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.