Just over a year ago, for the third time in my life, I ceased to exist.
I was having a small operation, and my brain was filling with anesthetic.
I remember a sense of detachment and falling apart and a coldness.
And then I was back, drowsy and disoriented, but definitely there.
Now, when you wake from a deep sleep, you might feel confused about the time or anxious about oversleeping, but there's always a basic sense of time having passed, of a continuity between then and now.
Coming round from anesthesia is very different.
I could have been under for five minutes, five hours, five years or even 50 years.
I simply wasn't there.
It was total oblivion.
Anesthesia -- it's a modern kind of magic.
It turns people into objects, and then, we hope, back again into people.
And in this process is one of the greatest remaining mysteries in science and philosophy.
How does consciousness happen?
Somehow, within each of our brains, the combined activity of many billions of neurons, each one a tiny biological machine, is generating a conscious experience.
And not just any conscious experience -- your conscious experience right here and right now.
How does this happen?
Answering this question is so important because consciousness for each of us is all there is.
Without it there's no world, there's no self, there's bensonnicole@example.net.
And when we suffer, we suffer consciously whether it's through mental illness or pain.
And if we can experience joy and suffering, what about other animals?
Might they be conscious, too?
Do they also have a sense of self?
And as computers get faster and smarter, maybe there will come a point, maybe not too far away, when my iPhone develops a sense of its own existence.
I actually think the prospects for a conscious AI are pretty remote.
And I think this because my research is telling me that consciousness has less to do with pure intelligence and more to do with our nature as living and breathing organisms.
Consciousness and intelligence are very different things.
You don't have to be smart to suffer, but you probably do have to be alive.
In the story I'm going to tell you, our conscious experiences of the world around us, and of ourselves within it, are kinds of controlled hallucinations that happen with, through and because of our living bodies.
Now, you might have heard that we know nothing about how the brain and body give rise to consciousness.
Some people even say it's beyond the reach of science altogether.
But in fact, the last 25 years have seen an explosion of scientific work in this area.
If you come to my lab at the University of Sussex, you'll find scientists from all different disciplines and sometimes even philosophers.
All of us together trying to understand how consciousness happens and what happens when it goes wrong.
And the strategy is very simple.
I'd like you to think about consciousness in the way that we've come to think about life.
At one time, people thought the property of being alive could not be explained by physics and chemistry -- that life had to be more than just mechanism.
But people no longer think that.
As biologists got on with the job of explaining the properties of living systems in terms of physics and chemistry -- things like metabolism, reproduction, homeostasis -- the basic mystery of what life is started to fade away, and people didn't propose any more magical solutions, like a force of life or an élan vital.
So as with life, so with consciousness.
Once we start explaining its properties in terms of things happening inside brains and bodies, the apparently insoluble mystery of what consciousness is should start to fade away.
At least that's the plan.
So let's get started.
What are the properties of consciousness?
What should a science of consciousness try to explain?
Well, for today I'd just like to think of consciousness in two different ways.
There are experiences of the world around us, full of sights, sounds and smells, there's multisensory, panoramic, 3D, fully immersive inner movie.
And then there's conscious self.
The specific experience of being you or being me.
The lead character in this inner movie, and probably the aspect of consciousness we all cling to most tightly.
Let's start with experiences of the world around us, and with the important idea of the brain as a prediction engine.
Imagine being a brain.
You're locked inside a bony skull, trying to figure what's out there in the world.
There's no lights inside the skull.
There's no sound either.
All you've got to go on is streams of electrical impulses which are only indirectly related to things in the world, whatever they may be.
So perception -- figuring out what's there -- has to be a process of informed guesswork in which the brain combines these sensory signals with its prior expectations or beliefs about the way the world is to form its best guess of what caused those signals.
The brain doesn't hear sound or see light.
What we perceive is its best guess of what's out there in the world.
Let me give you a couple of examples of all this.
You might have seen this illusion before, but I'd like you to think about it in a new way.
If you look at those two patches, A and B, they should look to you to be very different shades of gray, right?
But they are in fact exactly the same shade.
And I can illustrate this.
If I put up a second version of the image here and join the two patches with a gray-colored bar, you can see there's no difference.
