations in the mu/beta-band (8–30 Hz) in the processing of proprioceptive stimulation in PD patients, ON/OFF levodopa medication, as compared to that of healthy controls (HC). We used a proprioceptive stimulator that generated precisely controlled passive movements of the index finger and measured the induced cortical oscillatory responses following the proprioceptive stimulation using magnetoencephalography. Both PD patients and HC showed a typical beta-band desynchronization during the passive movement. However, the subsequent beta rebound after the passive movement that was almost absent in PD patients compared to HC. Furthermore, we found no difference in the degree of beta rebound attenuation between patients ON and OFF levodopa medication. The results demonstrate a disease-related deterioration in cortical processing of proprioceptive afference in PD.

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.

Abnormal Temporal Coupling of Tactile Perception and Motor Action in Parkinson's Disease

Evidence shows altered somatosensory temporal discrimination threshold (STDT) in Parkinson’s disease in comparison to normal subjects. In healthy subjects, movement execution modulates STDT values through mechanisms of sensory gating. We investigated whether STDT modulation during movement execution in patients with Parkinson’s disease differs from that in healthy subjects. In 24 patients with Parkinson’s disease and 20 healthy subjects, we tested STDT at baseline and during index finger abductions (at movement onset “0”, 100, and 200 ms thereafter). We also recorded kinematic features of index finger abductions. Fifteen out of the 24 patients were also tested ON medication. In healthy subjects, STDT increased significantly at 0, 100, and 200 ms after movement onset, whereas in patients with Parkinson’s disease in OFF therapy, it increased significantly at 0 and 100 ms but returned to baseline values at 200 ms. When patients were tested ON therapy, STDT during index finger abductions increased significantly, with a time course similar to that of healthy subjects. Differently from healthy subjects, in patients with Parkinson’s disease, the mean velocity of the finger abductions decreased according to the time lapse between movement onset and the delivery of the paired electrical stimuli for testing somatosensory temporal discrimination. In conclusion, patients with Parkinson’s disease show abnormalities in the temporal coupling between tactile information and motor outflow. Our study provides first evidence that altered temporal processing of sensory information play a role in the pathophysiology of motor symptoms in Parkinson’s disease.

The Effect of Dopaminergic Medication on Joint Kinematics during Haptic Movements in Individuals with Parkinson's Disease

This study examined whether altered joint angular motion during haptic exploration could account for a decline in haptic sensitivity in individuals with PD by analyzing joint position data during haptic exploration of a curved contour. Each participant's hand was passively moved by a robotic arm along the edges of a virtual box (5 cm × 15 cm) with a curved left wall. After each trial, participants indicated whether the contour was curved or straight. Visual, auditory, and tactile cues were occluded, and an electrogoniometer recorded shoulder and elbow joint angles during each trial. The PD group in the OFF state had a higher mean detection threshold (4.67 m⁻¹) than the control group (3.06 m⁻¹). Individuals with PD in the OFF state also had a significantly greater magnitude of shoulder abduction than those in the ON state (p = 0.003) and a smaller magnitude of elbow flexion than those in the ON state or compared to the control group (both p < 0.001). These findings suggest that individuals with PD employ joint configurations that may contribute to haptic insensitivity. Dopamine replacement therapy improved joint configurations during haptic exploration in patients with PD, suggesting a role for dopaminergic dysfunction in PD-related haptic insensitivity.

Evaluation of Knee Proprioception and Factors Related to Parkinson's Disease

Background. Changes in proprioception may contribute to postural instability in individuals with neurological disorders. Objectives. Evaluate proprioception in the lower limbs of patients with Parkinson's disease (PD) and the association between proprioception and cognitive ability, motor symptoms, postural instability, and disease severity. Methods. This is a cross-sectional, controlled study that evaluated proprioception in PD patients and healthy age- and sex-matched individuals. Kinetic postural proprioception of the knee was evaluated using an isokinetic dynamometer (Biodex® Multi-Joint System 4 Pro). Participants were evaluated using the Montreal Cognitive Assessment (MoCA), the Hoehn and Yahr rating scale and postural instability (pull test and stabilometric analysis), and motor function (UPDRS-III) tests. Results. A total of 40 individuals were enrolled in the study: 20 PD patients and 20 healthy controls (CG). The PD patients had higher angular errors on the proprioceptive ratings than the CG participants (p = 0.002). Oscillations of the center of pressure (p = 0.002) were higher in individuals with PD than in the controls. Proprioceptive errors in the PD patients were associated with the presence of tremors as the dominant symptom and more impaired motor performance. Conclusion. These findings show that individuals with PD have proprioceptive deficits, which are related to decreased cognitive ability and impaired motor symptoms.

