When viewed as a whole, these basal ganglia-thalamo-cortical circuits appear to play a modulating role in a wide range of behaviors. At the cortical level, given convergence upon specified regions within the frontal lobes, the behaviors in question would be those dependent upon SMA, premotor, frontal eye fields, dorsolateral, and orbitofrontal outflow targets. Broadly speaking, processes such as the generation, maintenance, switching, and blending of motor, mental, or emotional sets would be involved. Accordingly, in basal ganglia disease, the planning and the execution of the above behavioral domains can be affected. Given the diversity and complexity of activity within the basal ganglia, the consequences of disruption depend largely upon lesion site and the associated interplay of neurochemical factors. For example, in the motor domain, damage to various striatal circuitry levels can result in either hypo-or hyperkinetic disorders of movement. Following this analogy, it might also be said that diverse lesions, depending on site, can result in problems with the development and maintenance of behavioral sets (" hypophrenic") versus problems in relinquishing preferential sets (" hyperphrenic"). These contrasting patterns are best represented in PD and OCD, respectively. In the latter case, however, the" hyperphrenic" pattern would only apply to those behaviors which are part of the obsessional rituals. This suggests that procedural system" overdrive" remains domain-specific as is the case for most operations within the procedural system. To return to the broad principle of habituation, a process first described in the context of the visual system and its connections with the tail of the caudate nucleus, it would be tempting to view PD and OCD as disorders of" under" and" over" habituation to behavioral routines. Unfortunately, the situation has proven to be more complex in view of recent neuropsychological findings (Nicholson et al., in preparation). Using a variety of problem-solving and other cognitive tasks, both PD and OCD patients were found to require more practice and/or the provision of external guidelines to facilitate habit formation. Thus, in both cases, as in other disorders of the basal ganglia, the establishment of useful heuristics by which to direct adaptive behavior suffers. OCD patients therefore appear to have at least two compartmentalized types of basal ganglia dysfunction: the ritualistic compulsions and obsessions as well as the heuristic inefficiency (ie, poor procedural mobilization). PD patients would also suffer a similar fate as it is known that the degrees of motor versus nonmotor (ie, procedural) deficit are poorly correlated (42).(ABSTRACT TRUNCATED AT 400 WORDS)