raw_transcripts/lecture_7.txt

Okay.

Is this working?

Is this working.

On the back?

Okay.

The task in this week's lectures is try and help

you understand a little bit about what comes into the

brain and what goes down.

We're going to spend a lot of time in discussing

what goes on between those two stages, but the access

to the outside world and how we affect our muscles

are the two primary functions of the brain.

Yohannes Mueller was one of the parents of sensory science,

along with Helmholtz in the late 19th century, and it's

hard to put it better than this.

So I'll just read this out from his book in

1935, where he detailed a lot of specific nerve energies.

What Miller said was that the same cause, such as

electricity, can simultaneously affect all sensory organs, since they are

all sensitive to it, and yet every sensory nerve reacts

to it differently.

One never passes as light, another hears it a sound,

another smells it, another tastes.

The electricity, another one feels it as pain and shock.

One nerve perceives a luminous feature through mechanical irritation.

Another one hears it as buzzing.

Another one senses it as pain.

Sensation is not the conduction of a quality or state

of external body to consciousness or the conduction of quality

or state of our nerves to consciousness.

Excited by an external cause.

So in this lovely, flowery 19th century prose that we

don't use, unfortunately now what we were trying to say

is that we do not have access to the outside

world.

What we have access to in terms of our perceptions,

our cognition is the activity of the sensory nerve fibres

that sense the outside world and provide the signals to

the rest of the brain.

It seems rather commonplace now, but at the time it

was quite a revolutionary idea.

It has analogies in more modern sensory science, and when

we would talk about things called labelled lines, where we

think that individual nerve cells contribute to a particular quality

or sensation, for example, one nerve cell might signal the

readiness of something in the world, another one might signal

the greenness of something.

Another one might signal the fact that that object appeared

in a particular part of your visual field or on

a particular part of your skin.

Those are the labels that are attached to the activity

of those nerve cells.

One of the major challenges of neuroscience is to understand

how the activity of nerve cells is translated into perceptual

and cognitive states.

And we are not there yet.

But what we do know a lot about is how

it is that those nerve cells can provide the signals

that we need to access information about the outside world.

I can spend several lectures talking to you about the

structure of sensory nerve cells and cells, but I'm not.

And I'm going to try and instead try and communicate

to you three general principles, which I think for me

at least, are the basis for understanding sensation.

The first is that sensory receptors and we will understand

a bit more about sensory receptors in the moment and

not evenly distributed.

Different parts of the body have different densities of sensory

receptors, and for that reason, we use different parts of

our bodies for different things.

For example, we touch stuff with our fingers.

We have a high density of contraceptives.

We look at things in particular ways.

We trying to bring their gaze, our gaze on objects

so that the objects are projected onto the centre of

our visual field.

There are more photoreceptors in the centre of our visual

field, so these different densities of receptors have large implications

for how the brain is structured.

And I'll take you through that.

The second principle is that sensory signals are sent to

the cortex along parallel pathways.

This doesn't have to be the case.

We can imagine a sensory receptor trying to encode everything

it can about the outside world and sending all those

signals to the rest of the brain.

Instead, it seems that some receptors encode something the readiness

or the grain issues and others and other things the

blueish ness or the green regions of the brain of

the outside world.

These signals are therefore sent along parallel pathways to the

rest of the brain.

And this parallelism is the idea that different parts of

the outside world are represented within the same modality by

different nerve cells is key to understanding how the signals

get the cortex.

And may also be key to understanding how the cortex

is organised.

And the third thing I really want to get through

in this lecture is that cortex creates the cerebral cortex

creates topographic maps of the sensory periphery.

I hope you understand the sentence in detail by the

time we get through this next 15 minutes.

The idea is that the cortical represents representations that we

have that we use to see, to feel, to hear.

They are constructed representations of the outside world such that

the map of the body or the visual field is

projected onto the cerebral cortex.

And these topographic maps are key to understanding how at

least early parts of the cerebral cortex, the initial stages

of perception are organised.

So these are the three things I really want to

try and get through to you in the next 50

minutes.

As I said, I could spend five or six lectures

on sensory receptors themselves.

I'm not going to spend one slide.

This is because the basic structure of sensory receptors is

pretty similar, and there's only one thing you really need

to know about it.

That is that those sensory receptors take some form of

what we want to say.

