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 nothing at all. 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 difference at all. 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 changed at all. 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 do at home. 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 lab at Sussex. 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 that at all. 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. Nothing at all. Thank you. (Applause)
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