DOPAMINE NEURONS REPORT AN ERROR IN THE TEMPORAL PREDICTION OF REWARDDURING LEARNING

Many behaviors are affected by rewards, undergoing long-term changes w hen rewards are different than predicted but remaining unchanged when rewards occur exactly as predicted. The discrepancy between reward occ urrence and reward prediction is termed an 'error in reward prediction '. Dopamine neurons in the substantia nigra and the ventral tegmental area are believed to be involved in reward-dependent behaviors. Consis tent with this role, they are activated by rewards, and because they a re activated more strongly by unpredicted than by predicted rewards th ey may play a role in learning. The present study investigated whether monkey dopamine neurons code an error in reward prediction during the course of learning. Dopamine neuron responses reflected the changes i n reward prediction during individual learning episodes; dopamine neur ons were activated by rewards during early trials, when errors were fr equent and rewards unpredictable, but activation was progressively red uced as performance was consolidated and rewards became more predictab le. These neurons were also activated when rewards occurred at unpredi cted times and were depressed when rewards were omitted at the predict ed times. Thus, dopamine neurons code errors in the prediction of both the occurrence and the time of rewards. In this respect, their respon ses resemble the teaching signals that have been employed in particula rly efficient computational learning models.