Critical features of training that facilitate adaptive generalization of over ground locomotion

Learning a new balance task: the influence of prior motor practice on training adaptations

Prior motor experience is thought to aid in the acquisition of new skills. However, studies have shown that balance training does not promote learning of a subsequent balance task. These results stand in contrast to the learning-to-learn paradigm, which is well described for other tasks. We therefore tested if a coordinative affinity between tasks is needed to achieve a learning-to-learn for balance control.Three groups trained different motor tasks during training phase1 (coordination ladder (COOR); bipedal wobble board (2WB); single-leg wobble board (1WB)). During training phase2, all groups trained a tiltboard balance task. Task-specific and transfer effects were evaluated for phase1. A potential learning-to-learn effect was evaluated by comparing the acquisition rates from phase2 for the tiltboard task that was used for training and testing.The results indicate task-specific adaptations after phase1 for 1WB. In contrast, 2WB showed similar improvements than 1WB and COOR (effect sizes: -0.31 to -0.38) when tested on the wobble board with bipedal stance indicating no task-specific improvement for 2WB. For phase2, the linear regression analysis showed larger adaptations for 1WB and 2WB when compared to COOR. This effect implies some uncertainty due to overlapping confidence intervals.Task-specific adaptations after phase1 were found for 1WB but not 2WB. It is discussed that the difficulty of the training task could explain these contrasting results. During phase2, larger adaptations were found for both groups that trained balance tasks during phase1. Thus, despite some uncertainty, prior balance training appears to promote adaptations of a subsequently learned balance task.

Adaptation in Gait to Lunar and Martian Gravity Unloading During Long-Term Isolation in the Ground-Based Space Station Model

The aim of the experiment was to evaluate the adaptive responses of biomechanical and electromyographic parameters to vertical unloading (Lunar—0.15 G and Martian—0.35 G) when walking during the 4-month isolation experiment SIRIUS-19 in the ground-based space station model (GBI). The study involved 6 healthy international crew members of the SIRIUS-19 project aged 34 ± 6.2 years (3 women and 3 men). Body Weight Unloading (BWU) conditions was created by the h/p/cosmos airwalk system. The locomotor test included walking (3.5 ± 0.3 km/h) with a sequential change of BWU modes: 5-min walking with 0% BWU (1 G), 5-min walking with 65% BWU (0.35 G) and 5-min walking with 85% BWU (0.15 G). Ground Reaction Force was recorded by the h/p/cosmos treadmill device. Muscle Lab Model 4000e device was used to record the electromyographic signals of the hip and shin muscles. The locomotor test was performed twice before GBI, monthly during GBI and 1 week after leaving isolation. The results obtained before GBI demonstrate that the changes of support and proprioceptive afferentation signals play significant role in reorganizing of the biomechanical structure of motor acts and the development of new movement patterns. The results of the study are consistent with the previously obtained results of other studies in this direction. Despite the fact that during the GBI the participants of the experiment performed regular physical training, a decrease in the performance indicators values was detected, especially pronounced after 100 days of GBI. This is probably due to limited space of a space station model, as well as the development of a special motor stereotype in it. Noteworthy are the results obtained after the 4th session of the experiment, indicating the effect of sensorimotor learning. We think that the data obtained in this study will be useful in research both in gravitational physiology and in clinical medicine.

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

KEY POINTS

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

ABSTRACT

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

The Gait Disorientation Test: A New Method for Screening Adults With Dizziness and Imbalance

OBJECTIVE

To develop and evaluate a new method for identifying gait disorientation due to vestibular dysfunction.

DESIGN

The gait disorientation test (GDT) involves a timed comparison of the ability to walk 6.096 m with eyes open versus eyes closed. In this prospective study, participants were grouped based on vestibular function. All participants completed a clinical examination, self-report- and performance-based measures relevant to vestibular rehabilitation, and the tasks for the GDT. Vestibular-impaired participants underwent the criterion standard, videonystagmography and/or rotational chair testing.

SETTING

Ambulatory clinic, tertiary referral center.

PARTICIPANTS

Participants (N=40) (20 vestibular-impaired, 30 women, 49.9±16.1years old) were enrolled from a convenience/referral sample of 52 adults.

