Are avatars treated like human obstacles during aperture crossing in virtual environments?

RESEARCH OBJECTIVE: The current study set out to determine whether individuals walking in a virtual reality environment pass through apertures made of two avatars differently than apertures created by two pole obstacles, as previously observed between pole and human obstacles in real-world environments. METHODS: Eleven healthy young adults wore a head-mounted virtual reality display, walked along a 10 m path and passed through a virtual aperture located 5 m from the starting location. Participants were instructed to avoid colliding with the obstacles when passing through the aperture. The experiment was conducted in a block design, where the aperture was either created by two pole obstacles or by two avatars. In both conditions, the width of the aperture ranged between 1.0-1.8x each participant's shoulder width. RESULTS: Regardless of whether the aperture was created by the virtual poles or the avatars, participants rotated their shoulders for all aperture sizes and results found no significant differences in shoulder rotation angle, onset of rotation, walking speed or velocity at time of crossing between the two types of obstacles. Therefore, it appears that the differences in avoidance behaviours observed in real-world settings between people and pole obstacles is not translated to a virtual reality environment. SIGNIFICANCE: It is possible that during experiments in which the avatars do not move, they do not possess human-like qualities suggested to be responsible for the increased caution used when walking through real human obstacles and instead, are treated as any ordinary obstacle.

Action strategies for walking through multiple, misaligned apertures.

When avoiding obstacles, path selection is thought to be determined by the attraction of the end-goal. However for aperture crossing, it is unclear whether the attraction point originates in the center of the aperture or at the end-goal, as previous experiments align the aperture with the end-goal. The purpose of the current study was to decipher the possible location of the attraction point, by evaluating crossing behaviour for multiple, misaligned apertures. Participants were instructed to walk through three separate apertures while en route to an end-goal. The first and last apertures were fixed such that they were both either 0.9× or 1.7× shoulder width (SW) while the second aperture was either 0.9, 1.3 or 1.7× SW and shifted 25, 50 or 75cm off the midline. Findings revealed that the attraction of the end-goal, and not the middle of the aperture, guided crossing behaviour. The spatial margin decreased as the size of the shift increased. Furthermore, the frequency of rotation increased as the aperture was shifted away from midline, regardless of the aperture size. Since rotations would not normally occur for all of these aperture sizes when aligned with the end-goal, these results suggest that rotations were produced in an attempt to keep one's trajectory as close to the midline as possible. Therefore, not only does the attraction of the goal guide path trajectory, but individuals will choose to reduce the spatial margin and rotate the shoulders when walking through misaligned apertures, likely in attempt to maintain the straightest possible path.

Postural motor learning in Parkinson's disease: The effect of practice on continuous compensatory postural regulation.

INTRODUCTION: Although balance training is considered the most effective treatment for balance impairments in Parkinson's disease (PD), few studies have examined if learning for balance control remains intact with PD. This study aimed to determine if learning for automatic postural responses is preserved in people with PD. METHODS: Eleven participants with moderate PD (68±6.4years; H&Y: 2-3) on their usual medication maintained balance on a platform that oscillated forward and backward with variable amplitude and constant frequency. Participants completed 42 trials during one training session, and retention and transfer tests following a 24-h delay. Performance was measured by comparing spatial and temporal measures of whole-body centre of mass (COM) with platform displacements. Learning was compared between participants with PD and previously reported, age-matched older adults (Van Ooteghem et al., 2010). RESULTS: Although postural responses in participants with PD were impaired compared to control participants, a majority of PD participants improved their postural responses with practice as revealed by reduced COM displacements and improved phase relationships between COM and platform motion. Rates of improvement were comparable between groups demonstrating preserved adaptive capacity for participants with PD. Similar to control participants, the PD group moved toward anticipatory COM control as a strategy for improving stability, exhibited short-term retention of performance improvements, and demonstrated generalizability of the learned responses. Rate of improvement with practice, but not retention, was related to severity of motor impairments. CONCLUSIONS: Patients with moderate PD on medication demonstrate retention of improvements in automatic postural responses with practice suggesting that intrinsic postural motor learning is preserved in this group.

Reliability of the Achilles tendon tap reflex evoked during stance using a pendulum hammer.

