Rumination may promote ineffective behavior that ultimately harms mental health, although the mechanisms are unclear. Recent research suggests rumination, which is defined as dwelling on the possible significance and implications of one’s distress and problems, may promote ineffective behavior by disrupting reinforcement learning (RL). RL refers to the suite of processes that enable adjusting expectations, when an experience differs from expected, and then taking appropriate action. RL is a ubiquitous process that is necessary for effective behavior. Hence, rumination’s disruption of RL may represent an upstream process that helps explain rumination’s link to various ineffective behaviors. This raises the prospect of intervening to prevent rumination from disrupting RL, thereby making behavior more effective, and ultimately improving mental health. However, the recent research showing rumination disrupts RL is currently unclear on if rumination disrupts the RL mechanisms of learning (adjusting expectations) or choice (taking appropriate action). Different mechanisms would have distinct translational implications, hence clarity on the specific mechanisms is critically needed. Therefore, the objectives of this project are to dissociate rumination’s effects on the learning and choice components of RL, and to apply computational modeling and electroencephalography (EEG) to identify the specific mechanisms of these components it impairs. The central hypotheses are that rumination disrupts RL by impairing the following RL mechanisms: guidance by working memory that accelerates learning, long-term punishment learning, and cognitive control that enables effective choice. To test these hypotheses, the within- subject effects of ruminative (RUM) vs. neutral (NEU) thinking on mechanisms that guide learning (Aim 1) and choice (Aim 2) in RL will be investigated. To ensure generalization to clinical rumination, all participants (N=60) in this single-visit experimental study will meet criteria for Major Depressive Disorder or Generalized Anxiety Disorder. To ensure feasibility, recruitment will be embedded in a large-scale external project that conducts diagnostic interviews. Aim 1 will examine if RUM (vs. NEU) leads to slower acquisition of contingencies by disrupting working memory, and causes working memory to represent punishment experiences more strongly during initial learning, thereby disrupting long-term punishment learning. Aim 2 will examine if RUM (vs. NEU) disrupts cognitive control in a task where Pavlovian (instinctual) and instrumental (goal-directed) impulses conflict, and cognitive control is needed to override the former. These aims are integrated with a training plan to broaden and deepen the applicant’s background in computational neuroscience, to enable him to acquire specialized skills in this area (hierarchical Bayesian computational modeling and EEG), and to facilitate his development of a programmatic and competitive translational resea...