MAIN OUTCOME AND MEASURE(S)

We determined test-retest reliability using the intraclass correlation coefficient model 3,1; calculated the minimal detectable change (MDC); examined concurrent validity through Spearman correlation coefficients; assessed criterion validity with the area under the curve (AUC) from receiver operator characteristic analysis; and computed the sensitivity, specificity, diagnostic odds ratio (DOR), likelihood ratios for positive (LR+) and negative (LR-) tests, and posttest probabilities of a diagnosis of vestibulopathy. The 95% confidence interval demonstrates measurement uncertainty.

RESULTS

Test-retest reliability was 0.887 (0.815, 0.932). The MDC was 3.7 seconds. Correlations with other measures ranged from 0.59 (0.34, 0.76) to -0.85 (-0.92, -0.74). The AUC was 0.910 (0.822, 0.998), using a threshold of 4.5 seconds. The sensitivity and specificity were 0.75 (0.51, 0.91) and 0.95 (0.75, 1), respectively. The DOR=57 (6, 541.47), LR+ =15 (2.18, 103.0), and LR- =0.26 (0.12, 0.9). Positive posttest probabilities were 89%-94%.

CONCLUSIONS AND RELEVANCE

The GDT has good reliability, excellent discriminative ability, strong convergent validity, and promising clinical utility.

Screening for Vestibular Disorders Using the Modified Clinical Test of Sensory Interaction and Balance and Tandem Walking With Eyes Closed

OBJECTIVES

Determine accurate cut-points and optimal combinations of screening tests of balance to detect patients with vestibular disorders.

STUDY DESIGN

Case-control study.

SETTING

Out-patient tertiary care.

SUBJECTS AND METHODS

Community-dwelling adults, without known neurological deficits or significant musculoskeletal disorders, including patients with vestibular disorders and healthy controls without vestibular disorders were tested while standing on medium density compliant foam with feet together and eyes closed under three head movement conditions, head stationary, and head moving in yaw and pitch at 0.33 Hz, for up to 30 seconds per trial. Dependent measures were trial duration, number of head movements during head movement trials, trunk kinematic measures, and number of correct tandem steps during tandem walking trials.

RESULTS

Receiver operator characteristics (ROC), sensitivity and specificity, and specific cut-points were calculated. Individual tests had moderate ROC values, from 0.67 to 0.84. ROC values were higher in the head moving trials than the head stationary trial and best for subjects aged 40 to 79. Using combined analyses of two or more tests, including published data on tandem walking, ROC values were higher, 0.80 to 0.90. Age- and sex-related performance differences were found.

CONCLUSION

Balance skills in standing and walking differ, so testing both skills is optimal and increases the likelihood of finding a deficit. Patients should be compared to age-appropriate norms. Kinematics and number of head movements were not very useful. This combined set of rapid, low-tech balance tests is useful in an initial approach to screening patients who may have vestibular disorders.

Short-Term Intensive Therapy and Outcomes for Athletes With Paradoxical Vocal Fold Motion Disorder

Purpose The aim of the study was to develop a treatment for athletes with paradoxical vocal fold motion disorder (PVFMD) based on exercise physiology and learning theory principles and administer it over a preestablished time frame. Method A prospective, repeated-measures, within-subject group design was used. Eleven adolescent/teen athletes diagnosed with PVFMD via laryngoscopy received short-term intensive (STI) therapy. Eight of the athletes returned for extended follow-up. Changes in postexercise inspiratory ( R _i) and expiratory ( R _e) resistances and Modified Borg Dyspnea Scale (MBDS) ratings collected at baseline were compared immediately posttreatment and at extended follow-up. Dyspnea Index scores were collected at baseline and at extended follow-up. Two no-treatment control athletes with PVFMD participated in two exercise challenges-baseline and 6 weeks later. Results Immediately after STI therapy, athletes attained significant improvement in R _i, R _e, and MBDS ratings. These changes were maintained at extended follow-up as well as a significant change in Dyspnea Index scores. The 2 control athletes who were reassessed 6 weeks after baseline experienced negative changes in postexercise R _i and MBDS ratings. Conclusion STI therapy that incorporated individuality, specificity, and variable practice effectively changed outcome measures posttreatment with further improvement observed at extended follow-up. These results provide preliminary evidence for STI therapy for PVFMD.