The tendon tap reflex (T-reflex) is often evoked in relaxed muscles to assess spinal reflex circuitry. Factors contributing to reflex excitability are modulated to accommodate specific postural demands. Thus, there is a need to be able to assess this reflex in a state where spinal reflex circuitry is engaged in maintaining posture. The aim of this study was to determine whether a pendulum hammer could provide controlled stimuli to the Achilles tendon and evoke reliable muscle responses during normal stance. A second aim was to establish appropriate stimulus parameters for experimental use. Fifteen healthy young adults stood on a forceplate while taps were applied to the Achilles tendon under conditions in which postural sway was constrained (by providing centre of pressure feedback) or unconstrained (no feedback) from an invariant release angle (50°). Twelve participants repeated this testing approximately six months later. Within one experimental session, tap force and T-reflex amplitude were found to be reliable regardless of whether postural sway was constrained (tap force ICC=0.982; T-reflex ICC=0.979) or unconstrained (tap force ICC=0.968; T-reflex ICC=0.964). T-reflex amplitude was also reliable between experimental sessions (constrained ICC=0.894; unconstrained ICC=0.890). When a T-reflex recruitment curve was constructed, optimal mid-range responses were observed using a 50° release angle. These results demonstrate that reliable Achilles T-reflexes can be evoked in standing participants without the need to constrain posture. The pendulum hammer provides a simple method to allow researchers and clinicians to gather information about reflex circuitry in a state where it is involved in postural control.

Postural Motor Learning Deficits in People With MS in Spatial but Not Temporal Control of Center of Mass.

BACKGROUND: Multiple sclerosis (MS) is associated with balance deficits resulting in falls and impaired mobility. Although rehabilitation has been recommended to address these balance deficits, the extent to which people with MS can learn and retain improvements in postural responses is unknown. AIM: To determine the ability of people with MS to improve postural control with surface perturbation training. METHODS: A total of 24 patients with mild MS and 14 age-matched controls underwent postural control training with a set pattern of continuous, forward-backward, sinusoidal, and surface translations provided by a force platform. Postural control was then tested the following day for retention. The primary outcome measures were the relative phase and center-of-mass (CoM) gain between the body CoM and the platform motion. RESULTS: People with MS demonstrated similar improvements in acquiring and retaining changes in the temporal control of the CoM despite significant deficits in postural motor performance at the baseline. Both MS and control groups learned to anticipate the pattern of forward-backward perturbations, so body CoM shifted from a phase-lag (age-matched controls [CS] = -7.1 ± 1.3; MS = -12.9 ± 1.0) toward a phase-lead (CS = -0.7 ± 1.8; MS = -6.1 ± 1.4) relationship with the surface oscillations. However, MS patients were not able to retain the changes in the spatial control of the CoM acquired during training. CONCLUSIONS: People with MS have the capacity to improve use of a feed-forward postural strategy with practice and retain the learned behavior for temporal not spatial control of CoM, despite their significant postural response impairments.

Does the passability of apertures change when walking through human versus pole obstacles?

The current study set out to evaluate how individuals walk through apertures created by different stationary obstacles. Specifically, we examined whether the passability of apertures differed between human and pole obstacles by quantifying aperture crossing behaviors such as the critical point. Participants walked an 8m path toward a visible goal located at the end. Two obstacles were positioned 5m from the starting location and participants were instructed to pass between the obstacles without hitting them. The distance between the obstacles ranged between 1.0 and 1.8× the participant's shoulder width. Results revealed that, when the obstacles were humans, individuals rotated their shoulders more frequently at larger apertures, as evidenced by a larger critical point (1.7 vs 1.3 for poles), initiated shoulder rotations earlier, rotated to a larger degree, left a wider clearance between their shoulders and the obstacles at the time of crossing, and walked slower when approaching and passing through the obstacles compared to when the obstacles were poles. Furthermore, correlational analyses revealed that the amount of change between an individual's critical point for the poles and the critical point for the human obstacles was related to social risk-taking and changes in walking speed. Therefore, it appears that the passability of apertures changes when walking between two people versus two objects such that more space and greater caution are needed for human obstacles. It is possible that the greater caution observed for human obstacles is to account for the personal space needs of others that do not exist in the same extent for poles and that the degree of caution is related to social factors.

