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Imagine going to post a comment on this video, but out of nowhere, your hand decides to slap you instead of typing. That's basically what happened to a 57-year-old woman in 1908.

Much to her surprise, she was attacked by her left hand. And the hand was persistent— it took a lot of effort to restrain the thing and stop it from choking her. Her case was one of the first recorded instances of alien hand syndrome, an extremely rare condition where patients seem to lose all control of one of their limbs—usually a hand, hence the name.

And as creepy as that sounds, it can tell us a lot about how our brains control our actions. Now, if you're sitting there worried that your hand is suddenly going to turn on you, you can relax. Not all alien hands suddenly turn into assassins … although it's not uncommon for them to misbehave.

Some have casually undressed people, or poured freshly made coffee on the floor. Which, to be fair, is also not great, and kind of makes the syndrome sound like something someone made up to excuse their behavior. But it isn't made up.

It's a real syndrome, with a known physiological cause. Even though alien hand syndrome was first reported around 1908, the frequency of cases increased somewhat from the 1940s onwards, when neurosurgery for epilepsy began to take off. That's because the accepted surgical plan at the time involved cutting along the corpus callosum— the bundle of nerves that connects the left and right hemispheres of the brain and lets them talk to each other.

Neurosurgeons of the period quickly came to realize, however, that a remission in seizures wasn't the only result. There were some … less welcome side effects. Like patients whose left and right hands would face off against each other while they were trying to do stuff.

Later, in the 1950s, doctors began to notice that patients with damage to parts of their frontal lobe showed similar symptoms, with their hands reaching out and grasping things without conscious consent. These days, alien hand syndrome is more commonly associated with strokes, tumors, and neurodegenerative diseases. What all these causes have in common is that they can damage three main parts of the brain: the medial frontal cortex, the corpus callosum, and the parietal lobe.

Damage in each leads to slightly different variants of the syndrome. Medial frontal cortex damage usually leads to mischievous dominant hands. Patients usually find that their right hands grope impulsively, and seem reluctant to let go of anything they manage to grab.

They're also pretty likely to show difficulties with other behaviors that involve the frontal cortex, like speaking a language they aren't fluent in. With damage to the corpus callosum, like from brain surgery, strokes, and tumors, the left hand is usually the one that goes rogue. Those patients are also more likely to see their hands literally fight with each other—what doctors call diagnostic dyspraxia.

The left hand also tends to go rogue when the damage is in the parietal lobe. But unlike the other two, patients with this variant are said to feel a, quote, ‘strong sense of estrangement' from the limb. It's not just that they can't control it— it's like someone else's arm is glued onto their body.

Some people even name their alien hands, personifying them as someone else. The parietal lobe is often involved in body awareness, so it kinda makes sense that damage to it might prevent you from feeling ownership over a part of your body. Even though doctors have enough data to categorize these three types of alien hand syndrome, they still aren't sure exactly how the brain damage causes the syndrome.

Most agree that injuries to these areas somehow keep people from being able to inhibit their actions. If you look at a cup with a handle, part of your brain knows you can pick it up, even if you don't consciously think about it. But, if you see any old cup during the course of your day, you don't just reach out and grab it.

The kinds of damage associated with an alien hand seem to break down that barrier between ‘I could do this action' and ‘I will do this action' for one limb. So the person's hand just goes ahead and grabs the cup, with or without their consent. And having your hand do something you don't actually want it to do makes it feel … well, alien.

Unfortunately, there's no known cure for alien hand syndrome— partly because cases are so rare. There are some clever ways patients manage their condition, though. Some find it helpful to give their alien hands a cane to keep them occupied.

By letting them hold and play with something, they're distracted from acting on other objects in the environment and cause less trouble. Others have tried more direct, physical interventions. One case report suggested that symptoms improved after injecting botulinum toxin, better known as Botox, into the affected arm to weaken it.

But even if alien hands aren't easy to treat, every case is a learning opportunity, because doctors can examine how different types of brain damage lead to different behaviors and feelings. And the more we learn about how the different parts of our brains work together to coordinate actions and take ownership of our movements, the closer we'll get to finding a real cure someday. So next time you accidentally spill coffee down your shirt… just be glad that your hand is listening to you at all.

Thanks for watching this episode of SciShow Psych. If you were intrigued by the idea of your hand becoming not your own, you might like our episode on the opposite effect— how scientists can make you think a hand that isn't yours actually is. [OUTRO ♪].