Vibrotactile feedback as a countermeasure for spatial disorientation.

Spaceflight can make astronauts susceptible to spatial disorientation which is one of the leading causes of fatal aircraft accidents. In our experiment, blindfolded participants used a joystick to balance themselves while inside a multi-axis rotation device (MARS) in either the vertical or horizontal roll plane. On Day 1, in the vertical roll plane (Earth analog condition) participants could use gravitational cues and therefore had a good sense of their orientation. On Day 2, in the horizontal roll plane (spaceflight analog condition) participants could not use gravitational cues and rapidly became disoriented and showed minimal learning and poor performance. One potential countermeasure for spatial disorientation is vibrotactile feedback that conveys body orientation provided by small vibrating devices applied to the skin. Orientation-dependent vibrotactile feedback provided to one group enhanced performance in the spaceflight condition but the participants reported a conflict between the accurate vibrotactile cues and their erroneous perception of their orientation. Specialized vibrotactile training on Day 1 provided to another group resulted in significantly better learning and performance in the spaceflight analog task with vibrotactile cueing. In this training, participants in the Earth analog condition on Day 1 were required to disengage from the task of aligning with the gravitational vertical encoded by natural vestibular/somatosensory afference and had to align with randomized non-vertical directions of balance signaled by vibrotactile feedback. At the end of Day 2, we deactivated the vibrotactile feedback after both vibration-cued groups had practiced with it in the spaceflight analog condition. They performed as well as the group who did not have any vibrotactile feedback. We conclude that after appropriate training, vibrotactile orientation feedback augments dynamic spatial orientation and does not lead to any negative dependence.

Crash Prediction Using Deep Learning in a Disorienting Spaceflight Analog Balancing Task.

Were astronauts forced to land on the surface of Mars using manual control of their vehicle, they would not have familiar gravitational cues because Mars' gravity is only 0.38 g. They could become susceptible to spatial disorientation, potentially causing mission ending crashes. In our earlier studies, we secured blindfolded participants into a Multi-Axis Rotation System (MARS) device that was programmed to behave like an inverted pendulum. Participants used a joystick to stabilize around the balance point. We created a spaceflight analog condition by having participants dynamically balance in the horizontal roll plane, where they did not tilt relative to the gravitational vertical and therefore could not use gravitational cues to determine their position. We found 90% of participants in our spaceflight analog condition reported spatial disorientation and all of them showed it in their data. There was a high rate of crashing into boundaries that were set at ± 60° from the balance point. Our goal was to see whether we could use deep learning to predict the occurrence of crashes before they happened. We used stacked gated recurrent units (GRU) to predict crash events 800 ms in advance with an AUC (area under the curve) value of 99%. When we prioritized reducing false negatives we found it resulted in more false positives. We found that false negatives occurred when participants made destabilizing joystick deflections that rapidly moved the MARS away from the balance point. These unpredictable destabilizing joystick deflections, which occurred in the duration of time after the input data, are likely a result of spatial disorientation. If our model could work in real time, we calculated that immediate human action would result in the prevention of 80.7% of crashes, however, if we accounted for human reaction times (∼400 ms), only 30.3% of crashes could be prevented, suggesting that one solution could be an AI taking temporary control of the spacecraft during these moments.

The role of spatial acuity in a dynamic balancing task without gravitational cues.

In earlier studies, blindfolded participants used a joystick to orient themselves to the direction of balance in the horizontal roll plane while in a device programmed to behave like an inverted pendulum. In this spaceflight analog situation, position relevant gravitational cues are absent. Most participants show minimal learning, positional drifting, and failure of path integration. However, individual differences are substantial, some participants show learning and others become progressively worse. In Experiment 1, our goal was to determine whether spatial acuity could explain these individual differences in active balancing. We exposed blindfolded participants to passive movement profiles, with different frequency components, in the vertical and horizontal roll planes. They pressed a joystick trigger to indicate every time they passed the start point. We found greater spatial acuity for higher frequencies but no relation between passive spatial accuracy and active balance control in the horizontal roll plane, suggesting that spatial acuity in the horizontal roll plane does not predict performance in a disorienting spaceflight condition. In Experiment 2, we found significant correlations between passive spatial acuity in the vertical roll plane, where participants have task relevant gravitational cues, and early active balancing in the horizontal roll plane. These correlations appeared after participants underwent brief provocative vestibular stimulation by making a pitch head movement during vertical yaw rotation. Our findings suggest that vestibular stimulation may be a valuable part of assessments of individual differences in performance during initial exposure to disorienting spaceflight conditions where there are no reliable gravity dependent positional cues.

