Visual contribution to human standing balance during support surface tilts

What Is the Role of Sustained Visual Attention in the Maintenance of Postural Control in Young Adults?

Dual tasks requiring sustained visual attention and upright stance are common, yet their impact on standing balance is not well understood. We investigated the role of visual attention in the maintenance of postural control, using the multiple-object tracking (MOT) task. Healthy young adults (n = 12) performed the MOT task at three object movement speeds while seated or standing. MOT performance was assessed using tracking capacity (k). Metrics calculated to assess mediolateral (ML) and anterior-posterior (AP) postural control included: maximum difference between CoM and CoP position (CoM-CoP Max), root mean square distance for center of pressure and center of mass position (CoP and CoM RMS distance), and correlation between CoM and CoP time series signals (CoM/CoP correlation). As predicted, k decreased significantly as object movement speed increased for both standing and seated conditions. Object movement speed also significantly affected AP CoM-CoP Max in seated conditions (p = .021) and AP CoM/CoP correlation for standing conditions (p = .002). The results demonstrate utility of the MOT task in understanding the role of visual attention in postural control, even though healthy young adults were able to compensate for the addition of a sustained visual attention task, with minimal deficits to postural control.

Estimation of the visual contribution to standing balance using virtual reality

Sensory perturbations are a valuable tool to assess sensory integration mechanisms underlying balance. Implemented as systems-identification approaches, they can be used to quantitatively assess balance deficits and separate underlying causes. However, the experiments require controlled perturbations and sophisticated modeling and optimization techniques. Here we propose and validate a virtual reality implementation of moving visual scene experiments together with model-based interpretations of the results. The approach simplifies the experimental implementation and offers a platform to implement standardized analysis routines. Sway of 14 healthy young subjects wearing a virtual reality head-mounted display was measured. Subjects viewed a virtual room or a screen inside the room, which were both moved during a series of sinusoidal or pseudo-random room or screen tilt sequences recorded on two days. In a between-subject comparison of 10 [Formula: see text] 6 min long pseudo-random sequences, each applied at 5 amplitudes, our results showed no difference to a real-world moving screen experiment from the literature. We used the independent-channel model to interpret our data, which provides a direct estimate of the visual contribution to balance, together with parameters characterizing the dynamics of the feedback system. Reliability estimates of single subject parameters from six repetitions of a 6 [Formula: see text] 20-s pseudo-random sequence showed poor test-retest agreement. Estimated parameters show excellent reliability when averaging across three repetitions within each day and comparing across days (Intra-class correlation; ICC 0.7-0.9 for visual weight, time delay and feedback gain). Sway responses strongly depended on the visual scene, where the high-contrast, abstract screen evoked larger sway as compared to the photo-realistic room. In conclusion, our proposed virtual reality approach allows researchers to reliably assess balance control dynamics including the visual contribution to balance with minimal implementation effort.

Human body-sway steady-state responses to small amplitude tilts and translations of the support surface - Effects of superposition of the two stimuli

BACKGROUND

For humans, control of upright standing posture is a prerequisite for many physical activities. Experimentally, this control is often challenged by the motion of the support surface presented as tilt or translation, or some combination thereof. In particular, we have investigated subjects balancing in situations where tilt and translation stimuli were presented in isolation and compared it to a situation where such stimuli occurred simultaneously.

RESEARCH QUESTIONS

Is the human posture control system in the case of two or more superimposed external disturbances responding to these as if it were dealing with one disturbance? Or does it identify the disturbances individually and as such and respond to them specifically, as suggested in a current concept of disturbance-specific estimations and compensations?

METHODS

We had healthy human subjects controlling their balancing of upright stance on a motion platform while we presented them with different combinations of pseudorandom support surface tilt and translation stimuli alone or in superposition (with peak-to-peak amplitude of 0.5° and 1° for tilt, and 0.8 cm and 1.5 cm for translation). In one set of trials they kept their eyes closed and in a second set open. Furthermore, a simulation was performed to qualitatively evaluate the impact of sensory non-linearities and joint stiffness modulation.

RESULTS

We found that the experimental conditions 'eyes open' vs. 'eyes closed' always created significant differences (p < 0.05) between the frequency response functions. In contrast to this, with different combinations of the tilt and translation stimuli, significant differences between the responses were observed only in 5 cases over the 24 that have been tested. Significance The superposition of translation and tilt can be used to characterize the responses to both stimuli with one trial. When the amplitude of the stimuli is unbalanced (e.g. very small tilt superimposed with a larger translation) the effect of stiffness modulation can be studied.

Efficacy of Tai Chi on lower limb function of Parkinson's disease patients: A systematic review and meta-analysis

Background

At present, the effect of Tai Chi (TC) on lower limb function in patients with Parkinson's disease (PD) is controversial. Therefore, we conducted a meta-analysis on the influence of TC on lower limb function in PD patients.

Methods

According to the PRISMA guidelines, seven databases were searched. Randomized controlled trials (RCTS) were selected and screened according to inclusion and exclusion criteria. We assessed the quality of the studies using the Cochrane Risk of Bias tool and then extracted the characteristics of the included studies. The random effect model was adopted, and heterogeneity was measured by I ² statistic.

Results

A total of 441 articles were screened, and 10 high-quality RCTs were with a total of 532 patients with PD met Our inclusion criteria. Meta-analysis showed that compared To control groups TC improved several outcomes. TC significantly improved motor function (SMD = -0.70; 95% CI = -0.95, -0.45; p < 0.001; I ² = 35%), although The results were not statistically significant for The subgroup analysis of TC duration (SMD = -0.70; 95% CI = -0.95, -0.45; p = 0.88; I ² = 0%;). TC significantly improved balance function (SMD = 0.89; 95% CI = 0.51, 1.27; p < 0.001; I ² = 54%), functional walking capacity (SMD = -1.24; 95% CI = -2.40, -0.09; p = 0.04; I ² = 95%), and gait velocity (SMD = 0.48; 95% CI = -0.02, 0.94; p = 0.04; I ² = 78%), But Did Not improve endurance (SMD = 0.31; 95% CI = -0.12, 0.75; p = 0.16; I ² = 0%), step length (SMD = 0.01; 95% CI = -0.34, 0.37; p = 0.94; I ² = 29%), and cadence (SMD = 0.06; 95% CI = -0.25, 0.36; p = 0.70; I ² = 0%).

Conclusion

TC has beneficial effects on motor function, balance function, functional walking ability, and gait velocity, but does not improve walking endurance, stride length, and cadence.

