Optimization of Transcranial Direct Current Stimulation Protocols for Enhancing Cognitive Function in Healthy Older Adults: A Systematic Review

Abstract

Context: This systematic review evaluated the efficacy of transcranial direct current stimulation (tDCS) in enhancing cognitive function in healthy older adults. Objectives: The review focused on identifying optimal stimulation protocols and factors influencing individual responsiveness. Data Sources: A comprehensive literature search was conducted across PubMed, Google Scholar, Web of Science, and Scopus databases to identify English-language randomized controlled trials (RCTs) published between 2015 and 2025 involving healthy adults aged 65 and older receiving tDCS interventions targeting cognitive outcomes. Study Selection: Screening and selection were conducted according to the PRISMA guidelines. Two independent reviewers assessed the methodological quality of the included RCTs using the Cochrane Risk of Bias Tool version 2 (RoB 2). Data Extraction: Data extraction covered participant demographics, detailed tDCS parameters (intensity, session number, duration, electrode placement), and cognitive outcomes. Quantitative synthesis revealed a pooled standardized mean difference (SMD) of 0.35 [95% confidence interval (CI): 0.12 - 0.58], favoring active tDCS over sham, particularly for working memory improvement following interventions of at least ten sessions at an intensity of 2 mA. Results: From 2,359 initial records, 13 studies involving 647 participants met the inclusion criteria. Most studies applied tDCS at intensities of 1 - 2 mA over the prefrontal cortex and assessed working memory, executive function, and verbal fluency. Interventions with ten or more sessions at 2 mA showed more consistent working memory improvements compared to sham controls. Conclusions: The tDCS shows promise as a non-invasive intervention to support cognitive health in healthy older adults. However, outcome heterogeneity highlights the need for further research to optimize stimulation protocols and personalize interventions. Future studies should aim to standardize methodologies and examine the interplay between stimulation parameters and participant characteristics to maximize cognitive benefits.