Dynamic reweighting of three modalities for sensor fusion

Type of auditory cues and apparatus influence how healthy young adults integrate sounds for dynamic balance

It is unclear whether the brain handles auditory cues similarly to visual cues for balance. We investigated the influence of headphones and loudspeaker reproduction of sounds on dynamic balance performance when an individual is facing a cognitive challenge. Twenty participants (16 females, aged 19-36) were asked to avoid a ball according to a specific visual rule. Visuals were projected from the HTC Vive head-mounted display in an acoustically controlled space. We varied the environment by adding congruent sounds (sounds coincide with the visual rule) or incongruent sounds (sounds may or may not coincide with the visual rule) as well as creating a multimodal (visual and congruent sounds) vs. unimodal (visual or congruent sounds only) display of stimuli. Sounds were played over headphones or loudspeakers. We quantified reaction time (RT) and accuracy (choosing the correct direction to move) by capturing the head movement. We found that in the absence of sounds, RT was slower with headphones compared to loudspeakers, but the introduction of either congruent or incongruent sounds resulted in faster movements with headphones such that RT was no longer different between apparatus. Participants used congruent sounds to improve accuracy but disregarded incongruent sounds. This suggests that selective attention may explain how sounds are incorporated into dynamic balance performance in healthy young adults. Participants leveraged sounds played over loudspeakers, but not over headphones, to enhance accuracy in a unimodal dark environment. This may be explained by the natural listening conditions created by loudspeakers where sounds may be perceived as externalized.

Sensory conflicts through short, discrete visual input manipulations: Identification of balance responses to varied input characteristics

Human balance control relies on various sensory modalities, and conflict of sensory input may result in postural instability. Virtual reality (VR) technology allows to train balance under conflicting sensory information by decoupling visual from somatosensory and vestibular systems, creating additional demands on sensory reweighting for balance control. However, there is no metric for the design of visual input manipulations that can induce persistent sensory conflicts to perturb balance. This limits the possibilities to generate sustained sensory reweighting processes and design well-defined training approaches. This study aimed to investigate the effects that different onset characteristics, amplitudes and velocities of visual input manipulations may have on balance control and their ability to create persistent balance responses. Twenty-four young adults were recruited for the study. The VR was provided using a state-of-the-art head-mounted display and balance was challenged in two experiments by rotations of the visual scene in the frontal plane with scaled constellations of trajectories, amplitudes and velocities. Mean center of pressure speed was recorded and revealed to be greater when the visual input manipulation had an abrupt onset compared to a smooth onset. Furthermore, the balance response was greatest and most persistent when stimulus velocity was low and stimulus amplitude was large. These findings show clear dissociation in the state of the postural system for abrupt and smooth visual manipulation onsets with no indication of short-term adaption to abrupt manipulations with slow stimulus velocity. This augments our understanding of how conflicting visual information affect balance responses and could help to optimize the conceptualization of training and rehabilitation interventions.

Weighting of visual and auditory inputs in dancers with and without previous ankle injury

Elite dance requires highly controlled balance performance in dynamic visual and auditory environments characterized by the use of stage lighting and music. Perturbing visual input is known to influence balance in dancers, but the effects of sounds have not been clearly established. Ankle inversion injuries are common in dancers and may also play a role in dancers' ability to respond to sensory perturbations. The aims of this study were to identify changes in static balance in response to visual or auditory perturbation in elite collegiate dancers with and without a history of ankle injury, and to describe coupling of measurements obtained from the waist and head during balance. Thirty-seven dancers: 22 controls and 15 with a history of ankle sprain, performed single-leg balance under static and dynamic visual and auditory conditions. Dancers demonstrated increased waist sway when viewing a dynamic visual scene and when presented with moving sounds and increased head sway with dynamic visuals. These results did not vary by history of ankle sprain. While dancers appear to dampen the impact of waist sway on head motion during single leg stance, even highly trained dancers may experience challenges controlling balance under perturbing visual and auditory conditions like those present during stage performance.

Seeing does not mean processing: where we look and the visual information we rely on change independently as we learn a novel walking task

People use vision to inform motor control strategies during walking. With practice performing a target stepping task, people shift their gaze farther ahead, transitioning from watching their feet contact the target to looking for future target locations. The shift in gaze focus suggests the role of vision in motor control changes from emphasizing feedback to feedforward control. The present study examines whether changing visual fixation location is accompanied by a similar change in reliance upon visual information. Twenty healthy young adults practiced stepping on moving targets projected on the surface of a treadmill. Periodically, participants' visual reliance was probed by hiding stepping targets which inform feedback or feedforward (targets < or > 1.5 steps ahead, respectively) motor control strategies. We calculated visual reliance as the increase in step error when targets were hidden. We hypothesized that with practice, participant reliance on feedback visual information would decrease and their reliance on feedforward visual information would increase. Contrary to our hypothesis, participants became significantly more reliant on feedback visual information with practice (p < 0.001) but their reliance on feedforward visual information did not change (p = 0.49). Participants' reliance on visual information increased despite looking significantly farther ahead with practice (p < 0.016). Together, these results suggest that participants fixated on feedback information less. However, changes in fixation pattern did not reduce their reliance upon feedback information as stepping performance still significantly decreased when feedback information was removed after training. These findings provide important context for how the role of vision in controlling walking changes with practice.

Electrical Stimulation Intensity to Induce Sensory Reweighting Dynamics While Standing on Balance Board

In human postural control, maladaptation of sensory reweighting to sudden environmental changes is one of the main causes of postural instability. Providing sensory cues for body motion by means of stimulation could induce the sensory reweighting dynamics. In this paper, we aimed to investigate the intensity level of electrical stimulation to induce sensory reweighting dynamics while standing on a balance board under three conditions: no stimulation (control), electrotactile stimulation (ETS) at a low-intensity level, and electrical muscle stimulation (EMS) at a high-intensity level. A total of 30 participants (ten for each condition) controlled their posture to keep the board horizontal in a balance-board task, which included a pre-test without stimulation, a stimulation test, and a post-test without stimulation. The EMS and ETS groups received electrical stimulation to the tibialis anterior or soleus muscles based on the board tilt. Before and after the balance-board task, participants performed static standing with their eyes open and also with their eyes closed to evaluate the visual reweighting. In the EMS group, the visual reweighting showed a strong negative correlation with the balance-board sway ratio between the pre- and stimulation tests, indicating that EMS induced a tendency that requires visual up-weighting to improve postural balance. However, there were no significant correlations between either parameter in the control and ETS groups. These results suggest that high-intensity electrical stimulation at the level of directly contracting muscles may be effective in reliably inducing sensory reweighting dynamics, while low-intensity electrical stimulation may be insufficient.Clinical relevance- These findings will be helpful for designing stimulus conditions to reliably induce the reweighting during balance training, and for establishing a new balance training method utilizing EMS to induce visual up-weighting.

Does vestibular function correlate with objective MRI findings after vestibular schwannoma surgery?

Objective

Vestibular schwannoma surgery leads to acute unilateral vestibular loss. In some patients, however, the process of post-operatively initiated central compensation proceeds more rapidly than in others. This study aimed to evaluate post-operative vestibular function and correlate it with morphological findings of MRI scans.

