Task prioritization in aging: effects of sensory information on concurrent posture and memory performance

In older adults, cognitive resources play a key role in maintaining postural stability. In the present study, we evaluated whether increasing postural instability using sway referencing induces changes in resource allocation in dual-task performance leading older adults to prioritize the more age-salient posture task over a cognitive task. Young and older adults participated in the study which comprised two sessions. In the first session, three posture tasks (stable, sway reference visual, sway reference somatosensory) and a working memory task (n-back) were examined. In the second session, single- and dual-task performance of posture and memory were assessed. Postural stability improved with session. Participants were more unstable in the sway reference conditions, and pronounced age differences were observed in the somatosensory sway reference condition. In dual-task performance on the stable surface, older adults showed an almost 40% increase in instability compared to single-task. However, in the sway reference somatosensory condition, stability was the same in single- and dual-task performance, whereas pronounced (15%) costs emerged for cognition. These results show that during dual-tasking while standing on a stable surface, older adults have the flexibility to allow an increase in instability to accommodate cognitive task performance. However, when instability increases by means of compromising somatosensory information, levels of postural control are kept similar in single- and dual-task, by utilizing resources otherwise allocated to the cognitive task. This evidence emphasizes the flexible nature of resource allocation, developed over the life-span to compensate for age-related decline in sensorimotor and cognitive processing.     

The effects of aging on postural control and selective attention when stepping down while performing a concurrent auditory response task

Purpose

To investigate the effects of aging on postural control and cognitive performance in single- and dual-tasking.

Methods

A cross-sectional comparative design was conducted in a university motion analysis laboratory. Young adults (n = 30; age 21.9 ± 2.4 years) and older adults (n = 30; age 71.9 ± 6.4 years) were recruited. Postural control after stepping down was measured with and without performing a concurrent auditory response task. Measurement included: (1) reaction time and (2) error rate in performing the cognitive task; (3) total sway path and (4) total sway area after stepping down.

Results

Our findings showed that the older adults had significantly longer reaction times and higher error rates than the younger subjects in both the single-tasking and dual-tasking conditions. The older adults had significantly longer reaction times and higher error rates when dual-tasking compared with single-tasking, but the younger adults did not. The older adults demonstrated significantly less total sway path, but larger total sway area in single-leg stance after stepping down than the young adults. The older adults showed no significant change in total sway path and area between the dual-tasking and when compared with single-tasking conditions, while the younger adults showed significant decreases in sway.

Conclusion

Older adults prioritize postural control by sacrificing cognitive performance when faced with dual-tasking.     

Age-related changes in prefrontal activity during walking in dual-task situations: A fNIRS study

Background

Previous studies suggest that the human gait is under control of higher-order cognitive processes, located in the frontal lobes, such that an age-related degradation of cognitive capabilities has a negative impact on gait.

Methods

Using functional Near-Infrared-Spectroscopy (fNIRS) we investigate the frontocortical hemodynamic correlates of dual-task walking in two conditions. 15 young and 10 older individuals walked on a treadmill while completing concurrent tasks that had either visual (checking) or verbal-memory (alphabet recall) demands. We compared subjects' motor performance, as well as their prefrontal activity in single- and dual-task walking.

Results

Our behavioral data partly confirm previous accounts on higher dual-task costs in stepping parameters (i.e., decreased step duration) in old age, particularly with a visual task and negative dual-task cost (i.e., improved performance) during the verbal task in young adults. Functional imaging data revealed little change of prefrontal activation from single- to dual-task walking in young individuals. In the elderly, however, prefrontal activation substantially decreased during dual-task walking with a complex visual task.

Conclusion

We interpret these findings as evidence for a shift of processing resources from the prefrontal cortex to other brain regions when seniors face the challenge of walking and concurrently executing a visually demanding task.     

