Effect of competing attentional demands on perturbation-evoked stepping reactions and associated gaze behavior in young and older adults

Background. Rapid stepping reactions are a prevalent response to sudden loss of balance and are thought to play a crucial role in preventing falls. Previous dual-task studies, involving concurrent performance of step reactions and a visuomotor tracking task, indicated that online visual attention was not required to guide the step, even when nearby objects increased demands for accurate foot movement. However, the planning and execution of the step apparently required attentional resources initially allotted to the tracking task. Reallocation of these resources (‘‘attention switching’’) was delayed in older adults. The present study examined the influence of the competition for attentional resources by comparing trials performed with and without the concurrent task. Methods. Unpredictable platform perturbations were used to evoke rapid forward stepping reactions in healthy young and older adults. Challenging obstacles and/or step targets increased demands for accurate foot motion in some trials. A concurrent tracking task was performed in half of the trials. Results. Although participants looked down more frequently in the absence of the tracking task, the ability to clear the obstacle or land on the step target and other spatiotemporal features of the stepping reactions were largely unaffected. There was, however, one notable exception: In older adults, the duration and amplitude of the ‘‘anticipatory postural adjustment’’ that preceded foot lift were reduced in tracking trials, resulting in increased lateral center-of-mass motion. Conclusion. Impaired attention switching apparently compromised the control of lateral stability during stepping reactions in older adults, and may be an important contributor to increased risk of falling.This study was supported by an operating grant (MOP-13355) from the Canadian Institutes of Health Research (CIHR). B.E.M. was a CIHR Senior Investigator, and W.E.M. held a Canada Research Chair in Neurorehabilitation. J.L.Z. held scholarships from the Natural Sciences and Engineering Research Council and the Ontario Neurotrauma Foundation

Stepping to recover balance in complex environments: is online visual control of the foot motion necessary or sufficient?

Rapid step reactions evoked by balance perturbation must accommodate constraints on limb motion imposed by obstacles and other environmental features. Recent results suggest that the required visuospatial information (VSI) is acquired and stored “proactively”, prior to perturbation onset (PO); however, the extent to which “online” (post-PO) visual feedback can contribute is not known. To study this, we used large unpredictable platform perturbations to evoke rapid step reactions, while subjects wore liquid crystal goggles that occluded vision: (1) prior to PO (forcing use of online-VSI), (2) after PO (forcing use of stored-VSI), or (3) not at all (normal-VSI). Subjects stood behind a barrier in which the location of a narrow slot, through which the foot had to be moved during forward step reactions, was varied unpredictably between trials. Within subjects who were able to do the task (6 of 8 young adults tested), responses in stored-VSI and normal-VSI trials were very similar. However, in online-VSI trials, the foot-off time for the step through the slot was delayed (by ∼50 ms, on average). Presumably, this delay allowed more time to acquire and process online-VSI regarding the required foot trajectory, yet subjectswere still more likely to select the “wrong” foot (contralateral to the slot location) and to contact the barrier while moving the foot through the slot, in online-VSI trials. These results suggest a critical role for stored-VSI during the earliest phase of the step, in selecting the step limb and planning the initial trajectory. Online acquisition and processing of the required VSI may be too slow to allow effective control of this early phase, particularly in situations where the demands for accurate foot motion are high.This study was supported by an operating grant (#MOP-13355) from the Canadian Institutes of Health Research (CIHR)

Gaze behavior of older adults during rapid balance-recovery reactions

Background: Rapid stepping reactions are a prevalent response to sudden loss of balance and play a crucial role in preventing falls. A previous study indicated that young adults are able to guide these stepping reactions amid challenging environmental constraints using ‘stored’ visuospatial information. This study addressed whether healthy older adults also use ‘stored’ visuospatial information in this manner, or are more dependent on ‘online’ visual control. Methods: Gaze behavior was recorded during rapid forward stepping reactions evoked by unpredictable platform perturbation, as subjects performed a concurrent task demanding visual attention. Challenging obstacles and/or step targets were used to increase demands for accurate foot motion. Twelve healthy older adults (61-73yrs) were compared to 12 young adults (22- 29yrs) tested in a previous study. Results: Similar to young adults, older subjects seldom redirected gaze downward in response to the perturbation (11% of trials), yet were commonly able to clear the obstacle (74% of trials) or land on the target (41% of trials) while stepping to recover balance. The threat posed by the obstacle apparently prompted older adults to initiate early downward saccades during a small proportion (18%) of obstacle trials; however, this did not improve ability to clear the obstacle. Conclusion: Aging did not alter the predominant visual-control strategy used to guide the stepping reactions. Both young and older persons typically used ‘stored’ visuospatial information, thereby allowing vision/attention to be switched to other demands during the stepping reaction and minimizing head/eye movements that could exacerbate the destabilizing effect of the balance perturbation

Redirection of gaze and switching of attention during rapid stepping reactions evoked by unpredictable postural perturbation