Multiple roles of active stiffness in upright balance and multidirectional sway.

Both passive and active mechanisms are necessary to explain small amplitude forward-backward (FB) voluntary swaying. Parallel and symmetric leg inverted pendulum models with stiffness control are a simple way to replicate FB swaying during quiet stance. However, it has been more difficult to model lateral left-right (LR) voluntary swaying involving the dual mechanisms of hip loading-unloading and ankle pressure distribution. To assess these factors, we had subjects perform small amplitude FB and LR sways and circular rotation. We experimentally identified three parameters that characterized their two-dimensional stiffnesses: AP stiffness (KSAP), and lateral stiffness (KSML), at the ankles and a parameter we refer to as the engagement-disengagement rate (KED) of the legs. We performed simulations with our engaged leg model (Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2042-2060, 2019; Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2028-2041, 2019) to test its predictions about the limits of balance stability during sway in the three test conditions. Comparing the model's predictions with the experimental data, we found that KSAP has a task-dependent dual role in upright balance and is crucial to prevent falling; KSML helps overcome viscous drags but is not instrumental to stability; KED has a key role in stability and is dependent on the biomechanical geometry of the body, which is invariant across balance tasks. These findings provide new insights into balance control that have important clinical implications for falling, especially for patients who are unable to use a hip strategy during balance control.NEW & NOTEWORTHY Our previously published Engaged Leg Model here shows how stiffness plays complex multicausal roles in balance. In one role, it is crucial to stability, with task contingent influences over balance. In another, it overcomes viscous drag. Task-dependent stiffness alone does not explain stable balance; geometrical, invariant aspects of body biomechanics also matter. Our model is fully applicable to clinical balance pathologies involving asymmetries in movement and balance control.

The effect of hypergravity on upright balance and voluntary sway.

We compared voluntary oscillatory sway for eight subjects tested in 1.8-g and 1-g gravito-inertial force (GIF) levels of parabolic flight. Subjects performed voluntary forward-backward (FB) and lateral left-right (LR) swaying as the forces and moments under the soles of each foot were measured. We calculated the experimental values of three parameters: two ankle stiffness parameters KSAP and KSML acting in orthogonal FB and LR directions and one parameter KED related to leg pivot shifting. Simulations of the engaged leg model (Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2042-2060, 2019; Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2028-2041, 2019) correctly predicted the experimentally determined stability bounds of upright balance and also the scaling of the postural parameters as a function of GIF magnitude. The effective stiffness, KSAP, at the ankles played the primary role to prevent falling in FB swaying and both model predictions, and experimental data showed KSAP to scale up in proportion to GIF magnitude. For LR swaying, the model predicted a 3:4 scaling of anterior-posterior stiffness to change in GIF magnitude, which was borne out by the experimental data. Simulations predict stability (nonfalling) not to depend on lateral stiffness, KSML, which was experimentally found not to depend on the GIF magnitude. Both model and experiment showed that the geometry-dependent pivot shift parameter KED was invariant to a change in GIF magnitude. Thus the ELM explains voluntary sway and balance in altered GIF magnitude conditions, rotating environments with Coriolis perturbations of sway, as well as normal terrestrial conditions.NEW & NOTEWORTHY A nonparallel leg model of balance, the engaged leg model (ELM), was previously developed to characterize adaptive balance control in a rotating environment. Here we show the ELM also explains sway in hypergravity. It predicts the changes in balance control parameters with changes in gravity. ELM is currently the only balance model applicable to artificial and hypergravity conditions. ELM can also be applied to terrestrial clinical situations for pathologies that generate postural asymmetries.

Characterizing Individual Differences in a Dynamic Stabilization Task Using Machine Learning.

