Electrode montage dependent effects of transcranial Direct Current Stimulation on semantic fluency

Transcranial direct current stimulation (tDCS) has proved to be valuable in improving many language processes. However, its influence on verbal fluency still needs to be fully proved. In the present study, we explored the effects of different electrode montages on a semantic fluency task, aimed at comparing their effectiveness in affecting language production. 90 healthy, right-handed volunteers were randomly assigned to receive one of the following stimulation protocols: 1) anode over the left frontal cortex/cathode over the right supraorbital (rSO) area, 2) anode over the left fronto-temporal (lFT) cortex/cathode over the rSO area, 3) anode over the lFT cortex/cathode over the right FT cortex, 4) anode over the lFT cortex/big-size cathode over the rSO area, 5) Sham. In the active stimulation conditions, 2 mA current was delivered for 20 minutes. Participants performed the semantic fluency task before the stimulation, immediately after it, and 15 minutes after the first post-stimulation task. Although none of the different protocols improved language production immediately after the stimulation, anodal stimulation over the left frontal cortex (standard-size cathode over the rSO area) improved fluency at the second post-stimulation task. This proved that small differences in either active electrode positioning, or reference positioning/size can impact tDCS behavioral effects also in the cognitive domain. These findings, which can be sometimes missed when tested immediately after the stimulation only, add new information on tDCS spatial and temporal features, thus providing new indications to increase the effectiveness of stimulation protocols.