A portable and low-cost solution for real-time manipulation of the vestibular sense

BACKGROUND

The vestibular system encodes head motion in space which is naturally accompanied by other sensory cues. Electrical stimuli, applied across the mastoid processes, selectively activate primary vestibular afferents which has spurred clinical and biomedical applications of electrical vestibular stimulation (EVS). When properly matched to head motion, EVS may also manipulate the closed-loop relationship between actions and vestibular feedback to reveal the mechanisms of sensorimotor recalibration and learning.

NEW METHOD

We designed a portable, low-cost real-time EVS system using an Arduino microcontroller programmed through Simulink that provides electrical currents based on head angular motion. We used well-characterized vestibular afferent physiological responses to head angular velocity and electrical current to compute head-motion equivalent of real-time modulatory EVS currents. We also examined if our system induced recalibration of the vestibular system during human balance control.

RESULTS

Our system operated at 199.997 Hz ( ± 0.005 Hz) and delivered head-motion-equivalent electrical currents with ∼10 ms delay. The output driving the current stimulator matched the implemented linear model for physiological vestibular afferent dynamics with minimal background noise (<0.2% of ± 10 V range). Participants recalibrated to the modulated closed-loop vestibular feedback using visual cues during standing balance, replicating earlier findings.

COMPARISON WITH EXISTING METHODS

EVS is typically used to impose external perturbations that are independent of one's own movement. We provided a solution using open-source hardware to implement a real-time, physiology based, and task-relevant vestibular modulations using EVS.

CONCLUSIONS

Our portable, low-cost vestibular modulation system will make physiological closed-loop vestibular manipulations more accessible thus encouraging novel investigations and biomedical applications of EVS.

The Walls Are Closing In: Postural Responses to a Virtual Reality Claustrophobic Simulation

Background: Changes in the visual environment and thereby, the spatial orientation, can induce postural instability leading to falls. Virtual reality (VR) has been used to expose individuals to virtual environments (VE) that increase postural threats. Claustrophobia is an anxiety disorder categorized under situational phobias and can induce such postural threats in a VE. Purpose: The purpose of the study was to investigate if VR-generated claustrophobic simulation has any impact on postural threats that might lead to postural instability. Methods: Thirty healthy men and women (age: 20.7 ± 1.2 years; height: 166.5 ± 7.3 cm; mass: 71.7 ± 16.2 kg) were tested for postural stability while standing on a force platform, upon exposure to five different testing trials, including a normal stance (NoVR), in stationary VE (VR), and three consecutive, randomly initiated, unexpected claustrophobia trials (VR CP1, VR CP2, VR CP3). The claustrophobia trials involved all four walls closing in towards the center of the room. Center of pressure (COP)-derived postural sway variables were analyzed with a one-way repeated measures analysis of variance at an alpha level of 0.05. Results: Significant main effect differences existed in all but one dependent COP-derived postural sway variables, at p < 0.05. Post-hoc pairwise comparisons with a Bonferroni correction revealed that, predominantly, postural sway excursions were significantly lower in claustrophobia trials compared to NoVR and VR, but only accomplished with significantly increased sway velocity. Conclusion: The VR CP trials induced lower postural sway magnitude, but with increased velocity, suggesting a bracing and co-contraction strategy when exposed to virtual claustrophobic postural threats. Additionally, postural sway decreased with subsequent claustrophobia trials, suggesting potential motor learning effects. Findings from the study offer insights to postural control behavior under virtual claustrophobic simulations and can aid in VR exposure therapy for claustrophobia.

Mechanisms of motor symptom improvement by long-term Tai Chi training in Parkinson’s disease patients

Background

Tai Chi has been shown to improve motor symptoms in Parkinson’s disease (PD), but its long-term effects and the related mechanisms remain to be elucidated. In this study, we investigated the effects of long-term Tai Chi training on motor symptoms in PD and the underlying mechanisms.

Methods

Ninety-five early-stage PD patients were enrolled and randomly divided into Tai Chi (n = 32), brisk walking (n = 31) and no-exercise (n = 32) groups. At baseline, 6 months and 12 months during one-year intervention, all participants underwent motor symptom evaluation by Berg balance scale (BBS), Unified PD rating-scale (UPDRS), Timed Up and Go test (TUG) and 3D gait analysis, functional magnetic resonance imaging (fMRI), plasma cytokine and metabolomics analysis, and blood Huntingtin interaction protein 2 (HIP2) mRNA level analysis. Longitudinal self-changes were calculated using repeated measures ANOVA. GEE (generalized estimating equations) was used to assess factors associated with the longitudinal data of rating scales. Switch rates were used for fMRI analysis. False discovery rate correction was used for multiple correction.

Results

Participants in the Tai Chi group had better performance in BBS, UPDRS, TUG and step width. Besides, Tai Chi was advantageous over brisk walking in improving BBS and step width. The improved BBS was correlated with enhanced visual network function and downregulation of interleukin-1β. The improvements in UPDRS were associated with enhanced default mode network function, decreased L-malic acid and 3-phosphoglyceric acid, and increased adenosine and HIP2 mRNA levels. In addition, arginine biosynthesis, urea cycle, tricarboxylic acid cycle and beta oxidation of very-long-chain fatty acids were also improved by Tai Chi training.

Conclusions

Long-term Tai Chi training improves motor function, especially gait and balance, in PD. The underlying mechanisms may include enhanced brain network function, reduced inflammation, improved amino acid metabolism, energy metabolism and neurotransmitter metabolism, and decreased vulnerability to dopaminergic degeneration.

Trial registration This study has been registered at Chinese Clinical Trial Registry (Registration number: ChiCTR2000036036; Registration date: August 22, 2020).

Supplementary Information

The online version contains supplementary material available at 10.1186/s40035-022-00280-7.

The effect of computer mouse mass on target acquisition performance among action video gamers

Advances in human performance have shifted research attention from individuals towards understanding the effects that equipment can have on their performance. In esports, the effect of gaming peripherals on performance has only recently been explored. In this study, we investigated the effect of computer mouse mass on gaming skill. 72 video game players performed a target acquisition task, a pertinent composite skill in first-person shooter (FPS) video games, while using computer mice that varied only by their mass (50g, 60g, 90g & 100g). Results showed that participants were 4% faster and 9% more accurate with 50g, 60g, and 90g mice compared to the 100g mouse. They were also 34% more accurate and 14% more precise when using lower control-display gain and preferred lighter mice (50g and 60g) when performing the skill at a lower control-display gain. Taken together, using a lighter mouse at lower sensitivity may augment target acquisition performance among video gamers.

