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

Sensory reweighting for balance in people living with Parkinson's Disease: Postural adaptation, muscle co-contraction, and perceptual delays

BACKGROUND

Postural instability is common in people with Parkinson's Disease (PwPD), increasing their risk of injurious falls. Evidence suggests a sensory reweighting deficit in PwPD, along with compensatory muscle co-contraction in response to postural challenges. During balance tasks requiring sensory reweighting, older adults exhibit elevated postural sway and muscle co-contraction, as well as longer perceptual delays, compared to young adults. Such responses may be exacerbated in PwPD, with implications for fall risk.

RESEARCH QUESTION

The aim of this study was to assess postural sway, muscle co-contraction, and perceptual delays in PwPD and healthy age-matched controls during a sensory reweighting balance task.

METHODS

Eleven PwPD and 16 control participants completed a sensory reweighting protocol: standing without vision on a fixed platform (2-min), which then undergoes a period of body sway-referencing (3-min) before returning to its fixed position (2.5-min). Anteroposterior (AP) path length, co-contraction index (CCI), and perceptual delay were analysed across task phases.

RESULTS

PwPD showed a longer delay in perceiving when the body sway-referenced platform returned to a fixed position. This perceptual delay in PwPD (43.40-s) was over double that observed in control participants (21.25-s). AP path length and co-contraction aftereffects were longer in control participants than PwPD.

SIGNIFICANCE

Where conditions require it, PwPD can effectively adjust their reliance on proprioceptive information for postural control. However, the significant delay shown by PwPD in perceiving changes to sensory conditions could be detrimental during everyday sensory transitions, potentially increasing fall risk.

Still Functional but Limited Postural Adaptation for Individuals With Parkinson's Disease in Goal-Directed Visual Tasks

Patients with Parkinson's disease (PD) notably exhibit impairments in posture and visual attention. The objective of the present study was to determine whether PD patients were able to exhibit adaptive postural control in a goal-directed visual task. We hypothesized that the patients would reduce their centre of pressure (COP) movement and/or postural sway to a lesser extent than age-matched controls in the goal-directed visual (search) task, compared with the control free-viewing task (i.e., a lower degree of relative postural adaptation). We also expected the PD patients to sway more than controls in the goal-directed task (i.e., a lower degree of absolute adaptive postural control). The study included 39 PD patients (mean age: 59; mean Hoehn and Yahr stage: 2.1; mean Movement Disorder Society-Unified Parkinson's Disease Rating Scale score: 22; mean Montreal Cognitive Assessment score: 28 (on-drug)) and 40 age-matched adults (mean age: 62 years). The participants gazed at domestic ecological images (visual angle: 100°). Movements of the COP, head, upper back and lower back and variations in pupil dilatation were analysed. As expected, PD patients exhibited greater COP and body sway than controls in both tasks (p < 0.05). Unexpectedly, the difference in COP and/or body sway between the two tasks was greater in PD patients than in controls (p < 0.05). Our results showed that PD patients are able to exhibit adaptive postural control for goal-directed visual tasks. On a practical level and at a more general level, our findings emphasize the likely benefits of rehabilitation with goal-directed tasks requiring a visual attentional focus (walking on footprints on the ground, etc.).

The Effect of Sensory Reweighting on Postural Control and Cortical Activity in Parkinson's Disease: A Pilot Study

Objective

To investigate the effects of sensory reweighting on postural control and cortical activity in individuals with Parkinson's disease (PD) compared to age-matched controls using a virtual reality sensory organization test (VR-SOT).

Design

Cross-sectional pilot study.

Setting

University research laboratory.

Participants

Ten participants with idiopathic Parkinson's disease and 11 age- and sex-matched control participants without neurologic disorders.

Interventions

Not applicable.

Main Outcome Measures

Changes in center of pressure (COP) and electroencephalography (EEG) activity (ie, power) in the alpha band and the theta/beta ratio recorded during the VR-SOT were the main outcome variables.

Results

PD participants exhibited greater COP displacement, particularly in the mediolateral direction across sensory conditions. They also showed increased alpha power when relying on visual inputs and increased theta/beta ratio power when depending on somatosensory inputs.

Conclusion

PD affects sensory reweighting mechanisms involved in postural control, as evidenced by greater COP displacement and altered cortical activity. These findings emphasize the potential of EEG and VR-SOT in understanding and monitoring postural control impairments in PD.

Multisensory and biomechanical influences on postural control in children

Children's ability to maintain balance requires effective integration of multisensory and biomechanical information. The current project examined the interaction between such sensory inputs, manipulating visual input (presence vs. absence), haptic (somatosensory) input (presence vs. absence of contact with a stable or unstable finger support surface), and biomechanical (sensorimotor) input (varying stance widths). Analyses of mean velocity of the center of pressure and the percentage stability gain highlighted the role of varying multisensory inputs in postural control. Developmentally, older children (6-11 years) showed a multisensory integration advantage compared with their younger counterparts (3-5.9 years), with the impact of varying sensory inputs more closely akin to that seen in adults. Subsequent analyses of the impact of anthropometric individual difference parameters (e.g., height, leg length, weight, areas of base of support) revealed a shifting pattern across development. For younger children, these parameters were positively related to postural stability across experimental conditions (i.e., increasing body size was related to increasing postural control). This pattern transitioned for older children, who showed a nonsignificant relation between body size and balance. Interestingly, because adults show a negative relation between anthropometric factors and stability (i.e., increasing body size is related to decreasing postural control), this shift for the older children can be seen as a developmental transition from child-like to adult-like balance control.