Impact of limiting visual input on gait: Individuals with Parkinson disease, age-matched controls, and healthy young participants

Normal and limited vision gait was investigated in individuals with Parkinson disease (PD), healthy older and healthy young individuals. Participants walked a GAITRite mat with normal vision or vision of lower limbs occluded. Results indicate individuals with PD walked more slowly, with shorter and wider steps, and spent more time in double support with limited vision as compared to full vision. Healthy young and old individuals took shorter steps but were otherwise unchanged between conditions.

Wrist proprioceptive acuity: A comprehensive robot-aided assessment

Proprioception is the sense of the body awareness. Proprioceptive deficits represent frequent consequences of several neurological conditions like stroke, Parkinson's disease and others. The assessment of such somatosensory function is crucial, although the available clinical tests are not sensitive enough. The human wrist is a crucial joint for many activities of daily living and to address the lack of its characterization in terms of proprioceptive acuity the authors in previous studies proposed a novel method that combined the use of a 3-DoF robot and a threshold haunting paradigm. Further experiments were performed to characterize the proprioceptive acuity of the dominant wrist for adduction, extension, pronation and supination by using a 2-alternative-forced-choice test. The acuity thresholds obtained from six subjects (mean values ± standard deviation of 1.65±0.39 for extension, 1.13±0.34 for adduction, 1.90±0.58 for pronation and 1.70±0.30 for supination) were finally combined with the ones harvested in the previous studies for flexion and abduction in order to build the first comprehensive database of human wrist proprioceptive acuity.

The effects of subthalamic deep brain stimulation on mechanical and thermal thresholds in 6OHDA-lesioned rats

Chronic pain is a major complaint for up to 85% of Parkinson's disease patients; however, it often not identified as a symptom of Parkinson's disease. Adequate treatment of motor symptoms often provides analgesic effects in Parkinson's patients but how this occurs remains unclear. Studies have shown both Parkinson's patients and 6-hydroxydopamine-lesioned rats exhibit decreased sensory thresholds. In humans, some show improvements in these deficits after subthalamic deep brain stimulation, while others report no change. Differing methods of testing and response criteria may explain these varying results. We examined this effect in 6-hydroxydopamine-lesioned rats. Sprague-Dawley rats were unilaterally implanted with subthalamic stimulating electrodes in the lesioned right hemisphere and sensory thresholds were tested using von Frey, tail-flick and hot-plate tests. Tests were done during and off subthalamic stimulation at 50 and 150 Hz to assess its effects on sensory thresholds. The 6-hydroxydopamine-lesioned animals exhibited lower mechanical (left paw, P < 0.01) and thermal thresholds than shams (hot plate, P < 0.05). Both 50 and 150 Hz increased mechanical (left paw; P < 0.01) and thermal thresholds in 6-hydroxydopamine-lesioned rats (hot-plate test: 150 Hz, P < 0.05, 50 Hz, P < 0.01). Interestingly, during von Frey testing, low-frequency stimulation provided a more robust improvement in some 6OHDA lesioned rats, while in others, the magnitude of improvement on high-frequency stimulation was greater. This study shows that subthalamic deep brain stimulation improves mechanical allodynia and thermal hyperalgesia in 6-hydroxydopamine-lesioned animals at both high and low frequencies. Furthermore, we suggest considering using low-frequency stimulation when treating Parkinson's patients where pain remains the predominant complaint.

