The Striatum: Where Skills and Habits Meet

After more than a century of work concentrating on the motor functions of the basal ganglia, new ideas have emerged, suggesting that the basal ganglia also have major functions in relation to learning habits and acquiring motor skills. We review the evidence supporting the role of the striatum in optimizing behavior by refining action selection and in shaping habits and skills as a modulator of motor repertoires. These findings challenge the notion that striatal learning processes are limited to the motor domain. The learning mechanisms supported by striatal circuitry generalize to other domains, including cognitive skills and emotion-related patterns of action.The nuclei and interconnections of the basal ganglia are widely recognized for modulating motor behavior. Whether measured at the neuronal or regional level, the activities of neurons in the basal ganglia correlate with many movement parameters, particularly those that influence the vigor of an action, such as force and velocity. Pathology within different basal ganglia circuits predictably leads to either hypokinetic or hyperkinetic movement disorders. In parallel, however, the basal ganglia, and especially the striatum, are now widely recognized as being engaged in activity related to learning. Interactions between the dopamine-containing neurons of the midbrain and their targets in the striatum are critical to this function. A fundamental question is how these two capacities—(motor behavior and reinforcement-based learning)—relate to each other and what role the striatum and other basal ganglia nuclei have in forming new behavioral repertoires. Here, we consider relevant physiological properties of the striatum by contrasting two common forms of adaptation found in all mammals: the acquisition of behavioral habits and physical skills.Without resorting to technical definitions, we all have an intuition of what habits and skills are. Tying one’s shoes after putting them on is something we consider a habit—part of a behavioral routine. The capacity to tie the laces properly is a skill. Habits and skills have many common features. Habits are consistent behaviors triggered by appropriate events (typically, but not always, external stimuli) occurring within particular contexts. Physical skills are changes in a physical repertoire: new combinations of movements that lead to new capacities for goal-directed action. Both habits and skills can leverage reward-based learning, particularly during their initial acquisition. In either instance, after sufficient experience, the need for reward becomes lower and lower. With sufficient practice, both lead to “automaticity” and a resilience against competing actions that might lead to unlearning.