Investigating the brain regions involved in tDCS-Enhanced category learning using finite element modeling

Transcranial direct current stimulation (tDCS) influences performance in many cognitive domains. However, the question of which brain networks are involved in these effects is rarely examined. In prior experiments we identified tDCS protocols that produce a large improvement in category learning. Here we examined which brain regions were involved by modelling and comparing the behavioral effects of different electrode placements. In Experiment 1, we placed electrodes at two cephalic sites found the be most effective in our prior studies (F10 and T5/P7), expecting an increased combined effect. However, no effect was found, suggesting that stimulation of additional far field regions using extracephalic electrodes in our prior studies may have been necessary for producing these effects. In Experiment 2, we used finite element modeling (FEM) to compare the E-fields produced by these montages. One region with large differences and that is accessible to tDCS was the cerebellum. We then tested the involvement of the cerebellum by placing electrodes below the inion vs. the left arm in thirty-six participants who received anodal, cathodal, or sham stimulation during training. Neither anodal nor cathodal cerebellar tDCS led to significant changes when compared with sham. These results suggest that neither far-field stimulation of the cerebellum nor nearby cranial nerves played a large causal role in our previous tDCS studies. To our knowledge, this one of the first studies to systematically compare the behavioral and energetic effects produced by different montages to identify the specific brain regions involved in the behavioral responses to tDCS.