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Just over a year ago,

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for the third time in my life,
I ceased to exist.

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I was having a small operation,
and my brain was filling with anesthetic.

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I remember a sense
of detachment and falling apart

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and a coldness.

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And then I was back,
drowsy and disoriented,

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but definitely there.

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Now, when you wake from a deep sleep,

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you might feel confused about the time
or anxious about oversleeping,

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but there's always a basic sense
of time having passed,

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of a continuity between then and now.

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Coming round from
anesthesia is very different.

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I could have been under
for five minutes, five hours,

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five years or even 50 years.

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I simply wasn't there.

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It was total oblivion.

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Anesthesia --
it's a modern kind of magic.

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It turns people into objects,

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and then, we hope, back again into people.

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And in this process

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is one of the greatest remaining
mysteries in science and philosophy.

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How does consciousness happen?

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Somehow, within each of our brains,

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the combined activity
of many billions of neurons,

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each one a tiny biological machine,

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is generating a conscious experience.

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And not just any conscious experience --

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your conscious experience
right here and right now.

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How does this happen?

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Answering this question is so important

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because consciousness
for each of us is all there is.

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Without it there's no world,

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there's no self,

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there's nothing at all.

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And when we suffer, we suffer consciously

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whether it's through
mental illness or pain.

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And if we can experience
joy and suffering,

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what about other animals?

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Might they be conscious, too?

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Do they also have a sense of self?

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And as computers get faster and smarter,

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maybe there will come a point,
maybe not too far away,

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when my iPhone develops
a sense of its own existence.

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I actually think the prospects
for a conscious AI are pretty remote.

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And I think this because
my research is telling me

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that consciousness has less to do
with pure intelligence

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and more to do with our nature
as living and breathing organisms.

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Consciousness and intelligence
are very different things.

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You don't have to be smart to suffer,
but you probably do have to be alive.

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In the story I'm going to tell you,

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our conscious experiences
of the world around us,

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and of ourselves within it,

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are kinds of controlled hallucinations

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that happen with, through
and because of our living bodies.

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Now, you might have heard
that we know nothing

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about how the brain and body
give rise to consciousness.

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Some people even say it's beyond
the reach of science altogether.

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But in fact,

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the last 25 years have seen an explosion
of scientific work in this area.

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If you come to my lab
at the University of Sussex,

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you'll find scientists
from all different disciplines

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and sometimes even philosophers.

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All of us together trying to understand
how consciousness happens

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and what happens when it goes wrong.

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And the strategy is very simple.

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I'd like you to think about consciousness

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in the way that we've
come to think about life.

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At one time, people thought
the property of being alive

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could not be explained
by physics and chemistry --

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that life had to be
more than just mechanism.

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But people no longer think that.

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As biologists got on with the job

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of explaining the properties
of living systems

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in terms of physics and chemistry --

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things like metabolism,
reproduction, homeostasis --

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the basic mystery of what life is
started to fade away,

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and people didn't propose
any more magical solutions,

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like a force of life or an élan vital.

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So as with life, so with consciousness.

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Once we start explaining its properties

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in terms of things happening
inside brains and bodies,

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the apparently insoluble mystery
of what consciousness is

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should start to fade away.

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At least that's the plan.

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So let's get started.

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What are the properties of consciousness?

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What should a science
of consciousness try to explain?

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Well, for today I'd just like to think
of consciousness in two different ways.

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There are experiences
of the world around us,

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full of sights, sounds and smells,

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there's multisensory, panoramic,
3D, fully immersive inner movie.

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And then there's conscious self.

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The specific experience
of being you or being me.

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The lead character in this inner movie,

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and probably the aspect of consciousness
we all cling to most tightly.

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Let's start with experiences
of the world around us,

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and with the important idea
of the brain as a prediction engine.

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Imagine being a brain.

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You're locked inside a bony skull,

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trying to figure
what's out there in the world.

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There's no lights inside the skull.
There's no sound either.

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All you've got to go on
is streams of electrical impulses

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which are only indirectly related
to things in the world,

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whatever they may be.

