Electrophysiological responses of medial prefrontal cortex to feedback at different levels of hierarchy

Recent advances in computational reinforcement learning suggest that humans and animals can learn from different types of reinforcers in a hierarchically organised fashion. According to this theoretical framework, while humans learn to coordinate subroutines based on external reinforcers such as food rewards, simple actions within those subroutines are reinforced by an internal reinforcer called a pseudo-reward. Although the neural mechanisms underlying these processes are unknown, recent empirical evidence suggests that the medial prefrontal cortex (MPFC) is involved. To elucidate this issue, we measured a component of the human event-related brain potential, called the reward positivity, that is said to reflect a reward prediction error signal generated in the MPFC. Using a task paradigm involving reinforcers at two levels of hierarchy, we show that reward positivity amplitude is sensitive to the valence of low-level pseudo-rewards but, contrary to our expectation, is not modulated by high-level rewards. Further, reward positivity amplitude to low-level feedback is modulated by the goals of the higher level. These results, which were further replicated in a control experiment, suggest that the MPFC is involved in the processing of rewards at multiple levels of hierarchy.