Methods

The study included 29 patients who underwent surgery for vestibular schwannoma. Vestibular function was analysed post-operatively by video head impulse test (vHIT). Subjective symptoms were evaluated using validated questionnaires. All patients underwent MRI 3 months post-operatively, and the presence of the facial and vestibulocochlear nerves in the internal auditory canal was assessed.

Results

The vestibulo-ocular reflex gain measured by the vHIT correlated positively with audiological findings. Subjective perception of vestibular disorder did not correlate with objectively measured vestibular impairment or with MRI findings.

Conclusions

After the resection of vestibular schwannoma, some patients may still have preserved vestibular function as measured by vHIT. The preserved function does not correlate with subjective symptoms. Patients with partially deteriorated vestibular function showed lower sensitivity to combined stimuli.

Adaptation in motor strategies for postural control associated with sensory reweighting

The purpose of this study was to identify and differentiate the motor strategies associated with sensory reweighting adapted during specific sensory integration tasks by healthy young adults. Thirty-six subjects (age range: 21-33 years) performed standing computerized dynamic posturography balance tasks across progressively increasing amplitudes of visual (VIS), somatosensory (SOM) and both (VIS+SOM) systems perturbation conditions. Adaptation in the motor strategy was measured as changes in electromyographic (EMG) activities and joint angles. The contribution of the perturbed sensory input in maintaining postural stability was calculated to determine the sensory reweighting. A multivariate design was used to model a linear combination of motor adaptation variables that discriminates specific sensory integration tasks. Results showed a significant progressive decrease in postural sway per unit amplitude of sensory perturbation in each condition, indicating dynamic sensory reweighting. Linear discriminant function analysis indicated that the adaptation in motor strategy during the VIS condition was associated with increased activity of EMG and joint angles in the upper body compared to the lower body. Conversely, during the SOM and VIS+SOM conditions, the adaptation in motor strategy was associated with decreased activity of EMG and joint angles in the lower body compared to the upper body. Therefore, the adaptation in motor strategies associated with sensory reweighting were different for different sensory integration tasks.

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.

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

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

Visual feedback in the lower visual field affects postural control during static standing

BACKGROUND

The dorsal parietal visual system plays an important role in self-motion perception and spatial cognition. It also strongly responds to visual inputs from the lower visual field. Postural control is modified in a process called sensory reweighting based on the reliability of available sensory sources. The question of whether visual stimuli presented to either the lower or upper visual field affect postural control and sensory reweighting has not been resolved.

RESEARCH QUESTION

Do visual stimuli presented to the lower and upper visual fields affect postural control and sensory reweighting?

METHODS

Twenty-nine healthy young adults participated in the study. Four conditions (full visual field, upper visual field, lower visual field, and no optic flow condition) were simulated in a VR environment using a head-mounted display. The optic flow stimuli used were swarms of small white spheres originating from the central point of the visual field, moving radially towards the periphery, and expanding across the scene. Participants were instructed to stand quietly for 50 s under each visual condition. Using force plate signals, we measured the center of pressure (COP) signal in the horizontal plane and calculated its 95 % ellipse area, root mean square (RMS) deviations, the mean velocity, and power spectral density (PSD).

RESULTS

Optic flow in the full and lower visual fields produced significantly smaller 95 % ellipse area and RMS of COP in the anterior-posterior direction compared to optic flow in the upper visual field. Furthermore, the PSD of the lower frequency band (0-0.3 Hz) was decreased and that of higher frequency bands (0.3-1 Hz and 1-3 Hz) was increased for the lower compared to the upper visual field.

SIGNIFICANCE

Visual feedback affects static postural control more when presented in the lower visual field compared to the upper visual field.

A Comprehensive Review of Pain Interference on Postural Control: From Experimental to Chronic Pain

Motor control, movement impairment, and postural control recovery targeted in rehabilitation could be affected by pain. The main objective of this comprehensive review is to provide a synthesis of the effect of experimental and chronic pain on postural control throughout the available literature. After presenting the neurophysiological pathways of pain, we demonstrated that pain, preferentially localized in the lower back or in the leg induced postural control alteration. Although proprioceptive and cortical excitability seem modified with pain, spinal modulation assessment might provide a new understanding of the pain phenomenon related to postural control. The literature highlights that the motor control of trunk muscles in patient presenting with lower back pain could be dichotomized in two populations, where the first over-activates the trunk muscles, and the second under-activates the trunk muscles; both generate an increase in tissue loading. Taking all these findings into account will help clinician to provide adapted treatment for managing both pain and postural control.

Sinusoidal Optic Flow Perturbations Reduce Transient but Not Continuous Postural Stability: A Virtual Reality-Based Study

Optic flow perturbations induced by virtual reality (VR) are increasingly used in the rehabilitation of postural control and gait. Here, VR offers the possibility to decouple the visual from the somatosensory and vestibular system. By this means, it enables training under conflicting sensorimotor stimulation that creates additional demands on sensory reweighting and balance control. Even though current VR-interventions still lack a well-defined standardized metric to generate optic flow perturbations that can challenge balance in a repeatable manner, continuous oscillations of the VR are typically used as a rehabilitation tool. We therefore investigated if continuous sensory conflicts induced by optic flow perturbations can challenge the postural system sustainably. Eighteen young adults (m = 8, f = 10, age = 24.1 ± 2.0 yrs) were recruited for the study. The VR was provided using a state-of-the-art head-mounted display including the virtual replica of the real environment. After familiarization in quiet stance without and with VR, bipedal balance was perturbed by sinusoidal rotations of the visual scenery in the sagittal plane with an amplitude of 8° and a frequency of 0.2 Hz. Postural stability was quantified by mean center of mass speed derived from 3D-kinematics. A rmANOVA found increased postural instability only during the first perturbation cycle, i.e., the first 5 s. Succeeding the first perturbation cycle, visual afferents were downregulated to reduce the destabilizing influence of the sensory conflicts. In essence, only the transient beginning of sinusoidal oscillation alters balance compared to quiet standing. Therefore, continuous sinusoidal optic flow perturbations appear to be not suitable for balance training as they cannot trigger persisting sensory conflicts and hence challenge the postural system sustainably. Our study provides rationale for using unexpected and discrete optic flow perturbation paradigms to induce sustainable sensory conflicts.

Peripheral vestibular system: Age-related vestibular loss and associated deficits

Given the interdependence of multiple factors in age-related vestibular loss (e.g., balance, vision, cognition), it is important to examine the individual contributions of these factors with ARVL. While the relationship between the vestibular and visual systems has been well studied (Bronstein et al., 2015), little is known about the association of the peripheral vestibular system with neurodegenerative disorders (Cronin et al., 2017). Further, emerging research developments implicate the vestibular system as an opportunity for examining brain function beyond balance, and into other areas, such as cognition and psychological functioning. Additionally, the bidirectional impact of psychological functioning is understudied in ARVL. Recognition of ARVL as part of a multifaceted aging process will help guide the development of integrated interventions for patients who remain at risk for decline. In this review, we will discuss a wide variety of characteristics of the peripheral vestibular system and ARVL, how it relates to neurodegenerative diseases, and correlations between ARVL and balance, vision, cognitive, and psychological dysfunction. We also discuss clinical implications as well as future directions for research, with an emphasis on improving care for patients with ARVL.

