Geoffrey Schoenbaum, Ph.D.

Assistant Professor
University of Maryland School of Medicine

"Prediction Error Signaling in VTA Dopamine Neurons  and the Orbitofrontal Cortex in Conditioned Reinforcement  and Over-expectation"

In my talk, I will describe two somewhat related new research initiatives in our lab.  In the first, I will discuss firing patterns of dopamine neurons recorded in the VTA of rats making choices between differently valued rewards.  Activity in these neurons will be compared to what has been reported in primate dopamine neurons in tasks where responses to obtain rewards are typically trivial or not required.  The main take home point will be that in rats, as in primates, these neurons signal reward prediction errors, and that these error signals reflect the value of the best available reward even when it is not subsequently selected.  This latter result confirms predictions of some but not other models of reinforcement learning.  From there, I will turn to consider the potential role of the orbitofrontal cortex in reward and error processing.  A number of imaging studies - supported by anecdotal evidence from single-unit recording work - have reported error encoding in orbitofrontal cortex.  I will present single-unit data from orbitofrontal cortex in rats performing the same task used to characterize prediction errors in dopamine neurons.  These data show clearly that output of this area does not signal prediction errors, but rather seems to signal reward predictions themselves.  Behavioral data from several tasks, including a novel outcome-specific conditioned reinforcement task, will be presented to support the hypotheses that these predictive signals are critical to the behavior of the animal and perhaps also to calculation of reward prediction errors by dopamine neurons.