Intraindividual Variability in Executive Functions but Not Speed of Processing or Conflict Resolution Predicts Performance Differences in Gait Speed in Older Adults

The relationship between specific cognitive functions and gait performance in nondemented older adults is reproducible, robust, and evidenced by studies that involve different methodologies (1,2). Studies that utilize neuropsychological tests to assess the relationship between cognitive functions and mobility reveal that attention and executive functions (EF) play a key, though not exclusive role, in predicting variance in gait performance and decline (1,3–6). Dual-task paradigms that involve walking while performing a cognitive interference task can be conceptualized as an attention-demanding mobility stress test that was recently shown to predict the risk of frailty, disability, and mortality in older adults (7). EF is a multifaceted construct that subsumes cognitive abilities such as planning, organizing, set shifting, inhibition, and conflict resolution. Allocating attention to competing task demands, as assessed via dual-task paradigms, is a facet of EF (8) that is rooted in classic cognitive psychology (9) and is distinct from other aspects of this construct (10). EF are often evaluated using clinical neuropsychological tests that by design are multidetermined. Consequently, identifying specific cognitive component(s) that underlie the relationship between EF and gait has been an unresolved issue to date. Determining, however, the components of EF that are most relevant to gait has critical implications with respect to the discovery of shared brain substrates, etiology, risk assessment, and cognitive interventions for mobility decline and disability. One key cognitive determinant in many measures of EF is speed of processing. The effect of decreased processing speed on performance differences on a wide range of cognitive functions including EF in older adults has been well documented (11). It is, therefore, reasonable to expect that speed of processing might account for the relationship between EF and gait speed. Indeed, processing speed was a key component in latent neuropsychological factors (1,3) and individual measures such as the Digit Symbol Modalities Test (4,6) that were used to assess the relationship between EF and mobility outcomes. Furthermore, a recent structural magnetic resonance imaging study reported that the relationship between prefrontal cortex (PFC) volume and gait speed was attenuated by performance on the Digit Symbol Modalities Test (12). This finding was interpreted by the authors as evidence that shared brain substrates subserved cognitive and motor tasks that were dependent on processing speed. However, two important caveats should be considered. First, processing speed is one of several cognitive processes that are involved in EF in general and in Digit Symbol Modalities Test performance in particular. Second, regions in the PFC subserve complex cognitive functions that include but are not limited to conflict resolution, allocation of attention to competing task demands as well as executive control and monitoring of the efficiency of responses during cognitive task performance (13,14). We have recently demonstrated that functional involvement of the PFC in walking was increased under dual-task gait conditions (15). These findings suggest that processing speed alone might not account for the relationship between EF and gait in older adults, especially in attention-demanding locomotive tasks. One approach to identifying key EF component(s) that predict gait performance is to use more refined cognitive paradigms that empirically separate processing speed from other EF measures. The flanker task (16) requires individuals to inhibit prepotent responses and resolve conflicting visual information and is mediated by the anterior cingulate and dorsolateral PFC (17). Importantly, cognitive paradigms that employ variants of the flanker task separate processing speed from conflict resolution and other measures of attention and EF (18). Intraindividual variability, considered as inconsistency in cognitive performance within a person, can be directly assessed using the flanker task. The taxonomy of intraindividual variability includes several definitions. However, variability on repeated trials of a single cognitive task, which is increased in aging (13) and dementia (19), is considered a defined facet of EF that measures the individual’s ability to monitor and optimize the consistency of responses during acute mental effort (20). Using the flanker task, the current study was designed to determine whether processing speed, conflict resolution, and intraindividual variability in EF predicted variance in gait performance as assessed in single- and dual-task conditions in nondemented older adults. In light of recent studies implicating the functional involvement of the PFC in locomotion tasks (15), we hypothesized that when assessed simultaneously conflict resolution and intraindividual variability but not processing speed would predict gait performance in older adults. We further hypothesized that the relationship between EF measures and gait would be more pronounced in dual compared with single task conditions.