INTRODUCTION: Being able to identify individual differences in skilled motor learning during disorienting conditions is important for spaceflight, military aviation, and rehabilitation.METHODS: Blindfolded subjects (N = 34) were strapped into a device that behaved like an inverted pendulum in the horizontal roll plane and were instructed to use a joystick to stabilize themselves across two experimental sessions on consecutive days. Subjects could not use gravitational cues to determine their angular position and many soon became spatially disoriented.RESULTS: Most demonstrated minimal learning, poor performance, and a characteristic pattern of positional drifting during horizontal roll plane balancing. To understand the wide range of individual differences observed, we used a Bayesian Gaussian Mixture method to cluster subjects into three statistically distinct groups that represent Proficient, Somewhat Proficient, and Not Proficient performance. We found that subjects in the Not Proficient group exhibited a suboptimal strategy of using very stereotyped large magnitude joystick deflections. We also used a Gaussian Naive Bayes method to create predictive classifiers. As early as the second block of experimentation (out of ten), we could predict a subject's final group with 80% accuracy.DISCUSSION: Our findings indicate that machine learning can help predict individual performance and learning in a disorienting dynamic stabilization task and identify suboptimal strategies in Not Proficient subjects, which could lead to personalized and more effective training programs.Vimal VP, Zheng H, Hong P, Fakharzadeh LN, Lackner JR, DiZio P. Characterizing individual differences in a dynamic stabilization task using machine learning. Aerosp Med Hum Perform. 2020; 91(6):479-488.

Velocity storage: its multiple roles.

Our research described in this article was motivated by the puzzling finding of the Skylab M131 experiments: head movements made while rotating that are nauseogenic and disorienting on Earth are innocuous in a weightless, 0-g environment. We describe a series of parabolic flight experiments that directly addressed this puzzle and discovered the gravity-dependent responses to semicircular canal stimulation, consistent with the principles of velocity storage. We describe a line of research that started in a different direction, investigating dynamic balancing, but ended up pointing to the gravity dependence of angular velocity-to-position integration of semicircular canal signals. Together, these lines of research and the theoretical framework of velocity storage provide an answer to at least part of the M131 puzzle. We also describe recently discovered neural circuits by which active, dynamic vestibular, multisensory, and motor signals are interpreted as either appropriate for action and orientation or as conflicts evoking motion sickness and disorientation.

Learning and long-term retention of dynamic self-stabilization skills.

In earlier studies, we had subjects use a joystick to balance themselves when seated in a device programmed to behave like an inverted pendulum. Subjects tested in a vertically oriented roll plane showed rapid learning for dynamically stabilizing themselves about the direction of balance when it corresponded with the direction of gravity. Subjects tested in a horizontally oriented roll plane, unlike the vertical roll plane subjects, did not have gravitational cues to determine their angular positions and showed minimal learning and persistent cyclical drifting. We describe here a training program to enhance learning and performance of dynamic stabilization in the horizontal roll plane based on our previous finding that balance control involves two dissociable components: alignment using gravity-dependent positional cues and alignment using dynamic cues. We hypothesized that teaching subjects to balance in a vertical roll plane to directions of balance that did not correspond with the direction of gravity would enhance the ability to stabilize at the direction of balance in the horizontal roll plane where gravity-dependent cues are absent. All subjects trained in vertical roll later showed greatly improved performance in horizontal plane balance. Control subjects exposed only to horizontal roll plane balancing showed minimal improvements. When retested 4 months later, the training subjects showed further performance improvements during the course of the retest trials whereas the control group showed no further improvement. Our findings indicate that balance control can be enhanced in situations lacking gravitationally dependent position cues as in weightlessness, when initial training occurs with such cues present.

Rapid adaptation to Coriolis force perturbations of voluntary body sway.

Studying adaptation to Coriolis perturbations of arm movements has advanced our understanding of motor control and learning. We have now applied this paradigm to two-dimensional postural sway. We measured how subjects (n = 8) standing at the center of a fully enclosed rotating room who made voluntary anterior-posterior swaying movements adapted to the Coriolis perturbations generated by their sway. Subjects underwent four voluntary sway trials prerotation, 20 per-rotation at 10 rpm counterclockwise, and 10 postrotation. Each trial lasted 20 s, and subjects were permitted normal vision. Their voluntary sway during rotation generated Coriolis forces that initially induced rightward deviations of their forward sway paths and leftward deviations of their backward sway. Sagittal plane sway was gradually restored over per-rotation trials, and a mirror image aftereffect occurred in postrotation trials. Dual force plate data analysis showed that subjects learned to counter the Coriolis accelerations during rotation by executing a bimodal torque pattern that was asymmetric across legs and contingent on forward vs. backward movement. The experience-dependent acquisition and washout of this compensation indicate that an internal, feedforward model underlies the leg-asymmetric bimodal torque compensation, contingent on forward vs. backward movement. The learned torque asymmetry we observed for forward vs. backward sway is not consistent with parallel two-leg models of postural control. NEW & NOTEWORTHY This paper describes adaptation to Coriolis force perturbations of voluntary sway in a rotating environment. During counterclockwise rotation, sway paths are deviated clockwise, but full restoration of fore-aft sway is regained in minutes. Negative aftereffects are briefly present postrotation. Current parallel leg models of postural control cannot account for these findings, which show that postural control, like arm movement control, can adapt rapidly and completely to the Coriolis forces generated in artificial gravity environments.

