Picture this scenario: You’re napping on a breezy Sunday afternoon in the hammock of your backyard, enjoying the peace and quiet. Slowly, your left hand eases up, wraps around your neck and you awake to find your own hand locked in a kung-Fu grip on your throat. You pry it loose with your other hand, finger by finger, until it surrenders and you’re left there staring at a hand that suddenly doesn’t feel like your own. While it sounds like something from horror movie, it’s actually a very odd and very real medical condition known as alien hand syndrome.

Alien hand syndrome is the feeling that one’s hand is possessed by a force outside of one’s control. The syndrome typically arises after trauma to the brain, after brain surgery or after a stroke or an infection of the brain. A person with the alien hand syndrome can feel sensation in the affected hand but thinks that the hand is not part of their body and that they have no control over its movement, that it belongs to an alien.

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.

Think of the corpus callosum, the area of the brain which connects the two cerebral hemispheres, the two halves of the brain, 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. When the callosum is damaged, it leaves the different sections of the brain disconnected and unable to speak to each other. 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.

Symptoms can range from unbuttoning or tearing of clothes, compulsively grasping or releasing an object.

There is currently no treatment for alien hand. All a patient can do to control the problem is to keep the hand busy by having it hold an object. Although even trying to control the hand, the movements may be imprecise; for example, while trying to touch the tip of the nose, they touch the shoulder instead.

Most people feel that rather than living with AHS, they have a very strong desire to have the offending limb amputated.

Regardless of how few cases of alien hand syndrome exist, or how little we know about its cause, the mystery and intrigue of the condition will no doubt continue to inspire writers and filmmakers to explore its horrific and comedic potential.