Abstract
Animals exhibit a diverse behavioural repertoire when exploring new environments and can learn which actions or action sequences produce positive outcomes. Dopamine release after encountering a reward is critical for reinforcing reward-producing actions1-3. However, it has been challenging to understand how credit is assigned to the exact action that produced the dopamine release during continuous behaviour. Here we investigated this problem in mice using a self-stimulation paradigm in which specific spontaneous movements triggered optogenetic stimulation of dopaminergic neurons. Dopamine self-stimulation rapidly and dynamically changes the structure of the entire behavioural repertoire. Initial stimulations reinforced not only the stimulation-producing target action, but also actions similar to the target action and actions that occurred a few seconds before stimulation. Repeated pairings led to a gradual refinement of the behavioural repertoire to home in on the target action. Reinforcement of action sequences revealed further temporal dependencies of refinement. Action pairs spontaneously separated by long time intervals promoted a stepwise credit assignment, with early refinement of actions most proximal to stimulation and subsequent refinement of more distal actions. Thus, a retrospective reinforcement mechanism promotes not only reinforcement, but also gradual refinement of the entire behavioural repertoire to assign credit to specific actions and action sequences that lead to dopamine release.