[MUSIC] One of the things that the cerebellum is very tightly involved with is the control of eye movements. The control of eye movements and the vestibular system. And it, the vestibular system meets eye movements most, most closely in one reflex, called the vestibuloocular reflex, also known as the VOR. Most people call it the VOR. So, the VOR is a very important reflex that we all have. It's what enables you to sit on a train or a bus or, and read. It's what enables you, as you walk down the street, to see a steady image. In fact your head's moving all the time, why doesn't the scene move all the time? Well, the answer is because the vestibuloocular reflex moves your eyes in opposition to your head movements. It uses the vestibular signal that tells you about head movement and then it moves your eyes to oppose that head movement and to keep your gaze where you're looking at, your fixation point, steady. So, why do we do that? Because the vestibular system is so fast. So, you can do, you should do this at home. For yourself. If you simply look at your finger, and move your, head back and forth, you can keep your finger in focus, but in contrast if you move your finger and keep your head still, you cannot keep your finger in focus because the visual system is too slow to adapt to, to this. So it can't, your visual system is too slow but your vestibular system is very fast. So, you use your vestibular system to guide an eye movement that opposes the head movement. And we can just, we can illustrate that it in, in this way. If we move our head, from the right to the left and then back to the right and back to the left and back to the right. Our eyes, if we are fixating on a steady point, our eyes will do the opposite. They will go and the addition of these two is, means that our gaze is steady. Great. So the VOR is a very important reflex. We use it every day, all day long. But it turns out that the VOR is, it, it's like all reflexes. Don't, don't think that a reflex is written in stone. A reflex has to be modulated and the VOR is no exception. And so we're going to, I'm going to demonstrate to you one example of how the VOR is modulated. If I look at my finger, like this, and I move my head, look at how much my my eyes move. So my eyes are moving a fair amount. Now, I'm looking off in the distance, and I'm going to move my head, and you'll see that my eyes move much less. So the amount of excursion that my eyes have to make when I'm looking at a near object is much greater to steady my gaze than the amount of excursion my eyes have to make to look at a far object. How do we change? How do we change the VOR. What we're doing is we're changing this gain. We're saying that sometimes the eyes have to, move this much. But sometimes they have to move less. [BLANK_AUDIO] Same face but, they have to move, move less. So they might have to move this much or they might have to move this much. And how does that get changed? It gets changed by a part of the cerebellum called the vestibular cerebellum, the flocculus and the nodulus. So, the cerebellum is on the fly every moment of the day adapting the VOR to to serve a steady gaze. And, in fact, not only does it adapt the VOR to have the right gains so that we're moving our eyes by the right amount. It can actually also turn off the VOR. I may not want my eyes to move in opposition to my head. What, let's say that I want to start looking over there. Well, it's so far over there that I have to move my head. But I don't want my eyes to oppose my head movement. I actually want my eyes to stay steady in my head. And I want to change my gaze. So, if I go like that, I have turned off my VOR. So, I can not only change the gain, but I can make the gain zero. I can turn off the VOR, but the only way I do that is through the, is through the cerebellum. The cerebellum is a, is a learning and changing feed forward machine. So. In patients that are in reduced states of consciousness, we can have, one of the common ways to try and figure out how extensive the damage is in the brain stem is to test the VOR. If the person is unconscious. What would happen if you turn their head? Well, what you would expect is that if there's cerebellar damage the VOR is going to have a fixed gain and the, the default gain of the VOR is about one. So, meaning that it's going to move the eyes in opposition to a head movement. So, a comatose patient, as you move the head back and forth, they're going to have what's called doll's eyes. The, the VOR is fixed at one. It is always going to move. They never suppress it, they never change the gain on it. Now. That tells you that the cerebellum is not working, but it also tells you that the brain stem, which supports the vestibuloocular reflex is working. So one step down from a person who has dolls's eyes is a person who has no VOR. And in that situation, they're unconscious as you turn their head, their eyes move with their head. Okay. And that's a person who has much reduced function within, within the brain stem. It takes the pons and midbrain to make the, the vestibuloocular reflex. So the, this is a very important way in which we evaluate unconscious patients. Another way, another reason why the VOR is, is important is as we go about our daily lives we change our glasses, and, every time you change your glass, you're recalibrating your VOR, and you're using the cerebellum to do that. In the next segment, we're going to look at a different example of cerebellar function as, as our final segment on the cerebellum. [MUSIC]
