Comparing the effects of different circular vection stimuli on upright stance.

BACKGROUND: Upright quiet stance is maintained through the complex integration of sensory information from the visual, vestibular, and somatosensory systems [1]. Virtual reality (VR) is a well-established tool that has been used to study sensory contributions to balance and induce visual perturbations. Previous assessments of virtual environments have suggested that VR can be used to create various visual stimuli that affect balance [2]; however, there is limited work examining which dynamic visual stimulus, in the form of circular vection (CV), is the most effective at inducing whole body lean. RESEARCH QUESTION: Therefore, this study assessed the effects of two visual stimuli using VR to better understand their effects on postural control. METHODS: 33 healthy young adults between the ages of 18-40, free of neurological impairments, stood quietly on a force plate for 30 s while wearing a head-mounted display. Participants were exposed to a field of random white dots (DOTS) or a black and white striped tunnel (TUNNEL) that rotated in the roll plane at 60°/s clockwise or counterclockwise. Amplitude was calculated from head orientation data recorded from a head-mounted display, and centre of pressure (COP). RESULTS: Independent of visual stimuli, postural lean was in the same direction as the stimulus. The DOTS stimulus increased Head orientation and COP position compared to the TUNNEL stimulus. There was no significant main effect or interaction with direction for Head or COP data. SIGNIFICANCE: When comparing the effect of stimulus design on postural sway, a DOTS stimulus was most effective at inducing direction-modulated postural sway This study builds on our understanding of the VR-related destabilizing effects on postural control and shows evidence that a DOTS stimulus has a stronger effect than a TUNNEL stimulus. Overall, it is important to consider the design of visual stimuli when examining VR effects on upright stance.

Retrospective Analysis of Circumstances of Falls and Related Injuries across Levels of Care in Older Adult Retirement Home Facilities.

Towards developing more effective interventions for fall-related injuries, this study analysed a novel database from six retirement home facilities over a 4-year period comprising 1,877 fallers and 12,445 falls. Falls were characterized based on location, activity, injury site, and type, and the database was stratified across four levels of care: Independent Living, Retirement Care, Assisted Care, and Memory care. Falls most occurred within the bedroom (62.8%), and during unknown (38.1%), walking (20.2%), and transfer tasks (14.6%). Approximately one in three (37%) of all falls resulted in an injury, most commonly involving the upper limb (31.8%), head (26.3%), and lower limb (22.2%), resulting in skin tears (35.3%), aches/pains (29.1%), or bruises (28.0%). While fall location, activity, and injury site were different across levels of care, injury type was not. The data from this study can assist in targeting fall-related injury prevention strategies across levels of care within retirement facilities.

The Effect of Roll Circular Vection on Roll Tilt Postural Responses and Roll Subjective Postural Horizontal of Healthy Normal Subjects.

Background: Falls and related injuries are critical issues in several disease states, as well as aging, especially when interactions between vestibular and visual sensory inputs are involved. Slow support surface tilt (0.6 deg/s) followed by subjective postural horizontal (SPH) assessments have been proposed as a viable method for assessing otolith contributions to balance control. Previous assessments of perceived body alignment to vertical, including subjective visual vertical, have suggested that visual inputs are weighted more when vestibular information is near the threshold and less reliable during slow body tilt. To date, no studies have examined the influence of visual stimuli on slow roll-tilt postural responses and the SPH. Therefore, this study investigated how dynamic visual cues, in the form of circular vection (CV), influence postural responses and the perception of the horizontal during and after support surface tilt. Methods: Ten healthy young adults (6 female, mean age 23) wore a head-mounted display while standing on a tilting platform. Participants were asked to remain upright for 30 s, during which (1) the visual scene rotated, inducing roll CV clockwise (CW) or counter-clockwise (CCW) at 60°/s; (2) the platform only (PO) rotated in roll to test SPH (0.6°/s, 2°, CW or CCW); (3) a combination of both; or (4) neither occurred. During SPH trials, participants used a hand-held device to reset the position of the platform to 0.8°/s to their perceived SPH. The angular motion of body segments was measured using pairs of light-emitting diodes mounted on the head, trunk and pelvis. Segment motion, prior to platform motion, was compared to that at peak body motion induced by platform motion and when SPH had been set. Results: When the support surface was tilted 2°, peak upper body tilt significantly increased for congruent CV and platform tilt and decreased at the pelvis for incongruent CV when compared to PO, leading to significant differences across body segments for congruent and incongruent conditions (p ≤ 0.008). During PO, participants' mean SPH deviated from horizontal by 0.2°. The pelvis deviated 0.2°, the trunk 0.3°, and the head 0.5° in the direction of initial platform rotation. When platform tilt and CV directions were congruent or incongruent, only head tilt at SPH reset under congruent conditions was significantly different from the PO condition (1.7° vs. 0.5°). Conclusions: Roll CV has a significant effect on phasic body responses and a less significant effect on tonic body responses to lateral tilt. The SPH of the support surface was not altered by CV. Responses during tilt demonstrated enhanced reactions for congruent and reduced reactions for incongruent CV, both different from responses to CV alone. Tonic body displacements associated with SPH were changed less than those during tilt and were only slightly larger than displacements for CV alone. This study supports the hypothesis of weighted multisensory integration during dynamic postural tasks being highly dependent on the direction of visual cues during tilt and less dependent on tonic SPH offsets. These techniques could be used to examine vestibular and visual interactions within clinical populations, particularly those with visual vertigo and dizziness.

