Prefrontal unit activity during associative learning in the monkey.

Single unit activity was recorded from the dorsolateral prefrontal cortex of two monkeys which were trained on a stimulus-reward association task. The monkeys were trained on a reaction time task overlapped with a classical conditioning paradigm. The sequential events of the task were as follows: (1) lever pressing to start the trial; (2) presentation of a visual cue for 1 s; (3) delay period of 1 s; (4) imperative stimulus presentation; and (5) release of the lever by the animal. The visual cue signaled whether or not a drop of fruit juice would be given (its associative significance) for the animal's release response instead of signaling what response the animal should perform (its behavioral significance). In this task, the animal had to release the lever even on the trial where no juice was given in order to advance to the next trial. A total of 423 units showed activity changes in relation to one or more of the task events, such as the cue presentation, delay, release response and reward delivery. Among 313 units which showed cue-related activity changes, 179 units showed differential activity in relation to the different cues. A majority of them (Type M; n = 120) showed activity changes in relation to whether the cue indicated juice delivery or not, independent of its physical properties. The activity of 13 units (Type P) was related to the physical properties of the stimulus, and the activity of the remaining 46 units (Type MP) appeared to be related to both aspects of the stimulus. Sustained activity changes during the delay period were observed in 68 Type M, in 3 Type P and in 24 Type MP units. The results suggest that the prefrontal cortex plays important roles in the stimulus-reward association and that prefrontal units are involved in higher order information processing, extracting and retaining the "associative significance" of the stimulus independent of its physical properties.