Impact of Parkinson's disease on proprioceptively based on-line movement control

Evidence suggests that Parkinson's disease (PD) patients produce large spatial errors when reaching to proprioceptively defined targets. Here, we examined whether these movement inaccuracies result mainly from impaired use of proprioceptive inputs for movement planning mechanisms or from on-line movement guidance. Medicated and non-medicated PD patients and healthy controls performed three-dimensional reaching movements in four sensorimotor conditions that increase proprioceptive processing requirements. We assessed the influence of these sensorimotor conditions on the final accuracy and initial kinematics of the movements. If the patterns of final errors are primarily determined by planning processes before the initiation of the movement, the initial kinematics of reaching movements should show similar trends and predict the pattern of final errors. Medicated and non-medicated PD patients showed a greater mean level of final 3D errors than healthy controls when proprioception was the sole source of information guiding the movement, but this difference reached significance only for medicated PD patients. However, the pattern of initial kinematics and final spatial errors were markedly different both between sensorimotor conditions and between groups. Furthermore, medicated and non-medicated PD patients were less efficient than healthy controls in compensating for their initial spatial errors (hand distance from target location at peak velocity) when aiming at proprioceptively defined compared to visually defined targets. Considered together, the results are consistent with a selective deficit in proprioceptively based movement guidance in PD. Furthermore, dopaminergic medication did not improve proprioceptively guided movements in PD patients, indicating that dopaminergic dysfunction within the basal ganglia is not solely responsible for these deficits.

Robot-aided assessment of wrist proprioception

INTRODUCTION

Impaired proprioception severely affects the control of gross and fine motor function. However, clinical assessment of proprioceptive deficits and its impact on motor function has been difficult to elucidate. Recent advances in haptic robotic interfaces designed for sensorimotor rehabilitation enabled the use of such devices for the assessment of proprioceptive function.

PURPOSE

This study evaluated the feasibility of a wrist robot system to determine proprioceptive discrimination thresholds for two different DoFs of the wrist. Specifically, we sought to accomplish three aims: first, to establish data validity; second, to show that the system is sensitive to detect small differences in acuity; third, to establish test-retest reliability over repeated testing.

METHODOLOGY

Eleven healthy adult subjects experienced two passive wrist movements and had to verbally indicate which movement had the larger amplitude. Based on a subject's response data, a psychometric function was fitted and the wrist acuity threshold was established at the 75% correct response level. A subset of five subjects repeated the experimentation three times (T1, T2, and T3) to determine the test-retest reliability.

RESULTS

Mean threshold for wrist flexion was 2.15°± 0.43° and 1.52°± 0.36° for abduction. Encoder resolutions were 0.0075°(flexion-extension) and 0.0032°(abduction-adduction). Motor resolutions were 0.2°(flexion-extension) and 0.3°(abduction-adduction). Reliability coefficients were r T2-T1 = 0.986 and r T3-T2 = 0.971.

CONCLUSION

We currently lack established norm data on the proprioceptive acuity of the wrist to establish direct validity. However, the magnitude of our reported thresholds is physiological, plausible, and well in line with available threshold data obtained at the elbow joint. Moreover, system has high resolution and is sensitive enough to detect small differences in acuity. Finally, the system produces reliable data over repeated testing.

The effectiveness of proprioceptive training for improving motor function: a systematic review

OBJECTIVE

Numerous reports advocate that training of the proprioceptive sense is a viable behavioral therapy for improving impaired motor function. However, there is little agreement of what constitutes proprioceptive training and how effective it is. We therefore conducted a comprehensive, systematic review of the available literature in order to provide clarity to the notion of training the proprioceptive system.

METHODS

Four major scientific databases were searched. The following criteria were subsequently applied: (1) A quantified pre- and post-treatment measure of proprioceptive function. (2) An intervention or training program believed to influence or enhance proprioceptive function. (3) Contained at least one form of treatment or outcome measure that is indicative of somatosensory function. From a total of 1284 articles, 51 studies fulfilled all criteria and were selected for further review.

RESULTS

Overall, proprioceptive training resulted in an average improvement of 52% across all outcome measures. Applying muscle vibration above 30 Hz for longer durations (i.e., min vs. s) induced outcome improvements of up to 60%. Joint position and target reaching training consistently enhanced joint position sense (up to 109%) showing an average improvement of 48%. Cortical stroke was the most studied disease entity but no clear evidence indicated that proprioceptive training is differentially beneficial across the reported diseases.

CONCLUSIONS

There is converging evidence that proprioceptive training can yield meaningful improvements in somatosensory and sensorimotor function. However, there is a clear need for further work. Those forms of training utilizing both passive and active movements with and without visual feedback tended to be most beneficial. There is also initial evidence suggesting that proprioceptive training induces cortical reorganization, reinforcing the notion that proprioceptive training is a viable method for improving sensorimotor function.

