UC San Diego

Using past experience to inform future choice is fundamental to decision-making and behavior. Integrating experience by comparing choices and outcomes across time is a critical task, and requires a neural mechanism by which information may be assessed and accumulated. This is a widespread phenomenon in the brain, involving many cortical and subcortical structures. In this dissertation I show that one cortical area, the retrosplenial cortex (RSC), is particularly enriched in neurons that encode behaviorally-relevant history information, and is necessary for decision-making that relies on reward-history. RSC neurons exhibit a diversity of time-constants over which this history information is integrated, and I have found that the timescales encoded in RSC match the temporal characteristics of the behavior better than other cortical areas. I developed a novel behavioral model in which decision is reached as the weighted sum of multiple exponential integrators using the observed diversity of time-constants. Acutely inactivating RSC results in the attenuation of this combinatorial behavioral strategy, and a decreased reliance on reward-history. From these results, I propose a conceptual model where reward-history information is encoded in neurons with a simple update rule, but the time-constants are heterogenous and vary across the population. The combination of diverse temporal information produces a behavioral strategy which is sensitive to both recent experience and long-term trends, a feature observed as the hyperbolic discounting of past experience. In Chapter 1 I introduce the concepts of reinforcement learning theory relevant for this dissertation, and survey how reward-based value information is encoded in the brain. Chapter 2 identifies RSC as particularly important for the integration of past reward experience into actionable value information. Chapter 3 further examines and tests the role of RSC in integrating information across a diversity of timescales, and proposes a model of independent temporal integration in the brain that underlies the hyperbolic discounting of past experience. Chapter 4 discusses the properties of RSC that support the integration and maintenance of diverse information, and contextualizes the results in the broader decision-making context