[0:03]The most complex thing we've discovered in the universe is the human brain.
[0:13]For the past 20 years, I've been trying to understand how what happens in three pounds of jello-like material somehow becomes us. What we feel, what matters to us, our beliefs and our hopes, everything we are, happens in here.
[0:42]And for me, there's one mystery that's absolutely fundamental. What is reality? What if I told you that this world around us, this richly textured world, we're all just an illusion constructed in your head? What if I said that the real world has no smell or taste? No sound.
[1:17]What if I said there's no color? If you could perceive reality as it really is out there, you wouldn't recognize it at all.
[1:32]I want to show you how the brain takes in information, sifts through it to find patterns. And uses it to build the multi-sensory technicolor show that is your reality.
[2:10]When I'm in the world, my senses are flooded with sights and sounds and smells, and it seems obvious that reality is just out there. There's a person, there's a cab. All I have to do is show up and my senses let me experience it all. But there's a twist to this story. Let me show you something. So take a look at this middle square here. Does that look more similar to the light square or the dark? Well, it looks like a light square, yeah? You might be surprised if I move it. Now it looks like a dark square. Oh my God. They're the same! It's surprising, right? It is. Wow. It is. Seriously?
[3:01]Do you have a guess as to why there's an illusion here? Well, it seems like there's a shadow, so it makes this darker. That's exactly right. Your brain is trying to understand the colors of things irrespective of the lighting and the shadows. So somehow it's not about what's hitting your eyes, it's about your brain's interpretation. That's really trippy. You just messed up my whole day.
[3:28]Now, this is about more than just a visual illusion. It's about a fact that's central to our lives. Our perception of reality has less to do with what's happening out there, and more to do with what's happening in here.
[4:00]It feels as if sights and sounds just stream in through our eyes and our ears. But imagine if you could climb inside a human skull. When you step into the skull, you'll find there's no way for light or sound or smells to get directly in here.
[4:39]Your brain's never seen the outside world, but somehow you experience it. Now, this might seem straightforward because we have portals to the outside world, like your eyes and ears, but these aren't just piping in sights and sounds. Instead, photons of light or air compression waves, these are getting converted into the common currency of the brain, electrochemical signals.
[5:09]These signals travel through dense networks of brain cells called neurons. There are a hundred billion neurons in the human brain. And in every second of your life, each one of these is sending tens or hundreds of electrical pulses to thousands of other neurons. And somehow, all of this activity produces your sense of reality. So whether it's the bark of a dog, or the smell of coffee, or a view of a beautiful sunset, it's all made of the same stuff in here.
[5:53]And this is the stuff of reality. But how does the brain turn it into something meaningful? Well, it does it by sifting through the non-stop stream of incoming data to find patterns, which it then assembles into a reality. It's an operation which is the product of millions of years of evolution. So efficient, so powerful that its work seems effortless and instantaneous.
[6:32]Take as an example, sight. The act of seeing feels so natural that it's hard to appreciate the vast, sophisticated machinery running under the hood.
[6:50]For us to see clearly, many different systems need to be operating in concert. It's about more than just the eyes. The best way to understand this is to look at the extraordinary case of a man who lost his sight, and then was given the chance to get it back. I lost my sight when I was three and a half years old as the result of a chemical explosion. And oddly it didn't seem like it was a big deal. I guess as a three-and-a-half-year-old, my world according to vision was not as well established as it would be for somebody who lost their vision later in life. After over 40 years of blindness, Mike May had pioneering stem cell treatment that would repair the physical damage that the explosion caused to his eyes. Cameras were there to witness the moment when for the first time, the bandages came off. Dr. Goodman does the corneal transplant, and he peels back the bandages, and there's this woosh.
[8:06]Of light and bombardment of images onto my eye. Holy smokes. In surgical terms, the operation was a total success. But to Mike, it wasn't. What's across the room over here?
[8:23]There was something wrong. All of a sudden you turn on this flood of visual information, it's overwhelming. My brain is just going, oh my gosh.
[8:38]So that's how the world proceeded, one image at a time. Seeing cars as they whizzed by, and then I would see a sign ahead of us. It looked like we're going to smack right into it. And in fact, it's a sign over the freeway and we're not going to run into it, we're going under it.
[9:00]And that was only the first hour. It was going to get worse when Mike got home. If you put four blonde boys together all roughly the same height, and I looked at them, I couldn't tell you which two were mine. Don't go away. I'm not finished looking at you.
[9:19]Mike's new eyes were functioning perfectly, and they were sending signals to the brain, just like yours or mine do. But he couldn't see his sons in any meaningful way. I had no face recognition whatsoever, none. When he'd been totally blind, Mike was a Paralympic skier. But his first sighted attempt at skiing was a complete failure. When I skied for the first time, because of my depth perception difficulty, I had no time to figure out the difference between four dark things on the white snow. A person, a tree, shadow, or a hole.
[10:11]Ten years on, Mike still needs his guide dog to get around. He can detect light and motion and identify colors, but he struggles to gauge how far away things are. He still can't read the expressions on his son's faces. He still can't read words on a page.
[10:37]What Mike's story gives us is a glimpse of all the elements that have to be in place for the brain to construct a visual reality. Many regions of the brain are involved in vision. They specialize in different aspects, such as motion, edges, colors, face recognition. somehow the brain weaves all of this together, unifies it to form what we experience as an image. In Mike's case, decades of blindness caused these regions of his brain to be taken over for other tasks, like hearing and touch. They just weren't available for him to use, even when he was given a pair of new eyes.
