Why amputees still feel a phantom limb

HostMost of us have had that strange moment where we reach for a phone in our pocket, only to realize we left it on the kitchen table. For a second, you could've sworn you felt it vibrate against your leg. It's a tiny trick of the mind, but it hints at something much deeper. Some people feel that same thing, but with an entire arm or a leg that's no longer there. They call it a phantom limb, and it can feel just as real as a hand you can actually see. Why does the brain keep hold of something that has been lost?

GuestIt's because your brain has a very firm idea of what your body looks like, and it doesn't like to change its mind. Deep inside your head, there's a kind of map. Every part of your body has a dedicated spot on this map. There's a little patch for your thumb, a strip for your leg, and a big section for your face. When you touch your real thumb, that thumb spot in your brain lights up. The thing is, when someone loses a limb, the map doesn't just vanish. The part of the brain that used to talk to the hand is still there, sitting in the dark, waiting for a signal. If those brain cells fire for any reason, the brain doesn't think, oh, that's just a stray spark in my head. It thinks, my hand is moving. It's a bit like a ghost house where the lights keep turning on even though no one lives there anymore.

HostBut you would think the brain would get the message eventually. If you look down and see that your arm is gone, why doesn't the brain just update the map and delete that section?

GuestWell, the brain isn't like a computer that you can just wipe and restart. It's more like a dense forest where the paths have been trodden down for years. Those paths are hard to change. But something even weirder happens over time. Since the hand area of the map isn't getting any new signals, the neighbors start to move in. In the brain, the area for the face sits right next to the area for the hand. When the hand stops sending news, the face area starts to grow and take over that empty space. This leads to some truly wild results. There are cases where a person can be touched on their cheek, and they'll tell you they feel a tickle on the thumb of their ghost hand. Their brain has re-wired itself so that a touch on the face now triggers the old hand map. The brain is trying to be useful by using that idle space, but it ends up creating a strange crossover in what the person feels.

HostThat sounds like it would be incredibly confusing to live with. Is it just a harmless mix-up of signals, or does it actually cause problems for the person?

GuestIt can be a huge problem because the ghost limb often feels like it's in a lot of pain. Think about it this way. Your brain is used to sending a command to clench your fist and then getting a signal back saying, okay, the fist is closed. Then it sends a command to relax, and it gets a signal back saying the hand is open. But if the hand is gone, the brain sends the command to clench and hears... nothing. No signal comes back to say the job is done. So the brain just keeps shouting. It keeps sending the signal to clench harder and harder. To the person, it feels like their ghost hand is squeezed into a tight, painful fist that they can never, ever relax. They might feel like their nails are digging into a palm that isn't there. It's a kind of loop where the brain is stuck waiting for an answer that can never come.

HostIt seems like a cruel trick for the brain to play on itself. If the problem is that the brain is stuck in a loop because it can't see the hand relax, is there any way to trick it into thinking the hand is actually moving?

GuestThat's exactly what doctors started doing with a simple mirror. They put a mirror upright on a table and have the person put their remaining real hand on one side. When the person looks into the mirror, they see the reflection of their real hand, but it looks like their missing hand has come back to life. Then, the doctor tells them to move both hands at once. The person sees their ghost hand opening and closing in the reflection. For the first time in years, the brain gets a visual signal that says, hey, the hand is relaxing. For many people, that one look in the mirror is enough to break the loop. The ghost hand finally lets go, and the pain that has been there for years just drops away because the brain finally feels like it has been heard.

GuestDoctors are now testing ways to use virtual reality goggles to help people move their ghost fingers in a digital world to help them find that same sense of peace.

HostThat ghost phone in your pocket shows just how hard it's for the brain to rewrite the map it has been using since the day you were born.