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So perception --
figuring out what's there --

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has to be a process of informed guesswork

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in which the brain combines
these sensory signals

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with its prior expectations or beliefs
about the way the world is

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to form its best guess
of what caused those signals.

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The brain doesn't hear sound or see light.

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What we perceive is its best guess
of what's out there in the world.

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Let me give you a couple
of examples of all this.

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You might have seen this illusion before,

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but I'd like you to think
about it in a new way.

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If you look at those two patches, A and B,

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they should look to you to be
very different shades of gray, right?

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But they are in fact
exactly the same shade.

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And I can illustrate this.

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If I put up a second version
of the image here

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and join the two patches
with a gray-colored bar,

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you can see there's no difference.

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It's exactly the same shade of gray.

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And if you still don't believe me,

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I'll bring the bar across
and join them up.

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It's a single colored block of gray,
there's no difference at all.

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This isn't any kind of magic trick.

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It's the same shade of gray,

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but take it away again,
and it looks different.

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So what's happening here

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is that the brain
is using its prior expectations

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built deeply into the circuits
of the visual cortex

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that a cast shadow dims
the appearance of a surface,

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so that we see B as lighter
than it really is.

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Here's one more example,

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which shows just how quickly
the brain can use new predictions

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to change what we consciously experience.

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Have a listen to this.

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(Distorted voice)

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Sounded strange, right?

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Have a listen again
and see if you can get anything.

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(Distorted voice)

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Still strange.

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Now listen to this.

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(Recording) Anil Seth: I think Brexit
is a really terrible idea.

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(Laughter)

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Which I do.

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So you heard some words there, right?

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Now listen to the first sound again.
I'm just going to replay it.

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(Distorted voice)

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Yeah? So you can now hear words there.

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Once more for luck.

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(Distorted voice)

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OK, so what's going on here?

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The remarkable thing is the sensory
information coming into the brain

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hasn't changed at all.

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All that's changed
is your brain's best guess

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of the causes of that sensory information.

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And that changes
what you consciously hear.

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All this puts the brain
basis of perception

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in a bit of a different light.

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Instead of perception depending largely
on signals coming into the brain

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from the outside world,

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it depends as much, if not more,

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on perceptual predictions
flowing in the opposite direction.

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We don't just passively
perceive the world,

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we actively generate it.

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The world we experience
comes as much, if not more,

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from the inside out

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as from the outside in.

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Let me give you
one more example of perception

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as this active, constructive process.

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Here we've combined immersive
virtual reality with image processing

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to simulate the effects
of overly strong perceptual predictions

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on experience.

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In this panoramic video,
we've transformed the world --

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which is in this case Sussex campus --

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into a psychedelic playground.

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We've processed the footage using
an algorithm based on Google's Deep Dream

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to simulate the effects
of overly strong perceptual predictions.

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In this case, to see dogs.

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And you can see
this is a very strange thing.

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When perceptual
predictions are too strong,

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as they are here,

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the result looks very much
like the kinds of hallucinations

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people might report in altered states,

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or perhaps even in psychosis.

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Now, think about this for a minute.

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If hallucination is a kind
of uncontrolled perception,

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then perception right here and right now
is also a kind of hallucination,

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but a controlled hallucination

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in which the brain's predictions
are being reigned in

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by sensory information from the world.

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In fact, we're all
hallucinating all the time,

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including right now.

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It's just that when we agree
about our hallucinations,

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we call that reality.

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(Laughter)

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Now I'm going to tell you
that your experience of being a self,

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the specific experience of being you,

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is also a controlled hallucination
generated by the brain.

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This seems a very strange idea, right?

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Yes, visual illusions
might deceive my eyes,

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but how could I be deceived
about what it means to be me?

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For most of us,

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the experience of being a person

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is so familiar, so unified
and so continuous

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that it's difficult
not to take it for granted.

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But we shouldn't take it for granted.

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There are in fact many different ways
we experience being a self.

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There's the experience of having a body

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and of being a body.

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There are experiences
of perceiving the world

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from a first person point of view.

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There are experiences
of intending to do things

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and of being the cause of things
that happen in the world.