The Effect of Sensory Reweighting on Postural Control and Cortical Activity in Parkinson's Disease

Aims

Balance requires the cortical control of visual, somatosensory, and vestibular inputs. The aim of this cross-sectional study was to compare the contributions of each of these systems on postural control and cortical activity using a sensory reweighting approach between participants with Parkinson's disease (PD) and controls.

Methods

Ten participants with PD (age: 72 ± 9; 3 women; Hoehn & Yahr: 2 [1.5 - 2.50]) and 11 controls (age: 70 ± 3; 4 women) completed a sensory organization test in virtual reality (VR-SOT) while cortical activity was being recorded using electroencephalography (EEG). Conditions 1 to 3 were completed on a stable platform; conditions 4 to 6 on a foam. Conditions 1 and 4 were done with eyes open; conditions 2 and 5 in a darkened VR environment; and conditions 3 and 6 in a moving VR environment. Linear mixed models were used to evaluate changes in center of pressure (COP) displacement and EEG alpha and theta/beta ratio power between the two groups across the postural control conditions. Condition 1 was used as reference in all analyses.

Results

Participants with PD showed greater COP displacement than controls in the anteroposterior (AP) direction when relying on vestibular input (condition 5; p<0.0001). The mediolateral (ML) COP sway was greater in PD than in controls when relying on the somatosensory (condition 2; p = 0.03), visual (condition 4; p = 0.002), and vestibular (condition 5; p < 0.0001) systems. Participants with PD exhibited greater alpha power compared to controls when relying on visual input (condition 2; p = 0.003) and greater theta/beta ratio power when relying on somatosensory input (condition 4; p = 0.001).

Conclusions

PD affects reweighting of postural control, exemplified by greater COP displacement and increased cortical activity. Further research is needed to establish the temporal dynamics between cortical activity and COP displacement.

Instrumental assessment of dynamic postural stability in patients with unilateral vestibular hypofunction during straight, curved, and blindfolded gait

PURPOSE

To characterise dynamic postural stability of gait in patients with vestibular hypofunction (PwVH) using a sensor-based assessment while performing dynamic tasks and to correlate the results of this evaluation with clinical scales.

METHODS

This cross-sectional study involved 22 adults between 18 and 70 years old from a healthcare hospital centre. Eleven patients suffering from chronic vestibular hypofunction (PwVH) and eleven healthy controls (HC) were evaluated through a combined inertial sensor-based and clinical scale assessment. Participants were equipped with five synchronised inertial measurement units (IMUs) (128 Hz, Opal, APDM, Portland, OR, USA): three IMUs were located on the occipital cranium bone, near the lambdoid suture of the head, at the centre of the sternum, and at L4/L5 level, just above the pelvis, and were used to quantify gait quality parameters, while the other two were located slightly above lateral malleoli and used to perform stride and step segmentation. Three different motor tasks were performed in a randomized order: the 10-m Walk Test (10mWT), the Figure of Eight Walk Test (Fo8WT) and the Fukuda Stepping Test (FST). A set of gait quality parameters related to stability, symmetry and smoothness of gait were extracted from IMU data and correlated with the clinical scale scores. PwVH and HC results were compared to test for significant between-group differences.

RESULTS

Significant differences were found for the three motor tasks (10mWT, Fo8WT and FST) when comparing PwVH and HC groups. For the 10mWT and the Fo8WT, significant differences between the PwVH and HC groups were found for the stability indexes. Considering the FST, significant differences between the PwVH and HC groups were also found in the stability and symmetry of gait. A significant correlation was found between the Dizziness Handicap Inventory and gait indices during the Fo8WT.

CONCLUSIONS

In this study, we characterized the dynamic postural stability alterations during linear, curved, and blindfolded walking/stepping in PwVH combining an instrumental IMU-based with traditional clinical scales approach. Combining instrumental and clinical evaluation for dynamic stability of gait alterations in PwVH is useful in thoroughly evaluating the effects of unilateral vestibular hypofunction.

Vertical Ground Reaction Forces in Parkinson's Disease: A Speed-Matched Comparative Analysis with Healthy Subjects

This study aimed to investigate and compare the vertical Ground Reaction Forces (vGRFs) of patients with Parkinson's Disease (PwPD) and healthy subjects (HS) when the confounding effect of walking speed was absent. Therefore, eighteen PwPD and eighteen age- and linear walking speed-matched HS were recruited. Using plantar pressure insoles, participants walked along linear and curvilinear paths at self-selected speeds. Interestingly, PwPD exhibited similar walking speed to HS during curvilinear trajectories (p = 0.48) and similar vGRF during both linear and curvilinear paths. In both groups, vGRF at initial contact and terminal stance was higher during linear walking, while vGRF at mid-stance was higher in curvilinear trajectories. Similarly, the time to peak vGRF at each phase showed no significant group differences. The vGRF timing variability was different between the two groups, particularly at terminal stance (p < 0.001). In conclusion, PwPD and HS showed similar modifications in vGRF and a similar reduction in gait speed during curvilinear paths when matched for linear walking speed. This emphasized the importance of considering walking speed when assessing gait dynamics in PwPD. This study also suggests the possibility of the variability of specific temporal measures in differentiating the gait patterns of PwPD versus those of HS, even in the early stages of the disease.

