Impaired action control in patients with functional movement disorders

Objective: Despite being a major cause of neurological disability, the neural mechanisms of functional movement disorders (FMDs) remain poorly understood. Recent studies suggest that FMD is linked to dysfunctional motor and pre-frontal regions that could lead to motor and cognitive impairments. The aim of this study was to investigate different components of action control in FMD by using choice-reaction, stop-signal, and Simon tasks. Methods: Thirty patients with an FMD were prospectively recruited from the University of Louisville Movement Disorders Clinic and compared with 53 healthy control subjects, recruited from the Vanderbilt University Medical Center Movement Disorders Clinic. FMD motor symptom severity was rated with the Simplified Functional Movement Disorder Rating Scale (S-FMDRS). By using a computer and handheld response grips, participants completed three action-control tasks (choice-reaction task, stop-signal task, and Simon task) that tested action initiation, action cancelation, and interference control over actions. Action-control measures were compared between groups with analyses of variance. Results: Patients with FMD were less proficient in suppressing incorrect response impulses on the Simon task and were slower to stop on the stop-signal task compared with healthy control subjects. No significant correlation with neuropsychological measurements, S-FMDRS scores, and action-control measurements was observed. Conclusions: These results suggest that two forms of inhibitory control, selective impulse inhibition and global action cancelation, are impaired in patients with FMD, independent of slowing on go reaction times. Improved understanding of action control in FMD may help in the development of new diagnostic and therapeutic strategies.