Failure to Improve Verbal Fluency with Transcranial Direct Current Stimulation

Abstract

Previous studies in healthy populations have provided equivocal evidence whether the application of anodal transcranial direct current stimulation (tDCS) over the left prefrontal cortex (PFC) can improve performance in verbal fluency tasks. In this double-blind, randomised within-participant study, we investigated whether anodal tDCS over the left PFC improves verbal fluency performance relative to sham tDCS. Forty eight healthy native German speakers performed two verbal fluency tasks after having received 20 min of anodal or sham tDCS over the left PFC. During stimulation, participants performed a picture naming task, which was expected to increase neuronal activity in the targeted region. We found no modulation of verbal fluency performance following anodal tDCS, with virtually identical overall scores across tDCS conditions. Furthermore, initiation time (i.e., time to produce the first correct utterance) was not affected by tDCS. As an unexpected finding, picture naming latencies were significantly longer during anodal compared to sham tDCS. Yet, changes in the naming task were not predictive of performance changes in the fluency task. Overall, the current study found no evidence that verbal fluency performance in healthy speakers could be improved by excitatory stimulation of the left PFC. We argue that previously observed positive effects could be false positives and should be interpreted with caution. The findings from the current study thus cast further doubt on the utility of tDCS in enhancing cognitive performance in the healthy (young) brain.