He sat at the library sweating profusely, looking at his disobedient left hand, scared. Dude has been behaving as if it has a mind of its own since morning, his left hand.
Once again, he took a deep breath, and tried to focus on the book before him. "It's all in my head," he tried to reassure himself. Slowly he reached for the book and opened it to the page of interest, but almost immediately, his left had instinctively reached for it and closed it back.
It must be from the village,he thought; they have finally remembered him. How else could he explain this, the fact that he is 24 hours from his final exams and the rest of his life, and suddenly he could no longer read because his left had wouldn't let him.
In frustration he stormed out of the library to find a place to smoke and calm his nerves in order to figure out what to do next. But dude still won't let him; immediately he stuck the cigarette into his mouth, it instinctively reached for it and threw it away, the smouldering cigarette. And he stood there rooted to the ground, shocked beyond his wit ends. They have bewitched his left hand. Now he has an enemy in his own body fighting him. A house divided against itself.

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The scenario above depicts a clinical condition known as theAlien Hand Syndrome (AHS),in which one's hand develops "a mind of its own", and acts involuntarily, performing purposeful or semi-purposeful actions, without the owner being aware of it, or consciously initiating such actions.
In alien hand syndrome, while the sufferer recognises the "estranged hand" as his own, he doesn't feel responsible for the actions it performs, as he looses conscious control of said hand there's a sort of dissociation between action and thought.
This could happen in a number of medical conditions including corticobasal syndrome, stroke [ especially bilateral anterior cerebral artery stroke], certain brain infections, tumors, following a surgery [used to cure temporal lobe epilepsy] separating the two brain hemispheres, and in some neurodegenerative conditions like Alzheimer's and Creutzfeldt-Jacob diseases.

I would like to introduce two perceptions used in neuropsychology, the sense of ownership and the sense of agency.

THE SENSE OF OWNERSHIP AND THE SENSE OF AGENCY
The sense of ownership is a faculty that enables one to recognize an action to have been performed by him, or as the case may be, a part of him. It is more or less sensory, and depends on re-afferent feedback generated by the moving part to the brain.
The sense of agency on the other hand is the faculty that enables one to recognize an action as being consciously though out, planned, executed and controlled by him throughout the course of the action. Hence it's otherwise called sense of performance , in that it enables you to recognize yourself as the performer of the action.
In, AHS, there's a dissociation between these two senses; the sufferer retains his sense of ownership of the alienated hand, but loses sense of agency over the performed action.
To throw more light on this, imagine you are lying still on your bed and someone passively moves your hand. Of course you will recognize the part being moved as a part of your body sense of ownership, but you won't have a conscious control over the action, a memory of you planning and initiating the movement sense of agency.

The question is, how is this possible in the human body with out an external influence?

THE SEQUENCE OF VOLUNTARY MOTOR ACTION
Every volitional [voluntary] motor action is well thought out and planned before being embarked on, and certain brain parts are sequentially activated before such a thought can be successfully translated into an action.
Functional neuroimaging [imaging of the brain to map out areas of activity during an action in order to understand how the brain works] shows that the flow of information during a motor activity is in this order: from the supplemental motor area [A], to the premotor cortex [B], thence to the primary motor cortex [C], which in turn controls actual body movement through the spinal cord [D]. [NB: Don't bother yourself with the names, just remember the alphabets tagged to each name].
Actions are planned in A and B, and executed in [or rather through] C. Now, bearing in mind that the brain is a network of interconnecting neurones [wires] powered by electricity, and considering A, B and C as a circuit in series , if for any reason there's a lesion interrupting flow of information from A-B [where action is planned] to C [where it is executed], one would retain sense of ownership, but lose his sense of agency.

TYPES OF ALIEN HAND SYNDROME
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1. The FRONTAL Variant Of AHS
This usually manifests in the dominant hand, the hand one uses to perform tasks requiring high dexterity, like writing.
It is usually due to a lesion in the frontal lobe of the brain, involving areas A and B, separating them from C as described above.
It manifests as instinctive reaching, grasping, groping and other related purposeful movements. And once the alienated hand grasps an object, it holds tenaciously onto it, and it would be difficult to release the grip, so difficult the owner of the hand may resort to peeling off the fingers one by one to release the grasped object. This phenomenon is called MAGNETIC APRAXIA, in the sense that the hand sticks to the object it grasps, like a magnet to a metallic object.
Compare this phenomenon, of "magnetic apraxia", to the following:

• The next time you see a baby less than 5 months, try and tickle his hand with your finger. The baby would instinctively reach for your hand and grasp it so tightly that you can lift him well off the bed with the strength of the magnetism between your finger and his hand, without him slipping; so tightly you would have to forcefully remove your hand. This is called the grasp reflex and is due to the fact that the FRONTAL LOBE in babies is not yet well developed to inhibit such instinctive reaction. Thus a damage to the frontal lobe in an adult recreates such "magnetism" in an alienated hand.

2. The CALLOSAL Variant Of AHS.
The brain has two hemispheres, the right and the left, both of which has identical areas connected in the middle by a bundle of bundle of nerve fibres called the Corpus Callosum.
The Corpus callosum, by sharing information between the two hemispheres of the brain-- the right and the left, enables the two of them to be aware of what each other is doing for the sake of harmony, among other things.
Also, of the two parts of the brain, one-- the one that controls the dominant hand [the hand you use more often to perform tasks requiring high dexterity like writing], is the dominant hemisphere. The A and B of this dominant hemisphere is usually charged with planning motor functions. And the Corpus callosum ensures that the other hemisphere's A and B is intimated of its plans.
Now, if for any reason the Corpus callosum is severed [ example on purpose during an operation to treat severe epilepsy], or loses its function [for example after a stroke involving the artery that supplies it], the two parts of the brain will basically be functioning unaware of, and independent of each other. And this may lead to the type of AHS described in scenario above, in which one hand- the alienated hand, controlled by the non-dominant hemisphere, ANTAGONISES the other hand-- the one controlled by the dominant hemisphere.

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TREATMENT OPTIONS
There's presently no cure for the AHS, given that the causes are mostly permanent injuries to the brain.
However, there are treatment options to RESTRAIN the actions of the alienated hand.
For example, someone with the frontal variant could wear thick, wooly gloves without space for the fingers [just like an oven glove] to "muffle" the hand, and its ability to grasp things instinctively.
Also, the alienated hand could be gotten occupied with a walking stick, a briefcase, and what have you. While sitting, one can form the habit of wedging the hand in between the laps, to keep it from "misbehaving".
There's also the option of learning to control the alienated hand, which is however very difficult, and rarely rewarding.

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REFERENCES
Corticobasal degeneration
neurology times
The alien hand syndrome

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