Electrical Brain Activity during Human Walking with Parametric Variations in Terrain Unevenness and Walking Speed.

Mobile brain imaging with high-density electroencephalography (EEG) can provide insight into the cortical processes involved in complex human walking tasks. While uneven terrain is common in the natural environment and poses challenges to human balance control, there is limited understanding of the supraspinal processes involved with traversing uneven terrain. The primary objective of this study was to quantify electrocortical activity related to parametric variations in terrain unevenness for neurotypical young adults. We used high-density EEG to measure brain activity when thirty-two young adults walked on a novel custom-made uneven terrain treadmill surface with four levels of difficulty at a walking speed tailored to each participant. We identified multiple brain regions associated with uneven terrain walking. Alpha (8 - 13 Hz) and beta (13 - 30 Hz) spectral power decreased in the sensorimotor and posterior parietal areas with increasing terrain unevenness while theta (4 - 8 Hz) power increased in the mid/posterior cingulate area with terrain unevenness. We also found that within stride spectral power fluctuations increased with terrain unevenness. Our secondary goal was to investigate the effect of parametric changes in walking speed (0.25 m/s, 0.5m/s, 0.75 m/s, 1.0 m/s) to differentiate the effects of walking speed from uneven terrain. Our results revealed that electrocortical activities only changed substantially with speed within the sensorimotor area but not in other brain areas. Together, these results indicate there are distinct cortical processes contributing to the control of walking over uneven terrain versus modulation of walking speed on smooth, flat terrain. Our findings increase our understanding of cortical involvement in an ecologically valid walking task and could serve as a benchmark for identifying deficits in cortical dynamics that occur in people with mobility deficits.

Comparison of EEG Source Localization Using Simplified and Anatomically Accurate Head Models in Younger and Older Adults.

Accuracy of electroencephalography (EEG) source localization relies on the volume conduction head model. A previous analysis of young adults has shown that simplified head models have larger source localization errors when compared with head models based on magnetic resonance images (MRIs). As obtaining individual MRIs may not always be feasible, researchers often use generic head models based on template MRIs. It is unclear how much error would be introduced using template MRI head models in older adults that likely have differences in brain structure compared to young adults. The primary goal of this study was to determine the error caused by using simplified head models without individual-specific MRIs in both younger and older adults. We collected high-density EEG during uneven terrain walking and motor imagery for 15 younger (22±3 years) and 21 older adults (74±5 years) and obtained [Formula: see text]-weighted MRI for each individual. We performed equivalent dipole fitting after independent component analysis to obtain brain source locations using four forward modeling pipelines with increasing complexity. These pipelines included: 1) a generic head model with template electrode positions or 2) digitized electrode positions, 3) individual-specific head models with digitized electrode positions using simplified tissue segmentation, or 4) anatomically accurate segmentation. We found that when compared to the anatomically accurate individual-specific head models, performing dipole fitting with generic head models led to similar source localization discrepancies (up to 2 cm) for younger and older adults. Co-registering digitized electrode locations to the generic head models reduced source localization discrepancies by  âˆ¼  6 mm. Additionally, we found that source depths generally increased with skull conductivity for the representative young adult but not as much for the older adult. Our results can help inform a more accurate interpretation of brain areas in EEG studies when individual MRIs are unavailable.

Uneven terrain treadmill walking in younger and older adults.

We developed a method for altering terrain unevenness on a treadmill to study gait kinematics. Terrain consisted of rigid polyurethane disks (12.7 cm diameter, 1.3-3.8 cm tall) which attached to the treadmill belt using hook-and-loop fasteners. Here, we tested four terrain unevenness conditions: Flat, Low, Medium, and High. The main objective was to test the hypothesis that increasing the unevenness of the terrain would result in greater gait kinematic variability. Seventeen younger adults (age 20-40 years), 25 higher-functioning older adults (age 65+ years), and 29 lower-functioning older adults (age 65+ years, Short Physical Performance Battery score < 10) participated. We customized the treadmill speed to each participant's walking ability, keeping the speed constant across all four terrain conditions. Participants completed two 3-minute walking trials per condition. Using an inertial measurement unit placed over the sacrum and pressure sensors in the shoes, we calculated the stride-to-stride variability in step duration and sacral excursion (coefficient of variation; standard deviation expressed as percentage of the mean). Participants also self-reported their perceived stability for each condition. Terrain was a significant predictor of step duration variability, which roughly doubled from Flat to High terrain for all participant groups: younger adults (Flat 4.0%, High 8.2%), higher-functioning older adults (Flat 5.0%, High 8.9%), lower-functioning older adults (Flat 7.0%, High 14.1%). Similarly, all groups exhibited significant increases in sacral excursion variability for the Medium and High uneven terrain conditions, compared to Flat. Participants were also significantly more likely to report feeling less stable walking over all three uneven terrain conditions compared to Flat. These findings support the hypothesis that altering terrain unevenness on a treadmill will increase gait kinematic variability and reduce perceived stability in younger and older adults.

