Computational evidence for hierarchically-structured reinforcement learning in humans
Humans have the fascinating ability to achieve goals in a complex and constantly changing world, still surpassing modern machine learning algorithms in terms of flexibility and learning speed. It is generally accepted that a crucial factor for this ability is the use of abstract, hierarchical representations, which employ structure in the environment to guide learning and decision making. Nevertheless, how we create and use these hierarchical representations is poorly understood. This study presents evidence that human behavior can be characterized as hierarchical reinforcement learning (RL). We designed an experiment to test specific predictions of hierarchical RL using a series of subtasks in the realm of context-based learning, and observed several behavioral markers of hierarchical RL, such as asymmetric switch costs between changes in higher-level versus lower-level features, faster learning in higher-valued compared to lower-valued contexts, and preference for higher-valued compared to lower-valued contexts. We replicated these results across three independent samples. We simulated three models: a classic RL, a hierarchical RL, and a hierar-chical Bayesian model, and compared their behavior to human results. While the flat RL model captured some aspects of participants sensitivity to outcome values, and the hierarchical Bayesian model some markers of transfer, only hierarchical RL accounted for all patterns observed in human behavior. This work shows that hierarchical RL, a biologically-inspired and computationally simple algorithm, can capture human behavior in complex, hierarchical environments, and opens the avenue for future research in this field.