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What’s Inside Your Brain?

Who are ‘you’ and how do you make decisions? Neuroscience leader David Eagleman investigates the mysteries of the brain

In his latest book, The Brain, renowned neuroscientist David Eagleman unlocks the mysteries of the most complex creation on earth and how it makes sense of the world, while examining the effect of modern technology and how our brain might develop in the future.


“As far as we can tell, we’re the only species that has grown so sophisticated that we’ve thrown ourselves into this problem of trying to figure out what our own programming language is. So it would be as though your computer started controlling its own peripheral devices, pulled its cover off and pointed its webcam at its own circuitry and tried to figure out what it’s made out of. That’s the situation that we’re in.

What we have found is the most complicated device that we’ve ever found in the universe. This weighs about three pounds and it contains 100 billion neurons, and every single neuron is about as complicated as the City of London. Every single neuron has the entire human genome in it and it’s trafficking millions of proteins around very complicated cascades. Each one of these is connected to about 10,000 of its neighbours.

So what we have is 1,000 trillion connections here, each of which is popping off multiple times a second, every moment of your life. The most striking intellectual development that we’ve made as a species is realising that this is you — somehow, this, three pounds of wet, biological gushy stuff, is you. It’s your hopes, your dreams, your aspirations, the agony, the ecstasy, all of it is happening right here. How do we know that?

If you were to damage the tip of your pinkie, you would be sad about it but you wouldn’t be any different as a person. But if you were to damage an equivalently sized chunk of brain tissue, that could change you entirely. That can change your personality, your risk aversion, your capacity to see colours or name animals or speak language or understand music or 100 other things that we see every day when people get even very small damage to the brain. So that’s how we know that this is the densest representation of you.

Now, the reason I did this latest project of mine is because I see the brain as a very good lens for the project of knowing ourselves — of self-exploration.

I grew up watching Carl Sagan’s Cosmos, for me, it turned me on to the beauty of science and the power of science as a way of knowing the world. So I grew up, I became a neuroscientist, I run a research lab where I study lots of aspects about how the brain constructs reality and how we take in sensory information, and how the brain can rewire itself.

In parallel with that, I got interested in writing about science. That’s because I write lots of academic journal paper — I’ve got over 100 that are widely read by at least seven people on the planet [laughs]. I wanted to disseminate the magic a bit more widely, so I started writing books and I was very gratified to find out that people actually cared about this. It’s a really powerful inroad for knowing oneself.

So that’s what led to this television show and book, The Brain, and what it’s about is fleshing out the science. So at the same time I wrote this, I’d just finished writing a cognitive neuroscience textbook that’s 1,000 pages long — it’s a litany of facts and data. What I wanted to do with this was something totally on the opposite end of the spectrum, which is, ‘How can we start with the foundation of brain science and figure out what that tells us about what it means to be a human?’.

So I’ve structured this around six major questions, about our identity, how we perceive reality, our unconscious brains, how we make decisions, social neuroscience, and where our species is going. So let’s start with this question about identity.

Every time I hold a brain, it’s awe-inspiring. I’ve been holding human brains for a long time now, and every single time it gives me the same sense of awe because someone’s entire life took place right there. Every conversation they had, who they were, their experiences, all of that took place right here in this three pounds of tissue.

David Eagleman holding a brain

The shape of the hills and valleys of the brain is largely conserved across people. But the finer details give a personal and unique reflection of where you’ve been, where you are now and who you are now. Everything you’ve experienced has altered the physical structure of your brain — from the expression of genes to the position of molecules to the architecture of neurons — your family of origin, your culture, your friends, your work, every movie you’ve watched, every conversation you’ve had, these have all left their footprints in your nervous system. These indelible microscopic impressions accumulate to make you who you are.

So neurally speaking, who you are depends on where you’ve been. Your brain is a relentless shapeshifter, constantly rewriting its own circuitry. Because your experiences are unique, so are the vast, detailed patterns in your neural network. So what that means is that brains are as unique as snowflakes.

If you look around a room at the amount of variation in people’s faces, there’s a lot of variation. There’s that much variation on the inside of people’s brains as well, because we all come to the table with a certain set of genetic predispositions, and that mixes with all of our experiences. That sends brains off on very different developmental pathways.

As your trillions of new connections continually form and reform, the distinctive pattern means that no one like you has ever existed or will ever exist again. The experience of your conscious awareness right now, is unique to you.

Something that I found interesting, is as a result of this trajectory that you’re on with your genes and your experience, what it means is that the way we interpret the world has everything to do with that. Your interpretation of objects in the world actually has very little to do with them, it’s all about what you’re bringing to the table. The way we perceive objects in the world is not as they are, but as we are.


