Effects of HD-tDCS targeting prefrontal cortex, cerebellum, and motor cortex on dual-task gait in older adults.

Falls pose a significant health risk for older adults, with most incidents occurring during dual-task situations-when walking is combined with a secondary cognitive task. Reducing cognitive-motor interference and attentional demands during walking is therefore a key goal in effective fall-prevention and rehabilitation programs. High-definition transcranial direct current stimulation (HD-tDCS) is an emerging neuromodulation technique that may support this goal by enhancing neural efficiency. To date, no studies have directly compared the effects of HD-tDCS targeting different brain regions-such as the left dorsolateral prefrontal cortex (DLPFC), cerebellum, and primary motor cortex (M1)-to identify the most effective site for reducing dual-task cost (DTC) during walking in older adults. Thus, the present study aimed to compare the effects of these HD-tDCS montages on dual-task gait performance in older adults. This study utilized a randomized, double-blind, sham-controlled, counterbalanced crossover design. Each of the 19 older adult participants completed four sessions, with a minimum one-week washout period between sessions. In each session, HD-tDCS was applied for 20 min, targeting either the cerebellum, M1, left DLPFC, or a sham condition, with the order of stimulation counterbalanced across participants. Although the main effect of stimulation site was not significant (P ≥ 0.204), significant interaction effects were found for stride time variability under both single-task (P = 0.023) and dual-task (P < 0.001) conditions. Post hoc analyses showed significant reductions in stride time variability following stimulation of the left DLPFC, M1, and the cerebellum (P ≤ 0.001 for all), but not after sham stimulation (P ≥ 0.646). For the DTC of stride length variability, a significant interaction effect was also observed (P = 0.038), with reductions following M1 (P = 0.007) and cerebellar (P = 0.011) stimulation. Additionally, dual-task gait speed improved significantly after stimulation of the left DLPFC (P = 0.039) and M1 (P = 0.028). No significant effects were found for stride length, stride length variability, or the DTC of gait speed. Although M1, left DLPFC, and cerebellar stimulation showed favorable effects on gait parameters over time, no stimulation target demonstrated clear superiority. These findings suggest that HD-tDCS may modulate dual-task gait performance, but further research is required to confirm site-specific advantages.RCT registration: On the Iranian Registry of Clinical Trials (IRCT20220718055490N1). Registration date: 1/10/2022.