Head motion predictability explains activity-dependent suppression of vestibular balance control

Modulation of stepping balance reactions through the alignment of vestibular input with body perturbation axis

Balance reactions depend on sensing imbalances to direct recovery responses, though the role of vestibular feedback in scaling these responses remains uncertain. While vestibular input can alter anticipatory postural adjustments before a compensatory step (McMorran, Bent, and Zettel 2024), this study aimed to clarify whether this reflects vestibular-based scaling of step-recovery or if vestibular input acts as a stability reference in feedforward control of step execution. To investigate vestibular input's influence on step scaling, galvanic vestibular stimulation (GVS) was aligned to amplify or diminish the sense of perturbed postural motion in forward step recovery, through inducing the sensation of forward (FGVS) or backward (BGVS) postural motion. Effects of altered vestibular input was analyzed in terms of postural and step motion and restabilization. GVS modulated forward step responses asymmetrically, with BGVS exhibiting greater postural motion in advance of the step as indicated through forward stability, while FGVS evoked larger and faster forward steps relative to the body. Upon landing the step, these differences culminated as skewed stability according to GVS direction, with a smaller stability margin in BGVS compared to a larger one in FGVS. This stability shift continued post-recovery, with a forward-BGVS and backward-FGVS shift when re-establishing equilibrium. These results demonstrate step-recovery scaling according to the GVS direction, indicating vestibular-based modulation of compensatory stepping reactions.

A mobile electrical stimulator for therapeutic modulation of the vestibular system - design, safety, and functionality

Low-intensity noisy galvanic vestibular stimulation (nGVS) is a promising non-invasive treatment for enhancing vestibular perceptual performance and postural control in patients with chronic vestibular hypofunction. However, this approach has so far been studied mainly under laboratory conditions. Evidence indicates that continuous application of nGVS in daily life is necessary for it to be effective. To address this need, we have developed a mobile nGVS stimulator and conducted a series of pilot studies to evaluate its safety, tolerability, functionality, and therapeutic effects. The device is a lightweight, compact, and portable AC stimulator featuring a user-friendly interface for the individualized adjustment of nGVS parameters. It includes an integrated motion sensor that automatically activates stimulation during body movement and deactivates it during inactivity, optimizing its practical use in real-world settings. The stimulator adheres to strict safety standards and, in initial long-term use, has exhibited only mild side effects (e.g., skin irritation and headaches), likely attributable to the current electrode placement, which requires further optimization. As expected, the device consistently elicits known vestibular sensorimotor reflex responses in healthy individuals. Importantly, further pilot studies in healthy participants demonstrate that the device can reliably replicate known facilitating effects on vestibular perception and postural control. Together, these findings suggest that this mobile stimulation device can facilitate the translation of nGVS into therapeutic everyday use.

Visual Deprivation's Impact on Dynamic Posture Control of Trunk: A Comprehensive Sensing Information Analysis of Neurophysiological Mechanisms

Visual information affects static postural control, but how it affects dynamic postural control still needs to be fully understood. This study investigated the effect of proprioception weighting, influenced by the presence or absence of visual information, on dynamic posture control during voluntary trunk movements. We recorded trunk movement angle and angular velocity, center of pressure (COP), electromyographic, and electroencephalography signals from 35 healthy young adults performing a standing trunk flexion-extension task under two conditions (Vision and No-Vision). A random forest analysis identified the 10 most important variables for classifying the conditions, followed by a Wilcoxon signed-rank test. The results showed lower maximum forward COP displacement and trunk flexion angle, and faster maximum flexion angular velocity in the No-Vision condition. Additionally, the alpha/beta ratio of the POz during the switch phase was higher in the No-Vision condition. These findings suggest that visual deprivation affects cognitive- and sensory-integration-related brain regions during movement phases, indicating that sensory re-weighting due to visual deprivation impacts motor control. The effects of visual deprivation on motor control may be used for evaluation and therapeutic interventions in the future.

How spotting technique affects dizziness and postural stability after full-body rotations in dancers

Consecutive longitudinal axis rotations are very common in dance, ranging from head spins in break dance to pirouettes in ballet. They pose a rather formidable perceptuomotor challenge - and hence form an interesting window into human motor behaviour - yet they have been scarcely studied. In the present study, we investigated dancers' dizziness and postural stability after consecutive rotations. Rotations were performed actively or undergone passively, either with or without the use of a spotting technique in such an order that all 24 ordering options were offered at least once and not more than twice. Thirty-four dancers trained in ballet and/or contemporary dance (aged 27.2 ± 5.1 years) with a mean dance experience of 14.2 ± 7.1 years actively performed 14 revolutions in passé or coupé positions with a short gesture leg "foot down" after each revolution. In addition, they were passively turned through 14 revolutions on a motor-driven rotating chair. Participants' centre-of-pressure (COP) displacement was measured on a force-plate before and after the rotations. Moreover, the dancers indicated their subjective feeling of dizziness on a scale from 0 to 20 directly after the rotations. Both the active and passive conditions were completed with and without the dancers spotting. As expected, dizziness was worse after rotations without the adoption of the spotting technique, both in active and passive rotations. However, the pre-post difference in COP area after active rotations was unaffected by spotting, whereas in the passive condition, spotting diminished this difference. Our results thus suggest that adopting the spotting technique is a useful tool for dizziness reduction in dancers who have to perform multiple rotations. Moreover, spotting appears most beneficial for postural stability when it involves less postural control challenges, such as when seated on a chair and occurs in situations with limited somatosensory feedback (e.g., from the cutaneous receptors in the feet). However, the unexpected finding that spotting did not help postural stability after active rotations needs to be investigated further in future studies, for example with a detailed analysis of whole-body kinematics and eye-tracking.

