Changing task instructions on the WWT task resulted in reduced motor performance in the WWT-T than in the WWT-C condition but in no significant changes on the cognitive task in our study. Both WWT conditions used the same cognitive and motor tasks, and differed only in the instructions. Subjects walked slower when they were asked to focus on talking (WWT-T), diverting attention away from the walking. Uncontrolled self-task prioritization during WWT-C, where subjects were asked to pay attention to both walking and talking, did not slow down gait to the same extent as WWT-T. Subjects took longer strides in WWT-T than WWT-C, which may be a compensatory response (albeit unsuccessful) to the slower cadence and velocity. While subjects made fewer errors reciting alternate letters on the WWT-T, as predicted by shifting the focus to the verbal task, the difference was not significant. In future studies, increasing number of trials or increasing the difficulty on the verbal task may improve the sensitivity of the WWT test to detect differences on the verbal task during different test conditions.
There is a dynamic interplay between the cognitive and motor tasks during divided attention tasks such as the WWT. Walking at normal pace is said to require minimal cognitive involvement, and relies on automatic motor control processes.27
When additional cognitive demands are introduced during walking, attentional resources have to be shared between both the cognitive and the motor tasks as suggested by decrements on gait performance during our WWT conditions. In subjects with balance problems, poor or limited attentional resource reallocation may result in postural instability and increase risk of falls.8
Older adults are reported to show an innate preference to maintaining posture while attempting to simultaneously balance and perform a cognitive task.8,9
A previous study suggested that older adults prioritized walking over a secondary memory task.10
An interesting age related behavior was seen in this study; when given compensatory external aids during the dual task, older adults optimized walking, whereas younger adults optimized memory performance.10
Our findings show that when instructions on the WWT cognitive task were experimentally manipulated, subjects attempted to compensate by slowing down their gait to avoid instability.
Strengths of this study include the large sample size and reliability of study procedures.1,11-13
Selection bias was reduced by having subjects serving as their own controls. While age, sex, cognitive status, or nature of the cognitive task may influence overall WWT performance,10,28-30
in this study subjects served as their own controls for the 2 task conditions. We did not attempt to normalize gait variables by using transformations because this would make the results difficult to interpret, and also because the relevant comparisons were within and not between subjects. Performance bias was minimized because the WWT tests were done by clinical assistants independent of clinical and cognitive assessments, and blinded to study aims. The magnitude of gait slowing was similar for WWT-T irrespective of task order. While WWT-C velocity was slower done second, supporting task order effects, this should bias against the mean. Hence, the statistical differences represent a conservative estimate. Perhaps, subjects were primed by the WWT-T to divert less attention to their gait during the WWT-C condition that followed. Future studies may choose to employ either WWT-T or WWT-C depending on their outcomes of interest. If both conditions are used we suggest that WWT-C be done first to minimize possible task order effects.