Why do we see illusions? I'm going to tell you about some of my research, where I provided evidence for a different kind of hypothesis than the one that might be in the book on your coffee stand. Alright, so let's look at one of the illusions here. And this is a stand-in for many, many kinds of illusions that are explained by this hypothesis. I'm just going to walk through it for this particular one. As usual in these things, these two lines are, in fact, parallel, but you perceive them to bow outwards at their centers. At the center where those radial lines are, it's wider in your visual field than the parts above and below. And this is remarkable, because it's a remarkably simple stimulus. It's just a bunch of straight lines. Why should one of the most complicated objects in the universe be unable to render this incredibly simple image? When you want to answer questions like this, you need to ask, well, what might this mean to your brain? And what your brain is going to think this is, is not some lines on a page. Your brain has evolved to handle the kinds of natural stimuli that it encounters in real life. So when does the brain encounter stimuli like this? Well, it seems a bit odd, but in fact, you've been encountering this stimulus all day long. Whenever you move, whenever you move forward, in particular. When you move forward, you get optic flow, flowing outwards in your visual field, like when the Enterprise goes into warp. All of these objects flow outwards and they leave trails, or blur lines, on your retina. They're activating mini-neurons all in a row. So, this is a version of what happens in real life and this another version of what happens in real life all the time. In fact, cartoonists know about this. They put these blur lines in their cartoons and it means to your brain: motion. Now, it's not that in real life you see blur lines. The point is that it's the stimulus at the back of your eye that has these optic blurs in them, and that's what tells your brain that you're moving. When you move forward, your eyes fixate like cameras, like snapshot cameras, it fixates, it fixates, little (Snapshot sound) camera shots, and each time it fixates when you're moving forward, you get all this flowing outwards. So when you take a fixation, you end up with this weird optic blur stuff, and it tells you the direction you're moving. Alright, that's half the story. That's what this stimulus means. It means that your brain thinks, when it's looking at the first image, that you're actually on your way, moving towards the center. It still doesn't explain why you should perceive these straight lines as bowed outwards. To understand the rest of the story, you have to understand that our brains are slow. What you would like is that when light hits your eye, then -- ping! -- immediately you have a perception of what the world is like. But it doesn't work that way. It takes about a tenth of a second for your perception to be created. And a tenth of a second doesn't sound very long, but it's a long time in normal behaviors. If you're moving just at one meter per second, which is fairly slow, then in a tenth of second, you've moved 10 centimeters. So if you didn't correct for this delay, then anything that you perceived to be within 10 centimeters of you, by the time you perceived it, you would have bumped into it or just passed it. And of course, this is going to be much worse -- (Laughter) it's going to be much worse in a situation like this. Your perception is behind. What you want is that your perception should look like this. You want your perceptions at any time T to be of the world at time T. But the only way your brain can do that, is that it has to, instead of generating a perception of the way the world was when light hit your retina, it has to do something fancier. It can't passively respond and create a best guess, it has to create a best guess about the next moment. What will the world look like in a tenth of a second? Build a perception of that, because by the time your perception of the near future occurs in your brain, the near future will have arrived and you'll have a perception of the present, which is what you want. In my research, I provided a lot of evidence -- and there's other research areas that have provided evidence -- that the brain is filled with mechanisms that try to compensate for its slowness. And I've shown that huge swaths of illusions are explained by this, this just being one example. But let me finish by saying, how exactly does this explain this particular example? So, the question, really, we have to ask is: how do those two vertical lines in that first stimulus, how do they change in the next moment were I moving towards the center, that all those optical lines are suggesting that I'm