Age and CGG-repeat length are associated with neuromotor impairments in at-risk females with the FMR1 premutation

Recent studies report a higher risk of dementia and motor symptoms in females with the fragile X mental retardation 1 premutation (PM-carriers) than has hitherto been appreciated. Here, we use dual-task gait paradigms to identify potential markers of cognitive and motor decline in female PM-carriers. Spatiotemporal gait characteristics and variability of gait were assessed during single- and dual-task conditions in 28 female PM-carriers (mean age 41.32 ± 8.03 years) and 31 female controls with normal fragile X mental retardation 1 alleles (mean age 41.61 ± 8.30 years). Despite comparable gait characteristics at baseline, gait performance was significantly poorer for PM-carriers when performing concurrent working memory tasks (counting backwards by 3′s or 7′s) when compared with controls. Correlational analyses showed that low working memory capacity was significantly associated with dual-task interference for the gait domains of pace (speed, step length) and variability (step time, swing time) in PM-carriers. Multiple regression analyses further showed that the interaction between age and CGG repeat length was strongly predictive of gait variability during dual-task performance. These findings indicate for the first time that vulnerability in specific domains of gait control may act as sensitive surrogate markers of future decline in female PM-carriers.     

A cognitive dual task affects gait variability in patients suffering from chronic low back pain

Chronic pain and gait variability in a dual-task situation are both associated with higher risk of falling. Executive functions regulate (dual-task) gait variability. A possible cause explaining why chronic pain increases risk of falling in an everyday dual-task situation might be that pain interferes with executive functions and results in a diminished dual-task capability with performance decrements on the secondary task. The main goal of this experiment was to evaluate the specific effects of a cognitive dual task on gait variability in chronic low back pain (CLBP) patients. Twelve healthy participants and twelve patients suffering from CLBP were included. The subjects were asked to perform a cognitive single task, a walking single task and a motor-cognitive dual task. Stride variability of trunk movements was calculated. A two-way ANOVA was performed to compare single-task walking with dual-task walking and the single cognitive task performance with the motor-cognitive dual-task performance. We did not find any differences in both of the single-task performances between groups. However, regarding single-task walking and dual-task walking, we observed an interaction effect indicating that low back pain patients show significantly higher gait variability in the dual-task condition as compared to controls. Our data suggest that chronic pain reduces motor-cognitive dual-task performance capability. We postulate that the detrimental effects are caused by central mechanisms where pain interferes with executive functions which, in turn, might contribute to increased risk of falling.     

Neurological Gait Abnormalities Moderate the Functional Brain Signature of the Posture First Hypothesis

The posture first hypothesis suggests that under dual-task walking conditions older adults prioritize gait over cognitive task performance. Functional neural confirmation of this hypothesis, however, is lacking. Herein, we determined the functional neural correlates of the posture first hypothesis and hypothesized that the presence of neurological gait abnormalities (NGA) would moderate associations between brain activations, gait and cognitive performance. Using functional near-infrared spectroscopy we assessed changes in oxygenated hemoglobin levels in the pre-frontal cortex (PFC) during normal walk and walk while talk (WWT) conditions in a large cohort of non-demented older adults (n = 236; age = 75.5 ± 6.49 years; female = 51.7 %). NGA were defined as central (due to brain diseases) or peripheral (neuropathic gait) following a standardized neurological examination protocol. Double dissociations between brain activations and behavior emerged as a function of NGA. Higher oxygenation levels during WWT were related to better cognitive performance (estimate = 0.145; p < 0.001) but slower gait velocity (estimate = ?6.336, p < 0.05) among normals. In contrast, higher oxygenation levels during WWT among individuals with peripheral NGA were associated with worse cognitive performance (estimate = ?0.355; p < 0.001) but faster gait velocity (estimate = 14.855; p < 0.05). Increased activation in the PFC during locomotion may have a compensatory function that is designed to support gait among individuals with peripheral NGA.     

Introducing Statistical Persistence Decay: A Quantification of Stride-to-Stride Time Interval Dependency in Human Gait

Stride-to-stride time intervals during human walking are characterised by predictability and statistical persistence quantified by sample entropy (SaEn) and detrended fluctuation analysis (DFA) which indicates a time dependency in the gait pattern. However, neither analyses quantify time dependency in a physical or physiological interpretable time scale. Recently, entropic half-life (ENT½) has been introduced as a measure of the time dependency on an interpretable time scale. A novel measure of time dependency, based on DFA, statistical persistence decay (SPD), was introduced. The present study applied SaEn, DFA, ENT½, and SPD in known theoretical signals (periodic, chaotic, and random) and stride-to-stride time intervals during overground and treadmill walking in healthy subjects. The analyses confirmed known properties of the theoretical signals. There was a significant lower predictability (p = 0.033) and lower statistical persistence (p = 0.012) during treadmill walking compared to overground walking. No significant difference was observed for ENT½ and SPD between walking condition, and they exhibited a low correlation. ENT½ showed that predictability in stride time intervals was halved after 11–14 strides and SPD indicated that the statistical persistency was deteriorated to uncorrelated noise after ~50 strides. This indicated a substantial time memory, where information from previous strides affected the future strides.     