Adaptation to Coriolis force perturbations of postural sway requires an asymmetric two-leg model.

In the companion paper (Bakshi A, DiZio P, Lackner JR. J Neurophysiol. In press, 2019), we reported how voluntary forward-backward sway in a rotating room generated medial-lateral Coriolis forces that initially deviated intended body sway paths. Pure fore-aft sway was gradually restored over per-rotation trials, and a negative aftereffect occurred during postrotation sway. Force plate recordings showed that subjects learned to compensate for the Coriolis forces by executing a bimodal torque, the distribution of which was asymmetric across the two legs and of opposite sign for forward vs. backward sway. To explain these results, we have developed an asymmetric, nonparallel-leg, inverted pendulum model to characterize upright balance control in two dimensions. Fore-aft and medial-lateral sway amplitudes can be biomechanically coupled or independent. Biomechanical coupling occurs when Coriolis forces orthogonal to the direction of movement perturb sway about the ankles. The model includes a mechanism for alternating engagement/disengagement of each leg and for asymmetric drive to the ankles to achieve adaptation to Coriolis force-induced two-dimensional sway. The model predicts the adaptive control underlying the adaptation of voluntary postural sway to Coriolis forces. A stability analysis of the model generates parameter values that match those measured experimentally, and the parameterized model simulations reproduce the experimentally observed sway trajectories. NEW & NOTEWORTHY This paper presents a novel nonparallel leg model of postural control that correctly predicts the perturbations of voluntary sway that occur in a rotating environment and the adaptive changes that occur to restore faithful movement trajectories. This engaged leg model (ELM) predicts the asymmetries in force distribution and their patterns between the two legs to restore accurate movement trajectories. ELM has clinical relevance for pathologies that generate postural asymmetries and for altered gravitoinertial force conditions.

An Active Suspension System for Mitigating Motion Sickness and Enabling Reading in a Car.

INTRODUCTION: The advent of autonomous automobiles raises new challenges for maintaining passenger safety and comfort. The challenge addressed here is how to predict and mitigate motion sickness when passengers read in a moving vehicle. METHODS: We utilized a car equipped with a commercial active suspension system developed for attenuating the transmission of road surface fluctuations to passengers. The system was used to reproduce, in a parked car, either the vibrations that would be experienced in a moving car equipped with a conventional suspension system (unmitigated ride) or the attenuated vibrations that would occur on the road with the active cancellation system engaged (mitigated ride). We evaluated the consequences of these two simulated ride conditions for reading performance, comfort, and evocation of motion sickness. RESULTS: Both ride conditions reduced the 0 to 0.8 Hz vibrations to below threshold for evoking motion sickness during passive exposure [corrected]. Only the mitigated ride condition attenuated frequencies in the 0.8 to 8 Hz band where visual suppression of the vestibulo-ocular reflex is known to break down, and this condition also reduced the motion sickness induced by reading and increased reading comprehension and comfort relative to the unmitigated ride. DISCUSSION: The palliative effects of 0.8 to 8 Hz attenuation are discussed in terms of the different mechanisms underlying motion sickness evoked by reading in a vehicle versus mere exposure to vehicle motion without reading. Implications for ISO-2631 standards for human exposure to vibration are also discussed.DiZio P, Ekchian J, Kaplan J, Ventura J, Graves W, Giovanardi M, Anderson Z, Lackner JR. An active suspension system for mitigating motion sickness and enabling reading in a car. Aerosp Med Hum Perform. 2018; 89(9):822-829.

