Speaker 2: Qiyong Gong, China

Major depressive disorder (MDD) is characterized by persistent, pervasive feelings of sadness, guilt, and worthlessness and often results in an increased risk of suicide. Imaging evidences suggest that MDD is a kind of disconnection syndrome, however, the related brain circuitry and networks remain poorly understood. Magnetic resonance (MR) imaging allows noninvasive investigation of the circuits and brain networks in vivo. MR diffusion tensor tractography, for example, can track the fiber bundles and permits the assessment of the structural connectivity of the brain. In addition, the fast development of functional MR imaging (fMRI) opens a new window to explore how different parts of the brain functionally connect, interact and coordinate with each other. In particular, the functional connectivity MR imaging (fcMRI) enables the functional connectivity between the brain regions to be effectively investigated. Using high-field MR imaging, researches on MDD has generally focused on two major clinical issues, i.e., suicidality and refractoriness. Studies using diffusion tensor imaging have revealed microstructural abnormalities of the frontal-striatal circuits passing through the anterior limb of the internal capsule associated with suicidality among MDD patients. With respect to refractoriness of the MDD, study has revealed differences in functional connectivity related to treatment responsiveness, with the non refractory group showing a decrease mainly in the limbic-striatal-pallidal-thalamic circuits, and the refractory group showing a decrease mainly in thalamo-cortical circuits. In addition to the investigation of brain circuitry, brain connectome study of MDD using the high-field MR imaging is also promising. Given that MDD is a ‘disorders’ encompassing multiple, heterogeneous, behavioral phenotypic features, MDD is increasingly understood as a disorder of distributed effects of aberrant interaction in the brain, i.e. a network-based disorder. Several core brain networks have been identified using intrinsic physiological coupling in resting-state fMRI data, such as default mode network (DMN), salience network (SN) and central executive network (CEN). Depression is characterized by both stimulus-induced heightened activity and a failure to normally down-regulate activity broadly within the DMN. Study suggests that brain regions in DMN, SN and CEN are linked together through the dorsal nexus, and this help explain how symptoms of MDD arise in distinct networks--decreased ability to focus on cognitive tasks, rumination, excessive self-focus, increased vigilance, and emotional, visceral, and autonomic dysregulation--could occur concurrently and behave synergistically. In addition, the combination of resting-state fMRI and graph-based network analysis allows revealing the topological organization of whole-brain functional networks, such as small-world properties and network modularity, and study has revealed disrupted topological organization of intrinsic functional brain networks during rest in MDD patients. In summary, the alterations of the brain circuitry and connectome in MDD can be investigated using the high-field MR imaging. In conjunction with the advanced imaging analysis, MR imaging of these circuit-based abnormalities not only provides unique insight into the underlying psychopathologies, but also is potentially of translation value in assisting early detection, therapeutic intervention and prognostic prediction of the patients with MDD.