The Illusion of Motion: Why Stationary Objects Sometimes Appear to Move
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Ever watched like a basketball spinger, a car speeding down the street and wondered like, how does your brain actually make sense of all that movement? Yeah.

Yeah, it's pretty incredible, isn't it? Well, today we are going on a deep dive into the world of visual motion perception. Okay, cool. We've got some lecture notes here from a Stanford psych class. Oh, wow. And we break down how our brains process motion, why it's so important, and maybe even throw in a few mind-bending illusions along the way. Sounds fun. I'm in. So these notes start off by saying that our visual systems don't actually see motion directly. Oh, really? It has to, like, infer it. Hmm.

I'm no neuroscientist, but that sounds kind of counterintuitive to me. Yeah, I mean, you would think we just like see things move and that's it, right? Right. But it's actually way more complex than that. So basically our eyes are constantly taking in light and those patterns of light change as things move. Okay. And your brain uses those changes on your retinas to kind of make an educated guess about what's moving and how.

So our brains are like detectives piecing together clues from these changing light patterns to solve the mystery of motion. Exactly. And most of the time it works amazingly well. But sometimes those inferences can lead to some pretty wild illusions. Okay, so like the motion without movement illusion that's in these notes. Right. There's a figure that appears to be moving, but it never actually goes anywhere. It's wild, isn't it?

It is. How does that even work? It's a great example of how our brains can be tricked. So you have these things called direction-selective neurons in our visual cortex, and they're wired to respond to edges or lines that move in specific directions. So like tiny motion detectors, each tuned to a specific direction. Exactly.

And so in this illusion, the shapes are arranged in a way that kind of creates a pattern of light that activates those neurons. And it makes us think we're seeing motion even though nothing is actually changing position. So even when our eyes are telling us one thing...

our brains can create a completely different perception. It's like a visual short circuit making us see something that isn't really there. That's crazy. And it's not just static images either, right? Yeah. You have things like the classic waterfall illusion or the motion after effect. Ever heard of that? Oh, yeah. You stare at a waterfall for a bit, then you look at something stationary and it looks like it's moving upwards. It's so bizarre. It's like my brain is messing with me. It always makes me wonder if something is wrong with my eyes.

No, no, not at all. It's totally normal, and it shows just how adaptable our brains are. So what's happening is the neurons that detect downward motion, they get tired after you stare at the waterfall for a while. They're basically like, okay, we've seen enough downward motion for now. Yeah, exactly. Then when you look at something stationary...

Those downward motion neurons are kind of taking a break. And the ones that detect upward motion are relatively more active. And that's what creates that illusion of upward movement. So our brains are constantly recalibrating based on what they're seeing. All the time. It makes you realize that what we see isn't always an accurate reflection of reality. For sure. Our brains are always trying to make sense of the world.

And sometimes they take shortcuts or make assumptions, which lead to these fascinating perceptual quirks. So aside from illusions, these notes say motion perception is super important for survival. Oh, yeah. Totally. Not just cool visual tricks. It's essential for so many things we do. Take attention, for example. Okay. Like any sudden movement instantly grabs our attention, right? Right. Could be a predator or a car swerving into your lane. It's like a built-in alarm system. Exactly. Exactly.

But it's not just avoiding danger. Motion also helps us separate objects from their backgrounds. Like imagine a cheetah chasing its prey across the savanna. It's the cheetah's movement that helps us see it against the grass and understand what's going on. So motion helps us make sense of a busy visual scene by highlighting what's important.

What about depth perception? Does motion play a role there too? Definitely. There's this cool demo in the notes using two transparent sheets of dots. When they're both still, it just looks like a random jumble of dots. Right. But when you move one sheet, suddenly a shape pops out.

Wow. And that's because the motion gives us depth information. So our brains are using motion to create a three-dimensional understanding of the world. Exactly. It's the same principle that makes movies so immersive, especially on those giant IMAX screens. It's true. I've always wondered how they make those special effects look so real. It's all about understanding how our brains process motion and depth. So cool. Speaking of understanding...

