The role of the prefrontal cortex in the controlof dual-task gait

Prefrontal cortex is frequently linked to dual-task gait performance; however, its precise role is unknown. The purpose of this thesis was to examine the role of prefrontal cortex in the control of dual-task gait. Using transcranial direct stimulation (tDCS) to alter prefrontal cortex activity, the influence of prefrontal cortex on dual-task gait performance and the corticospinal system was examined across four experiential studies using the guided activation framework of prefrontal cortex function (Miller and Cohen, 2001). The first study examined the role of cognitive task type and walking speed on stride time variability and trunk range of motion during dual-task walking. Results revealed the greatest dual-task cost on gait occurred when walking at a slow speed whilst simultaneously performing a serial subtraction task, compared to performance of a working memory task, providing a rationale for the use of this paradigm in later studies. The second study examined the effect of prefrontal tDCS on dual-task gait performance during both normal and slow walking. Anodal tDCS reduced the dualtask cost on both gait and cognitive task performance, and these effects were not dependent on walking speed. These results indicating that prefrontal tDCS may alter the allocation of cognitive control across tasks during dual-task gait, in accordance with established models of prefrontal cortex function. The third study examined the effect of prefrontal tDCS on corticospinal excitability and working memory performance. Results revealed that cathodal tDCS reduced corticospinal excitability. However, there was no effect of tDCS on working memory performance. Because prefrontal tDCS altered the activity in remote motor networks, these results indicated a possible mechanism by which prefrontal cortex exerts control over gait performance. In addition, because this study failed to replicate previous reports of working memory improvement following tDCS, these results also suggested a degree of inter-individual variability in response to tDCS. The final study examined the influence of walking modality and task difficulty on the effects of prefrontal tDCS on dual-task gait performance. tDCS altered the allocation of cognitive control during over-ground dual-task gait performance, and 3 these effects were mediated by task difficulty. In contrast to the second study, there was no effect of tDCS on treadmill dual-task gait. A secondary aim of the final study was to examine whether cognitive and walking task performance were coordinated. Results revealed that participants articulated answers during the initial swing phase of the gait cycle more frequently than other phases, indicating a degree of coordination between the performance of these tasks. Overall the finding of this thesis indicate that prefrontal cortex is involved in the allocation of cognitive control processes during dual-task walking, in accordance with the guided activation and flexible hub accounts of frontal cortex function (Miller and Cohen, 2001; Cole et al., 2013). These findings may have implications for the design and validation of strategies aimed at improving the cognitive control of gait.