Primate basal ganglia system
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.A separate consideration of the primate basal ganglia is also warranted by the fact that different types of information are available than for other species. Large areas of the primate brain are devoted to vision; consequently the role of the basal ganglia in controlling eye movements has been studied almost exclusively in primates. Functional imaging studies have been performed mainly using human subjects. Also, several major degenerative diseases of the basal ganglia, including Parkinson's disease and Huntington's disease, are specific to humans, although ""models"" of them have been proposed for other species.