Coding of Behavioral Sequences in the Basal Ganglia

The idea that the basal ganglia have a role in motor control is well accepted. Yet the fundamental question remains, what exactly do the basal ganglia do for movement? One clue can be obtained from studies of Huntington’s and Parkinson’s diseases. The devastating impact on movement caused by these degenerative disorders of the basal ganglia strongly supports a motor function. However, close scrutiny suggests that basal ganglia pathology is not restricted to the elemental properties of motor control. Even more affected are higher-level organizational aspects of motor control. An important aspect of organization is sequential coordination. For example, Parkinson’s patients can perform motor tasks that require them to control kinematic and dynamic features of movement such as force and direction; however, their difficulty in performing sequences of movements suggests that a higher, organizational aspect of motor control is disturbed by this disorder. Huntington’s patients also have deficits in related high-level “ideomotor” aspects of movement coordination. The neostriatum has even been suggested to be crucial to sequential aspects of human language. Marsden suggested “The sequencing of motor action and the sequencing of thought could be a uniform function carried out by the basal ganglia.” Linguistic and cognitive evidence supports the idea that injury to the basal ganglia may produce an inability to control behavioral sequences in general. The pathological repetitions of spoken words in Tourette’s syndrome and the tormenting habits and thoughts of obsessive-compulsive disorder, both of which are associated with pathology of the basal ganglia. These disorders suggest that the basal ganglia might even participate in the organization of the sequential aspects of “cognitive” behavior. Lieberman in particular