Effects of a secondary task on obstacle avoidance in healthy young adults

Research on attention and gait stability has suggested that the process of recovering gait stability requires attentional resources, but the effect of performing a secondary task on stability during obstacle avoidance is poorly understood. Using a dual-task paradigm, the present experiment investigated the extent to which young adults are able to respond to a secondary auditory Stroop task (requiring executive attentional network resources) concurrently with obstacle crossing during gait when compared with performing unobstructed walking or sitting (control task). Our results demonstrated that as the level of difficulty in the postural task increased, there was a significant reduction in verbal response time from congruent to incongruent conditions in the auditory Stroop task, but no differences in gait parameters, indicating that these postural tasks require attention, and that young adults use a strategy of modulating the auditory Stroop task performance while keeping stable gait performance under the dual-task situations. Our findings suggest the existence of a hierarchy of control within both postural task (obstacle avoidance requires the most information processing resources) and dual-task (with gait stability being a priority) conditions.     

The contribution of postural control and bilateral coordination to the impact of dual tasking on gait

The simultaneous performance of a cognitive task while walking typically alters the gait pattern. In some populations, these alterations have been associated with an increased risk of falls, motivating study of this response from the clinical perspective. The mechanisms responsible for these effects are not fully understood. The concurrent requirement to control upright posture and stepping, a bilaterally coordinated rhythmic task, may be the cause of this so-called dual-tasking effect. To evaluate this possibility, the present study was designed to isolate the individual contribution of these two demands by assessing the effects of cognitive loading on standing (i.e., postural control without bilateral coordination of stepping), cycling (i.e., bilateral coordination similar to stepping, but with minimal postural demands), and walking. We also investigated the effects of aging and parkinsonism on the performance of these three tasks in response to cognitive loading, also referred to as a dual task. Twenty-one healthy young adults, 15 healthy older adults, and 18 patients with Parkinson’s disease were assessed while walking, standing, and cycling, with and without an additional cognitive load. In the young adults, the performance on the two motor tasks that involved bilateral coordination deteriorated significantly in response to the dual task, while standing was not impacted. Similar results, although less robust, were observed among the healthy older adults. In contrast, among the patients with Parkinson’s disease, the dual-task costs, i.e., the impact of the simultaneously performed cognitive task on the gait pattern, were high in all motor tasks. These findings suggest that walking is especially vulnerable to cognitive loading, in part, because of the unique sensitivity of bilateral coordination of limb movements to the effects of dual tasking.     

The ability of people with Parkinson’s disease to modify dual-task performance in response to instructions during simple and complex walking tasks

Gait impairments are a common and consequential motor symptom in Parkinson’s disease (PD). A cognitive strategy that incorporates instructions to concentrate on specific parameters of walking is an effective approach to gait rehabilitation for persons with PD during single-task and simple dual-task walking conditions. This study examined the ability to modify dual-task walking in response to instructions during a complex walking task in people with PD compared to healthy older adults (HOA). Eleven people with PD and twelve HOA performed a cognitive task while walking with either a usual base or a narrow base of support. Dual-task walking and cognitive task performance were characterized under two conditions—when participants were instructed focus on walking and when they were instructed to focus on the cognitive task. During both usual base and narrow base walking, instructions affected cognitive task response latency, with slower performance when instructed to focus on walking compared to the cognitive task. Regardless of task or instructions, cognitive task performance was slower in participants with PD compared to HOA. During usual base walking, instructions influenced gait speed for both people with PD and HOA, with faster gait speed when instructed to focus on walking compared to the cognitive task. In contrast, during the narrow base walking, instructions affected gait speed only for HOA, but not for people with PD. This suggests that among people with PD the ability to modify walking in response to instructions depends on the complexity of the walking task.     