Altered Vestibular Balance Function in Combat Sport Athletes

Combat sports pose a risk for accumulative injuries to the nervous system, yet fighters have remained an understudied population. Here, our purpose was to determine whether repetitive blows to the head have an effect on vestibular balance reflexes in combat sports athletes. We compared lower-limb muscle responses evoked with electrical vestibular stimuluation (EVS) between fighters (boxing/muay thai) and non-fighter controls. Each participant received stochastic vestibular stimulation (0-25 Hz, ±3 mA) over their mastoid processes while they stood relaxed with their head to the left or right. Surface electromyography was recorded from the medial gastrocnemius and soleus muscles bilaterally. Short and medium latency response (SLR/MLR) peaks were significantly delayed in the fighter group compared to controls. SLR and MLR peak amplitudes were also significantly lower in fighters. Fighter-estimated cumulative repetitive head impact (RHI) events demonstrated strong positive correlations with the timing of SLR and MLR peaks. Cumulative RHI events also negatively correlated with peak MLR amplitude and response gain at frequencies above 5 Hz. Our results provide evidence of a progressive vestibular impairment in combat sports athletes, potentially resulting from blows to the head accumulated in sparring practice and competitive bouts throughout their careers. Taken together, EVS-based vestibular assessments may provide a valuable clinical diagnostic tool and help better inform "return-to-play" and career-length decisions for not only combat sports athletes, but potentially other populations at risk of RHIs.

Adaptation of the Compensatory Stepping Response Following Predictable and Unpredictable Perturbation Training

Background

Effective training of the backward step response could be beneficial to improve postural stability and prevent falls. Unpredicted perturbation-based balance training (PBT), widely known as compensatory-step training, may enhance the fear of falling and the patterns of postural muscle co-contraction. Contrastingly, PBT with predictable direction or both direction and timing would suppress the fear and the co-contraction patterns during training, but the efficacy of predictable PBT for unpredictable perturbations is still unknown.

Objective

To compare the adaptation effects of compensatory-step training with and without predictable perturbations on backward stepping against unpredictable perturbations.

Methods

Thirty-three healthy young adults were randomly assigned to one of the following step training groups: Unpredicted, Predicted, and Self-initiated. In training sessions, participants were perturbed to induce a compensatory step with (Predicted group) or without (Unpredicted group) knowledge of the perturbation's direction or while knowing both the direction and timing of the perturbation (Self-initiated group). In test sessions (pre- and post-training), participants were instructed to recover their postural stability in response to an unpredicted perturbation. The margin of stability (MOS), center of mass (COM) shift, and step characteristics were measured during a backward step in both test and training sessions.

Results

All three groups showed a significant increase in the step length and velocity in the post-training sessions compared to those in the pre-training sessions. Moreover, in the Unpredicted and Predicted groups, but not in the Self-initiated group, the MOS at step contact was significantly increased following the training session. In addition, the Self-initiated group showed a significant increase in COM shift at 50 ms after slip onset during training compared to the Unpredicted and Predicted groups.

Conclusion

Unpredicted and predicted PBT improve step characteristics during backward stepping against unpredictable perturbations. Moreover, the unpredictable PBT and PBT with direction-predictable perturbations enhance the feedback postural control reflected as the postural stability at step contact.

Gymnastics Experience Enhances the Development of Bipedal-Stance Multi-Segmental Coordination and Control During Proprioceptive Reweighting

Performance and control of upright bipedal posture requires a constant and dynamic integration of relative contributions of different sensory inputs (i. e., sensory reweighting) to enable effective adaptations as individuals face environmental changes and perturbations. Children with gymnastic experience showed balance performance closer to that of adults during and after proprioceptive alteration than children without gymnastic experience when their center of pressure (COP) was analyzed. However, a particular COP sway can be achieved through performing and coordinating different postural movements. The aim of this study was to assess how children and adults of different gymnastic experience perform and control postural movements while they have to adjust balance during and after bilateral tendon vibration. All participants were equipped with spherical markers attached to their skin and two vibrators strapped over the Achilles tendons. Bipedal stance was performed in three 45-s trials in two visual conditions (eyes open, EO, and eyes closed, EC) ordered randomly in which vibration lasted 10 s. Posture movements were analyzed by a principal component analysis (PCA) calculated on normalized and weighted markers coordinates. The relative standard deviation of each principal movement component (principal position, PP-rSTD) quantified its contribution to the whole postural movements, i.e., quantified the coordinative structure. The first (principal velocities, PV-rSTD) and second (principal accelerations, PA-rSTD) time-derivatives characterized the rate-dependent sensory information associated with and the neuromuscular control of the postural movements, respectively. Children without gymnastic experience showed a different postural coordinative structure and different sensory-motor control characteristics. They used less ankle movements in the anterior-posterior direction but increased ankle movements in medio-lateral direction, presented larger hip and trunk velocities, and exhibited more hip actions. Gymnastic experience during childhood seemed to benefit the development of proprioceptive reweighting processes in children, leading to a more mature form of coordinating and controlling posture similarly to adults.

Effects of simulated peripheral visual field loss on the static postural control in young healthy adults

BACKGROUND

Integration of visual, vestibular, and proprioceptive sensations contributes to postural control. People with peripheral visual field loss have serious postural instability. However, the directional specificity of postural stability and sensory reweighting caused by gradual peripheral visual field loss remain unclear.

RESEARCH QUESTION

What are the effects of peripheral visual field loss on static postural control?

METHODS

Fifteen healthy young adults participated in this study. The participants were asked to stand quietly on a foam surface. Three conditions of virtual visual field loss (90°, 45°, and 15°) were provided by a head-mounted display, and ground reaction forces were collected using a force plate to calculate the displacements of the center of pressure (COP).

RESULTS

The root mean square (RMS), mean velocity, and 95% ellipse area of COP displacements in the horizontal plane increased, and RMS in the anteroposterior (AP) direction was unchanged under the smallest visual field condition compared to the largest one. The power spectrum density of COP displacements in the low-frequency band was decreased and that in the medium-frequency band was increased in the AP direction.

SIGNIFICANCE

During quiet standing of young healthy adults with peripheral visual field loss, increased peripheral visual field loss resulted in lower postural stability. Postural stability in the AP direction was maintained contrary to the functional sensitivity hypothesis. Peripheral visual field loss reduced the weighting of the visual input and increased that of the vestibular input in the AP direction to maintain equilibrium.

Perceptual-motor styles

Even for a stereotyped task, sensorimotor behavior is generally variable due to noise, redundancy, adaptability, learning or plasticity. The sources and significance of different kinds of behavioral variability have attracted considerable attention in recent years. However, the idea that part of this variability depends on unique individual strategies has been explored to a lesser extent. In particular, the notion of style recurs infrequently in the literature on sensorimotor behavior. In general use, style refers to a distinctive manner or custom of behaving oneself or of doing something, especially one that is typical of a person, group of people, place, context, or period. The application of the term to the domain of perceptual and motor phenomenology opens new perspectives on the nature of behavioral variability, perspectives that are complementary to those typically considered in the studies of sensorimotor variability. In particular, the concept of style may help toward the development of personalised physiology and medicine by providing markers of individual behaviour and response to different stimuli or treatments. Here, we cover some potential applications of the concept of perceptual-motor style to different areas of neuroscience, both in the healthy and the diseased. We prefer to be as general as possible in the types of applications we consider, even at the expense of running the risk of encompassing loosely related studies, given the relative novelty of the introduction of the term perceptual-motor style in neurosciences.

