A Methodological Evaluation of Meta-Analyses in tDCS - Motor Learning Research

With transcranial direct-current stimulations (tDCS) rising popularity both in motor learning research and as a commercial product, it is becoming increasingly important that the quality of evidence on its effectiveness be evaluated. Special attention should be paid to meta-analyses, as they usually have a large impact on research and clinical practice. The aim of this study was to evaluate the methodological quality of meta-analyses estimating the effect of tDCS on motor learning with respect to reproducibility as the main focus, and reporting quality and publication bias control as secondary aspects. The three meta-analyses we reviewed largely adhered to PRISMA reporting guidelines and reported the primary effect sizes and sampling variances / confidence intervals they calculated, enabling successful reproductions of pooled effect size estimates. However, akin to previous meta-research reviews with similar aims, we found the methods and results sections of the meta-analyses to be severely underreported, which compromises the ability to judge the soundness of the methodological procedure adopted as well as its reproducibility. While publication bias detection methods were applied, the approaches chosen do not allow for well informed decisions about the presence or extent of publication bias. These results reemphasise the need to transparently report methods in meta-analyses and to meticulously evaluate their quality before and after publication.