Visuomotor adaptation in Parkinson's disease: effects of perturbation type and medication state

To perform simple everyday tasks, we use visual feedback from our external environment to generate and guide movements. However, tasks like reaching for a cup may become extremely difficult in movement disorders such as Parkinson's disease (PD), and it is unknown whether PD patients use visual information to compensate for motor deficiencies. We tested adaptation to changes in visual feedback of the hand in three subject groups, PD patients on daily levodopa (l-dopa) therapy (PD ON), PD patients off l-dopa (PD OFF), and age-matched control subjects, to determine the effects of PD on the visual control of movement. Subjects were tested on two classes of visual perturbations, one that altered visual direction of movement and one that altered visual extent of movement, allowing us to test adaptive sensitivity to changes in both movement direction (visual rotations) and extent (visual gain). The PD OFF group displayed more complete adaptation to visuomotor rotations compared with control subjects but initial, transient difficulty with adaptation to visual gain perturbations. The PD ON group displayed feedback control more sensitive to visual error compared with control subjects but compared with the PD OFF group had mild impairments during adaptation to changes in visual extent. We conclude that PD subjects can adapt to changes in visual information but that l-dopa may impair visual-based motor adaptation.

Assessing proprioceptive function: evaluating joint position matching methods against psychophysical thresholds

BACKGROUND

The importance of assessing proprioceptive function for rehabilitation after neurological or orthopedic injury has long been recognized. Yet, neither the validity nor the accuracy of the available tests is firmly established. Testing typically involves repeated matching of a given joint position with the same or opposite limb where the difference between the 2 positions indicates proprioceptive acuity.

OBJECTIVES

The aim of this study was to compare position sense acuity between ipsilateral and contralateral matching methods against a psychophysical threshold method to establish the accuracy and relationships between these models.

DESIGN

A repeated-measures design was used.

METHOD

Assessment of forearm position sense for a 10-degree reference position in 27 young adults who were healthy.

RESULTS

Psychophysical thresholds were revealed to be the most precise and least variable acuity measure. The mean (±SD) threshold (1.05°±0.47°) was significantly lower than mean position errors obtained by both joint position matching tasks (ipsilateral: 1.51°±0.64°; contralateral: 1.84°±0.73°)-a 44% to 75% difference in measurement accuracy. Individual participant position errors correlated poorly with respective thresholds, indicating a lack of concurrent validity. Position errors for both matching methods correlated only mildly with each other.

LIMITATIONS

The data represent performance of a healthy, young adult cohort. Differences between methods will likely be more pronounced in aging and clinical populations.

CONCLUSIONS

Threshold testing and joint position matching methods examine different physiological aspects of proprioceptive function. Because threshold testing is based on passive motion, it most closely reflects afferent sensory feedback processing (ie, proprioception). Matching methods require active motion and are consequently influenced by additional sensorimotor processes. Factors such as working memory and transmission between brain hemispheres also influence joint matching task outcomes.

Age-related differences in proprioceptive and visuo-proprioceptive function in relation to fine motor behaviour

Leversen et al. (PLoS One 7(6):e38830, 2012) emphasise the importance of understanding the principles of life-long development. In their study of motor control, they found a common tendency towards improved motor performance from childhood to adulthood and a subsequent deterioration. The aim of our study was to examine this issue further by investigating fine motor behaviour (tracing a model line) in 196 participants (age range 12-95 years old) in two sensory conditions-proprioceptive + visual (PV) and proprioceptive only-in both hands and in two types of movement, frontal and transversal. Regression analyses of line length and task performance speed in relation to age were conducted for the different test conditions. The best performance was found in middle age, and a quadratic function provided the best fit for most of the test conditions. The corresponding inflection points (the age at which graphical analysis showed a change in performance as a peak of maturation before decline due to ageing) showed earlier ages in the proprioceptive condition. For most types of movement analysed, performance speed was slower under the PV condition. Paired correlation analysis showed that the symmetry of precision performance between hands became stronger with age. The results provide information on age-dependent differences in proprioception based on fine motor performance. They may be of use in the design of preventive strategies for preserving proprioceptive function by reducing the risk of falls and accidents or diseases such as Parkinson's.

Sensory aspects of movement disorders

Movement disorders, which include disorders such as Parkinson's disease, dystonia, Tourette's syndrome, restless legs syndrome, and akathisia, have traditionally been considered to be disorders of impaired motor control resulting predominantly from dysfunction of the basal ganglia. This notion has been revised largely because of increasing recognition of associated behavioural, psychiatric, autonomic, and other non-motor symptoms. The sensory aspects of movement disorders include intrinsic sensory abnormalities and the effects of external sensory input on the underlying motor abnormality. The basal ganglia, cerebellum, thalamus, and their connections, coupled with altered sensory input, seem to play a key part in abnormal sensorimotor integration. However, more investigation into the phenomenology and physiological basis of sensory abnormalities, and about the role of the basal ganglia, cerebellum, and related structures in somatosensory processing, and its effect on motor control, is needed.