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And there are experiences

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of being a continuous
and distinctive person over time,

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built from a rich set
of memories and social interactions.

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Many experiments show,

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and psychiatrists
and neurologists know very well

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that these different ways
in which we experience being a self

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can all come apart.

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What this means is
the basic background experience

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of being a unified self is a rather
fragile construction of the brain.

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Another experience,
which just like all others,

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requires explanation.

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So let's return to the bodily self.

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How does the brain generate
the experience of being a body

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and of having a body?

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Well, just the same principles apply.

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The brain makes its best guess

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about what is and what is not
part of its body.

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And there's a beautiful experiment
in neuroscience to illustrate this.

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And unlike most neuroscience experiments,

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this is one you can do at home.

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All you need is one of these.

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(Laughter)

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And a couple of paintbrushes.

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In the rubber hand illusion,

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a person's real hand is hidden from view,

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and that fake rubber hand
is placed in front of them.

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Then both hands are simultaneously
stroked with a paintbrush

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while the person stares at the fake hand.

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Now, for most people, after a while,

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this leads to the very uncanny sensation

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that the fake hand
is in fact part of their body.

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And the idea is that the congruence
between seeing touch and feeling touch

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on an object that looks like hand
and is roughly where a hand should be,

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is enough evidence for the brain
to make its best guess

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that the fake hand
is in fact part of the body.

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(Laughter)

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So you can measure
all kinds of clever things.

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You can measure skin conductance
and startle responses,

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but there's no need.

255
00:11:10,120 --> 00:11:13,056
It's clear the guy in blue
has assimilated the fake hand.

256
00:11:13,080 --> 00:11:16,496
This means that even experiences
of what our body is

257
00:11:16,520 --> 00:11:18,056
is a kind of best guessing --

258
00:11:18,080 --> 00:11:20,560
a kind of controlled
hallucination by the brain.

259
00:11:21,480 --> 00:11:22,680
There's one more thing.

260
00:11:24,120 --> 00:11:27,776
We don't just experience our bodies
as objects in the world from the outside,

261
00:11:27,800 --> 00:11:29,536
we also experience them from within.

262
00:11:29,560 --> 00:11:33,800
We all experience the sense
of being a body from the inside.

263
00:11:35,000 --> 00:11:37,576
And sensory signals
coming from the inside of the body

264
00:11:37,600 --> 00:11:41,656
are continually telling the brain
about the state of the internal organs,

265
00:11:41,680 --> 00:11:44,336
how the heart is doing,
what the blood pressure is like,

266
00:11:44,360 --> 00:11:45,576
lots of things.

267
00:11:45,600 --> 00:11:48,080
This kind of perception,
which we call interoception,

268
00:11:48,840 --> 00:11:50,096
is rather overlooked.

269
00:11:50,120 --> 00:11:51,536
But it's critically important

270
00:11:51,560 --> 00:11:54,896
because perception and regulation
of the internal state of the body --

271
00:11:54,920 --> 00:11:56,520
well, that's what keeps us alive.

272
00:11:57,400 --> 00:11:59,816
Here's another version
of the rubber hand illusion.

273
00:11:59,840 --> 00:12:01,336
This is from our lab at Sussex.

274
00:12:01,360 --> 00:12:04,776
And here, people see
a virtual reality version of their hand,

275
00:12:04,800 --> 00:12:06,176
which flashes red and back

276
00:12:06,200 --> 00:12:09,456
either in time or out of time
with their heartbeat.

277
00:12:09,480 --> 00:12:11,936
And when it's flashing
in time with their heartbeat,

278
00:12:11,960 --> 00:12:15,080
people have a stronger sense
that it's in fact part of their body.

279
00:12:15,720 --> 00:12:19,216
So experiences of having a body
are deeply grounded

280
00:12:19,240 --> 00:12:21,640
in perceiving our bodies from within.

281
00:12:23,680 --> 00:12:26,336
There's one last thing
I want to draw your attention to,

282
00:12:26,360 --> 00:12:30,016
which is that experiences of the body
from the inside are very different

283
00:12:30,040 --> 00:12:32,096
from experiences of the world around us.