Visual and vestibular integration in Parkinson's disease while walking

Postural control requires effective sensory integration. People with Parkinson's disease (PD) are reported to have impaired visual and vestibular perception. While self-motion perception is a key aspect of locomotion, visual-vestibular integration has not been directly characterized in people with PD during gait. We compared the ability of people with PD and healthy older adults (OA) to integrate multi-sensory information during straight-line walking in response to visual and vestibular perturbations, using continuous translations of the visual surround and galvanic vestibular stimulation within a virtual reality environment. We measured their endpoint deviations from midline and changes in gait parameters. We found that people with PD deviated more than OA when walking in a dark environment but did not show differences in deviations when walking in a virtual room with visual information. With visual and vestibular perturbations, people with PD did not differ from OA in endpoint deviations nor variabilities. However, people with PD did not adopt a more cautious gait when GVS was applied in a virtual room, unlike OA. Overall, we showed that people with mild PD did not perform worse than OA but did show differences in gait patterns, suggesting that visual-vestibular integration is relatively preserved during gait in PD.

Margins of postural stability in Parkinson's disease: an application of control theory

Introduction: Postural instability is a restrictive feature in Parkinson's disease (PD), usually assessed by clinical or laboratory tests. However, the exact quantification of postural stability, using stability theorems that take into account human dynamics, is still lacking. We investigated the feasibility of control theory and the Nyquist stability criterion-gain margin (GM) and phase margin (PM)-in discriminating postural instability in PD, as well as the effects of a balance-training program. Methods: Center-of-pressure (COP) data of 40 PD patients before and after a 4-week balance-training program, and 20 healthy control subjects (HCs) (Study1) as well as COP data of 20 other PD patients at four time points during a 6-week balance-training program (Study2), collected in two earlier studies, were used. COP was recorded in four tasks, two on a rigid surface and two on foam, both with eyes open and eyes closed. A postural control model (an inverted pendulum with a Proportional-integral-derivative (PID) controller and time delay) was fitted to the COP data to subject-specifically identify the model parameters thereby calculating |GM| and PM for each subject in each task. Results: PD patients had a smaller margin of stability (|GM| and PM) compared with HCs. Particularly, patients, unlike HCs, showed a drastic drop in PM on foam. Clinical outcomes and margins of stability improved in patients after balance training. |GM| improved early in week 4, followed by a plateau during the rest of the training. In contrast, PM improved late (week 6) in a relatively continuous-progression form. Conclusion: Using fundamental stability theorems is a promising technique for the standardized quantification of postural stability in various tasks.

Cholinergic system correlates of postural control changes in Parkinson's disease freezers

Postural instability and freezing of gait are the most debilitating dopamine-refractory motor impairments in advanced stages of Parkinson's disease because of increased risk of falls and poorer quality of life. Recent findings suggest an inability to efficaciously utilize vestibular information during static posturography among people with Parkinson's disease who exhibit freezing of gait, with associated changes in cholinergic system integrity as assessed by vesicular acetylcholine transporter PET. There is a lack of adequate understanding of how postural control varies as a function of available sensory information in patients with Parkinson's disease with freezing of gait. The goal of this cross-sectional study was to examine cerebral cholinergic system changes that associate with inter-sensory postural control processing features as assessed by dynamic computerized posturography and acetylcholinesterase PET. Seventy-five participants with Parkinson's disease, 16 of whom exhibited freezing of gait, underwent computerized posturography on the NeuroCom© Equitest sensory organization test platform, striatal dopamine, and acetylcholinesterase PET scanning. Findings demonstrated that patients with Parkinson's disease with freezing of gait have greater difficulty maintaining balance in the absence of reliable proprioceptive cues as compared to those without freezing of gait [β = 0.28 (0.021, 0.54), P = 0.034], an effect that was independent of disease severity [β = 0.16 (0.062, 0.26), P < 0.01] and age [β = 0.092 (-0.005, 0.19), P = 0.062]. Exploratory voxel-based analysis revealed an association between postural control and right hemispheric cholinergic network related to visual-vestibular integration and self-motion perception. High anti-cholinergic burden predicted postural control impairment in a manner dependent on right hemispheric cortical cholinergic integrity [β = 0.34 (0.065, 0.61), P < 0.01]. Our findings advance the perspective that cortical cholinergic system might play a role in supporting postural control after nigro-striatal dopaminergic losses in Parkinson's disease. Failure of cortex-dependent visual-vestibular integration may impair detection of postural instability in absence of reliable proprioceptive cues. Better understanding of how the cholinergic system plays a role in this process may augur novel treatments and therapeutic interventions to ameliorate debilitating symptoms in patients with advanced Parkinson's disease.

Central sensorimotor integration assessment reveals deficits in standing balance control in people with chronic mild traumatic brain injury

Imbalance is common following mild Traumatic Brain Injury (mTBI) and can persist months after the initial injury. To determine if mTBI subjects with chronic imbalance differed from healthy age- and sex-matched controls (HCs) we used both the Central SensoriMotor Integration (CSMI) test, which evaluates sensory integration, time delay, and motor activation properties and the standard Sensory Organization Test (SOT). Four CSMI conditions evoked center-of-mass sway in response to: surface tilts with eyes closed (SS/EC), surface tilts with eyes open viewing a fixed visual surround (SS/EO), visual surround tilts with eyes open standing on a fixed surface (VS/EO), and combined surface and visual tilts with eyes open (SS+VS/EO). The mTBI participants relied significantly more on visual cues during the VS/EO condition compared to HCs but had similar reliance on combinations of vestibular, visual, and proprioceptive cues for balance during SS/EC, SS/EO, and SS+VS/EO conditions. The mTBI participants had significantly longer time delays across all conditions and significantly decreased motor activation relative to HCs across conditions that included surface-tilt stimuli with a sizeable subgroup having a prominent increase in time delay coupled with reduced motor activation while demonstrating no vestibular sensory weighting deficits. Decreased motor activation compensates for increased time delay to maintain stability of the balance system but has the adverse consequence that sensitivity to both internal (e.g., sensory noise) and external disturbances is increased. Consistent with this increased sensitivity, SOT results for mTBI subjects showed increased sway across all SOT conditions relative to HCs with about 45% of mTBI subjects classified as having an “Aphysiologic” pattern based on published criteria. Thus, CSMI results provided a plausible physiological explanation for the aphysiologic SOT pattern. Overall results suggest that rehabilitation that focuses solely on sensory systems may be incomplete and may benefit from therapy aimed at enhancing rapid and vigorous responses to balance perturbations.

