Alien hand syndrome: The feeling that one's hand is possessed by a force outside of one's control. The syndrome typically arises after trauma to the brain, after brain surgery or after a stroke or an infection of the brain. A person with the alien hand syndrome can feel sensation in the affected hand but thinks that the hand is not part of their body and that they have no control over its movement, that it belongs to an alien.

Different types of brain injuries cause different subtypes of alien hand syndrome. For example, take an injury to the corpus callosum (the area of the brain which connects the two cerebral hemispheres, the two halves of the brain). Such an injury in a right-handed person can give rise to purposeful movements of the left hand, while injury to the brain's frontal lobe of the brain can trigger grasping and other purposeful movements in the dominant right hand. More complex hand movements such as unbuttoning or tearing of clothes are usually associated with brain tumors, aneurysms or strokes.

There is currently no treatment for alien hand syndrome. All a patient can do to control the problem is to keep the hand busy by having it hold an object.

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