Reinforcement-based processes actively regulate motor exploration along redundant solution manifolds.

From a baby's babbling to a songbird practising a new tune, exploration is critical to motor learning. A hallmark of exploration is the emergence of random walk behaviour along solution manifolds, where successive motor actions are not independent but rather become serially dependent. Such exploratory random walk behaviour is ubiquitous across species' neural firing, gait patterns and reaching behaviour. The past work has suggested that exploratory random walk behaviour arises from an accumulation of movement variability and a lack of error-based corrections. Here, we test a fundamentally different idea-that reinforcement-based processes regulate random walk behaviour to promote continual motor exploration to maximize success. Across three human reaching experiments, we manipulated the size of both the visually displayed target and an unseen reward zone, as well as the probability of reinforcement feedback. Our empirical and modelling results parsimoniously support the notion that exploratory random walk behaviour emerges by utilizing knowledge of movement variability to update intended reach aim towards recently reinforced motor actions. This mechanism leads to active and continuous exploration of the solution manifold, currently thought by prominent theories to arise passively. The ability to continually explore muscle, joint and task redundant solution manifolds is beneficial while acting in uncertain environments, during motor development or when recovering from a neurological disorder to discover and learn new motor actions.

Imaging the lower limb network in Parkinson's disease.

BACKGROUND: Despite the significant impact of lower limb symptoms on everyday life activities in Parkinson's disease (PD), knowledge of the neural correlates of lower limb deficits is limited. OBJECTIVE: We ran an fMRI study to investigate the neural correlates of lower limb movements in individuals with and without PD. METHODS: Participants included 24 PD and 21 older adults who were scanned while performing a precisely controlled isometric force generation task by dorsiflexing their ankle. A novel MRI-compatible ankle dorsiflexion device that limits head motion during motor tasks was used. The PD were tested on their more affected side, whereas the side in controls was randomized. Importantly, PD were tested in the off-state, following overnight withdrawal from antiparkinsonian medication. RESULTS: The foot task revealed extensive functional brain changes in PD compared to controls, with reduced fMRI signal during ankle dorsiflexion within the contralateral putamen and M1 foot area, and ipsilateral cerebellum. The activity of M1 foot area was negatively correlated with the severity of foot symptoms based on the Movement Disorder Society-Sponsored Revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS-III). CONCLUSION: Overall, current findings provide new evidence of brain changes underlying motor symptoms in PD. Our results suggest that pathophysiology of lower limb symptoms in PD appears to involve both the cortico-basal ganglia and cortico-cerebellar motor circuits.

Rate control deficits during pinch grip and ankle dorsiflexion in early-stage Parkinson's disease.

BACKGROUND: Much of our understanding of the deficits in force control in Parkinson's disease (PD) relies on findings in the upper extremity. Currently, there is a paucity of data pertaining to the effect of PD on lower limb force control. OBJECTIVE: The purpose of this study was to concurrently evaluate upper- and lower-limb force control in early-stage PD and a group of age- and gender-matched healthy controls. METHODS: Twenty individuals with PD and twenty-one healthy older adults participated in this study. Participants performed two visually guided, submaximal (15% of maximum voluntary contractions) isometric force tasks: a pinch grip task and an ankle dorsiflexion task. PD were tested on their more affected side and after overnight withdrawal from antiparkinsonian medication. The tested side in controls was randomized. Differences in force control capacity were assessed by manipulating speed-based and variability-based task parameters. RESULTS: Compared with controls, PD demonstrated slower rates of force development and force relaxation during the foot task, and a slower rate of relaxation during the hand task. Force variability was similar across groups but greater in the foot than in the hand in both PD and controls. Lower limb rate control deficits were greater in PD with more severe symptoms based on the Hoehn and Yahr stage. CONCLUSIONS: Together, these results provide quantitative evidence of an impaired capacity in PD to produce submaximal and rapid force across multiple effectors. Moreover, results suggest that force control deficits in the lower limb may become more severe with disease progression.

Effect of Increasing Obstacle Distances Task on Postural Stability Variables During Gait Initiation in Older Nonfallers and Fallers.

