MODIFICATIONS OF REWARD EXPECTATION-RELATED NEURONAL-ACTIVITY DURING LEARNING IN PRIMATE STRIATUM

This study investigated neuronal activity in the anterior striatum whi le monkeys repeatedly learned to associate new instruction stimuli wit h known behavioral reactions and reinforcers. In a delayed go-nogo tas k with several trial types, an initial picture instructed the animal t o execute or withhold a reaching movement and to expect a liquid rewar d or not. During learning, new instruction pictures were presented, an d animals guessed and performed one of the trial types according to a trial-and-error strategy. Learning of a large number of pictures resul ted in a learning set in which learning took place in a few trials and correct performance exceeded 80% in the first 60-90 trials. About 200 task-related striatal neurons studied in both familiar and learning c onditions showed three forms of changes during learning. Activations r elated to the preparation and execution of behavioral reactions and th e expectation of reward were maintained in many neurons but occurred i n inappropriate trial types when behavioral errors were made. The acti vations became appropriate for individual trial types when the animals ' behavior adapted to the new task contingencies. In particular, rewar d expectation-related activations occurred initially in both rewarded and unrewarded movement trials and became subsequently restricted to r ewarded trials. These changes occurred in parallel with the visible ad aptation of reward expectations by the animals. The second learning ch ange consisted in decreases of task-related activations that were eith er restricted to the initial trials of new learning problems or persis ted during the subsequent consolidation phase. They probably reflected reductions in the expectation and preparation of upcoming task events , including reward. The third learning change consisted in transient o r sustained increases of activations. These might reflect the increase d attention accompanying learning and serve to induce synaptic changes underlying the behavioral adaptations. Both decreases and increases o ften induced changes in the trial selective occurrence of activations. In conclusion, neurons in anterior striatum showed changes related to adaptations or reductions of expectations in new task situations and displayed activations that might serve to induce structural changes du ring learning.