What is it?
Alien hand syndrome (AHS) is a neurological disorder that causes hand movement without the person being aware of or able to control it. Sometimes, the alien hand can do unwanted things like grabbing and manipulating objects, to the extent that the person has to grab the alien hand with their normal hand to restrain it. The person feels as if the hand has a will of its own, or that someone else is moving it – as such, AHS is an extremely interesting phenomenon to study, when it comes to the link between thought and action.
AHS is rare, but the exact rates of occurrence are not known.
What causes it?
AHS is the result of abnormal functioning of the brain. This can be brought about in a number of ways, but the most common ones include strokes, tumors, and neurodegenerative diseases.
What happens during it?
In normal circumstances, movement arises when a stimulus causes the brain to decide to move, and then through a complex series of activation, the brain sends signals down the spinal cord to the muscles, causing them to contract.
The area most associated with conscious control of movement lies in the motor cortex – an area in the cerebral cortex, and practically the “highest” part of the our brain. Although in reality the functions of these brain areas are not as clear-cut, you could say that the premotor cortex plans movement and the primary motor cortex actually initiates it. It is thought that in AHS, the ability of the primary motor cortex to cause movement is separated from the conscious planning of the premotor cortex. The connection between these two areas can be damaged through things like strokes or tumors. The sense of a “self” initiating movement is thought to arise with an orderly sequence of activation in the brain, before the message reaches the primary motor cortex. The failure of this system might cause the person to be unable to distinguish who it was that caused the movement. The movement of an alien hand might therefore arise when the primary motor cortex is activated independently of the influence of the primary motor cortex.
How can it be treated?
There is no known cure for AHS. The hand can be prevented from doing any unwanted damage, however, by giving it something that keeps it occupied – this could be things like making it grasp an object. Sometimes the brain heals from damage if it’s given enough time, and this sometimes leads to a reduction of symptoms. Patients can often be taught to control the alien hand gradually, beginning with simple tasks like grabbing objects, and potentially leading to an ability to control most symptoms.