OBJECTIVE: To compare the scaling of the postural stability variables between older nonfallers and fallers during gait initiation (GI) while stepping over increasing obstacle distances. DESIGN: Cross-sectional study. SETTING: University research laboratory. PARTICIPANTS: A sample of participants (N=24) divided into 2 groups: older nonfallers (n=12) and older fallers (n=12). Participants had no known neurologic, musculoskeletal, or cardiovascular conditions that could have affected their walking, and all were independent walkers. All the participants had an adequate cognitive function to participate as indicated by a score of more than 24 on the Mini-Mental State Examination. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The primary dependent variables were peak anterior-posterior (AP) center of mass (CoM)-center of pressure (CoP) separation during anticipatory postural adjustments (APAs), AP CoM-CoP separation at the toe-off, and peak AP CoM-CoP separation during the swing. Secondary dependent variables were AP trunk angle during GI. Within- and between-repeated measures analysis of variance was used to compare means between groups across different task conditions for all the dependent variables. RESULTS: There was a main effect of group for peak AP CoM-CoP separation during APA (P=.018), an interaction effect between group and condition for AP CoM-CoP separation at toe-off (P=.009), and a main effect of condition for peak AP CoM-CoP separation during the swing (P<.001). We also found a main effect of group for peak AP trunk angle during the swing (P=.028). CONCLUSIONS: For GI while stepping over increasing obstacle distances, older fallers adopt a more conservative strategy of AP CoM-CoP separation than nonfallers prior to toe-off and demonstrate increased peak AP trunk lean during the swing. AP CoM-CoP separation prior to toe-off during the GI task may be a critical marker to identify fallers and warrants additional investigation.

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.

Posterior fall-recovery training applied to individuals with chronic stroke: A single-group intervention study.

BACKGROUND: To assess the effects of the initial stepping limb on posterior fall recovery in individuals with chronic stroke, as well as to determine the benefits of fall-recovery training on these outcomes. METHODS: This was a single-group intervention study of 13 individuals with chronic stroke. Participants performed up to six training sessions, each including progressively challenging, treadmill-induced perturbations from a standing position. Progressions focused on initial steps with the paretic or non-paretic limb. The highest perturbation level achieved, the proportion of successful recoveries, step and trunk kinematics, as well as stance-limb muscle activation about the ankle were compared between the initial stepping limbs in the first session. Limb-specific outcomes were also compared between the first and last training sessions. FINDINGS: In the first session, initial steps with the non-paretic limb were associated with a higher proportion of success and larger perturbations than steps with the paretic limb (p = 0.02, Cohen's d = 0.8). Paretic-limb steps were wider relative to the center of mass (CoM; p = 0.01, d = 1.3), likely due to an initial standing position with the CoM closer to the non-paretic limb (p = 0.01, d = 1.4). In the last training session, participants recovered from a higher proportion of perturbations and advanced to larger perturbations (p < 0.05, d > 0.6). There were no notable changes in kinematic or electromyography variables with training (p > 0.07, d < 0.5). INTERPRETATION: The skill of posterior stepping in response to a perturbation can be improved with practice in those with chronic stroke, we were not able to identify consistent underlying kinematic mechanisms behind this adaptation.

Effects of purposeful soccer heading on circulating small extracellular vesicle concentration and cargo.

BACKGROUND: Considering the potential cumulative effects of repetitive head impact (HI) exposure, we need sensitive biomarkers to track short- and long-term effects. Circulating small extracellular vesicles (sEVs) (<200 nm) traffic biological molecules throughout the body and may have diagnostic value as biomarkers for disease. The purpose of this study was to identify the microRNA (miRNA) profile in circulating sEVs derived from human plasma following repetitive HI exposure. METHODS: Healthy adult (aged 18-35 years) soccer players were randomly assigned to one of 3 groups: the HI group performed 10 standing headers, the leg impact group performed 10 soccer ball trapping maneuvers over 10 min, and the control group did not participate in any soccer drills. Plasma was collected before testing and 24 h afterward, and sEVs were isolated and characterized via nanoparticle tracking analysis. Next-generation sequencing was utilized to identify candidate miRNAs isolated from sEVs, and candidate microRNAs were analyzed via quantitative polymerase chain reaction. In silico target prediction was performed using TargetScan (Version 7.0; targetscan.org) and miRWalk (http://mirwalk.umm.uni-heidelberg.de/) programs, and target validation was performed using luciferase reporter vectors with a miR-7844-5p mimic in human embryonic kidney (HEK) 293T/17 cells. RESULTS: Plasma sEV concentration and size were not affected across time and group following repetitive HI exposure. After 24 h, the HI read count from next-generation sequencing showed a 4-fold or greater increase in miR-92b-5p, miR-423-5p, and miR-24-3p and a 3-fold or greater decrease in miR-7844-5p, miR-144-5p, miR-221-5p, and miR-22-3p. Analysis of quantitative polymerase chain reaction revealed that leg impact did not alter the candidate miRNA levels. To our knowledge, miR-7844-5p is a previously unknown miRNA. We identified 8 miR-7844-5p mRNA targets: protein phosphatase 1 regulatory inhibitor subunit 1B (PPP1R1B), LIM and senescent cell antigen-like domains 1 (LIMS1), autophagy-related 12 (ATG12), microtubule-associated protein 1 light chain 3 beta (MAP1LC3B), integrin subunit alpha-1 (ITGA1), mitogen-activated protein kinase 1 (MAPK1), glycogen synthase kinase 3ß (GSK3ß), and mitogen-activated protein kinase 8 (MAPK8). CONCLUSION: Collectively, these data indicate repetitive HI exposure alters plasma sEV miRNA content, but not sEV size or number. Furthermore, for the first time we demonstrate that previously unknown miR-7844-5p targets mRNAs known to be involved in mitochondrial apoptosis, autophagy regulation, mood disorders, and neurodegenerative disease.

