Basal ganglia disorders are a heterogeneous group of clinical syndromes with a common anatomic locus within the basal ganglia. To account for the variety of clinical manifestations associated with insults to various parts of the basal ganglia we propose a mode/in which specific types of basal ganglia disorders are associated with changes in the function of subpopulations of striata/projection neurons. This mode/is based on a synthesis of experimental animal and post-mortem human anatomic and neurochemical data. Hyperkinetic disorders, which are characterized by an excess of abnormal movements, are postulated to result from the selective impairment of striatal neurons projecting to the lateral globus pa/lidus. Hypokinetic disorders, such as Parkinson's disease, are hypothesized to result from a complex series of changes in the activity of striatal projection neuron subpopu/ations resulting in an increase in basal ganglia output. This mode/suggests that the activity of subpopulations of striata/projection neurons is differentially regulated by striata/afferents and that different striata/projection neuron subpopu/ations may mediate different aspects of motor control

Speculation about the function (s) of the basal ganglia have been strongly influenced by descriptions of the clinical phenomenology of human basal ganglia disorders. The study of Parkinson's disease (PD), in particular, has had a strong influence on speculation about basal ganglia function. The observation that parkinsonian patients have considerable difficulty in initiating movements led to the hypothesis that the basal ganglia are involved in the'automatic execution of learned motor movements'1. In this scheme, the basal ganglia function as an entity devoted to sequencing individual motor programs into a smooth series of actions. This close linkage of conceptual schemes about function and the clinical details of human disease is virtually unique in contemporary neuroscience.