The alien hand syndrome: What makes the alien hand alien?

The alien hand syndrome is a deeply puzzling phenomenon in which brain-damaged patients experience their limb performing seemingly purposeful acts without their intention. Furthermore, the limb may interfere with the actions of their normal limb. We report a case of alien hand syndrome following a left medial frontal and corpus callosum ischemic lesion. From our clinical observations and the patient's performances on experimental tasks, we postulate that three factors contribute to the sense of alienness: First, the errant limb must be disinhibited and disproportionately reactive to external environmental stimuli. Second, the limb is under less volitional control and produces perseverative movements in which motor stereotypies are concatenated. Consequently, the disinhibited limb perseverates on external stimuli and appears purposeful, despite not being engaged in true goal-directed intentions. Finally, the patient needs to have a relatively intact action-monitoring system to be aware of the abnormal movements as they are occurring.