Control of structural redundancy from the head to trunk in the human upright standing revealed using a data-driven approach

The human being dynamically and highly controls the head-trunk with redundant mechanical structures to maintain a stable upright standing position that is inherently unstable. The posture control strategies are also affected by the differences in the conditions of sensory inputs. However, it is unclear how the head-trunk segmental properties are altered to respond to situations that require appropriate changes in standing posture control strategies. We used a data-driven approach to conduct a multipoint measurement of head-trunk sway control in a quiet standing position with differences in the conditions of sensory inputs. Healthy young subjects with 22 accelerometers attached to their backs were evaluated for head-trunk vibration during quiet standing under two conditions: one with open eyes and one with closed eyes. The synchronization of the acceleration and the instantaneous phase was then calculated. The results showed that the synchronization of acceleration and instantaneous phase varied depending on the visual condition, and there were some continuous coherent patterns in each condition. Findings were that the structural redundancy of the head-trunk, which is multi-segmental and has a high mass ratio in the whole body, must be adjusted adaptively according to the conditions to stabilize upright standing in human-specific bipeds.

Cumulative additional information does not improve the neuromuscular control during postural responses to perturbations in postural instability/gait disorders subtype of Parkinson's disease

BACKGROUND

Postural response impairments in postural instability and gait disorders (PIGD) subtype patients may be attributed to Parkinson's disease (PD)-deterioration in central-set (programing/modulating of central outputs during motor responses). Although additional information improves some PD motor impairments, an unanswered question is whether additional information can benefit postural response in PIGD subtype.

OBJECTIVE

To analyze the effect of cumulative additional information on postural responses after perturbation in PIGD and neurologically healthy older adults (CG).

METHODS

Perturbations were applied in 16 PIGD and 19 CG by the support-base translation. Participants performed 3 blocks of 5 trials without additional information (B1-B3, Day 1) and 5 trials of each cumulative additional information (C1-C4, Day 2): information about perturbation (C1), visual (C2), verbal (C3), and somatosensory information (C4). Electromyography and center of pressure (CoP) parameters were analyzed by ANOVAs with Group (PIGD × CG) and Block (B1 × B2 × B3) and with Group (PIGD × CG) and Condition (B3 × C1 × C2 × C3 × C4).

RESULTS

PIGD decreased the range of CoP in B3 while CG decreased both range of CoP and the integral of antagonist's muscle activity (iEMG) in B2. Also, PIGD decreased the recovery time in C4 while CG increased the iEMG of agonist's muscle in C2 and antagonist's muscle in all conditions except C2.

CONCLUSION

Additional information provided before postural control assessment influences the postural response in PIGD and CG differently. PIGD demonstrated inflexibility of central-set in modulating the neuromuscular control regardless of additional information. CG presents a flexible system evidenced by the increase of agonist muscle iEMG when provided visual information.

The effects of the pressure on the coccygeal skin on the perception of backward-leaning sitting positions in stroke patients

The present study investigated the effects of an additional pressure stimulus on coccygeal skin using an original tool to evaluate the perceptibility of sitting while leaning backward in 13 chronic stroke patients who were able to walk independently and 12 age-matched healthy subjects. Each participant's perception of the trunk reference angle at which they felt the highest-pressure stimulation of the coccygeal skin while leaning backward from a quiet sitting position was evaluated based on the accuracy of each reproduction under both normal and additional pressure conditions. The absolute error under the pressure condition was significantly smaller than that under the normal condition in the control group, while no marked difference between conditions was found in the stroke group. The relationship between the absolute error under the normal condition and the pressure effect index showed a significant negative correlation in the stroke group. In stroke patients with a high trunk position perceptibility under the normal condition, the additional pressure information may have functioned as a disturbance and reduced the position perceptibility. In contrast, stroke patients with a low perceptibility in the normal condition may have been able to re-weight and prioritize the additional pressure information in the reference frame. In the control group, the added pressure information may have been re-weighted as prior position information in the reference frame.

Use of Real-time Multimodal Sensory Feedback Home Program Improved Backward Stride and Retention for People with Parkinson Disease: a Pilot Study

•

Multimodal sensory feedback with home exercises increased backward stride for PwPD.

•

Retention of gains occurred 6 weeks after exercise ended for participants using MMSF.

•

The MMSF home program improvements were likely due to integration of proprioception.

•

Outcomes were highly rated by MMSF participants on a Perceived Outcome Scale.

Introduction

Parkinson disease (PD) impairs sensory integration, contributes to motor dysfunction, loss of gait automaticity, and increased fall risk. Employing multimodal sensory feedback (MMSF) has the potential to improve proprioceptive integration and gait safety while reducing exercise burden especially for backward gait.

Methods

This single-blinded, randomized controlled pilot study used a home program with or without real-time visual, proprioceptive, and auditory feedback with stepping exercises which progressed in speed and distance. Both groups completed a six-week intervention followed by 6 weeks without exercise to assess long-term retention. Six additional weeks of exercises were completed to assess recovery of potential losses after the washout session.

