INFLUENCE OF REWARD EXPECTATION ON BEHAVIOR-RELATED NEURONAL-ACTIVITYIN PRIMATE STRIATUM

Rewards constitute important goals for voluntary behavior. This study aimed to investigate how expected rewards influence behavior-related n euronal activity in the anterior striatum. In a delayed go-nogo task, monkeys executed or withheld a reaching movement and obtained liquid o r sound as reinforcement. An initial instruction picture indicated the behavioral reaction to be performed and the reinforcer to be obtained after a subsequent trigger stimulus. Movements varied according to th e reinforcers predicted by the instructions, suggesting that animals d ifferentially expected the two outcomes. bout 250 of nearly 1,500 neur ons in anterior parts of caudate nucleus, putamen, and ventral striatu m showed typical task-related activations that reflected the expectati on of instructions and trigger, and the preparation, initiation, and e xecution of behavioral reactions. Strikingly, most task-related activa tions occurred only when liquid reward was delivered at trial end, rat her than the reinforcing sound. Activations close to the time of rewar d showed similar preferences for Liquid reward over the reinforcing so und, suggesting a relationship to the expectation or detection of the motivational outcome of the trial rather than to a ''correct'' or ''en d-of-trial'' signal. By contrast, relatively few activations in the pr esent task occurred irrespective of the type of reinforcement. In conc lusion, many of the behavior-related neurons investigated in the anter ior striatum were influenced by an upcoming primary liquid reward and did not appear to code behavioral acts in a motivationally neutral man ner. Rather, these neurons incorporated information about the expected outcome into their behavior-related activity. The activations influen ced by reward several seconds before its occurrence may constitute a n euronal basis for the retrograde effects of rewards on behavioral reac tions.