Neurophysiological mechanisms of dual-tasking in people with multiple sclerosis

Abstract

Saskatchewan has one of the highest rates of multiple sclerosis (MS) globally, affecting approximately 4,000 people within the province. An emerging topic of study within MS rehabilitation is dual-tasking (i.e., the performance of two tasks simultaneously). Reports on dual-task performance among people with MS (PwMS) vary in the literature, with past research limited by methodological differences and minimal consideration of the underlying neurophysiology. Studies suggest that changes in inhibitory neural activity in the motor cortex may support dual-task performance in healthy adults, as assessed by transcranial magnetic stimulation (TMS) techniques. Other TMS work indicates that MS alters corticospinal inhibition, but how and whether it is modulated during dual-tasking in PwMS is unknown. The primary objective of this cross-sectional study was to determine whether changes in corticospinal inhibition that occur during dual-tasking are different in PwMS compared to non-MS controls. Six PwMS (4F; 45.17 ± 15.74 years) and three non-MS controls (2F; 42.33 ± 16.62 years) participated and performed motor and cognitive tasks under single-task and dual-task conditions. Each dual-task included a core motor task, which involved maintaining a pinch grip at a steady-force level. Performance of this core motor task allowed for assessment of corticospinal inhibition during task performance via measurement of the cortical silent period elicited by TMS. Tasks combined with the core motor task included holding a string of numbers and/or number letter combinations in working memory and a foot-tapping task. Several versions of these tasks were presented alongside the core motor task, each providing different levels of task novelty and complexity. Dual-task performance was measured as a dual-task cost with consideration of task performance and cortical silent period duration. Several key findings emerged from this exploratory study: i) PwMS demonstrated only minor negative impacts to cognitive performance under dual-task conditions, with some indication of a ‘cognitive-first’ prioritization strategy, ii) both PwMS and non-MS controls generally displayed some level of an interference effect on force-grip performance indicated by negative dual-task costs; and iii) PwMS displayed greater changes in cortical silent period dual-task costs that were particularly apparent under the motor-motor dual-task conditions. Findings from this study indicate that while PwMS and non-MS controls may experience similar performance changes during dual-tasking, it appears that the neurophysiological mechanisms may be different. Future work should continue to investigate the impact of MS-related changes in the corticospinal system on dual-task performance.