Task-dependent changes in the cortical control of postural dual tasks: effects of secondary cognitive and motor tasks

This study investigated whether the motor cortical control underlying dual task performance depends on the type and difficulty of the dual task. Forty-four adults stood on a balance platform with or without performing a secondary motor (i.e., maintain a ball in the middle of a tray) or cognitive task (i.e., 2-back test) task. Balance task difficulty was manipulated by changing the resistance on the balance platform. Transcranial magnetic stimuli (TMS) were applied to elicit motor-evoked potentials to assess corticospinal excitability and short-interval intracortical inhibition (SICI) from the tibialis anterior. Results indicated that TMS-evoked measures differed based on the type of secondary task. When the motor task was involved, individuals responded with an increase in corticospinal excitability (32.2±5.4%; p<0.001) but no change in SICI compared to the balance-only task. In contrast, when the secondary cognitive task was added, both corticospinal excitability and SICI were unaltered compared to when balancing on its own. These task-specific changes occurred in tandem with differential changes in balance performance. Compared to the balance-only task, performance worsened by 34.2-41.9% (p<0.001) when dual tasking with the secondary motor task but was either unaffected (hard balance task) or improved by 9.1±4.0% (easy balance task; p=0.028) when dual tasking with the cognitive task. Increasing balance task difficulty resulted in 38.7±6.9% larger MEPs (p<0.001) and 21.9±9.4% reduced SICI (p<0.001) but were unaffected by dual-tasking. Together, these findings provide insight into how individuals adopt different motor control strategies, leading to differences in performance, depending on the secondary task.