[11:34]We often get our best view of how the brain operates when that operation is disrupted.
[11:43]Hey, Ryan. Hey. That's why neuroscientists sometimes disrupt things deliberately. Brian is part of an experiment being conducted by Alyssa Brewer at the University of California. Jeez, so good to see you. Welcome. You're ready to try some goggles on? Oh, I'm ready. Volunteers wear these goggles for weeks at a time. Their brains are forced to cope with a new view of the world that's dramatically altered. Now what these have inside are two prisms that take the whole visual world and flip it. So whatever you see normally on the left side of the world will now be on the right side of the world. So as you move through the world, you're going to have a problem figuring out where things are. As you see them on one side, but reach for them in the opposite side. What the world looks like is this. But what I'm seeing is this. It's a straightforward change, but it's also a massive mind mash.
[13:56]It's very difficult to figure out which way to go, and so his motor system and feeling of touch were sending him in one direction while his visual system was sending him in the other direction.
[14:17]Brian's doing well. Unlike me.
[14:24]I have to consciously reconstruct my reality. This morning my brain could rely on automated interactions. But now it can't. Interestingly, I've broken out in a sweat and I'm hot and I'm super dizzy and nauseated. You know, I got to take a break. I'm so sorry. I got to take these off for a second. Is that okay?
[14:48]Boy, that is really nauseating.
[14:53]We're going to go out in the maze down here and see how you guys do in navigating your way through a spatial map that's... Oh, my God. I'm just trying to get my head start.
[15:10]So how do I get as good as Brian? Well, it happens intuitively.
[15:19]Just look at my hands. I cross-reference what I see with what I can touch.
[15:28]In fact, all my senses come into play. This is what Brian's been doing for the last seven days.
[15:40]And the result is that his brain is now starting to decode that new visual input automatically.
[15:52]Brian's not simply getting better at making conscious adjustments. His whole reality is changing.
[16:09]If you take those subjects and put goggles on them for two weeks, we find that it takes about a week to start behaving normally. They start being able to figure out how to interact with the world to constructing a new reality around them, a new way of dealing with these incoming perceptions. And they say that initially they can tell there's a new left and an old left and a new right and an old right. But by about a week and a half, they even lose the concept of which right and left were the old ones and the new ones. So it's like the whole spatial map of the world is altering. And by two weeks in, they can write well, read without a problem, do all of our walking tasks and reaching tasks. Um, and then when they remove the goggles, it actually takes them about a day to go back to normal behavior. What this exposes for me is how much effort the brain goes through to construct our world. Because normally, you're walking through the world, and it feels like there's reality out there. But, in fact, there's so much work happening behind the scenes to allow that reality to happen. Seeing requires an intensive training program. But new recruits come on board every day. We call them babies. When babies reach out to touch what's in front of them, they're not just learning what an object feels like. They're learning how to see. They're establishing pathways in the brain that'll be used for the rest of their lives. Because vision is a whole-body experience. The data coming in from our eyes only means something if we can cross-reference it. If from birth, you weren't able to interact with the world, if you couldn't work out through feedback, what the sensory information meant. In theory, you'd never be able to see. This cross-referencing doesn't stop when we're fully grown.
[18:19]It continues throughout our lives. What we touch influences how we see.
[18:30]Taste is affected by our sense of smell. Our sight informs how we hear. Our senses depend on each other, and our reality is built by comparing these streams of data. When they're woven together, we get our perception of this moment.
[18:57]It's an astonishing feat to pull off. But there's one factor which really adds complication. Timing.
[19:09]All those streams of sensory data are processed by the brain at different speeds. For our reality to be constructed, they have to be synchronized. What do I mean by this? Well, the easiest way for me to show you is right here at a racetrack.
[19:33]When there's a loud sound, it feels as though you react to it instantly. But you don't.
[19:48]Watching sprinters in slow motion, we can see that there's a gap between the gun going off and their start.
[20:02]They may train to make this gap as small as possible, but their biology imposes limits. Processing that sound, then sending out signals to the muscles to move, will take around two tenths of a second.
[20:30]And that time really can't be improved on. In a sport where thousands of a second can be the difference between winning and losing, It seems surprisingly slow. So why do we use a pistol to start sprinters? Everyone knows that light travels faster than sound. So why not use a light? We set up a test to show you. In the top screen we're triggered by a light. In the bottom screen, we're triggered by the gun.
[21:07]You can see that when our start is triggered by a flash of light, we respond more slowly.
[21:17]It takes 40 milliseconds longer to process. Why? Because the visual system is more complex. It's bigger, it involves almost a third of the brain. So while all of the electrical signals inside the brain travel at the same speed, the ones related to sight go through more complex processing, and that takes time.
[21:51]And this isn't just about hearing and seeing. Every type of sensory information takes a different amount of time to process. For our reality to be constructed, they have to be synchronized.
[22:18]When I clap my hands, everything seems synchronized. Why? Well, your brain is pulling off fancy editing tricks. What it takes to be reality is actually a delayed version. It collects up all the information from the senses before it decides on a story of what happened. And that means you live in the past. By the time you think the moment now occurs, it's already long gone.
