Project Summary/Abstract What produces individual differences in behavior? This fundamental question has classically been given two answers: nature and nurture. Here, we suggest that those two answers, while both critical and correct, are insufficient to fully explain individual variability. Instead, we propose that the vast differences in behavior between individuals arise in part from different individuals forming different reward associations within the same environment. This results from the fact that the world is complex and high-dimensional, in that there are almost always multiple possible actions or events that could be attributed to reward. Given the key role of dopamine neurons as the brain’s positive feedback system for behavioral control, the specific hypothesis is that small differences across individuals in initial conditions ultimately produce large differences in which features of the environment that the individual attributes to reward. This hypothesis is inspired in part by complex systems theory, which emphasizes the role of positive feedback in generating and amplifying small differences, creating outcomes that seem stochastic. To address this hypothesis, we will leverage our recent finding that different dopamine neurons calculate reward prediction error across different dimensions of the environment. Specifically, we will use dopamine neuron recordings to infer the time-varying features of the environment that each animal uses to predict reward, and then build reinforcement learning models of each individual based on these features. Ultimately, this testable framework aims to explain both normal variation across individuals, as well as the ubiquitous contribution of dopamine in mediating a disparate range of neuropsychiatric diseases.

Project Narrative Understanding the origin of individual differences in behavior is not only a question of deep fundamental interest, but is also central to understanding the variable penetrance of neuropsychiatric disease. We propose and intend to test a new hypothesis regarding the origin of individual differences: that they arise in part from small differences in initial conditions caused to be amplified by positive feedback, such that different individuals from different reward associations within the same environment. This results from the fact that the world is high-dimensional, in that there are almost always multiple possible actions or events that could be attributed to reward.