A deficit in the adjustment of grip force responses in schizophrenia.

Delusions of control in schizophrenia may be due to a deficit in the generation of an efference copy, used to distinguish between self generated and externally imposed changes in the environment. This hypothesis was tested using a framework that differentiated automatic and controlled levels of motor behavior. Subjects resisted collisions that were either self or externally imposed. The grip to load force correlation (response accuracy) and the overall grip force level used (response efficiency) were measured. Controls improved both accuracy and efficiency of their grip force responses in self compared to externally imposed collisions. Patients improved accuracy but not efficiency of motor response. There was no difference between patients with and without delusions of control. These results refute the hypothesis of a perturbed efference copy in patients with delusions of control. We rather propose that schizophrenia globally preserves the automatic level but affects the controlled, more voluntary level of motor behavior.