Reaching to proprioceptively defined targets in Parkinson's disease: effects of deep brain stimulation therapy

Deep brain stimulation of the subthalamic nucleus (STN DBS) provides a unique window into human brain function since it can reversibly alter the functioning of specific brain circuits. Basal ganglia-cortical circuits are thought to be excessively noisy in patients with Parkinson's disease (PD), based in part on the lack of specificity of proprioceptive signals in basal ganglia-thalamic-cortical circuits in monkey models of the disease. PD patients are known to have deficits in proprioception, but the effects are often subtle, with paradigms typically restricted to one or two joint movements in a plane. Moreover, the effects of STN DBS on proprioception are virtually unexplored. We tested the following hypotheses: first, that PD patients will show substantial deficits in unconstrained, multi-joint proprioception, and, second, that STN DBS will improve multi-joint proprioception. Twelve PD patients with bilaterally implanted electrodes in the subthalamic nucleus and 12 age-matched healthy subjects were asked to position the left hand at a location that was proprioceptively defined in 3D space with the right hand. In a second condition, subjects were provided visual feedback during the task so that they were not forced to rely on proprioception. Overall, with STN DBS switched off, PD patients showed significantly larger proprioceptive localization errors, and greater variability in endpoint localizations than the control subjects. Visual feedback partially normalized PD performance, and demonstrated that the errors in proprioceptive localization were not simply due to a difficulty in executing the movements or in remembering target locations. Switching STN DBS on significantly reduced localization errors from those of control subjects when patients moved without visual feedback relative to when they moved with visual feedback (when proprioception was not required). However, this reduction in localization errors without vision came at the cost of increased localization variability.

Is DOPA-Responsive Hypokinesia Responsible for Bimanual Coordination Deficits in Parkinson's Disease?

Bradykinesia is a well-documented DOPA-responsive clinical feature of Parkinson’s disease (PD). While amplitude deficits (hypokinesia) are a key component of this slowness, it is important to consider how dopamine influences both the amplitude (hypokinesia) and frequency components of bradykinesia when a bimanually coordinated movement is required. Based on the notion that the basal ganglia are associated with sensory deficits, the influence of dopaminergic replacement on sensory feedback conditions during bimanual coordination was also evaluated. Bimanual movements were examined in PD and healthy comparisons in an unconstrained three-dimensional coordination task. PD were tested “off” (overnight withdrawal of dopaminergic treatment) and “on” (peak dose of dopaminergic treatment), while the healthy group was evaluated for practice effects across two sessions. Required cycle frequency (increased within each trial from 0.75 to 2 Hz), type of visual feedback (no vision, normal vision, and augmented vision), and coordination pattern (symmetrical in-phase and non-symmetrical anti-phase) were all manipulated. Overall, coordination (mean accuracy and standard deviation of relative phase) and amplitude deficits during bimanual coordination were confirmed in PD participants. In addition, significant correlations were identified between severity of motor symptoms as well as bradykinesia to greater coordination deficits (accuracy and stability) in PD “off” group. However, even though amplitude deficits (hypokinesia) improved with dopaminergic replacement, it did not improve bimanual coordination performance (accuracy or stability) in PD patients from “off” to “on.” Interestingly, while coordination performance in both groups suffered in the augmented vision condition, the amplitude of the more affected limb of PD was notably influenced. It can be concluded that DOPA-responsive hypokinesia contributes to, but is not directly responsible for bimanual coordination impairments in PD. It is likely that bimanual coordination deficits in PD are caused by the combination of dopaminergic system dysfunction as well as other neural impairments that may be DOPA-resistant or related to non-dopaminergic pathways.