Transient visual perturbations boost short-term balance learning in virtual reality by modulating electrocortical activity

Immersive virtual reality can expose humans to novel training and sensory environments, but motor training with virtual reality has not been able to improve motor performance as much as motor training in real-world conditions. An advantage of immersive virtual reality that has not been fully leveraged is that it can introduce transient visual perturbations on top of the visual environment being displayed. The goal of this study was to determine whether transient visual perturbations introduced in immersive virtual reality modify electrocortical activity and behavioral outcomes in human subjects practicing a novel balancing task during walking. We studied three groups of healthy young adults (5 male and 5 female for each) while they learned a balance beam walking task for 30 min under different conditions. Two groups trained while wearing a virtual reality headset, and one of those groups also had half-second visual rotation perturbations lasting ~10% of the training time. The third group trained without virtual reality. We recorded high-density electroencephalography (EEG) and movement kinematics. We hypothesized that virtual reality training with perturbations would increase electrocortical activity and improve balance performance compared with virtual reality training without perturbations. Our results confirmed the hypothesis. Brief visual perturbations induced increased theta spectral power and decreased alpha spectral power in parietal and occipital regions and improved balance performance in posttesting. Our findings indicate that transient visual perturbations during immersive virtual reality training can boost short-term motor learning by inducing a cognitive change, minimizing the negative effects of virtual reality on motor training. NEW & NOTEWORTHY We found that transient visual perturbations in virtual reality during balance training can boost short-term motor learning by inducing a cognitive change, overcoming the negative effects of immersive virtual reality. As a result, subjects training in immersive virtual reality with visual perturbations have equivalent performance improvement as training in real-world conditions. Visual perturbations elicited cortical responses in occipital and parietal regions and may have improved the brain's ability to adapt to variations in sensory input.

Motor learning of a dynamic balance task: Influence of lower limb power and prior balance practice

OBJECTIVES

We wanted to verify if the "learning to learn" effect observed in the learning of visuomotor tasks is also present when learning a balance task, i.e., whether the learning rate of a balance task is improved by prior practice of similar balance tasks.

DESIGN

Single centre, parallel group, controlled training study.

METHODS

32 young healthy participants were divided into a control and a training group. The training group's practice consisted of 90 trials of three balance tasks. Forty-eight hours after the training, we recorded performance during the acquisition (90 trials) of a novel balance task in both groups, and 24h thereafter we measured its retention (10 trials).

RESULTS

Mixed models statistical analysis showed that the learning rate of both the acquisition and the retention phase was not influenced by the 90 prior practice trials performed by the training group. However, participants with high lower limb power had a higher balance performance than participants with low power, which can be partly explained by the higher learning rate observed during the acquisition phase for participants with high power.

CONCLUSIONS

Contrary to visuomotor or perceptual tasks, we did not find a "learning to learn" effect for balance tasks. The correlation between learning rate and lower limb power suggests that motor learning of dynamic balance tasks may depend on the physical capability to execute the correct movement. Thus, a prior strength and conditioning program with emphasis on lower limb power should be considered when designing a balance training, especially in fall prevention.

Efficacy of Stochastic Vestibular Stimulation to Improve Locomotor Performance During Adaptation to Visuomotor and Somatosensory Distortion

Astronauts exposed to microgravity face sensorimotor challenges affecting balance control when readapting to Earth's gravity upon return from spaceflight. Small amounts of electrical noise applied to the vestibular system have been shown to improve balance control during standing and walking under discordant sensory conditions in healthy subjects, likely by enhancing information transfer through the phenomenon of stochastic resonance. The purpose of this study was to test the hypothesis that imperceptible levels of stochastic vestibular stimulation (SVS) could improve short-term adaptation to a locomotor task in a novel sensory discordant environment. Healthy subjects (14 males, 10 females, age = 28.7 ± 5.3 years, height = 167.2 ± 9.6 cm, weight = 71.0 ± 12.8 kg) were tested for perceptual thresholds to sinusoidal currents applied across the mastoids. Subjects were then randomly and blindly assigned to an SVS group receiving a 0–30 Hz Gaussian white noise electrical stimulus at 50% of their perceptual threshold (stim) or a control group receiving zero stimulation during Functional Mobility Tests (FMTs), nine trials of which were done under conditions of visual discordance (wearing up/down vision reversing goggles). Time to complete the course (TCC) was used to test the effect of SVS between the two groups across the trials. Adaptation rates from the normalized TCCs were also compared utilizing exponent values of power fit trendline equations. A one-tailed independent-samples t-test indicated these adaptation rates were significantly faster in the stim group (n = 12) than the control (n = 12) group [t_(16.18) = 2.00, p = 0.031]. When a secondary analysis was performed comparing “responders” (subjects who showed faster adaptation rates) of the stim (n = 7) group to the control group (n = 12), independent-samples t-tests revealed significantly faster trial times for the last five trials with goggles in the stim group “responders” than the controls. The data suggests that SVS may be capable of improving short-term adaptation to a locomotion task done under sensory discordance in a group of responsive subjects.