Interactions Between Different Age-Related Factors Affecting Balance Control in Walking.

Maintaining balance during walking is a continuous sensorimotor control problem. Throughout the movement, the central nervous system has to collect sensory data about the current state of the body in space, use this information to detect possible threats to balance and adapt the movement pattern to ensure stability. Failure of this sensorimotor loop can lead to dire consequences in the form of falls, injury and death. Such failures tend to become more prevalent as people get older. While research has established a number of factors associated with higher risk of falls, we know relatively little about age-related changes of the underlying sensorimotor control loop and how such changes are related to empirically established risk factors. This paper approaches the problem of age-related fall risk from a neural control perspective. We begin by summarizing recent empirical findings about the neural control laws mapping sensory input to motor output for balance control during walking. These findings were established in young, neurotypical study populations and establish a baseline of sensorimotor control of balance. We then review correlates for deteriorating balance control in older adults, of muscle weakness, slow walking, cognitive decline, and increased visual dependency. While empirical associations between these factors and fall risk have been established reasonably well, we know relatively little about the underlying causal relationships. Establishing such causal relationships is hard, because the different factors all co-vary with age and are difficult to isolate empirically. One option to analyze the role of an individual factor for balance control is to use computational models of walking comprising all levels of the sensorimotor control loop. We introduce one such model that generates walking movement patterns from a short list of spinal reflex modules with limited supraspinal modulation for balance. We show how this model can be used to simulate empirical studies, and how comparison between the model and empirical results can indicate gaps in our current understanding of balance control. We also show how different aspects of aging can be added to this model to study their effect on balance control in isolation.

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.

Anterior fall-recovery training applied to individuals with chronic stroke.

BACKGROUND: To study the effects of the initial stepping limb on anterior fall-recovery performance and kinematics, as well as to determine the benefits of fall-recovery training on those outcomes in individuals with chronic stroke. METHODS: Single-group intervention of 15 individuals with chronic stroke who performed up to six sessions of fall-recovery training. Each session consisted of two progressions of treadmill-induced perturbations to induce anterior falls from a standing position. Progressions focused on initial steps with the paretic or non-paretic limb. Fall-recovery performance (the highest disturbance level achieved and the proportion of successful recoveries), as well as step and trunk kinematics were compared between the initial stepping limbs on the first session. Limb-specific outcomes were also compared between the first and last training sessions. FINDINGS: There were no between-limb differences in fall-recovery performance in the first session. With training, participants successfully recovered from a higher proportion of falls (p's = 0.01, Cohen's d's > 0.7) and progressed to larger perturbation magnitudes (p's < 0.06, d's > 0.5). Initial steps with the paretic limb were wider and shorter relative to the center of mass (p's < 0.06, d's > 0.5). With training, initial paretic-limb steps became longer relative to the CoM (p = 0.03, d = 0.7). Trunk forward rotation was reduced when first stepping with the non-paretic limb (p = 0.03, d = 0.6). INTERPRETATION: The initial stepping limb affects relevant step kinematics during anterior fall recovery. Fall-recovery training improved performance and select kinematic outcomes in individuals with chronic stroke.

Phase-Dependency of Medial-Lateral Balance Responses to Sensory Perturbations During Walking.

