Tracking progress toward a goal in corticostriatal ensembles.

When performing sequences of actions, we constantly keep track of our current position in the sequence relative to the overall goal. The present study searched for neural representations of sequence progression in corticostriatal circuits. Neurons within the anterior cingulate cortex (ACC) and its target region in the dorsal striatum (DS) were recorded from simultaneously as rats performed different sequences of lever presses. We analyzed the responses of the neurons to presses occurring in the "first," "second," or "third" serial position regardless of the particular sequence or physical levers. Principal component analysis revealed that the main source of firing rate variance in the ACC was a smooth ramp-like change as the animal progressed through the sequence toward the reward. No such smooth-ramping activity was observed in DS ensembles as firing tended to be tightly linked to each action. In the ACC, the progression in firing was observed only for correct choices and not errors, whereas in the DS, firing associated with each action in a sequence was similar regardless of whether the action was correct or not. Therefore, different forms of a signal exist within corticostriatal circuits that evolve across a sequence of actions, with DS ensembles tracking every action and ACC ensembles tracking actual progress toward the goal.