The role of the basal ganglia in motivated behavior.

The present paper reviews foundational and contemporary theories of motivated behaviors and the growing body of evidence that they require specific functional interactions within the basal ganglia. Such evidence suggests that unconditioned responses (UR), conditioned responses (CR), goal-directed actions and stimulus-response (S-R) habits are selected in the basal ganglia. Such selection depends on activation of striatal neurons by cortical and subcortical neurons encoding unconditioned stimuli (US), conditioned stimuli (CS), goals and neutral stimuli (S). These neurons project respectively to the medial nucleus accumbens (NAc) shell/olfactory tubercle, NAc core/lateral olfactory tubercle, dorsolateral striatum and dorsomedial striatum. The strength of these synapses is altered when the levels of extracellular dopamine in the basal ganglia undergo phasic increases or decreases, which signal outcomes that are, respectively, better or worse than expected. In addition, dopamine release in response to salient USs and to CSs with incentive salience increases the signal-to-noise ratio of corticostriatal neurotransmission, thus 'energizing' the performance of selected actions. Different actions can be selected in the striatum because the striatal neurons of the so-called direct and indirect pathways can respectively initiate and end actions through pallidum/nigral-thalamic projections to premotor and motor areas of the cortex. According to this view, the basal ganglia is thought to play a role in the action-selection processes needed for the expression of both declarative and procedural memories, but the memories of the contexts, predictive stimuli or neutral stimuli associated with free rewards or with an action's outcomes are stored elsewhere.