Eleven people with PD exercised with real-time MMSF and 7 exercised without MMSF. Outcome measures included backward stride length, velocity, cadence, and double support time. The Dual Timed Up and Go measured automaticity. Self-perceived improvements in gait, activities of daily living, participation, and quality of life were registered by a questionnaire.

Results

Analysis was by repeated measures ANOVA. Using MMSF significantly improved backward stride length at 12 and 18 weeks, p = .007, η² = 0.239. Both groups improved in all outcome measures after the initial 6-week exercise program, supporting efficacy of stepping exercises. The MMSF + ex group's significant improvements after a 6-week washout supported automaticity development. Questionnaire items received higher agreement percentages from MMSF + ex participants.

Conclusion

Using real-time MMSF in a home program for pwPD provided significant and lasting improvements in backward stride, and potentially decreased fall risk and exercise burden compared to the same program without MMSF.

Deep brain stimulation in the subthalamic nuclei alters postural alignment and adaptation in Parkinson's disease

Parkinson’s disease (PD) can produce postural abnormalities of the standing body position such as kyphosis. We investigated the effects of PD, deep brain stimulation (DBS) in the subthalamic nucleus (STN), vision and adaptation on body position in a well-defined group of patients with PD in quiet standing and during balance perturbations. Ten patients with PD and 25 young and 17 old control participants were recruited. Body position was measured with 3D motion tracking of the ankle, knee, hip, shoulder and head. By taking the ankle as reference, we mapped the position of the joints during quiet standing and balance perturbations through repeated calf muscle vibration. We did this to explore the effect of PD, DBS in the STN, and vision on the motor learning process of adaptation in response to the repeated stimulus. We found that patients with PD adopt a different body position with DBS ON vs. DBS OFF, to young and old controls, and with eyes open vs. eyes closed. There was an altered body position in PD with greater flexion of the head, shoulder and knee (p≤0.042) and a posterior position of the hip with DBS OFF (p≤0.014). With DBS ON, body position was brought more in line with the position taken by control participants but there was still evidence of greater flexion at the head, shoulder and knee. The amplitude of movement during the vibration period decreased in controls at all measured sites with eyes open and closed (except at the head in old controls with eyes open) showing adaptation which contrasted the weaker adaptive responses in patients with PD. Our findings suggest that alterations of posture and greater forward leaning with repeated calf vibration, are independent from reduced movement amplitude changes. DBS in the STN can significantly improve body position in PD although the effects are not completely reversed. Patients with PD maintain adaptive capabilities by leaning further forward and reducing movement amplitude despite their kyphotic posture.

Strategic alterations of posture are delayed in Parkinson's disease patients during deep brain stimulation

Parkinson’s disease (PD) is characterized by rigidity, akinesia, postural instability and tremor. Deep brain stimulation (DBS) of the subthalamic nucleus (STN) reduces tremor but the effects on postural instability are inconsistent. Another component of postural control is the postural strategy, traditionally referred to as the ankle or hip strategy, which is determined by the coupling between the joint motions of the body. We aimed to determine whether DBS STN and vision (eyes open vs. eyes closed) affect the postural strategy in PD in quiet stance or during balance perturbations. Linear motion was recorded from the knee, hip, shoulder and head in 10 patients with idiopathic PD with DBS STN (after withdrawal of other anti-PD medication), 25 younger adult controls and 17 older adult controls. Correlation analyses were performed on anterior–posterior linear motion data to determine the coupling between the four positions measured. All participants were asked to stand for a 30 s period of quiet stance and a 200 s period of calf vibration. The 200 s vibration period was subdivided into four 50 s periods to study adaptation between the first vibration period (30–80 s) and the last vibration period (180–230 s). Movement was recorded in patients with PD with DBS ON and DBS OFF, and all participants were investigated with eyes closed and eyes open. DBS settings were randomized and double-blindly programmed. Patients with PD had greater coupling of the body compared to old and young controls during balance perturbations (p ≤ 0.046). Controls adopted a strategy with greater flexibility, particularly using the knee as a point of pivot, whereas patients with PD adopted an ankle strategy, i.e., they used the ankle as the point of pivot. There was higher flexibility in patients with PD with DBS ON and eyes open compared to DBS OFF and eyes closed (p ≤ 0.011). During balance perturbations, controls quickly adopted a new strategy that they retained throughout the test, but patients with PD were slower to adapt. Patients with PD further increased the coupling between segmental movement during balance perturbations with DBS ON but retained a high level of coupling with DBS OFF throughout balance perturbations. The ankle strategy during balance perturbations in patients with PD was most evident with DBS OFF and eyes closed. The increased coupling with balance perturbations implies a mechanism to reduce complexity at a cost of exerting more energy. Strategic alterations of posture were altered by DBS in patients with PD and were delayed. Our findings therefore show that DBS does not fully compensate for disease-related effects on posture.

Quantitative evaluation of posture control in rats with inferior olive lesions

Impairment of inferior olivary neurons (IONs) affects whole-body movements and results in abnormal gait and posture. Because IONs are activated by unpredicted motion rather than regular body movements, the postural dysfunction caused by ION lesions is expected to involve factors other than simple loss of feedback control. In this study, we measured the postural movements of rats with pharmacological ION lesions (IO rats) trained to stand on their hindlimbs. The coordination of body segments as well as the distribution and frequency characteristics of center of mass (COM) motion were analyzed. We determined that the lesion altered the peak properties of the power spectrum density of the COM, whereas changes in coordination and COM distribution were minor. To investigate how the observed properties reflected changes in the control system, we constructed a mathematical model of the standing rats and quantitatively identified the control system. We found an increase in linear proportional control and a decrease in differential and nonlinear control in IO rats compared with intact rats. The dystonia-like changes in body stiffness explain the nature of the linear proportional and differential control, and a disorder in the internal model is one possible cause of the decrease in nonlinear control.

