Can You Perceive Acceleration?

Have you ever considered how your eye scans across a landscape? I think most people assume that, like a video camera, it pans smoothly across the scene. But I don't actually think that's what happens. In truth, I think the eye jumps from image to image as it moves across this scene.

So I'm going to try to find a volunteer to help me demonstrate that. What I'd like you to do is, um, scan across this landscape as smoothly as you can. Can you try to tell me about the process of scanning across? You know, from the point, it just feels like you're stopping and going, and then like stopping and going. Yeah, it just feels like there's a pause all the time constantly when you're going across. It's not like you're just looking straight across.

Yeah, it's not smooth. The eye is capable of tracking smoothly; however, if something moves across your field of view, your eye can lock onto it and move smoothly with it. What I'd like to do is repeat that little game, but I would like to ask you just to walk across on those rocks, M, and then I'd like you to follow her along with your eyes. Is that possible?

Yeah, okay, let's give it a shot. Do you want to go over there? Okay, all right, go! Yeah, go for it!

Can I just get your reaction on what that was like? How was that the same or different?

Felt that felt a lot smoother because I was following something instead of just looking at something. Yeah, I don't know, just like if you follow something, your brain focuses on that one thing, and you just kind of just go with it.

Why would that be useful to be able to pan with a moving object? How does that affect our perception of motion?

Well, I think it makes most motion seem quite smooth and makes it more difficult for us to determine when things are accelerating.

One last question, since I've got you guys here. So, what I want to do is drop the ball. I want you guys to watch it, and then tell me how the ball moved, okay? Is that fair enough?

Okay, so you want us to watch it till it hits the ground, and then afterward, you're going to tell us how it moved?

Okay, three, two, one... I'll do it just one more. Three, two, one. Could you tell me about the motion of the ball? What was it doing as it went down?

Fast. Um, I don't know. It just seemed like it just dropped, like it wasn't even moving. You just seen the ball dropped to the floor, and that was it. You didn't see it like, here; you just saw it from there to there.

Yeah. Would you say its speed was constant, or do you think its speed was changing?

Constant.

Yeah, it was the same, yeah, the whole way down.

Yeah, no acceleration.

The goal of my whole experiment today, I'll let you guys know the secret, is to point out that the eye tracks motion very smoothly. So when the ball, like, goes down, it's really easy for us to track that, and it looks like it's just doing one constant thing. But, in fact, it is speeding up the whole time when I release it. It's not moving at all, and when it hits the ground, it's actually going quite fast.

So that was kind of my point. If you scan across, like, a stationary scene, then your eye is doing this weird jumpy thing.

Yeah, but then if you're actually moving with an object or with a person or something, then your eyes are so smooth you don't even notice.

Yeah, but something that's still and you have to move, that's when it starts to jump. But if there's something moving, your eyes have to follow that; it's much easier.

Yeah, I understand.