Consolidation of visuomotor adaptation memory with consistent and noisy environments

Our understanding of how we learn and retain motor behaviors is still limited. For instance, there is conflicting evidence as to whether the memory of a learned visuomotor perturbation consolidates; i.e., the motor memory becomes resistant to interference from learning a competing perturbation over time. Here, we sought to determine the factors that influence consolidation during visually guided walking. Subjects learned a novel mapping relationship, created by prism lenses, between the perceived location of two targets and the motor commands necessary to direct the feet to their positions. Subjects relearned this mapping 1 wk later. Different groups experienced protocols with or without a competing mapping (and with and without washout trials), presented either on the same day as initial learning or before relearning on day 2 We tested identical protocols under constant and noisy mapping structures. In the latter, we varied, on a trial-by-trial basis, the strength of prism lenses around a non-zero mean. We found that a novel visuomotor mapping is retained at least 1 wk after initial learning. We also found reduced foot-placement error with relearning in constant and noisy mapping groups, despite learning a competing mapping beforehand, and with the exception of one protocol, with and without washout trials. Exposure to noisy mappings led to similar performance on relearning compared with the equivalent constant mapping groups for most protocols. Overall, our results support the idea of motor memory consolidation during visually guided walking and suggest that constant and noisy practices are effective for motor learning.

NEW & NOTEWORTHY

The adaptation of movement is essential for many daily activities. To interact with targets, this often requires learning the mapping to produce appropriate motor commands based on visual input. Here, we show that a novel visuomotor mapping is retained 1 wk after initial learning in a visually guided walking task. Furthermore, we find that this motor memory consolidates (i.e., becomes more resistant to interference from learning a competing mapping) when learning in constant and noisy mapping environments.

Treadmill exercise within lower-body negative pressure attenuates simulated spaceflight-induced reductions of balance abilities in men but not women

Spaceflight causes sensorimotor adaptations that result in balance deficiencies on return to a gravitational environment. Treadmill exercise within lower-body negative pressure (LBNP) helps protect physiological function during microgravity as simulated by bed rest. Therefore, we hypothesized that treadmill exercise within LBNP would prevent balance losses in both male and female identical twins during 30 days of 6° head-down tilt bed rest. Fifteen (seven female and eight male) identical twin sets participated in this simulation of microgravity. Within each twin pair, one twin was randomly assigned to an exercise group that performed 40 min of supine treadmill exercise within LBNP set to generate 1.0–1.2 body weight, followed by 5 min of static feet-supported LBNP, 6 days per week. Their identical sibling was assigned to a non-exercise control group with all other bed rest conditions equivalent. Before and immediately after bed rest, subjects completed standing and walking rail balance tests with eyes open and eyes closed. In control subjects, standing rail balance times (men: −42%, women: −40%), rail walk distances (men: −44%, women: −32%) and rail walk times (men: −34%, women: −31%) significantly decreased after bed rest. Compared with controls, treadmill exercise within LBNP significantly attenuated losses of standing rail balance time by 63% in men, but the 41% attenuation in women was not significant. Treadmill exercise within LBNP did not affect rail walk abilities in men or women. Treadmill exercise within LBNP during simulated spaceflight attenuates loss of balance control in men but not in women.

A Career in Inquiry

This article, based on the 52nd Eleanor Clarke Slagle lecture given at the 95th American Occupational Therapy Association Annual Conference & Expo, explores the concept of inquiry as the basis for a career and as an activity of daily living. Using the heliocentric theory and the space program at NASA as examples, the broad concept of inquiry is discussed, because it has led to important changes in society over the course of history. The article describes how a career as a clinician-scientist can be grounded in the concept of inquiry and explains how all occupational therapists and occupational therapy assistants can base their own careers in inquiry, using examples from the early history of the profession of occupational therapy and from work by current investigators. Practical suggestions applicable to every clinician are provided.