Persistent Visual and Vestibular Impairments for Postural Control Following Concussion: A Cross-Sectional Study in University Students

OBJECTIVE

To examine how concussion may impair sensory processing for control of upright stance.

METHODS

Participants were recruited from a single university into 3 groups: 13 participants (8 women, 21 ± 3 years) between 2 weeks and 6 months post-injury who initiated a return-to-play progression (under physician management) by the time of testing (recent concussion group), 12 participants (7 women, 21 ± 1 years) with a history of concussion (concussion history group, > 1 year post-injury), and 26 participants (8 women, 22 ± 3 years) with no concussion history (control group). We assessed sensory reweighting by simultaneously perturbing participants' visual, vestibular, and proprioceptive systems and computed center of mass gain relative to each modality. The visual stimulus was a sinusoidal translation of the visual scene at 0.2 Hz, the vestibular stimulus was ± 1 mA binaural monopolar galvanic vestibular stimulation (GVS) at 0.36 Hz, the proprioceptive stimulus was Achilles' tendon vibration at 0.28 Hz.

RESULTS

The recent concussion (95% confidence interval 0.078-0.115, p = 0.001) and the concussion history (95% confidence interval 0.056-0.094, p = 0.038) groups had higher gains to the vestibular stimulus than the control group (95% confidence interval 0.040-0.066). The recent concussion (95% confidence interval 0.795-1.159, p = 0.002) and the concussion history (95% confidence interval 0.633-1.012, p = 0.018) groups had higher gains to the visual stimulus than the control group (95% confidence interval 0.494-0.752). There were no group differences in gains to the proprioceptive stimulus or in sensory reweighting.

CONCLUSION

Following concussion, participants responded more strongly to visual and vestibular stimuli during upright stance, suggesting they may have abnormal dependence on visual and vestibular feedback. These findings may indicate an area for targeted rehabilitation interventions.

Effects of Shank Vibration on Lean After-Effect

Postural adaptability is related to central sensory integration and reweighting efficiency. Incline-interventions lead to lean after-effect (LAE), but it is not fully known how sensory reweighting may affect the magnitude and duration of LAE. We tasked fifteen young and healthy subjects with performing incline-interventions under conditions designed to perturb proprioception during or after the incline-intervention. We found that support surface configuration affected responses to tendon vibration. Additionally, vibration during an incline-intervention did not inhibit LAE, but vibration during an after-effect significantly affected LAE. Results reinforce claims that postural adaptation is based on modifications of central mechanisms of perception, not peripheral shank proprioceptors and improve our understanding of the role of sensory reweighting and sensory integration into postural adaptability.

Sensory Reweighting for Upright Stance in Soccer Players: A Comparison of High and Low Exposure to Soccer Heading

The purpose of this study was to compare sensory reweighting for upright stance between soccer players who report higher soccer heading exposure to those who report lower soccer heading exposure. Thirty participants completed a self-reported questionnaire to estimate the number of soccer headers experienced over the previous year and were divided into "low exposure" and "high exposure" groups based on their responses. Sensory reweighting for upright stance was assessed by simultaneously perturbing visual, vestibular, and proprioceptive systems. The visual stimulus was a sinusoidal translation of the visual scene at 0.2 Hz, the vestibular stimulus was ±1mA binaural monopolar galvanic vestibular stimulation (GVS) at 0.36 Hz, and the proprioceptive stimulus was Achilles tendon vibration at 0.28 Hz. The visual stimulus was presented at two amplitudes (0.2 m, 0.8 m). Center of mass (COM) gain/phase to each modality, total power, 95% area and velocity were compared between low exposure (N = 15, six males, 21.5 ± 1.9 years, 27.7 ± 31.6 headers) and high exposure groups (N = 15, 10 males, 22.1 ± 3.5years, 734.9 ± 877.7 headers). Without vibration, COM 95% area (F = 5.861, p = 0.022*, partial η² = 0.173), velocity (F = 14.198, p = 0.001, partial η² = 0.336), and total power (F = 13.491, p = 0.001, partial η² = 0.325) for the "high exposure" group were higher than for the "low exposure" group, and postural sway lagged the vestibular stimulus in the "high exposure" group rather than leading it as in the "low exposure" group (F = 4.765, p = 0.038, partial η² = 0.145). There were no differences in sensory reweighting and no differences in COM gain/phase between groups. These findings lend empirical evidence to a detrimental effect of soccer heading exposure on balance control during upright stance.

Age of First Exposure to Soccer Heading and Sensory Reweighting for Upright Stance

US Soccer eliminated soccer heading for youth players ages 10 years and younger and limited soccer heading for children ages 11-13 years. Limited empirical evidence associates soccer heading during early adolescence with medium-to-long-term behavioral deficits. The purpose of this study was to compare sensory reweighting for upright stance between college-aged soccer players who began soccer heading ages 10 years and younger (AFE ≤ 10) and those who began soccer heading after age 10 (AFE > 10). Thirty soccer players self-reported age of first exposure (AFE) to soccer heading. Sensory reweighting was compared between AFE ≤ 10 and AFE > 10. To evaluate sensory reweighting, we simultaneously perturbed upright stance with visual, vestibular, and proprioceptive stimulation. The visual stimulus was presented at two different amplitudes to measure the change in gain to vision, an intra-modal effect; and change in gain to galvanic vestibular stimulus (GVS) and vibration, both inter-modal effects. There were no differences in gain to vision (p=0.857, η2=0.001), GVS (p=0.971, η2=0.000), or vibration (p=0.974, η2=0.000) between groups. There were no differences in sensory reweighting for upright stance between AFE ≤ 10 and AFE > 10, suggesting that soccer heading during early adolescence is not associated with balance deficits in college-aged soccer players, notwithstanding potential deficits in other markers of neurological function.

Foot and Ankle Somatosensory Deficits Affect Balance and Motor Function in Children With Cerebral Palsy

Sensory dysfunction is prevalent in cerebral palsy (CP). Evidence suggests that sensory deficits can contribute to manual ability impairments in children with CP, yet it is still unclear how they contribute to balance and motor performance. Therefore, the objective of this study was to investigate the relationship between lower extremity (LE) somatosensation and functional performance in children with CP. Ten participants with spastic diplegia (Gross Motor Function Classification Scale: I-III) and who were able to stand independently completed the study. Threshold of light touch pressure, two-point discriminatory ability of the plantar side of the foot, duration of cutaneous vibration sensation, and error in the joint position sense of the ankle were assessed to quantify somatosensory function. The balance was tested by the Balance Evaluation System Test (BESTest) and postural sway measures during a standing task. Motor performance was evaluated by using a battery of clinical assessments: (1) Gross Motor Function Measure (GMFM-66-IS) to test gross motor ability; (2) spatiotemporal gait characteristics (velocity, step length) to evaluate walking ability; (3) Timed Up and Go (TUG) and 6 Min Walk (6MWT) tests to assess functional mobility; and (4) an isokinetic dynamometer was used to test the Maximum Volitional Isometric Contraction (MVIC) of the plantar flexor muscles. The results showed that the light touch pressure measure was strongly associated only with the 6MWT. Vibration and two-point discrimination were strongly related to balance performance. Further, the vibration sensation of the first metatarsal head demonstrated a significantly strong relationship with motor performance as measured by GMFM-66-IS, spatiotemporal gait parameters, TUG, and ankle plantar flexors strength test. The joint position sense of the ankle was only related to one subdomain of the BESTest (Postural Responses). This study provides preliminary evidence that LE sensory deficits can possibly contribute to the pronounced balance and motor impairments in CP. The findings emphasize the importance of developing a thorough LE sensory test battery that can guide traditional treatment protocols toward a more holistic therapeutic approach by combining both motor and sensory rehabilitative strategies to improve motor function in CP.

