The operation was to treat Karen's epilepsy, so her doctors put her to sleep and cut into her brain.

And Karen says when she woke up, her speech was a little funny, but basically she felt great.

"I had woken up, and I was sitting on the hospital bed, talking to my surgeon, and —"

And then all of a sudden, her left hand picked itself up and started moving towards her shirt, started delicately undoing its pearly top button.

"My hand was taking my clothes off!"

"Taking your clothes off?"

"Yeah, my hand was taking my shirt off!"

"The surgeon was like, 'Karen? Do you realize what's happening?'"

And then, all of a sudden, the hand seemed to get angry.

"It was tearing the buttons off the shirt."

Karen kept telling it to stop.

"Knock it off, knock it off, knock it off."

Her surgeon was screaming at her to control it.

"Try to make it stop!"

I said, "I can't! It won't stop it!"

And then I started to just cry, I just didn't know what to do.

When Karen went home from the hospital, the hand — this new hand, with a mind of its own — went with her.

A lot of the time, the hand was okay.

But it would also get really upset with her.

Like sometimes, when Karen did something it didn't like? "Smacked me, right across my face."

"Your own hand?"

Her problem, the doctors explained, was Alien Hand Syndrome.

See, to cure her epilepsy, they'd had to seperate the two halves of her brain.

So, without her corpus callosum, her right hand seemed to be getting directions primarily from her left hemisphere, and her left hand — this new alien hand — seemed to be getting directions primarily from her right hemisphere.

"Half my brain just didn't particularly care for me too much."

Now this is not science, this is just how Karen sees it.

She says, she thinks her hand wants her to be more moral.

Take, for example, what her alien hand does when she tries to smoke.

"When I go to light a cigarette, the hand will either put the cigarette out, or flick the ashes around."

And though she's learned to live with it — she's in her fifties now — she's not always happy with it.

"I understand that you want me to quit, but, cut out the crap."

Hanna Rosin, NPR News.