Alien Hand Syndrome is an unusual mental disorder in which one of the sufferer's hands seems to take on a life of its own. AHS is best documented in cases where a person has had the two hemispheres of their brain surgically separated, a procedure sometimes used to relieve the symptoms of extreme cases of epilepsy. It also occurs in some cases after other brain surgery, strokes, or infections.
An Alien Hand sufferer can feel normal sensation in the hand, but believe that it is not part of their body and that they have no control over its movements. Alien hands can perform complex acts such as undoing buttons or removing clothing. Sometimes the sufferer will not be aware of what the hand is doing until it is brought to his or her attention. Sufferers of Alien Hand will often personify the rogue limb, for example believing it "possessed" by some intelligent spirit, and may fight or punish it in an attempt to control it.
There are several distinct subtypes of Alien Hand that appear to be associated with specific types of triggering brain injury. Damage to the corpus callosum can give rise to "purposeful" actions in the sufferer's non-dominant hand (a right-handed sufferer's left hand will turn alien, and the right hand will turn alien in the left-handed), whereas injury to the brain's frontal lobe can trigger grasping and other purposeful movements in the dominant hand. Injuries to the cerebral cortex can give rise to aimless movements of either hand, and more complex alien hand movements are usually associated with brain tumors, aneurysm or stroke.
It is theorized that Alien Hand Syndrome results when disconnection occurs between different parts of the brain that control bodily movement. As a result, different regions of the brain are able to command bodily movements without being "aware" of what the other brain regions are doing. There is no known treatment for Alien Hand Syndrome at this time though the symptoms can be reduced by keeping the alien hand occupied, for example by giving it an object to hold.