So this leads us to the second big question of the series, which is, what is reality? So you show up in the world and you open your eyes every day, and your senses are flooded with reality. There’s a person, there’s a lamp, there’s a door. It’s all out there and all you have to do is show up. It’s actually not quite so straightforward. One of the ways that neuroscience gets in at this is very simple visual illusions. There’s this mismatch between what you know is out there and what you’re perceiving internally.

All your brain sees are electrochemical signals that are coming in along different data cables and it is locked in silence and darkness inside the vault of your skull. Your eyes and your eyes, they’re not piping in photons and air compression waves, instead what these peripherals are doing is converting everything into the common currency of electrochemical signals and that’s it. That’s all it’s got to work with and nothing more.

But even though your brain is encased in darkness, it literally lights up the world for you. All of the seeing you’re doing right now is taking place inside of you, it’s not about what’s out here. So what this means is that your reality is all happening in this closed system, it’s a narrative played out inside of your cranium. It can be very different inside different heads.

So for example when [someone] is in the thick of a schizophrenic delusion, [their] reality is every bit as real as yours is right now. Every night when you go to bed and dream, you’re having the same sort of reality playing out in your internal theatre, and you completely believe whatever your brain is serving up to you.

Something that’s been very interesting to me lately is that because of the differences in brains, and because reality is constructed internally, reality can be really, really different inside different heads. So I’ve started to think about is as though we are the inhabitants of this wondrous world, but there’s a sense where we are each the sole inhabitant of our own wondrous world. We have enough in common that we can generally communicate between our planets — that’s why literature works, because there’s enough in common — but it can be quite different from person to person based on our own experiences and reality.

So the third issue is this question about who is in control. It feels like we’re consciously in control of our lives. We wake up in the morning and you say, ‘Okay, look, I’m going to go do this with my day, this is the kind of person I want to be’, and so on. But in fact, most of what we do and feel and act and believe is generated by massive machinery running under the hood, below the surface of conscious awareness. We don’t have access to how we’re doing most things.

When I lift a cup of coffee to my mouth, that’s actually a really complicated motor act that’s underpinned by a lightning storm of neural activity at this level, but it’s totally invisible to you. If I weren’t a biologist I’d have no reason to suspect the existence of muscles and tendons and nerves and electrical signals, because all I’d know is if I’ve got the glass to my mouth or spilled it on myself. It’s everything in life, it’s recognising a friend’s face, or falling in love, or getting a joke, or driving a car — any of the things we do are all underpinned by massive machinery. It’s very hard to get robots to do any of this, in fact, we’ve failed on most of those fronts. But this is actually what’s happening under the hood. So when we think about the conscious mind, it’s actually a broom closet in the mansion of the brain — it’s the smallest bit of what your brain is doing — and your brain can do many things without the conscious mind being involved at all.

An example is that a fastball travels from the pitcher’s mound to the home plate in 4/10ths of a second. Lots of us can hit fastballs, but conscious awareness takes at least 5/10ths of a second. So you are always living in the past — your conscious awareness takes a long time before it comes along. When you think the moment ‘now’ happens, it’s already happened a long time ago because your brain has to collect up lots of signals through these different sensory channels, stitch it all together, figure out what happened, it serves up a story to your conscious awareness and you say, ‘Oh, that just happened’, when in fact you’re living in the past.

So the interesting part about what happens when we automatise things is that the conscious mind is setting the long-term goal and gets you to practise over and over, and it becomes burned into the circuitry of the brain. Interestingly, once that happens, we no longer have conscious access to how we do something.

So riding a bicycle took a lot of attention when you were first learning how to do it, but then after you learn, you have no idea how you’re making all these micro-corrections and shifting your body weight and so on, you no longer have access to that.

Onto the fourth chapter: how do I decide? So we live in a world where we’re making hundreds of thousands of decisions every day [and] we’re typically unaware of it. But the brain does a lot of work to choose between possible options.

Listening to activity recorded deep in the brain, it’s impossible not to be awed, after all, this is what every decision in the history of our species sounded like. Every marriage proposal, every declaration of war, every leap of the imagination, every mission launched into the unknown, every act of kindness, every lie, every euphoric breakthrough, every decisive moment, it all happened right here in the darkness of the skull, emerging from patterns of activity and networks of biological cells.

So to my mind, the key thing to understanding decision making in the brain is this issue that when we think about who we are and how we make decisions, it’s easy to imagine that what’s going on is we are essentially single-minded. We go into a situation, we make a cost-benefit analysis and make a decision that way. But in fact, you’re built of multiple drives that are all competing to be in control.

I suggest that the right way to think about the brain is like a neural parliament, where you have different political parties all of whom are battling it out to steer the ship of state. All the parties love their country, but they just have different ways of going about it.

Humans are so subtle and nuanced and interesting because we’re a machine built out of conflicting parts. So when we think about neurobiological conflict, it’s like, this is the thing that’s happening under the hood, and who you are emerges from brain-wide battles that are raging every moment of your life in the darkness of your skull.

