Do variations in emotional state modulate the effects of transcranial direct current stimulation (tDCS) on memory in healthy human participants?

Abstract

Implementation of non-invasive brain stimulation (NIBS) methods in research has been extremely successful in expanding the knowledge of neural mechanisms behind distinct cognitive processes. A form of NIBS, Transcranial Direct Current Stimulation (tDCS), has increasingly been used over the past decade as a potential tool for the modulation of cognitive functions. Although tDCS has been shown to have neurophysiological and behavioural impacts on a variety of cognitive functions, the results are frequently inconsistent. Among many potential factors that could contribute to the variability of the findings, the participants' emotional state is one that seems to be frequently overlooked. In the present study, we aimed to determine whether participants' emotional states influenced tDCS effects on associative (AM) and working memory (WM) performance. We analysed the results from six within-subject, sham-controlled tDCS experiments involving a total of 144 young, healthy volunteers. In each experiment, parallel forms of either AM or WM tasks were used to assess the effects of real vs. sham tDCS. The emotional state was assessed before each tDCS session using the Depression Anxiety Stress Scales (DASS) questionnaire. The results did not show a systematic effect of variations in the emotional state across sessions on the tDCS effects. Namely, neither variations in depression, anxiety, nor stress acted as a systemic modulator of tDCS effects on memory outcomes across different experiments. Nevertheless, in some of the experiments, either depressiveness or stress was found to modulate the tDCS effects. This would suggest that variability in the emotional state should be taken into account when assessing tDCS effects, especially at the individual level.