The human body is mechanically unstable during walking. Maintaining upright stability requires constant regulation of muscle force by the central nervous system to push against the ground and move the body mass in the desired way. Activation of muscles in the lower body in response to sensory or mechanical perturbations during walking is usually highly phase-dependent, because the effect any specific muscle force has on the body movement depends upon the body configuration. Yet the resulting movement patterns of the upper body after the same perturbations are largely phase-independent. This is puzzling, because any change of upper-body movement must be generated by parts of the lower body pushing against the ground. How do phase-dependent muscle activation patterns along the lower body generate phase-independent movement patterns of the upper body? We hypothesize that when a sensory system detects a deviation of the body in space from a desired state that indicates the onset of a fall, the nervous system generates a functional response by pushing against the ground in any way possible with the current body configuration. This predicts that the changes in the ground reaction force patterns following a balance perturbation should be phase-independent. Here we test this hypothesis by disturbing upright balance in the frontal plane using Galvanic vestibular stimulation at three different points in the gait cycle. We measure the resulting changes in whole-body center of mass movement and the location of the center of pressure of the ground reaction force. We find that the magnitude of the initial center of pressure shift in the direction of the perceived fall is larger for perturbations late in the gait cycle, while there is no statistically significant difference in onset time. These results contradict our hypothesis by showing that even the initial CoP shift in response to a balance perturbation depends upon the phase of the gait cycle. Contrary to expectation, we also find that the whole-body balance response is not phase-independent. Both the onset time and the magnitude of the whole-body center of mass shift depend on the phase of the perturbation. We conclude that the central nervous system recruits any available mechanism to generate a functional balance response by pushing against the ground as fast as possible in response to a perturbation, but that the different mechanisms available at different phases in the gait cycle are not equally strong, leading to phase-dependent differences in the overall response.

Neural Control of Balance During Walking.

Neural control of standing balance has been extensively studied. However, most falls occur during walking rather than standing, and findings from standing balance research do not necessarily carry over to walking. This is primarily due to the constraints of the gait cycle: Body configuration changes dramatically over the gait cycle, necessitating different responses as this configuration changes. Notably, certain responses can only be initiated at specific points in the gait cycle, leading to onset times ranging from 350 to 600 ms, much longer than what is observed during standing (50-200 ms). Here, we investigated the neural control of upright balance during walking. Specifically, how the brain transforms sensory information related to upright balance into corrective motor responses. We used visual disturbances of 20 healthy young subjects walking in a virtual reality cave to induce the perception of a fall to the side and analyzed the muscular responses, changes in ground reaction forces and body kinematics. Our results showed changes in swing leg foot placement and stance leg ankle roll that accelerate the body in the direction opposite of the visually induced fall stimulus, consistent with previous results. Surprisingly, ankle musculature activity changed rapidly in response to the stimulus, suggesting the presence of a direct reflexive pathway from the visual system to the spinal cord, similar to the vestibulospinal pathway. We also observed systematic modulation of the ankle push-off, indicating the discovery of a previously unobserved balance mechanism. Such modulation has implications not only for balance but plays a role in modulation of step width and length as well as cadence. These results indicated a temporally-coordinated series of balance responses over the gait cycle that insures flexible control of upright balance during walking.

Light touch compensates peripheral somatosensory degradation in postural control of older adults.

The present study aimed to investigate the sensitivity of detecting lower limb passive motion and use of additional sensory information from fingertip light touch for the postural control of older adults in comparison with young adults. A total of 11 older and 11 young adults (aged 68.1 ±â€¯5.2 and 24.2 ±â€¯2.2 years, respectively) underwent two tasks. We evaluated their sensitivity to passive ankle joint movement while seated in the first task. Participants then stood quietly on a force plate in a semi-tandem stance, for 30 s under two fingertip contact force conditions (no touch and light touch limited to 1 N). The results showed that the threshold of passive ankle displacement and body sway is higher in older adults than in young adults. The body sway reduced for both older and young adults with the addition of light touch at the fingertips. The maximum cross-correlation coefficient and time lags between body sway and fingertip light touch center of pressure was similar between both groups, suggesting that older adults used light touch to reduce body sway, similar to young adults. A higher threshold in detecting passive ankle joint movement may contribute to the increased body sway observed in older adults. These deficits may be compensated by additional sensory cues that would provide enhanced information used to control the upright stance.

Sensory-Challenge Balance Exercises Improve Multisensory Reweighting in Fall-Prone Older Adults.