Let's talk about how motion helps us recognize things. Could motion actually help us identify what we're seeing? Well, have you ever seen those point light walker videos? You mean the ones where it's just a bunch of moving dots, but you can instantly tell it's a person walking? Exactly. Those are a perfect example of how sensitive our brains are to like biological motion. We can recognize human movement from just a few moving dots.

It's incredible how much meaning our brains can pull from such limited information. It really is. And it highlights the importance of motion for object recognition. Okay. It's not just shape and color. It's also about how things move. So our brains are constantly analyzing movement to help us understand our surroundings, recognize objects, and experience the world in 3D. Pretty much. That's amazing. It is.

And to make things even more complex, there are actually two types of motion our brains have to deal with. Two types. Okay. I need you to break that down for me. Yeah. I'm all about keeping things simple. No problem. So you have object motion, which is

Pretty straightforward. Okay. It's when something in the world is moving, like that Cheeto we were talking about or a car driving by. Okay. Yeah, that makes sense. Then you have observer motion. Observer motion. Yeah. That's when we're the ones moving. Like, imagine you're sitting on a train watching the scenery go by. Hmm.

That's observer motion. Okay, so if I'm walking down the street, that's observer motion. But if a bird flies past me, that's object motion. Exactly. And our brains need to be able to tell the difference between those two to understand what's happening. Otherwise, we'd feel like we're on a roller coaster all the time. Uh-huh, yeah. And this is where optical flow comes in. Optical flow, okay.

That sounds pretty technical. It's basically a visual representation of motion in the world. Can you give me an example? Sure. Like when you're driving down a road and the lines on the road seem to be flowing past you towards a point in the distance, that's optical flow. So it's like a visual speedometer for our brains. Kind of. It helps us judge speed and direction, navigate our surroundings, and even anticipate what's going to happen next. Wow. That's really cool.

These notes mention a researcher named J.J. Gibson who was super into optical flow. What did he think about it? Gibson was a really interesting guy. He believed that our visual system could use optical flow to figure out motion and depth perfectly. Yeah. He thought all the information we need to perceive the world accurately is already there in those changing light patterns. But wait, our perception isn't always perfect, right? Right. Like, what about optical illusions? Yeah, good point.

It turns out Gibson was a little too optimistic. Okay. So optical flow gives us a lot of information, but there's something called the distance speed ambiguity that can sometimes mess us up. Distance speed ambiguity. Yeah, basically a small object close to you moving slowly moves

can create the same pattern of motion on your retina as a large object far away that's moving quickly. I see. So your brain has to decide which one it is. That's why we need speedometers in our cars, I guess. Exactly. We're better at judging relative speed and distance, like how fast one car is moving compared to another. Okay, so optical flow is powerful, but it has its limits. Right. Now, I'm wondering how all of this processing happens in the brain.

Are there specific parts of the brain that handle motion perception? Oh, yeah, there are. And these notes talk about some fascinating research on that. There's this idea called the functional specialization hypothesis. Okay. Which basically says that different brain areas handle different parts of visual motion. So like a team effort. Exactly.

Exactly. Like a well-coordinated team where each member has a special role. Okay. So who are the key players on this motion processing team? Well, first you have V1, which is the primary visual cortex. Think of it as the entry point for all visual information in the brain.

Okay, so V1 is like the welcome center greeting all the incoming visuals. Exactly. And it's in V1 that we find those direction selective neurons we talked about. Oh, right. The ones that respond to edges and lines moving in certain directions. So V1 is laying the groundwork for motion processing. Then what happens? From there, things get more specialized.

Next, we have area MT, which is also known as V5. MT or V5. Okay. And MT is all about velocity. So it takes those basic motion signals from V1 and figures out how fast and in what direction things are moving. Exactly. Each MT neuron has like a preferred speed and direction that it responds to most strongly. Hmm. Interesting. They help us process those more complex motion patterns beyond just basic edges and lines. Okay. So V1.

So V1 is detecting basic motion and MT is adding in speed and direction. What other areas are involved? Well, right next door to MT, you have MST. MST. Which stands for the medial superior temporal area. Wow. That's a mouthful. Uh-huh. Yeah. So what does MST do? MST neurons have much larger receptive fields. Okay. Meaning they take in information from a wider area of your visual field. So they're seeing the bigger picture. Right.

