BACKGROUND: Conversion disorder (motor type) describes weakness that is not due to recognized disease or conscious simulation but instead is thought to be a "psychogenic" phenomenon. It is a common clinical problem in neurology but its neural correlates remain poorly understood. OBJECTIVE: To compare the neural correlates of unilateral functional weakness in conversion disorder with those in healthy controls asked to simulate unilateral weakness. METHODS: Functional magnetic resonance imaging (fMRI) was used to examine whole brain activations during ankle plantarflexion in four patients with unilateral ankle weakness due to conversion disorder and four healthy controls simulating unilateral weakness. Group data were analyzed separately for patients and controls. RESULTS: Both patients and controls activated the motor cortex (paracentral lobule) contralateral to the "weak" limb less strongly and more diffusely than the motor cortex contralateral to the normally moving leg. Patients with conversion disorder activated a network of areas including the putamen and lingual gyri bilaterally, left inferior frontal gyrus, left insula, and deactivated right middle frontal and orbitofrontal cortices. Controls simulating weakness, but not cases, activated the contralateral supplementary motor area. CONCLUSIONS: Unilateral weakness in established conversion disorder is associated with a distinctive pattern of activation, which overlaps with but is different from the activation pattern associated with simulated weakness. The overall pattern suggests more complex mental activity in patients with conversion disorder than in controls.
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