Visual perturbation of balance suggests impaired motor control but intact visuomotor processing in Parkinson's disease

Postural instability marks one of the most disabling features of Parkinson's disease (PD), but it only reveals itself after affected brain areas have already been significantly damaged. Thus there is a need to detect deviations in balance and postural control before visible symptoms occur. In this study, we visually perturbed balance in the anterior-posterior direction using sinusoidal oscillations of a moving room in virtual reality at different frequencies. We tested three groups: individuals with PD under dopaminergic medication, an age-matched control group, and a group of young healthy adults. We tracked their center of pressure and their full-body motion, from which we also extracted the center of mass. We investigated sway amplitudes and applied newly introduced phase-locking analyses to investigate responses across participants' bodies. Patients exhibited significantly higher sway amplitudes as compared with the control subjects. However, their sway was phase locked to the visual motion like that of age-matched and young healthy adults. Furthermore, all groups successfully compensated for the visual perturbation by phase locking their sway to the stimulus. As frequency of the perturbation increased, distribution of phase locking (PL) across the body revealed a shift of the highest PL values from the upper body toward the hip region for young healthy adults, which could not be observed in patients and elderly healthy adults. Our findings suggest an impaired motor control, but intact visuomotor processing in early stages of PD, while less flexibility to adapt postural strategy to different perturbations revealed to be an effect of age rather than disease.NEW & NOTEWORTHY A better understanding of visuomotor control in Parkinson's disease (PD) potentially serves as a tool for earlier diagnosis, which is crucial for improving patient's quality of life. In our study, we assess body sway responses to visual perturbations of the balance control system in patients with early-to-mid stage PD, using motion tracking along with recently established phase-locking techniques. Our findings suggest patients at this stage have an impaired muscular stability but intact visuomotor control.

Using virtual reality to assess vestibulo-visual interaction in people with Parkinson's disease compared to healthy controls

People with Parkinson's disease (PD) have increased visual dependency for balance and suspected vestibular dysfunction. Immersive virtual reality (VR) allows graded manipulation of visual sensory inputs during balance tasks, and hence VR coupled with portable force platforms have emerged as feasible, affordable, and validated tools for assessing sensory-motor integration of balance. This study aims to determine (i) how people with PD perform on a VR-based visual perturbation standing balance task compared to healthy controls (HC), and (ii) whether balance performance is influenced by vestibular function, when other known factors are controlled for. This prospective observational study compared the balance performance under varying sensory conditions in 40 people with mild to moderate PD with 40 age-matched HC. Vestibular function was assessed via Head Impulse Test (HIMP), cervical and ocular vestibular evoked myogenic potentials (cVEMPs and oVEMPs) and subjective visual vertical (SVV). Regression analyses were used to determine associations between VR balance performance on firm and foam surfaces with age, group, vestibular function, and lower limb proprioception. PD failed at significantly lower levels of visual perturbation than HC on both surfaces. In PD, greater disease severity was significantly associated with lower fall thresholds on both surfaces. Multiple PD participants failed prior to visual perturbation on foam. On firm, PD had a greater visual dependency. Increasing age, impaired proprioception, impaired SVV, abnormal HIMP and cVEMP scores were associated with worse balance performance. The multivariate model containing these factors explained 29% of the variability in balance performance on both surfaces. Quantitative VR-based balance assessment is safe and feasible in PD. Balance performance on both surfaces was associated with age, HIMP abnormality and proprioception.

Functional limits of stability and standing balance in people with Parkinson's disease with and without freezing of gait using wearable sensors

BACKGROUND

People with from Parkinson's disease (PD) and freezing of gait (FoG) have more frequent falls compared to those who do not freeze but there is no consensus on which, specific objective measures of postural instability are worse in freezers (PD + FoG) than non-freezers (PD-FoG).

RESEARCH QUESTION

Are functional limits of stability (fLoS) or postural sway during stance measured with wearable inertial sensors different between PD + FoG versus PD-FoG, as well as between PD versus healthy control subjects (HC)?

METHODS

Sixty-four PD subjects with FoG (MDS-UPDRS Part III: 45.9 ± 12.5) and 80 PD subjects without FoG (MDS-UPDRS Part III: 36.2 ± 10.9) were tested Off medication and compared with 79 HC. Balance was quantified with inertial sensors worn on the lumbar spine while performing the following balance tasks: 1) fLoS as defined by the maximum displacement in the forward and backward directions and 2) postural sway area while standing with eyes open on a firm and foam surface. An ANOVA, controlling for disease duration, compared postural control between groups.

RESULTS

PD + FoG had significantly smaller fLoS compared to PD-FoG (p =  0.004) and to healthy controls (p <  0.001). However, PD-FoG showed similar fLoS compared to healthy controls (p =  0.48). Both PD+FoG and PD-FoG showed larger postural sway on a foam surface compared to healthy controls (p =  0.001) but there was no significant difference in postural sway between PD+FoG and PD-FoG.