Enhancing astronaut performance using sensorimotor adaptability training

Astronauts experience disturbances in balance and gait function when they return to Earth. The highly plastic human brain enables individuals to modify their behavior to match the prevailing environment. Subjects participating in specially designed variable sensory challenge training programs can enhance their ability to rapidly adapt to novel sensory situations. This is useful in our application because we aim to train astronauts to rapidly formulate effective strategies to cope with the balance and locomotor challenges associated with new gravitational environments—enhancing their ability to “learn to learn.” We do this by coupling various combinations of sensorimotor challenges with treadmill walking. A unique training system has been developed that is comprised of a treadmill mounted on a motion base to produce movement of the support surface during walking. This system provides challenges to gait stability. Additional sensory variation and challenge are imposed with a virtual visual scene that presents subjects with various combinations of discordant visual information during treadmill walking. This experience allows them to practice resolving challenging and conflicting novel sensory information to improve their ability to adapt rapidly. Information obtained from this work will inform the design of the next generation of sensorimotor countermeasures for astronauts.

Trends in sensorimotor research and countermeasures for exploration-class space flights

Research in the area of sensorimotor and neurovestibular function has played an important role in enabling human space flight. This role, however, is changing. One of the key aspects of sensorimotor function relevant to this role will build on its widespread connections with other physiological and psychological systems in the body. The firm knowledge base in this area can provide a strong platform to explore these interactions, which can also provide for the development of effective and efficient countermeasures to the deleterious effects of space flight.

Human manual control performance in hyper-gravity

Hyper-gravity provides a unique environment to study how misperceptions impact control of orientation relative to gravity. Previous studies have found that static and dynamic roll tilts are perceptually overestimated in hyper-gravity. The current investigation quantifies how this influences control of orientation. We utilized a long-radius centrifuge to study manual control performance in hyper-gravity. In the dark, subjects were tasked with nulling out a pseudo-random roll disturbance on the cab of the centrifuge using a rotational hand controller to command their roll rate in order to remain perceptually upright. The task was performed in 1, 1.5, and 2 G's of net gravito-inertial acceleration. Initial performance, in terms of root-mean-square deviation from upright, degraded in hyper-gravity relative to 1 G performance levels. In 1.5 G, initial performance degraded by 26 % and in 2 G, by 45 %. With practice, however, performance in hyper-gravity improved to near the 1 G performance level over several minutes. Finally, pre-exposure to one hyper-gravity level reduced initial performance decrements in a different, novel, hyper-gravity level. Perceptual overestimation of roll tilts in hyper-gravity leads to manual control performance errors, which are reduced both with practice and with pre-exposure to alternate hyper-gravity stimuli.

Sensorimotor Adaptation Training’s Effect on Head Stabilization in Response to a Lateral Perturbation in Older Adults

The goal of this study was to determine if exposure to sensorimotor adaptation training improved head stabilization in older adults. Sixteen participants, age 66–81 yr, were assigned at random to the control group (n= 8) or the experimental group (n= 8). Both groups first completed 6 trials of walking a foam pathway consisting of a moveable platform that induced a lateral perturbation during walking. Head-in-space and trunk-in-space angular velocities were collected. Participants from both groups then trained twice per week for 4 wk. Both groups walked on a treadmill for 20 min. The control group viewed a static scene. The experimental group viewed a rotating visual scene that provided a perceptual-motor mismatch. After training, both groups were retested on the perturbation pathway test. The experimental group used a movement strategy that preserved head stabilization compared with the controls (p< .05). This training effect was not retained after 4 wk.

Posturography and locomotor tests of dynamic balance after long-duration spaceflight

The currently approved objective clinical measure of standing balance in astronauts after space flight is the Sensory Organization Test battery of computerized dynamic posturography. No tests of walking balance are currently approved for standard clinical testing of astronauts. This study determined the sensitivity and specificity of standing and walking balance tests for astronauts before and after long-duration space flight. Astronauts were tested on an obstacle avoidance test known as the Functional Mobility Test (FMT) and on the Sensory Organization Test using sway-referenced support surface motion with eyes closed (SOT 5) before and six months after (n=15) space flight on the International Space Station. They were tested two to seven days after landing. Scores on SOT tests decreased and scores on FMT increased significantly from pre- to post-flight. In other words, post-flight scores were worse than pre-flight scores. SOT and FMT scores were not significantly related. ROC analyses indicated supra-clinical cut-points for SOT 5 and for FMT. The standard clinical cut-point for SOT 5 had low sensitivity to post-flight astronauts. Higher cut-points increased sensitivity to post-flight astronauts but decreased specificity to pre-flight astronauts. Using an FMT cut-point that was moderately highly sensitive and highly specific plus SOT 5 at the standard clinical cut-point was no more sensitive than SOT 5, alone. FMT plus SOT 5 at higher cut-points was more specific and more sensitive. The total correctly classified was highest for FMT, alone, and for FMT plus SOT 5 at the highest cut-point. These findings indicate that standard clinical comparisons are not useful for identifying problems. Testing both standing and walking balance will be more likely to identify balance deficits.

