Behavioral reactions reflecting differential reward expectations in monkeys

Learning theory emphasizes the importance of expectations in the control of instrumental action. This study investigated the variation of behavioral r eactions toward different rewards as an expression of differential expectat ions of outcomes in primates. We employed several versions of two basic beh avioral paradigms, the spatial delayed response task and the delayed reacti on task. These tasks are commonly used in neurobiological studies of workin g memory, movement preparation, and event expectation involving the frontal cortex and basal ganglia. An initial visual instruction stimulus indicated to the animal which one of several food or liquid rewards would be deliver ed after each correct behavioral response, or whether or not a reward could be obtained. We measured the reaction times of the operantly conditioned a rm movement necessary for obtaining the reward, and the durations of antici patory licking prior to liquid reward delivery as a Pavlovian conditioned r esponse. The results showed that both measures varied depending on the rewa rd predicted by the initial instruction. Arm movements were performed with significantly shorter reaction times for foods or liquids that were more pr eferred by the animal than for less preferred ones. Still larger difference s were observed between rewarded and unrewarded trials. An interesting effe ct was found in unrewarded trials, in which reaction times were significant ly shorter when a highly preferred reward was delivered in the alternative rewarded trials of the same trial block as compared to a less preferred rew ard. Anticipatory licks preceding the reward were significantly longer when highly preferred rather than less preferred rewards, or no rewards, were p redicted. These results demonstrate that behavioral reactions preceding rew ards may vary depending on the predicted future reward and suggest that mon keys differentially expect particular outcomes in the presently investigate d tasks.