Kohonen Neural Network Investigation of the Effects of the Visual, Proprioceptive and Vestibular Systems to Balance in Young Healthy Adult Subjects

Kohonen neural network (KNN) was used to investigate the effects of the visual, proprioceptive and vestibular systems using the sway information in the mediolateral (ML) and anterior-posterior (AP) directions, obtained from an inertial measurement unit, placed at the lower backs of 23 healthy adult subjects (10 males, 13 females, mean (standard deviation) age: 24.5 (4.0) years, height: 173.6 (6.8) centimeter, weight: 72.7 (9.9) kg). The measurements were based on the modified Clinical Test of Sensory Interaction and Balance (mCTSIB). KNN clustered the subjects’ time-domain sway measures by processing their sway’s root mean square position, velocity, and acceleration. Clustering effectiveness was established using external performance indicators such as purity, precision-recall, and F-measure. Differences in these measures, from the clustering of each mCTSIB condition with its condition, were used to extract information about the balance-related sensory systems, where smaller values indicated reduced sway differences. The results for the parameters of purity, precision, recall, and F-measure were higher in the AP direction as compared to the ML direction by 7.12%, 11.64%, 7.12%, and 9.50% respectively, with their differences statistically significant (p < 0.05) thus suggesting the related sensory systems affect majorly the AP direction sway as compared to the ML direction sway. Sway differences in the ML direction were lowest in the presence of the visual system. It was concluded that the effect of the visual system on the balance can be examined mostly by the ML sway while the proprioceptive and vestibular systems can be examined mostly by the AP direction sway.

Multisensory postural control in adults: Variation in visual, haptic, and proprioceptive inputs

Maintaining balance is fundamentally a multisensory process, with visual, haptic, and proprioceptive information all playing an important role in postural control. The current project examined the interaction between such sensory inputs, manipulating visual (presence versus absence), haptic (presence versus absence of contact with a stable or unstable finger support surface), and proprioceptive (varying stance widths, including shoulder width stance, Chaplin [heels together, feet splayed at approximately 60°] stance, feet together stance, and tandem stance) information. Analyses of mean velocity of the Centre of Pressure (CoP) revealed significant interactions between these factors, with stability gains observed as a function of increasing sensory information (e.g., visual, haptic, visual + haptic), although the nature of these gains was modulated by the proprioceptive information and the reliability of the haptic support surface (i.e., unstable versus stable finger supports). Subsequent analyses on individual difference parameters (e.g., height, leg length, weight, and areas of base of support) revealed that these variables were significantly related to postural measures across experimental conditions. These findings are discussed relative to their implications for multisensory postural control, and with respect to inverted pendulum models of balance. (185 words).

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.

Does Regular Dancing Improve Static Balance?

The aim of the study was to compare the static balance of dancers and non-dancers in a bipedal and unipedal stance. Twenty-three female dancers (mean age: 21.3 ± 1.7) and 24 age and sex-matched subjects (mean age 22.3 ± 1.0) participated in this study. A force platform was used to assess balance. The tests on the balance platform were performed in several positions with different foot placement, such as normal standing (NS) eyes open and eyes closed positions, semi-tandem position (ST), tandem position (TP), and one-leg standing (1L) eyes open and eyes closed position. Significant differences in balance between the dancers and the control group, especially in the tandem position and one-leg standing position with eyes closed were found. We observed higher results for the velocity of the COP in the frontal plane in the TP with a dominant limb in front (p = 0.04) and higher results for the velocity of the COP in the frontal plane (p = 0.01) and in the sagittal plane (p < 0.01) in the TP with a dominant limb in front in the control group. We also observed significant differences between groups in the mean velocity of COP sway in the sagittal plane in the 1 L position with eyes closed (p = 0.04). We concluded that dancing regularly for several years improves static balance.

Rolling balance board of adjustable geometry as a tool to assess balancing skill and to estimate reaction time delay

The relation between balancing performance and reaction time is investigated for human subjects balancing on rolling balance board of adjustable physical parameters: adjustable rolling radius R and adjustable board elevation h. A well-defined measure of balancing performance is whether a subject can or cannot balance on balance board with a given geometry (R, h). The balancing ability is linked to the stabilizability of the underlying two-degree-of-freedom mechanical model subject to a delayed proportional-derivative feedback control. Although different sensory perceptions involve different reaction times at different hierarchical feedback loops, their effect is modelled as a single lumped reaction time delay. Stabilizability is investigated in terms of the time delay in the mechanical model: if the delay is larger than a critical value (critical delay), then no stabilizing feedback control exists. Series of balancing trials by 15 human subjects show that it is more difficult to balance on balance board configuration associated with smaller critical delay, than on balance boards associated with larger critical delay. Experiments verify the feature of the mechanical model that a change in the rolling radius R results in larger change in the difficulty of the task than the same change in the board elevation h does. The rolling balance board characterized by the two well-defined parameters R and h can therefore be a useful device to assess human balancing skill and to estimate the corresponding lumped reaction time delay.

Visual Modulation of Human Responses to Support Surface Translation

Vision is known to improve human postural responses to external perturbations. This study investigates the role of vision for the responses to continuous pseudorandom support surface translations in the body sagittal plane in three visual conditions: with the eyes closed (EC), in stroboscopic illumination (EO/SI; only visual position information) and with eyes open in continuous illumination (EO/CI; position and velocity information) with the room as static visual scene (or the interior of a moving cabin, in some of the trials). In the frequency spectrum of the translation stimulus we distinguished on the basis of the response patterns between a low-frequency, mid-frequency, and high-frequency range (LFR: 0.0165-0.14 Hz; MFR: 0.15–0.57 Hz; HFR: 0.58–2.46 Hz). With EC, subjects’ mean sway response gain was very low in the LFR. On average it increased with EO/SI (although not to a significant degree p = 0.078) and more so with EO/CI (p < 10⁻⁶). In contrast, the average gain in the MFR decreased from EC to EO/SI (although not to a significant degree, p = 0.548) and further to EO/CI (p = 0.0002). In the HFR, all three visual conditions produced, similarly, high gain levels. A single inverted pendulum (SIP) model controlling center of mass (COM) balancing about the ankle joints formally described the EC response as being strongly shaped by a resonance phenomenon arising primarily from the control’s proprioceptive feedback loop. The effect of adding visual information in these simulations lies in a reduction of the resonance, similar as in the experiments. Extending the model to a double inverted pendulum (DIP) suggested in addition a biomechanical damping effective from trunk sway in the hip joints on the resonance.

