Working memory (WM) is important for everyday planning and decision-making and has been shown to decline in older age. The dorsolateral prefrontal cortex (DLPFC) is a functional neuroanatomical region that undergoes functional alterations associated with age-related WM deficits. The Scaffolding Theory of Aging and Cognition (STAC) proposes that cognitive training (CT) and transcranial direct current stimulation (tDCS) interventions may enhance older adults’ ability to compensate for such declines. Few studies have evaluated intervention effects on compensation in older age, operationalized as regional activation changes to maintain higher cognitive function. The present study therefore evaluated combined CT+tDCS effects on DLPFC function and WM associations in older adults without dementia.

172 older adults (age: M=71.2 years; 63.4% female; 75.0% obtained a bachelor’s degree) completed CT and were randomly assigned to undergo either sham (n=81) or active tDCS (n=91) as part of a double-blind randomized controlled intervention. CT involved 8 attention/processing speed and working memory paradigms using the POSIT Science Brain HQ training program. tDCS was applied through electrodes placed at the F3 and F4 positions using the 10-20 electrode placement system. All stimulation procedures were identical across conditions except stimulation duration. Active tDCS involved 2mA direct current applied for 20 minutes. Sham tDCS participants received 30 seconds of 2mA direct current and habituated to the tDCS sensation within 30-60 seconds of stimulation. All participants completed demographic questionnaires, cognitive testing, and an fMRI 2-back task pre- and post-intervention. A priori spherical DLPFC regions of interest  in an established working memory network were used to quantify blood-oxygen level dependent (BOLD) signal. Statistical models adjusted for age, sex, educational attainment, study site, and intervention adherence.

Across groups, participants demonstrated 88.34% mean baseline target accuracy and 91.57% accuracy post-intervention. A binary logistic regression revealed no significant tDCS group differences in the probability of improved 2-back performance (Exp(B)=.821, 95% CI=[.435, 1.551], p=.544). Separate ANCOVAs further demonstrated no significant tDCS group differences in right or left DLPFC BOLD signal (p’s >.05). Separate binary logistic regressions revealed that increased BOLD signal in only the left DLPFC significantly predicted the probability of improved 2-back performance following CT+tDCS (Exp(B)=1.465, 95% CI=[1.044, 2.056], p=.027).

Results demonstrated no significant intervention effects on 2-back task performance. Given high target accuracy across time points, ceiling effects potentially limited the ability to detect intervention effects. Sociocontextual factors (e.g., educational attainment, race, and ethnicity) likely contributed to ceiling effects. High performance combined with observed lateralized activation patterns may also reflect greater brain and cognitive reserve. Specifically, older adults are thought to exhibit bilateral activation patterns compared to lateralized activation in younger adults. Moreover, results demonstrating increased BOLD activation across tDCS groups suggest CT may facilitate enhanced recruitment and engagement of additional neural resources necessary to meet task demands in older age. Cognitive training and tDCS therefore may still serve as effective interventions for working memory in older age. Future work in more diverse, representative samples may help further characterize CT and tDCS effects in older adults without dementia.