Learning dynamic control of body yaw orientation.

To investigate the role of gravitational cues in the learning of a dynamic balancing task, we placed blindfolded subjects in a device programmed with inverted pendulum dynamics about the yaw axis. Subjects used a joystick to try and maintain a stable orientation at the direction of balance during 20 100 s-long trials. They pressed a trigger button on the joystick to indicate whenever they felt at the direction of balance. Three groups of ten subjects each participated. One group balanced with their body and the yaw axis vertical, and thus did not have gravitational cues to help them to determine their angular position. They showed minimal learning, inaccurate indications of the direction of balance, and a characteristic pattern of positional drifting away from the balance point. A second group balanced with the yaw axis pitched 45° from the gravitational vertical and had gravity relevant position cues. The third group balanced with their yaw axis horizontal where they had gravity-dependent cues about body position in yaw. Groups 2 and 3 showed better initial balancing performance and more learning across trials than Group 1. These results indicate that in the absence of vision, the integration of transient semicircular canal and somatosensory signals about angular acceleration is insufficient for determining angular position during dynamic balancing; direct position-dependent gravity cues are necessary.

Gravitational and Somatosensory Influences on Control and Perception of Roll Balance.

INTRODUCTION: Blindfolded subjects used a joystick to orient themselves to the direction of balance in a device programmed to exhibit inverted pendulum behavior in the roll plane; they indicated with a trigger press when they were at that location. Our goal was to determine how otolith and somatosensory information about the gravitational vertical influenced the ability to locate the direction of balance. METHODS: The subjects (N = 12) were tested in each of three orientations of the body roll plane: vertical (Upright), 45° back (45_Degree), and 90° back (Supine), which provided progressively less salient otolith and somatosensory information about roll orientation with regard to the direction of gravity. For each pitch plane, subjects were tested with three directions of balance: 0° (aligned with the gravitational vertical in the Upright condition) and 30° right or left. RESULTS: The mean achieved and indicated orientations for the Upright and 45_Degree conditions were significantly displaced away from the direction of balance in the direction of gravity, with indicated angles less displaced. In the Supine condition, the mean achieved and indicated angles were closer to the direction of balance, but their within-trial standard deviations were significantly larger than in the Upright and 45_Degree conditions, which did not differ. This greater variability resulted from the frequent side to side "drifting" behavior that was a characteristic feature of the Supine condition only. DISCUSSION: These findings indicate that in the absence of vision accurate dynamic orientation requires gravity dependent shear forces on the otolith organs and body surface.Panic AS, Panic H, DiZio P, Lackner JR. Gravitational and somatosensory influences on control and perception of roll balance. Aerosp Med Hum Perform. 2017; 88(11):993-999.

Learning dynamic balancing in the roll plane with and without gravitational cues.

We determined the relative contributions of gravity-dependent positional cues and motion cues to the learning of roll balance control. We hypothesized that gravity-dependent otolith and somatosensory shear forces related to body orientation would yield better initial performance, more rapid learning, and better retention. Blindfolded subjects rode in a device programmed to roll with inverted pendulum dynamics in a vertical (UPRIGHT) or horizontal plane (SUPINE), and used a joystick to align themselves with the direction of balance. Each subject completed five blocks of four 100 s long trials on two consecutive days in one of four groups (n = 10 per group): Group 1, UPRIGHT balancing both days; Group 2, SUPINE both days; Group 3, UPRIGHT then SUPINE; and Group 4, SUPINE then UPRIGHT. On Day 1, UPRIGHT subjects showed better initial performance and greater improvement in performance than SUPINE subjects, who showed improvements only in having fewer deviations exceeding ±60 deg from the direction of balance. Subjects tested UPRIGHT on both days showed full retention of learning across days and additional Day 2 learning, but subjects tested SUPINE on both days showed partial retention of their marginal learning from Day 1 and little improvement on Day 2. Subjects tested SUPINE on Day 2 after being tested UPRIGHT on Day 1 showed no better performance than subjects tested SUPINE on Day 1. By contrast, there was transfer from SUPINE on Day 1 to UPRIGHT on Day 2. We conclude that absence of gravitationally dependent otolith and somatosensory cues degrades balance performance.

The influence of sleep deprivation and oscillating motion on sleepiness, motion sickness, and cognitive and motor performance.

