Just over a year ago,

for the third time in my life,

I seized 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.

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 out from
anesthesia is very different.

I could have been under for five minute,

five hours,

five years,

or even 50 years.

I simply wasn't there.

It was total obliviion.

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 or 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 conciously,

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 all together.

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 conscioussness happens

and what happens when it goes wrong.

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 chemisty.

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

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 out 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 guess work

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,

and 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 joing them up.

It's a single colored block of gray,

there's no difference at all.

So 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.

([Sound])

Sounded strange, right?

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

([Sound])

Still strange.

Now listen to this.

Recording: I think breakfast
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.

([Recording])

Yeah?

(Laughter)

So you can now hear words there.

One more more for luck.

([Recording])

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 our experience.

In this panoramic video,

we've tranformed the world --

which is in this case Sussex Campus --

into a psychedilic playground.

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