Effects of postural threat on perceptions of lower leg somatosensory stimuli during standing.

Height-induced postural threat affects emotional state and standing balance behaviour during static, voluntary, and dynamic tasks. Facing a threat to balance also affects sensory and cortical processes during balance tasks. As sensory and cognitive functions are crucial in forming perceptions of movement, balance-related changes during threatening conditions might be associated with changes in conscious perceptions. Therefore, the purpose of this study was to examine the changes and potential mechanisms underlying conscious perceptions of balance-relevant information during height-induced postural threat. A combination of three experimental procedures utilized height-induced postural threat to manipulate emotional state, balance behavior, and/or conscious perceptions of balance-related stimuli. Experiment 1 assessed conscious perception of foot position during stance. During continuous antero-posterior pseudorandom support surface rotations, perceived foot movement was larger while actual foot movement did not change in the High (3.2 m, at the edge) compared to Low (1.1 m, away from edge) height conditions. Experiment 2 and 3 assessed somatosensory perceptual thresholds during upright stance. Perceptual thresholds for ankle rotations were elevated while foot sole vibrations thresholds remained unchanged in the High compared to Low condition. This study furthers our understanding of the relationship between emotional state, sensory perception, and balance performance. While threat can influence the perceived amplitude of above threshold ankle rotations, there is a reduction in the sensitivity of an ankle rotation without any change to foot sole sensitivity. These results highlight the effect of postural threat on neurophysiological and cognitive components of balance control and provide insight into balance assessment and intervention.

Visual feedback-dependent modulation of arousal, postural control, and muscle stretch reflexes assessed in real and virtual environments.

Introduction: The mechanisms regulating neuromuscular control of standing balance can be influenced by visual sensory feedback and arousal. Virtual reality (VR) is a cutting-edge tool for probing the neural control of balance and its dependence on visual feedback, but whether VR induces neuromodulation akin to that seen in real environments (eyes open vs. closed or ground level vs. height platform) remains unclear. Methods: Here we monitored 20 healthy young adults (mean age 23.3 ± 3.2 years; 10 females) during four conditions of quiet standing. Two real world conditions (eyes open and eyes closed; REO and REC) preceded two eyes-open virtual 'low' (ground level; VRL) and 'high' (14 m height platform; VRH) conditions. We measured arousal via electrodermal activity and psychosocial questionnaires rating perceived fear and anxiety. We recorded surface electromyography over the right soleus, medial gastrocnemius, and tibialis anterior, and performed force plate posturography. As a proxy for modulations in neural control, we assessed lower limb reflexive muscle responses evoked by tendon vibration and electrical stimulation. Results: Physiological and perceptual indicators of fear and anxiety increased in the VRH condition. Background soleus muscle activation was not different across conditions; however, significant increases in muscle activity were observed for medial gastrocnemius and tibialis anterior in VRH relative to REO. The mean power frequency of postural sway also increased in the VRH condition relative to REO. Finally, with a fixed stimulus level across conditions, mechanically evoked reflexes remained constant, while H-reflex amplitudes decreased in strength within virtual reality. Discussion: Notably, H-reflexes were lower in the VRL condition than REO, suggesting that these ostensibly similar visual environments produce different states of reflexive balance control. In summary, we provide novel evidence that VR can be used to modulate upright postural control, but caution that standing balance in analogous real and virtual environments may involve different neural control states.

