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 re quiring one to three bar release trials to be completed successfully b efore a reward was given, they seemed more motivated as the rewarded t rials approached; they responded more quickly and accurately. When the monkeys were cued as to the progress of the schedule, 89 out of 150 v entral 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, an d/or (3) near the time of reward delivery, When the cue signaled progr ess through the schedule, the neuronal activity was related to the pro gress through the schedule, For example, one large group of these neur ons responded in the first trial of every schedule, another large grou p 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 car ried the information about how the monkey was progressing through the task. The differential activity disappeared on the first trial after r andomizing the relation of the cue to the schedule. Considering the an atomical loop structure that includes ventral striatum and prefrontal cortex, we suggest that the ventral striatum might be part of a circui t that supports keeping track of progress through learned behavioral s equences that, when successfully completed, lead to reward.