| The ability to form and maintain associations between environmental cues, actions, and rewarding stimuli is an elementary yet fundamental aspect of learned behavior. Moreover, in order for organisms to optimize behavioral allocation after learning has occurred, such associations must be able to guide decision making processes as animals weigh the benefits and costs of potential actions. Multiple lines of research have identified that reward-related learning and decision making are mediated by a distributed network of brain nuclei that includes the nucleus accumbens (NAc) and its innervation from dopamine neurons located in the midbrain. However, the precise neural processing that underlies this function is unclear. The first set of experiments detailed in this dissertation took advantage of technological advances to characterize patterns of NAc dopamine release in real time, during behavioral performance. The results of the first experiment demonstrate for the first time that rapid dopamine release in the NAc is dramatically altered during stimulus-reward learning. Before learning, reward delivery produced robust increases in NAc dopamine concentration. After learning, these increases had completely transferred to the predictive cue and were no longer present when rewards were delivered. Further experiments revealed that cue-evoked increases in NAc dopamine concentration did not signal reward prediction alone, but reflected the work required to obtain rewards. Together, these results suggest that NAc dopamine encodes both the benefits and costs of predicted rewards. A second set of experiments used electrophysiological techniques to measure neural activity within the nucleus accumbens during decision making tasks. These experiments show that when rats were choosing between rewards with different effort requirements, a subset of NAc neurons tracked the degree of effort predicted by cues, while other neurons exhibited prolonged activation or inhibition as animals overcame large effort requirements to obtain rewards. Finally, when rats were choosing between rewards that came at different temporal delays, many NAc neurons exhibited changes in activity that correlated with reward delay. Such activity represents a candidate mechanism for linking actions with outcomes, and may also provide insight into the role of the NAc in psychiatric disorders characterized by maladaptive goal-directed behavior and decision making processes. |