The effects of narrow and elevated path walking on aperture crossing.

The study investigated the impact that action capabilities have on identifying possibilities for action, particularly how postural threat influences the passability of apertures. To do this, the ability to maintain balance was challenged by manipulating the level of postural threat while walking. First, participants walked along a 7m path and passed through two vertical obstacles spaced 1.1-1.5×the shoulder width apart during normal walking. Next, postural threat was manipulated by having participants complete the task either walking on a narrow, ground level path or on an elevated/narrow path. Despite a decrease in walking speed as well as an increase in trunk sway in both the narrow and elevated/narrow walking conditions, the passability of apertures was only affected when the consequence of instability was greatest. In the elevated/narrow walking condition, individuals maintained a larger critical point (rotated their shoulders for larger aperture widths) compared to normal walking. However, this effect was not observed for the narrow path walking suggesting that the level of postural threat was not enough to impose similar changes to the critical point. Therefore, it appears that manipulating action capabilities by increasing postural threat does indeed influence aperture crossing behavior, however the consequence associated with instability must be high before both gait characteristics and the critical point are affected.

Parkinson's disease and segmental coordination during turning: I. Standing turns.

OBJECTIVE: Many of the falls among people with Parkinson's disease (PD) occur during sudden, on-the-spot turning which requires systematic reorientation of axial segments towards the new direction. We examined whether a disturbance in the coordination of segmental reorientation is an important cause of turning difficulty in individuals with PD and is altered by dopaminergic medication. METHODS: The sequence and timing of segmental reorientation during 45° and 90° on-the-spot turns was examined in fourteen individuals with PD while "off" and "on" medication and nineteen healthy controls (HC). RESULTS: Regardless of the magnitude of the turn, HC reoriented their head, shoulder, and pelvis simultaneously followed by mediolateral foot displacement. PD patients displayed temporal coordination patterns similar to the HC. PD however, reduced the velocity and early magnitude of reorientation of each body segment which were both slightly improved by dopaminergic medication. CONCLUSION: Our finding that the HC and PD patients turn en bloc when the turn is predictable and there are no time constraints shows that the strategy of en bloc turning is not wrong if the movement parameters are unconstrained. However, in real life situations, which usually require quick and unpredictable turns, the en bloc strategy may be unsafe and more likely to result in falls. While in such situations HC are able to change the strategy from en bloc to sequential segmental turning, PD patients may not be able to do so and continue to turn en bloc.

Parkinson's disease and segmental coordination during turning: II. Walking turns.

OBJECTIVE: Individuals with Parkinson's disease (PD) show poorer balance and greater incidence of falls while turning. We investigated whether a disturbance in timing and sequence of reorientation of body segments is a potential cause of turning difficulty in PD and is altered by levodopa. METHODS: The sequence and timing of segmental reorientation during 45° and 90° walking turns were recorded in nineteen healthy controls and fourteen individuals with PD "off" and "on" medication. RESULTS: Both healthy elderly and PD patients "off" medication displayed a top-down sequence of segment reorientation, but differed with respect to the delay time between segments: PD "off" medication displayed a shorter delay between the onset of head and shoulder reorientation and longer delays for pelvis and foot reorientation. Furthermore, for all segments the peak angular velocities were lower for PD patients than healthy controls, with greater difference between the two groups during larger turns. While for both groups the velocity and magnitude of rotation of all segments were greater during larger turns, the relative timing of reorientation of segments remained the same during small and large turns. Medication had no significant effect on the timing and sequence of reorientation of segments and caused only a small and non-significant increase to segment velocities. CONCLUSION: This study further characterized the turning performance of individuals with PD. Our findings have clinical applications and therapeutic value for PD patients with difficulty turning. Understanding the specific deficiencies of turning performance of PD patients allows the therapists to opt for the most effective rehabilitation techniques.

Effects of age and pathology on stance modifications in response to increased postural threat.