Tests of walking balance for screening vestibular disorders

Few reliable tests are available for screening people rapidly for vestibular disorders although such tests would be useful for a variety of testing situations. Balance testing is widely performed but of unknown value for screening. The goal of this study was to determine the value of tests of walking balance for screening people with vestibular impairments. We tested three groups of patients with known vestibular impairments: benign paroxysmal positional vertigo, unilateral vestibular weakness, and post-acoustic neuroma resection. We compared them to normal subjects. All subjects were independently ambulatory without gait aids. Subjects were tested on tandem walking (TW) with eyes open and eyes closed for 10 steps, walking with no additional head motions and with augmented head rotations in yaw for 7 m (WwHT), and an obstacle avoidance task, the Functional Mobility Test (FMT). Subjects wore a 3-D motion sensor centered at mid-torso to capture kinematic measures. Patients and normals differed significantly on some behavioral measures, such as the number of steps to perform TW, and on some but not all kinematic measures. ROC analyses, however, were at best only moderate, and failed to find strong differences and cut-points that would differentiate the groups. These findings suggest that although patients and normals differ in performance of these tests in some interesting ways the groups are not sufficiently different on these tests for easy use as screening tests to differentiate the populations.

Structural learning in feedforward and feedback control

For smooth and efficient motor control, the brain needs to make fast corrections during the movement to resist possible perturbations. It also needs to adapt subsequent movements to improve future performance. It is important that both feedback corrections and feedforward adaptation need to be made based on noisy and often ambiguous sensory data. Therefore, the initial response of the motor system, both for online corrections and adaptive responses, is guided by prior assumptions about the likely structure of perturbations. In the context of correcting and adapting movements perturbed by a force field, we asked whether these priors are hard wired or whether they can be modified through repeated exposure to differently shaped force fields. We found that both feedback corrections to unexpected perturbations and feedforward adaptation to a new force field changed, such that they were appropriate to counteract the type of force field that participants had experienced previously. We then investigated whether these changes were driven by a common mechanism or by two separate mechanisms. Participants experienced force fields that were either temporally consistent, causing sustained adaptation, or temporally inconsistent, causing little overall adaptation. We found that the consistent force fields modified both feedback and feedforward responses. In contrast, the inconsistent force field modified the temporal shape of feedback corrections but not of the feedforward adaptive response. These results indicate that responses to force perturbations can be modified in a structural manner and that these modifications are at least partly dissociable for feedback and feedforward control.

Gait adaptability training is affected by visual dependency

As part of a larger gait adaptability training study, we designed a program that presented combinations of visual flow and support-surface manipulations to investigate the response of healthy adults to walking on a treadmill in novel discordant sensorimotor conditions. A visual dependence score was determined for each subject, and this score was used to explore how visual dependency was linked to locomotor performance (1) during three training sessions and (2) in a new discordant environment presented at the conclusion of training. Performance measures included reaction time (RT), stride frequency (SF), and heart rate (HR), which respectively served as indicators of cognitive load, postural stability, and anxiety. We hypothesized that training would affect performance measures differently for highly visually dependent individuals than for their less visually dependent counterparts. A seemingly unrelated estimation analysis of RT, SF, and HR revealed a significant omnibus interaction of visual dependency by session (p < 0.001), suggesting that the magnitude of differences in these measures across training day 1 (TD1), training day 3 (TD3), and exposure to a novel test is dependent on subjects' levels of visual dependency. The RT result, in particular, suggested that highly visually dependent subjects successfully trained to one set of sensory discordant conditions but were unable to apply their adapted skills when introduced to a new sensory discordant environment. This finding augments rationale for developing customized gait training programs that are tailored to an individual. It highlights one factor--personal level of visual dependency--to consider when designing training conditions for a subject or patient. Finally, the link between visual dependency and locomotor performance may offer predictive insight regarding which subjects in a normal population will require more training when preparing for specific novel locomotor conditions.