Adaptation of balancing behaviour during continuous perturbations of stance. Supra-postural visual tasks and platform translation frequency modulate adaptation rate

When humans are administered continuous and predictable perturbations of stance, an adaptation period precedes the steady state of balancing behaviour. Little information is available on the modulation of adaptation by vision and perturbation frequency. Moreover, performance of supra-postural tasks may modulate adaptation in as yet unidentified ways. Our purpose was to identify differences in adaptation associated to distinct visual tasks and perturbation frequencies. Twenty non-disabled adult volunteers stood on a platform translating 10 cm in antero-posterior (AP) direction at low (LF, 0.18 Hz) and high frequency (HF, 0.56 Hz) with eyes open (EO) and closed (EC). Additional conditions were reading a text fixed to platform (EO-TP) and reading a text stationary on ground (EO-TG). Peak-to-peak (PP) displacement amplitude and AP position of head and pelvis markers were computed for each of 27 continuous perturbation cycles. The time constant and extent of head and pelvis adaptation and the cross-correlation coefficients between head and pelvis were compared across visual conditions and frequencies. Head and pelvis mean positions in space varied little across conditions and perturbation cycles but the mean head PP displacements changed over time. On average, at LF, the PP displacement of the head and pelvis increased progressively. Adaptation was rapid or ineffective with EO, but slower with EO-TG, EO-TP, EC. At HF, the head PP displacement amplitude decreased progressively with fast adaptation rates, while the pelvis adaptation was not apparent. The results show that visual tasks can modulate the adaptation rate, highlight the effect of the perturbation frequency on adaptation and provide evidence of priority assigned to pelvis stabilization over visual tasks at HF. The effects of perturbation frequency and optic flow and their interaction with other sensory inputs and cognitive tasks on the adaptation strategies should be investigated in impaired individuals and considered in the design of rehabilitation protocols.

Haemodialysis patients have worse postural balance with an associated risk of falls

INTRODUCTION

Postural balance is the result of a complex interaction of sensory input which keeps us upright. Haemodialysis patients have alterations which can lead to postural instability and a risk of falls. Our objective was to analyse postural stability and its relationship with the risk of falls in haemodialysis patients using a force platform.

MATERIAL AND METHODS

This was a prospective cross-sectional study. Postural balance was recorded using a force platform in prevalent haemodialysis patients. We collected epidemiological, dialysis, analytical and treatment data. The incidence of falls was recorded over the 6 months following the tests. The postural stability analysis was performed with a portable strain gauge platform (AMTI AccuGait®) and a specific software unit for stabilometry (Balance Trainer® program). We measured 31 balance parameters; the balance variables used were: Area95; AreaEffect; VyMax; Xrange and Yrange. The stabilometry studies were performed in 3 situations: with eyes open; with eyes closed; and with the patient performing a simultaneous task. We performed one study at the start of the dialysis session, and a second study at the end. Stabilometry was measured in a control group under similar conditions.

RESULTS

We studied 32 patients with a mean age of 68 years old; of this group, 20 subjects were male and 12 were female. Their mean weight was 74kg, with a mean BMI of 27.6kg/m². In the controls, there were no significant differences in the stabilometry between the 3 situations studied. Both pre- and post-haemodialysis, patients with closed eyes showed greater imbalance, and there were significant differences with the other situations and controls. We found a significant increase in instability after the haemodialysis session, and greater instability in the 13 patients with diabetes (P<.05). The 4 patients with hyponatraemia (Na<136mmol/l) had worse balance in the simultaneous task situation (P=.038). Various drugs, such as insulin (P=.022), antiplatelet agents (P=.036) and beta-blockers (P=.029), were associated with imbalance. The 10 patients who suffered falls had greater imbalance, Yrange, Xrange, Area95 and AreaEffect, both pre- and post-haemodialysis (P<.05) than those without falls.

CONCLUSIONS

Haemodialysis patients have alterations which can lead to postural instability and a risk of falls. Prevention programmes which include specific exercises to improve balance could be beneficial in reducing the risk of falls in this population.

The Influence of Proprioceptive Training with the Use of Virtual Reality on Postural Stability of Workers Working at Height

The aim of the study was to assess the impact of proprioceptive training with the use of virtual reality (VR) on the level of postural stability of high-altitude workers. Twenty-one men working at height were randomly assigned to the experimental group (EG) with training (n = 10) and control group (CG) without training (n = 11). Path length of the displacement of the center of pressure (COP) signal and its components in the anteroposterior and medial-lateral directions were measured with use of an AccuGaitTM force plate before and after intervention (6 weeks, 2 sessions × 30 min a week). Tests were performed at two different platform heights, with or without eyes open and with or without a dual task. Two-way ANOVA revealed statistically significant interaction effects for low-high threat, eyes open-eyes closed, and single task-dual task. Post-training values of average COP length were significantly lower in the EG than before training for all analyzed parameters. Based on these results, it can be concluded that the use of proprioceptive training with use of VR can support, or even replace, traditional methods of balance training.

Reductions in body sway responses to a rhythmic support surface tilt perturbation can be caused by other mechanisms than prediction

Studies investigating balance control often use external perturbations to probe the system. These perturbations can be administered as randomized, pseudo-randomized, or predictable sequences. As predictability of a given perturbation can affect balance performance, the way those perturbations are constructed may affect the results of the experiments. In the present study, we hypothesized that subjects are able to adapt to short, rhythmic support surface tilt stimuli, but not to long pseudo-random stimuli. 19 subjects were standing with eyes closed on a servo-controlled platform tilting about the ankle joint axis. Pre and post to the learning intervention, pseudo-random tilt sequences were applied. For the learning phase, a rhythmic and easy-to-memorize 8-s long sequence was applied 75 times, where subjects were instructed to stand as still as possible. Body kinematics were measured and whole body center of mass sway was analyzed. Results showed reduced sway and less forward lean of the body across the learning phase. The sway reductions were similar for stimulus and non-stimulus frequencies. Surprisingly, for the pseudo-random sequences, comparable changes were found from pre- to post-tests. In summary, results confirmed that considerable adaptations exist when exposing subjects to an 8-s long rhythmic perturbation. No indications of predictions of the learning tilt sequence were found, since similar changes were also observed in response to pseudo-random sequences. We conclude that changes in body sway responses following 75 repetitions of an 8-s long rhythmic tilt sequence are due to adaptations in the dynamics of the control mechanism (presumably stiffness).

