Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 81-93).The frontal-striatal reward network is involved in many reward-related behaviors, including decision-making and those related to psychiatric disease. One important class of decisions involves the conflict between immediate rewards and delayed gratification. Temporal discounting preferences reflect how a person makes decisions that involve tradeoffs over time. A fundamental question is how people vary and what accounts for this variation in temporal discounting preferences, both behaviorally and neurobiologically. In addition, psychiatric diseases, such as social anxiety, are associated with deficits in behaviors that involve social reward. Here I report two experiments that provide evidence for two major factors that contribute differences in temporal discounting preferences: personality traits and the underlying frontal-striatal reward network, both during task and at rest. Finally, I report underlying differences in the organization of the frontal-striatal reward network in social anxiety disorder. In the first study, I investigated the frontal-striatal reward network that underlies personality traits and the association of personality with temporal discounting preferences. Higher neuroticism was associated with a greater preference for immediate rewards and greater impulsivity, and higher conscientiousness with a greater preference for delayed rewards and less impulsivity. Executive-control and reward regions in the frontal-striatal reward network were more activated when higher conscientiousness participants selected a smaller-sooner reward and, conversely, when higher neuroticism participants selected a larger-later reward. Both cases involved choices that went against predispositions implied by personality. In the second study, I investigated how restingstate intrinsic functional brain organization (functional connectivity) of the frontal-striatal reward network varies with temporal discounting preferences. Increased patience and decreased impulsivity were associated with stronger functional connectivity between the nucleus accumbens and prefrontal executive control regions, including the dorsolateral prefrontal cortex. These findings reveal that the intrinsic strength of the frontal-striatal network is associated with differences in temporal discounting preferences. In the third network in social anxiety disorder. There was decreased functional connectivity between the nucleus accumbens and other reward regions, including the ventromedial prefrontal cortex, and decreased functional connectivity between the ventromedial prefrontal cortex and executive control regions of the prefrontal cortex. Taken together, these results indicate that the frontal-striatal reward network is associated with individual differences in reward-related behavior.by Joshua Manning.Ph. D.