A Meta-analysis of Transcranial Direct Current Stimulation Studies Examining the Reliability of Effects on Language Measures

- We report a meta-analysis of the effects from single-session tDCS on language studies.
- There were many significant effects from single-session tDCS in healthy adults.
- Anodal tDCS produced significant effects on verbal fluency and word learning.
- Anodal tDCS using prefrontal stimulation produced significant effects on verbal fluency.

BackgroundTranscranial direct current stimulation (tDCS) is a brain stimulation technique used to examine causal relationships between brain regions and cognitive functions. The effects from tDCS are complex, and the extent to which stimulation reliably affects different cognitive domains is not fully understood and continues to be debated.Objective/hypothesisTo conduct a meta-analysis of studies examining the effects of single-session anodal tDCS on language.MethodsThe meta-analysis examined the behavioral results from eleven experiments of single-session anodal tDCS and language processing in healthy adults. The means and standard deviations of the outcome measures were extracted from each experiment and entered into the meta-analyses. In the first analysis, we examined the effects of single-session tDCS across all language studies. Next, a series of sub-analyses examined the effects of tDCS on specific tasks and stimulation protocols.ResultsThere was a significant effect from anodal single-session tDCS in healthy adults compared to sham (P = 0.001) across all language measures. Next, we found significant effects on specific stimulation protocols (e.g., offline measures, P = 0.002), as well as specific tasks and electrode montages (e.g., verbal fluency measures and left prefrontal cortex, P = 0.035).ConclusionsThe results indicate that single-session tDCS produces significant and reliable effects on language measures in healthy adults.