To Enhance Cognition in Late Life Depression Using Transcranial Direct Current Stimulation

tDCS is a non-invasive brain stimulation method that utilizes low intensity electrical current either to increase cortical excitability with an anodal electrode or to suppress cortical excitability with a cathodal electrode. tDCS produces its effects through long lasting changes in neuroplasticity: in animal studies, tDCS results in long-term potentiation and brain-derived neurotrophic factor secretion; in human studies, tDCS results in potentiation of neurophysiologic markers of neuroplasticity. Consistent with these preclinical findings, tDCS to the temporoparietal cortex or left DLPFC improves memory in patients with mild to moderate Alzheimer disease(AD). tDCS has also been tested among patients with depression - including by our group - but mainly as a treatment for depressive symptoms. In one recent large study one session of bilateral anodal tDCS was shown to enhance working memory among midlife participants with major depression prior to receiving any treatment. However, it is still not known (1) whether tDCS improves cognition beyond the partial improvement that is associated with resolution of depressive symptoms; or (2) whether it has a more fundamental and long lasting effect on cognition.

This trial will assess the acute and long-term effects of a 2-week course of daily (5 days/week) bilateral anodal tDCS of the DLPFC on cognition, and in particular working memory among 36 patients with LLD. We will recruit and randomize participants who have already been treated for LLD prior to consenting to the study and who still meet criteria for remission while on antidepressant medications.

This trial will also be used as a platform to explore the effect of tDCS on neurophysiologic measures of plasticity during working memory performance. Working memory is supported by re-entrant circuits, some are local within the DLPFC and others are long-range connecting the DLPFC to more posterior cortices. These networks result in high and low frequency neuronal oscillations that can be captured by electroencephalography (EEG). Using novel neurophysiologic analyses, our group and others have demonstrated that the modulation of high-frequency (particularly gamma) oscillations by low-frequency (particularly theta) oscillations - "theta-gamma coupling" - is a neurophysiologic marker of working memory. Thus, we will explore the effect of tDCS on theta-gamma coupling.