Modifications of reward expectation-related neuronal activity during learning in primate orbitofrontal cortex.

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This study investigated how neuronal activity in orbitofrontal cortex related to the expectation of reward changed while monkeys repeatedly learned to associate new instruction pictures with known behavioral reactions and reinforcers. In a delayed go-nogo task with several trial types, an initial picture instructed the animal to execute or withhold a reaching movement and to expect a liquid reward or a conditioned auditory reinforcer. When novel instruction pictures were presented, animals learned according to a trial-and-error strategy. After experience with a large number of novel pictures, learning occurred in a few trials, and correct performance usually exceeded 70% in the first 60-90 trials. About 150 task-related neurons in orbitofrontal cortex were studied in both familiar and learning conditions and showed two major forms of changes during learning. Quantitative changes of responses to the initial instruction were seen as appearance of new responses, increase of existing responses, or decrease or complete disappearance of responses. The changes usually outlasted initial learning trials and persisted during subsequent consolidation. They often modified the trial selectivities of activations. Increases might reflect the increased attention during learning and induce neuronal changes underlying the behavioral adaptations. Decreases might be related to the unreliable reward-predicting value of frequently changing learning instructions. The second form of changes reflected the adaptation of reward expectations during learning. In initial learning trials, animals reacted as if they expected liquid reward in every trial type, although only two of the three trial types were rewarded with liquid. In close correspondence, neuronal activations related to the expectation of reward occurred initially in every trial type. The behavioral indices for reward expectation and their neuronal correlates adapted in parallel during the course of learning and became restricted to rewarded trials. In conclusion, these data support the notion that neurons in orbitofrontal cortex code reward information in a flexible and adaptive manner during behavioral changes after novel stimuli.