That Neuroscience Guy - The Neuroscience of Phantom Limbs

Episode Date: January 22, 2023

Some people with missing limbs report sensations of that limb still being there, which we have termed Phantom Limb Syndrome. As it turns out, this is an excellent example of how our brain processes our body image. In today's episode of That Neuroscience Guy, we discuss the neuroscience behind phantom limbs and body image. 

Hi, my name is Olof Kregolsen, and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. Have you ever heard of phantoms in the brain? Now, there's actually a really cool book by a guy named Ramachandran, and that's kind of what I'm talking about, but this isn't a book review. It's more about people that have phantom limbs, limbs that don't exist. So on today's podcast, the neuroscience of phantom limbs, phantom pain, and body image. All right. So what do I mean by a phantom limb? Well, there's all these examples

of people that have had something traumatic happen to them. So imagine a car accident. And when they awaken, although their left arm might be gone, and this is one of the cases I'm going to discuss, they still feel like their left arm is there. So it's a phantom limb, a limb that's not there, but it feels like it's there. And what gets really interesting with these people is some of the things that you can do. So for instance, with the case I'm referring to right now, if you actually touch the person's face, they would tell you that it feels like they're touching their arm and their hand, depending on where you were on their face.

And when their face moves, they feel like their hand is moving. And even to the point that if you put a warm drop of water on their face and it slowly drips down their face, they can tell you that they're sensing a sense of warmth in their phantom limb, and they can also feel that water trickling along. So a phantom limb. Now, how can you explain phantom limbs, and what does it tell us about the way our brain works? Well, the original medical explanations were tied to the fact that if, say, the left arm was severed, just to stick with that example, then the neurons had been severed and those neurons were still activated somehow. The fact that they had been severed off forced them into a state of activation. Well, that has been largely discarded now. It doesn't make sense for any number of reasons.

So how do you actually explain phantom limbs? Well, to understand phantom limbs, we have to talk a little bit about the nervous system. Now, we've talked about the sensory system a little bit, but let's do a quick review. There's the central nervous system, which is effectively the brain and the spinal cord. And then there's the peripheral nervous system, which are the motor and sensory neurons that go out to the muscles of the body. Now, let's just talk about the sensory system. Let's just try this. You can try this while you're listening. If you tap the palm of your right hand right now and just tap it gently, you can feel that. Now, why can you feel that? Well, because

I'm tapping the palm of my hand. Thanks, neuroscience guy. But no, what you're actually doing is underneath the surface of your hand, there are pressure receptors. All right. And these are a form of neuron. The ending of them, the dendrite, if you will, has a specialized receptor that when it's compressed, it causes the neuron to fire. And that sensation or that firing is carried by the sensation and it goes to the spinal cord. Then it goes up to the brain into the primary sensory cortex. And in the case of tapping on your right hand, it actually crosses over and goes to the left primary sensory cortex. So when you start tapping your hand gently, then there's firing in the primary sensory cortex. And that's your brain's way of registering that

the left hand has been tapped. And underneath the surface of your skin, there's all sorts of receptors. There's heat receptors, there's pain receptors, there's cold receptors. Even the hairs have receptors on them that are sensitive to movement. And all of this information, like I said, is sent up through the spinal cord to the thalamus. Eventually, we've talked about the thalamus and to the primary sensory cortex where there is a representation of the body a one-for-one mapping of all these receptors now i'll point out here this also includes the muscles within muscles and tendons there are stretch receptors and they're firing as well and their message is being sent up the spinal cord to the thalamus and then on to the

primary sensory cortex so the primary sensory cortex. So the primary sensory cortex is always receiving information. It's continuously coming in and basically giving the brain a state of the body. So what's going on? Am I sitting? Am I standing? Am I moving? Am I cold? Am I hot? You know, is something touching me? Am I touching something? All of this information is encoded by sensory neurons firing underneath the skin and in the muscles and in the tendons and sending their messages up to the brain. And it's important to realize they're always firing. They're never not firing. Even if you're not using the biceps muscle, let's say if you contract your right biceps muscle,