284
00:12:32,120 --> 00:12:34,816
When I look around me,
the world seems full of objects --

285
00:12:34,840 --> 00:12:37,016
tables, chairs, rubber hands,

286
00:12:37,040 --> 00:12:38,776
people, you lot --

287
00:12:38,800 --> 00:12:40,776
even my own body in the world,

288
00:12:40,800 --> 00:12:43,056
I can perceive it
as an object from the outside.

289
00:12:43,080 --> 00:12:45,136
But my experiences
of the body from within,

290
00:12:45,160 --> 00:12:46,576
they're not like that at all.

291
00:12:46,600 --> 00:12:48,656
I don't perceive my kidneys here,

292
00:12:48,680 --> 00:12:50,256
my liver here,

293
00:12:50,280 --> 00:12:51,536
my spleen ...

294
00:12:51,560 --> 00:12:53,456
I don't know where my spleen is,

295
00:12:53,480 --> 00:12:54,856
but it's somewhere.

296
00:12:54,880 --> 00:12:56,776
I don't perceive my insides as objects.

297
00:12:56,800 --> 00:13:00,440
In fact, I don't experience them
much at all unless they go wrong.

298
00:13:01,400 --> 00:13:02,880
And this is important, I think.

299
00:13:03,600 --> 00:13:05,696
Perception of the internal
state of the body

300
00:13:05,720 --> 00:13:07,576
isn't about figuring out what's there,

301
00:13:07,600 --> 00:13:09,736
it's about control and regulation --

302
00:13:09,760 --> 00:13:13,656
keeping the physiological variables
within the tight bounds

303
00:13:13,680 --> 00:13:15,520
that are compatible with survival.

304
00:13:16,640 --> 00:13:19,496
When the brain uses predictions
to figure out what's there,

305
00:13:19,520 --> 00:13:22,616
we perceive objects
as the causes of sensations.

306
00:13:22,640 --> 00:13:26,256
When the brain uses predictions
to control and regulate things,

307
00:13:26,280 --> 00:13:29,440
we experience how well
or how badly that control is going.

308
00:13:29,880 --> 00:13:32,696
So our most basic experiences
of being a self,

309
00:13:32,720 --> 00:13:34,776
of being an embodied organism,

310
00:13:34,800 --> 00:13:38,760
are deeply grounded in the biological
mechanisms that keep us alive.

311
00:13:40,760 --> 00:13:43,096
And when we follow this idea
all the way through,

312
00:13:43,120 --> 00:13:46,816
we can start to see
that all of our conscious experiences,

313
00:13:46,840 --> 00:13:51,576
since they all depend on the same
mechanisms of predictive perception,

314
00:13:51,600 --> 00:13:55,016
all stem from this basic
drive to stay alive.

315
00:13:55,040 --> 00:13:57,656
We experience the world and ourselves

316
00:13:57,680 --> 00:14:00,760
with, through and because of
our living bodies.

317
00:14:01,560 --> 00:14:03,960
Let me bring things together step-by-step.

318
00:14:04,560 --> 00:14:06,136
What we consciously see depends

319
00:14:06,160 --> 00:14:08,456
on the brain's best guess
of what's out there.

320
00:14:08,480 --> 00:14:10,856
Our experienced world
comes from the inside out,

321
00:14:10,880 --> 00:14:12,120
not just the outside in.

322
00:14:12,600 --> 00:14:15,856
The rubber hand illusion shows
that this applies to our experiences

323
00:14:15,880 --> 00:14:17,720
of what is and what is not our body.

324
00:14:18,560 --> 00:14:22,136
And these self-related predictions
depend critically on sensory signals

325
00:14:22,160 --> 00:14:23,920
coming from deep inside the body.

326
00:14:24,400 --> 00:14:25,616
And finally,

327
00:14:25,640 --> 00:14:30,016
experiences of being an embodied self
are more about control and regulation

328
00:14:30,040 --> 00:14:31,720
than figuring out what's there.