The effect of modified optic flow gain on quiet stance.

Visual feedback provides critical information to support postural stability. Previous work has shown that magnifying visual feedback, such as by presenting individuals with biofeedback during balance tasks, can improve postural control. When studies manipulate the availability of optic flow directly, the conditions are often restricted to include an absence of visual feedback or sway referenced paradigms. Therefore, the aim of this study was to understand how manipulating the gain of optic flow contributes to quiet standing balance control among healthy adults. Optic flow was amplified or reduced relative to head motion using a virtual reality head-mounted display while participants stood quietly on either a firm or foam surface. Overall, when there was an increased reliance placed on the visual system by standing on foam, a tighter control of upright stance was observed as the gain of optic flow increased. Further, this study provided evidence that visual contributions to balance control may extend to higher frequencies of postural sway than previously theorized (greater than 0.1 Hz).

Exploring emotional-modulation of visually evoked postural responses through virtual reality.

Exposure to postural threat has been documented to influence the sensory contributions of proprioceptive and vestibular information in standing balance control. Contributions from the visual system to balance are also crucial, yet the degree to which postural threat may modulate visual control of balance is not well characterized. Therefore, the aims of this study were to assess the feasibility of eliciting visual evoked postural responses (VEPRs) using head-mounted virtual reality (VR) and use this method to examine the potential influence of virtual postural threat on the visual control of balance. Thirty-six healthy young adults were exposed to a pseudorandom, translational visual stimulus of a real-world environment in VR. The visual stimulus was presented in virtual conditions of LOW and HIGH postural threat in which participants stood at ground level, and on a 7 m elevated platform, respectively. VEPRs were successfully produced in both postural threat conditions. When exposed to the visual stimulus while at an elevated surface height, participants demonstrated significant changes to their physiological arousal and emotional state. Despite significant coherence across the stimulus' frequency range, stimulus correlated VEPRs were not significantly modulated during exposure to the visual stimulus under virtual postural threat. This study supports the future utility of VR head-mounted displays in examining emotional influences on the visual control of balance.

Vestibulo-ocular reflex gain improvements at peak head acceleration and velocity following onset of unilateral vestibular neuritis: Insights into neural compensation mechanisms.

BACKGROUND AND AIMS: An acute unilateral peripheral vestibular deficit (aUPVD) due to vestibular neuritis causes deficient yaw axis vestibular ocular reflex (VOR) gains. Using video head impulse tests (vHITs), we examined phasic and tonic velocity gains of the VOR over time to determine if these differed at onset and during subsequent improvement. METHODS: The VOR responses of 61 patients were examined within 5 days of aUPVD onset, and 3 and 7 weeks later using vHIT with mean peak yaw angular velocities of 177°/s (sd 45°/s) and mean peak accelerations of 3660°/s2 (sd 1300°/s2). The phasic velocity or acceleration gain (aG) was computed as the ratio of eye to head velocity around peak head acceleration, and the tonic velocity gain (vG) was calculated as the same ratio around peak head velocity. RESULTS: aG increased ipsi-deficit from 0.45 at onset to 0.67 at 3 weeks and 7 weeks later, and vG increased ipsi-deficit from 0.29 to 0.51 and 0.53, respectively, yielding a significant time effect (p < 0.001). Deficit side aG was significantly greater (p < 0.001) than vG at all time points. Deficit side gain improvements in aG and vG were similar. Contra-deficit aG increased from 0.86 to 0.95 and 0.94 at 3 weeks and 7 weeks, and vG contra-deficit increased from 0.84, to 0.89 and 0.87, respectively, also yielding a significant time effect (p = 0.004). Contra-deficit aG and vG were normal at 3 weeks. Mean canal paresis values improved from 91% to 67% over the 7 weeks. CONCLUSIONS: Acceleration and velocity VOR gains on the deficit side are reduced by aUPVD and improve most in the first 3 weeks after aUPVD onset. Deficit side aG is consistently higher than deficit side vG following an aUPVD, suggesting that acceleration rather than velocity sensitive compensatory neural mechanisms are predominant during the compensation process for aUPVD.

Brain connectivity during simulated balance in older adults with and without Parkinson's disease.