Our goal was to determine how sleep deprivation, nauseogenic motion, and a combination of motion and sleep deprivation affect cognitive vigilance, visual-spatial perception, motor learning and retention, and balance. We exposed four groups of subjects to different combinations of normal 8h sleep or 4h sleep for two nights combined with testing under stationary conditions or during 0.28Hz horizontal linear oscillation. On the two days following controlled sleep, all subjects underwent four test sessions per day that included evaluations of fatigue, motion sickness, vigilance, perceptual discrimination, perceptual learning, motor performance and learning, and balance. Sleep loss and exposure to linear oscillation had additive or multiplicative relationships to sleepiness, motion sickness severity, decreases in vigilance and in perceptual discrimination and learning. Sleep loss also decelerated the rate of adaptation to motion sickness over repeated sessions. Sleep loss degraded the capacity to compensate for novel robotically induced perturbations of reaching movements but did not adversely affect adaptive recovery of accurate reaching. Overall, tasks requiring substantial attention to cognitive and motor demands were degraded more than tasks that were more automatic. Our findings indicate that predicting performance needs to take into account in addition to sleep loss, the attentional demands and novelty of tasks, the motion environment in which individuals will be performing and their prior susceptibility to motion sickness during exposure to provocative motion stimulation.

Learning dynamic control of body roll orientation.

Our objective was to examine how the control of orientation is learned in a task involving dynamically balancing about an unstable equilibrium point, the gravitational vertical, in the absence of leg reflexes and muscle stiffness. Subjects (n = 10) used a joystick to set themselves to the gravitational vertical while seated in a multi-axis rotation system (MARS) device programmed with inverted pendulum dynamics. The MARS is driven by powerful servomotors and can faithfully follow joystick commands up to 2.5 Hz with a 30-ms latency. To make the task extremely difficult, the pendulum constant was set to 600°/s(2). Each subject participated in five blocks of four trials, with a trial ending after a cumulative 100 s of balancing, excluding reset times when a subject lost control. To characterize performance and learning, we used metrics derived from joystick movements, phase portraits (joystick deflections vs MARS position and MARS velocity vs angular position), and stabilogram diffusion functions. We found that as subjects improved their balancing performance, they did so by making fewer destabilizing joystick movements and reducing the number and duration of joystick commands. The control strategy they acquired involved making more persistent short-term joystick movements, waiting longer before making changes to ongoing motion, and only intervening intermittently.

Direction of balance and perception of the upright are perceptually dissociable.

We examined whether the direction of balance rather than an otolith reference determines the perceived upright. Participants seated in a device that rotated around the roll axis used a joystick to control its motion. The direction of balance of the device, the location where it would not be accelerated to either side, could be offset from the gravitational vertical, a technique introduced by Riccio, Martin, and Stoffregen (J Exp Psychol Hum Percept Perform 18: 624-644, 1992). Participants used the joystick to align themselves in different trials with the gravitational vertical, the direction of balance, the upright, or the direction that minimized oscillations. They pressed the joystick trigger whenever they thought they were at the instructed orientation. Achieved angles for the "align with gravity" and "align with the upright" conditions were not different from each other and were significantly displaced past the gravitational vertical opposite from the direction of balance. Mean indicated angles for align with gravity and align with the upright coincided with the gravitational vertical. Both mean achieved and indicated angles for the "minimize oscillations" and "align with the direction of balance" conditions were significantly deviated toward the gravitational vertical. Three control experiments requiring self-settings to instructed orientations only, perceptual judgments only, and perceptual judgments during passive exposure to dynamic roll profiles confirmed that perception of the upright is determined by gravity, not by the direction of balance.

Parkinsonian gait ameliorated with a moving handrail, not with a banister.