Human Balance in Response to Continuous, Predictable Translations of the Support Base: Integration of Sensory Information, Adaptation to Perturbations, and the Effect of Age, Neuropathy and Parkinson’s Disease

This short narrative review article moves from early papers that described the behaviour of healthy subjects balancing on a motorized platform continuously translating in the antero-posterior direction. Research from the laboratories of two of the authors and related investigations on dynamic balancing behaviour are briefly summarized. More recent findings challenging time-honoured views are considered, such as the statement that vision plays a head-in-space stabilizing role. The time interval to integrate vision or its withdrawal in the balancing pattern is mentioned as well. Similarities and differences between ageing subjects and patients with peripheral or central disorders are concisely reported. The muscle activities recorded during the translation cycles suggest that vision and amplitude changes of the anticipatory postural activities play a predominant role in controlling dynamic balance during prolonged administration of the predictable perturbation. The potential of this paradigm for rehabilitation of balance problems is discussed.

Virtual-Reality-Induced Visual Perturbations Impact Postural Control System Behavior

Background: Virtual reality (VR) is becoming a widespread tool in rehabilitation, especially for postural stability. However, the impact of using VR in a “moving wall paradigm” (visual perturbation), specifically without and with anticipation of the perturbation, is unknown. Methods: Nineteen healthy subjects performed three trials of static balance testing on a force plate under three different conditions: baseline (no perturbation), unexpected VR perturbation, and expected VR perturbation. The statistical analysis consisted of a 1 × 3 repeated-measures ANOVA to test for differences in the center of pressure (COP) displacement, 95% ellipsoid area, and COP sway velocity. Results: The expected perturbation rendered significantly lower (p < 0.05) COP displacements and 95% ellipsoid area compared to the unexpected condition. A significantly higher (p < 0.05) sway velocity was also observed in the expected condition compared to the unexpected condition. Conclusions: Postural stability was lowered during unexpected visual perturbations compared to both during baseline and during expected visual perturbations, suggesting that conflicting visual feedback induced postural instability due to compensatory postural responses. However, during expected visual perturbations, significantly lowered postural sway displacement and area were achieved by increasing the sway velocity, suggesting the occurrence of postural behavior due to anticipatory postural responses. Finally, the study also concluded that VR could be used to induce different postural responses by providing visual perturbations to the postural control system, which can subsequently be used as an effective and low-cost tool for postural stability training and rehabilitation.

Vision Does Not Necessarily Stabilize the Head in Space During Continuous Postural Perturbations

Vision favors head stabilization in space during perturbations of standing balance. This is particularly obvious under conditions of continuous predictable perturbations as during sinusoidal antero-posterior (A-P) translations of the supporting platform. We tested here the hypothesis that under this condition the head can instead undergo large A-P oscillations, when a precision visual task is concurrently performed. We compared the head oscillations across four conditions while standing on a continuously translating platform. Eyes open (EO, no visual task), EO while reading a text fixed to the moving platform (EO-TP), EO while reading a text fixed to earth-ground (EO-TG), eyes-closed (EC). The platform translated at 0.2 and 0.6 Hz. Participants were young adult subjects, who received no particular instruction except reading the text aloud when required. Markers fixed on head, platform and text-sheet were captured by an optoelectronic device. We found that head oscillations were larger with EC than under all EO conditions. The oscillations were the least with EO and EO-TG, and intermediate with EO-TP. This was true under both low and high translation frequency, in spite of broadly smaller head oscillations at high frequency, common to all visual conditions. The distance between the head and the text was quite constant with EO-TP but fluctuated with EO-TG. The basic whole-body coordination features were moderately similar under all conditions, as assessed by the head-platform correlation coefficients and time lags. It appears that vision does not produce head stabilization in space when a concurrent visual task requiring focusing on a reading-text moving with the platform is performed. Contrary to traditional views centered on the stabilizing effect of vision under both static and dynamic conditions, the results show that head stabilization, normally ensuring a reference for inertial guidance for body balance, can be revoked by the CNS to allow performance of a non-postural task. This novel paradigm can shift long-standing views on the effect of vision on equilibrium control and be considered a potential exercise treatment for enhancing the multisensory integration process in people with balance problems.

Efficacy Of Dual-Task Training With Two Different Priorities Instructional Sets On Gait Parameters In Patients With Chronic Stroke

PURPOSE

Balance is controlled through a complex process involving sensory, visual, vestibular and cerebral functioning which get affected by various neurological disorders such as in stroke. Various types of exercises are designed to address the imbalance that is developed due to these neurological disorders. This study aimed to compare the efficacy of dual-task training using two different priority instructional sets in improving gait parameters in patients with chronic stroke.

METHODS

This study was a randomized, pretest-posttest experimental group design that compared between two different priority instructional sets (fixed versus variable) of the dual-task training. A convenience sample of thirty patients with chronic stroke due to ruptured middle cerebral artery (mean age±SD = 55.76±5.23; range 48-65 years) was recruited and equally allocated into two groups. Group 1 received dual-task training with fixed priority instructional sets and group 2 received dual-task training with variable priority instructional sets. Both groups were trained for a period of 45 mins each session, 3-sessions per week for 4 weeks. The timed 10-m walk test and foot prints on walkway paper were used to assess the gait parameters (walking speed, stride length and step length) before and after the training session.

RESULTS

Within-group analysis revealed a significant improvement (p<0.05) on gait parameters for both the groups. Furthermore, Cohen's d calculation for the treatment effect size revealed highly larger effect size on gait parameters in group 2 (Cohen's d>2 SD) than group 1 (Cohen's d<2 SD) for the all variables.

CONCLUSION

The dual-task training with variable priority instructional sets (group 2) was more effective than dual-task training with fixed priority instructional sets (group 1) in improving gait parameters such as gait speed, stride length, and step length in patients with chronic stroke. Physiotherapists should spread awareness and use this specific set of exercises (variable priority instructional sets) while performing a dual-task balance training program among patients with chronic stroke.

