Project Summary/Abstract Network models are increasingly invoked to characterize the neurobiological underpinnings of mental illnesses. Dysfunction within specific circuits promotes the formation of specific symptoms. This suggests an opportunity to treat specific symptoms by modulating specific circuits. Repetitive transcranial magnetic stimulation (rTMS) is capable of circuit-specific neuromodulation. It is also an established treatment for Major Depressive Disorder (MDD). Clinical experience suggests that rTMS treats different symptom constructs by stimulating different circuits. However, there remains a critical lack of mechanistic evidence to support putative network mechanisms of rTMS, limiting its ability to treat patients with more personalized and optimized approaches. This mechanistic proposal will first use resting-state functional connectivity (FC) MRI and customized analytic pipelines to characterize functional network topography in healthy and depressed individuals at high resolution. This data will be used to derive rTMS targets functionally situated in discrete prefrontal networks (e.g., control, default, salience, limbic/reward). Next, patients will take part in a within-subject design in which they undergo rTMS to each target on separate days. Each target will be stimulated four times on a given day, and after each stimulation changes will be measured with: (1) REST-BOLD MRI (to assess FC changes), (2) TASK-BOLD MRI (to assess changes in BOLD activation on paradigms validated to test RDoC constructs), (3) state-based questionnaires or (4) neuropsychological tests. This work will facilitate individualized neuromodulation approaches based on network topography. This will pollinate large-scale clinical trials assessing the effects of differential circuit modulation. It will also illuminate circuit-construct relationships across neuropsychiatric disorders.