In the past, the basal ganglia were considered to act as funnels and as an integration center for many cortical inputs. Based on research with rats, this view was changed to one in which the basal ganglia consist of functionally segregated thalamocortical circuits or loops that are involved in the parallel processing of different information. So far, five such circuits have been identified:
• motor circuit (see figure below) that plays an indirect role in kinematics, but not dynamics, of ramp movements (viz., their initiation, their direction and the scaling of their amplitude and velocity). An example of a ramp movement is tracking the displacements of cursor on a screen with a hand-held stylus
• oculomotor circuit whose main function is the voluntary control of saccadic eye movements (also ramp-like movements)
• association circuit hypothesized as having an influence on ‘higher’ mental functions
• dorsolateral prefrontal circuit implicated in spatial memory
• lateral orbitofrontal (limbic) circuit thought to be involved in the regulation of emotions
From this more recent perspective on the functions of the basal ganglia, it can be appreciated that they not only have an important role in motor control, but that they are also ascribed involvement in an array of quite diverse behaviors, many of which were previously thought to be those controlled by the cerebral cortex. In fact, it has been said “… the basal ganglia subserve many functions, perhaps all of the functions served by the cortex itself!” [Kandel, E.R., Schwartz, J.H., & Jesell, T.M. (Eds.). (1995). Essentials of neural science and behavior. Norfolk:Appleton & Lange, p. 652). In addition, there are also similarities (and differences) in both connectivity and functions between the basal ganglia and the cerebellum.