moving. What happens to them? Well, let's imagine. Imagine you've got a doorway. You've got a doorway. Imagine it's a cathedral doorway, to make it more concrete -- it'll be helpful in a second. When you're very far away from it, the sides are perfectly parallel. But now imagine what happens when you get closer. It all flows outwards in your visual field, flowing outwards. But when you're really close -- imagine the sides of the doorway are here and here, but if you look up at this cathedral doorway and do your fingers like this, the sides of the doorway are going up, like railroad tracks in the sky. What started off as two parallel lines, in fact, bows outwards at eye level, and doesn't go outwards nearly as much above. So in the next moment, you have a shape that's more like this next picture. The projective geometry -- that is, the way the things project, in fact, change in this way in the next moment. So when you have a stimulus like this, well, your brain has no problem, there's just two vertical lines and no cues that there'll be a change in the next moment, so just render it as it is. But if you add cues -- and this is just one of many kinds of cues that can lead to these kinds of illusions, this very strong optic blur cue -- then you're going to perceive instead exactly how it will appear in the next moment. All of our perceptions are always trying to be about the present, but you have to perceive the future to, in fact, perceive the present. And these illusions are failed perceptions of the future, because they're just static images on the page, they're not changing like in real life. And let me just end by showing one illusion here. If I can, I'll quickly show two. This one's fun. If you just fixate at the middle there, and make stabbing motions with your head, looming towards it like this. Everybody do that. Make short, stabbing motions. Because I've added blur to these optic flow lines, your brain says, "They're probably already moving, that's why they're blurry." When you do it, they should be bursting out in your visual field faster than they should. They shouldn't be moving that much. And a final one I'll just leave in the background is this. Here are the cues of motion, the kinds of cues that you get on your retina when things are moving. You don't have to do anything -- just look at it. Raise your hand if things are moving when they shouldn't be. It's weird, right? But what you have now are the cues that, from your brain's point of view, you have the stimulus on your eyes, like, "Oh, these things are moving." Render a perception of what they'll do in the next moment -- they should be moving and they should have shifted. Alright, thank you very much. (Applause)
人为什么会有错觉? 让我来通过调查中 获得的一些证据 来证明一项假设 这项假设 完全不同于你在枕边书中读到的 好了,我们来看看这张错觉图 许多错觉图都能用这项假设来解释原因 这张图是其中的代表 所以我将以此图为例做详细说明 如图所示 两条线实际上相互平行 但你的感觉是它们的中间部分向外弯曲 在你的视野中 有射线穿过的中间部分 比上下两头都宽 太不可思议了 其实它给你的刺激非常简单 因为它们只是些直线而已 为什么宇宙中结构最复杂的人 无法辨识这幅简单至极的图像? 要回答这个问题 我们首先要问 “我的大脑看到了什么?” 这幅图对于大脑来说 绝不仅仅是一些线条而已 人脑经过进化 已经能处理现实生活中遇到的 一些自然刺激 那么当大脑遇到此类刺激时,会如何反应呢? 虽然它看上去有些奇怪 但实际上你一直都在经历这种刺激 只要你在移动 特别是向前移动时 你会获得视觉流 景象在你的视野中向外扩张 就像进取号开始曲速飞行一样 一切物体都在向外扩张 它们在你的视网膜上留下轨迹或残影 不断刺激着你的迷你神经元 以上是你在日常生活中遇到的一个例子 下面还有另一个一直遇到的刺激图像 漫画家十分了解这种刺激 他们把残影运用到漫画中 你看到时,大脑的反应就是运动 重点并非你看到的残影 而是你眼睛后部受到的刺激 眼睛接收到这些视觉残影后 大脑就做出了“你在移动”的反应 当你向前移动时 眼睛就像自动照相机一样 捕捉图像 眼睛不停地 捕捉图像 就像相机在拍摄 每次你在前进时 捕捉图像 就会看到景象向外扩张 你捕捉到图像后 它会留下怪异的视觉残影 残影让你知道了自己移动的方向 好了,以上并非对错觉的全部解释 只不过告诉你什么是刺激 在第一次看到这幅图时 你的大脑做出了什么反应 还有,你实际上正在朝 图像中部移动 然而,你还是不知道 为什么自己会感觉直线向外弯曲 要回答这个问题 你得知道,我们的大脑不够快 也许你以为当光照进眼睛 “唰”地一下 就看到了 世界的面貌 事实上并非如此 大脑要花十分之一秒的时间 来形成图像 十分之一听上去不长 但对普通行为来说不短 假设你每秒移动一米 这速度很慢 那十分之一秒你就移动了10厘米 所以如果不修正这种延迟 那么在你感知到 任何身边10厘米范围内的东西时 你就已经 撞上或直接错过它了 而且还有更糟的情况 更糟的情况 就是 自己的感知有延后 你希望自己 在“t”时间获得感知 此时外部时间也为“t” 要想让大脑做到这点 它必须要在光照到视网膜时 给出更出色的反应 而不是对外部世界 做实际反馈 大脑不能被动地对外界作出反应 而要最准确地预估 下个时点会发生什么 世界在十分之一秒后会怎么样? 大脑会先对此作出描绘 因为 当大脑感知到未来时 未来已经发生 这样你就会得到一个对现在的感知 这是你希望大脑做到的 在我和其他领域的调查中 有大量证据表明 大脑中充满了这类机制 以弥补自身反应速度上的缺失 许多错觉都可以用 大脑的这种机制来解释 这幅图只是个例子 但最后我还是想说明 大脑机制如何来解释这幅错觉图 因此,我们的问题就变为 第一眼看到的两条竖直线 在下一刻将如何变化 如果我向中部移动 因为在视觉上 这些线条让我觉得自己在移动 两条直线会怎么样? 好了,我们来看另一种情形 想象你面前有一扇门 这是一扇 大教堂的门 形象越具体 对我们接下去的说明越有帮助 你在远处看着门时 门框完全平行 想象自己向门靠近 当你靠得很近 在你的视野中门框会向外凸 越靠近变形越厉害 如果你靠得非常非常近 门框就在你两边时 你抬头看大教堂的门 同时两手像这样向上移 你会发现门框 像两条轨道一样直指天空 一开始,门框是两条平行线 而在视平面上向外突出 到顶部又没有变形 于是下一刻 你脑中就形成了 如下幅图所示的图形 这牵涉到投影几何学 在下一时刻 事物本来的投影方式发生了改变 因此如果你接收到这样的刺激 对大脑来说 毫无疑问这只是两条竖直线 没有任何线索显示 它们接下去会发生变化 大脑就不会对图像进行加工 但如果有附加线索 只要有一条 可以导致错觉的线 比如上面提到的明显光学残影 你就能感知到 下一刻将看到什么 我们总试图感知现在 而实际上我们感知的是未来 而不是现在 错觉其实是对未来的错误感知 仅仅是脑中的静态图 并没有真的发生变化 最后,我想用一幅错觉图来收尾 如果时间允许就再加一幅 这张图很有意思 如果你盯着中间那点 前后摆动你的头 就像这样 大家一起来 快速摆动头 由于我在视觉流动方向加入了残影 你的大脑会想:“它们是残影 所以它们应该在移动” 你在摆动头的同时 视野中的图像会比实际动得更快 它们本来没有这么快 这是最后一幅图 这些是运动的线索 事物移动时 会在视网膜上留下这样的线索 不做别的 你只要看着它就会有错觉 看到它们在移动的 举起你们的手 是不是很奇怪? 但你的大脑 接收到了线索 而你的眼睛 受到刺激 感觉“它们在移动” 这是对它们下一刻状态的感知 下一刻,它们应该在动 不停地移动 好了,谢谢大家