Downbeat nystagmus: a clinical and pathophysiological review

Downbeat nystagmus (DBN) is a neuro-otological finding frequently encountered by clinicians dealing with patients with vertigo. Since DBN is a finding that should be understood because of central vestibular dysfunction, it is necessary to know how to frame it promptly to suggest the correct diagnostic-therapeutic pathway to the patient. As knowledge of its pathophysiology has progressed, the importance of this clinical sign has been increasingly understood. At the same time, clinical diagnostic knowledge has increased, and it has been recognized that this sign may occur sporadically or in association with others within defined clinical syndromes. Thus, in many cases, different therapeutic solutions have become possible. In our work, we have attempted to systematize current knowledge about the origin of this finding, the clinical presentation and current treatment options, to provide an overview that can be used at different levels, from the general practitioner to the specialist neurologist or neurotologist.

Comparison Between Effects of Galvanic and Vibration-Based Vestibular Stimulation on Postural Control and Gait Performance in Healthy Participants: A Systematic Review of Cross-Sectional Studies

Electricity and vibration were two commonly used physical agents to provide vestibular stimulation in previous studies. This study aimed to systematically review the effects of galvanic (GVS) and vibration-based vestibular stimulation (VVS) on gait performance and postural control in healthy participants. Five bioscience and engineering databases, including MEDLINE via PubMed, CINAHL via EBSCO, Cochrane Library, Scopus, and Embase, were searched until March 19th, 2023. Studies published between 2000 and 2023 in English involving GVS and VVS related to gait performance and postural control were included. The procedure was followed via the Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines. The methodological quality of included studies was assessed using the NIH study quality assessment tool for observational cohort and cross-sectional studies. A total of 55 cross-sectional studies met the inclusion criteria and were included in this study. Five studies were good-quality while 49 were moderate-quality and 1 was poor-quality. There were 50 included studies involving GVS and 5 included studies involving VVS. GVS and VVS utilized different physical agents to provide vestibular stimulation and demonstrated similar effects on vestibular perception. Supra-threshold GVS and VVS produced vestibular perturbation that impaired gait performance and postural control, while sub-threshold GVS and VVS induced stochastic resonance phenomenon that led to an improvement. Bilateral vestibular stimulation demonstrated a greater effect on gait and posture than unilateral vestibular stimulation. Compared to GVS, VVS had the characteristics of better tolerance and fewer side effects, which may substitute GVS to provide more acceptable vestibular stimulation.

Roles for cerebellum and subsumption architecture in central pattern generation

Within vertebrates, central pattern generators drive rhythmical behaviours, such as locomotion and ventilation. Their pattern generation is also influenced by sensory input and various forms of neuromodulation. These capabilities arose early in vertebrate evolution, preceding the evolution of the cerebellum in jawed vertebrates. This later evolution of the cerebellum is suggestive of subsumption architecture that adds functionality to a pre-existing network. From a central-pattern-generator perspective, what additional functionality might the cerebellum provide? The suggestion is that the adaptive filter capabilities of the cerebellum may be able to use error learning to appropriately repurpose pattern output. Examples may include head and eye stabilization during locomotion, song learning, and context-dependent alternation between learnt motor-control sequences.

The DizzyQuest Combined with Accelerometry: Daily Physical Activities and Limitations among Patients with Bilateral Vestibulopathy Due to DFNA9

BACKGROUND

DFNA9 is a genetic disease of the inner ear, causing progressive bilateral sensorineural deafness and bilateral vestibulopathy (BV). In this study, DizzyQuest, a mobile vestibular diary, and the MOX accelerometer were combined to assess the daily life functional limitations and physical activity of patients with DFNA9 suffering from BV. These parameters might be appropriate as potential candidacy criteria and outcome measures for new therapeutic interventions for BV.

METHODS

Fifteen DFNA9 patients with BV and twelve age-matched healthy controls were included. The DizzyQuest was applied for six consecutive days, which assessed the participants' extent of functional limitations, tiredness, types of activities performed during the day, and type of activity during which the participant felt most limited. The MOX accelerometer was worn during the same six days of DizzyQuest use, measuring the participants intensity and type of physical activity. Mixed-effects linear and logistic regression analyses were performed to compare the DFNA9 patients and control group.

RESULTS

DFNA9 patients with BV felt significantly more limited in activities during the day compared to the age-matched controls, especially in social participation (p < 0.005). However, these reported limitations did not cause adjustment in the types of activities and did not reduce the intensity or type of physical activity measured with accelerometry. In addition, no relationships were found between self-reported functional limitations and physical activity.

CONCLUSIONS

This study demonstrated that self-reported functional limitations are significantly higher among DFNA9 patients with BV. As a result, these limitations might be considered as part of the candidacy criteria or outcome measures for therapeutic interventions. In addition, the intensity or type of physical activity performed during the day need to be addressed more specifically in future research.

Real-world characterization of vestibular contributions during locomotion

Introduction

The vestibular system, which encodes our head movement in space, plays an important role in maintaining our balance as we navigate the environment. While in-laboratory research demonstrates that the vestibular system exerts a context-dependent influence on the control of balance during locomotion, differences in whole-body and head kinematics between indoor treadmill and real-world locomotion challenge the generalizability of these findings. Thus, the goal of this study was to characterize vestibular-evoked balance responses in the real world using a fully portable system.

Methods

While experiencing stochastic electrical vestibular stimulation (0-20 Hz, amplitude peak ± 4.5 mA, root mean square 1.25 mA) and wearing inertial measurement units (IMUs) on the head, low back, and ankles, 10 participants walked outside at 52 steps/minute (∼0.4 m/s) and 78 steps/minute (∼0.8 m/s). We calculated time-dependent coherence (a measure of correlation in the frequency domain) between the applied stimulus and the mediolateral back, right ankle, and left ankle linear accelerations to infer the vestibular control of balance during locomotion.

Results

In all participants, we observed vestibular-evoked balance responses. These responses exhibited phasic modulation across the stride cycle, peaking during the middle of the single-leg stance in the back and during the stance phase for the ankles. Coherence decreased with increasing locomotor cadence and speed, as observed in both bootstrapped coherence differences (p < 0.01) and peak coherence (low back: 0.23 ± 0.07 vs. 0.16 ± 0.14, p = 0.021; right ankle: 0.38 ± 0.12 vs. 0.25 ± 0.10, p < 0.001; left ankle: 0.33 ± 0.09 vs. 0.21 ± 0.09, p < 0.001).

