PROJECT SUMMARY A growing body of research implicates memory systems, such as the hippocampus, in reward computations related to exploration and reward-seeking behaviors. Episodic memory formation relies on flexible neural coding to form memories in changing environments. How are reward mechanisms involved in memory formation within the hippocampus, a region required for episodic memory formation? Previous work has suggested that hippocampal place cells—cells that fire at a specific location in the environment—overrepresent goal locations where animals receive rewards. Further studies show that a subpopulation of place cells will form fields and potentiate (i.e., increase their firing rate) at locations of head scanning, an investigatory behavior. These studies, however, did not examine the mechanisms of rewards alone or unexpected reward changes in evoking head scanning and place field formation. Although it is known that reward inputs preferentially project to CA1 over CA3, how rewards evoke head scanning and subsequent place field formation across hippocampal subfields CA1 and CA3 have not been explored. To test this relationship, I will use in vivo electrophysiology recordings via silicon probes in hippocampal CA1 and CA3 during navigation tasks. Rewards will be delivered using an intracranial stimulating electrode to the medial forebrain bundle, a region that projects from dopaminergic neurons in the ventral tegmental area to the nucleus accumbens. For our first aim, I will evaluate the timescale and presence of goal overrepresentation in CA1 and CA3. I will explore the hypothesis that head scans predict place field formation at rewarded goals rapidly across both regions. For our second aim, I will assess the role of reward expectation in head scanning exploration and place field potentiation. I will test the hypothesis that unexpected rewards will evoke more head scanning and place field potentiation than expected rewards. Further, I will test how place field potentiation in CA1 differs from that of CA3 at expected and unexpected reward locations. I expect that CA1 place fields will potentiate more to unexpected increases in reward and weaken more to unexpected removal of reward than CA3 place fields. By controlling reward delivery timing, this project will reveal the role of expected and unexpected rewards in head scan initiation and place field formation. Together, the results of this project will establish a spatiotemporal framework for reward exploration in episodic memory encoding. This project directly investigates mechanisms of complex behaviors important for developing therapeutics aligned with the NIMH mission. This proposal will support technical training endeavors, in electrophysiology, animal pose tracking, and intracranial electrical stimulation. This fellowship will also provide training on science communication, reward learning theory, data interpretation, sophisticated methods of statistical analysis, and academic career d...