Individuals with Parkinson's disease often experience postural instability, a debilitating and largely treatment-resistant symptom. A better understanding of the neural substrates contributing to postural instability could lead to more effective treatments. Constraints of current functional neuroimaging techniques, such as the horizontal orientation of most MRI scanners (forcing participants to lie supine), complicates investigating cortical and subcortical activation patterns and connectivity networks involved in healthy and parkinsonian balance control. In this cross-sectional study, we utilized a newly-validated MRI-compatible balance simulator (based on an inverted pendulum) that enabled participants to perform balance-relevant tasks while supine in the scanner. We utilized functional MRI to explore effective connectivity underlying static and dynamic balance control in healthy older adults (n = 17) and individuals with Parkinson's disease while on medication (n = 17). Participants performed four tasks within the scanner with eyes closed: resting, proprioceptive tracking of passive ankle movement, static balancing of the simulator, and dynamic responses to random perturbations of the simulator. All analyses were done in the participant's native space without spatial transformation to a common template. Effective connectivity between 57 regions of interest was computed using a Bayesian Network learning approach with false discovery rate set to 5%. The first 12 principal components of the connection weights, binomial logistic regression, and cross-validation were used to create 4 separate models: contrasting static balancing vs {rest, proprioception} and dynamic balancing vs {rest, proprioception} for both controls and individuals with Parkinson's disease. In order to directly compare relevant connections between controls and individuals with Parkinson's disease, we used connections relevant for predicting a task in either controls or individuals with Parkinson's disease in logistic regression with Least Absolute Shrinkage and Selection Operator regularization. During dynamic balancing, we observed decreased connectivity between different motor areas and increased connectivity from the brainstem to several cortical and subcortical areas in controls, while individuals with Parkinson's disease showed increased connectivity associated with motor and parietal areas, and decreased connectivity from brainstem to other subcortical areas. No significant models were found for static balancing in either group. Our results support the notion that dynamic balance control in individuals with Parkinson's disease relies more on cortical motor areas compared to healthy older adults, who show a preference of subcortical control during dynamic balancing.

The Effect of Unilateral Vestibular Loss on Standing Balance During Postural Threat.

OBJECTIVE: Vestibular deficit patients have an increased fall risk and fear of falling. Postural threat, known to increase balance-related fear and anxiety, influences vestibular gains during quiet standing in young healthy adults. The current study examined whether there is a similar relationship for peripheral unilateral vestibular loss (UVL) patients in comparison to age-matched healthy controls (HC). SETTING: University laboratory. STUDY DESIGN: Prospective laboratory study. PATIENTS AND CONTROLS: Eleven UVL patients, nine with vestibular neurectomy. Eleven aged-matched HCs. MAIN OUTCOME MEASURES: Subjects stood on a hydraulic lift placed at two heights: low (0.8 m, away from the edge) and high (3.2 m, at the edge). Amplitude (root mean square), mean power frequency (MPF), and mean position were analyzed for center of foot pressure (COP) and 90% ranges for angle amplitude and velocity for trunk sway. RESULTS: Group interactions were strongest for anterior-posterior (AP) COP and trunk pitch angle. AP lean away from the edge was greater in HCs than UVLs. HCs, but not UVLs had a decrease in root mean square AP COP with height. Trunk pitch sway was changed similarly. Both groups had increased trunk pitch velocity at height. Changes with height were less for roll: MPF of lateral COP increased with height for UVLs with no changes for HCs, and trunk roll amplitude decreased for both groups. CONCLUSIONS: This report provides evidence for a differential effect of height induced postural threat on balance control between UVLs and HCs presumably due to the reduced vestibular-spinal gain in UVL subjects.

Monocular 3D Sway Tracking for Assessing Postural Instability in Cerebral Hypoperfusion During Quiet Standing.

Postural instability is prevalent in aging and neurodegenerative disease, decreasing quality of life and independence. Quantitatively monitoring balance control is important for assessing treatment efficacy and rehabilitation progress. However, existing technologies for assessing postural sway are complex and expensive, limiting their widespread utility. Here, we propose a monocular imaging system capable of assessing sub-millimeter 3D sway dynamics during quiet standing. Two anatomical targets with known feature geometries were placed on the lumbar and shoulder. Upper and lower trunk 3D kinematic motion were automatically assessed from a set of 2D frames through geometric feature tracking and an inverse motion model. Sway was tracked in 3D and compared between control and hypoperfusion conditions in 14 healthy young adults. The proposed system demonstrated high agreement with a commercial motion capture system (error [Formula: see text], [-0.52, 0.52]). Between-condition differences in sway dynamics were observed in anterior-posterior sway during early and mid stance, and medial-lateral sway during mid stance commensurate with decreased cerebral perfusion, followed by recovered sway dynamics during late stance with cerebral perfusion recovery. This inexpensive single-camera system enables quantitative 3D sway monitoring for assessing neuromuscular balance control in weakly constrained environments.