Number of Trials Needed to Assess Postural Control of Young Adults in Single and Dual-Task.

Despite its popularity, there is a lack of standardization when assessing postural control. This study aimed to suggest how many trials should be used when assessing young adults' postural control with a specific single-task and dual-task quiet stance protocol. Two groups of 15 participants performed 20 trials of 60 s (feet together, eyes open) with or without a dual-task. The number of trials needed to obtain two consecutive intra-class correlation coefficients (ICC(2,k)) ≥0.900 was then assessed for seven center of pressure variables. Although inconsistency was observed between variables and tasks, five trials seems to be a good compromise.

Motion and Muscle Artifact Removal Validation Using an Electrical Head Phantom, Robotic Motion Platform, and Dual Layer Mobile EEG.

Motion and muscle artifacts can undermine signal quality in electroencephalography (EEG) recordings during locomotion. We evaluated approaches for recovering ground-truth artificial brain signals from noisy EEG recordings. We built an electrical head phantom that broadcast four brain and four muscle sources. Head movements were generated by a robotic motion platform. We recorded 128-channel dual layer EEG and 8-channel neck electromyography (EMG) from the head phantom during motion. We evaluated ground-truth electrocortical source signal recovery from artifact contaminated data using Independent Component Analysis (ICA) to determine: (1) the number of isolated noise sensor recordings needed to capture and remove motion artifacts, (2) the ability of Artifact Subspace Reconstruction to remove motion and muscle artifacts at contrasting artifact detection thresholds, (3) the number of neck EMG sensor recordings needed to capture and remove muscle artifacts, and (4) the ability of Canonical Correlation Analysis to remove muscle artifacts. We also evaluated source signal recovery by combining the best practices identified in aims 1-4. By including isolated noise and EMG recordings in the ICA decomposition, we more effectively recovered ground-truth artificial brain signals. A reduced subset of 32-noise and 6-EMG channels showed equivalent performance compared to including the complete arrays. Artifact Subspace Reconstruction improved source separation, but this was contingent on muscle activity amplitude. Canonical Correlation Analysis also improved source separation. Merging noise and EMG recordings into the ICA decomposition, with Artifact Subspace Reconstruction and Canonical Correlation Analysis preprocessing, improved source signal recovery. This study expands on previous head phantom experiments by including neck muscle source activity and evaluating artificial electrocortical spectral power fluctuations synchronized with gait events.

Effect of Bilateral and Unilateral Plantarflexor Muscle Fatigue on Blind Navigation Precision and Gait Parameters.

The objective was to evaluate the impact of bilateral and unilateral fatigue of the plantarflexor muscles on blind navigation. Thirty-eight young adults walked 8-m without vision before fatigue (pre-fatigue), then fatigued either one or both of their plantarflexor muscles by performing isometric contractions. After each fatigue, two blind navigation trials were performed (post-fatigue trials 1 and 2). Results revealed no effect of bilateral muscle fatigue on navigation precision and gait parameters. Unilateral muscle fatigue led to longer linear distance travelled during post-fatigue trial 2 compared to pre-fatigue and to a change in angular deviation between pre- and post-fatigue. In general, results suggest that participants were able to make adaptive changes to counter muscle fatigue during blind navigation.

Absence of Ankle Stiffening While Standing in Focus and Cognitive Task Conditions in Older Adults.

Research suggests that an external focus or cognitive task may improve postural control. Removing attention from movement production may promote automaticity, or the tasks may promote ankle stiffening. To investigate these two theories, twenty older adults stood while performing baseline standing, internal focus, external focus, and two cognitive tasks. Changes in postural control occurred in external focus and cognitive task conditions compared to baseline and internal focus, while no change occurred in cocontraction indices. This suggests that an external focus and cognitive task can improve postural control in older adults. Since no change occurred in cocontraction indices across conditions, this suggests that stiffening cannot explain these changes. Instead, changes could be due to automaticity of sway.

Automaticity of Postural Control while Dual-tasking Revealed in Young and Older Adults.