Gait adaptations to simultaneous cognitive and mechanical constraints

Previous studies have shown that walking is not a purely automatic motor task but places demands on sensory and cognitive systems. We set out to investigate whether complex walking tasks, as when walking down a steeper gradient while performing a concurrent cognitive task, would demand gait adaptation beyond those required for walking under low-challenge conditions. Thirteen healthy young individuals walked at their self-selected speed on a treadmill at different inclinations (0, −5 and −10%). Gait spatio-temporal measures, pelvis angular excursion, and sacral centre of mass (CoM) motion were acquired while walking or while walking and performing a mental tracking task. Repeated-measures ANOVAs revealed that decreasing treadmill inclination from 0 to −10% resulted in significant decreased walking speed (P < 0.001), decreased stride length (P < 0.001), increased pelvis tilt (P = 0.006) and obliquity variability (P = 0.05), decreased pelvis rotation (P = 0.02), and increased anterio-posterior (A-P) CoM displacement (P = 0.015). Compared to walking alone, walking under dual-task condition resulted in increased step width (P < 0.001), and increased medio-lateral (M-L) CoM displacement (P = 0.039) regardless of inclination grade, while sagittal plane dynamics did not change. Findings suggest that gait adapts differently to cognitive and mechanical constraints; the cognitive system is more actively involved in controlling frontal than sagittal plane gait dynamics, while the reverse is true for the mechanical system. Finally, these findings suggest that gait adaptations maintain the ability to perform concurrent tasks while treadmill walking in healthy young adults.     

Footfall Placement Variability and Falls in Multiple Sclerosis

Gait variability (i.e., fluctuations in walking) provides unique information about the control of movement and is associated with falls. This investigation examined the association between gait variability and falls in persons with multiple sclerosis (MS) and healthy controls. Traditional distributional metrics of gait variability (i.e., coefficient of variation (CV)) and a novel metric based on Fourier series analysis of footfall placement variability were determined for 41 individuals with MS and 20 age- and sex-matched controls. Spatiotemporal parameters of gait were collected using a 7.9 m electronic walkway that recorded individual footfalls during steady state comfortable walking. Persons with MS were divided into two groups based on fall history (non-fallers and recurrent fallers). Overall, persons with MS had greater gait variability than controls as indexed by CV and Fourier-based variability (p’s < 0.05). Moreover, recurrent fallers with MS had greater Fourier-based variability than non-fallers with MS (p = 0.025), whereas there was no difference in MS groups in traditional gait variability metrics (p > 0.05). These observations highlight that footfall placement variability is related to fall status in MS. Future work determining the sensitivity of footfall placement variability to dysfunction is warranted.     

Balance and gait performance after maximal and submaximal endurance exercise in seniors: is there a higher fall-risk?

Impaired balance and gait performance increase fall-risk in seniors. Acute effects of different exercise bouts on gait and balance were not yet addressed. Therefore, 19 healthy seniors (10 women, 9 men, age: 64.6 ± 3.2 years) were examined on 3 days. After exhaustive treadmill testing, participants randomly completed a 2-km treadmill walking test (76 ± 8 % VO_2max) and a resting control condition. Standing balance performance (SBALP) was assessed by single limb-eyes opened (SLEO) and double limb-eyes closed (DLEC) stance. Gait parameters were collected at comfortable walking velocity. A condition × time interaction of center of pressure path length (COP_path) was observed for both balance tasks (p < 0.001). Small (Cohen’s d = 0.42, p = 0.05) and large (d = 1.04, p < 0.001) COP_path increases were found after 2-km and maximal exercise during DLEC. Regarding SLEO, slightly increased COP_path occurred after 2-km walking (d = 0.29, p = 0.65) and large increases after exhaustive exercise (d = 1.24, p < 0.001). No significant differences were found for gait parameters. Alterations of SBALP after exhaustive exercise might lead to higher fall-risk in seniors. Balance changes upon 2-km testing might be of minor relevance. Gait is not affected during single task walking at given velocities.     

