Effects of anodal transcranial direct current stimulation on short-term memory and delayed recall in patients with memory impairment due to progressive and non-progressive brain diseases: a meta-analysis

Introduction:
Treating memory disorders poses a significant challenge for neuropsychologists, who increasingly employ non-invasive brain stimulation alongside traditional cognitive training. This meta-analysis (registered in PROSPERO: CRD42023460773) investigated the impact of anodal transcranial direct current stimulation (a tDCS) on memory in individuals with both progressive and non-progressive brain damage.

Material and methods:
Eligible randomized controlled studies (RCT) were identified from publicly available databases. Two independent reviewers assessed bias risks using the Cochrane criteria, and Hedges’ g coefficient values were calculated for memory outcomes.

Results:
Data from 22 RCTs (23 experiments, 577 participants) were used in the analysis. There were minor concerns about the methodological quality of some studies. Most experiments used active a-tDCS over the dorsolateral prefrontal cortices, with an average current density of 0.1 mA/cm². Analysis of effect sizes revealed significant improvement in short-term memory (g = 0.58, 95% CI = 0.27-0.88) and delayed recall (g = 0.45, p < 0.001, 95% CI = 0.23-0.67). Bilateral stimulation was significantly associated with the overall effect, but concerns were raised about publication bias and study heterogeneity. Subgroup analysis revealed a slightly larger effect size for short-term memory (g = 0.4 and 0.72 for progressive and non-progressive groups, respectively) compared to delayed recall (g = 0.45 and 0.44).

Conclusions:
A-tDCS has a small-to-medium positive effect on memory across neurological disorders. However, due to small sample sizes, low statistical power, and possible publication bias in the analysed data, it is premature to endorse a-tDCS as a reliable adjunct to standard neuropsychological interventions.