In 1908, the German neurologist and psychiatrist Dr Kurt Goldstein was bewildered by the strange behaviors manifest in one of his patients. The fifty-seven year old woman reported that her left hand had a “will of its own,” and indeed it seemed to possess its own sinister agenda. The rogue limb moved around involuntarily, and its searching fingers tended to fiddle with the various objects that they chanced upon. If she didn’t keep an eye on her arm, she couldn’t quite be sure what it was up to. Even more alarmingly, her conjoined tormentor occasionally wrapped its fingers tightly around her throat, forcing her to defend herself with her more obedient arm. The eminent neurologist was at a loss for how to treat this unsettling abnormality.

Although Dr Goldstein was the first to officially record the phenomenon, his patient wasn’t the first to be afflicted with this rare and astonishing malady, nor would she be the last. It was not until 1972, however, that it was officially recognized as a legitimate disease. It came to be known as Alien Hand Syndrome (AHS).

This neurological disorder is now known to be the result of specific kinds of brain damage. The majority of recorded cases are a side effect of a brain operation called a corpus callosotomy; a surgery which helps to relieve extreme cases of epilepsy by severing the bundle of nerve fibers that carry information between the two hemispheres of the brain. On occasion, a hand can also become alienated due to head trauma, stroke, aneurysm, or tumor, or due to certain degenerative brain conditions such as Alzheimer’s disease and Creutzfeldt-Jakob disease.

The specific symptoms and severity of Alien Hand Syndrome depend on which sectors of the brain have been compromised. Although patients typically retain sensation in the alien hand, they feel strongly that the rebel limb does not belong to them. They will often dissociate themselves from the hand by personifying it—sometimes assigning it a name—and attribute its inexplicable movements to ghosts or gods. Some sufferers only experience a few isolated episodes, but in most cases the alien arm is active whenever its owner is conscious.

The mildest form of the syndrome is caused by damage to the brain’s parietal and/or occipital lobe, causing a hand to unconsciously avoid contact with objects by “levitating” with its fingers extended. When damage occurs to the corpus callosum—the data link between the brain’s two hemispheres—the non-dominant hand can develop what seems to be an independent sense of purpose, groping around its environment and manipulating the objects it finds. Sometimes the sufferer is completely unaware of what the hand is doing until it is brought to his attention, or until he happens to notice the shenanigans. Alien hands have been implicated in a variety of misdeeds, such as unbuttoning one’s own shirt, adjusting a thermostat, or involuntarily stuffing food in the mouth. In some cases it will grasp onto an object and hold on stubbornly, requiring the patient to peel back the fingers in order to let go. Such incorrigible behavior prompts many AHS sufferers to speak to the unruly arm in an attempt to reason with it, some becoming so frustrated that they try to modify its behavior through punishment. Others simply use their obedient hand to restrain the strangeness.

In a report published in the Journal of Neurology, Neurosurgery, and Psychiatry, Dr Ramón Leiguarda and his colleagues recorded several first-hand accounts from AHS sufferers. A sixty-five year old man reported the following:

While I was travelling on a bus I noticed that a hand was approaching me on the right from behind, trying to catch me. After grasping my trouser leg, the hand did not release it. First, I thought somebody was assaulting me, but then I realised that it was my own right hand, although I did not feel it belonged to me. Thereafter, the fingers developed creeping movements, and repetitive jerks involving the whole arm soon followed. I was unable to control my right hand and I had to grasp and hold it with my left hand. My right arm felt heavy and awkward. I was very anguished, anxious, and frightened, and had palpitations…

The researchers also recounted the experience of a fifty year old woman who developed AHS following a brain hematoma:

Suddenly I had a strange feeling on my left side; later I could not recognise the left arm as my own; I felt it belonged to someone else and wanted to hurt me because it moved towards me; I saw it quite big and distorted like a monster; I was terrified.

When the arm approached the woman’s face during a session with one of the authors, she begged for him to stop the ‘monster’, pleading, “Look, it’s coming… please help me.” A quick injection of Valium put her at ease.

One particularly unorthodox subtype of Alien Hand Syndrome can arise due to lesions on the corpus callosum, producing a phenomenon called intermanual conflict. For these frustrated few, the alien hand constantly meddles in the affairs of the obedient hand. For instance, one particular patient had difficulty operating a TV remote because the wayward hand would snatch it away as soon as he picked it up with his good hand. In another case, a sufferer experienced difficulty getting dressed because the alien hand would unhook the buttons on her blouse immediately after she buttoned them. Yet another AHS sufferer was unable to smoke because her rogue hand would pluck the cigarette from her lips and toss it away before she could light it, leading the woman to surmise that ‘he’ didn’t want her to smoke just then.

Although the alien hand’s actions are seldom threatening, there are a few recorded cases where the dissident limb seems to wish harm upon its host body. The hand can occasionally grab and tear its owner’s clothes, attempt to strangle the sufferer, punch the patient in the face, or grab a steering wheel and turn the vehicle unpredictably. The out-of-control hand can also be a source of profound embarrassment, such as the case of a right-handed man who developed AHS in the aftermath of a stroke. His left hand would involuntarily grasp at objects and cause considerable mischief, and on occasion it would unfasten its owner’s trousers and “gratify” him without regard for his surroundings. Needless to say, the stroke victim was frustrated and dismayed when he discovered the hand doing so in public.

The nature of the brain malfunction that leads to Alien Hand Syndrome is not yet known to science. Under ordinary circumstances, intentional movements of the arm and hand are driven by a combination of signals from several regions of the brain: the parietal lobe provides the body’s position in space, the temporal lobe provides data about past motion strategies, and so on. All of this information is then processed by the frontal lobe into a specific plan of action, and the instructions are relayed to the brain’s motor strip for execution.

Evidence suggests that the brain’s dominant hemisphere—the half that controls an individual’s dominant hand—maintains some indirect control over the subordinate hand in carrying out motion; so when the corpus callosum is damaged, the hemispheres’ data link is lost, and the subordinate hand is deprived of guidance. Doctors conducting fMRI scans of Alien Hand Syndrome patients found that the liberated limbs seem to be driven by the motor strip directly, neither receiving orders from the frontal lobe nor sending a signal to inform the host that the movement is occurring. The actual source of these motor strip signals remains a mystery. When AHS is triggered by lesions to the frontal lobe itself, the involuntary movements are most likely caused by corruption in the brain’s motor instructions.

Since its discovery in 1908, only fifty or so cases of Alien Hand Syndrome have been confirmed. It is believed, however, that many other instances have gone undiagnosed, written off as part of a psychiatric disorder.

There is no known treatment to restore control over the estranged limbs, however some patients will gradually reacquire voluntary use of the arm as the brain mends its delicate pathways. For the others, the disease can be disarmed for a time by satisfying its primitive tactile urges, and giving the fidgety fingers an object to grasp and grope.

A version of this article appeared in our actual, physical, paper book. You should buy a copy.