Discussion

These results replicate previous in-laboratory studies, thus providing further insight into the vestibular control of balance during naturalistic movements and validating the use of this portable system as a method to characterize real-world vestibular responses. This study will help support future work that seeks to better understand how the vestibular system contributes to balance in variable real-world environments.

Repetitive Low-Intensity Vestibular Noise Stimulation Partly Reverses Behavioral and Brain Activity Changes following Bilateral Vestibular Loss in Rats

Low-intensity noisy galvanic vestibular stimulation (nGVS) can improve static and dynamic postural deficits in patients with bilateral vestibular loss (BVL). In this study, we aimed to explore the neurophysiological and neuroanatomical substrates underlying nGVS treatment effects in a rat model of BVL. Regional brain activation patterns and behavioral responses to a repeated 30 min nGVS treatment in comparison to sham stimulation were investigated by serial whole-brain 18F-FDG-PET measurements and quantitative locomotor assessments before and at nine consecutive time points up to 60 days after the chemical bilateral labyrinthectomy (BL). The 18F-FDG-PET revealed a broad nGVS-induced modulation on regional brain activation patterns encompassing biologically plausible brain networks in the brainstem, cerebellum, multisensory cortex, and basal ganglia during the entire observation period post-BL. nGVS broadly reversed brain activity adaptions occurring in the natural course post-BL. The parallel behavioral locomotor assessment demonstrated a beneficial treatment effect of nGVS on sensory-ataxic gait alterations, particularly in the early stage of post-BL recovery. Stimulation-induced locomotor improvements were finally linked to nGVS brain activity responses in the brainstem, hemispheric motor, and limbic networks. In conclusion, combined 18F-FDG-PET and locomotor analysis discloses the potential neurophysiological and neuroanatomical substrates that mediate previously observed therapeutic nGVS effects on postural deficits in patients with BVL.

Locomotor efference copy signaling and gaze control: An evolutionary perspective

Neural replicas of the spinal motor commands that drive locomotion have become increasingly recognized as an intrinsic neural mechanism for producing gaze-stabilizing eye movements that counteract the perturbing effects of self-generated head/body motion. By pre-empting reactive signaling by motion-detecting vestibular sensors, such locomotor efference copies (ECs) provide estimates of the sensory consequences of behavioral action. Initially demonstrated in amphibian larvae during spontaneous fictive swimming in deafferented in vitro preparations, direct evidence for a contribution of locomotor ECs to gaze stabilization now extends to the ancestral lamprey and to tetrapod adult frogs and mice. Supporting behavioral evidence also exists for other mammals, including humans, therefore further indicating the mechanism's conservation during vertebrate evolution. The relationship between feedforward ECs and vestibular sensory feedback in ocular movement control is variable, ranging from additive to the former supplanting the latter, depending on vestibular sensing ability, and the intensity and regularity of rhythmic locomotor movements.

The Neural Basis for Biased Behavioral Responses Evoked by Galvanic Vestibular Stimulation in Primates

Noninvasive electrical stimulation of the vestibular system in humans has become an increasingly popular tool with a broad range of research and clinical applications. However, common assumptions regarding the neural mechanisms that underlie the activation of central vestibular pathways through such stimulation, known as galvanic vestibular stimulation (GVS), have not been directly tested. Here, we show that GVS is encoded by VIIIth nerve vestibular afferents with nonlinear dynamics that differ markedly from those predicted by current models. GVS produced asymmetric activation of both semicircular canal and otolith afferents to the onset versus offset and cathode versus anode of applied current, that in turn produced asymmetric eye movement responses in three awake-behaving male monkeys. Additionally, using computational methods, we demonstrate that the experimentally observed nonlinear neural response dynamics lead to an unexpected directional bias in the net population response when the information from both vestibular nerves is centrally integrated. Together our findings reveal the neural basis by which GVS activates the vestibular system, establish that neural response dynamics differ markedly from current predictions, and advance our mechanistic understanding of how asymmetric activation of the peripheral vestibular system alters vestibular function. We suggest that such nonlinear encoding is a general feature of neural processing that will be common across different noninvasive electrical stimulation approaches.SIGNIFICANCE STATEMENT Here, we show that the application of noninvasive electrical currents to the vestibular system (GVS) induces more complex responses than commonly assumed. We recorded vestibular afferent activity in macaque monkeys exposed to GVS using a setup analogous to human studies. GVS evoked notable asymmetries in irregular afferent responses to cathodal versus anodal currents. We developed a nonlinear model explaining these GVS-evoked afferent responses. Our model predicts that GVS induces directional biases in centrally integrated head motion signals and establishes electrical stimuli that recreate physiologically plausible sensations of motion. Altogether, our findings provide new insights into how GVS activates the vestibular system, which will be vital to advancing new clinical and biomedical applications.

EFEITOS DO EXERCÍCIO DE REABILITAÇÃO ACELERADA DEPOIS DE TBM SOBRE A FUNÇÃO E A FORÇA NA INSTABILIDADE DO TORNOZELO

RESUMO Introdução: Os atletas de taekwondo com instabilidade lateral crônica do tornozelo (ILCT) podem apresentar dificuldades com propriocepção e força muscular do tornozelo. Depois da cirurgia, um programa de exercícios de reabilitação convencional pode ser ineficaz, porque a propriocepção ou a força muscular podem não se restaurar e, portanto, resultar em melhora mínima. Objetivos: Este estudo teve como objetivo avaliar os efeitos de um programa de ERA depois de TBM sobre a propriocepção, força isocinética e resistência do tornozelo. Métodos: Trinta atletas com diagnóstico de ILCT foram submetidos à TBM. Eles foram divididos em grupo ERA (n=15) e grupo controle CON (n = 15). O grupo ERA realizou exercícios ERA precoces seis vezes por semana durante 4 semanas. Os parâmetros aplicados para testar a força muscular do tornozelo foram velocidade angular de 30°/s e 180°/s (inversão: Inv e eversão: Eve, respectivamente). A propriocepção foi medida pela capacidade de sentir a posição articular de 15° de Inv e 5° de Eve Resultados: Foram observadas diferenças significativas entre os grupos ERA e CON na força do tornozelo 30°/s (Inv: p < 0,001, Eve: p < 0,001), 180°/s (Inv: p < 0,001, Eve: p < 0,001), e propriocepção a 15° (Inv: p < 0,001) e 5° (Eve: p < 0,001). Conclusões: Registramos efeitos de curto prazo significativos com ERA precoce em atletas de taekwondo com ILCT depois da TBM. Os resultados fornecem dados de referência para o reabilitador esportivo ou ATC (Certified athletic trainer) na avaliação da fase de reabilitação e informar os pacientes sobre as expectativas depois da TBM em termos de condições de desempenho e momento de retorno ao esporte. Nível de evidência III; Estudos terapêuticos – Investigação dos resultados do tratamento - Estudo de caso-controle.