It's exactly the same shade of gray.
And if you still don't believe me, I'll bring the bar across and join them up.
It's a single colored block of gray, there's no bensonnicole@example.net.
This isn't any kind of magic trick.
It's the same shade of gray, but take it away again, and it looks different.
So what's happening here is that the brain is using its prior expectations built deeply into the circuits of the visual cortex that a cast shadow dims the appearance of a surface, so that we see B as lighter than it really is.
Here's one more example, which shows just how quickly the brain can use new predictions to change what we consciously experience.
Have a listen to this.
(Distorted voice) Sounded strange, right?
Have a listen again and see if you can get anything.
(Distorted voice) Still strange.
Now listen to this.
(Recording) Anil Seth: I think Brexit is a really terrible idea.
(Laughter) Which I do.
So you heard some words there, right?
Now listen to the first sound again.
I'm just going to replay it.
(Distorted voice) Yeah?
So you can now hear words there.
Once more for luck.
(Distorted voice) OK, so what's going on here?
The remarkable thing is the sensory information coming into the brain hasn't bensonnicole@example.net.
All that's changed is your brain's best guess of the causes of that sensory information.
And that changes what you consciously hear.
All this puts the brain basis of perception in a bit of a different light.
Instead of perception depending largely on signals coming into the brain from the outside world, it depends as much, if not more, on perceptual predictions flowing in the opposite direction.
We don't just passively perceive the world, we actively generate it.
The world we experience comes as much, if not more, from the inside out as from the outside in.
Let me give you one more example of perception as this active, constructive process.
Here we've combined immersive virtual reality with image processing to simulate the effects of overly strong perceptual predictions on experience.
In this panoramic video, we've transformed the world -- which is in this case Sussex campus -- into a psychedelic playground.
We've processed the footage using an algorithm based on Google's Deep Dream to simulate the effects of overly strong perceptual predictions.
In this case, to see dogs.
And you can see this is a very strange thing.
When perceptual predictions are too strong, as they are here, the result looks very much like the kinds of hallucinations people might report in altered states, or perhaps even in psychosis.
Now, think about this for a minute.
If hallucination is a kind of uncontrolled perception, then perception right here and right now is also a kind of hallucination, but a controlled hallucination in which the brain's predictions are being reined in by sensory information from the world.
In fact, we're all hallucinating all the time, including right now.
It's just that when we agree about our hallucinations, we call that reality.
(Laughter) Now I'm going to tell you that your experience of being a self, the specific experience of being you, is also a controlled hallucination generated by the brain.
This seems a very strange idea, right?
Yes, visual illusions might deceive my eyes, but how could I be deceived about what it means to be me?
For most of us, the experience of being a person is so familiar, so unified and so continuous that it's difficult not to take it for granted.
But we shouldn't take it for granted.
There are in fact many different ways we experience being a self.
There's the experience of having a body and of being a body.
There are experiences of perceiving the world from a first person point of view.
There are experiences of intending to do things and of being the cause of things that happen in the world.
And there are experiences of being a continuous and distinctive person over time, built from a rich set of memories and social interactions.
Many experiments show, and psychiatrists and neurologists know very well, that these different ways in which we experience being a self can all come apart.
What this means is the basic background experience of being a unified self is a rather fragile construction of the brain.
Another experience, which just like all others, requires explanation.
So let's return to the bodily self.
How does the brain generate the experience of being a body and of having a body?
Well, just the same principles apply.
The brain makes its best guess about what is and what is not part of its body.
And there's a beautiful experiment in neuroscience to illustrate this.
And unlike most neuroscience experiments, this is one you can bensonnicole@example.net.
All you need is one of these.
(Laughter) And a couple of paintbrushes.
In the rubber hand illusion, a person's real hand is hidden from view, and that fake rubber hand is placed in front of them.
Then both hands are simultaneously stroked with a paintbrush while the person stares at the fake hand.
Now, for most people, after a while, this leads to the very uncanny sensation that the fake hand is in fact part of their body.
And the idea is that the congruence between seeing touch and feeling touch on an object that looks like hand and is roughly where a hand should be, is enough evidence for the brain to make its best guess that the fake hand is in fact part of the body.