Larger Head Displacement to Optic Flow Presented in the Lower Visual Field

Optic flow that simulates self-motion often produces postural adjustment. Although literature has suggested that human postural control depends largely on visual inputs from the lower field in the environment, effects of the vertical location of optic flow on postural responses are not well investigated. Here, we examined whether optic flow presented in the lower visual field produces stronger responses than optic flow in the upper visual field. Either expanding or contracting optic flow was presented in upper, lower, or full visual fields through an Oculus Rift head-mounted display. Head displacement and vection strength were measured. Results showed larger head displacement under the optic flow presentation in the full visual field and the lower visual field than the upper visual field, during early period of presentation of the contracting optic flow. Vection was strongest in the full visual field and weakest in the upper visual field. Our findings of lower field superiority in head displacement and vection support the notion that ecologically relevant information has a particularly important role in human postural control and self-motion perception.

Weighting and reweighting of visual input via head mounted display given unilateral peripheral vestibular dysfunction

We translated a well-established laboratory paradigm to study sensory integration into a Head-Mounted-Display (HMD). In the current study, a group of 23 individuals with unilateral vestibular dysfunction and 16 age-matched controls observed moving spheres projected from the Oculus Rift. We confirmed increased visual weighting with an unstable surface and decreased visual weighting (i.e., reweighting) with increased visual amplitude. We did not observe significant differences in gains and phases between individuals with vestibular dysfunction and age-matched controls. The vestibular group increased sway in mid and high frequencies significantly more than controls with the change in surface or visual amplitude. Mild visual perturbations within HMDs carry the potential to become a useful portable assessment of postural control in individuals with vestibular disorders.

Virtual Walking Sensation by Prerecorded Oscillating Optic Flow and Synchronous Foot Vibration

This article reports the first psychological evidence that the combination of oscillating optic flow and synchronous foot vibration evokes a walking sensation. In this study, we first captured a walker's first-person-view scenes with footstep timings. Participants observed the naturally oscillating scenes on a head-mounted display with vibrations on their feet and rated walking-related sensations using a Visual Analogue Scale. They perceived stronger sensations of self-motion, walking, leg action, and telepresence from the oscillating visual flow with foot vibrations than with randomized-timing vibrations or without vibrations. The artificial delay of foot vibrations with respect to the scenes diminished the walking-related sensations. These results suggest that the oscillating visual scenes and synchronous foot vibrations are effective for creating virtual walking sensations.

Group-level cortical and muscular connectivity during perturbations to walking and standing balance

Maintaining balance is a complex process requiring multisensory processing and coordinated muscle activation. Previous studies have indicated that the cortex is directly involved in balance control, but less information is known about cortical flow of signals for balance. We studied source-localized electrocortical effective connectivity dynamics of healthy young subjects (29 subjects: 14 male and 15 female) walking and standing with both visual and physical perturbations to their balance. The goal of this study was to quantify differences in group-level corticomuscular connectivity responses to sensorimotor perturbations during walking and standing. We hypothesized that perturbed visual input during balance would transiently decrease connectivity between occipital and parietal areas due to disruptive visual input during sensory processing. We also hypothesized that physical pull perturbations would increase cortical connections to central sensorimotor areas because of higher sensorimotor integration demands. Our findings show decreased occipito-parietal connectivity during visual rotations and widespread increases in connectivity during pull perturbations focused on central areas, as expected. We also found evidence for communication from cortex to muscles during perturbed balance. These results show that sensorimotor perturbations to balance alter cortical networks and can be quantified using effective connectivity estimation.

Sensory integration for human balance control

The upright stance position is inherently unstable since the smallest deviation from a perfect upright orientation produces forces due to gravity that accelerate the body toward the ground. Stability is achieved by generating appropriate joint torques that correct for deviations from a desired orientation with orientation changes detected by sensory systems (primarily somatosensory/proprioceptive, visual, and vestibular systems). Functionally, balance control can be viewed as a closed-loop feedback control system with the integration of different sources of sensory orientation information being one component of the overall system, but with the system's feedback nature placing constraints on the sensory integration process. Analysis of body sway evoked by balance perturbations allows for the measurement of "sensory weights" that represent the relative contributions of different sensory systems to an internal estimate of orientation that, in turn, is used to generate corrective actions. Experiments reveal that sensory weights are not fixed quantities, but vary as a function of environmental and experimental conditions as well as neurologic disorders that affect the quality of sensory information available from different sensory systems. Because environmental conditions can change rapidly, sensory reweighting must also occur rapidly enough to prevent instability due to an under- or overproduction of corrective action.

Stochastic resonance stimulation improves balance in children with cerebral palsy: a case control study

Background

Stochastic Resonance (SR) Stimulation has been used to enhance balance in populations with sensory deficits by improving the detection and transmission of afferent information. Despite the potential promise of SR in improving postural control, its use in individuals with cerebral palsy (CP) is novel. The objective of this study was to investigate the immediate effects of electrical SR stimulation when applied in the ankle muscles and ligaments on postural stability in children with CP and their typically developing (TD) peers.

Methods

Ten children with spastic diplegia (GMFCS level I- III) and ten age-matched TD children participated in this study. For each participant the SR sensory threshold was determined. Then, five different SR intensity levels (no stimulation, 25, 50, 75, and 90% of sensory threshold) were used to identify the optimal SR intensity for each subject. The optimal SR and no stimulation condition were tested while children stood on top of 2 force plates with their eyes open and closed. To assess balance, the center of pressure velocity (COPV) in anteroposterior (A/P) and medial-lateral (M/L) direction, 95% COP confidence ellipse area (COPA), and A/P and M/L root mean square (RMS) measures were computed and compared.

Results

For the CP group, SR significantly decreased COPV in A/P direction, and COPA measures compared to the no stimulation condition for the eyes open condition. In the eyes closed condition, SR significantly decreased COPV only in M/L direction. Children with CP demonstrated greater reduction in all the COP measures but the RMS in M/L direction during the eyes open condition compared to their TD peers. The only significant difference between groups in the eyes closed condition was in the COPV in M/L direction.

Conclusions

SR electrical stimulation may be an effective stimulation approach for decreasing postural sway and has the potential to be used as a therapeutic tool to improve balance. Applying subject-specific SR stimulation intensities is recommended to maximize balance improvements. Overall, balance rehabilitation interventions in CP might be more effective if sensory facilitation methods, like SR, are utilized by the clinicians.

Trial registration

ClinicalTrials.gov identifier NCT02456376; 28 May 2015 (Retrospectively registered); https://clinicaltrials.gov/ct2/show/NCT02456376.