A Novel MRI Compatible Balance Simulator to Detect Postural Instability in Parkinson's Disease.

Background: Postural instability is a debilitating and largely treatment-resistant symptom of Parkinson's disease (PD). A better understanding of the neural substrates contributing to postural instability could lead to new targets for improved pharmacological and neurosurgical interventions. However, investigating these neural substrates necessitates the use of functional MRI scanners, which are almost exclusively horizontally-based. Objective: We aimed to develop, and validate the use of, an MRI compatible balance simulator to study static and dynamic balance control in PD patients and elderly controls. Methods: Our MRI compatible balance simulator allowed participants to actively balance an inverted pendulum by activating postural muscles around the ankle joint while supine. Two studies were performed to compare static and dynamic balance performance between upright stance and simulated stance in PD patients and controls. Study 1 (14 PD; 20 controls) required participants to maintain static balance during upright and simulated stance for 120 s with eyes open and closed. In study 2 (20 PD; 22 controls) participants repeated the static balance task (80 s, eyes closed only), and also completed a dynamic balance task which required maintaining balance while experiencing random anterior-posterior perturbations applied to the trunk/pendulum. Postural sway of the body/pendulum was measured using an angular velocity sensor (SwayStarTM, study 1) and Optotrak motion capture (study 2). Outcome measures were amplitude and frequency of center of mass sway for static balance, and peak and time-to-peak of center of mass displacement and velocity for dynamic balance. Results: PD patients had larger sway amplitude during both upright and simulated static balance compared to controls. PD patients had larger peak and time-to-peak sway, and larger time-to-peak sway velocity, during simulated, but not upright, dynamic balance compared to controls. Conclusions: Deficits in static and dynamic balance control can be detected in PD patients using a novel MRI compatible balance simulator. This technique allows for functional neuroimaging to be combined with balance-relevant tasks, and provides a new means to create insights into the neural substrates contributing to postural instability in PD.

Postural Threat Modulates Perceptions of Balance-Related Movement During Support Surface Rotations.

Postural threat decreases center of pressure displacements yet increases the magnitude of movement-related conscious sway perception during quiet standing. It is unknown how these changes influence perception of whole body movement during dynamic stance. The aim of this study was to examine how postural threat influences whole-body movements and conscious perception of these movements during continuous pseudo-random support surface perturbations to stance. Sixteen healthy young adults stood on a moveable platform with their eyes closed for 7 min in a low threat (1.1 m above ground, away from edge) then high threat (3.2 m above ground, near edge) condition. Continuous pseudorandom roll platform rotations (± 4.5°, < 0.5 Hz) evoked large amplitude sway in the medio-lateral (ML) direction. Participants were asked to remain upright and avoid a fall at all times while tracking their ML body movements using a hand-held rotary encoder. Kinematic data was recorded using three markers placed on the upper trunk. Questionnaires assessed anxiety, fear and confidence. Electrodermal activity (EDA) was recorded as an indicator of arousal. Height-induced threat increased fear, anxiety and EDA, and decreased confidence. Trunk sway amplitude remained constant, while tracked movement amplitude increased at height. The gain for perceived to trunk movement was significantly increased at height across frequencies. Threat-related increases in sensitivity of sensory systems related to postural control and changes in cognitive and attention processes may lead to misperceptions of actual movement amplitudes, which may be important when examining increased fall risk in those with a fear of falling.

Postural threat influences the conscious perception of body position during voluntary leaning.

