Update on models of basal ganglia function and dysfunction.

Circuit models of basal ganglia function and dysfunction have undergone significant changes over time. The previous view that the basal ganglia are centers in which massive convergence of cortical information occurred has now been replaced by a view in which these structures process information in a highly specific manner, participating in anatomical and functional modules that also involve cortex and thalamus. In addition, much has been learned about the intrinsic connections of the basal ganglia. While the basal ganglia-thalamocortical circuitry was originally seen almost exclusively in its relationship to the control of movement, these structures are now viewed as essential for higher level behavioral control, for instance in the regulation of habit learning or action selection. Probably the greatest benefit of these models has been that they have motivated a wealth of studies of the pathophysiology of movement disorders of basal ganglia origin, such as Parkinson's disease. Such studies, in turn, have helped to reshape the existing circuit models. In this paper we review these fascinating changes of our appreciation of the basal ganglia circuitry, and comment on the current state of our knowledge in this field.