ABSTRACT/PROJECT SUMMARY Time does not heal all wounds equally. Approximately ten to twenty percent of people who have suffered the death of a close loved one will develop a serious prolonged response to bereavement known as complicated grief ? in which grief?s severity and duration continue to exert a substantially detrimental impact on daily life many years, even decades, later. Older adults are at high risk for negative consequences of complications in the grief process that can accelerate age-related decline. By the age of 65, approximately one in four married adults in the United States will have faced the death of a spouse, making complicated grief a serious public health concern for the aging population. To develop effective biobehavioral strategies for prevention and/or intervention, it is imperative to better understand the nature and mechanisms of complicated grief. With the very recent inclusions of a provisional grief disorder in both the DSM-5 (in 2013) and the ICD-11 (in June 2018), the field is currently grappling with the question of how to define ?disordered? grief. The prevailing hypothesis is that effective coping with bereavement requires a person to be able to flexibly shift focus between the tasks of processing the loss and restoring one?s life. In contrast, maladaptive spontaneous thought processes (i.e., yearning, rumination) prevent the loss from being successfully integrated into ongoing daily living. However, the time scale at which these mental shifts take place and how they are enacted in the brain is unknown ? and this limits our ability to effectively detect and intervene when older adults are showing signs of complicated grief. Broadly, the proposed study aims to test spontaneous thought dynamics and their implementation in large-scale brain networks as a mechanism of grief (mal)adaptation. Intrinsic brain networks (default mode, salience, frontoparietal control networks) function to limit or support the capacity for flexible spontaneous thought, and their function in grief may relate to how people think about and process the death of a loved one. The proposed study will investigate the role of dynamic functional connectivity, or time-varying interactions between large-scale brain networks, using resting state fMRI data from widowed older adults with and without complicated grief representing a full range of grief symptom severity. In order to better characterize how this brain activity might differ in complicated grief, the proposed study also leverages data from an intranasal oxytocin manipulation to probe hypothesized motivation- and affect-driven influences. The proposed training will significantly advance the applicant?s progression towards an independent research career in clinical neuroscience through goals that emphasize research rigor and replicability (a current priority for psychological science and neuroscience); advanced quantitative and neuroimaging skills; and knowledge of cognitive-affective processes in psychopathology and aging. The fellowship will provide a foundation for the applicant to catalyze a research program addressing mechanisms of adaptation to interpersonal stressors (like the death of a partner) that can negatively influence older adults? social, cognitive, and emotional functioning.