Task exposure and stimulation duration modulate HD-tDCS effects on working memory in older adults

Introduction Age-related working-memory decline has been associated with altered neural oscillations, and high-definition transcranial direct current stimulation (HD-tDCS) may improve cognition. However, responses vary and may depend on prior task engagement and stimulation duration. Methods We examined these factors in a randomized, double-blind, sham-controlled crossover study of 39 healthy older adults (65.4 ± 3.0 years). Participants received real and sham 2-mA HD-tDCS over the left dorsolateral prefrontal cortex for 10 or 20 min. Task exposure before real stimulation was determined by stimulation sequence, creating less- and more-exposure groups. Working-memory performance and EEG spectral power were assessed during two-back tasks before and after stimulation. Results Change in discrimination sensitivity (Δd’) showed a Stimulation × Duration × Exposure interaction. Exploratory comparisons indicated that 10-min real stimulation improved Δd’ relative to sham in the less-exposure group but reduced Δd’ in the more-exposure group; neither 20-min subgroup showed a significant effect. Secondary inverse-efficiency analyses showed improved efficiency after real stimulation under less exposure and poorer efficiency under more exposure. The more-exposure, 10-min group also showed increased central beta power following real stimulation. Associations between beta-power changes and poorer behavioral performance did not remain significant after false-discovery-rate correction. Discussion These findings suggest that HD-tDCS effects vary with stimulation duration and prior task context. However, the small subgroups and confounding of task exposure with stimulation order and session position require cautious interpretation. Larger studies independently manipulating exposure and stimulation order are needed to confirm these potentially state-dependent effects.