Effect of attentional interference on balance recovery in older adults

Since most working memory (WM) tasks used in dual-task (DT) postural paradigms include both storage and processing of information, it is difficult to determine the extent to which each of these contributes to interference with balance control. In the current study, a change-detection task (changes in colored squares between two presentation events) that estimates visual working memory capacity (VWMC) was paired with tasks of increasing postural demand (stance, perturbation) in young adults (YAs) and older adults (OAs) and performance compared between the two postural conditions and across the two populations. The change-detection task was selected as it requires storage of information without updating or manipulation; 34 YAs, 34 OAs, and five frail OAs were recruited. A significant reduction in VWMC occurred with increasing postural demand during the perturbation condition for both YAs (p < 0.01) and OAs (p < 0.001). VWMC was also significantly lower for OAs than YAs in the control condition (1.8 ± 0.7 vs. 2.8 ± 0.6) (p < 0.001). OAs showed a significant increase in the number of trials in which steps or rise to toes occurred during recovery between single-task (ST) and DT (p < 0.05; p < 0.05). OAs also showed a significant increase in normalized tibialis anterior amplitude (p < 0.001) following perturbations. YAs showed an increase in normalized area under the center of pressure trajectory and in AP forces (nAcopx1: p < 0.001; nFap1: p < 0.05) during the DT condition.     

Transtibial Amputee Joint Motion has Increased Attractor Divergence During Walking Compared to Non-Amputee Gait

The amputation and subsequent prosthetic rehabilitation of a lower leg affects gait. Dynamical systems theory would predict the use of a prosthetic device should alter the functional attractor dynamics to which the system self-organizes. Therefore, the purpose of this study was to compare the largest Lyapunov exponent (a nonlinear tool for assessing attractor dynamics) for amputee gait compared to healthy non-amputee individuals. Fourteen unilateral, transtibial amputees and fourteen healthy, non-amputee individuals ambulated on a treadmill at preferred, self-selected walking speed. Our results showed that the sound hip (p = 0.013), sound knee (p = 0.05), and prosthetic ankle (p = 0.023) have significantly greater largest Lyapunov exponents than healthy non-amputees. Furthermore, the prosthetic ankle has a significantly greater (p = 0.0.17) largest Lyapunov exponent than the sound leg ankle. These findings indicate attractor states for amputee gait with increased divergence. The increased attractor divergence seems to coincide with decreased ability for motor control between the natural rhythms of the individual and those of the prosthetic device. Future work should consider the impact of different prostheses and rehabilitation on the attractor dynamics.     

Error performances of a model-based biplane fluoroscopic system for tracking knee prosthesis during treadmill gait task

Roentgen stereophotogrammetry analysis technique allows an accurate measurement of knee joint prosthesis position and orientation using two X-ray images. Although this technique is used generally during static procedure, it is possible to use it with a biplane fluoroscopic system to measure the prosthesis kinematics during functional tasks (e.g., gait, squat, jump) performed in a laboratory environment. However, the performance of the system in terms of errors for the measurements and the model-based matching algorithm are not well known for dynamic tasks such as walking. The goal of this study was to estimate the static and dynamic errors of a model-based biplane fluoroscopic system for a treadmill gait task and analyze the error performance according to the speed and location of the knee joint prosthesis relative to X-ray sources. The results show a static maximum error (RMSE) of 0.13° for orientation and 0.06 mm for position for prosthesis components. The dynamic errors were different for each axis of the acquisition system and each prosthesis component. The largest dynamic error was along the vertical axis for the position (RMSE = 2.42 mm) and along the medio-lateral axis (perpendicular to movement) for the orientation (RMSE = 0.95°). As expected, the error depends on the distance between the prosthesis and the source in the acquisition system as well as the linear and angular velocity of the movement. The most accurate dynamic measure was around the centroid of the acquisition system, while kinematics measurements close to the X-rays detectors gave the worst errors.     

Gait speed is more challenging than cognitive load on the stride-to-stride variability in individuals with anterior cruciate ligament deficiency

Background

Several investigations have studied gait variability of individuals with anterior cruciate ligament (ACL) deficiency; however, the effect of dual-tasking on the gait variability of these individuals remained unclear. The aim of the present study was to determine the effect of gait speed and dual-tasking on knee flexion–extension variability in subjects with and without ACL deficiency.