OBJECTIVE: To determine whether haptic (touch and proprioception) cues from touching a moving handrail while walking can ameliorate the gait symptoms of Parkinson disease (PD), such as slowness and small stride length. DESIGN: Nonrandomized, controlled before-after trial. SETTING: Physical therapy clinic. PARTICIPANTS: People with PD (n=16) and healthy age-matched control subjects (n=16) with no neurologic disorders volunteered. No participants withdrew. INTERVENTIONS: We compared gait using a moving handrail as a novel assistive aid (speed self-selected) versus a banister and unassisted walking. Participants with PD were tested on and off dopaminergic medication. MAIN OUTCOME MEASURES: Mean gait speed, stride length, stride duration, double-support duration, and medial-lateral excursion. RESULTS: With the moving handrail, participants with PD increased gait speed relative to unassisted gait by 16% (.166m/s, P=.009, d=.76; 95% confidence interval [CI], .054-.278m/s) and increased stride length by 10% (.053m, P=.022, d=.37; 95% CI, .009-.097m) without significantly changing stride or double-support duration. The banister reduced speed versus unassisted gait by 11% (-.097m/s, P=.040, d=.40; 95% CI, .002-.193m/s) and reduced stride length by 8% (.32m, P=.004, d=.26; 95% CI, .010-.054m), whereas it increased stride duration by 3% (.023s, P=.022, d=.21; 95% CI, .004-.041s) and double-support duration by 35% (.044s, P=.031, d=.58; 95% CI, .005-.083s). All medication × condition interactions were P>.05. CONCLUSIONS: Using haptic speed cues from the moving handrail, people with PD walked faster by spontaneously (ie, without specific instruction) increasing stride length without altering cadence; banisters slowed gait. Haptic cues from the moving handrail can be used by people with PD to engage biomechanical and neural mechanisms for interpreting tactile and proprioception changes related to gait speed to control gait better than static cues afforded by banisters.

Statistical analysis of quiet stance sway in 2-D.

Subjects exposed to a rotating environment that perturbs their postural sway show adaptive changes in their voluntary spatially directed postural motion to restore accurate movement paths but do not exhibit any obvious learning during passive stance. We have found, however, that a variable known to characterize the degree of stochasticity in quiet stance can also reveal subtle learning phenomena in passive stance. We extended Chow and Collins (Phys Rev E 52(1):909-912, 1995) one-dimensional pinned-polymer model (PPM) to two dimensions (2-D) and then evaluated the model's ability to make analytical predictions for 2-D quiet stance. To test the model, we tracked center of mass and centers of foot pressures, and compared and contrasted stance sway for the anterior-posterior versus medio-lateral directions before, during, and after exposure to rotation at 10 rpm. Sway of the body during rotation generated Coriolis forces that acted perpendicular to the direction of sway. We found significant adaptive changes for three characteristic features of the mean square displacement (MSD) function: the exponent of the power law defined at short time scales, the proportionality constant of the power law, and the saturation plateau value defined at longer time scales. The exponent of the power law of MSD at a short time scale lies within the bounds predicted by the 2-D PPM. The change in MSD during exposure to rotation also had a power-law exponent in the range predicted by the theoretical model. We discuss the Coriolis force paradigm for studying postural and movement control and the applicability of the PPM model in 2-D for studying postural adaptation.

Adaptation to Coriolis perturbations of voluntary body sway transfers to preprogrammed fall-recovery behavior.

In a rotating environment, goal-oriented voluntary movements are initially disrupted in trajectory and endpoint, due to movement-contingent Coriolis forces, but accuracy is regained with additional movements. We studied whether adaptation acquired in a voluntary, goal-oriented postural swaying task performed during constant-velocity counterclockwise rotation (10 RPM) carries over to recovery from falling induced using a hold and release (H&R) paradigm. In H&R, standing subjects actively resist a force applied to their chest, which when suddenly released results in a forward fall and activation of an automatic postural correction. We tested H&R postural recovery in subjects (n = 11) before and after they made voluntary fore-aft swaying movements during 20 trials of 25 s each, in a counterclockwise rotating room. Their voluntary sway about their ankles generated Coriolis forces that initially induced clockwise deviations of the intended body sway paths, but fore-aft sway was gradually restored over successive per-rotation trials, and a counterclockwise aftereffect occurred during postrotation attempts to sway fore-aft. In H&R trials, we examined the initial 10- to 150-ms periods of movement after release from the hold force, when voluntary corrections of movement path are not possible. Prerotation subjects fell directly forward, whereas postrotation their forward motion was deviated significantly counterclockwise. The postrotation deviations were in a direction consistent with an aftereffect reflecting persistence of a compensation acquired per-rotation for voluntary swaying movements. These findings show that control and adaptation mechanisms adjusting voluntary postural sway to the demands of a new force environment also influence the automatic recovery of posture.