Specificity and variability of trunk kinematics on a mechanical horse

As perturbation training is gaining popularity, it is important to better understand postural control during complex three-dimensional stimuli. One clinically relevant and commonly used three-dimensional stimulus is found in hippotherapy and simulated hippotherapy on a mechanical horse. We tested nine healthy participants on a horse simulator, measured head and trunk kinematics, and characterized data in time (root-mean-square and variability) and frequency (amplitude spectra, gains, and phases) domains. We addressed three fundamental questions: 1) What is the specificity of postural responses to the simulator? 2) Which plane of motion is associated with the most and least variability (repeatable movements across repeated stimuli and across participants)? 3) To what extent are postural responses influenced by different degrees of stability (addition of pelvis straps and trunk support)? We found head and trunk responses were highly specific to the three-dimensional simulator perturbation direction and frequency. Frontal plane responses had the least variability across repetitions and participants whereas transverse motion was most variable. Head motion was more variable than the trunk at low frequencies and exhibited a marked decrease in tilt in the sagittal plane. Finally, the inclusion of pelvis straps had minimal effect on kinematics at low frequencies but altered higher frequencies; whereas added trunk support reduced head and trunk responses to perturbations and altered timing characteristics in all three planes. In conclusion, the present study suggests that frontal plane motion was under a high level of control, and results support the idea that specific head and trunk postural responses can be elicited from a complex three-dimensional stimuli, such as those found in hippotherapy. Researchers and clinicians can use results from this study to help interpret variability, implement mechanical adjustments to stability, and assess responses in pathological populations.

A Standing Function Evaluation System Based on Virtual Light Touch Contact and Factor Analysis

This paper proposes a standing function evaluation system on the basis of factor analysis for indices related to standing characteristics. For this approach, five standing function factors were checked in a large-scale experiment using the results of extraction from 48 indices based on factor analysis. Five factors were extracted from these indices, and the related score radar charts indicated that most factors changed with age. The evaluation results suggest that the system facilitates intuitive comprehension of standing-related factors based on these charts.

Relationship between vestibular sensitivity and multisensory temporal integration

A single event can generate asynchronous sensory cues due to variable encoding, transmission, and processing delays. To be interpreted as being associated in time, these cues must occur within a limited time window, referred to as a "temporal binding window" (TBW). We investigated the hypothesis that vestibular deficits could disrupt temporal visual-vestibular integration by determining the relationships between vestibular threshold and TBW in participants with normal vestibular function and with vestibular hypofunction. Vestibular perceptual thresholds to yaw rotation were characterized and compared with the TBWs obtained from participants who judged whether a suprathreshold rotation occurred before or after a brief visual stimulus. Vestibular thresholds ranged from 0.7 to 16.5 deg/s and TBWs ranged from 13.8 to 395 ms. Among all participants, TBW and vestibular thresholds were well correlated ( R² = 0.674, P < 0.001), with vestibular-deficient patients having higher thresholds and wider TBWs. Participants reported that the rotation onset needed to lead the light flash by an average of 80 ms for the visual and vestibular cues to be perceived as occurring simultaneously. The wide TBWs in vestibular-deficient participants compared with normal functioning participants indicate that peripheral sensory loss can lead to abnormal multisensory integration. A reduced ability to temporally combine sensory cues appropriately may provide a novel explanation for some symptoms reported by patients with vestibular deficits. Even among normal functioning participants, a high correlation between TBW and vestibular thresholds was observed, suggesting that these perceptual measurements are sensitive to small differences in vestibular function. NEW & NOTEWORTHY While spatial visual-vestibular integration has been well characterized, the temporal integration of these cues is not well understood. The relationship between sensitivity to whole body rotation and duration of the temporal window of visual-vestibular integration was examined using psychophysical techniques. These parameters were highly correlated for those with normal vestibular function and for patients with vestibular hypofunction. Reduced temporal integration performance in patients with vestibular hypofunction may explain some symptoms associated with vestibular loss.

Neck muscle vibration produces diverse responses in balance and gait speed between individuals with and without neck pain

BACKGROUND

Neck muscle vibration can influence cervical proprioception and sensorimotor function. It is hypothesized to affect motor performance differently in persons with and without neck pain.

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

To clarify the extent to which vibration-induced motor responses of neck muscles affect static standing balance and gait speed in persons with and without neck pain.

METHODS

Thirty participants with chronic neck pain and 30 healthy controls were recruited. Balance and gait were measured before and after 30 s of suboccipital neck muscle vibration. Balance was measured in a confortable stance with eyes closed using a swaymeter and gait using the timed 10 m walk test.

RESULTS

At baseline, neck pain participants had greater postural sway, particularly in the anterior-posterior direction and slower gait speed than healthy controls (p < 0.001). Immediately after vibration, neck pain participants displayed decreased postural sway, and increased gait speed (p < 0.001). Healthy controls had increased postural sway and decreased gait speed (p < 0.001).

CONCLUSION

Neck muscle vibration improved standing balance and gait speed in participants with neck pain but reduced performance in healthy controls. The study supports the importance of cervical proprioceptive information in postural control. Use of vibration could be considered neck pain patients' rehabilitation.

The effect of spatial auditory landmarks on ambulation

The maintenance of balance and posture is a result of the collaborative efforts of vestibular, proprioceptive, and visual sensory inputs, but a fourth neural input, audition, may also improve balance. Here, we tested the hypothesis that auditory inputs function as environmental spatial landmarks whose effectiveness depends on sound localization ability during ambulation. Eight blindfolded normal young subjects performed the Fukuda-Unterberger test in three auditory conditions: silence, white noise played through headphones (head-referenced condition), and white noise played through a loudspeaker placed directly in front at 135 centimeters away from the ear at ear height (earth-referenced condition). For the earth-referenced condition, an additional experiment was performed where the effect of moving the speaker azimuthal position to 45, 90, 135, and 180° was tested. Subjects performed significantly better in the earth-referenced condition than in the head-referenced or silent conditions. Performance progressively decreased over the range from 0° to 135° but all subjects then improved slightly at the 180° compared to the 135° condition. These results suggest that presence of sound dramatically improves the ability to ambulate when vision is limited, but that sound sources must be located in the external environment in order to improve balance. This supports the hypothesis that they act by providing spatial landmarks against which head and body movement and orientation may be compared and corrected. Balance improvement in the azimuthal plane mirrors sensitivity to sound movement at similar positions, indicating that similar auditory mechanisms may underlie both processes. These results may help optimize the use of auditory cues to improve balance in particular patient populations.

Imbalance: Objective measures versus subjective self-report in clinical practice

BACKGROUND

Dizziness and imbalance are very common complaints in clinical practice. One of the challenges is to evaluate the 'real' risk of falls. Two tools are available: the patient's self-report and the measure of the patient's balance. We evaluated the relationship between these methods using the Dizziness Handicap Inventory (DHI) and measures of balance while visual inputs are perturbed with Virtual Reality (VR).

METHODS

90 consecutive patients underwent the DHI questionnaire and the balance test. The DHI questionnaire was used to measure the subject's perception of handicap associated with dizziness. The balance test measured the postural sway in several visual conditions: eyes open, eyes closed, and with an unpredictable visual perturbation using VR at several amplitudes of movement.

RESULTS

No correlation was found between the DHI score and the balance measurement. The visual perturbations allow us to characterize patients into three groups: one group with a high DHI score who did not fall on the balance test (5.5%), one group with a low DHI score who failed eyes closed on a compliant surface (9.0%), and one group of the remaining patients (85.5%). The correlation between the DHI score and the balance performance became significant on the remaining group of patients.

