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 reactions toward different rewards as an expression of differential expectations of outcomes in primates. We employed several versions of two basic behavioral paradigms, the spatial delayed response task and the delayed reaction task. These tasks are commonly used in neurobiological studies of working 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 delivered after each correct behavioral response, or whether or not a reward could be obtained. We measured the reaction times of the operantly conditioned arm movement necessary for obtaining the reward, and the durations of anticipatory licking prior to liquid reward delivery as a Pavlovian conditioned response. The results showed that both measures varied depending on the reward predicted by the initial instruction. Arm movements were performed with significantly shorter reaction times for foods or liquids that were more preferred by the animal than for less preferred ones. Still larger differences were observed between rewarded and unrewarded trials. An interesting effect was found in unrewarded trials, in which reaction times were significantly shorter when a highly preferred reward was delivered in the alternative rewarded trials of the same trial block as compared to a less preferred reward. Anticipatory licks preceding the reward were significantly longer when highly preferred rather than less preferred rewards, or no rewards, were predicted. These results demonstrate that behavioral reactions preceding rewards may vary depending on the predicted future reward and suggest that monkeys differentially expect particular outcomes in the presently investigated tasks.