抄録
Although both the cerebellum and basal ganglia are known to be involved in motor control and learning, the basal ganglia are unique in their dependence on the information of motivation. In order to access the role of midbrain dopamine (DA) system in reward-based decision and learning, we recorded activity of DA neurons in an instrumental conditioning task in which macaque monkeys made a series of behavioral decisions based on the distinct reward expectations. DA neurons responded to the first visual cue appeared in each trial (conditioned stimulus, CS) through which monkeys initiated trial for decision while expecting trial-specific reward probability and volume. The magnitude of CS responses appeared to represent motivational properties rather than the reward expectation, because their magnitude at trials with identical reward expectation had significant negative correlation with reaction times of animal after the CS. DA neurons responded also to reinforcers occurred after behavioral decisions, and the responses precisely encoded positive and negative reward expectation errors (REEs). This supported roles as teaching signals in reinforcement learning theories. The magnitude of CS responses was positively correlated with that to reinforcers. This suggested a modulation of the effectiveness of REEs as a teaching signal by a motivation. For instance, rate of learning could be faster when animals are motivated, while slower when less motivated, even at identical REEs. Therefore, the DA system is critically involved in reward-based decision-making and learning by encoding both motivational signal and reinforcement signal. [Jpn J Physiol 54 Suppl:S51 (2004)]
