So, if some of you are still feeling a bit too certain, I'd like to do this one. So, if we have the lights down. And what we have here -- Can everyone see 25 purple surfaces on your left, 00:15:03 and 25, call it yellowish, surfaces on your right? So now, what I want to do, I'm going to put the middle nine surfaces here under yellow illumination, by simply putting a filter behind them. Now you can see that changes the light that's coming through there, right? Because now the light is going through a yellowish filter and then a purplish filter. I'm going to do the opposite on the left here. 00:15:31 I'm going to put the middle nine under a purplish light. Now, some of you will have noticed that the consequence is that the light coming through those middle nine on the right, or your left, is exactly the same as the light coming through the middle nine on your right. Agreed? Yes? Okay. So they are physically the same. 00:15:56 Let's pull the covers off. Now remember -- you know that the middle nine are exactly the same. Do they look the sa

also true for complex perceptions of motion. So, here we have -- let's turn this around -- a diamond. And what I'm going to do is, I'm going to hold it here, 00:08:53 and I'm going to spin it. And for all of you, you'll see it probably spinning this direction. Now I want you to keep looking at it. Move your eyes around, blink, maybe close one eye. And suddenly it will flip, and start spinning the opposite direction. Yes? Raise your hand if you got that. Yes? Keep blinking. Every time you blink, it will switch. So I can ask you, which direction is it rotating? 00:09:20 How do you know? Your brain doesn't know, because both are equally likely. So depending on where it looks, it flips between the two possibilities

Here we have that exact same illusion. We have two identical tiles on the left, one in a dark surround, one in a light surround. And the same thing over on the right. Now, I'll reveal those two scenes, but I'm not going to change anything within those boxes, except their meaning. 00:07:11 And see what happens to your perception. Notice that on the left the two tiles look nearly completely opposite: one very white and one very dark, right? Whereas on the right, the two tiles look nearly the same. And yet there is still one on a dark surround, and one on a light surround. Why? Because if the tile in that shadow were in fact in shadow, and reflecting the same amount of light to your eye 00:07:35 as the one outside the shadow, it would have to be more reflective -- just the laws of physics. So you see it that way. Whereas on the right, the information is consistent with those two tiles being under the same light. If they're under the same light reflecting the same amount of light to your eye, then they must be equally reflective. So you see it that way. Which means we can bring all this information together to create some incredibly strong illusions.

let me show you how quickly our brains can redefine normality, even at the simplest thing the brain does, which is color. 00:05:29 So if I could have the lights down up here. I want you to first notice that those two desert scenes are physically the same. One is simply the flipping of the other. Now I want you to look at that dot between the green and the red. And I want you to stare at that dot. Don't look anywhere else. We're going to look at it for about 30 seconds, which is a bit of a killer in an 18-minute talk. (Laughter) But I really want you to learn. 00:05:55 And I'll tell you -- don't look anywhere else -- I'll tell you what's happening in your head. Your brain is learning, and it's learning that the right side of its visual field is under red illumination; the left side of its visual field is under green illumination. That's what it's learning. Okay? Now, when I tell you, I want you to look at the dot between the two desert scenes. So why don't you do that now? (Laughter) 00:06:21 Can I have the lights up again? I take it from your response they don't look the same anymore, right? (Applause) Why? Because your brain is seeing that same information as if the right one is still under red light, and the left one is still under green light. That's your new normal. Okay? So, what does this mean for context? It means I can take two identical squares, put them in light and dark surrounds, and the one on the dark surround looks lighter than on the light surround. 00:06:47 What's significant is not simply the light and dark surrounds that matter. It's what those light and dark surrounds meant for your behavior in the past.