| Summary: | <p>It has recently been proposed that deficits in reinforcement learning are a core feature of depressive disorders. This thesis aimed to investigate whether transcranial direct current stimulation (tDCS) could be applied to alter these learning processes in healthy adults and individuals with symptoms of depression. A first study assessed the psychometric properties of a computerised reinforcement learning task, which was the main paradigm used in the tDCS studies. Next, a neurostimulation study was designed to examine the effects of tDCS of the dorsolateral prefrontal cortex (DLPFC) on reinforcement learning parameters in healthy adults. It was found that prefrontal stimulation was associated with faster learning from positive but not negative outcomes when applied during task performance. The subsequent study investigated whether this effect of neurostimulation could be harnessed to augment a cognitive training paradigm aimed to enhance learning from either positive or negative events. No evidence was found that tDCS could boost effects of cognitive training on either reinforcement learning or affective biases in information processing in healthy adults. Finally, a pilot study examined whether tDCS of the DLPFC could affect reward learning in a sample of individuals with subclinical depression. While interpretations of the findings were limited by the study’s small sample size, there were indications that active tDCS might enhance learning rates for both positive and negative outcomes during stimulation in this population. Altogether, this work presents preliminary evidence that non-invasive brain stimulation can transiently alter reinforcement learning processes in healthy adults and subclinical depression. Additional research is needed to examine the potential of this technique as a clinical intervention for depression and related psychiatric disorders.</p>
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