EFFECTS OF ACCELERATED REHABILITATION EXERCISE AFTER MBO ON ANKLE FUNCTION AND STRENGTH IN ANKLE INSTABILITY

ABSTRACT Background: Taekwondo athletes with lateral chronic ankle instability (LCAI) may experience difficulties with proprioception and ankle muscle strength. After surgery, a conventional rehabilitation exercise program can be ineffective, as it may not restore proprioception or muscle strength and, thus, result in minimal improvement. Objective: This study aimed to assess the effects of an ARE program following MBO on the proprioception, isokinetic strength, and endurance of ankle. Methods: Thirty athletes diagnosed with LCAI underwent MBO. They were divided into the ARE group (n=15) and the control group (CON, n=15). The ARE group performed early ARE exercises six times per week for 4 weeks. The parameters applied to test ankle muscle strength were angular speed of 30°/sec and 180°/sec (inversion: Inv. and eversion: Eve., respectively). Proprioception was measured as being able to sense a joint position of 15° of Inv. and 5° of Eve. Results: Significant differences were observed between the ARE and CON groups in ankle strength 30°/sec (Inv.: p<0.001, Eve.: p<0.001), 180°/sec (Inv.: p<0.001, Eve.: p<0.001), and proprioception at 15° (Inv.: p<0.001) and 5° (Eve.: p<0.001). Conclusions: We recorded significant short-term effects from early ARE in Taekwondo athletes with LCAI after MBO. The results provide reference data for the sports rehabilitator or ATC in evaluating the rehabilitation phase and informing patients about expectations after MBO in terms of performance status and the timing of return to sports. Level of evidence III; Therapeutic studies–Investigation of treatment outcomes - Case-control Study.

Role of locomotor efference copy in vertebrate gaze stabilization

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

Locomotion-induced ocular motor behavior in larval Xenopus is developmentally tuned by visuo-vestibular reflexes

Locomotion in vertebrates is accompanied by retinal image-stabilizing eye movements that derive from sensory-motor transformations and predictive locomotor efference copies. During development, concurrent maturation of locomotor and ocular motor proficiency depends on the structural and neuronal capacity of the motion detection systems, the propulsive elements and the computational capability for signal integration. In developing Xenopus larvae, we demonstrate an interactive plasticity of predictive locomotor efference copies and multi-sensory motion signals to constantly elicit dynamically adequate eye movements during swimming. During ontogeny, the neuronal integration of vestibulo- and spino-ocular reflex components progressively alters as locomotion parameters change. In young larvae, spino-ocular motor coupling attenuates concurrent angular vestibulo-ocular reflexes, while older larvae express eye movements that derive from a combination of the two components. This integrative switch depends on the locomotor pattern generator frequency, represents a stage-independent gating mechanism, and appears during ontogeny when the swim frequency naturally declines with larval age.

Detection of balance disorders using rotations around vertical axis and an artificial neural network

Vestibular impairments affect patients' movements and can result in difficulties with daily life activities. The main aim of this study is to answer the question whether a simple and short test such as rotation about a vertical axis can be an objective method of assessing balance dysfunction in patients with unilateral vestibular impairments. A 360˚ rotation test was performed using six MediPost devices. The analysis was performed in three ways: (1) the analytical approach based only on data from one sensor; (2) the analytical approach based on data from six sensors; (3) the artificial neural network (ANN) approach based on data from six sensors. For approaches 1 and 2 best results were obtained using maximum angular velocities (MAV) of rotation and rotation duration (RD), while approach 3 used 11 different features. The following sensitivities and specificities were achieved: for approach 1: MAV—80% and 60%, RD—69% and 74%; for approach 2: 61% and 85% and RD—74% and 56%; for approach 3: 88% and 84%. The ANN-based six-sensor approach revealed the best sensitivity and specificity among parameters studied, however one-sensor approach might be a simple screening test used e.g. for rehabilitation purposes.

Downbeat nystagmus becomes attenuated during walking compared to standing

Downbeat nystagmus (DBN) is a common form of acquired fixation nystagmus related to vestibulo-cerebellar impairments and associated with impaired vision and postural imbalance. DBN intensity becomes modulated by various factors such as gaze direction, head position, daytime, and resting conditions. Further evidence suggests that locomotion attenuates postural symptoms in DBN. Here, we examined whether walking might analogously influence ocular-motor deficits in DBN. Gaze stabilization mechanisms and nystagmus frequency were examined in 10 patients with DBN and 10 age-matched healthy controls with visual fixation during standing vs. walking on a motorized treadmill. Despite their central ocular-motor deficits, linear and angular gaze stabilization in the vertical plane were functional during walking in DBN patients and comparable to controls. Notably, nystagmus frequency in patients was considerably reduced during walking compared to standing (p < 0.001). The frequency of remaining nystagmus during walking was further modulated in a manner that depended on the specific phase of the gait cycle (p = 0.015). These attenuating effects on nystagmus intensity during walking suggest that ocular-motor control disturbances are selectively suppressed during locomotion in DBN. This suppression is potentially mediated by locomotor efference copies that have been shown to selectively govern gaze stabilization during stereotyped locomotion in animal models.

