Bifrontal transcranial direct current stimulation normalises learning rate adjustment in low mood

Background Transcranial direct current stimulation (tDCS) applied to dorsolateral prefrontal cortex has mild to moderate antidepressant effects. Little is known about the mechanisms of action. Other antidepressant treatments have been shown to act in part by reducing negative biases, which are thought to play a causal role in the maintenance of depression. Negative biases are hypothesized to stem from aberrant reinforcement learning processes, more precisely from overestimation of the informativeness of negative outcomes. The aim of this study was to test whether bifrontal tDCS might normalise such aberrant reinforcement learning processes in depressed mood. Methods 85 community volunteers with low mood received tDCS during (or before) the performance of a reinforcement learning task that manipulated the informativeness (volatility) of positive and negative outcomes. In two sessions participants received real or sham tDCS in counter-balanced order. Baseline performance (sham tDCS) was compared to a sample of healthy individuals (n = 40) to identify the effect of low mood on task performance. The impact of tDCS on task performance was assessed by contrasting real and sham tDCS. Results Low mood was characterised by decreased adjustment of loss relative to win learning rates in response to changes in informativeness. Bifrontal tDCS applied during task performance normalised this deficit by increasing the adjustment of loss learning rates to informativeness. Bifrontal tDCS applied before task performance had no effect indicating that the stimulation effect is cognitive state dependent. Conclusions Our study provides preliminary evidence that bifrontal tDCS can normalise aberrant reinforcement learning processes in low mood. Crucially, this was only the case if stimulation was applied during task performance, suggesting that combining tDCS with a concurrent cognitive manipulation might increase the functional impact on cognitive functions and potentially on emotional symptoms. Future studies are needed to test if the effect on learning processes might have a beneficial effect on mood itself.