Exactly. And they respond to even more complex types of motion, like expansion-contraction rotation. Wow. They help us understand how objects are moving relative to each other and to us. So it's like a hierarchy of motion processing from simple to complex.

Yeah, you could say that. Each area building on the information from the one before it. Right. What about biological motions? Is there a specific area for that? Yep. That's where STS comes in. The Superior Temporal Sulcus. STS, okay. STS is all about recognizing and understanding the movements of living things. So STS is what helps us understand those Point Lightwalker videos and recognize our friends from their walk, even from far away. Exactly. It's a really cool area of the brain that scientists are still studying.

This is all so fascinating. But I have one big question. Yeah. How does the brain know what's actually moving? What do you mean? Like, is it us moving or is it the world around us that's moving? Right. How do we separate our own movement from the movement of everything else?

Yeah. It really is a tough question to think about. Like if everything we see is moving, how do we know if it's because we're moving or if it's because things around us are moving? Right. It's seriously impressive how the brain figures that out. And it uses more than just our eyes to do it. Oh, okay. Interesting. So what else does it use? Well, our brain combines the visual information with signals from other parts of our body. Like what?

Like our sense of balance, for example. Okay, yeah, that makes sense. Our vestibular system, which is in our inner ear, it tells us about how our head and body are moving in space. So if we're spinning, our vestibular system is basically telling our brain, hey, you're the one spinning, not the room. Exactly. But what about when we move our eyes? Oh, yeah, good point. Like if I'm watching a bird fly across the sky, my eyes are moving, but the whole world doesn't look like it's spinning. Right. And that brings us to another piece of the puzzle.

eye movement signals. Okay. So whenever we move our eyes, our brain sends a copy of those movement commands to the parts of the brain that process vision. So it's like our brain is keeping a record of our eye movement. Exactly. It's called the corollary discharge. Corollary discharge. Okay. And by comparing that copy with what our eyes are actually seeing, our brain can figure out what's really moving. So my eyes are following a moving object. My brain knows it's the object that's moving, not me. Right.

But if my eyes are still up and something moves across my vision, then my brain knows I'm not moving. But that object is. Pretty clever, huh? It really is. It's like a system for separating our own movement from the movement of everything else. Yep.

Want to try a little experiment to see how this works? Sure. I'm always up for a good brain teaser. What do I got to do? Just gently push on the side of one of your eyes. Whoa. Everything just like shifted. Right. What you're experiencing is a conflict between what you're seeing and that corollary discharge we were talking about. Huh. So you're not actually moving your eye. Right. But by pushing on it, you're sending that signal to your brain as if you were. That is true.

That's so trippy. Like I'm fooling my own brain. Exactly. And this demo perfectly illustrates that corollary discharge model, which explains how our brains figure out self-motion versus object motion. Wow, this whole conversation has been mind-blowing. I never realized how much is going on in our brains just to perceive motion.

Pretty incredible, isn't it? It is. Signals from our eyes, our inner ear, even our own eye movements all working together. It's like a symphony of neural activity just to make sense of a world in motion. And we started this deep dive by talking about how our brains infer motion.

How illusions can trick us and why motion perception is so important for survival. Right. We even learned about optical flow and how our brains use it to understand the world around us. It's amazing how much ground we cover. It is. But I feel like we've only scratched the surface. Uh-huh. There's always more to explore. That's true. I think the big takeaway is that motion perception isn't just about passively taking in information. Yeah. It's about actively interpreting and integrating all these different signals.

Our brains are constantly working to make sense of a world in motion. Absolutely. So the next time you see a basketball spinning or a bird flying through the air, take a moment to appreciate all the amazing things happening in your brain to let you experience that. Yeah. It's really incredible when you think about it. It really is. And with that, we'll leave you with one final thought. We talked about how we perceive real motion.

But what about illusory motion? Like, how do filmmakers and animators create those illusions of movement on a screen? And how do our brains make sense of that? That's something to think about. Definitely. Until next time, keep exploring the wonders of your own perception.