SIGNIFICANCE

People with PD and FoG showed task-specific, postural impairments with smaller fLoS compared to non-freezers, even when controlling for disease duration. However, individuals with PD with or without FoG had similar difficulties standing quietly on an unreliable surface compared to healthy controls. Wearable inertial sensors can reveal worse fLoS in freezers than non-freezers that may contribute to FoG and help explain their more frequent falls.

Differences in the Effect of Sleep Deprivation on the Postural Stability among Men and Women

Objective: Sleepiness caused by sleep deprivation may increase the risk of injuries and damages during physical activity. Individual data so far indicate a generally better static postural stability of women regardless of sleeping conditions. The main aim of this study was to assess the impact of sleep deprivation on postural stability according to gender after 24 h of sleep deprivation. Methods: Participants included 83 students (36 men and 47 women). Postural stability was measured with eyes open and closed eyes before and after sleep deprivation. Data from posturographic platform were used to assess postural stability objectively. Results: The type of test determined the size of observed changes in postural stability. The data suggest that women are better able to cope with the effects of sleep deprivation than men. Conclusion: Postural control system is very important in sport and in physically active people. The results show that men are more sensitive to sleep deprivation than women because they had higher COP (center of pressure) values in tests. Less postural stability of the body due to sleep deprivation indicates a higher risk of injury during physical activity.

Adapting gait with asymmetric visual feedback affects deadaptation but not adaptation in healthy young adults

Split-belt treadmill walking allows researchers to understand how new gait patterns are acquired. Initially, the belts move at two different speeds, inducing asymmetric step lengths. As people adapt their gait on a split-belt treadmill, left and right step lengths become more symmetric over time. Upon returning to normal walking, step lengths become asymmetric in the opposite direction, indicating deadaptation. Then, upon re-exposure to the split belts, step length asymmetry is less than the asymmetry at the start of the initial exposure, indicating readaptation. Changes in step length symmetry are driven by changes in step timing and step position asymmetry. It is critical to understand what factors can promote step timing and position adaptation and therefore influence step length asymmetry. There is limited research regarding the role of visual feedback to improve gait adaptation. Using visual feedback to promote the adaptation of step timing or position may be useful of understanding temporal or spatial gait impairments. We measured gait adaptation, deadaptation, and readaptation in twenty-nine healthy young adults while they walked on a split-belt treadmill. One group received no feedback while adapting; one group received asymmetric real-time feedback about step timing while adapting; and the last group received asymmetric real-time feedback about step position while adapting. We measured step length difference (non-normalized asymmetry), step timing asymmetry, and step position asymmetry during adaptation, deadaptation, and readaptation on a split-belt treadmill. Regardless of feedback, participants adapted step length difference, indicating that walking with temporal or spatial visual feedback does not interfere with gait adaptation. Compared to the group that received no feedback, the group that received temporal feedback exhibited smaller early deadaptation step position asymmetry (p = 0.005). There was no effect of temporal or spatial feedback on step timing. The feedback groups adapted step timing and position similarly to walking without feedback. Future work should investigate whether asymmetric visual feedback also results in typical gait adaptation in populations with altered step timing or position control.

The Role of Mental Imagery in Parkinson's Disease Rehabilitation

Parkinson’s disease (PD) is a disabling neurodegenerative disease whose manifestations span motor, sensorimotor, and sensory domains. While current therapies for PD include pharmacological, invasive, and physical interventions, there is a constant need for developing additional approaches for optimizing rehabilitation gains. Mental imagery is an emerging field in neurorehabilitation and has the potential to serve as an adjunct therapy to enhance patient function. Yet, the literature on this topic is sparse. The current paper reviews the motor, sensorimotor, and sensory domains impacted by PD using gait, balance, and pain as examples, respectively. Then, mental imagery and its potential for PD motor and non-motor rehabilitation is discussed, with an emphasis on its suitability for addressing gait, balance, and pain deficits in people with PD. Lastly, future research directions are suggested.

The Effects of Leg Preference on Transient Characteristics of Body Sway During Single-Leg Stance: A Cross-Sectional Study

Instrumented assessments of quiet-stance postural control typically involve recording and analyzing of body sway signal, most often the center of pressure (CoP) movement. It has been recently suggested that transient characteristics of body sway may offer additional information regarding postural control. In this study, we explored the relationship between whole-trial estimates of body sway (CoP velocity, amplitude, and frequency) and corresponding transient behavior indexes, as well as the effects of leg preference. A total of 705 healthy young athletes performed 30 s single-leg body sway trials for both legs. It was found that the transient characteristics of the body sway (expressed as relative differences between individual time intervals within the trial) are in negligible or weak correlation (r ≤ 0.26) with the corresponding variables, averaged across the whole trial. All CoP variables showed transient characteristics, reflected in statistically significant decrease (CoP velocity and amplitude) or increase (CoP frequency) throughout the trial. The preferred leg showed smaller body sway; however, the effect sizes were very small. Moreover, differences between the legs were also noted in terms of transient characteristics of body sway. In particular, the preferred leg showed earlier reduction in anterior–posterior body sway and larger reduction in medial–lateral body sway. Further studies should focus on examining the clinical utility of indexes of transient behavior of body sway, for instance, their sensitivity to aging-related changes and risk of falling.

Effect of Different Intensities of Transcranial Direct Current Stimulation on Postural Response to External Perturbation in Patients With Parkinson’s Disease

Background

Habituation of postural response to perturbations is impaired in people with Parkinson’s disease (PD) due to deficits in cortico-basal pathways. Although transcranial direct current stimulation (tDCS) modulate cortico-basal networks, it remains unclear if it can benefit postural control in PD.

