That Neuroscience Guy - The Neuroscience of Why You Can't Tickle Yourself

Episode Date: November 11, 2023

Have you ever wondered why you can't tickle yourself? In today's episode of That Neuroscience Guy, we discuss how the brain's motor system prevents you from being able to tickle yourself. 

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. I have to confess that all of my life I've been really ticklish, like the bottom of my feet, the backs of my knees, just about anywhere. If you tickle me, I'm going to laugh. And I remember when my son was younger, he took advantage of that. He would tickle me and I would just, I did try not to fight back and I was just perpetually laughing and laughing and laughing.

But we were talking the other day and he asked me an interesting question and I do know the answer and I thought I'd share it with you. What he asked me was, dad, why can't you tickle yourself? So to understand this question, it's actually kind of a deep dive into motor control. And I mentioned it briefly back in season one, but I thought I'd get back into it and take it to a sort of different level because it's about how your brain actually works. So the reason you can't tickle yourself starts with the way that motor commands are executed. So when you decide to move, so you're deciding to tickle yourself, the premotor areas of the brain put together what we call a motor command. It's a set of instructions that is basically what gets sent to the primary

motor cortex, and then that's how your body ends up moving. Those instructions are executed, and those instructions contain information about the muscles you're going to use, how much force you're going to exert, and the timing of those things. And when those instructions are sent to the primary motor cortex, information is sent down the spinal cord, and you begin to move. So you've got your hand, and you've programmed it to do a tickling thing, and let's say you're trying to tickle the back of your knee. Well, obviously you can physically do that, but you don't respond, right? Like, you know, you don't get tickled and at least you shouldn't get tickled. It's pretty rare that someone can actually tickle themselves. For most of us, we can't. So what's happening and why can't we do this? Well, interestingly enough, every time

you put together a motor command, it doesn't matter what it's for, your brain makes a copy of it. So imagine that it is this set of instructions. It's like the brain takes a photocopy of that set of instructions. And that photocopy has a very specific name. It's called the efference copy. And the efference copy is sent to a different part of the brain. It's sent to the cerebellum. At least that's the working theory. Some of this stuff is still being pondered and debated, but most people believe that the efference copy is sent to the cerebellum.

So what does the cerebellum do with this efference copy? Well, this is where it gets kind of cool. What the cerebellum does is it takes that set of instructions and it simulates them. Yeah, that's right. It simulates them. It's like, if I do this, what's going to happen? So the motor command is simulated and your brain doesn't just think about the outcome. It actually also simulates the sensory feedback. So it's almost like it's saying, hey, what is my limb going to feel like when it's in a certain position? And that makes sense because if you think back to the times I've talked about sensory feedback,

your brain is constantly getting information from joints and muscles, basically telling the brain where they are in space. And that's how we get our body position and understanding of our body and what's going on. So the forward model in the cerebellum simulates you tickling yourself and it simulates what it's going to feel like. But it also does something interesting. It actually negates the sensory feedback that would lead to you being tickled. It's like your brain is saying, well, hey, I don't want to do that. So I know it's coming. So I'm just not going to respond that way. Hopefully that makes sense. Your brain is simulating the act of tickling. All right. And it's predicting the sensory consequences

and it's basically negating them. And that's why you can't tickle yourself. All right. Because your brain is literally negating the sensory feedback that it knows is coming because the forward models predicted it. And this is why someone else can tickle you. If someone else goes to tickle you, their brain's going to put together a motor command that is them tickling. But your forward model doesn't know what they're going to do. All right command that is them tickling. But your forward

model doesn't know what they're going to do, all right? It can't tell what's happening next, so it has no ability to predict what's going to happen. So you're just forced to respond, and because you're being tickled, your sensory system takes that as something it should response to, and you start laughing and giggling or whatever your tickle response is. So someone else can tickle you because your brain can't make the prediction about what they're going to do and specifically the forward model. But you can't tickle yourself because your brain can make the prediction and it's able to accommodate the tickling. And you can try this right now at home. Not the tickling bit. I'll give you something else to try. Close your eyes and touch your nose. Now, most people are incredibly

accurate with this, even if you do it very quickly. And why are you accurate? Well, because the sensory system knows the exact position of your body in space and knows where your hand is and it knows where your nose is. So when you go to tickle yourself, the forward model is actually already examining the movement and making sure that it does what it's supposed to do. And if it isn't, let's say you had programmed a movement that it would have ended up with your finger in your eye, the forward model goes, well, that's not what we want to have happen. So it helps the brain adjust the movement so you always end up touching your nose. Now, you could try this if someone else was around you, but don't actually do it unless it's a close friend or someone that's okay with it. If you're sitting by someone, close your eyes and try and touch their nose. Well, you can't touch their nose because you can't see.

