Motor agency: a new and highly sensitive measure to reveal agency disturbances in early psychosis.

Background: Early diagnosis of young adults at risk of schizophrenia is essential for preventive approaches of the illness. Nevertheless, classic screening instruments are difficult to use because of the non-specific nature of the signs at this pre-onset phase of illness. The objective of the present contribution was to propose an innovating test that can probe the more specific symptom of psychosis, i.e., the sense of agency, which is defined as being the immediate experience of oneself as the cause of an action. More specifically, we tested whether motor agency is abnormal in early psychosis.

Methods: Thirty-two young symptomatic patients and their age-matched controls participated in the study. 15 of these patients were at ultra high-risk for developing psychosis (UHR), and 17 patients were suffering from first-episode psychosis (FEP). Patients' neurocognitive capacities were assessed through the use of seven neuropsychological tests. A motor agency task was also introduced to obtain an objective indicator of the degree of sense of agency, by contrasting force levels applied during other and self-produced collisions between a hand-held objet and a pendulum.

Results: As reported in the literature for adult controls, healthy adolescents used more efficient force levels in self than in other-imposed collisions. For both UHR and FEP patients, abnormally high levels of grip force were used for self-produced collisions, leading to an absence of difference between self and other. The normalized results revealed that motor agency differentiated patients from controls with a higher level of sensitivity than the more classic neuropsychological test battery.

Conclusions: This study is in favour of the existence of an abnormal sense of agency early in the psychotic illness. Because it is quick and none verbal, motor agency may be a valuable tool to use in complement to classic interviews, especially when investigating complex ineffable experiences that are difficult to explicitly describe.