CONCLUSION

Both subjective self-report and objective measure are important to characterize a patient. The use of VR visual perturbations allowed us to define three important groups of patients. VR visual perturbations provided additional information that helps explain the lack of correlation between DHI and objective test results.

Visual Sensory Signals Dominate Tactile Cues during Docked Feeding in Hummingbirds

Animals living in and interacting with natural environments must monitor and respond to changing conditions and unpredictable situations. Using information from multiple sensory systems allows them to modify their behavior in response to their dynamic environment but also creates the challenge of integrating different, and potentially contradictory, sources of information for behavior control. Understanding how multiple information streams are integrated to produce flexible and reliable behavior is key to understanding how behavior is controlled in natural settings. Natural settings are rarely still, which challenges animals that require precise body position control, like hummingbirds, which hover while feeding from flowers. Tactile feedback, available only once the hummingbird is docked at the flower, could provide additional information to help maintain its position at the flower. To investigate the role of tactile information for hovering control during feeding, we first asked whether hummingbirds physically interact with a feeder once docked. We quantified physical interactions between docked hummingbirds and a feeder placed in front of a stationary background pattern. Force sensors on the feeder measured a complex time course of loading that reflects the wingbeat frequency and bill movement of feeding hummingbirds, and suggests that they sometimes push against the feeder with their bill. Next, we asked whether the measured tactile interactions were used by feeding hummingbirds to maintain position relative to the feeder. We created two experimental scenarios-one in which the feeder was stationary and the visual background moved and the other where the feeder moved laterally in front of a white background. When the visual background pattern moved, docked hummingbirds pushed significantly harder in the direction of horizontal visual motion. When the feeder moved, and the background was stationary, hummingbirds generated aerodynamic force in the opposite direction of the feeder motion. These results suggest that docked hummingbirds are using visual information about the environment to maintain body position and orientation, and not actively tracking the motion of the feeder. The absence of flower tracking behavior in hummingbirds contrasts with the behavior of hawkmoths, and provides evidence that they rely primarily on the visual background rather than flower-based cues while feeding.

Vestibular Deficits in Neurodegenerative Disorders: Balance, Dizziness, and Spatial Disorientation

The vestibular system consists of the peripheral vestibular organs in the inner ear and the associated extensive central nervous system projections—from the cerebellum and brainstem to the thalamic relays to cortical projections. This system is important for spatial orientation and balance, both of critical ecological importance, particularly for successful navigation in our environment. Balance disorders and spatial disorientation are common presenting features of neurodegenerative diseases; however, little is known regarding central vestibular processing in these diseases. A ubiquitous aspect of central vestibular processing is its promiscuity given that vestibular signals are commonly found in combination with other sensory signals. This review discusses how impaired central processing of vestibular signals—typically in combination with other sensory and motor systems—may account for the impaired balance and spatial disorientation in common neurodegenerative conditions. Such an understanding may provide for new diagnostic tests, potentially useful in detecting early disease while a mechanistic understanding of imbalance and spatial disorientation in these patients may enable a vestibular-targeted therapy for such problems in neurodegenerative diseases. Studies with state of the art central vestibular testing are now much needed to tackle this important topic.

Assessment via the Oculus of Visual "Weighting" and "Reweighting" in Young Adults

Substantial advances in virtual reality technology open an exciting window toward better understanding of subdomains of balance control. Here, we studied whether a portable virtual reality headset can be used to test sensory integration for balance. Twenty young adults stood on a both-sides-up ball or floor. Moving spheres were projected from an Oculus Development Kit 2 at various amplitudes and frequencies. Participants' gains indicated visual "weighting" when standing on both-sides-up but not on the floor and "reweighting" with increased visual amplitude. Intraclass correlations showed acceptable to good reliability for all floor conditions and for some of the both-sides-up conditions when we repeated the protocol a week later. Future steps to further develop our paradigm into a clinical assessment of sensory integration for postural control are discussed.

Loss of Peripheral Sensory Function Explains Much of the Increase in Postural Sway in Healthy Older Adults

Postural sway increases with age and peripheral sensory disease. Whether, peripheral sensory function is related to postural sway independent of age in healthy adults is unclear. Here, we investigated the relationship between tests of visual function (VISFIELD), vestibular function (CANAL or OTOLITH), proprioceptive function (PROP), and age, with center of mass sway area (COM) measured with eyes open then closed on firm and then a foam surface. A cross-sectional sample of 366 community dwelling healthy adults from the Baltimore Longitudinal Study of Aging was tested. Multiple linear regressions examined the association between COM and VISFIELD, PROP, CANAL, and OTOLITH separately and in multi-sensory models controlling for age and gender. PROP dominated sensory prediction of sway across most balance conditions (β's = 0.09–0.19, p's < 0.001), except on foam eyes closed where CANAL function loss was the only significant sensory predictor of sway (β = 2.12, p < 0.016). Age was not a consistent predictor of sway. This suggests loss of peripheral sensory function explains much of the age-associated increase in sway.

Control of vertical posture while elevating one foot to avoid a real or virtual obstacle

The purpose of this study is to investigate the control of vertical posture during obstacle avoidance in a real versus a virtual reality (VR) environment. Ten healthy participants stood upright and lifted one leg to avoid colliding with a real obstacle sliding on the floor toward a participant and with its virtual image. Virtual obstacles were delivered by a head mounted display (HMD) or a 3D projector. The acceleration of the foot, center of pressure, and electrical activity of the leg and trunk muscles were measured and analyzed during the time intervals typical for early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs). The results showed that the peak acceleration of foot elevation in the HMD condition decreased significantly when compared with that of the real and 3D projector conditions. Reduced activity of the leg and trunk muscles was seen when dealing with virtual obstacles (HMD and 3D projector) as compared with that seen when dealing with real obstacles. These effects were more pronounced during APAs and CPAs. The onsets of muscle activities in the supporting limb were seen during EPAs and APAs. The observed modulation of muscle activity and altered patterns of movement seen while avoiding a virtual obstacle should be considered when designing virtual rehabilitation protocols.

Body sway adaptation to addition but not withdrawal of stabilizing visual information is delayed by a concurrent cognitive task

The aim of this study was to test the effects of a concurrent cognitive task on the promptness of the sensorimotor integration and reweighting processes following addition and withdrawal of vision. Fourteen subjects stood in tandem while vision was passively added and removed. Subjects performed a cognitive task, consisting of counting backward in steps of three, or were "mentally idle." We estimated the time intervals following addition and withdrawal of vision at which body sway began to change. We also estimated the time constant of the exponential change in body oscillation until the new level of sway was reached, consistent with the current visual state. Under the mentally idle condition, mean latency was 0.67 and 0.46 s and the mean time constant was 1.27 and 0.59 s for vision addition and withdrawal, respectively. Following addition of vision, counting backward delayed the latency by about 300 ms, without affecting the time constant. Following withdrawal, counting backward had no significant effect on either latency or time constant. The extension by counting backward of the time interval to stabilization onset on addition of vision suggests a competition for allocation of cortical resources. Conversely, the absence of cognitive task effect on the rapid onset of destabilization on vision withdrawal, and on the relevant reweighting time course, advocates the intervention of a subcortical process. Diverting attention from a challenging standing task discloses a cortical supervision on the process of sensorimotor integration of new balance-stabilizing information. A subcortical process would instead organize the response to removal of the stabilizing sensory input.NEW & NOTEWORTHY This study is the first to test the effect of an arithmetic task on the time course of balance readjustment following visual withdrawal or addition. Performing such a cognitive task increases the time delay following addition of vision but has no effect on withdrawal dynamics. This suggests that sensorimotor integration following addition of a stabilizing signal is performed at a cortical level, whereas the response to its withdrawal is "automatic" and accomplished at a subcortical level.