Facilitation of learning induced by both random and gradual visuomotor task variation

Motor task variation has been shown to be a key ingredient in skill transfer, retention, and structural learning. However, many studies only compare training of randomly varying tasks to either blocked or null training, and it is not clear how experiencing different nonrandom temporal orderings of tasks might affect the learning process. Here we study learning in human subjects who experience the same set of visuomotor rotations, evenly spaced between −60° and +60°, either in a random order or in an order in which the rotation angle changed gradually. We compared subsequent learning of three test blocks of +30°→−30°→+30° rotations. The groups that underwent either random or gradual training showed significant (P < 0.01) facilitation of learning in the test blocks compared with a control group who had not experienced any visuomotor rotations before. We also found that movement initiation times in the random group during the test blocks were significantly (P < 0.05) lower than for the gradual or the control group. When we fit a state-space model with fast and slow learning processes to our data, we found that the differences in performance in the test block were consistent with the gradual or random task variation changing the learning and retention rates of only the fast learning process. Such adaptation of learning rates may be a key feature of ongoing meta-learning processes. Our results therefore suggest that both gradual and random task variation can induce meta-learning and that random learning has an advantage in terms of shorter initiation times, suggesting less reliance on cognitive processes.

Improving balance function using vestibular stochastic resonance: optimizing stimulus characteristics

Stochastic resonance (SR) is a phenomenon whereby the response of a non-linear system to a weak periodic input signal is optimized by the presence of a particular non-zero level of noise. Stochastic resonance using imperceptible stochastic vestibular electrical stimulation, when applied to normal young and elderly subjects, has been shown to significantly improve ocular stabilization reflexes in response to whole-body tilt; improved balance performance during postural disturbances and optimize covariance between the weak input periodic signals introduced via venous blood pressure receptors and the heart rate responses. In our study, 15 subjects stood on a compliant surface with their eyes closed. They were given low-amplitude binaural bipolar stochastic electrical stimulation of the vestibular organs in two frequency ranges of 1-2 and 0-30 Hz over the amplitude range of 0 to ±700 μA. Subjects were instructed to maintain an upright stance during 43-s trials, which consisted of baseline (zero amplitude) and stimulation (non-zero amplitude) periods. Measures of stability of the head and trunk using inertial motion unit sensors attached to these segments and the whole body using a force plate were measured and quantified in the mediolateral plane. Using a multivariate optimization criterion, our results show that the low levels of vestibular stimulation given to the vestibular organs improved balance performance in normal healthy subjects in the range of 5-26% consistent with the stochastic resonance phenomenon. In our study, 8 of 15 and 10 of 15 subjects were responsive for the 1-2- and 0-30-Hz stimulus signals, respectively. The improvement in balance performance did not differ significantly between the stimulations in the two frequency ranges. The amplitude of optimal stimulus for improving balance performance was predominantly in the range of ±100 to ±400 μA. A device based on SR stimulation of the vestibular system might be useful as either a training modality to enhance adaptability or skill acquisition, or as a miniature patch-type stimulator that may be worn by people with disabilities due to aging or disease to improve posture and locomotion function.

Gait training improves performance in healthy adults exposed to novel sensory discordant conditions

Recent evidence shows that the ability to adapt to a novel discordant sensorimotor environment can be increased through prior training. We aimed to determine whether gait adaptability could be increased and then retained using a training system comprised of a treadmill placed on a motion base facing a virtual visual scene that provided a variety of sensory challenges that served as training modalities. Ten healthy adults participated in three training sessions during which they walked on a treadmill at 1.1 m/s while receiving discordant support-surface and visual manipulations. Upon completion, participants were presented with a novel sensorimotor challenge designed to test for transfer of adaptive skills. During this test, stride frequency, reaction time, and heart rate data were collected as measures of postural stability, cognitive load, and anxiety, respectively. Compared to 10 untrained controls, trained participants showed enhanced overall performance on the Novel Test, which was administered 20 min after their final training session. Subjects in both groups had greater stride frequency, reaction time, and heart rate when exposed to the new sensory environment; however, these increases were less pronounced in the trained subjects than in the controls. The Novel Test was re-administered to both groups 1 week, 1 month, 3 months, and 6 months later. Trained subjects maintained their level of performance for 6 months. Untrained subjects continued to improve in these measures at each subsequent test session, suggesting that a lasting sensorimotor adaptability training effect can be achieved with very short, repeated exposures to discordant sensory conditions.