Background: Postural control improvements in external focus and cognitive task conditions are thought to occur because directing attention away from postural control allows greater automaticity. We aimed to support this theory by using three dynamic measures of postural control that may reveal changes in the structure or composition of sway: the discrete wavelet transform, sample entropy, and rambling trembling analyses.Methods: We analyzed the center of pressure data from twenty-two healthy young adults (20.8 ± 2.82 years) and twenty healthy older adults (69.02 ± 3.47 years). Participants stood with feet together in five conditions: baseline standing, internal focus, external focus, easy cognitive task, and difficult cognitive task. Analyses of variance were used to examine the effect of condition and age on the three dynamic measures.Results: The wavelet transform revealed a shift toward greater contributions from higher frequency bands in cognitive task conditions, suggesting greater automaticity. Sample entropy was higher in cognitive task conditions, suggesting more complex sway and automatic control. The external focus and difficult cognitive tasks increased trembling in young adults, suggesting increased contributions from spinal reflex components.Conclusion: Results support the theory that stability improvements in cognitive task conditions were due to automaticity in young and older adults. They also suggest that tasks that are more difficult are better at promoting automaticity than tasks requiring less cognitive involvement.

The influence of carrying an anterior load on attention demand and obstacle clearance before, during, and after obstacle crossing.

Carrying an anterior load during obstacle negotiation increases attention demand, which may differ at various crossing stages. Less is known on the impact of lower visual field obstruction and the weight of the anterior load on obstacle negotiation and attention demand. The objectives of this study were to: (1) determine if carrying a weighted anterior load, lower visual field occlusion, or both, modify obstacle clearance and/or reaction time (RT); and (2) examine whether RT is modulated across obstacle crossing phases as measured by a probe RT protocol. Sixteen young adults crossed an obstacle while carrying no load, a clear 5 kg load, and an opaque 5 kg load, while performing a simple RT task. Auditory stimuli were presented at five locations: (1) two steps before the obstacle; (2) one step before the obstacle; (3) as the leading limb crossed the obstacle; (4) as the lead limb touched down after the obstacle; and (5) as the trail limb crossed the obstacle. The toe clearance height of the leading limb was greatest for the weighted opaque box load type followed by the weighted clear box type compared to the no box load type. Carrying an anterior load during obstacle crossing did not influence RT. RTs were longer at the pre-crossing and beginning of the crossing phases compared to after-crossing phases. Results suggest that carrying a weighted anterior load and lower visual field occlusion increase the risk for tripping. Attention demands differ across obstacle crossing phases during dual-tasking and should be considered in fall-risk assessments.

Cognitive task modality influences postural control during quiet standing in healthy older adults.

BACKGROUND: The interstimulus interval of a cognitive task was found to have a limited effect on postural control in young adults, while visual cognitive tasks were found to improve stability compared to auditory tasks. It is of interest to investigate whether postural control in healthy older adults is sensitive to these types of cognitive task manipulations. AIMS: The objectives of the present experiment were to evaluate the impact of interstimulus interval and modality of a continuous cognitive task on postural control in healthy older adults. METHODS: Fifteen healthy older adults (70 ± 3.2 years, 3 male) were asked to stand with feet together on a force platform while performing auditory and visual cognitive tasks performed with interstimulus intervals of 2 and 5 s. RESULTS: Visual tasks led to reductions in sway area and sway variability in the anterior-posterior direction compared to auditory tasks (ps ≤ 0.05). The interstimulus interval did not lead to a change in sway, except for a small change in the medial-lateral direction for the 2-s interval compared to the 5-s interval (p = 0.05). DISCUSSION AND CONCLUSIONS: Results suggest that the interstimulus interval had a very limited effect on postural sway. The modality of the cognitive task had a greater effect on postural sway, as visual cognitive tasks yielded smaller sway area and anterior-posterior sway variability than auditory conditions. Visual stimuli may have acted as an anchor, yielding reduced sway.

Reaction Time of Healthy Older Adults Is Reduced While Walking Fast.

Attentional requirements of walking at various speeds in older adults were examined. Twenty healthy older adults (69.9 ± 2.77 years; 8 males) were asked to walk a distance of 10 m at a self-selected speed as well as 30% quicker and 30% slower. Concurrently, reaction time (RT) was evaluated by having participants respond as fast as possible to randomly presented auditory stimuli. Results reveal that an accelerated walking speed generated faster RT than slow and self-selected speeds, while no difference was found between the latter. Faster RTs during an accelerated walking speed may have been precipitated by the reduced equilibrium demands of the task.

Cognitive tasks promote automatization of postural control in young and older adults.

