Ventral striatum: a critical look at models of learning and evaluation.

Extensive evidence implicates the ventral striatum in multiple distinct facets of action selection. Early work established a role in modulating ongoing behavior, as engaged by the energizing and directing influences of motivationally relevant cues and the willingness to expend effort in order to obtain reward. More recently, reinforcement learning models have suggested the notion of ventral striatum primarily as an evaluation step during learning, which serves as a critic to update a separate actor. Recent computational and experimental work may provide a resolution to the differences between these two theories through a careful parsing of behavior and the instrinsic heterogeneity that characterizes this complex structure.