The strategy and motivational influences on the beneficial effect of neurostimulation: a tDCS and fNIRS study

Abstract

The use and public knowledge of noninvasive neurostimulation is rapidly increasing. Transcranial direct current stimulation (tDCS) is a noninvasive technique in which small amounts of current are passed through the cortex in order to change the resting state of underlying neurons. This technique has wide use in rehabilitation and research settings. Here we studied the use of tDCS in healthy younger adults. Our previous findings demonstrated that tDCS can improve working memory (WM) performance in some individuals. We learned that individual differences in education level and WM capacity modulate tDCS effects. In Experiment 1 and 2 we investigated why low WM capacity participants do not benefit or have reduced performance after tDCS. We also explored how tDCS affects cortical blood flow using functional near infrared spectroscopy (fNIRS). In Experiment 1 we examined how strategy use influences tDCS effects. The results demonstrated that active strategy use does not facilitate tDCS effects in low WM capacity participants. Conversely, the high WM capacity participants continued to improve. Furthermore, we found that only the high WM capacity participants had an increase in oxygenated blood flow following anodal tDCS regardless of strategy use. In Experiment 2 we investigated how motivation level modified tDCS effects. We found that motivation level promoted enhanced performance across tDCS conditions for both WM capacity groups. Interestingly, only the low WM capacity participants had an increase in oxygenated blood flow across all motivation and tDCS conditions. The results from all four experiences have important implications for future successful use of neurostimulation in both clinical and healthy populations.