New perspectives for the modulation of mind-wandering using transcranial electric brain stimulation.

When our attention is decoupled from an ongoing task and becomes coupled to thoughts and feelings not being subject to task engagement, we are mind-wandering. This transient and pervasive mental process can occupy a considerable amount of our waking hours. Mind-wandering is understood to exert both positive and negative effects on well-being, and has been shown to play a role in mood disorders and depression. Here we summarize recent research aiming to investigate whether states of mind-wandering can be modulated using transcranial direct current stimulation (tDCS), a non-invasive, reversible means of altering neuronal excitability and in turn, cortical activity. We examine and compare the methodologies underlying the existing studies on this topic, and evaluate the commonalities and contrasts of their outcomes. So far, existing studies tentatively suggest an influence of tDCS on the contents and propensity to mind-wander. However, these studies exhibit considerable methodological differences and changes in the propensity to mind-wander are inconsistent with task performance. Moreover, replication of the results of two studies from the same group by another group has recently failed. We discuss the implications of these findings, in particular, regarding therapeutic targets in mood disorders, and propose perspectives for future investigations. For instance, tDCS effects on deliberate versus undeliberate mind-wandering should be disentangled. The hippocampus as an important hub for mind-wandering-related processes may be targeted. Most importantly, research efforts related to mind-wandering and rumination should be integrated.