Researchers looking at the effects of performing a concurrent cognitive task on postural control in young and older adults using traditional center-of-pressure measures and complexity measures found discordant results. Results of experiments showing improvements of stability have suggested the use of strategies such as automatization of postural control or stiffening strategy. This experiment aimed to confirm in healthy young and older adults that performing a cognitive task while standing leads to improvements that are due to automaticity of sway by using sample entropy. Twenty-one young adults and twenty-five older adults were asked to stand on a force platform while performing a cognitive task. There were four cognitive tasks: simple reaction time, go/no-go reaction time, equation and occurrence of a digit in a number sequence. Results demonstrated decreased sway area and variability as well as increased sample entropy for both groups when performing a cognitive task. Results suggest that performing a concurrent cognitive task promotes the adoption of an automatic postural control in young and older adults as evidenced by an increased postural stability and postural sway complexity.

Continuous and difficult discrete cognitive tasks promote improved stability in older adults.

Directing attention away from postural control and onto a cognitive task affords the emergence of automatic control processes. Perhaps the continuous withdrawal of attention from the postural task facilitates an automatization of posture as opposed to only intermittent withdrawal; however this is unknown in the aging population. Twenty older adults (69.9±3.5years) stood with feet together on a force platform for 60s while performing randomly assigned discrete and continuous cognitive tasks. Participants were instructed to stand comfortably with their arms by their sides while verbally responding to the auditory stimuli as fast as possible during the discrete tasks, or mentally performing the continuous cognitive tasks. Participants also performed single-task standing. Results demonstrate significant reductions in sway amplitude and sway variability for the difficult discrete task as well as the continuous tasks relative to single-task standing. The continuous cognitive tasks also prompted greater frequency of sway in the anterior-posterior direction compared to single-standing and discrete tasks, and greater velocity in both directions compared to single-task standing, which could suggest ankle stiffening. No differences in the simple discrete condition were shown compared to single-task standing, perhaps due to the simplicity of the task. Therefore, we propose that the level of difficulty of the task, the specific neuropsychological process engaged during the cognitive task, and the type of task (discrete vs. continuous) influence postural control in older adults. Dual-tasking is a common activity of daily living; this work provides insight into the age-related changes in postural stability and attention demand.

The effects of attentional focus and cognitive tasks on postural sway may be the result of automaticity.

Research reveals improvements in postural control when focus is placed on movement effects rather than movement production, and further improvements during the performance of a concurrent cognitive task. It has yet to be determined if these changes are due to the use of an ankle stiffening strategy or to the use of more automatic postural control processes. The objectives of the present study were to replicate the effect of attentional focus and cognitive tasks on postural control and to test that no change occurs in lower leg muscle activity in these conditions. Twenty five healthy young adults (20.7±2.76years, 10 male) were asked to stand still while performing various tasks: baseline standing, internally focusing on minimizing movement of the ankles, externally focusing on minimizing movement of an apparatus placed on their ankle joint, and two cognitive tasks consisting of counting and simultaneously summing one or two single digits in a series of three-digit numbers. Compared to baseline and internal focus, sway decreased in external focus conditions and decreased further in cognitive task conditions. Furthermore, sway velocity increased in cognitive task conditions and sway frequency increased in the medial-lateral direction in the more difficult cognitive task. Finally, no effect of condition was found on muscle activity around the ankle joint. Collectively, the findings lend support to the hypothesis that changes in postural control were the result of an automatic type of postural control rather than due to stiffening occurring at the ankle joint.

Continuous Cognitive Task Promotes Greater Postural Stability than an Internal or External Focus of Attention in Older Adults.

Background/Study Context: Recent evidence suggests that removing attention from postural control using either an external focus or a cognitive task will improve stability in healthy young adults. Due to increases in attentional requirements of upright stance in older adults, it is unclear if similar benefits would be observed in this population. The aim of the present study was to examine the effect of attentional focus and of a continuous cognitive task on postural control in older adults. METHODS: Sixteen healthy older adults (71.9 ± 4.32 years) were asked to stand quietly on a force platform with feet together in three different conditions: internal focus (minimizing movement of the hips), external focus (minimizing movement of markers placed on the hips), and cognitive task (silently counting the occurrence of a single digit in a 3-digit number sequence). A one-way analysis of variance with repeated measures on condition was performed for each postural control measure. RESULTS: Hypotheses were partially supported because the cognitive task led to greater stability than both focus conditions, as evidenced by a smaller sway area (p < .01, ηp2 = .41), reduced sway variability (anterior-posterior: p = .001, ηp2 = .37; medial-lateral: p < .0001, ηp2 = .49), and higher mean power frequency in the anterior-posterior direction (p = .01, ηp2 = .78). However, no difference was observed between internal and external focus conditions. CONCLUSIONS: A continuous, attention-demanding cognitive task significantly improved stability in older adults compared with an internal or external focus of attention. This suggests that older adults were able to effectively allocate their attention away from postural control, allowing a more automatic type of control to operate. Future studies should investigate a variety of cognitive tasks to determine the degree of postural improvement that can be observed in older adults.

