ABSTRACT

A standard assumption about the basal ganglia is that they are implicated in the regulation of motor activity. A large literature has been devoted to their cellular properties: Neuronal types, neurotransmitters and single-cell anatomy and neurophysiology have all been extensively studied (for a recent review, see Mink, 1996). However, the functional role of these subcortical structures has been difficult to pinpoint. The anatomy of the circuitry in which the basal ganglia are embedded is well known. In essence, information originating in the neocortex passes through the basal ganglia, and returns, via the thalamus, to specific areas in the frontal lobe (Alexander,

DeLong and Strick, 1986; Alexander and Crutcher, 1990a). The neocortical input areas include various domains that subserve sensory, motor, associative and limbic functions (Parent and Hazrati, 1995a,b). How information is modified, before and after its transit through the basal ganglia, is mostly unknown. Why this circuit utilizes two inhibitory synapses in sequence is not known either. On the basis of lesion and neurophysiological studies (DeLong and Georgopoulos, 1981; DeLong, 1990), it has been suggested that the basal ganglia are crucially involved in the control, and especially in the planning of goal-driven movements, which is consistent with the massive input they receive from frontal cortex (Parent and Hazrati, 1995a,b). The wide diversity of the sources that send information to them also suggests integrative functions.