And the forward model, while it's good, it can't handle that problem. So it doesn't know exactly where their nose is in space. It's just got to guess based on its last memory. That memory is probably a bit inaccurate. So your limb is going to probably get somewhere near where their nose is. But it can't make those adjustments and predictions that it needs to be able to touch someone else's nose. So within the cerebellum, there's this cool thing, the forward model, and it's why you can't tickle yourself because it's making predictions about what you're going to do. There are a lot

of other cool consequences of forward models in the brain. One of them is it's why our movements are so smooth. If you think about it, when you reach out for a fork or an apple or your cell phone, you never miss unless you're not looking. You have this very smooth movement that always gets you to where you want to be. Now, your brain isn't able to program those perfect movements. It does have mistakes in them. Your brain is trying its best to put together that movement, and it does a pretty good job, but it's not perfect, at least on the initial programming of the movement, the initial motor command. But what happens? The efferent copy is sent to the cerebellum, and the cerebellum makes a prediction about what's going to happen. And if it detects

that the movement isn't perfect, as in you're a little bit off course for that apple, it's going to happen. And if it detects that the movement isn't perfect, as in you're a little bit off course for that apple, it's going to make adjustments. And because the Ford model doesn't do this once, it does it over and over again. So no one knows the exact time course, but let's say it's updating every half second. Your movement is smooth because if there's any errors in the movement, the Ford model is going to fix it for you. And this is why those movements that we will practice over and over again are smooth. Now, in the case of sports, if you think of expert performance, experts probably have forward models for their sport skills. But if you're just learning a new sport, the forward model is probably not that well

developed. And that's why you might not be able to perform the movement perfectly every time. There's of course certain other factors you know baseball would be an example that would be good. The reason baseball players don't hit 100% is because the forward model can get the bat to the right spot, but that its estimate of that right spot is based on information about the pitch that it's gotten quite a bit earlier in time. If you think of a major league pitcher pitching close to 100 miles an hour, the last time you actually see the ball, it's still some distance from the plate. And the reason for that is it takes time to process visual information and update the motor system. So what's happening is the forward model has to make a guess as to where the ball is going to be,

and then it makes fixes to the motor command to get there. So based on that information that's a bit far out, the forward model is doing it perfectly, but of course the pitch might change. It might be a slider, it might be a curveball, and any number of things that baseball players do. I'm sort of highlighting my ignorance of baseball there, but you get the idea. But if you took a major league player and you got them playing tee ball, that's when they put the ball on top of the stationary tee, they would knock it out of the park every single time. And this is kind of what happens in batting practice. In Major League batting practice, one of the reasons they hit the ball so often is the pitchers are instructed to throw the ball very

specifically directly over the plate. And because the forward model has a pretty good estimate of what's happening, when the player makes the swing, they're going to be successful most of the time. In a real game, however, the pitcher changes it up and the batter doesn't know what's coming and that throws the forward model off. So the reason you can't tickle yourself is your motor system and specifically the forward model and the cerebellum is making predictions about what's going to happen and it's negating that sensory feedback. It also leads to some advice I could give you. If you want the neuroscience advice on how to be good at tickling, you have to be unpredictable. Think about it. If someone's

tickling you and they do the exact same motion over and over and over again, your forward model is going to learn. It's going to anticipate the movement as best it can, and it's going to try to dial down the sensory response. However, if you're tickling someone and you're incredibly unpredictable, guess what? The forward model can't do anything and the person is going to get tickled really well. Anyway, that's a little bit on the neuroscience of why you can't tickle yourself, but it was a review of this concept of forward models and the fact that your brain spends an awful amount of time making predictions about what's going to happen next. spends an awful amount of time making predictions about what's going to happen next.

All right. Don't forget the website, thatneuroscienceguy.com. There's links to our Etsy store. There's links to Patreon where you can support us directly. I'm available on X at that neuroscience guy and threads at that neuroscience guy. If you want to send us ideas, it would be great just to DM me. We are planning episodes all the time, and this idea was one of my own, but a lot of the episodes this season have come from you, the listeners. We really want to know what you want to know about the neuroscience of daily life. And if you don't like those two mediums, you can always email us, thatneuroscienceguy at gmail.com.

And finally, thank you so much for listening to the podcast. Please subscribe if you haven't already. My name's Olof Kreg Olsson, and I'm that neuroscience guy. I'll see you soon for another neuroscience bite.