Perceptual errors increase with movement duration and may contribute to hypokinesia in Parkinson's disease

People with Parkinson's disease (PD) perceive that their movement amplitude is greater than what they actually perform. The neural mechanisms underlying one's perception of movement are believed to involve the sensorimotor integration process (SIP). How PD affects the SIP is not well understood. A previous study interrogating the SIP showed healthy adults (HAs) overestimated their limb position in the direction of movement and the error and its variance (VOE) depended on movement duration. We asked if PDs showed errors in perceived limb position and if the dependence on movement duration was different from HAs. We used an existing computational model of the SIP to explore mechanisms for the error and VOE as a function of movement duration. Twenty PDs, off medication, and 20 age-matched HAs were asked to estimate the position of their hand after performing 50, slow, non-visually guided wrist flexion or extension movements for a random period of time (<4.0 s). Both groups overestimated the amount they moved; however, the PDs' error and VOE were larger (p<0.001). HAs showed increasing error/VOE for small movement durations that reduced/stabilized for longer movement durations. PDs however showed increasing error/VOE with increasing movement duration that did not significantly improve/stabilize. The model suggested that the basis for such perceptual deficits may be abnormal proprioceptive feedback and/or processing of an abnormal internal impression (prediction) that underestimates movement amplitude. Simulation results imply that the PD's SIP could no longer effectively access sensory (proprioceptive) feedback to correct errors in other components of the SIP due to the abnormal processing of sensory feedback. We suggest from this study that an impaired perception of movement amplitude and sensory processing deficits contribute to hypokinesia in PD.

Dissociating the cognitive effects of levodopa versus dopamine agonists in a neurocomputational model of learning in Parkinson's disease

BACKGROUND/AIMS

Levodopa and dopamine agonists have different effects on the motor, cognitive, and psychiatric aspects of Parkinson's disease (PD).

METHODS

Using a computational model of basal ganglia (BG) and prefrontal cortex (PFC) dopamine, we provide a theoretical synthesis of the dissociable effects of these dopaminergic medications on brain and cognition. Our model incorporates the findings that levodopa is converted by dopamine cells into dopamine, and thus activates prefrontal and striatal D(1) and D(2) dopamine receptors, whereas antiparkinsonian dopamine agonists directly stimulate D(2) receptors in the BG and PFC (although some have weak affinity to D(1) receptors).

RESULTS

In agreement with prior neuropsychological studies, our model explains how levodopa enhances, but dopamine agonists impair or have no effect on, stimulus-response learning and working memory.

CONCLUSION

Our model explains how levodopa and dopamine agonists have differential effects on motor and cognitive processes in PD.

Preserved spatial memory for reaching to remembered three-dimensional targets in aging

BACKGROUND/STUDY CONTEXT: Compared with the large literature on the impact of aging on spatial memory span, far fewer studies have examined the influence of aging on spatial memory processes required to reach a remembered target. This study assessed the ability of seniors to accurately reach to three-dimensional (3D) memorized targets in four conditions in which the memory delay and the attentional demands varied.

METHODS

The accuracy and variability of reaching movements (3D absolute, 3D variable, and spatial component errors) were analyzed to evaluate the performance of 12 young adults aged 20 to 30 and 12 older adults aged 62 to 69 in the different delay conditions (short passive delay, long passive delay, long cognitive delay, and long spatial delay). Variance analyses were applied on each error measure as well as on kinematic features of the movements (movement time, deceleration time, and peak velocity).

RESULTS

Results revealed that older participants were as capable as their younger counterpart to maintain target location in memory regardless of task complexity.

CONCLUSION

Although memory deficits have been found in older adults in several previous studies, the current results support the idea that healthy aging does not produce a breakdown in all memory tasks. Hence, a specific spatial memory channel seems to remain unaffected in normal aging.

Dopamine alters tactile perception in Parkinson's disease

BACKGROUND

Abnormal somatosensory processing may contribute to motor impairments observed in Parkinson's disease (PD). Dopaminergic medications have been shown to alter somatosensory processing such that tactile perception is improved. In PD, it remains unclear whether the temporal sequencing of tactile stimuli is altered and if dopaminergic medications alter this perception.

METHODS

Somatosensory tactile perception was investigated using temporal order judgment in patients with Parkinson's disease on and off dopaminergic medications and in aged-matched healthy controls. Measures of temporal order judgment were acquired using computer controlled stimulation to digits 2 and 3 on the right hand and subjects were required to determine which stimuli occurred first. Two experimental tasks were compared, temporal order judgment without and with synchronization whereby digits 2 and 3 were vibrated synchronously in advance of the temporal order judgment sequence of stimuli.

RESULTS

Temporal order judgment in PD patients of and on medications were similar to controls. Temporal order judgment preceded by synchronous vibration impaired tactical acuity in controls and in PD patients off medications to similar degrees, but this perceptual impairment by synchronous vibration was not present in PD patients on medications.

