Transcranial Direct Current Stimulation targeting the Left Dorsolateral Prefrontal Cortex Increases Gamma Power and Standing Balance in Older Adults

BACKGROUND: Standing is a complex task often performed concurrently with a cognitive task. This type of "dual-tasking" requires attentional processing and related patterns of brain activation, particularly in the gamma frequency band of electrographic (EEG) signals (30-50 Hz). In older adults, transcranial direct current stimulation (tDCS) designed to facilitate the excitability of the left dorsolateral prefrontal cortex (L-DLPFC) improves attentional control and dual-task standing performance when tested just after stimulation. However, the effects of tDCS on underlying brain activity during single- and dual-task standing are unknown. METHODS: Thirty older adults completed three visits during which they received 20 minutes of tDCS targeting the L-DLPFC, the primary sensorimotor cortex (SM1, active control), or sham (inactive control) in randomized order. Before and immediately after each stimulation session, 32-channel EEG and body-worn motion sensors were used as participants completed trials of standing with and without verbalized serial subtractions. RESULTS: tDCS targeting L-DLPFC, as compared to SM1 and sham stimulation, increased gamma power during single-task standing in the anterior left (p = 0.02) and anterior right regions (p = 0.03). A similar trend (p = 0.08) towards increased gamma power in these regions following tDCS targeting the L-DLPFC was also observed during dual-task standing. Participants who demonstrated a greater increase in gamma power after tDCS targeting L-DLPFC exhibited greater reduction (improvement) in postural sway during single-task standing (r=-0.53, p = 0.007) and dual-task standing (r=-0.51, p = 0.003). CONCLUSIONS: These results suggest that tDCS designed to facilitate L-DLPFC excitability modulates gamma power during single-task standing, which was in turn correlated with improved postural control, in older adults.