BACKGROUND: Height-related changes in postural control can alter feedback used to control balance, which may lead to a mismatch in perceived and actual sway changes during quiet stance. However, there is still a need to examine how these changes affect the ability to detect limits of stability (and movements related to base of support limits). RESEARCH QUESTION: The aim of this study was to examine how changes in height-related threat influence conscious perceptions of body position during voluntary balance tasks. METHODS: Twenty young healthy adults, fitted with kinematic markers on the right side of the body, stood on a forceplate mounted to a hydraulic lift placed at two heights (0.8 m and 3.2 m). At height (completed first), participants leaned as far forward as possible, at the ankle joint, while trying to remain as an inverted pendulum. Then, at each height, participants stood with eyes open, and voluntarily leaned to one of ten targets (10%-100% maximum lean) displayed visually as angular displacement of body segments on a screen. Once on target, participants reported a perceived position relative to their maximum lean. Balance confidence, fear and anxiety, and physiological arousal (hand electrodermal activity, EDA) were recorded and statistically tested using paired sample t-tests. Actual and perceived body positions were tested using repeated measures ANOVAs (height x target). RESULTS: Height significantly increased EDA, fear and anxiety, and decreased balance confidence. Participants voluntarily leaned to all target positions equally across heights. However, at any given target position, the perceived lean changed with height. When participants are asked to lean to a target in at height, their amount of perceived lean was larger by 4.9%, on average (range: 1.8%-9.7%). SIGNIFICANCE: This modulation in perceived limits of stability may increase the risk of falls in those who have an increased fear of falling.

Influence of emotional stimuli on lower limb cutaneous reflexes during human gait.

Previous research has shown that cutaneous reflexes are modulated when walking with a threat to stability. It is unclear if this reflex modulation is purely related to the context of the imposed threat or if emotional changes associated with the threat exert an independent influence on reflex excitability. This study investigated the influence of emotional stimuli on lower limb cutaneous reflexes during treadmill walking. Twenty-eight healthy young adults walked at a self-selected pace while viewing pictures that manipulated emotional arousal and valence (confirmed with electrodermal and self-report measures). Throughout each trial, cutaneous reflexes were evoked by electrically stimulating the sural nerve at heel contact, mid-stance, or toe off. Surface electromyography of the ipsilateral soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), biceps femoris (BF), and vastus lateralis (VL) was recorded to assess reflexes. Highly arousing pictures, independent of valence, significantly facilitated TA, and trended toward facilitating SOL and BF reflexes during mid-stance. Unpleasant pictures, independent of arousal, significantly reduced reflex amplitudes in BF during mid-stance and TA during toe off. While changes in background muscle activity and step cadence were observed, they did not correlate with reflex changes. This study provides the first evidence that emotional stimuli exert an independent influence on cutaneous reflex excitability during gait. As cutaneous reflexes contribute to stability during gait, these findings support the notion that emotional state influences important sensorimotor processes underlying balance control.

Differences in head impulse test results due to analysis techniques.

BACKGROUND: Different analysis techniques are used to define vestibulo-ocular reflex (VOR) gain between eye and head angular velocity during the video head impulse test (vHIT). Comparisons would aid selection of gain techniques best related to head impulse characteristics and promote standardisation. OBJECTIVE: Compare and contrast known methods of calculating vHIT VOR gain. METHODS: We examined lateral canal vHIT responses recorded from 20 patients twice within 13 weeks of acute unilateral peripheral vestibular deficit onset. Ten patients were tested with an ICS Impulse system (GN Otometrics) and 10 with an EyeSeeCam (ESC) system (Interacoustics). Mean gain and variance were computed with area, average sample gain, and regression techniques over specific head angular velocity (HV) and acceleration (HA) intervals. RESULTS: Results for the same gain technique were not different between measurement systems. Area and average sample gain yielded equally lower variances than regression techniques. Gains computed over the whole impulse duration were larger than those computed for increasing HV. Gain over decreasing HV was associated with larger variances. Gains computed around peak HV were smaller than those computed around peak HA. The median gain over 50-70 ms was not different from gain around peak HV. However, depending on technique used, the gain over increasing HV was different from gain around peak HA. Conversion equations between gains obtained with standard ICS and ESC methods were computed. For low gains, the conversion was dominated by a constant that needed to be added to ESC gains to equal ICS gains. CONCLUSIONS: We recommend manufacturers standardize vHIT gain calculations using 2 techniques: area gain around peak HA and peak HV.

Postural threat influences vestibular-evoked muscular responses.

