Amputation, brain changes and phantom pain
Different regions of the central nervous system are specialised for performing certain tasks. And in areas like the somatosensory system, for example, the nerve cells are organised into a map that represents the body surface. And a stimulus delivered to any part of the body produces activity in the corresponding brain area. So what happens if someone undergoes an amputation? Does the brain still incorporate the now-missing anatomy in its neural map? And how does that compare with people born without that same body part in the first place? Chris Smith got the answers from Oxford University's Daan Wesselink...
Daan - A few years ago we had invited an amputee who had very vivid phantom sensations. And what I mean with phantom sensations is that, even though they'd lost their hand a long time ago, they could still feel their phantom hand as vividly as their other hand that they still had. And when we asked him in a scanner to move their fingers - that they don't have - their brain was activated exactly the same as this other hand. And we then wanted to do a big study where we compared every type of one-hander, both feeling their phantom out very vividly, and not feeling their phantom at all.
Chris - Now we've been brought up that practice makes perfect, and if you're a fine musician, or you learn another language, that actually this leads to remodelling of the brain - the cortex - so that we can generate more connections and have that ability. Having a phantom symptom that won't disappear kind of flies in the face of that, because it says the opposite is true; that once your brain has gotten an established map of a part of your body, it doesn't get rid of it.
Daan - Right. And I think both things are true: so the brain is able to save some information it's learned at a very young age and keep that going through the rest of the life; but also is plastic, and able to adapt through the things we learn. So with this study we really looked at the most basic area in the brain, dealing with touch, and say that that area...actually the fine details might be slightly different, but the overall architecture just stays the same once you've developed a hand.
Chris - How did you reach that conclusion? What was actually involved in doing the study?
Daan - We’re a neuroscience lab, and what we do is we use fMRI, which is basically this large machine in the hospital where we put people in. And then when they do a certain action in this scanner, we can see how active these different brain areas are. And so we were particularly focused in the touch area. So we put two groups of participants inside the scanner: so both people who had lost their hands later in life, and people who are born without a hand. And then when we looked at this brain area, which in you and I is a hand area, there was nothing there in people who had never had that han. And that's not so surprising, because they never had that hand. But then when we actually looked at the people who also didn't have that hand, but did have it when they were children, we couldn't distinguish it from that of people who still have both hands.
Chris - What that's telling you then is, as you surmised, that when you have a hand to start with, the brain establishes a map and a representation of that body part. If you then lose that body part, the brain doesn't magically remove that representation. It stays there, and is still capable of creating a mental image for that person of the part of the body that no longer have. So this must be the underpinning of when people have say, phantom sensations, it's because that brain representation is still there.
Daan - Yeah, exactly. We think that this somehow is produced by this area that still has all this information. However, we were even more surprised when we also put amputees, people who have lost their hand later in life. But some amputees do not feel this phantom hand anymore, they just lose it as well. But even in these amputees, we could find all the hand information in this brain area.
Chris - Now what do you think contributes to the maintenance of that representation? Is it just that, once you've laid down a patch of brain architecture when you're young, it's locked in and you're stuck with it? Or do you think that, because the sensory system has these reciprocal connections to many other regions of the brain - both the motor areas, so that you can anticipate what you're going to feel when you move, for example; also, the equivalent motor area on the opposite side of the brain, because the two reciprocally linked, aren't they. Do you think that it's the inputs from those other brain regions which helps to maintain this map?
Daan - Yeah, it must be somehow that the brain is able to keep this input from other areas. We're looking into it right now with more experiments. The only thing we know: it's not the other hand. So the two hands are in that aspect sort of independent. But like you said, motor areas could be true.
Chris - So why do you think, then, that some people do have more vivid phantom sensations? Why is it that people who don't have that appear to be better at remodelling their brain?
Daan - There are some behavioural experiments suggesting that what is important is not necessarily this most fundamental brain area, dealing with touch, creating sort of a map of the body, but some other type of readout. That there is another brain area that reads whatever information is in that area, and that brain area just stops reading it. and people lose this phantom sensations. But it's hard to say for sure unfortunately at this point.
Chris - Essentially what you've managed to do here is to confirm what we suspected might be going on, which is you've got this fairly fixed map, which is established early. Because we had clues that that was there from the days of the visual system, didn't we, with the Nobel Prize winners Hubel and Wiesel who actually did similar experiments in animals, showing that if you deprive the animal of visual input from a very early stage, you get permanent changes to the architecture of the nervous system. So that sort of agrees what you're showing here. But what can you now do next in order to take this further?
Daan - Right. I think the motivation really to move forward now is in neuroprosthetics. There's been a lot of research going on, on how to get this information from the brain to some kind of mechanical device that will replace a hand, and there's sort of two teams here. One group of engineers, they try to just access the brain, and try to have the controls make somehow sense to you and I - just be as natural as moving a real hand. And other people try to use other types of information, maybe like movement of the arm muscles, or movement of the eyebrows, that then will control this robotic hand. And I think we really like to say, well, in these people who have lost their hand later in life, all this information is still there, so you should probably use this information in the brain. Whereas in people who have never had this other hand, you would need to use other types of information. So other types of controls...