Recent neuroanatomical studies undertaken with various powerful neural tracing methods have radically changed our concept of the organization of the basal ganglia. This paper briefly reviews some of the findings that have led to the conclusion that the major components of the basal ganglia can no longer be considered as single undifferentiated entities. Instead, each of these structures is characterized by several distinct afferent and efferent chemospecific subsystems by which they can modulate and convey the multifarious information that flows through the basal ganglia. This paper focuses mainly on data obtained in primates, but also stresses the importance of comparison with non-primate species.

The circuitry of the basal ganglia is classically known as being composed of multiple intrinsic loops, through which most of the key structures of the system are reciprocally linked, and of a smaller number of projection pathways, which allow the basal ganglia to exert their influence upon distant target structures. Although this general concept of the basal ganglia circuitry is still valid, the numerous neuroanatomical studies undertaken during the past decade, with ever more powerful and sophisticated methods, have greatly expanded our knowledge of the anatomical and functional organization of the basal ganglia. This paper summarizes some of these findings, particularly those that have challenged and even radically changed our way of ttfinking about the arrangement of this set of structures, which plays a crucial role in the control of psychomotor behavior.