then a whole bunch of stretch receptors in the muscle are going to start firing and send the message up to the brain saying, hey, we're contracting. Well, even if you're not contracting the muscle, even if it's completely relaxed, a message is still being sent every once in a while. It's just the occasional little firing. Just every once in a while. It's just the occasional little firing, just every once in a while, the neurons in the stretch receptor fire. And your brain interprets that as well. It goes, well, hey, the biceps muscle is still there. All right, I still have that biceps muscle. It's still a part of it. And when the firing rate picks up, it detects that as movement. And like I said, there's just this massive array of receptor neurons in the primary sensory

cortex bringing in all this information. Now, here's something interesting about this. There's regions in the somatosensory cortex for every muscle or stretch receptor, every tendon, every touch receptor. But what happens, for instance, let's say we lose a limb, as in the story of phantom limbs. So if you lose your left arm, you've got this massive stretch of sensory cortex that's expecting input from the left arm. And all of a sudden, there's no more input. And remember what I said, the muscles and tendons and the receptors in that arm would have always been sending a message.

But all of a sudden, they're not. All right. They don't check in. They don't do anything. Well, what actually happens is your brain sort of reorganizes itself. And the primary sort of level of the sensory cortex sort of expands. So the regions that are beside the arm,

and if you look at a map of the body on the brain, the areas that are beside the arm are the face on one side and then the shoulders, head and neck in the trunk of the body on the other side. So the arm occupies this region of the somatosensory cortex between the face and the trunk. Well, that's interesting. So what happens is the face region grows in to take over part of the region that was used by the arm,

and then the head and neck region grows in to use part of the region that was used by the arm. Because remember, the arm is gone. So that primary receptor level isn't needed anymore. So it's taken over by the face region and by the trunk and head and neck regions. So what does that tell us about phantom limbs? Like how do you explain a phantom limb? All right, you might have already guessed it, but there's multiple layers in the primary sensory cortex.

There's that first input layer, which is just receiving those signals from the muscles, the joints, the tendons, and all those other receptors. But then there's a secondary level, and a third level, and a fourth level. And those levels, the secondary level, the association levels we call them, they're sort of the representation of the body within the brain. So you've got the sensory level, which is this outer level, which is what's actually happening. And you've got these inner levels, which are more like your hardwired model of the brain. Now, if you lose that limb, like I said, then that primary sensory layer sort of takes

over the region that's not being stimulated, but the secondary association layer still stays the same. So in that secondary association layer, there still is a representation of the left arm, but in the primary sensory layer, there isn't anymore. But the connections are still there. And this is where it gets really crazy. The connections from the face now project to the face representation in the secondary layer, but also to the arm representation in the secondary layer. So if you think about that example I gave you, when you stroke that person's face, the primary sensory layer is sending input to both the arm representation in the secondary layer and the face. Thus, the person feels the sensation of the arm still being

present. And that is the explanation of the phantom limb. So the idea is that the primary area takes over the region for the left arm, but the connections are still to the secondary level representation of the left arm. So when the face is touched, the secondary level of the arm is stimulated. And there's another example of this. There's something called gaze tinnitus. And basically, people with gaze tinnitus, if they look to the left or right, they hear a ringing sound. Now, why is this?

Well, there's damage to the auditory nerve. And the eye movement neurons are next to auditory neurons. So when the auditory nerve neurons get damaged, the eye neurons take over the space and you get gaze tinnitus. So when the eye neurons fire, they trigger off these damaged auditory nerves and you hear a ringing sound. So this idea of taking over neural territory is quite profound. Now this tells us something else about how the way our brain works, and that's this idea of body image. So, within the primary sensory cortex, there's this initial layer,

which is just a representation of the world, and then there's these secondary layers, which might be a representation of the arm as a whole, or parts of the arm, so the forearm, and the biceps, and the triceps. And then as you move through the layers, the third layer, the fourth layer, you might get to more complex things. And eventually you end up with, hey, I'm standing or hey, I'm sitting or I'm lying down. So it's important to realize this buildup of information. So the primary layer to the secondary layer, to the third layer, to the fourth layer. And by the time you're firing at, and no one's ever counted all the layers, so don't ask. By the time you're firing at that highest layer, you've got basically a neuron that's saying, I am standing.

