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 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