Effects of Transcranial Direct Current Stimulation on Episodic Memory in Older Adults: A Meta-analysis

Objectives: In the last two decades, the number of intervention studies using transcranial direct current stimulation (tDCS) has grown enormously. Though some studies have shown positive influences on episodic memory among older adults, disagreement exists in the literature. Therefore, the current meta-analysis aimed to provide a quantitative assessment of the efficacy of tDCS in modulating episodic memory functions in older adults. Method: Eligible studies were sham-controlled trials examining the effects of anodal tDCS on episodic memory in older adults. Twenty-four articles comprising 566 participants aged over 60 qualified for inclusion. Results: Compared to the sham tDCS group, the active tDCS group showed significant memory improvements at both immediate poststimulation (Hedges' g = 0.625, p = .001) and long-term follow-up (Hedges' g = 0.404, p = .002). There were no differences in effect sizes between cognitively healthy and impaired older adults. Moderator analyses suggested that tDCS having a duration of 20 min or less, bilateral stimulation, or a larger stimulation area would produce greater benefits for episodic memory performance in older adults. Discussion: These findings suggest that tDCS holds great promise to ameliorate memory decline in older individuals. In the future, well-designed randomized controlled trials are expected to verify the optimal stimulation protocols and determine the factors impacting the long-term effects of tDCS in enhancing episodic memory.