Stress and decision making: neural correlates of the interaction between stress,executive functions,and decision making under risk

Stress and additional load on the executive system, produced by a parallel working memory task, impair decision making under risk. However, the combination of stress and a parallel task seems to preserve the decision-making performance [e.g., operationalized by the Game of Dice Task (GDT)] from decreasing, probably by a switch from serial to parallel processing. The question remains how the brain manages such demanding decision-making situations. The current study used a 7-tesla magnetic resonance imaging (MRI) system in order to investigate the underlying neural correlates of the interaction between stress (induced by the Trier Social Stress Test), risky decision making (GDT), and a parallel executive task (2-back task) to get a better understanding of those behavioral findings. The results show that on a behavioral level, stressed participants did not show significant differences in task performance. Interestingly, when comparing the stress group (SG) with the control group, the SG showed a greater increase in neural activation in the anterior prefrontal cortex when performing the 2-back task simultaneously with the GDT than when performing each task alone. This brain area is associated with parallel processing. Thus, the results may suggest that in stressful dual-tasking situations, where a decision has to be made when in parallel working memory is demanded, a stronger activation of a brain area associated with parallel processing takes place. The findings are in line with the idea that stress seems to trigger a switch from serial to parallel processing in demanding dual-tasking situations.     

Biofeedback Training Effects on Minimum Toe Clearance Variability During Treadmill Walking

A number of variability analysis techniques, including Poincaré plots and detrended fluctuation analysis (DFA) were used to investigate minimum toe clearance (MTC) control during walking. Ten young adults walked on a treadmill for 10 min at preferred speed in three conditions: (i) no-intervention baseline, (ii) with biofeedback of MTC within a target range, and (iii) no-biofeedback retention. Mean, median, standard deviation (SD), and inter quartile range of MTC during biofeedback (45.57 ± 11.65, 44.98 ± 11.57, 7.08 ± 2.61, 8.58 ± 2.77 mm, respectively) and retention (56.95 ± 20.31, 56.69 ± 20.94, 10.68 ± 5.41, 15.38 ± 10.19 mm) were significantly greater than baseline (30.77 ± 9.49, 30.51 ± 9.49, 3.04 ± 0.77, 3.66 ± 0.91 mm). Relative to baseline, skewness was reduced in biofeedback and retention but only significantly for retention (0.88 ± 0.51, 0.63 ± 0.55, and 0.40 ± 0.40, respectively). Baseline Poincaré measures (SD1 = 0.25, SD2 = 0.34) and DFA (α1 = 0.72 and α2 = 0.64) were lower than biofeedback (SD1 = 0.58, SD2 = 0.83, DFA α1 = 0.76 and α2 = 0.92) with significantly greater variability in retention compared to biofeedback only in the long-term SD2 and α2 analyses. Increased DFA longer-term correlations α2 in retention confirm that a novel gait pattern was acquired with a longer-term variability structure. Short- and long-term variability analyses were both useful in quantifying gait adaptations with biofeedback. The findings provide evidence that MTC can be modified with feedback, suggesting future applications in gait training procedures for impaired populations designed to reduce tripping risk.     

Cognitive task effects on gait stability following concussion

The purpose of this study was to determine how two different types of concurrent tasks affect gait stability in patients with concussion and how balance is maintained. Fourteen individuals suffering from a grade II concussion and 14 matched controls performed a single task of level walking and two types of concurrent tasks during level walking: a discrete reaction time task and a continuous sequential question and answer task. Common gait spatial/temporal measurements, whole-body center of mass motion, and the center of pressure trajectory were recorded. Concussed individuals demonstrated differences in gait while performing single-task level walking and while being challenged with a more difficult secondary task compared to normal controls. Concussed individuals adopted a slower, more conservative gait strategy to maintain stability, but still exhibited signs of instability with center of mass deviations in the coronal plane increasing by 13% during the question and answer dual-task and 26% more than control subjects. Trends of attentional deficits were present with the question and answer task, while the reaction time task seemed to help concussed individuals be more alert to their gait and stability. Recommendations for a sensitive testing protocol of deficits following concussion are explained.