tDCS Effects on Verbal Fluency: A Response to Vannorsdall et al (2016)

In a prior study (Cattaneo et al, 2011. Neuroscience. 183:64-70), we demonstrated that anodal transcranial direct current stimulation (tDCS) over the left inferior frontal gyrus enhanced verbal fluency in healthy young adults. Although our data are in line with the results of other published studies, another research group recently failed to report anodal tDCS effects on verbal fluency using a paradigm similar to ours (Vannorsdall et al, 2016. Cogn Behav Neurol. 29:11-17). Here we discuss aspects of study design and interpretation of results that should be considered in replications, focusing particularly on homogeneity of procedures. Notwithstanding the possibility that our study may indeed not be replicable, we hypothesize that Vannorsdall et al found an interesting modifier of the tDCS effects on verbal production by introducing a critical methodologic difference from our original study. We demonstrate this difference by presenting the results of an additional experiment. We believe that the sharing of data between research groups and constructive debate on possible differences in results should be encouraged because they help define the boundaries of applicability of an experimental paradigm. This is even more important for research findings that may have clinical implications, as is the case here.