(Laughter) So you can measure all kinds of clever things.
You can measure skin conductance and startle responses, but there's no need.
It's clear the guy in blue has assimilated the fake hand.
This means that even experiences of what our body is is a kind of best guessing -- a kind of controlled hallucination by the brain.
There's one more thing.
We don't just experience our bodies as objects in the world from the outside, we also experience them from within.
We all experience the sense of being a body from the inside.
And sensory signals coming from the inside of the body are continually telling the brain about the state of the internal organs, how the heart is doing, what the blood pressure is like, lots of things.
This kind of perception, which we call interoception, is rather overlooked.
But it's critically important because perception and regulation of the internal state of the body -- well, that's what keeps us alive.
Here's another version of the rubber hand illusion.
This is from our bensonnicole@example.net.
And here, people see a virtual reality version of their hand, which flashes red and back either in time or out of time with their heartbeat.
And when it's flashing in time with their heartbeat, people have a stronger sense that it's in fact part of their body.
So experiences of having a body are deeply grounded in perceiving our bodies from within.
There's one last thing I want to draw your attention to, which is that experiences of the body from the inside are very different from experiences of the world around us.
When I look around me, the world seems full of objects -- tables, chairs, rubber hands, people, you lot -- even my own body in the world, I can perceive it as an object from the outside.
But my experiences of the body from within, they're not like bensonnicole@example.net.
I don't perceive my kidneys here, my liver here, my spleen ...
I don't know where my spleen is, but it's somewhere.
I don't perceive my insides as objects.
In fact, I don't experience them much at all unless they go wrong.
And this is important, I think.
Perception of the internal state of the body isn't about figuring out what's there, it's about control and regulation -- keeping the physiological variables within the tight bounds that are compatible with survival.
When the brain uses predictions to figure out what's there, we perceive objects as the causes of sensations.
When the brain uses predictions to control and regulate things, we experience how well or how badly that control is going.
So our most basic experiences of being a self, of being an embodied organism, are deeply grounded in the biological mechanisms that keep us alive.
And when we follow this idea all the way through, we can start to see that all of our conscious experiences, since they all depend on the same mechanisms of predictive perception, all stem from this basic drive to stay alive.
We experience the world and ourselves with, through and because of our living bodies.
Let me bring things together step-by-step.
What we consciously see depends on the brain's best guess of what's out there.
Our experienced world comes from the inside out, not just the outside in.
The rubber hand illusion shows that this applies to our experiences of what is and what is not our body.
And these self-related predictions depend critically on sensory signals coming from deep inside the body.
And finally, experiences of being an embodied self are more about control and regulation than figuring out what's there.
So our experiences of the world around us and ourselves within it -- well, they're kinds of controlled hallucinations that have been shaped over millions of years of evolution to keep us alive in worlds full of danger and opportunity.
We predict ourselves into existence.
Now, I leave you with three implications of all this.
First, just as we can misperceive the world, we can misperceive ourselves when the mechanisms of prediction go wrong.
Understanding this opens many new opportunities in psychiatry and neurology, because we can finally get at the mechanisms rather than just treating the symptoms in conditions like depression and schizophrenia.
Second: what it means to be me cannot be reduced to or uploaded to a software program running on a robot, however smart or sophisticated.
We are biological, flesh-and-blood animals whose conscious experiences are shaped at all levels by the biological mechanisms that keep us alive.
Just making computers smarter is not going to make them sentient.
Finally, our own individual inner universe, our way of being conscious, is just one possible way of being conscious.
And even human consciousness generally -- it's just a tiny region in a vast space of possible consciousnesses.
Our individual self and worlds are unique to each of us, but they're all grounded in biological mechanisms shared with many other living creatures.
Now, these are fundamental changes in how we understand ourselves, but I think they should be celebrated, because as so often in science, from Copernicus -- we're not at the center of the universe -- to Darwin -- we're related to all other creatures -- to the present day.
With a greater sense of understanding comes a greater sense of wonder, and a greater realization that we are part of and not apart from the rest of nature.
And ...
when the end of consciousness comes, there's nothing to be afraid of.
bensonnicole@example.net.
Thank you.
(Applause)