All right. And that's what we call our body image. It's our, basically our internal image, and it's our memory of our body. And this is generated deep within the parietal cortex, where this is all integrated together. And phantom limbs, there's more on them. For instance, there's a patient named Mirabelle, and she was born with phantom limbs. So by poor luck, I guess, if nothing else, she had no limbs at birth, but she had phantom limbs. So she was missing her left and right arm, and she had phantom limbs. So she was missing her left and right arm and she had phantom limbs. And she even reported as she got older, she could move them, but only in certain situations. So what does that tell us? It tells us that those primary

neurons aren't there, but the secondary layer, that hardwired body representation is still present. And you can have multiple body images. You could have the original one that exists at birth, and you could have a more up-to-date version. And that's what we think is what's happening with phantom limbs. Basically, there is the original hardwired version that includes that left arm. There's an up-to-date version, which is the primary layer, which doesn't. And because of that mismatch, you get the sensation of a phantom limb. Now, phantom limbs can go away. A classic way to treat them is with what's called a mirror box. And effectively, if it's your arms, it works the best, but you stick your right hand into a mirror box. And if you look at it the right way, you can actually see a reflection of your

right arm, but it looks like it's your left arm. And people have used this kind of therapy to treat phantom limbs to the point that they can make them go away. And in even more modern versions of this, people use virtual reality. So in a virtual reality environment, you see both your limbs. And by working with that phantom limb, somehow that causes the brain to rewire again, and that secondary level updates itself and eventually realizes that, guess what, I don't have a left arm anymore. Now, this was a request from a listener, and one of the things they asked about was,

what about phantom limbs, but also phantom pain? A lot of people that have phantom limbs report phantom pain. Now, how can you have pain if there's no limb there? Well, one of the best theories on this is imagine that person with the left arm missing and imagine that they start grasping their hand as hard as they can. They're trying to make a fist. Now, when we make a fist, we're pushing our fingers into the palm of our hand and we stop because we can feel the hand. Now, if you keep going, if you literally try to push your fingers through the palm of your hand, well, one, you physically can't, but at some point it hurts, so you stop. So one theory is that people with

phantom limbs, their hand is clenched so tight or their toes or whatever other body part that it actually is penetrating the surface of the phantom limb, which your brain perceives as pain. Because of course, imagine if you could push your fingers through the palm of your hand, it would really hurt. And again, the mirror box or virtual reality is used to treat this. You basically get people to unclasp their phantom hand. So you'd put your right hand, the mirror box or virtual reality is used to treat this. You basically get people to unclasp their phantom hand. So you'd put your right hand into the mirror box and have it clasped. You would see a clasped reflected hand, which would be your phantom limb hand in principle. And then as you open your right hand, you see your left hand opening up.

And people have reported this mirror box treatment of phantom pain as working. They feel that their phantom limb is released and they no longer feel pain. Now it tends to come back but through repeated treatment people are able to cure phantom pain. So there's quite a bit on the neuroscience of phantom limbs and phantom pain but it all ties down to body image and this idea that our brain builds up representations through layers. An initial layer, which is our sensation of the outside world, and then subsequent layers, which have more and more complex representations of the body. And if you think back to talking about the ventral visual stream, I talked, that's way back in season one, this same concept is used to explain visual processing. All right, that's way back in season one. This same concept is used to explain visual processing.

All right, that's the neuroscience of phantom limbs and phantom pain. Remember, check out our website, thatneuroscienceguy.com. There's links to our Etsy store where you can buy some of our merch, a couple of cheesy t-shirts and show t-shirts. I'll be meaning to get some more up and I will as soon as I have some free time. There's also links to Patreon. If you're not familiar with Patreon, you basically sign up and you create an account. And if you search for that neuroscience guy, you can support us.

Even a dollar a month or a dollar a week really helps. All of the money goes to graduate students in the Kregolson Lab. None of it goes to me. It's not that I'm rich. It's just my graduate students need it more. And of course, this episode was entirely based on a couple of users saying they were interested in phantom limbs and phantom pain. So follow me on Twitter at that NeuroSci guy, DM me, and I'll try to get your ideas onto the show, whether it's this season or next season. And of course, thank you so much

for listening. Please subscribe to the podcast podcast no matter what platform you're on. It really, really does help. My name is Olive Kregolson, and I'm that neuroscience guy. Thank you so much for listening, and I'll see you on Wednesday for another Neuroscience Bite.