Dopamine neuronal responses in monkeys performing visually cued reward schedules.

Dopamine neurons are important for reward-related behaviours. They have been recorded during classical conditioning and operant tasks with stochastic reward delivery. However, daily behaviour, although frequently complex in the number of steps, is often very predictable. We studied the responses of 75 dopamine neurons during schedules of trials in which the events and related reward contingencies could be well-predicted, within and across trials. In this visually cued reward schedule task, a visual cue tells the monkeys exactly how many trials, 1, 2, 3, or 4, must be performed to obtain a reward. The number of errors became larger as the number of trials remaining before the reward increased. Dopamine neurons frequently responded to the cues at the beginning and end of the schedules. Approximately 75% of the first-cue responsive neurons did not distinguish among the schedules that were beginning even though the cues were different. Approximately half of the last-cue responsive neurons depended on which schedule was ending, even though the cue signalling the last trial was the same in all schedules. Thus, the responses were related to what the monkey knew about the relation between the cues and the schedules, not the identity of the cues. These neurons also frequently responded to the go signal and/or to the OK signal indicating the end of a correctly performed trial whether a reward was forthcoming or not, and to the reward itself. Thus, dopamine neurons seem to respond to behaviourally important, i.e. salient, events even when the events have been well-predicted.