Imagine being attacked by one of your own hands, which repeatedly tries to slap and punch you. Or you go into a shop and when you try to turn right, one of your legs decides it wants to go left, leaving you walking round in circles.
Last summer I met 55-year-old Karen Byrne in New Jersey, who suffers from Alien Hand Syndrome.
Her left hand, and occasionally her left leg, behaves as if it were under the control of an alien intelligence.
Karen's condition is fascinating, not just because it is so strange but because it tells us something surprising about how our own brains work.
It started after Karen had surgery at 27 to control her epilepsy, which had dominated her life since she was 10.
Surgery to cure epilepsy usually involves identifying and then cutting out a small section of the brain, where the abnormal electrical signals originate.
When this does not work, or when the damaged area cannot be identified, patients may be offered something more radical. In Karen's case her surgeon cut her corpus callosum, a band of nervous fibres which keeps the two halves of the brain in constant contact.
Cutting the corpus callosum cured Karen's epilepsy, but left her with a completely different problem. Karen told me that initially everything seemed to be fine. Then her doctors noticed some extremely odd behaviour.
"Dr O'Connor said 'Karen what are you doing? Your hand's undressing you'. Until he said that I had no idea that my left hand was opening up the buttons of my shirt.
"So I start rebuttoning with the right hand and, as soon as I stopped, the left hand started unbuttoning them. So he put an emergency call through to one of the other doctors and said, 'Mike you've got to get here right away, we've got a problem'."
Out of control
Karen had emerged from the operation with a left hand that was out of control.
"I'd light a cigarette, balance it on an ashtray, and then my left hand would reach forward and stub it out. It would take things out of my handbag and I wouldn't realise so I would walk away. I lost a lot of things before I realised what was going on."
Karen's problem was caused by a power struggle going on inside her head. A normal brain consists of two hemispheres which communicate with each other via the corpus callosum.
The left hemisphere, which controls the right arm and leg, tends to be where language skills reside. The right hemisphere, which controls the left arm and leg, is largely responsible for spatial awareness and recognising patterns.
Usually the more analytical left hemisphere dominates, having the final say in the actions we perform.
The discovery of hemispherical dominance has its roots in the 1940s, when surgeons first decided to treat epilepsy by cutting the corpus callosum. After they had recovered, the patients appeared normal. But in psychology circles they became legends.
That is because these patients would, in time, reveal something that to me is truly astonishing - the two halves of our brains each contain a kind of separate consciousness. Each hemisphere is capable of its own independent will.
The man who did many of the experiments that first proved this was neurobiologist, Roger Sperry.
In a particularly striking experiment, which he filmed, we can watch one of the split brain patients trying to solve a puzzle. The puzzle required rearranging blocks so they matched the pattern on a picture.
First the man tried solving it with his left hand (controlled by the right hemisphere), and that hand was pretty good at it.
Then Sperry asked the patient to use his right hand (controlled by the left hemisphere). And this hand clearly did not have a clue what to do. So the left hand tried to help, but the right hand did not want help, so they ended up fighting like two young children.
Experiments like this led Sperry to conclude that "each hemisphere is a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting".
In 1981 Sperry received a Nobel prize for his work. But in a cruel twist of fate, by then he was suffering from a fatal degenerative brain disease himself.
Most people who have had their corpus collosum cut appear normal afterwards. You could cross them in the street and you would not know anything had happened.
Karen was unlucky. After the operation, the right side of her brain refused to be dominated by the left.
She has suffered from Alien Hand Syndrome for 18 years, but fortunately for Karen her doctors have now found a medication that seems to have brought the right side of her brain back under some form of control.
Even so I felt it was tactful, when I said goodbye, to give both hands a firm "thank you" shake.
Karen's story features in The Brain: A Secret History - Broken Brains BBC Four, Thu 20 Jan 2100GMT, repeated Tue 25 Jan 2300GMT or online via iPlayer (UK only) at the above link.