Laboratory testing of the vestibular system

PURPOSE OF REVIEW

Recent reports on vestibular testing, relevant to clinical diagnosis, are reviewed.Besides the case history and bedside examination, objective measurement of the vestibuloocular reflex in all of its facets remains the cornerstone in the diagnostic process.

RECENT FINDINGS

In recent years, this has been enhanced considerably by reliable unilateral tests for the otolith organs, most notably by vestibular-evoked myogenic potential recording and estimation of subjective visual vertical. In addition, progress has been made in the investigation of multisensory interaction, involving visual acuity and posturography.Technological developments include improved eye movement measurement techniques, electrotactile and vibrotactile sensory enhancement or substitution, the use of virtual reality devices and motion stimulators such as hexapods and the rediscovery of galvanic vestibular stimulation as a research and diagnostic tool.

SUMMARY

The recent introduction of new tests, together with the development of novel technologies, is gradually increasing the scope of the physical and bedside examination of the dizzy patient (see chapter 'Medical management of peripheral disorders' in this issue). The use of more complex equipment, such as rotating chairs, linear sleds, hexapods and posturography platforms, is likely to become limited to specialized laboratories and rehabilitation centers in future years. Further, high resolution magnetic resonance tomography (MRT) and computed tomography have allowed insight into the morphology and determination of malformations of the human labyrinth.

Structure learning in action

'Learning to learn' phenomena have been widely investigated in cognition, perception and more recently also in action. During concept learning tasks, for example, it has been suggested that characteristic features are abstracted from a set of examples with the consequence that learning of similar tasks is facilitated-a process termed 'learning to learn'. From a computational point of view such an extraction of invariants can be regarded as learning of an underlying structure. Here we review the evidence for structure learning as a 'learning to learn' mechanism, especially in sensorimotor control where the motor system has to adapt to variable environments. We review studies demonstrating that common features of variable environments are extracted during sensorimotor learning and exploited for efficient adaptation in novel tasks. We conclude that structure learning plays a fundamental role in skill learning and may underlie the unsurpassed flexibility and adaptability of the motor system.

Improvement of obstacle avoidance on a compliant surface during transfer to a novel visual task after variable practice under unusual visual conditions

Previous work has shown that variable practice facilitates adaptation to novel visuomotor changes during throwing tasks and obstacle avoidance on a solid floor. To assess whether locomotor skill on an obstacle-avoidance task performed on a compliant surface and in a novel visuomotor environment improved after training with variable practice, 61 normal adults practiced traversing the obstacle course. Half the trials were performed with no visual changes and half with either sham lenses or visually distorting lenses; the latter were either single or multiple lenses. On transfer tests on the obstacle course while wearing novel lenses, scores were significantly better with multiple lenses than sham; the single-lens group did not differ from sham or multiple-lens groups. Thus, performance in a novel visual environment on a compliant surface improved most with variable practice.

Usefulness of some current balance tests for identifying individuals with disequilibrium due to vestibular impairments

The goal of this study was to determine which of several clinical balance tests best identifies patients with vestibular disorders. We compared the scores of normals and patients on the Berg Balance Scale (Berg), Dynamic Gait Index (DGI), Timed Up and Go (TUG), Computerized Dynamic Posturography Sensory Organization Test (SOT), and a new obstacle avoidance test: the Functional Mobility Test (FMT). The study was performed in an out-patient balance laboratory at a tertiary care center. Subjects were 40 normal adults, and 40 adults with vestibular impairments. The main outcome measures were the sensitivity of tests to patients and specificity to normals. When adjusted for age the Berg, TUG, DGI and FMT had moderate sensitivity and specificity. SOT had moderately high sensitivity and specificity. SOT and FMT, combined, had high sensitivity and moderate specificity. Therefore, the kinds of tests of standing and walking balance that clinicians may use to screen patients for falling are not as good for screening for vestibular disorders as SOT. SOT combined with FMT is better. When screening patients for vestibular disorders, when objective diagnostic tests of the vestibular system, itself, are unavailable, tests of both standing and walking balance, together, give the most information about community-dwelling patients. These tests may also indicate the presence of sub-clinical balance problems in community-dwelling, asymptomatic adults.