 OK, welcome back. Let's continue for the next lecture. We're continuing onward with the topic of the human eye and the human vision system. And so last time we finished talking about different kinds of eye movement. I went into a bit of detail on saccades and smooth pursuit. I'm going to cover other eye movements in just a moment. I first want to make a quick correction that was pointed out during the break. So I gave these six different eye muscle names. And these two, superior oblique and inferior oblique, I had them swapped in the previous lecture. So this is the way they should be. Superior means on top and inferior means below. So that's all. So I just have these inverted. Rest is correct. So eye movement number three that I want to talk about is the vestibulo-ocular reflex. You can see it has the word vestibular in it. So this is sometimes abbreviated as VOR. And it's called the VOR, pronounced letter by letter. Here's a diagram showing how the vestibulo-ocular reflex works as part of our physiology. And these circles in the bottom correspond to canals inside of our vestibular organ. So they're inside your ear canal. And they're measuring angular accelerations. And so for example, you have the right vestibular organ. And that's connected through a very short kind of loop, let's say, that does not use your higher level brain functions at all, a very tight loop that controls these left eye muscles. And then your left vestibular organ is controlling the right eye muscles. It's exactly these muscles that we've drawn on the board there for the eye movements. And in less than 10 milliseconds of delay, you get these eye rotations that counter the rotation of your head. There's also translational movement as well. So it's even occurring when I do this. If I just translate my head back and forth without doing any significant rotating. But the important thing is, the most common thing is that when I put an object in front of me and I go back and forth like this, I have the vestibulo-ocular reflex working for me that's causing me to perceive this as stationary. And it's again stabilizing images on my retina for that. So in one case, the smooth pursuit, it was the object of interest that is moving while the viewer is remaining still. And in this case, it's the viewer who's moving and the object is remaining still. Of course, you can imagine some combinations of motion of both, but I'm just trying to separate them out nicely. So it's fascinating that it bypasses the brain, at least the higher level parts of the brain. So the response time is around 10 milliseconds, which makes it the fastest reflex in your body. Purpose is to keep image stability. You can imagine right away, if you have a virtual reality headset that has a lot of flaws in it with regard to latency, pixelation, all sorts of things going on. This reflex is trying to maintain image stability and you're interfering with it by presenting this artificial stimulus, trying to fool your brain. If you don't get it right, your perceptions of stationarity are not going to be matched perfectly. So you get counter rotations and also translations. Well, you get counter rotations to compensate for the sixth degree of freedom head motion. So you're getting rotations because of these high movement muscles being connected to the vestibular organ. And so all we can do is rotation, right? I cannot translate the eyeballs inside of our heads. So it's doing rotations to compensate for full sixth degree of freedom motions while I remain fixated on something. Here's something fascinating that happens. You should definitely be aware. It's very important for virtual reality. It's called VOR gain adaptation. So in this case, I'm looking at this bottle and I move my eyes accordingly with the vestibular ocular reflex as I rotate my head. The counter rotation rate of my eyes is exactly matching the rotation rate of my head, right? So it's a perfect one-to-one correspondence. That corresponds to a gain of 1. If I were to put on some glasses and then do this very quickly, I would perceive the bottle as swaying back and forth. We would call that a swimmy kind of motion. Now for those of you who are wearing glasses for a long period of time, if you do this experiment, the bottle should look stationary. And so what's happening is the optics of your glasses are causing a distortion, but your brain is learning to compensate for that. So your vestibular ocular reflex, this gain parameter, will adapt and change to your glasses. If you take your glasses off immediately and then do this, you might see the real world, an object in the real world looking like it's not stationary, which is quite incredible, right? If you put on a virtual reality headset where the optics have distortion in them and it has not been correctly compensated, your brain may adapt to that. And then when you take the headset off and you do this in the real world, the real world might look like it's swimming back and forth. So when I was doing development at Oculus, I saw this very frequently. I would spend hours trying to fix distortion, tracking other kinds of things, and then I would go look at the menu board for lunch, turn my head back and