Persistence in postural dynamics is dependent on constraints of vision, postural orientation, and the temporal structure of support surface translations

Activities of daily living require maintaining upright posture within a variety of environmental constraints. A healthy postural control system can adapt to different environmental constraints. Afferent sensory information is used to determine where the body is in relation to the gravitational vertical and efferent motor commands make corrections with the goal of keeping the center of mass within the base of support. The purpose of this research was to understand how vision, direction of translation, and the temporal correlation of the support surface stimuli affected the persistence characteristics of postural dynamics on short and long time scales. Ten healthy young adults performed a standing task with either eyes open or closed, oriented anteriorly or mediolaterally while the support surface underwent structured translations based on different levels of temporal correlation-white noise (no correlation), pink noise (moderate correlation), and red noise and sinusoidal movements (strong correlations). Center of pressure velocity was analyzed using fractal analysis to determine the dynamics of postural control. On the short time scale, persistence was shown to be stronger with eyes closed, in the mediolateral direction, and when the structure of translation contained stronger temporal correlation. On the long time scale, anti-persistence was stronger with eyes closed, in the mediolateral direction, and for all structures of movement except red noise. This study provides deeper insight into the flexibility existing in human movement responses to structured environmental stimuli through the fractal analysis of movement variability.

Sensory integration of a light touch reference in human standing balance

In upright stance, light touch of a space-stationary touch reference reduces spontaneous sway. Moving the reference evokes sway responses which exhibit non-linear behavior that has been attributed to sensory reweighting. Reweighting refers to a change in the relative contribution of sensory cues signaling body sway in space and light touch cues signaling finger position with respect to the body. Here we test the hypothesis that the sensory fusion process involves a transformation of light touch signals into the same reference frame as other sensory inputs encoding body sway in space, or vice versa. Eight subjects lightly gripped a robotic manipulandum which moved in a circular arc around the ankle joint. A pseudo-randomized motion sequence with broad spectral characteristics was applied at three amplitudes. The stimulus was presented at two different heights and therefore different radial distances, which were matched in terms of angular motion. However, the higher stimulus evoked a significantly larger sway response, indicating that the response was not matched to stimulus angular motion. Instead, the body sway response was strongly related to the horizontal translation of the manipulandum. The results suggest that light touch is integrated as the horizontal distance between body COM and the finger. The data were well explained by a model with one feedback loop minimizing changes in horizontal COM-finger distance. The model further includes a second feedback loop estimating the horizontal finger motion and correcting the first loop when the touch reference is moving. The second loop includes the predicted transformation of sensory signals into the same reference frame and a non-linear threshold element that reproduces the non-linear sway responses, thus providing a mechanism that can explain reweighting.

Posture Control-Human-Inspired Approaches for Humanoid Robot Benchmarking: Conceptualizing Tests, Protocols and Analyses

Posture control is indispensable for both humans and humanoid robots, which becomes especially evident when performing sensorimotor tasks such as moving on compliant terrain or interacting with the environment. Posture control is therefore targeted in recent proposals of robot benchmarking in order to advance their development. This Methods article suggests corresponding robot tests of standing balance, drawing inspirations from the human sensorimotor system and presenting examples from robot experiments. To account for a considerable technical and algorithmic diversity among robots, we focus in our tests on basic posture control mechanisms, which provide humans with an impressive postural versatility and robustness. Specifically, we focus on the mechanically challenging balancing of the whole body above the feet in the sagittal plane around the ankle joints in concert with the upper body balancing around the hip joints. The suggested tests target three key issues of human balancing, which appear equally relevant for humanoid bipeds: (1) four basic physical disturbances (support surface (SS) tilt and translation, field and contact forces) may affect the balancing in any given degree of freedom (DoF). Targeting these disturbances allows us to abstract from the manifold of possible behavioral tasks. (2) Posture control interacts in a conflict-free way with the control of voluntary movements for undisturbed movement execution, both with "reactive" balancing of external disturbances and "proactive" balancing of self-produced disturbances from the voluntary movements. Our proposals therefore target both types of disturbances and their superposition. (3) Relevant for both versatility and robustness of the control, linkages between the posture control mechanisms across DoFs provide their functional cooperation and coordination at will and on functional demands. The suggested tests therefore include ankle-hip coordination. Suggested benchmarking criteria build on the evoked sway magnitude, normalized to robot weight and Center of mass (COM) height, in relation to reference ranges that remain to be established. The references may include human likeness features. The proposed benchmarking concept may in principle also be applied to wearable robots, where a human user may command movements, but may not be aware of the additionally required postural control, which then needs to be implemented into the robot.

Evidence in Support of the Independent Channel Model Describing the Sensorimotor Control of Human Stance Using a Humanoid Robot

The Independent Channel (IC) model is a commonly used linear balance control model in the frequency domain to analyze human balance control using system identification and parameter estimation. The IC model is a rudimentary and noise-free description of balance behavior in the frequency domain, where a stable model representation is not guaranteed. In this study, we conducted firstly time-domain simulations with added noise, and secondly robot experiments by implementing the IC model in a real-world robot (PostuRob II) to test the validity and stability of the model in the time domain and for real world situations. Balance behavior of seven healthy participants was measured during upright stance by applying pseudorandom continuous support surface rotations. System identification and parameter estimation were used to describe the balance behavior with the IC model in the frequency domain. The IC model with the estimated parameters from human experiments was implemented in Simulink for computer simulations including noise in the time domain and robot experiments using the humanoid robot PostuRob II. Again, system identification and parameter estimation were used to describe the simulated balance behavior. Time series, Frequency Response Functions, and estimated parameters from human experiments, computer simulations, and robot experiments were compared with each other. The computer simulations showed similar balance behavior and estimated control parameters compared to the human experiments, in the time and frequency domain. Also, the IC model was able to control the humanoid robot by keeping it upright, but showed small differences compared to the human experiments in the time and frequency domain, especially at high frequencies. We conclude that the IC model, a descriptive model in the frequency domain, can imitate human balance behavior also in the time domain, both in computer simulations with added noise and real world situations with a humanoid robot. This provides further evidence that the IC model is a valid description of human balance control.

