Depression is characterized by disruption in the regulation of emotion and mood. Converging evidence from multiple brain imaging studies indicates that depression is associated with dysfunction in the top-down, emotion-regulating frontoparietal brain regions that include the hypoactive left dorsolateral prefrontal cortex (DLPFC) brain region and the hyperactive right DLPFC. Dysfunction in additional brain regions including the cingulate cortex, hippocampus and amygdala has also been linked to depressive symptoms. Together, these brain regions form the dorso-fronto-limbic brain network, and beneficial modulation of this specific network has been hypothesized to improve depressive symptoms.

Network modulation is feasible using a wide range of brain stimulation techniques, including transcranial direct current stimulation (tDCS). The latter technique has recently received high interest in the context of moderate depression due to its demonstrated safety, low-cost and non-invasive nature. A typical tDCS setup consists of non-invasive electrodes placed on the scalp to administer tolerable electric currents at specific brain targets. The administered electric current has been shown to modulate brain activity depending on the direction of the applied electricity, with anodal tDCS increasing neuronal activity at the brain-target, and cathodal tDCS decreasing activity at the targeted brain region. Consequently, tDCS setups in depression have typically targeted the aforementioned hypoactive left DLPFC using anodal tDCS and/or the hyperactive right DLPFC using cathodal tDCS. However, the downstream effects of tDCS on the dorso-fronto-limbic network are not fully understood. Understanding the engagement of this depression-relevant brain network by tDCS could provide important insights into optimizing tDCS setups for antidepressant applications. Consequently, the current study is proposing to use MRI techniques to map and investigate network engagement by tDCS in depression, as described below. Note that while Study arm 1 will use data from an existing study, arms 2-4 will involve recruitment of depressed participants de-novo.

1. Study arm 1 will investigate the engagement of the dorso-fronto-limbic brain network in depression when 'active' or 'sham' anodal tDCS is administered at the left DLPFC brain region at rest. 'Sham' tDCS provides a control condition to facilitate the measurement of tDCS-specific effects, and besides involving minimal electricity, is designed to be identical to 'active' tDCS in every other way (e.g. electrode placement, etc.). For this arm, de-identified data from an ongoing clinical trial in depression with separate aims will be acquired for analysis (NCT04507243, N=48 participants, randomized to 24 'active' and 24 'sham' groups).
2. Study arm 2 will investigate network engagement when anodal tDCS is administered at the left DLPFC brain region during the performance of a mental task (2-back working memory). N=24 subjects will be enrolled using a single group experimental design incorporating a within-session control condition (instead of a separate 'sham' group as in Arm 1). Participation will involve one assessment over a secure videoconference call to confirm eligibility (lasting no more than 3 hours), and two in-person study visits (lasting 30 min and 2.5 hours respectively). Both in-person visits will involve MRI's: the first visit's MRI scans will help determine tDCS electrode positioning for the second visit, and the second visit's MRI scans will help us map changes in brain activity and connectivity during tDCS.
3. Study arms 3 and 4 will investigate network engagement during right DLPFC cathodal tDCS. N=48 participants will be recruited for arm 3 (randomized to 24 'active' and 24 'sham') and N=24 participants will be recruited for arm 4. In all other respects, study arms 3 and 4 will be identical to study arms 1 and 2 respectively.