This review is an attempt to explain the role of basal ganglia (BG) outputs in generating movements. Recent work showed that opponent outputs from the BG represent instantaneous body position coordinates during behavior. On the other hand, projection neurons in the striatum, the major input nucleus, as well as dopaminergic neurons that form the nigrostriatal pathway, can represent movement velocity. To explain these findings, a new model is proposed, in which the BG implement the level of transition control in an extended control hierarchy. BG outputs represent descending reference signals that command diverse lower-level position controllers. This model not only explains major neurological symptoms but also makes quantitative and testable predictions.
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