Stepping over obstacles: Attention demands and aging

Abstract

Older adults have been shown to trip on obstacles despite taking precautions to step carefully. It has been demonstrated in dual-task walking that age-related decline in cognitive and attentional mechanisms can compromise postural management. This is yet to be substantiated during obstacle negotiation when walking. Forty-six healthy volunteers (aged 20–79 years) stepped over obstacles in their path whilst walking and performing a verbal fluency task. Using 3D kinematic analysis we compared obstacle crossing during single-task (obstacle crossing only) and dual-task (obstacle crossing with verbal task) conditions. We grouped the participants into three age groups and examined age-related changes to cognitive interference on obstacle crossing. During dual-task trials, the 20–29 and 60–69 groups stepped closer to the obstacles prior to crossing, increased vertical toe-obstacle clearance, and had reduced gait variability. In these two groups there was a small dual-task decrease in verbal output. The 70–79 group applied similar dual-task stepping strategies during pre-crossing. However, during crossing they showed reduced vertical toe-to-obstacle clearance and increased variability of obstacle-to-heel distance. Additionally, this group did not show any significant change to verbal output across trials. These results suggest that with advanced age, increased cognitive demands are more likely to have a detrimental impact on motor performance, leading to compromised safety margins and increased variability in foot placement. We conclude that younger adults utilise a posture-preserving strategy during complex tasks but the likelihood of this strategy being used decreases with advanced age.

Introduction

It is estimated that one in three older adults fall each year and this figure includes those who are healthy and active [1]. In this age group, a large proportion of falls are caused by stepping on or tripping over obstacles [2]. Whilst it is accepted that a number of cognitive functions decline with advanced age, it is still unclear how such age-related changes impact on the control of complex motor skills such as walking and stepping. The relationship between cognitive activity and posture control can be examined using dual-task methods that apply a cognitively demanding task during a posture task (see Woollacott and Shumway-Cook [3] for a review). These studies suggest that concurrent cognitive activity can interfere with motor control, and that interference is greater in older adults [4], [5]. These findings have important implications for complex motor tasks that require large cognitive resources, as the addition of a cognitive task may lead to misallocation of resources, and impaired motor control. For example, a number of studies have shown that older adults who would typically be classified as independently mobile under single-task conditions demonstrate limitations to walking as the task becomes more complex [6], [7], [8].

Despite the apparent interplay between cognition and posture, the nature of dual-task interference on posture is not always clear. When walking, dual-task interference tends to manifest as reduced performance (e.g. reduced velocity and shorter stride length) [9]. In upright standing it can be difficult to interpret dual-task effects because concurrent cognitive activity has been shown to both increase and decrease postural sway [10], [11]. Few studies have examined the effect of concurrent cognitive activity or aging on foot placement when negotiating obstacles [12], [13], [14]. The results from dual-task walking studies suggest that concurrent cognitive performance should have a detrimental effect on the control of foot placement during obstacle crossing. There has been some indication of this in the literature [12], [13], [14] but these results are not conclusive. For example, Chen et al. [13] observed participants walking and avoiding a suddenly appearing band of light projected on the walkway. Concurrently participants completed a verbal reaction task, responding when a red light appeared at the end of the walkway. The results showed increased foot-light contacts when completing the verbal reaction task, especially for older adults. Since the reaction task in this study directed visual attention from the immediate walkway, it is not possible to infer that increased attention demands led to impaired stepping control. Schrodt et al. [12] demonstrated that a concurrent memory task altered stride positioning when crossing obstacles but reported no other changes to gait. Brown et al. [14] examined changes to a verbal reaction time task during obstacle crossing. The reaction task was presented before or during obstacle crossing. They reported increased reaction times in young and older adults when the task was presented before obstacle crossing. Additionally, the older adults had increased reaction times when the task was presented during obstacle crossing. Despite reporting significant effects to the verbal reaction task, they did not report data on obstacle clearance.

These results suggest that cognitive interference may influence obstacle crossing but the strength and nature of this influence is unclear. The difficulty in ascertaining the effect of cognitive performance on stepping control may be linked to the small critical period of obstacle crossing (∼2 s) that would allow uncompleted cognitive tasks to be temporarily suspended during obstacle crossing. It could also be due to insufficient cognitive demands used in some of these dual-task scenarios, since with increased attention demands dual-task interference on walking is typically observed [15], [16], [17].

Here we increase the likelihood of detecting genuine cognitive interference during obstacle crossing by increasing the duration of trials to record multiple steps and employ a continuous cognitive task. By maintaining the motor task over a longer duration it becomes possible to implement a concurrent cognitive task that is continuous. For this purpose we selected a word generation task that requires the continuous generation of novel words. Word generation tasks of this nature are cognitively demanding and have been shown to interfere with walking performance [17]. Critically, verbal fluency requires sustained attention to organise verbal output and to keep track of the words that have already been generated [18], [19] but it can be performed without the complication of visual distraction. We anticipate that the demands of sustaining attention on obstacle crossing whilst carrying out the verbal task will lead to observable changes in foot placement and/or cognitive performance. Since older adults have a natural decline in cognitive function [15] we predict that any observed interference effects would be greater with advanced age.