BACKGROUND AND PURPOSE: Multisensory reweighting (MSR) deficits in older adults contribute to fall risk. Sensory-challenge balance exercises may have value for addressing the MSR deficits in fall-prone older adults. The purpose of this study was to examine the effect of sensory-challenge balance exercises on MSR and clinical balance measures in fall-prone older adults. METHODS: We used a quasi-experimental, repeated-measures, within-subjects design. Older adults with a history of falls underwent an 8-week baseline (control) period. This was followed by an 8-week intervention period that included 16 sensory-challenge balance exercise sessions performed with computerized balance training equipment. Measurements, taken twice before and once after intervention, included laboratory measures of MSR (center of mass gain and phase, position, and velocity variability) and clinical tests (Activities-specific Balance Confidence Scale, Berg Balance Scale, Sensory Organization Test, Limits of Stability test, and lower extremity strength and range of motion). RESULTS: Twenty adults 70 years of age and older with a history of falls completed all 16 sessions. Significant improvements were observed in laboratory-based MSR measures of touch gain (P = 0.006) and phase (P = 0.05), Berg Balance Scale (P = 0.002), Sensory Organization Test (P = 0.002), Limits of Stability Test (P = 0.001), and lower extremity strength scores (P = 0.005). Mean values of vision gain increased more than those for touch gain, but did not reach significance. DISCUSSION AND CONCLUSIONS: A balance exercise program specifically targeting multisensory integration mechanisms improved MSR, balance, and lower extremity strength in this mechanistic study. These valuable findings provide the scientific rationale for sensory-challenge balance exercise to improve perception of body position and motion in space and potential reduction in fall risk.

A Tool to Quantify the Functional Impact of Oscillopsia.

BACKGROUND: Individuals with bilateral vestibular hypofunction (BVH) often report symptoms of oscillopsia during walking. Existing assessments of oscillopsia are limited to descriptions of severity and symptom frequency, neither of which provides a description of functional limitations attributed to oscillopsia. A novel questionnaire, the Oscillopsia Functional Impact scale (OFI) was developed to describe the impact of oscillopsia on daily life activities. Questions on the OFI ask how often individuals are able to execute specific activities considered to depend on gaze stability in an effort to link functional mobility impairments to oscillopsia for individuals with vestibular loss. METHODS: Subjective reports of oscillopsia and balance confidence were recorded for 21 individuals with BVH and 48 healthy controls. Spearman correlation coefficients were calculated to determine the relationship between the OFI and oscillopsia visual analog scale (OS VAS), oscillopsia severity questionnaire (OSQ), and Activities-Specific Balance Confidence scale to demonstrate face validity. Chronbach's α was calculated to determine internal validity for the items of the OFI. A one-way MANOVA was conducted with planned post hoc paired t-tests for group differences on all oscillopsia questionnaires using a corrected α = 0.0125. RESULTS: The OFI was highly correlated with measures of oscillopsia severity (OS VAS; r = 0.69, p < 0.001) and frequency (OSQ; r = 0.84, p < 0.001) and also with the Activities-Specific Balance Confidence scale (r = -0.84, p < 0.001). Cronbach's α for the OFI was 0.97. Individuals with BVH scored worse on all measures of oscillopsia and balance confidence compared to healthy individuals (p's < 0.001). CONCLUSION: The OFI appears to capture the construct of oscillopsia in the context of functional mobility. Combining with oscillopsia metrics that quantify severity and frequency allows for a more complete characterization of the impact of oscillopsia on an individual's daily behavior. The OFI discriminated individuals with BVH from healthy individuals.

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 (ß'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 (ß = 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.

Eye Movements Are Correctly Timed During Walking Despite Bilateral Vestibular Hypofunction.

Individuals with bilateral vestibular hypofunction (BVH) often report symptoms of oscillopsia (the perception that the world is bouncing or unstable) during walking. Efference copy/proprioception contributes to locomotion gaze stability in animals, sometimes inhibiting the vestibulo-ocular reflex (VOR). Gaze stability requires both adequate eye velocity and appropriate timing of eye movements. It is unknown whether eye velocity (VOR gain), timing (phase), or both are impaired for individuals with BVH during walking. Identifying the specific mechanism of impaired gaze stability can better inform rehabilitation options. Gaze stability was measured for eight individuals with severe BVH and eight healthy age- and gender-matched controls while performing a gaze fixation task during treadmill walking. Frequency response functions (FRF) were calculated from pitch eye and head velocity. A one-way ANOVA was conducted to determine group differences for each frequency bin of the FRF. Pearson correlation coefficients were calculated to determine the relationship between the real and imaginary parts of the FRF and the Oscillopsia Visual Analog Scale (oVAS) scores. Individuals with BVH demonstrated significantly lower gains than healthy controls above 0.5 Hz, but their phase was ideally compensatory for frequencies below 3 Hz. Higher oVAS scores were correlated with lower gain. Individuals with BVH demonstrated ideal timing for vertical eye movements while walking despite slower than ideal eye velocity when compared to healthy controls. Rehabilitation interventions focusing on enhancing VOR gain during walking should be developed to take advantage of the intact timing reported here. Specifically, training VOR gain while walking may reduce oscillopsia severity and improve quality of life.