The alien hand: cases, categorizations, and anatomical correlates.

The clinical characteristics and neuroanatomical damage reported in more than 50 published cases of observed "alien-hand" signs are reviewed. The terms alien-hand sign and alien-hand syndrome describe phenomena experienced by patients in which an upper limb performs complex motor activities outside of volitional control. The categories of frontal and callosal subtypes and their relation to behavior and neuropathology are evaluated with reference to the dual premotor system theory, which emphasizes the role of the supplementary motor areas in alien-hand phenomena. Detailed consideration is given to the more recently described posterior subtype, which is purported to result from damage to the parietal lobe or other posterior brain areas. The lack of uniformity in reported assessment methods (behavioral tests, neuroimaging) in published cases contributes to the difficulty in establishing clear subtypes of alien-hand phenomena. Suggestions are made regarding current categorizations and available assessment methods.