This study investigated modifications to standing posture in response to elevated postural anxiety evoked by a potential physical threat to stability. Sixteen young adults, 16 older adults and 16 patients diagnosed with Parkinson's disease (PD) stood with or without the expectation of a threat to their posture (i.e., external trunk perturbation). This method allowed for the assessment of the effects of anticipatory anxiety on standing posture associated with an ecologically valid and direct threat to stability. Our manipulation was successful as all participants, independent of age and disease, reported significant increases in postural anxiety when anticipating a threat to their posture. The trunk sway modifications observed in response to elevated postural anxiety were dependent on age and disease. Young adults showed increased trunk sway in both pitch and roll directions while older adults demonstrated decreased trunk sway but only in the roll direction when standing and expecting a threat to posture compared to standing without this threat. Individuals with PD showed no significant changes in trunk pitch or roll sway when anticipating a threat to posture compared to standing without this threat. Our findings suggest that the effects of postural anxiety on postural control are dependent on the context associated with the postural threat, and age and disease status.

The effects of skill focused instructions on walking performance depend on movement constraints in Parkinson's disease.

Previous research has shown that skill focused attention may be beneficial for the performance of complicated motor tasks in individuals with Parkinson's disease (PD). The objective of this study was to assess the impact of skill focused attention instructions on gait under temporal movement constraints that may reflect common challenges experienced in daily life. Eighteen patients with PD walked a straight pathway under two different attention focus conditions (no instruction, skill focused instruction) and two different walking speeds (preferred pace, as fast as possible). In the no instruction condition, patients were not told "where" attention should be directed. In the skill focused instruction condition, patients were told to focus on the foot contacting the floor with each step. Spatial and temporal gait measures, as well as, trunk sway were used to quantify walking performance. The results showed that when walking at a preferred pace, skill focused instructions benefited gait performance (e.g., increased gait velocity, larger steps, more trunk sway). However, when walking as fast as possible, skill focused instructions had the opposite effect on gait performance (e.g., decreased gait velocity, smaller steps, and less trunk sway). This study demonstrates that skill focused instructions may contribute to the prioritization of stability under imposed temporal movement constraints. Clinicians should be aware of the processes involved in prioritization of movement components versus task goals in PD and the potential application of an attention based instructional set in altering priorities in this population.

Coordination of segments reorientation during on-the-spot turns in healthy older adults in eyes-open and eyes-closed conditions.

Turning has frequent occurrence in everyday activities. Despite the prevalence of turning in everyday life and the challenge it poses to older adults, there is far less known about the multisegmental control of turning than the control of standing and straight walking, especially in elderly individuals. The purpose of this study was to examine the timing and sequence of segments reorientation in healthy older adults during 90° on-the-spot turns. The role of vision on segments coordination was also examined by testing the participants in eyes-open and eyes-closed conditions. When turning on-the-spot, healthy elderly reoriented their head, shoulder and pelvis simultaneously, followed by foot displacement. This was a robust behavior not affected by visual condition. Axial segments turned slower and more synchronously when vision was not available. While all segments started to turn together in both visual conditions, head turned faster and reached its peak velocity earlier than shoulder and pelvis. However, the difference in segmental velocity and the time to reach the peak velocity was smaller in eyes-closed than eyes-open condition. Without vision, the functional importance of a faster head turn is diminished. Participants may have adopted a tighter control of segments to simplify the control of movement by reducing the degrees of freedom.

Effect of walking velocity on segment coordination during pre-planned turns in healthy older adults.

BACKGROUND: Despite the prevalence of turning in daily activities and the challenges it poses to mobility-impaired individuals, far less is known about the multi-segmental control of turning than the control of straight walking. Gait slows with aging and neurological disorders such as Parkinson's disease and falls in these populations frequently occur when turning. Nevertheless, the influence of walking velocity on the complex inter-segmental coordination of the head, trunk and lower limbs during turning has not been examined. The purpose of this study was to examine the effect of walking velocity on the coordination of segment reorientation during turns embedded in locomotion in healthy older adults. METHODS: Nineteen healthy older adults volunteered to participate. Participants made a 45 degrees or 90 degrees turn to their right while walking either at their natural self-selected speed or slower or faster than their natural speed. We quantified the timing and sequence of segments reorientation during the turns. RESULTS: There was a top-down temporal sequence in initiation of segments reorientation during turning, i.e., head turned first, followed by shoulder, pelvis, and mediolateral foot displacement. Furthermore, results indicate that the top-down temporal sequence in segments reorientation during turning was a robust behavior which was not affected by the walking velocity or magnitude of the turn. CONCLUSIONS: Walking velocity does not affect segment coordination during pre-planned turns in healthy elderly. Therefore, we conclude that changes in coordination of segments reorientation during pre-planned turns in individuals with neurological disorders such as Parkinson's disease is not due to their slower gait.