Reaction Time Is Slower When Walking at a Slow Pace in Young Adults.

Limited research has examined attentional requirements of walking at various speeds. Twenty young adults were asked to walk 10 m at their preferred pace, 30% faster or 30% slower while verbally responding "top" as fast as possible to random auditory stimuli. Slow walking demonstrated significantly longer reaction time (RT; 457 ± 91 ms) than preferred (423 ± 80 ms) and fast (396 ± 73 ms) walking speeds, F(2, 38) = 13.4, p < .001; η(2)p = .414. Walking at a preferred pace also led to longer RT than walking at a fast pace (p < .05). Slower RT during slow walking may be attributed to increased task complexity, energy requirements and equilibrium demands. Faster RTs during fast walking could be due to familiarity of the task, higher arousal levels, and similar task instructions compared to slower speeds.

Continuous Cognitive Tasks Improve Postural Control Compared to Discrete Cognitive Tasks.

Research suggests that postural control synergies are sensitive to cognitive manipulations; however, the impact of different types of cognitive tasks on postural control remains inconclusive. The authors examined the effect of discrete and continuous tasks on postural control. Sixteen healthy young adults (M age = 22.7 ± 2.2 years) stood with feet together on a force platform while performing randomly assigned discrete and continuous cognitive tasks. Results demonstrated marked improvements in the area of 95% confidence ellipse and the standard deviation of the center of pressure in the anterior-posterior and medial-lateral directions for continuous compared to discrete tasks. This reinforces the notion that continuous tasks are sufficient in providing less opportunity to consciously attend to postural control, thereby facilitating automatic postural control.

Continuous cognitive task promotes greater postural stability than an internal or external focus of attention.

Research has demonstrated clear advantages of using an external focus of attention in postural control tasks, presumably since it allows a more automatic control of posture to emerge. However, the influence of cognitive tasks on postural stability has produced discordant results. This study aimed to compare the effects of an internal focus of attention, an external focus of attention and a continuous cognitive task on postural control. Twenty healthy participants (21.4±2.6 years) were recruited for this study. They were asked to stand quietly on a force platform with their feet together in three different attentional focus conditions: an internal focus condition (minimizing movements of the hips), an external focus condition (minimizing movements of markers placed on the hips) and a cognitive task condition (silently counting the total number of times a single digit was verbalized in a 3-digit sequence comprised of 30 numbers). Results demonstrated improved stability while performing the cognitive task as opposed to the internal and external focus conditions, as evidenced by a reduction in sway area, sway variability in the anterior-posterior (AP) and medial-lateral (ML) directions, and mean velocity (ML only). Results suggest that the use of a continuous cognitive task permits attention to be withdrawn from the postural task, thereby facilitating a more automatic control of posture.

Impact of age and obstacles on navigation precision and reaction time during blind navigation in dual-task conditions.

Navigation without vision is a skill that is often employed in our daily lives, such as walking in the dark at night. Navigating without vision to a remembered target has previously been studied. However, little is known about the impact of age or obstacles on the attentional demands of a blind navigation task. This study examined the impacts of age and obstacles on reaction time (RT) and navigation precision during blind navigation in dual-task conditions. The aims were to determine the effects of age, obstacles, and auditory stimulus location on RT and navigation precision in a blind navigation task. Ten healthy young adults (24.5±2.5 years) and ten healthy older adults (69.5±2.9 years) participated in the study. Participants were asked to walk to a target located 8m ahead. In half the trials, the path was obstructed with hanging obstacles. Participants performed this task in the absence of vision, while executing a discrete RT task. Results demonstrated that older adults presented increased RT, linear distance travelled (LDT), and obstacle contact; that obstacle presence significantly increased RT compared to trials with no obstacles; and that an auditory stimulus emitted early versus late in the path increased LDT. Results suggest that the attentional demands of blind navigation are higher in older than young adults, as well as when obstacles are present. Furthermore, navigation precision is affected by age and when participants are distracted by the secondary task early in navigation, presumably because the secondary task interferes with path estimation.