Low intensity vibration of ankle muscles improves balance in elderly persons at high risk of falling

In our study we examined postural performance of young healthy persons (HY), elderly healthy persons (HE), and elderly persons at high risk of falling (FR). Anterio-posterior (AP) and medio-lateral (ML) ankle and hip angular deviations, as well as linear displacements of the center of mass (COM) were assessed in persons standing with eyes either open or closed, while none, and 40 and 30 Hz vibrations were applied bilaterally to the ankle muscle gastrocnemius. During quiet standing with eyes open, balance parameters in FR group differed from those in healthy groups. ML ankle and hip angular deviations, as well as COM linear displacements were noticeably larger in FR group. During quiet standing with eyes closed, all balance parameters in participants of all groups had a clear trend to increase. During standing with eyes open, 40 Hz vibration increased all but one balance parameter within HY group, ankle angular deviations in HE group, but none in FR group. In response to 30 Hz vibration, only ankle angular deviations and COM linear displacements increased in HY group. There were no changes in both elderly groups. During standing with eyes closed, 40 and 30 Hz vibrations did not produce consistent changes in balance parameters in HY and HE groups. In FR persons, 40 Hz vibration did not change balance parameters. However, in FR groups, 30 Hz vibration decreased ankle and hip angular deviations, and COM linear displacements. The major result of the study is a finding that low intensity vibration of ankle muscles makes balance better in elderly persons at high risk of falling. This result is clinically relevant because it suggests that applying mild vibration to ankle muscles while standing and walking might benefit elderly persons, improving their postural performance and reducing a risk of unexpected falls.

Transitions in persistence of postural dynamics depend on the velocity and structure of postural perturbations

The sensorimotor system prefers sway velocity information when maintaining upright posture. Sway velocity has a unique characteristic of being persistent on a short time-scale and anti-persistent on a longer time-scale. The time where the transition from persistence to anti-persistence occurs provides information about how sway velocity is controlled. It is, however, not clear what factors affect shifts in this transition point. This research investigated postural responses to support surface movements of different temporal correlations and movement velocities. Participants stood on a force platform that was translated according to three different levels of temporal correlation. White noise had no correlation, pink noise had moderate correlation, and sine wave movements had very strong correlation. Each correlation structure was analyzed at five different average movement velocities (0.5, 1.0, 2.0, 3.0, and 4.0 cm·s^-1), as well as one trial of quiet stance. Center of pressure velocity was analyzed using fractal analysis to determine the transition from persistent to anti-persistent behavior, as well as the strength of persistence. As movement velocity increased, the time to transition became longer for the sine wave and shorter for the white and pink noise movements. Likewise, during the persistent time-scale, the sine wave resulted in the strongest correlation, while white and pink noise had weaker correlations. At the highest three movement velocities, the strength of persistence was lower for the white noise compared to pink noise movements. These results demonstrate that the predictability and velocity of support surface oscillations affect the time-scale threshold between persistent and anti-persistent postural responses. Consequently, whether a feedforward or feedback control is utilized for appropriate postural responses may also be determined by the predictability and velocity of environmental stimuli. The study provides new insight into flexibility and adaptability in postural control. This information has implications for the design of rehabilitative protocols in neuromuscular control.

Human-Derived Disturbance Estimation and Compensation (DEC) Method Lends Itself to a Modular Sensorimotor Control in a Humanoid Robot

The high complexity of the human posture and movement control system represents challenges for diagnosis, therapy, and rehabilitation of neurological patients. We envisage that engineering-inspired, model-based approaches will help to deal with the high complexity of the human posture control system. Since the methods of system identification and parameter estimation are limited to systems with only a few DoF, our laboratory proposes a heuristic approach that step-by-step increases complexity when creating a hypothetical human-derived control systems in humanoid robots. This system is then compared with the human control in the same test bed, a posture control laboratory. The human-derived control builds upon the identified disturbance estimation and compensation (DEC) mechanism, whose main principle is to support execution of commanded poses or movements by compensating for external or self-produced disturbances such as gravity effects. In previous robotic implementation, up to 3 interconnected DEC control modules were used in modular control architectures separately for the sagittal plane or the frontal body plane and successfully passed balancing and movement tests. In this study we hypothesized that conflict-free movement coordination between the robot's sagittal and frontal body planes emerges simply from the physical embodiment, not necessarily requiring a full body control. Experiments were performed in the 14 DoF robot Lucy Posturob (i) demonstrating that the mechanical coupling from the robot's body suffices to coordinate the controls in the two planes when the robot produces movements and balancing responses in the intermediate plane, (ii) providing quantitative characterization of the interaction dynamics between body planes including frequency response functions (FRFs), as they are used in human postural control analysis, and (iii) witnessing postural and control stability when all DoFs are challenged together with the emergence of inter-segmental coordination in squatting movements. These findings represent an important step toward controlling in the robot in future more complex sensorimotor functions such as walking.

Head Stability and Head-Trunk Coordination in Horseback Riders: The Contribution of Visual Information According to Expertise

Maintaining equilibrium while riding a horse is a challenging task that involves complex sensorimotor processes. We evaluated the relative contribution of visual information (static or dynamic) to horseback riders' postural stability (measured from the variability of segment position in space) and the coordination modes they adopted to regulate balance according to their level of expertise. Riders' perceptual typologies and their possible relation to postural stability were also assessed. Our main assumption was that the contribution of visual information to postural control would be reduced among expert riders in favor of vestibular and somesthetic reliance. Twelve Professional riders and 13 Club riders rode an equestrian simulator at a gallop under four visual conditions: (1) with the projection of a simulated scene reproducing what a rider sees in the real context of a ride in an outdoor arena, (2) under stroboscopic illumination, preventing access to dynamic visual cues, (3) in normal lighting but without the projected scene (i.e., without the visual consequences of displacement) and (4) with no visual cues. The variability of the position of the head, upper trunk and lower trunk was measured along the anteroposterior (AP), mediolateral (ML), and vertical (V) axes. We computed discrete relative phase to assess the coordination between pairs of segments in the anteroposterior axis. Visual field dependence-independence was evaluated using the Rod and Frame Test (RFT). The results showed that the Professional riders exhibited greater overall postural stability than the Club riders, revealed mainly in the AP axis. In particular, head variability was lower in the Professional riders than in the Club riders in visually altered conditions, suggesting a greater ability to use vestibular and somesthetic information according to task constraints with expertise. In accordance with this result, RFT perceptual scores revealed that the Professional riders were less dependent on the visual field than were the Club riders. Finally, the Professional riders exhibited specific coordination modes that, unlike the Club riders, departed from pure in-phase and anti-phase patterns and depended on visual conditions. The present findings provide evidence of major differences in the sensorimotor processes contributing to postural control with expertise in horseback riding.

