Damage to the corpus collosum is at the heart of this rare disorder.

Brain Function and Alien Hand

In order to understand what we know about alien hand syndrome, we should take a brief look at the brain and how it works. The human brain is divided into two hemispheres, each consisting of four different lobes, all working together to create, control and regulate speech, movement, emotion and about a billion other sub-functions. The frontal lobe is the section responsible for motor skills, like movement and speech, and cognitive functions, like planning and organization, and it's a good place to start in our understanding of AHS [source: Mayo Clinic].

Let's concentrate on the planning and organization. Let's say you want to take a sip of your morning coffee. What seems like a simple task is really a complex sequence of brain functions, starting from the moment you think, "Mmmm, coffee," until it hits your lips.

When you make the decision to sip the coffee, a signal originates in the frontal lobe that plans and organizes what must take place in order for that action to be completed. You need to reach for the cup, grasp the handle, bring it to your lips, sip and swallow, then return the cup and release your grasp.

These signals are then sent to the motor strip, the area that runs from the top of your head down to the ear, and is responsible for controlling all of your body's movement.

The frontal lobe tells the motor strip, "Hey, I need some coffee, do your thing," and before you know it, you're enjoying your nice morning roast. The key in making this happen is the successful sending of messages, thanks to the corpus callosum [source: Mayo Clinic].

Think of the corpus callosum as the brain's e-mail server, a bundle of message sending nerves that connect and share information with the two hemispheres. Alien hand syndrome is a result of damage to these nerves. This damage most often occurs in brain aneurysms, stroke patients and those with infections of the brain, but can also manifest as a side effect of brain surgery, most commonly after a radical procedure that treats extreme cases of epilepsy. When the callosum is damaged, it leaves the different sections of the brain disconnected and unable to speak to each other -- its e-mail is permanently down. With AHS, one hand functions normally, carrying out purposeful tasks without signaling the other hand, resulting in a limb that can act on its own, sometimes in opposition to the functioning side [source: Turkington].

In the next section, we'll look at pop culture and how alien hand syndrome, despite its infrequent diagnoses, has often been portrayed in books, TV and movies.