Towards optimized methodological parameters for maximizing the behavioral effects of transcranial direct current stimulation

Abstract

Introduction: Transcranial direct current stimulation (tDCS) administers low-intensity direct current electrical stimulation to brain regions via electrodes arranged on the surface of the scalp. The core promise of tDCS is its ability to modulate brain activity and affect performance on diverse cognitive functions (affording causal inferences regarding regional brain activity and behavior), but the optimal methodological parameters for maximizing behavioral effects remain to be elucidated. Here we sought to examine the effects of 10 stimulation and experimental design factors across a series of five cognitive domains: motor performance, visual search, working memory, vigilance, and response inhibition. The objective was to identify a set of optimal parameter settings that consistently and reliably maximized the behavioral effects of tDCS within each cognitive domain.

Methods: We surveyed tDCS effects on these various cognitive functions in healthy young adults, ultimately resulting in 721 effects across 106 published reports. Hierarchical Bayesian meta-regression models were fit to characterize how (and to what extent) these design parameters differentially predict the likelihood of positive/negative behavioral outcomes.

Results: Consistent with many previous meta-analyses of tDCS effects, extensive variability was observed across tasks and measured outcomes. Consequently, most design parameters did not confer consistent advantages or disadvantages to behavioral effects-a domain-general model suggested an advantage to using within-subjects designs (versus between-subjects) and the tendency for cathodal stimulation (relative to anodal stimulation) to produce reduced behavioral effects, but these associations were scarcely-evident in domain-specific models.

Discussion: These findings highlight the urgent need for tDCS studies to more systematically probe the effects of these parameters on behavior to fulfill the promise of identifying causal links between brain function and cognition.

Keywords: meta-analysis; meta-regression; non-invasive brain stimulation; research methods; transcranial direct current stimulation.

Figure 1

Summary flowchart of literature search and screening procedure.

Figure 2

Contrasting estimates of multiple neurostimulation and experimental parameters on behavioral performance outcomes, both within-domain and between-domain. For categorical predictors (e.g., target laterality), contrasts were specified in an A – B fashion, such that level B served as the reference point. Thus, positive values suggest larger effect sizes (on average) for level A compared to B; negative values indicate the reverse. For continuous predictors (e.g., stimulation intensity), positive values suggest larger effect sizes with increasing values of the predictor (e.g., stronger stimulation dosages); negative values suggest the inverse association. Values closer to zero indicate weak or no association with performance outcomes, either at the level of mean differences between factor levels or a linear relationship with continuous predictors. Point intervals give 90–95% credibility intervals (i.e., highest density intervals) around the median posterior estimates (thick lines = 90% CI; thin lines = 95% CI; solid gradient areas indicate the 95% CI). (A) Motor performance, (B) Visual search, (C) Working memory, (D) Vigilance, (E) Inhibition, (F) Domain-General.