Aging does not affect generalized postural motor learning in response to variable amplitude oscillations of the support surface.

Postural motor learning for dynamic balance tasks has been demonstrated in healthy older adults (Van Ooteghem et al. in Exp Brain Res 199(2):185-193, 2009). The purpose of this study was to investigate the type of knowledge (general or specific) obtained with balance training in this age group and to examine whether embedding perturbation regularities within a balance task masks specific learning. Two groups of older adults maintained balance on a translating platform that oscillated with variable amplitude and constant frequency. One group was trained using an embedded-sequence (ES) protocol which contained the same 15-s sequence of variable amplitude oscillations in the middle of each trial. A second group was trained using a looped-sequence (LS) protocol which contained a 15-s sequence repeated three times to form each trial. All trials were 45 s. Participants were not informed of any repetition. To examine learning, participants performed a retention test following a 24-h delay. LS participants also completed a transfer task. Specificity of learning was examined by comparing performance for repeated versus random sequences (ES) and training versus transfer sequences (LS). Performance was measured by deriving spatial and temporal measures of whole body center of mass (COM) and trunk orientation. Both groups improved performance with practice as characterized by reduced COM displacement, improved COM-platform phase relationships, and decreased angular trunk motion. Furthermore, improvements reflected general rather than specific postural motor learning regardless of training protocol (ES or LS). This finding is similar to young adults (Van Ooteghem et al. in Exp Brain Res 187(4):603-611, 2008) and indicates that age does not influence the type of learning which occurs for balance control.

The relationship between physiological arousal and cortical and autonomic responses to postural instability.

Postural instability evokes cortical and autonomic reactions in addition to the primary compensatory response, and it is hypothesized that these responses may be related to underlying affective influences such as tonic physiological arousal. The purpose of this study was to determine whether perturbation-evoked cortical potentials (N1 and P2) and electrodermal responses (EDRs) were related to each other or to tonic electrodermal level (EDL). Ten healthy individuals received sixty perturbations while standing at ground level (LOW) and at the edge of an elevated platform (HIGH), where an unsuccessful reaction could lead to a fall from a height of 160 cm. Postural responses, tonic EDLs and N1 potentials were all significantly larger (p < or = 0.01) at the HIGH height relative to LOW. EDR amplitudes did not show a main effect of condition, but habituated less at the HIGH height (interaction p = 0.04). P2 potentials were not different between conditions (p > 0.05). There was no statistically significant relationship between the magnitude of change in N1 amplitude between conditions and the change in EDR amplitude between conditions (R = 0.25, p = 0.5), between magnitude of N1 change and magnitude of EDL change (R = -0.23, p = 0.25), or between magnitude of EDR change and EDL change (R = -0.51, p = 0.16). Altered cortical and autonomic responses at the HIGH height were independent of elevated physiological arousal. These findings have implications for understanding the role of cortical and autonomic responses in compensatory balance control, and alleviate concerns about the potential influence of underlying arousal on electrodermal reactivity.

Turning behavior in healthy older adults: Is there a preference for step versus spin turns?

This research examined the prevalence of step turns and spin turns during turning while walking in healthy older adults. The potential effect of magnitude of the turn and walking velocity on the prevalence of the step and spin turns were also investigated by examining the participants' performance as they made 45 degrees and 90 degrees turns while walking at three different velocities. Results showed that healthy older adults in our study preferred spin turns while walking either slower or faster than their natural walking speed. Only during 90 degrees turns while walking fast the participants showed a preference for step turns over spin turns. Spin turns are less stable and have a greater biomechanical cost than step turns. The high incidence of spin turns in older adults may contribute to the higher risk of falling in this population.

