The effect of mood in modulating prefrontal transcranial Direct Current Stimulation (tDCS) cognitive outcomes: A systematic review and meta-analysis

In recent years, several studies in cognitive neuroscience investigated the impact of prefrontal transcranial Direct Current Stimulation (tDCS) on executive functioning. However, the mixed results obtained so far suggest the need for meta-analytic approaches and meta-regression methods, aimed to both synthesize outcomes across studies and potentially identify the reasons for their variability. The present work aimed to test the hypothesis of mood as a possible moderating factor in the effect of prefrontal tDCS on executive functions. This hypothesis stems from the evidence showing that the region targeted by tDCS studies on executive functions, i.e. the dorsolateral prefrontal cortex (dlPFC), is also involved in mood and emotional regulation. Hence, our hypothesis suggests that dlPFC is the shared neural underpinnings of the circuits of mood and executive functioning, and that mood influences the performance of cognitive tasks. Mood disturbances are often linked to cognitive deficits, and there is some evidence in research that suggests that mood states could influence cognitive performance. Moreover, different models attempt to explain how mood modulates cognition, for example by making some contents more available or by triggering different process strategies during cognitive tasks. To test this hypothesis, a systematic review and meta-analysis have been conducted by selecting anodal tDCS studies targeting the dlPFC. The included articles were 19 within-group, sham-controlled and single-session experiments. Every article included at least a mood measurement at baseline and a cognitive task performed after or during stimulation. In this study, both clinical and healthy populations were included. Results discarded mood as a modulating factor of cognitive performance in dlPFC-tDCS. Furthermore, tDCS led to no effect on cognitive outcomes. In conclusion, our analysis adds to the literature by supporting the evidence for a lack of relevant effect of dlPFC-tDCS on bettering cognitive performance, as well as ruling out the mood as a potential factor that explains the variety of findings.