329
00:14:32,840 --> 00:14:36,056
So our experiences of the world
around us and ourselves within it --

330
00:14:36,080 --> 00:14:38,336
well, they're kinds
of controlled hallucinations

331
00:14:38,360 --> 00:14:41,256
that have been shaped
over millions of years of evolution

332
00:14:41,280 --> 00:14:44,040
to keep us alive in worlds
full of danger and opportunity.

333
00:14:44,480 --> 00:14:47,160
We predict ourselves into existence.

334
00:14:48,520 --> 00:14:51,000
Now, I leave you with three
implications of all this.

335
00:14:51,640 --> 00:14:53,856
First, just as we can
misperceive the world,

336
00:14:53,880 --> 00:14:55,656
we can misperceive ourselves

337
00:14:55,680 --> 00:14:57,816
when the mechanisms
of prediction go wrong.

338
00:14:57,840 --> 00:15:02,056
Understanding this opens many new
opportunities in psychiatry and neurology,

339
00:15:02,080 --> 00:15:04,696
because we can finally
get at the mechanisms

340
00:15:04,720 --> 00:15:06,616
rather than just treating the symptoms

341
00:15:06,640 --> 00:15:09,160
in conditions like
depression and schizophrenia.

342
00:15:09,720 --> 00:15:10,936
Second:

343
00:15:10,960 --> 00:15:14,776
what it means to be me
cannot be reduced to or uploaded to

344
00:15:14,800 --> 00:15:17,176
a software program running on a robot,

345
00:15:17,200 --> 00:15:19,416
however smart or sophisticated.

346
00:15:19,440 --> 00:15:22,056
We are biological, flesh-and-blood animals

347
00:15:22,080 --> 00:15:25,336
whose conscious experiences
are shaped at all levels

348
00:15:25,360 --> 00:15:28,336
by the biological mechanisms
that keep us alive.

349
00:15:28,360 --> 00:15:32,000
Just making computers smarter
is not going to make them sentient.

350
00:15:33,000 --> 00:15:34,296
Finally,

351
00:15:34,320 --> 00:15:36,296
our own individual inner universe,

352
00:15:36,320 --> 00:15:37,976
our way of being conscious,

353
00:15:38,000 --> 00:15:40,760
is just one possible
way of being conscious.

354
00:15:41,840 --> 00:15:43,816
And even human consciousness generally --

355
00:15:43,840 --> 00:15:47,816
it's just a tiny region in a vast space
of possible consciousnesses.

356
00:15:47,840 --> 00:15:51,496
Our individual self and worlds
are unique to each of us,

357
00:15:51,520 --> 00:15:54,736
but they're all grounded
in biological mechanisms

358
00:15:54,760 --> 00:15:57,360
shared with many other living creatures.

359
00:15:57,920 --> 00:16:00,880
Now, these are fundamental changes

360
00:16:01,760 --> 00:16:04,016
in how we understand ourselves,

361
00:16:04,040 --> 00:16:05,896
but I think they should be celebrated,

362
00:16:05,920 --> 00:16:08,296
because as so often in science,
from Copernicus --

363
00:16:08,320 --> 00:16:10,320
we're not at the center of the universe --

364
00:16:10,344 --> 00:16:11,576
to Darwin --

365
00:16:11,600 --> 00:16:13,816
we're related to all other creatures --

366
00:16:13,840 --> 00:16:15,040
to the present day.

367
00:16:16,320 --> 00:16:19,056
With a greater sense of understanding

368
00:16:19,080 --> 00:16:21,496
comes a greater sense of wonder,

369
00:16:21,520 --> 00:16:23,376
and a greater realization

370
00:16:23,400 --> 00:16:28,000
that we are part of
and not apart from the rest of nature.

371
00:16:28,920 --> 00:16:30,120
And ...

372
00:16:30,720 --> 00:16:33,096
when the end of consciousness comes,

373
00:16:33,120 --> 00:16:35,920
there's nothing to be afraid of.

374
00:16:36,560 --> 00:16:37,760
Nothing at all.

375
00:16:38,480 --> 00:16:39,696
Thank you.

376
00:16:39,720 --> 00:16:47,746
(Applause)