ABSTRACT
Schizophrenic patients can perform most routine mental activities in a satisfactory manner when circumstances provide sufficient external guidance. However, difficulties in initiation and organisation of activity become apparent when circumstances do not adequately constrain responses; that is, when the activity might be described as arising from the patient’s own volition. Using positron emission tomography (PET) it is possible to delineate the brain regions involved in the performance of voluntary acts. Studies of normal individuals demonstrate that the prefrontal cortex is implicated in the initiation of simple motor acts and in the generation of verbal responses. In schizophrenic patients who have impaired ability to initiate activity, there is underactivity of dorsolateral prefrontal cortex at the site which is active in normal individuals performing self-directed activity. Patients with difficulties in the organisation of activity have underactivity of the orbital prefrontal cortex and overactivity at a site in the right anterior cingulate which is implicated in the suppression of inappropriate responses. Furthermore, there is substantial evidence that dopamine plays a role in the modulation of voluntary activity. However, the effect of dopamine appears to be determined by a complex interplay of internal and external circumstances. These observations suggest that adequate treatment demands more than the simple titration of the dose of dopaminergic agents.