Predicting UPDRS Motor Symptoms in Individuals with Parkinsons Disease from Force Plates Using Machine Learning

Parkinsons disease (PD) is a neurodegenerative disease that affects motor abilities with increasing severity as the disease progresses. Traditional methods for diagnosing PD include a section where a trained specialist scores qualitative symptoms using the motor subscale of the Unified Parkinsons Disease Rating Scale (UPDRS-III). The aim of this feasibility study was twofold. First, to evaluate quiet standing as an additional, out-of-clinic, objective feature to predict UPDRS-III subscores related to motor symptom severity; and second, to use quiet standing to detect the presence of motor symptoms. Force plate data were collected from 42 PD patients and 43 healthy controls during quiet standing (a task involving standing still with eyes open and closed) as a feasible task in clinics. Predicting each subscore of the UPDRS-III could aid in identifying progression of PD and provide specialists additional tools to make an informed diagnosis. Random Forest feature importance indicated that features correlated with range of center of pressure (i.e. the medial-lateral and anterior-posterior sway) were most useful in the prediction of the top PD prediction subscores of postural stability (r = 0.599; p = 0.014), hand tremor of the left hand (r = 0.650; p = 0.015), and tremor at rest of the left upper extremity (r = 0.703; p = 0.016). Quiet standing can detect body bradykinesia (AUC-ROC = 0.924) and postural stability (AUC-ROC = 0.967) with high predictability. Although there are limited data, these results should be used as a feasibility study that evaluates the predictability of individual UPDRS-III subscores using quiet standing data.

How Does the Central Nervous System for Posture and Locomotion Cope With Damage-Induced Neural Asymmetry?

In most vertebrates, posture and locomotion are achieved by a biomechanical apparatus whose effectors are symmetrically positioned around the main body axis. Logically, motor commands to these effectors are intrinsically adapted to such anatomical symmetry, and the underlying sensory-motor neural networks are correspondingly arranged during central nervous system (CNS) development. However, many developmental and/or life accidents may alter such neural organization and acutely generate asymmetries in motor operation that are often at least partially compensated for over time. First, we briefly present the basic sensory-motor organization of posturo-locomotor networks in vertebrates. Next, we review some aspects of neural plasticity that is implemented in response to unilateral central injury or asymmetrical sensory deprivation in order to substantially restore symmetry in the control of posturo-locomotor functions. Data are finally discussed in the context of CNS structure-function relationship.

Context-independent encoding of passive and active self-motion in vestibular afferent fibers during locomotion in primates

The vestibular system detects head motion to coordinate vital reflexes and provide our sense of balance and spatial orientation. A long-standing hypothesis has been that projections from the central vestibular system back to the vestibular sensory organs (i.e., the efferent vestibular system) mediate adaptive sensory coding during voluntary locomotion. However, direct proof for this idea has been lacking. Here we recorded from individual semicircular canal and otolith afferents during walking and running in monkeys. Using a combination of mathematical modeling and nonlinear analysis, we show that afferent encoding is actually identical across passive and active conditions, irrespective of context. Thus, taken together our results are instead consistent with the view that the vestibular periphery relays robust information to the brain during primate locomotion, suggesting that context-dependent modulation instead occurs centrally to ensure that coding is consistent with behavioral goals during locomotion.

Using experimental and computational approaches the authors show that the vestibular efferent system does not modulate peripheral coding during locomotion. Instead, vestibular afferents unambiguously convey information in a context independent manner.

Inflight head stabilization associated with wingbeat cycle and sonar emissions in the lingual echolocating Egyptian fruit bat, Rousettus aegyptiacus

Sensory processing of environmental stimuli is challenged by head movements that perturb sensorimotor coordinate frames directing behaviors. In the case of visually guided behaviors, visual gaze stabilization results from the integrated activity of the vestibuloocular reflex and motor efference copy originating within circuits driving locomotor behavior. In the present investigation, it was hypothesized that head stabilization is broadly implemented in echolocating bats during sustained flight, and is temporally associated with emitted sonar signals which would optimize acoustic gaze. Predictions from these hypotheses were evaluated by measuring head and body kinematics with motion sensors attached to the head and body of free-flying Egyptian fruit bats. These devices were integrated with ultrasonic microphones to record sonar emissions and elucidate the temporal association with periods of head stabilization. Head accelerations in the Earth-vertical axis were asymmetric with respect to wing downstroke and upstroke relative to body accelerations. This indicated that inflight head and body accelerations were uncoupled, outcomes consistent with the mechanisms that limit vertical head acceleration during wing downstroke. Furthermore, sonar emissions during stable flight occurred most often during wing downstroke and head stabilization, supporting the conclusion that head stabilization behavior optimized sonar gaze and environmental interrogation via echolocation.

Differences in the Structure and Function of the Vestibular Efferent System Among Vertebrates

The role of the mammalian vestibular efferent system in everyday life has been a long-standing mystery. In contrast to what has been reported in lower vertebrate classes, the mammalian vestibular efferent system does not appear to relay inputs from other sensory modalities to the vestibular periphery. Furthermore, to date, the available evidence indicates that the mammalian vestibular efferent system does not relay motor-related signals to the vestibular periphery to modulate sensory coding of the voluntary self-motion generated during natural behaviors. Indeed, our recent neurophysiological studies have provided insight into how the peripheral vestibular system transmits head movement-related information to the brain in a context independent manner. The integration of vestibular and extra-vestibular information instead only occurs at next stage of the mammalian vestibular system, at the level of the vestibular nuclei. The question thus arises: what is the physiological role of the vestibular efferent system in mammals? We suggest that the mammalian vestibular efferent system does not play a significant role in short-term modulation of afferent coding, but instead plays a vital role over a longer time course, for example in calibrating and protecting the functional efficacy of vestibular circuits during development and aging in a role analogous the auditory efferent system.

