(upbeat music) - I made a video a while back about the interesting ways that our eyes move. It's called how to move one eye on its own, but really it's an excuse to talk about the way our brains control our eyes, 'cause it's really weird. But I actually glossed over something really interesting.
Consider this perfect scale model of a human eye and think about the way that your eyes can move. So your eyes can move up and down like this. So you've got two muscles here and here, the top muscle pulls your eye upwards, the bottom muscle pulls your eye downwards like that.
Your eyes can also move side to side like that. So you've got a muscle here and a muscle here on the sides. This muscle pulls your eye that way, this muscle pulls your eye that way.
But you've actually got two more muscles. So you've got six in total. You've got one on the top and one at the buttons like this.
The top muscle pulls your eye around that way, the bottom muscle pulls your eye around that way. In other words, you can roll your eye. Now if you think about the terminology used in aviation, you've got pitch of an airplane, the yaw of an airplane and the roll of an airplane.
So these muscles roll your eye. In other words, they rotate your eye around an axis that is, your line of sight. But why would you need to move your eye in that way?
There are two reasons actually, the first reason is to maintain a stable image on your retina. That's really important for clear vision. Actually, it's why when you move your head side to side and up and down, your eyes move in the opposite direction.
It's a counter movement that maintains the orientation of your eyes. So they're always pointing at the same thing as you move your head around. That maintains a stable image on your retina and reduces motion blur.
The same thing happens when you tilt your head from side to side, your eyes counter rotate to maintain a stable image on your retina. It actually happens in steps. So your eyes smoothly rotate and then jump back and then smoothly rotate and then jump back.
And it repeats that as you tilt your head, it's a bit like if you're on a train and you're watching the world go by, your eyes will smoothly follow a tree and then jump back and then smoothly follow the next tree and then jump back. And again, it's to maintain a stable image on your retina. It's not that easy to see an eye rotating actually, unless it's really bloodshot.
So lucky me. The second reason is to maintain good depth perception. I've created a stereogram image here of a pencil.
To view the image you need to go cross-eyed and overlap the two pencils, and it should pop out as a three-dimensional image. If you're watching this on a big monitor you might need to step back from the one until or make the video smaller on the screen or switch to viewing on a phone. Hopefully, then you can go cross-eyed enough to overlap those pencils and see a 3D version of it.
Pause the video now if you need some time to achieve that. But now I'm going simulate your left eye rotating slightly. I want you to see what happens.
Because at the moment, the bottom of the pencil is coming towards you, and the top of the pencil is receding away from you. But look, as I rotate one of those images now the top of the pencil is poking towards you, and the bottom of the pencil is receding away from you. You have a false impression of the depth of this image.
So it's important to keep your eyes in alignment with each other for correct depth perception In summary then there are two types of this torsional eye movement. There's this type called cycloversion that maintains a steady image on the retina. And there's this type called cyclovergence that maintains good depth perception.
What's really interesting is this cyclovergence seems to be an evolutionary exaptation. So an evolutionary adaptation is a trait that solves a certain problem. But an exaptation is where a pre-existing trait is co-opted to do something else.
And this trait of being able to move eyes in this way evolves in our ancestors that had eyes on the sides of their heads like this one here. So by the way, prey animals have their eyes on the side of their heads like this. So they've got wraparound vision they can see predators coming.
Whereas vicious predator animals have their eyes at the front like this. (dog bark) So they lose that wraparound vision but they gain that extra depth perception. So think about this prey animal here as it pitches its body forwards and backwards it needs to maintain a stable image on its retina.
So the eyes counter rotate in its head which I've tried to demonstrate here using after effects. It's not perfect, but it gives you an idea, doesn't it? Kind of, so think about what that eye movement is like in the prey animal, the eyes are moving like this.
And as those eyes evolve to point forwards it's equivalent to that, all right cyclovergence. So this eye movement that was evolved to maintain a stable image on the retina has been co-opted to maintain good depth perception. There are two bits of evidence to support the idea that this cyclovergence movement of the eyes is an exaptation.
In other words, a co-opting of a pre-existing behavior in our evolutionary ancestors that had eyes on the sides of their heads. The first bit of evidence is that there's a vestigial behavior that remains in humans. A vestigial behavior or a trait is just a thing that we retain from our evolutionary ancestors that we don't need any more.
And in this case, it's been shown experimentally that when a human bends forwards or backwards their eyes actually do move very slightly like that as if their eyes are on the sides of their heads and they were trying to stabilize the image on the retina. It's only very slight. I wasn't able to record it on my camera, just with me in my studio, but it has been noticed experimentally.
The second bit of evidence is that certain neurological diseases or injuries will cause a patient to revert back to the behavior of their evolutionary ancestors. In other words, when the patient tilts forwards or backwards, their eyes do that cyclovergence thing. And you know, and not just that subtle vestigial behavior but like properly doing it as if their eyes were on the sides of their heads.
And that's because within our brains we have the neurological framework for the behavior of our evolutionary ancestors. And then on top of that we have some extra neurological processing to subdue that behavior and get it to do the thing that we want it to do. It's as if you've got this legacy code in your brain that you have to maintain.
So you just wait a software patch to stick on top of it. And if that patch gets broken then you revert back to the ancient behavior. It's kind of a metaphor for how cancer works actually within ourselves there's this ancient programming that says just replicate, replicate, replicate.
And then on top of that you add the control mechanisms that subdue that behavior. But if any of those control mechanisms get damaged then the sound reverts back to that ancient behavior replicate, replicate, replicate. I read some really interesting research papers where they were able to train people to voluntarily twist their eyes around, which is amazing really like 30 degrees of twist after just like 10 days of practice.
It was, I couldn't really figure out the experimental setup and what I could figure out it seemed it was gonna be really difficult like it needed like, you know flashbulbs and like special equipment and things like that. So I wasn't able to do that. Instead and I actually, I did this for experimental reasons not just to cheat.
I did it in three different ways. So the first way was to wear a helmet with a camera mount and a macro lens on my eye, and then just, you know tilt my eyes so the camera is on my eye like that. The second way was to literally just stand in front of the camera tilting my head and then stabilize that footage and after effects to remove the rotator.
And finally, I just looked at an image that was rotating and filmed that. The reason I wanted to do those three options is because your brain decides how to move your eyes based on feedback from two different systems. There's the equilibrioception or sense of balance that you have.
In other words your inner ear that tells you the orientation of your head relative to a gravitational field also tells you about acceleration and things like that. So your brain knows about the orientation of your head in space, and it can feed that information back to your eyes to tell them to rotate. It can also get information from what's coming into the eyes.
So if what the eye sees is rotating then your brain can interpret that rotation and tell your eyes to rotate the opposite way to keep a steady image on the retina. How important are those two things? So by putting a camera on my head mounted like that with a really bright ring light, that's all I could see.
So when I was tilting my head I was only getting information from my sense of balance. Compare that to when I was just watching a rotating image in that scenario, I was only getting information from the eyes. But when I simply stood in front of the camera and tilted my head, I was combining both systems.
I was getting information from my sense of balance and information from my eyes. And what I found was, you get hardly any irritation at all when it's just the information from your visual system you get much more rotation when it's just your sense of balance on its own. But the most movement you get is when you have both at the same time, which makes sense, right?
Because that's the most likely way someone is going to encounter a rotating image is because their head's rotating as well. So there you go. The weird evolutionary history of eye rotation.
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