forth, and the menu board looked like it was swaying back and forth and it wasn't right. So when I started to really question reality in many ways, right? It was very strange to have this happen. So these things are somewhat invisible to us. They're happening all the time. If you wear glasses, you know these things are happening. Question, yes? For 60 years of head, where eyes are only 2 years. So how can it compensate? So we're compensating for 6 degrees of freedom of head motion, but we only have 2 or 3 degrees of freedom, I guess, right? With the eyes in general, I guess, right? But I guess we cannot compensate completely in some cases, right? So that's a good observation. There's more degrees of freedom in the motions than there are in the possible rotations that we can apply. That's correct. Let's see. So maintaining visibility of you while I move my head and body around, that's a good question. What's really wrong here? How many? It's interesting. When I'm doing these motions, it's very hard. I guess I have to think about designing a particular motion that's going to be hard, right? If I do that, I think my VOR is not working very well. Anyone want to try that? Do some very strange figure 8 motions like that? I don't feel too well, no. So I think that's the, I think, yes, it's possible to design some complex motion patterns that cannot be compensated directly by the VOR. That's my guess to your answer. It's a great question. I haven't considered that before. Anyone else? I did sort of energize everyone towards paying attention to degrees of freedom very carefully. So you did that well. That's a good question. I should have anticipated it. Let's see. So another case, 4 is optokinetic. And this is an alternation between pursuit and saccade. And the reason why this happens is for a very fast moving rather large object that you may want to track. And you keep jumping from feature to feature on it. So the most common example I can think of is you're standing on the ground and you're watching a train go by. So your brain will go into a mode where your vision system will go into a mode where it tracks a feature on the train and then jumps to another one and then jumps to another one. You ever do this before? I don't find it very comfortable at all. But nevertheless, it's interesting to watch a fast moving train when you're fairly close to it. Has everyone done this before? So this is some kind of mode. I cannot think of too many more examples where this occurs. And let's see. Example 5, it's a convergence. And I'll put divergence together. Now, you put divergence there. I'm not saying that your eyes are diverging outward when you're looking. Some people may have such a condition, but what I mean to say is that when I'm converged and they're looking together, then when I look at an object that's far away, they diverge. The motion is diverging. So your muscles are either pulling them towards convergence or pulling them away towards divergence. So that's two different directions, but it's the same phenomenon. So I may have my eyes rotated to look at something very close or not rotated together as much, not oriented together as much, to look at something far away. So it's these motions between these two. So if I go this way, then I'm converging. If I go the other direction, then it's diverging. But it doesn't mean the result is fully diverged. The eyes are converged in both cases until the extreme case when you're looking at something, let's say, at infinity, then they should be parallel. Let me see where I'm at on this. And I have one final case to cover. This is the most mysterious. I think much, very much is mentioned about it in the book, but there's a lot of current literature on it. It's called microsiccades. That's number 6. So these are tiny motions. They're around 1 30th to 2 degrees of motion. They're involuntary. And this may be one of the reasons why I showed you some images in the first class. And one of the images I showed you had looked like some kind of fractal pattern. And it looked like it was moving all by itself, but clearly it wasn't moving. And microsiccades might be one of the reasons for this. So I've done some research on this. I've found half a dozen different possible explanations of what microsiccades are good for. Some of them may have to do with further stabilization, perhaps refreshing photoreceptors that have become saturated over time, all sorts of possibilities. But largely, researchers are debating this over the last few years, but just something to point out. There are all these additional motions. So the amount of instability in the images that are falling onto our retinas, I really find astounding. And the brain is compensating for all of that. So the microsiccades category could have a half a dozen subcategories where microsiccades are occurring for various purposes. These have been known for a very, very long time. Robert Darwin, father of Charles Darwin, actually discovered them first in the 19th century. So a very long time they've been known, but we still don't have an explanation of what exactly they're good for. Why are they occurring? They're clearly intentional. It's not some kind of accident, it seems.