Alien hand syndrome (AHS) is a neurological disorder in which a patient has only partial control over one of their limbs, according to ScienceDaily. This causes the limb to move sporadically. Studies using fMRI (functional magnetic resonance imaging) tests have shown that alien hand movements result from activity in the primary motor cortex, and that alien hand movements are associated with selective activity in the contralateral primary motor cortex, as described in the Archives of Neurology.
Researchers believe that within the brains of patients that suffer from AHS, the primary motor cortex is isolated from the premotor cortex, but still has the ability to control the alien hand at most times, according to ScienceDaily. The primary, occipital and parietal lobes of the brain are thought to be involved in alien hand syndrome. Some events that may trigger AHS are the separation of the brain's two hemispheres, strokes, brain surgery, Alzheimer's disease, tumors, infections and aneurysms, as stated by Wikipedia.
AHS patients may refer to their alien hand in third-person. They often dissociate from the affected hand and don't acknowledge that it is a part of their body.