• Question: I’m fascinated by cortical reorganisation and how it is suggested to be linked to phantom limb pain. What causes cortical reorganisation to occur? And will there ever potentially be a way of reversing this process or will it always remain a bit of a mystery?

    Asked by anon-252064 to Yewande, Siddharth, Shenghong, Patricia, Luke on 28 Apr 2020.
    • Photo: Luke Bryden

      Luke Bryden answered on 28 Apr 2020:

      Hi there, and thanks for the great question. If we take for example a hand, the brain represents the information about touch and pain (what is called somatosensation) in an amazing map in the part of the cortex that represents touch and pain (somatosensory cortex). That means that each area of the hand has a dedicated part of the somatosensory cortex that represents the information sensed by that area of the hand. If a finger was amputated and no cortical reorganisation occurred, we would expect the region of the brain representing that finger to be silent – that is, not have any activity there because it is receiving no information about touch, since the finger is gone. In fact, scientists have found this area to be far from silent! The part of the somatosensory cortex that once represented the amputated finger now represents the sensations of the fingers either side – in effect, they have taken over the part of the cortex once dedicated to the amputated finger. This is called cortical remapping.

      Why does this happen? To be honest, I am not sure of the exact reason this happens, maybe some others will have more understanding than me. It is possible we do not fully understand why this happens, but we just know that it does happen. I am sure there are lots of people researching this topic as we speak! And as more research is carried out, we will learn more about the precise mechanisms that cause cortical remapping.

      As regards phantom limb pain, there is some evidence that the pain felt by a person after the limb is removed is because the process of removing the limb damages a lot nerves. Information about touch and pain get to the brain via nerves that go from the limb and through the spinal cord. If the limb is amputated, the nerves connecting the limb to the spinal cord are cut. This damage can cause what is left of the nerves to begin sending constant messages to the brain that are felt as pain. Why does this happen? Because the nerve is damaged, it may become hyperactive and this activity means pain signals are being sent to the brain all the time, even though no physical pain is happening. Others, as you point out, have suggested that cortical reorganisation happening after amputation of the limb is what causes the pain – the new map in the brain is such that pain is now felt.

      Can this pain be reversed? Some scientists have shown that phantom limb pain can be reduced in people who have had an arm amputated and replaced with a prosthetic one. In an experiment, every time these people used the prosthetic arm, the skin in the part of the upper arm (not amputated) was stimulated with electricity depending how strongly they gripped items. They could use the sensation of this electricity to improve their control of grip strength, and this also had the effect of reducing the phantom limb pain. We still don’t know why this sort of thing happens. It could well be that the process of learning to grip items reorganises the cortical maps and as a result the phantom pain sensation is no longer part of that map.

      I hope that helps, and sorry if the answer is longer than you expected! Feel free to reply if you are confused by anything I have said, or would like to ask more questions about this topic.

    • Photo: Shenghong He

      Shenghong He answered on 28 Apr 2020:

      Sorry I am not familiar with this topic. I think although different areas in our brain are associated with different functions, the functional structure or connectivity in the brain is not fixed. If something go wrong somewhere in the brain, it will impact the corresponding functions. In contrast, if we keep doing something, we can get better and better, and the functional connectivity in our brain is being trained. I am not quite sure whether this is reorganisation your have mentioned, but for sure, we have reinforcement learning processes in our brain.

    • Photo: Patricia Brown

      Patricia Brown answered on 29 Apr 2020:

      What an interesting question! I’m not an expert on this topic but I think Luke gave an amazing answer! I don’t know what causes this reorganization but it’s probably a way for the brain to adapt to a new situation (like the loss of a limb). There are still a lot of things we don’t know about how and why this happens!
      I’ve also heard of cortical reorganization in the context of blindness, where the visual cortex (the area of the brain normally responsible for processing visual inputs from the retina) can reorganize and get used while reading Braille (these are little bumps that the blind can read using their fingertips).

Comments

  • Photo: anon-252064

    anon-252064 commented on :

    That’s brilliant thank you!! I think it’s amazing how our brains can do that.
    Is there a possibility that the cortex can be remapped to a completely different part of the body then? Or is it just the neighbouring body parts?

    • Photo: Shenghong

      Shenghong commented on :

      For that I think it would be very difficult. We know there is a mapping between different parts of our body and different areas in the brain, but I think it would not be very precise. Even if a tiny area is damaged in the brain, it would very likely to impact a big part of the body, e.g., hemiplegia. There are many studies suggesting that a single movement is not controlled by a specific area of the brain, but by a collaboration of several brain areas. If an area goes wrong in this collaboration network, it will impact this movement, even though the other areas are functional. Based on this hypothesis, a very cool idea is to bypass the damaged area using a technique named brain-machine interface (BMI), and to reactivate the collaboration network and thus rehabilitate the function. If we think the brain is just like a super complex network circuit, when a subset circuit is collapsed, and if we know the input and output of this collapsed circuit, then it is possible to bypass it with a BMI.

    • Photo: Luke

      Luke commented on :

      My pleasure. Very good question! You are right, it really is amazing that this happens. The brain is an extremely ‘plastic’ organ, meaning that it is very responsive to changes and can adapt itself to deal with those. This is really important for learning, as well as for processes such as cortical remapping.
      –––
      Well, I learnt about cortical remapping during my undergraduate degree and my understanding is that it is usually only neighbouring regions of the cortex that can remap on to an area that is no longer active. So, in the case of the amputated finger, the neighbouring fingers are represented in neighbouring parts of the cortex, hence the remapping happens. If you look at your hand and imagine the amputated finger in my example is the middle finger. Now, if the forefinger were also to be removed, experiments have shown that remapping of the other fingers (so, that is the fingers either side, the index and little fingers) happens in the cortex. This means that the part of the cortex that once represented the middle and forefinger has now been taken over to represent the index and little finger. BUT, this wouldn’t have been as extensive as if only one finger is removed and there would be ‘silent’ parts of the cortex that had not been remapped. The idea being that the further away the cortical representations are from each other, the less extensive the remapping is. This is probably just because the distance is too great for remapping to happen, due to the way the cortex is organised (I think…).
      ––––
      It is also interesting to know that the maps of sensation in the somatosensory cortex mean that some quite weird and amazing remapping can occur! Some patients who have had an arm amputated report feeling phantom limb sensations when their face is touched. For example, if a drop of water is run down their face, they report also feeling a sensation on their left arm – but their left arm is no longer there! That’s because the representation of the face and arms are in neighbouring parts of the somatosensory cortex, and the area representing the face in the cortex takes over the area once dedicated to representing the left arm!

  • Photo: anon-252064

    anon-252064 commented on :

    Thank you !! This is really helpful.