Effects of acute transcranial direct current stimulation in hot and cold working memory tasks in healthy and depressed subjects

Dorsolateral prefrontal cortex (DLPFC) hypoactivity and subcortical hyperactivity have been associated to cognitive impairment for non-emotional (“cold”) and emotional (“hot”) working memory tasks in major depressive disorder (MDD). We investigated whether an increase of DLPFC activity using transcranial direct current stimulation (tDCS) would differently influence the performance in working memory tasks in depressed and healthy subjects. Forty young adult participants (20 with MDD and 20 healthy controls) were randomized to a single, sham-controlled, bifrontal (left anodal/right cathodal), 2 mA, 30 min tDCS session in a parallel design. The n-back and the internal shift task (IST) were used as proxies of cold and hot working memory performance, respectively. Active tDCS compared to sham promoted more accurate and faster responses to the n-back task for both patients and controls. Conversely, only patients presented an improvement in response times for the IST task. Our findings suggest that the mechanisms of tDCS in MDD involve modulation of both cold and hot working memory. We discuss these findings considering the modulatory top-down effects of tDCS on subcortical structures via prefrontal activation, and how spreading of activation might be different for healthy volunteers versus depressed patients. We also discuss the role of tDCS in cognitive amelioration for depressed patients. Finally, the distinct effects of tDCS in the “hot” cognition task for healthy and depressed participants are indicative that tDCS outcomes are also regulated by differences in baseline activity of the stimulated network.