Project summary: The goal of Core B (Structural Connections Core) is to provide support for mapping brain pathways relevant to the OCD network, in non-human primates and human subjects. This involves bridging between projects to translate information from anatomical tracing studies to neuroimaging studies that utilize diffusion MRI. The first aim of Core B is to collect diffusion MRI data with very high resolution and signal-to- noise ratio on post mortem macaque brains that have previously received tracer injections for P1; and to use these data to determine which of the pathways identified by diffusion tractography are accurate and which are artifacts due to the inherent limitations of diffusion MRI. The second aim of core B is to use the insights generated from the aforementioned macaque studies to guide the analysis of diffusion MRI data from human subjects in P3-5. Core B will provide these projects with optimized data processing pipelines for mapping the pathways likely to be involved in OCD, with the ultimate goal of investigating how these pathways differ between patients and healthy controls (P3), and how they are affected by noninvasive or invasive interventions (P4-5). Core B will advise the projects on all aspects of diffusion MRI data acquisition, quality assurance, and analyses to reconstruct and characterize bundles of the OCD network.