No evidence for stochastic resonance effects on standing balance when applying noisy galvanic vestibular stimulation in young healthy adults

Noisy galvanic vestibular stimulation (nGVS) at imperceptible levels has been shown to reduce body sway. This reduction was commonly attributed to the mechanism of stochastic resonance (SR). However, it has never been explicitly tested whether nGVS-induced effects on body sway consistently follow a SR-like bell-shaped performance curve with maximal reductions in a particular range of noise intensities. To test this, body sway in 21 young healthy participants was measured during varying nGVS amplitudes while standing with eyes closed in 3 conditions (quiet stance, sway referencing, sinusoidal platform tilts). Presence of SR-like response dynamics in each trial was assessed (1) by a goodness-of-fit analysis using an established SR-curve model and (2) by ratings from 3 human experts. In accordance to theory, we found reductions of body sway at one nGVS amplitude in most trials (75–95%). However, only few trials exhibited SR-like bell-shaped performance curves with increasing noise amplitudes (10–33%). Instead, body sway measures rather fluctuated randomly across nGVS amplitudes. This implies that, at least in young healthy adults, nGVS effects on body sway are incompatible with SR. Thus, previously reported reductions of body sway at particular nGVS intensities more likely result from inherent variations of the performance metric or by other yet unknown mechanisms.

[Concepts for diagnosis, course and fall risk assessment in neurological gait disorders]

Quantitative gait assessment is increasingly applied in the diagnosis, disease monitoring, and risk stratification of neurological gait disorders. However, it is unclear, which measurement approaches, examination conditions, and gait characteristics are appropriate for answering specific clinical questions. The aim of this review was to provide generally applicable concepts and strategies for the measurement, analysis, and interpretation of gait function in the clinical context and to discuss their implementation in clinical practice. The first part of the article introduces currently available stationary and mobile measurement technologies that enable assessment of gait in clinical environments and to continuously track patients' mobility in the context of everyday life. Furthermore, the selection of adequate examination conditions and concepts that facilitate the parametrization of gait are discussed. The subsequent parts of the article address concrete clinical fields of application for quantitative gait analysis. With the help of exemplary cases from current research, the following issues are dicussed: (1) how specific patterns in gait assessments can guide differential diagnosis; (2) how quantitative gait measures can support the early diagnosis as well as the monitoring of disease progression and intervention outcomes in neurological gait disorders and finally, (3) the contribution of stationary gait and mobile mobility assessment for fall risk prognosis in patients with neurological gait impairments.

Assessing head acceleration to identify a motor threshold to galvanic vestibular stimulation

Galvanic vestibular stimulation (GVS) is used to assess vestibular system function, but vestibulospinal responses can exhibit variability depending on protocols or intensities used. Here, we measured head acceleration in healthy subjects to identify an objective motor threshold on which to base GVS intensity when assessing standing postural responses. Thirteen healthy right-handed subjects stood on a force platform, eyes closed, and head facing forward. An accelerometer was placed on the vertex to detect head acceleration, and electromyography activity of the right soleus was recorded. GVS (200 ms; current steps 0.5, from 1 mA to 4 mA) was applied in a binaural and bipolar configuration. 1) GVS induced a biphasic accelerometer response at a latency of 15 ms. Based on response amplitude, we constructed a recruitment curve for all participants and determined the motor threshold. In parallel, the method of limits was used to devise a more rapid approach to determine motor threshold. 2) We observed significant differences between motor threshold based on a recruitment curve and all perceptual thresholds reported either by the subject (sensation of movement) or a standing experimenter observing the participant (perception of movement). No significant difference was observed between the motor threshold based on the method of limits and perceptual thresholds of movement. 3) Using orthogonal polynomial contrasts, we observed a linear progression between multiples of the objective motor threshold (0.5, 0.75, 1, 1.5× motor threshold) and the 95% confidence ellipse area, the first peak of center of pressure displacement velocity, and the short and medium latency responses in the soleus. Hence, an objective motor threshold for GVS based on head acceleration was identified in standing participants and a recruitment curve could be constructed for all participants. These novel approaches could enable better understanding of changes in the vestibular system in different conditions or over time.NEW & NOTEWORTHY Galvanic vestibular stimulation (GVS) has been used to assess the vestibular system, but the significant interindividual variability in the responses makes it difficult to quantitatively compare them between individuals or conditions. Using an accelerometer to quantify head movement induced by GVS, we were able to determine an objective motor threshold and construct a recruitment curve for all participants. These methods could help assess changes in the vestibular system under different conditions.

The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions

The oblique extraocular muscles (EOMs) were dissected in 19 cetacean species and 10 non-cetacean mammalian species. Both superior oblique (SO) and inferior oblique (IO) muscles in cetaceans are well developed in comparison to out-groups and have unique anatomical features likely related to cetacean orbital configurations, swimming mechanics, and visual behaviors. Cetacean oblique muscles originate at skeletal locations typical for mammals: SO, from a common tendinous cone surrounding the optic nerve and from the medially adjacent bone surface at the orbital apex; IO, from the maxilla adjacent to lacrimal and frontal bones. However, because of the unusual orbital geometry in cetaceans, the paths and relations of SO and IO running toward their insertions onto the temporal ocular sclera are more elaborate than in humans and most other mammals. The proximal part of the SO extends from its origin at the apex along the dorsomedial aspect of the orbital contents to a strong fascial connection proximal to the preorbital process of the frontal bone, likely the cetacean homolog of the typical mammalian trochlea. However, the SO does not turn at this connection but continues onward, still a fleshy cylinder, until turning sharply as it passes through the external circular muscle (ECM) and parts of the palpebral belly of the superior rectus muscle. Upon departing this "functional trochlea" the SO forms a primary scleral insertion and multiple accessory insertions (AIs) onto adjacent EOM tendons and fascial structures. The primary SO scleral insertions are broad and muscular in most cetacean species examined, while in the mysticete minke whale (Balaenoptera acutorostrata) and fin whale (Balaenoptera physalus) the muscular SO bellies transition into broad fibrous tendons of insertion. The IO in cetaceans originates from an elongated fleshy attachment oriented laterally on the maxilla and continues laterally as a tubular belly before turning caudally at a sharp bend where it is constrained by the ECM and parts of the inferior rectus which form a functional trochlea as with the SO. The IO continues to a fleshy primary insertion on the temporal sclera but, as with SO, also has multiple AIs onto adjacent rectus tendons and connective tissue. The multiple IO insertions were particularly well developed in pygmy sperm whale (Kogia breviceps), minke whale and fin whale. AIs of both SO and IO muscles onto multiple structures as seen in cetaceans have been described in humans and domesticated mammals. The AIs of oblique EOMs seen in all these groups, as well as the unique "functional trochleae" of cetacean SO and IO seem likely to function in constraining the lines of action at the primary scleral insertions of the oblique muscles. The gimble-like sling formed by SO and IO in cetaceans suggest that the "primary" actions of the cetacean oblique EOMs are not only to produce ocular counter-rotations during up-down pitch movements of the head during swimming but also to rotate the plane containing the functional origins of the rectus muscles during other gaze changes.