In many situations successful execution of a balance-recovery reaction requires visual information about the environment. In particular, reactions that involve rapid limb movements, such as stepping, must be controlled to avoid obstacles and accommodate other constraints on limb trajectory. However, it is unknown whether the central nervous system can acquire the necessary visuospatial information prior to perturbation onset or must, instead, redirect gaze at the floor during the execution of the stepping reaction. To study this we examined gaze behaviour, during rapid forward-directed stepping reactions triggered by unpredictable platform perturbation, in 12 healthy young adults. We also monitored switching of attention, as inferred from onset of significant error in performing a concurrent visuomotor tracking task. Obstacles and/or step targets were used as constraints, to increase demands for accurate foot movement. Downward gaze shifts towards the floor almost never occurred during stepping reactions when foot motion was unconstrained but did occur more frequently as the demands for accurate foot movement increased. Nonetheless, even in the most challenging condition (target plus obstacle), downward redirection of gaze occurred in less than 40% of the trials, and subjects were commonly well able to clear the obstacle and land the foot on the target without redirecting their gaze towards the floor. An apparent switching of attention, subsequent to perturbation onset, occurred frequently (>80% of trials) in all task conditions, independent of the gaze shifts. The findings indicate that visual fixation of the foot or floor was not essential for accurate control of the foot movement, nor was the apparent switching of attention that followed perturbation onset linked, in any consistent way, to overt changes in visual fixation. Spatial features of the support surface were apparently “remembered” prior to perturbation onset, thereby allowing both vision and attention to be directed to other demands during the execution of the balance reaction.This study was supported by grants from the Canadian Institutes of Health Research (CIHR) and the Ontario Neurotrauma Foundation. B.E.M. was a CIHR Senior Investigator and W.E.M. held a Canada Research Chair in neuro-rehabilitation. J.L.Z. held scholarships from the Natural Sciences and Engineering Research Council and from the Health Care, Technology and Place Program (University of Toronto)

Gaze behavior governing balance recovery in an unfamiliar and complex environment

Visuospatial information regarding obstacles and other environmental constraints on limb movement is essential for the successful planning and execution of stepping movements. Visuospatial control strategies used during gait and volitional stepping have been studied extensively; however, the visuospatial strategies that are used when stepping rapidly to recover balance in response to sudden postural perturbation are not well established. To study this, rapid forward stepping reactions were evoked by unpredictable support-surface acceleration while subjects stood amid multiple obstacles that moved intermittently and unpredictably prior to perturbation onset (PO). To prevent predictive control, subjects performed only one trial (their very first exposure to the perturbation and environment). Visual scanning of the obstacles and surroundings occurred prior to PO in all subjects; however, gaze was never redirected at the obstacles, step foot or landing site in response to the perturbation. Surprisingly, the point of gaze at time of foot-contact was consistently and substantially anterior to the step-landing site. Despite the apparent absence of 'online' visual feedback related to the foot movement, the compensatory step avoided obstacle contact in 10 of 12 young adults and 9 of 10 older subjects. The results indicate that the balance-recovery reaction was typically modulated on the basis of visuospatial environmental information that was acquired and continually updated prior to perturbation, as opposed to a strategy based on 'online' visual control. The capacity to do this was not adversely affected by aging, despite a tendency for older subjects to look downward less frequently than young adults.Supported by the Canadian Institutes of Health Research and the Ontario Neurotrauma Foundation

Determinants and consequences for standing balance of spontaneous weight-bearing on the paretic side among individuals with chronic stroke

Hemiparetic stroke patients commonly bear more weight on the non-paretic side which seems intuitively linked to unilateral control deficits. However, there is evidence that some post-stroke favour weighting the paretic side, which may be problematic given altered capacity of the paretic limb to contribute to the control of upright posture. This study explores the prevalence and clinical determinants of stance asymmetry, and the relationship between stance asymmetry and postural control among chronic stroke patients. Subjects (n=147; >6 months post-stroke) stood on two force plates in eyes-open and eyes-closed conditions; 59 were symmetric, 18 had paretic asymmetry (PA), and 70 had non-paretic asymmetry (NPA). Root mean square (RMS) of antero-posterior and medio-lateral centre-of-pressure under each limb and both limbs combined were compared. RMS of total medio-lateral centre-of-pressure was greater for both asymmetric groups compared with the symmetric group. PA subjects relied less on the loaded limb for control than NPA subjects and relied more on visual information for postural control than those who were symmetric. There were no differences in the characteristics of individuals between the PA and NPA groups. The loading of the paretic limb was not related to impaired postural control during stationary standing which was attributable, in part, to individuals relying on control from the non-paretic limb, in spite of lower vertical load, and a greater dependence on visual contributions. There was no evidence that greater loading on the paretic limb was related to persisting dyscontrol but may rather reflect a learned strategy.This study was supported by the Heart and Stroke Foundation Centre for Stroke Recovery, the Heart and Stroke Foundation of Canada, the Canadian Institutes of Health Research, and the Canadian Stroke Network

Reducing fall risk by improving balance control: Development, evaluation and knowledge-translation of new approaches

Problem: Falling is a leading cause of serious injury, loss of independence, and nursing-home admission in older adults. Impaired balance control is a major contributing factor. Methods: Results from our balancecontrol studies have been applied in the development of new and improved interventions and assessment tools. Initiatives to facilitate knowledge-translation of this work include setting up a new network of balance clinics, a research-user network and a research-user advisory board. Results: Our findings support the efficacy of the developed balance-training methods, balance-enhancing footwear, neuro-prosthesis, walker design, handrail-cueing system, and handrail-design recommendations in improving specific aspects of balance control. Impact on Knowledge Users: A new balance-assessment tool has been implemented in the first new balance clinic, a new balance-enhancing insole is available through pharmacies and other commercial outlets, and handrail design recommendations have been incorporated into 10 Canadian and American building codes. Work in progress is expected to have further impact.The studies by the authors described in this paper were supported by grants #NET-54025, #MAT-91865, #MOP-13355, #MOP-77772, #PPP-52016, #PPP-53686 and #IAP79705 from the Canadian Institutes of Health Research, aswell as funding from the Canadian Foundation for Innovation, the Ontario Neurotrauma Foundation, the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Stairway Manufacturers Association, the Sunnybrook Trust for Medical Research, Sunnybrook Health Sciences Centre, the Toronto Rehabilitation Institute and the University of Toronto (Vision Science Research Program; Health Care Technology and Place Program; Institute of Biomaterials and Biomedical Engineering)