Reproducibility of tDCS results in a randomized trial: Failure to replicate findings of tDCS-induced enhancement of verbal fluency

Background: Transcranial direct current stimulation (tDCS) has been shown to enhance verbal productivity, but the finding and extent of enhancement vary across studies. Few attempts to replicate positive tDCS findings have been reported, suggesting the possibility of publication bias. Objective: We aimed to replicate the tDCS methodology and findings of Cattaneo, Pisoni, and Papagno (2011, Neuroscience 183:64-70) in a new population sample. We hypothesized that our study of anodal tDCS would improve verbal fluency production similarly to the original study. Methods: In our single-blind, sham-controlled crossover experiment, 14 healthy young adults were randomly assigned to receive 2 mA of anodal and sham stimulation to the Broca area in counterbalanced order before completing verbal fluency tasks. Results: Participants tolerated the stimulation well. Despite closely mirroring the original study methods, we saw no main effect of stimulation condition: F_1,13=0.002, P=0.97, letter fluency sham mean (standard deviation)=16.8 (2.3), letter fluency anodal=17.5 (3.8), category fluency sham=25.3 (5.4), or category fluency anodal=24.7 (5.2), η²≤0.01. Conclusions: While tDCS may enhance cerebral functions in general, the lack of consistency between studies suggests either that this tDCS protocol does not affect verbal fluency or, at minimum, that tDCS may be more sensitive to experimental conditions than has been thought. Our findings also highlight the need for replication studies in brain stimulation research.