Balance in Virtual Reality: Effect of Age and Bilateral Vestibular Loss

Background

Quantitative balance measurement is used in clinical practice to prevent falls. The conditions of the test were limited to eyes open, eyes closed, and sway-referenced vision. We developed a new visual perturbation to challenge balance using virtual reality (VR), measuring postural stability by a Wii Balance Board (WBB).

Methods

In this study, we recorded balance performance of 116 healthy subjects and of 10 bilateral vestibular loss patients using VR to assess the effect of age and the effect of total loss of vestibular function. We used several conditions: eyes open (normal visual inputs), eyes closed (no visual inputs), stable visual world (vision referenced), and perturbed visual world (visual perturbation) at different amplitudes of perturbation. Balance under these visual conditions was assessed on the WBB (stable support surface) and on the WBB plus foam rubber (unstable support surface).

Results

In healthy subjects, we found that the percentage of falls increased with age and with the amplitude of perturbation for both conditions: WBB or WBB + foam. Moreover, we can define a threshold for falls in each age group as the amplitude of perturbation which induced falls. For bilateral vestibular loss patients, on the WBB + foam, all of them failed with eyes closed and with perturbed visual world even at the minimal amplitude of perturbation. Finally, we observed that stable visual world induced fewer falls than eyes closed whatever the subject’s group (healthy or bilateral vestibular loss) and whatever the age decade.

Conclusion

VR allowed us to develop a useful new tool with a wide range of visual perturbations. Rather than only two levels of visual condition (eyes open and eyes closed), the VR stimulus can be continuously adjusted to produce a visual perturbation powerful enough to induce falls even in young healthy subjects and which has allowed us to determine a threshold for falls.

Altered functional brain connectivity in patients with visually induced dizziness

Background

Vestibular patients occasionally report aggravation or triggering of their symptoms by visual stimuli, which is called visually induced dizziness (VID). These patients therefore experience dizziness, discomfort, disorientation and postural unsteadiness. The underlying pathophysiology of VID is still poorly understood.

Objective

The aim of the current explorative study was to gain a first insight in the underlying neural aspects of VID.

Methods

We included 10 VID patients and 10 healthy matched controls, all of which underwent a resting state fMRI scan session. Changes in functional connectivity were explored by means of the intrinsic connectivity contrast (ICC). Seed-based analysis was subsequently performed in visual and vestibular seeds.

Results

We found a decreased functional connectivity in the right central operculum (superior temporal gyrus), as well as increased functional connectivity in the occipital pole in VID patients as compared to controls in a hypothesis-free analysis. A weaker functional connectivity between the thalamus and most of the right putamen was measured in VID patients in comparison to controls in a seed-based analysis. Furthermore, also by means of a seed-based analysis, a decreased functional connectivity between the visual associative area and the left parahippocampal gyrus was found in VID patients. Additionally, we found increased functional connectivity between thalamus and occipital and cerebellar areas in the VID patients, as well as between the associative visual cortex and both middle frontal gyrus and precuneus.

Conclusions

We found alterations in the visual and vestibular cortical network in VID patients that could underlie the typical VID symptoms such as a worsening of their vestibular symptoms when being exposed to challenging visual stimuli. These preliminary findings provide the first insights into the underlying functional brain connectivity in VID patients. Future studies should extend these findings by employing larger sample sizes, by investigating specific task-based paradigms in these patients and by exploring the implications for treatment.

•

Visually-induced patients present decreased functional connectivity of vestibular-related brain regions.

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Visually-induced dizziness patients present increased functional connectivity of visual and cerebellar brain regions.

•

These findings might underlie typically seen symptoms in visually-induced dizziness, i.e. an overreliance on visual cues.

•

This is the first exploratory study investigating the underlying neural aspects of visually-induced dizziness.

•

These preliminary findings should be extended by larger sample sizes and by supplementing rsfMRI with task-based paradigms.

Vestibular Dysfunction after Subconcussive Head Impact

Current thinking views mild head impact (i.e., subconcussion) as an underrecognized phenomenon that has the ability to cause significant current and future detrimental neurological effects. Repeated mild impacts to the head, however, often display no observable behavioral deficits based on standard clinical tests, which may lack sensitivity. The current study investigates the effects of subconcussive impacts from soccer heading with innovative measures of vestibular function and walking stability in a pre- 0-2 h, post- 24 h post-heading repeated measures design. The heading group (n = 10) executed 10 headers with soccer balls projected at a velocity of 25 mph (11.2 m/sec) over 10 min. Subjects were evaluated 24 h before, immediately after, and 24 h after soccer heading with: the modified Balance Error Scoring System (mBESS); a walking stability task with visual feedback of trunk movement; and galvanic vestibular stimulation (GVS) while standing with eyes closed on foam. A control group (n = 10) followed the same protocol with no heading. The results showed significant decrease in trunk angle, leg angle gain, and center of mass gain relative to GVS for the heading group compared with controls. Medial-lateral trunk orientation displacement and velocity during treadmill walking increased immediately after mild head impact for the heading group compared with controls. Controls showed an improvement in mBESS scores over time, indicating a learning effect, which was not observed with the heading group. These results suggest that mild head impact leads to a transient dysfunction in vestibular processing, which deters walking stability during task performance.

Segmental trunk and head dynamics during frontal plane tilt stimuli in healthy sitting adults

A more detailed understanding of trunk behavior during upright sitting is needed to create a foundation to address functional posture impairments. Therefore, we characterized the dynamics of the trunk and head during perturbed sitting. A three-link inverted pendulum model of head and trunk segments was used to analyze kinematics of eight healthy sitting adults. Magnetic sensors were placed at the head and two locations of the trunk (C7 and T7). Six surface tilt stimuli (two spontaneous sway tests [no surface stimulus; eyes open, EO/eyes closed, EC] and four tests with continuous pseudorandom surface tilts [2 peak-to peak amplitudes of 2° or 8°; EO/EC]) were applied in the frontal plane. We used frequency-response functions (FRFs) to analyze sway across ~0.045-3Hz and found systematic differences in sway dynamics across segments. Superior segments exhibited larger fluctuations in gain and phase values across frequencies. FRF gains in superior segments were attenuated compared to other segments only at low frequencies but were larger at the higher frequencies. We also tested the influence of stimulus amplitude and visual availability on FRFs. Across all segments, increasing stimulus amplitude and visual availability (EO) resulted in lower gains, however, these effects were most prominent in superior segments. These changes in gain were likely influenced by changes in sensory reliance across test conditions. In conclusion, these results provide a benchmark for future comparisons to segmental responses from individuals with impaired trunk control. We suggest that a frequency-based approach provides detail needed to characterize multi-segment dynamics related to sensorimotor control.