Absence of Nonlinear Coupling Between Electric Vestibular Stimulation and Evoked Forces During Standing Balance

The vestibular system encodes motion and orientation of the head in space and is essential for negotiating in and interacting with the world. Recently, random waveform electric vestibular stimulation has become an increasingly common means of probing the vestibular system. However, many of the methods used to analyze the behavioral response to this type of stimulation assume a linear relationship between frequencies in the stimulus and its associated response. Here we examine this stimulus-response frequency linearity to determine the validity of this assumption. Forty-five university-aged subjects stood on a force-plate for 4 min while receiving vestibular stimulation. To determine the linearity of the stimulus-response relationship we calculated the cross-frequency power coupling between a 0 and 25 Hz bandwidth limited white noise stimulus and induced postural responses, as measured using the horizontal forces acting at the feet. Ultimately, we found that, on average, the postural response to a random stimulus is linear across stimulation frequencies. This result supports the use of analysis methods that depend on the assumption of stimulus-response frequency linearity, such as coherence and gain, which are commonly used to analyze the body’s response to random waveform electric stimuli.

Loss of peripheral vestibular input alters the statistics of head movement experienced during natural self-motion

KEY POINTS

Sensory systems are adapted to the statistical structure of natural stimuli, thereby optimizing neural coding. Head motion during natural activities is first sensed and then processed by central vestibulo-motor pathways to influence subsequent behaviour, thereby establishing a feedback loop. To investigate the role of this vestibular feedback on the statistical structure of the head movements, we compared head movements in patients with unilateral vestibular loss and healthy controls. We show that the loss of vestibular feedback substantially alters the statistical structure of head motion for activities that require rapid online feedback control and predict this change by modelling the effects of increased movement variability. Our findings suggest that, following peripheral vestibular loss, changes in the reliability of the sensory input to central pathways impact the statistical structure of head motion during voluntary behaviours.

ABSTRACT

It is widely believed that sensory systems are adapted to optimize neural coding of their natural stimuli. Recent evidence suggests that this is the case for the vestibular system, which senses head movement and contributes to essential functions ranging from the most automatic reflexes to voluntary motor control. During everyday behaviours, head motion is sensed by the vestibular system. In turn, this sensory feedback influences subsequent behaviour, raising the questions of whether and how real-time feedback provided by the vestibular system alters the statistical structure of head movements. We predicted that a reduction in vestibular feedback would alter head movement statistics, particularly for tasks reliant on rapid vestibular feedback. To test this proposal, we recorded six-dimensional head motion in patients with variable degrees of unilateral vestibular loss during standard balance and gait tasks, as well as dynamic self-paced activities. While distributions of linear accelerations and rotational velocities were comparable for patients and age-matched healthy controls, comparison of power spectra revealed significant differences during more dynamic and challenging activities. Specifically, consistent with our prediction, head movement power spectra were significantly altered in patients during two tasks that required rapid online vestibular feedback: active repetitive jumping and walking on foam. Using computational methods, we analysed concurrently measured torso motion and identified increases in head-torso movement variability. Taken together, our results demonstrate that vestibular loss significantly alters head movement statistics and further suggest that increased variability and impaired feedback to internal models required for accurate motor control contribute to the observed changes.

Gait speed in clinical and daily living assessments in Parkinson's disease patients: performance versus capacity

Gait speed often referred as the sixth vital sign is the most powerful biomarker of mobility. While a clinical setting allows the estimation of gait speed under controlled conditions that present functional capacity, gait speed in real-life conditions provides the actual performance of the patient. The goal of this study was to investigate objectively under what conditions during daily activities, patients perform as well as or better than in the clinic. To this end, we recruited 27 Parkinson's disease (PD) patients and measured their gait speed by inertial measurement units through several walking tests in the clinic as well as their daily activities at home. By fitting a bimodal Gaussian model to their gait speed distribution, we found that on average, patients had similar modes in the clinic and during daily activities. Furthermore, we observed that the number of medication doses taken throughout the day had a moderate correlation with the difference between clinic and home. Performing a cycle-by-cycle analysis on gait speed during the home assessment, overall only about 3% of the strides had equal or greater gait speeds than the patients' capacity in the clinic. These strides were during long walking bouts (>1 min) and happened before noon, around 26 min after medication intake, reaching their maximum occurrence probability 3 h after Levodopa intake. These results open the possibility of better control of medication intake in PD by considering both functional capacity and continuous monitoring of gait speed during real-life conditions.

The potential of noisy galvanic vestibular stimulation for optimizing and assisting human performance

Noisy galvanic vestibular stimulation (nGVS) is an emerging non-invasive brain stimulation technique. It involves applying alternating currents of different frequencies and amplitudes presented in a random, or noisy, manner through electrodes on the mastoid bones behind the ears. Because it directly activates vestibular hair cells and afferents and has an indirect effect on a variety of brain regions, it has the potential to impact many different functions. The objective of this review is twofold: (1) to review how nGVS affects motor, sensory, and cognitive performance in healthy adults; and (2) to discuss potential clinical applications of nGVS. First, we introduce the technique. We then describe the regions receiving and processing vestibular information. Next, we discuss the effects of nGVS on motor, sensory, and cognitive function in healthy adults. Subsequently, we outline its potential clinical applications. Finally, we highlight other electrical stimulation technologies and discuss why nGVS offers an alternative or complementary approach. Overall, nGVS appears promising for optimizing human performance and as an assistive technology, though further research is required.