Control of dynamic stability during adaptation to gait termination on a slippery surface.

An unexpected slip during gait termination results in a generalised slip response designed to regain stability and prevent a fall. With knowledge of and experience with a slippery surface, locomotor behaviour adapts to proactively diminish the effect of the slip and improve the reactive control during the slip. Our purpose was to examine the organisation of the adaptation to a slippery surface during gait termination. After receiving an unexpected slip during gait termination, participants (N = 8) experienced cued gait termination trials in which they were given knowledge of the surface characteristics (i.e., slippery or non-slippery). The observed strategy used to repeatedly stop on a slippery surface involves proactively diminishing the size of the slip perturbation through a flattened foot at heel contact, anterior shift of the COM, shorter step, stance leg extension and swing limb slowing, as well as improving the reaction to the slippery surface through decreased muscle activity and an appropriate decrease in the braking force generation. The implications of this research are that a combination of knowledge of and experience with a slippery surface enables proactive and reactive adjustments in behaviour to effectively and more safely stop walking on a slippery surface.

Practice-related improvements in posture control differ between young and older adults exposed to continuous, variable amplitude oscillations of the support surface.

Healthy older adults were repeatedly exposed to continuous, variable amplitude oscillations of the support surface to determine (1) whether age affects the capacity for postural motor learning under continuous perturbation conditions with limited predictability and (2) whether practice leads to modifications in the control strategy used to maintain balance in older adults. During training, a translating platform underwent 45-s trials of constant frequency (0.5 Hz) and seemingly random amplitude oscillations (range +\- 2 to 15 cm). The middle 15 s of each trial contained the same sequence of oscillation amplitudes. This repeated middle segment was the same as the repeated segment used in Van Ooteghem et al. (Exp Brain Res 187(4): 603-611, 2008) and was therefore used for analyses. To examine learning, participants performed a retention test following a 24-h delay. Kinematic data were used to derive spatial and temporal measures of whole body centre of mass (COM), trunk, thigh, and shank segment orientation, and ankle and knee angle from performance during the repeated middle segment. Results showed that with training, older adults maintained the capacity to learn adaptive postural responses in the form of improved temporal control of the COM and minimization of trunk instability a a rate comparable to young adults. With practice, however older adults maintained a more rigid, 'platform-fixed' control strategy which differed from young adults who shifted towards 'gravity-fixed' control and decreased COM motion. This study provides important insight into the ability of older adults to demonstrate longer-term improvements in postural regulation.

Stilt walking: how do we learn those first steps?

This study examined how young healthy adults learn stilt walking. Ten healthy male university students attended two sessions of testing held on two consecutive days. In each session participants performed three blocks of 10 stilt-walking trials. Angular movements of head and trunk and the spatial and temporal gait parameters were recorded. When walking on stilts young adults improved their gait velocity through modifications of step parameters while maintaining trunk movements close to that observed during normal over-ground walking. Participants improved their performance by increasing their step frequency and step length and reducing the double support percentage of the gait cycle. Stilts are often used for drywall installation, painting over-the-head areas and raising workers above the ground without the burden of erecting scaffolding. This research examines the locomotor adaptation as young healthy adults learn the complex motor task of stilt walking; a task that is frequently used in the construction industry.

Cognitive demands of postural control during continuous rotational perturbations of the support surface.

This research explored the effect of the level of balance challenge on the cognitive demands of dynamic balance. The postural task was maintaining balance while standing on a rotating platform that moved about the axis of the ankle joint in the pitch plane. Different frequencies and amplitudes of perturbation were employed to introduce different levels of balance challenge. The cognitive task was a silent word identification task. Results showed no significant difference in the participants' performance on the cognitive task in any of the dual-task conditions in comparison with their performance in the "cognitive only" condition. Furthermore, regardless of the level of balance challenge concurrent performance of the cognitive task did not affect the balance control strategies adopted by our participants. The lack of interference between the cognitive task and the present postural task might be due to the fact that the rotational perturbations induced very small Center of Mass (COM) displacements at all frequencies which were under the automatic control.