The Dynamics of Striatum Circuitry

A commentary on Dynamic reorganization of striatal circuits during the acquisition and consolidation of a skill. Learning new motor skills is an integral and essential part of our life. Remember the fi rst time riding a bike or learning to ski? After a new skill is acquired, the once daunting task becomes effortless and automatic. How does our brain accomplish such a feat? Which brain systems are involved, and how? A recent article by Yin et al. (2009) provides novel mechanistic insights on this issue. Behavioral evidence indicates that motor skill learning takes place over an initial phase of fast improvements, followed by a slower phase of gradual refi nement (Karni et al., 1998; Costa et al., 2004). An interesting parallel that similarly consists of a fast and slow learning phase is reward-guided instrumental learning (Yin and Knowlton, 2006; Yin et al., 2008). The initial phase of reward-guided instrumental learning is deliberate and sensitive to both the action-outcome contingency and the value of the outcome. Prolonged training transforms an action into a habit, such that the once deliberate action becomes automatic and insensitive to both the action-outcome contingency and to the outcome value. Recently, the roles of different striatal subregions in reward-guided instrumental learning have been extensively investigated. Two subregions of the striatum, specifi cally the dorsomedial striatum (DMS) and the dorsolateral striatum (DLS), have been shown to play crucial roles in the learning of goal-directed actions and habit, respectively. Lesions and pharmacological inactivation of the DMS (DLS) impairs the acquisition of goal-directed actions (habit) (Yin et al., 2005, 2006). Such roles are consistent with their anatomical connection patterns, with the DMS receiving inputs from the prefrontal cortex and the DLS receiving inputs from sensorimotor cortices. Since the striatum is also essential for motor skill learning, the fi ndings that the DMS and DLS are important for reward-guided instrumental learning raises the question whether the DMS and DLS play similar roles in the fast and slow phases of motor skill learning. This question is what Yin et al. set out to address. Using a rotarod task, the authors found that lesions of the DMS impaired skill learning only in the early phase, similar to the effect of DMS lesions in reward-guided instrumental learning. Moreover, with simultaneous in vivo extracellular recording in the DMS and DLS of the same animal, the authors showed that the activity of DMS neurons is preferentially …