The effect of simple and complex dual-tasks on ambulation in individuals with Alzheimer's disease and healthy older adults: the role of divided attention and other higher brain functions in gait dual-task performance

Research using gait-dual task methodology suggests that the ability to divide attention during walking appears to be particularly vulnerable to the effects of Alzheimer’s disease (AD), even in the earliest stages of the illness. However, these previous studies are limited by the variability in the types of gait-dual tasks employed, as well as by the inclusion of heterogeneous groups of patients at different stages of disease severity. Study 1 aimed to address these methodological concerns by examining the effects of a simple and complex counting task on gait speed in healthy older adults and individuals with early stage AD. In contrast to previous findings reported in the literature, Study 1 found that when compared to an age appropriate control group individuals with early stage AD were not differentially impaired by a gait dual-task, regardless of the level of task complexity. Study 2 was designed to be a replication and extension of Study 1. In Study 2, sixteen individuals diagnosed with amnestic Mild Cognitive Impairment (aMCI; Petersen, 1999; 2001), 15 individuals with early-stage AD, 17 individuals with moderate stage AD, and 27 healthy older adults performed a timed walking task and simple and complex verbal counting tasks in single and dual-task combinations. In keeping with the results of Study 1, there were no significant differences among the early stage AD group, aMCI group, and healthy older adults on the gait dual-task, regardless of task complexity. However, significant differences were detected between the moderate AD group and the healthy normal control group on the complex dual-task. Study 3 examined the relationship between other higher brain functions and gait speed, with and without interference, in the same group of participants as Study 2. Neuropsychological test scores were used to create theoretically derived cognitive composite scores (i.e., Executive Functioning/Attention/Speed; Episodic Memory; Language) that were used as predictors of gait speed, with and without interference. As expected, The Executive Functioning/Attention/Speed composite was the most potent predictor of gait speed across conditions; however this relationship varied as a function of task complexity and all three factors predicted gait interference in the complex condition, even after controlling for disease severity. In contrast to previous gait dual-task studies, the current research suggests that aMCI and early stage AD are not associated with impaired gait dual-task performance. Rather, these results suggest that when overall degree of dementia severity is controlled for by subdividing patients based on diagnostic criteria, the specific deficit in attention appears later in the progression of AD than previously theorized. Furthermore, these results provide evidence that the relationship between cognition and gait is likely built upon components of cognitive, physical and task prioritization processes that appear to be modulated by task complexity and disease severity