We are relentless time-travellers; you’re constantly simulating possible futures to figure out which path to take. If you’re faced with different options, you can’t try them all, so what you do is you mentally simulate along those paths — ‘If I do this, bla bla, if I do that, bla bla bla’. You live those futures, you live them, you feel how they are, and that’s how you navigate your decision making. We also travel a lot to the past, but this is how we navigate our decisions moving forward.

One of the interesting parts about this is we’re so good at simulating futures, the thing that often gets in the way of that is the present. A future simulation is really just a pale ghost compared to something right in front of you. So that’s the thing that tends to trip up good decision making about the future.

The part of the light spectrum that you see is actually one ten trillionth of the available light spectrum. We call that little sliver ‘visible light’, but all the rest of it — gamma waves, x-rays, microwaves, cell phone conversations, television waves — all of that is passing through your body right now, it’s the same stuff. It’s light, it’s electromagnetic radiation, it’s passing through your body because you don’t have the proper biological sensors to pick up on that part of the spectrum. So as far as you’re concerned it’s completely invisible to you.

It’s the same with hearing, we hear in a very limited range — from 20hz to 20,000hz — and all of the other frequencies that we could be hearing are totally invisible to us.

So one of the things we’ve been doing in my lab is seeing how we can open up our window of sensory reality. We’ve built a vest that’s covered in vibratory motors — just like the little buzzing motors in your cell phone — and we can turn data streams into real-time patterns of vibration on your torso.

So the first thing we’ve done with this is cured deafness. So for somebody who’s deaf, we capture sounds in the world, convert it into patterns of vibration, and a deaf person can come to understand the auditory world that way by feeling it in their torso. Because the brain is locked in darkness, it doesn’t hear anything, all it ever gets is signals coming along data cables and it figures out what to do with them.

Then our big area of interest for our next step is beyond deafness, feeding any kind of information stream into anybody’s brain. So we’re experimenting right now with feeding in real-time stock market data, or weather data from the surrounding 200 miles, or Twitter data, so you’re plugged directly into the consciousness of the planet. All of these things are things that the brain can figure out the patterns for and have completely new perceptual experiences that humans haven’t had before. Many people are interested in this question of how do answers in neuroscience reflect on issues of our mortality? So you may be aware that many people now are cryogenically freezing themselves — the techniques have gotten better over the years — and it’s just a throw to the future. It might not work, but the idea is if you have a terminal illness and you’re going to die from this, the idea is, ‘Okay, well maybe people 200 years from now will have come up with the cure, the solution, so I’ll deep freeze myself now, they’ll thaw me out in 200 years, fix the problem and I’ll get a second life cycle’.

There’s plenty of controversy about this, everyone’s got their opinions, and no one knows if it’ll work. Maybe in 200 years no one will feel obligated to thaw you out, or maybe they’ll thaw you out and eat you because they’re cannibals. You don’t know what’s going to happen, but it’s an interesting play for the future, the idea of, ‘I understand that I am my brain and so if I freeze it and thaw it out later it should still be me’, and there’s even a higher level of that that’s of great interest to a lot of people.

The hypothesis is [that] the mind emerges from all the operation of the brain — very complex system, but nonetheless, the idea’s that the wet, biological gushy stuff doesn’t matter, what matters is the algorithm that’s being run on top of that stuff. So the idea is if you could reproduce that in a different medium, that would still be you. So if I could reproduce exactly your brain out of beer cans and tennis balls, and it was doing the same operations, then that would be you.

If that’s true — we don’t know yet, but I think operationally that’s how most of us go into the lab — we should also be able to reproduce in zeros and ones and you live in silico like Neo in The Matrix. You could live potentially forever that way. That’s the step that’s probably not that far. We’re all going to miss it, but it’s probably 100 years off and we’re already on that path.

The European Union has put €1bn into this project called the Human Brain Project, which is to collect up all of the data from different labs and put together a fully-fledged software simulation of a human brain. That project’s going very well along this path, but it’s an extremely long path. It’s at least a century because in order to capture all the data from a human brain would require a zettabyte of capacity. A zettabyte is essentially the computational capacity of our entire planet right now.

Data from the Human Brain Project

So we’re still a long way off from having the power to run a system of that enormity and complexity, but if it works, that’s the thing that will allow us to do interstellar travel. Meat-puppet bodies aren’t going to make it, but if you can transfer yourself onto a different medium like a jump drive, shoot you out to interstellar space, and then when you get somewhere and you reboot, that’s like a beaming in Star Trek.

So it may be that we have more in common with our stone-age ancestors than we do with our near-future descendants because things are changing really fast in terms of us being able to build new kinds of bodies and inhabit new kinds of sensory realities, and maybe achieve immortality. The situation we’re in now — for good and for bad — is we’re at a point where our species is able to define our own trajectory into the future. Right now we’re entering this stage that’s often called post-humanism or transhumanism.”

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