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

Abstract With transcranial direct-current stimulation’s (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. Competing Interest Statement The authors have declared no competing interest. Funding Statement This study did not receive any funding. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes Affiliations, last sentence of the abstract, and acknowledgements. Data Availability All data and code necessary for reproducing our results are available on https://github.com/TaymAlsalti/tDCS_meta-analysis