Objective

To analyze the effect of different intensities of anodal tDCS on postural responses and prefrontal cortex (PFC) activity during the habituation to the external perturbation in patients with PD (n = 24).

Methods

Anodal tDCS was applied over the primary motor cortex (M1) with 1 mA, 2 mA, and sham stimulation in 3 different sessions (~2 weeks apart) during 20 minutes immediately before the postural assessment. External perturbation (7 trials) was applied by a support base posterior translation (20 cm/s and 5 cm). Primary outcome measures included lower limb electromyography and center of pressure parameters. Measures of PFC activity are reported as exploratory outcomes. Analyses of variance (Stimulation Condition × Trial) were performed.

Results

Habituation of perturbation was evidenced independent of the stimulation conditions. Both active stimulation intensities had shorter recovery time and a trend for lower cortical activity in the stimulated hemisphere when compared to sham condition. Shorter onset latency of the medial gastrocnemius as well as lower cortical activity in the nonstimulated hemisphere were only observed after 2 mA concerning the sham condition.

Conclusions

tDCS over M1 improved the postural response to external perturbation in PD, with better response observed for 2 mA compared with 1 mA. However, tDCS seems to be inefficient in modifying the habituation of perturbation.

New insight into Parkinson's disease-related impairment of the automatic control of upright stance

Parkinson's disease (PD) affects the automatic control of body movements. In our study, we tested PD-related impairments in automatic postural control in quiet upright stance. Twenty PD patients (mean age: 60 ± 8 years; Hoehn and Yahr: 2.00 ± 0.32, on-drug) and twenty age-matched controls (61 ± 7 years) were recruited. We studied interrelations between center-of-pressure movements, body movements (head, neck, and lower back), eye movements and variability of pupil size. Participants performed two fixation tasks while standing, during which they looked at: (a) a cross surrounded by a white background; and (b) a cross surrounded by a structured visual background (images used: rooms in houses). PD patients exhibited stronger and weaker correlations between eye and center-of-pressure/body movement variables than age-matched controls in the white and structured fixation tasks, respectively. Partial correlations, controlling for variability of pupil size showed that PD patients used lower and greater attentional resources than age-matched controls to control their eye and center-of-pressure/body movements simultaneously in the white fixation and structured fixation tasks, respectively. In the white fixation task, PD patients used attentional resources to optimize visuomotor coupling between eye and body movements to control their posture. In the structured fixation task, the salient visual stimuli distracted PD patients' attention and that possibly affected postural control by deteriorating the automatic visuomotor coupling. In contrast, age-matched controls were able to use surrounding visual background to improve the automatic coupling between eye and center-of-pressure movements to control their posture. These results suggest that cluttered environments may distract PD patients and deteriorate their postural control.

People in early stages of Parkinson's disease are able to intentionally reweight the use of visual information for postural control

Background

Parkinson’s disease (PD) leads to several changes in motor control, many of them related to informational or cognitive overload. The aim of this study was to investigate the influence of knowledge and intention on the postural control performance and on the coupling between visual information and body sway in people with and without PD standing upright.

Methods

Participants were 21 people with PD (62.1 ± 7.2 years), stages 1 and 2 (Hoehn & Yahr scale), under dopaminergic medication, and 21 people in the control group (62.3 ± 7.1 years). Participants stood upright inside a moving room, performing seven trials of 60 s. In the first trial, the room remained motionless. In the others, the room oscillated at 0.2 Hz in the anterior-posterior direction: in the first block of three trials, the participants were not informed about the visual manipulation; in the second block of three trials, participants were informed about the room movement and asked to resist the visual influence. An OPTOTRAK system recorded the moving room displacement and the participants’ sway. The variables mean sway amplitude (MSA), coherence and gain were calculated.

Results

With no visual manipulation, no difference occurred between groups for MSA. Under visual manipulation conditions, people with PD presented higher MSA than control, and both groups reduced the sway magnitude in the resisting condition. Control group reduced sway magnitude by 6.1%, while PD group reduced by 11.5%. No difference was found between groups and between conditions for the coupling strength (coherence). For the coupling structure (gain), there was no group difference, but both groups showed reduced gain in the resisting condition. Control group reduced gain by 12.0%, while PD group reduced by 9.3%.

Conclusions

People with PD, under visual manipulation, were more influenced than controls, but they presented the same coupling structure between visual information and body sway as controls. People in early stages of PD are able to intentionally alter the influence of visual information.

How to Select Balance Measures Sensitive to Parkinson's Disease from Body-Worn Inertial Sensors-Separating the Trees from the Forest

This study aimed to determine the most sensitive objective measures of balance dysfunction that differ between people with Parkinson’s Disease (PD) and healthy controls. One-hundred and forty-four people with PD and 79 age-matched healthy controls wore eight inertial sensors while performing tasks to measure five domains of balance: standing posture (Sway), anticipatory postural adjustments (APAs), automatic postural responses (APRs), dynamic posture (Gait) and limits of stability (LOS). To reduce the initial 93 measures, we selected uncorrelated measures that were most sensitive to PD. After applying a threshold on the Standardized Mean Difference between PD and healthy controls, 44 measures remained; and after reducing highly correlated measures, 24 measures remained. The four most sensitive measures were from APAs and Gait domains. The random forest with 10-fold cross-validation on the remaining measures (n = 24) showed an accuracy to separate PD from healthy controls of 82.4%—identical to result for all measures. Measures from the most sensitive domains, APAs and Gait, were significantly correlated with the severity of disease and with patient-related outcomes. This method greatly reduced the objective measures of balance to the most sensitive for PD, while still capturing four of the five domains of balance.