CONCLUSIONS

These findings suggest that dopamine in PD reduces cortico-cortical connectivity with SI and this leads to changes in tactical sensitivity.

Evaluating dopaminergic system contributions to cued pattern switching during bimanual coordination

Switching between different coordinated movements has been shown to be slow, with delayed responses and even freezing deficits in individuals with Parkinson's disease (PD). While it is well accepted that the dopaminergic system responds to dopamine replacement to ameliorate overall slowness (bradykinesia) and other motor symptoms of PD, it is unknown whether the dopaminergic system can influence overall coordination between limbs and if this may be impacted by the availability of sensory feedback. In the current study, PD and healthy age-matched control participants performed a rhythmic coordination task that required a cued voluntary switch between movement patterns (in-phase and anti-phase). PD participants performed the task first after overnight withdrawal ('off'), and subsequently after administration ('on') of dopamine replacement. Coordinated movements were performed while paced by an auditory metronome in two sensory conditions: 'no vision' or 'normal vision'. Measures of voluntary switch time and delayed responses revealed that PD 'off' required significantly more time than healthy participants to switch between movement patterns. Interestingly, PD 'off' demonstrated disrupted coordination, as revealed by mean (accuracy) and standard deviation (stability) of absolute error of relative phase. Dopamine replacement improved the time needed to switch and amount of delayed responses in PD participants, but had no influence on coordination itself. It is concluded that although modulation of the dopaminergic system improves the slowness during switching, coordination deficits may be the result of secondary impairments (possibly attention-related) that cannot be improved with dopamine replacement.

Multisensory determinants of orientation perception in Parkinson's disease

Perception of the relative orientation of the self and objects in the environment requires integration of visual and vestibular sensory information, and an internal representation of the body's orientation. Parkinson's disease (PD) patients are more visually dependent than controls, implicating the basal ganglia in using visual orientation cues. We examined the relative roles of visual and non-visual cues to orientation in PD using two different measures: the subjective visual vertical (SVV) and the perceptual upright (PU). We tested twelve PD patients (nine both on- and off-medication), and thirteen age-matched controls. Visual, vestibular and body cues were manipulated using a polarized visual room presented in various orientations while observers were upright or lying right-side-down. Relative to age-matched controls, patients with PD showed more influence of visual cues for the SVV but were more influenced by the direction of gravity for the PU. Increased SVV visual dependence corresponded with equal decreases of the contributions of body sense and gravity. Increased PU gravitational dependence corresponded mainly with a decreased contribution of body sense. Curiously however, both of these effects were significant only when patients were medicated. Increased SVV visual dependence was highest for PD patients with left-side initial motor symptoms. PD patients when on and off medication were more variable than controls when making judgments. Our results suggest that (i) PD patients are not more visually dependent in general, rather increased visual dependence is task specific and varies with initial onset side, (ii) PD patients may rely more on vestibular information for some perceptual tasks which is reflected in relying less on the internal representation of the body, and (iii) these effects are only present when PD patients are taking dopaminergic medication.

Tactile/proprioceptive integration during arm localization is intact in individuals with Parkinson's disease

It has been theorized that sensorimotor processing deficits underlie Parkinson's disease (PD) motor impairments including movement under proprioceptive control. However, it is possible that these sensorimotor processing deficits exclude tactile/proprioception sensorimotor integration: prior studies show improved movement accuracy in PD with endpoint tactile feedback, and good control in tactile-driven precision-grip tasks. To determine whether tactile/proprioceptive integration in particular is affected by PD, nine subjects with PD (off-medication, UPDRS motor=19-42) performed an arm-matching task without visual feedback. In some trials one arm touched a static tactile cue that conflicted with dynamic proprioceptive feedback from biceps brachii muscle vibration. This sensory conflict paradigm has characterized tactile/proprioceptive integration in healthy subjects as specific to the context of tactile cue mobility assumptions and the intention to move the arm. We found that the individuals with PD had poorer arm-matching accuracy than age-matched control subjects. However, PD-group accuracy improved with tactile feedback. Furthermore, sensory conflict conditions were resolved in the same context-dependent fashion by both subject groups. We conclude that the somatosensory integration mechanism for prioritizing tactile and proprioception feedback in this task are not disrupted by PD, and are not related to the observed proprioceptive deficits.