Standing balance is significantly influenced by postural threat. While this effect has been well established, the underlying mechanisms of the effect are less understood. The involvement of the vestibular system is under current debate, and recent studies that investigated the effects of height-induced postural threat on vestibular-evoked responses provide conflicting results based on kinetic (Horslen BC, Dakin CJ, Inglis JT, Blouin JS, Carpenter MG. J Physiol 592: 3671-3685, 2014) and kinematic (Osler CJ, Tersteeg MC, Reynolds RF, Loram ID. Eur J Neurosci 38: 3239-3247, 2013) data. We examined the effect of threat of perturbation, a different form of postural threat, on coupling (cross-correlation, coherence, and gain) of the vestibulo-muscular relationship in 25 participants who maintained standing balance. In the "No-Threat" conditions, participants stood quietly on a stable surface. In the "Threat" condition, participants' balance was threatened with unpredictable mediolateral support surface tilts. Quiet standing immediately before the surface tilts was compared to an equivalent time from the No-Threat conditions. Surface EMG was recorded from bilateral trunk, hip, and leg muscles. Hip and leg muscles exhibited significant increases in peak cross-correlation amplitudes, coherence, and gain (1.23-2.66×) in the Threat condition compared with No-Threat conditions, and significant correlations were observed between threat-related changes in physiological arousal and medium-latency peak cross-correlation amplitude in medial gastrocnemius (r = 0.408) muscles. These findings show a clear threat effect on vestibular-evoked responses in muscles in the lower body, with less robust effects of threat on trunk muscles. Combined with previous work, the present results can provide insight into observed changes during balance control in threatening situations. NEW & NOTEWORTHY: This is the first study to show increases in vestibular-evoked responses of the lower body muscles under conditions of increased threat of postural perturbation. While robust findings were observed in hip and leg muscles, less consistent results were found in muscles of the trunk. The present findings provide further support in the ongoing debate for arguments that vestibular-evoked balance responses are influenced by fear and anxiety and explain previous threat-related changes in balance.

The threat of a support surface translation affects anticipatory postural control.

This study examined how postural threat in the form of a potential perturbation affects an individual's ability to perform a heel raise. Seventeen adults completed three conditions: i) low threat, where participants performed a heel raise in response to a "go" tone, ii) high threat, where participants either heard the same "go" tone, for which they performed a heel raise, or experienced a support surface translation in the medio-lateral direction that disturbed their balance, and iii) choice reaction time task, where participants either completed a heel raise in response to the same "go" tone or a toe raise in response to a lower pitched tone. For all heel raise trials, anticipatory postural adjustments (APAs) were quantified from center of pressure (COP) recordings and electromyographic (EMG) activity from the tibialis anterior (TA) and soleus (SOL). Results indicated that participants exhibited larger APAs, as reflected by the greater backward COP displacement (p=0.038) and velocity (p=0.022) as well as a larger TA EMG amplitude (p=0.045), during the high threat condition. During the execution phase of the heel raise, an earlier (p=0.014) and larger (p=0.041) SOL EMG activation were observed during the high threat condition. These results contrast with previous findings of reduced APAs when the postural threat was evoked through changes in surface height. Therefore, the characteristics of the postural threat must be considered to isolate the effects of threat on anticipatory movement control.

Influence of virtual height exposure on postural reactions to support surface translations.

As fear of falling is related to the increased likelihood of falls, it is important to understand the effects of threat-related factors (fear, anxiety and confidence) on dynamic postural reactions. Previous studies designed to examine threat effects on dynamic postural reactions have methodological limitations and lack a comprehensive analysis of simultaneous kinetic, kinematic and electromyographical recordings. The current study addressed these limitations by examining postural reactions of 26 healthy young adults to unpredictable anterior-posterior support-surface translations (acceleration=0.6m/s(2), constant velocity=0.25m/s, total displacement=0.75m) while standing on a narrow virtual surface at Low (0.4cm) and High (3.2m) virtual heights. Standing at virtual height increased fear and anxiety, and decreased confidence. Prior to perturbations, threat led to increased tonic muscle activity in tibialis anterior, resulting in a higher co-contraction index between lower leg muscles. For backward perturbations, muscle activity in the lower leg and arm, and center of pressure peak displacements, were earlier and larger when standing at virtual height. In addition, arm flexion significantly increased while leg, trunk and center of mass displacements remained unchanged across heights. When controlling for leaning, threat-related factors can influence the neuro-mechanical responses to an unpredictable perturbation, causing specific characteristics of postural reactions to be facilitated in young adults when their balance is threatened.