Primates have an ability to make an adaptive judgment even in novel situations by reasoning. Categorical knowledge may underlie the ability of abstract thinking like reasoning. To investigate the neuronal activity related to decision-making using category information, we trained two Japanese monkeys (Macaca fuscata) to perform a group reversal task. In this behavioral task, one of eight visual stimuli, half of which were associated with juice and the rest with saline, was chosen to serve as a cue to predict which type of liquid was to be given at the end of the trial. As monkeys learned the stimulus-outcome relation, they showed anticipatory licking after the presentation of a juice-predicting stimulus, and no licking after the presentation of a saline-predicting stimulus. We occasionally reversed the stimulus-outcome relation in all stimuli without giving any explicit cue. The monkeys showed quick adaptation to the change of the stimulus-outcome relation after making a single error at the reversal, although they had not experienced the relational change except for the stimulus presented in the first trial after the reversal. This result suggests that the monkeys had recognized the stimuli associated with the same outcome as a category, and used the category information to adapt their behavior when the stimulus-outcome relations changed.
We recorded neuronal activity in dorsolateral prefrontal cortex (DLPFC), ventrolateral prefrontal cortex (VLPFC), and orbitofrontal cortex (OFC) while the monkeys performed the task. We analyzed the neuronal activity by multiple regression analysis with variables of stimulus category, reward contingency, and task rule (as the interaction of category and contingency). Prefrontal neurons were found to code one or more than one of these types of information. A larger population of VLPFC neurons coded category information. DLPFC neurons showed sustained information coding of the task rule. DLPFC neurons showed higher and sustained, while VLPFC and OFC neurons showed weaker and phasic, responses related to the reward contingency. These results suggest that DLPFC, VLPFC, and OFC play different roles in behavioral adaptation in group reversal learning.