Sensory reweighting dynamics following removal and addition of visual and proprioceptive cues

Removing or adding sensory cues from one sensory system during standing balance causes a change in the contribution of the remaining sensory systems, a process referred to as sensory reweighting. While reweighting changes have been described in many studies under steady-state conditions, less is known about the temporal dynamics of reweighting following sudden transitions to different sensory conditions. The present study changed sensory conditions by periodically adding or removing visual (lights On/Off) or proprioceptive cues (surface sway referencing On/Off) in 12 young, healthy subjects. Evidence for changes in sensory contributions to balance was obtained by measuring the time course of medial-lateral sway responses to a constant-amplitude 0.56-Hz sinusoidal stimulus, applied as support surface tilt (proprioceptive contribution), as visual scene tilt (visual contribution), or as binaural galvanic vestibular stimulation (vestibular contribution), and by analyzing the time course of sway variability. Sine responses and variability of body sway velocity showed significant changes following transitions and were highly correlated under steady-state conditions. A dependence of steady-state responses on upcoming transitions was observed, suggesting that knowledge of impending changes can influence sensory weighting. Dynamic changes in sway in the period immediately following sensory transitions were very inhomogeneous across sway measures and in different experimental tests. In contrast to steady-state results, sway response and variability measures were not correlated with one another in the dynamic transition period. Several factors influence sway responses following addition or removal of sensory cues, partly instigated by but also obscuring the effects of reweighting dynamics.

Reliability of System Identification Techniques to Assess Standing Balance in Healthy Elderly

Objectives

System identification techniques have the potential to assess the contribution of the underlying systems involved in standing balance by applying well-known disturbances. We investigated the reliability of standing balance parameters obtained with multivariate closed loop system identification techniques.

Methods

In twelve healthy elderly balance tests were performed twice a day during three days. Body sway was measured during two minutes of standing with eyes closed and the Balance test Room (BalRoom) was used to apply four disturbances simultaneously: two sensory disturbances, to the proprioceptive and the visual system, and two mechanical disturbances applied at the leg and trunk segment. Using system identification techniques, sensitivity functions of the sensory disturbances and the neuromuscular controller were estimated. Based on the generalizability theory (G theory), systematic errors and sources of variability were assessed using linear mixed models and reliability was assessed by computing indexes of dependability (ID), standard error of measurement (SEM) and minimal detectable change (MDC).

Results

A systematic error was found between the first and second trial in the sensitivity functions. No systematic error was found in the neuromuscular controller and body sway. The reliability of 15 of 25 parameters and body sway were moderate to excellent when the results of two trials on three days were averaged. To reach an excellent reliability on one day in 7 out of 25 parameters, it was predicted that at least seven trials must be averaged.

Conclusion

This study shows that system identification techniques are a promising method to assess the underlying systems involved in standing balance in elderly. However, most of the parameters do not appear to be reliable unless a large number of trials are collected across multiple days. To reach an excellent reliability in one third of the parameters, a training session for participants is needed and at least seven trials of two minutes must be performed on one day.

Perspectives on Aging Vestibular Function

Much is known about age-related anatomical changes in the vestibular system. Knowledge regarding how vestibular anatomical changes impact behavior for older adults continues to grow, in line with advancements in diagnostic testing. However, despite advancements in clinical diagnostics, much remains unknown about the functional impact that an aging vestibular system has on daily life activities such as standing and walking. Modern diagnostic tests are very good at characterizing neural activity of the isolated vestibular system, but the tests themselves are artificial and do not reflect the multisensory aspects of natural human behavior. Also, the majority of clinical diagnostic tests are passively applied because active behavior can enhance performance. In this perspective paper, we review anatomical and behavioral changes associated with an aging vestibular system and highlight several areas where a more functionally relevant perspective can be taken. For postural control, a multisensory perturbation approach could be used to bring balance rehabilitation into the arena of precision medicine. For walking and complex gaze stability, this may result in less physiologically specific impairments, but the trade-off would be a greater understanding of how the aging vestibular system truly impacts the daily life of older adults.

Changes in sensory reweighting of proprioceptive information during standing balance with age and disease

With sensory reweighting, reliable sensory information is selected over unreliable information during balance by dynamically combining this information. We used system identification techniques to show the weight and the adaptive process of weight change of proprioceptive information during standing balance with age and specific diseases. Ten healthy young subjects (aged between 20 and 30 yr) and 44 elderly subjects (aged above 65 yr) encompassing 10 healthy elderly, 10 with cataract, 10 with polyneuropathy, and 14 with impaired balance, participated in the study. During stance, proprioceptive information of the ankles was disturbed by rotation of the support surface with specific frequency content where disturbance amplitude increased over trials. Body sway and reactive ankle torque were measured to determine sensitivity functions of these responses to the disturbance amplitude. Model fits resulted in a proprioceptive weight (changing over trials), time delay, force feedback, reflexive stiffness, and damping. The proprioceptive weight was higher in healthy elderly compared with young subjects and higher in elderly subjects with cataract and with impaired balance compared with healthy elderly subjects. Proprioceptive weight decreased with increasing disturbance amplitude; decrease was similar in all groups. In all groups, the time delay was higher and the reflexive stiffness was lower compared with young or healthy elderly subjects. In conclusion, proprioceptive information is weighted more with age and in patients with cataract and impaired balance. With age and specific diseases the time delay was higher and reflexive stiffness was lower. These results illustrate the opportunity to detect the underlying cause of impaired balance in the elderly with system identification.

Processing time of addition or withdrawal of single or combined balance-stabilizing haptic and visual information

We investigated the integration time of haptic and visual input and their interaction during stance stabilization. Eleven subjects performed four tandem-stance conditions (60 trials each). Vision, touch, and both vision and touch were added and withdrawn. Furthermore, vision was replaced with touch and vice versa. Body sway, tibialis anterior, and peroneus longus activity were measured. Following addition or withdrawal of vision or touch, an integration time period elapsed before the earliest changes in sway were observed. Thereafter, sway varied exponentially to a new steady-state while reweighting occurred. Latencies of sway changes on sensory addition ranged from 0.6 to 1.5 s across subjects, consistently longer for touch than vision, and were regularly preceded by changes in muscle activity. Addition of vision and touch simultaneously shortened the latencies with respect to vision or touch separately, suggesting cooperation between sensory modalities. Latencies following withdrawal of vision or touch or both simultaneously were shorter than following addition. When vision was replaced with touch or vice versa, adding one modality did not interfere with the effect of withdrawal of the other, suggesting that integration of withdrawal and addition were performed in parallel. The time course of the reweighting process to reach the new steady-state was also shorter on withdrawal than addition. The effects of different sensory inputs on posture stabilization illustrate the operation of a time-consuming, possibly supraspinal process that integrates and fuses modalities for accurate balance control. This study also shows the facilitatory interaction of visual and haptic inputs in integration and reweighting of stance-stabilizing inputs.