Currently popular theories implicate the basal ganglia primarily in action selection, that is, the decision of which of several possible behaviors to execute at a given time.
The basal ganglia form a major brain system in all species of vertebrates, but the basal ganglia of primates (including humans) have special features that justify a separate consideration. As in other vertebrates, the primate basal ganglia can be divided into striatal, pallidal, nigral, and subthalamic components. In primates, however, the two pallidal subdivisions are called the external and internal (or sometimes lateral and medial) segments of the globus pallidus, whereas in other species they are called the globus pallidus and entopeduncular nucleus. Also in primates, the striatum is divided by a large tract of white matter called the internal capsule into two masses of gray matter that early anatomists named the caudate nucleus and putamen -- in most other species no such division exists, and only the striatum as a whole is recognized. Beyond this, the complex topography of connections between the striatum and cortex means that functions are segregated within the primate striatum in ways that do not apply to other species.