Neuronal signals in the monkey ventral striatum related to progress through a predictable series of trials.

Single neurons in the ventral striatum of primates carry signals that are related to reward and motivation. When monkeys performed a task requiring one to three bar release trials to be completed successfully before a reward was given, they seemed more motivated as the rewarded trials approached; they responded more quickly and accurately. When the monkeys were cued as to the progress of the schedule, 89 out of 150 ventral striatal neurons responded in at least one part of the task: (1) at the onset of the visual cue, (2) near the time of bar release, and/or (3) near the time of reward delivery. When the cue signaled progress through the schedule, the neuronal activity was related to the progress through the schedule. For example, one large group of these neurons responded in the first trial of every schedule, another large group responded in trials other than the first of a schedule, and a third large group responded in the first trial of schedules longer than one. Thus, these neurons coded the state of the cue, i.e., the neurons carried the information about how the monkey was progressing through the task. The differential activity disappeared on the first trial after randomizing the relation of the cue to the schedule. Considering the anatomical loop structure that includes ventral striatum and prefrontal cortex, we suggest that the ventral striatum might be part of a circuit that supports keeping track of progress through learned behavioral sequences that, when successfully completed, lead to reward.