Abnormalities of motor imagery associated with somatic passivity phenomena in schizophrenia.

Some patients with schizophrenia report that their limbs are under the control of an alien force (motor passivity). This is hypothesised to be due to the dysfunction of an internal self-monitoring system that normally permits distinctions between internally generated and external influences on intentional behaviour. Motor imagery is the mental simulation of specific motor actions and it is based upon the internal representation of intended but unexecuted motor actions. Therefore, the generation of motor imagery should be impaired in schizophrenia characterised by passivity phenomena. The generation of motor imagery was compared using the visually guided pointing task (VGPT) and the Florida praxis imagery questionnaire (FPIQ) between patients with schizophrenia characterised by high levels of passivity symptoms (passivity) and patients without passivity symptoms (no-passivity). In both the passivity and no-passivity groups, the speed of real motor sequences on the VGPT was constrained by the distance of the movement and the width of the target in accordance with Fitts' law. For the no-passivity group, the same relationship was found for imagined movements. However, in the passivity group, imagined movements were not constrained by Fitts' law. The effect of a 2-kg load to the limb performing real or imagined movements on the VGPT was identical in both groups. The duration of imagined movements was slowed although the duration of real movements was unaffected. The FPIQ showed that the passivity group had difficulty answering questions that required them to imagine kinaesthetic aspects of performing simple gestures. These results suggest that passivity phenomena in schizophrenia are associated with a specific inability to represent the timing of motor actions internally. This is consistent with the hypothesis that patients with passivity phenomena have difficulty with maintaining an internal representation of intentional behaviour.