Bilateral vestibulopathy: beyond imbalance and oscillopsia

Objective

To optimize the current diagnostic and treatment procedures for patients with bilateral vestibulopathy (BV), this study aimed to determine the complete spectrum of symptoms associated with BV.

Method

A prospective mixed-method study design was used. Qualitative data were collected by performing semi-structured interviews about symptoms, context, and behavior. The interviews were recorded and transcribed until no new information was obtained. Transcriptions were analyzed in consensus by two independent researchers. In comparison to the qualitative results, quantitative data were collected using the Dizziness Handicap Inventory (DHI), Hospital Anxiety and Depression Scale (HADS) and a health-related quality of life questionnaire (EQ-5D-5L).

Results

Eighteen interviews were transcribed. Reported symptoms were divided into fourteen physical symptoms, four cognitive symptoms, and six emotions. Symptoms increased in many situations, such as darkness (100%), uneven ground (61%), cycling (94%) or driving a car (56%). These symptoms associated with BV often resulted in behavioral changes: activities were performed more slowly, with greater attention, or were avoided. The DHI showed a mean score of severe handicap (54.67). The HADS questionnaire showed on average normal results (anxiety = 7.67, depression = 6.22). The EQ-5D-5L demonstrated a mean index value of 0.680, which is lower compared to the Dutch age-adjusted reference 0.839 (60–70 years).

Conclusion

BV frequently leads to physical, cognitive, and emotional complaints, which often results in a diminished quality of life. Importantly, this wide range of symptoms is currently underrated in literature and should be taken into consideration during the development of candidacy criteria and/or outcome measures for therapeutic interventions such as the vestibular implant.

Electronic supplementary material

The online version of this article (10.1007/s00415-020-10243-5) contains supplementary material, which is available to authorized users.

The gait disorder in primary orthostatic tremor

OBJECTIVE

To uncover possible impairments of walking and dynamic postural stability in patients with primary orthostatic tremor (OT).

METHODS

Spatiotemporal gait characteristics were quantified in 18 patients with primary OT (mean age 70.5 ± 5.9 years, 10 females) and 18 age-matched healthy controls. One-third of patients reported disease-related fall events. Walking performance was assessed on a pressure-sensitive carpet under seven conditions: walking at preferred, slow, and maximal speed, with head reclination or eyes closed, and while performing a cognitive or motor dual-task paradigm.

RESULTS

Patients exhibited a significant gait impairment characterized by a broadened base of support (p = 0.018) with increased spatiotemporal gait variability (p = 0.010). Walking speed was moderately reduced (p = 0.026) with shortened stride length (p = 0.001) and increased periods of double support (p = 0.001). Gait dysfunction became more pronounced during slow walking (p < 0.001); this was not present during fast walking. Walking with eyes closed aggravated gait disability as did walking during cognitive dual task (p < 0.001).

CONCLUSION

OT is associated with a specific gait disorder with a staggering wide-based walking pattern indicative of a sensory and/or a cerebellar ataxic gait. The aggravation of gait instability during visual withdrawal and the normalization of walking with faster speeds further suggest a proprioceptive or vestibulo-cerebellar deficit as the primary source of gait disturbance in OT. In addition, the gait decline during cognitive dual task may imply cognitive processing deficits. In the end, OT is presumably a complex network disorder resulting in a specific spino-cerebello-frontocortical gait disorder that goes beyond mere tremor networks.

Quantitative Evaluation of a New Posturo-Locomotor Phenotype in a Rodent Model of Acute Unilateral Vestibulopathy

Vestibular pathologies are difficult to diagnose. Existing devices make it possible to quantify and follow the evolution of posturo-locomotor symptoms following vestibular loss in static conditions. However, today, there are no diagnostic tools allowing the quantitative and spontaneous analysis of these symptoms in dynamic situations. With this in mind, we used an open-field video tracking test aiming at identifying specific posturo-locomotor markers in a rodent model of vestibular pathology. Using Ethovision XT 14 software (Noldus), we identified and quantified several behavioral parameters typical of unilateral vestibular lesions in a rat model of vestibular pathology. The unilateral vestibular neurectomy (UVN) rat model reproduces the symptoms of acute unilateral peripheral vestibulopathy in humans. Our data show deficits in locomotion velocity, distance traveled and animal mobility in the first day after the injury. We also highlighted alterations in several parameters, such as head and body acceleration, locomotor pattern, and position of the body, as well as “circling” behavior after vestibular loss. Here, we provide an enriched posturo-locomotor phenotype specific to full and irreversible unilateral vestibular loss. This test helps to strengthen the quantitative evaluation of vestibular disorders in unilateral vestibular lesion rat model. It may also be useful for testing pharmacological compounds promoting the restoration of balance. Transfer of these novel evaluation parameters to human pathology may improve the diagnosis of acute unilateral vestibulopathies and could better follow the evolution of the symptoms upon pharmacological and physical rehabilitation.

Key gait findings for diagnosing three syndromic categories of dynamic instability in patients with balance disorders

With the emergence of affordable, clinical-orientated gait analysis techniques, clinicians may benefit from a general understanding of quantitative gait analysis procedures and their clinical applications. This article provides an overview of the potential of a quantitative gait analysis for decision support in three clinically relevant scenarios of early stage gait disorders: scenario I: gait ataxia and unsteadiness; scenario II: hypokinesia and slow gait; scenario III: apparently normal gait with a specific fall tendency in complex mobility situations. In a first part, we justify the advantages of standardized data collection and analysis procedures including data normalization and dimensionality reduction techniques that facilitate clinical interpretability of instrument-based gait profiles. We then outline typical patterns of pathological gait and their modulation during different walking conditions (variation of speed, sensory perturbation, and dual tasking) and highlight key aspects that are particularly helpful to support and guide clinical decision-making.