Interruption of the sensory surface and convert that into a

nerve nerve signal, a spiking action potential.

For example, photons of light come through in the eye

and hit the back of the retina where they sign

photoreceptors.

And those photoreceptors in turn, transduced that light into an

electrochemical energy, which they then pass on to the rest

of the brain touch receptors and specialised nerve endings which

are sensitive to the displacement of the membrane.

So when there's a pressure on to the skin that

membrane displaces.

And that in turn is converted into electrochemical energy and

sent to the rest of the brain ordering auditory receptors.

Hence those in the ear specialised receptors which in which

those things sense the vibration of the membrane, the tympanic

membrane, and they then transform that vibration into magical electrochemical

energy that is sent to the rest of the brain.

So these sensory nerve endings are all just simply basically

transmuting that external stimulus into something that is an action

potential.

Effectively.

I've learned a lot in that sentence, but it's what

you need to think about.

There's a couple of definitions I just want to get

us through as well.

The first is that essentially receptors signals the presence, that

is the actual detects the presence of an object and

signals at a location on the body.

So if we can imagine a stimulus saying, let's just

think the easiest to think of touching your skin, if

you touch it very lightly, you can't feel it.

Or you can because if someone else touches in very

lightly, you would feel that is only when you make

a strong enough indentation of the skin that you can

feel pressure.

And so as we discussed in a couple of lectures

ago, now, those have thresholds.

They need a minimum intensity of a stimulus to generate

an action potential.

And so you can imagine gradually increasing the amount of

pressure that someone applies to your skin.

And at some point, you will notice that.

And that's because as the stimulus intensity increases, so there

are small changes in the resting membrane potential of these

nerve cells.

And at some point in time, that small change is

sufficient to drive the occurrence of an action potential.

As we talked about in the third or fourth lecture.

So there's a threshold, there's a minimum intensity which are

below which you're not sensitive.

INVISION Minimum intensity is several photons.

In a perfectly black environment.

We turn all life off here.

We did what we call dark adaptation that is sat

in the dark for half an hour.

And your world photoreceptors incredibly sensitive.

They can actually detect the presence of a few photons

is really useful if you're running around the savanna late

at night trying to avoid lions.

And so they can be very sensitive, these receptors, but

they still need a minimum level before they can sense

something and signal something that would be, say, the threshold.

The next point is that as you increase the intensity,

the stimulus, at some point in time, you generate a

number of action potentials.

So at this point in time below here, below the

stimulus intensity, this neurone may not be signalling the presence

of the stimulus above that it does signal the presence

of the stimulus and indeed the action potentials that the

neurone produces increase with the intensity of the stimulus.

So the number of action potentials is neurone produces signal

something about the intensity of the stimulus in the outside

world.

This function is often called a sigmoid because it looks

a bit like an S, and you'll be encountering it

several times over the next few weeks.

The second related concept is that all sensory neurones have

receptive fields.

While sometimes we try and work this out, but it's

really, really simple.

The idea is that sensory stimuli, even if it's an

effective stimulus, let's just say it's a finger press on

your arm.

Each individual nerve cell is sensitive to a particular location

on the arm that that finger is put.

And if the finger is put somewhere else and the

rest of the body at nerve cell is not going

to sense it.

So it has a receptive field.

It has an area of the skin within which this

object, your finger, needs to be placed for the pressure

to elicit a response from that nerve so that its

receptive field will go through that in the second.

The same concept can be thought of in terms of

vision.

In that case, nerve cells in a visual pathway may

only respond if a light stimulus is placed in a

certain part of the visual field, say up here and

to the right.

It won't respond if in space over here or down

here or up here is the location in the visual

field or exactly equivalently the location on the retina where

that image is projected, where those receptive fields are.

So that's a receptive field location on your body, whether

on your body sensory surface, whether it is in the

eye, the cochlea, wherever, whereabouts.

So it takes information from.

It only takes you from a limited part of the

visual world, from limb, part of the body, etc..

And you can think of this.

We often think of these receptive fields as having a

non-uniform sensitivity across that, and we'll get into that in

a second.

But the idea here is that basically at the periphery

of the recipe field, the nerve cell is not very

sensitive.

You have to make very strong indentations for the nerve

cell to respond, whereas in the centre of the receptive

field, it's very sensitive in response to slightly weaker indentations.

We can measure that by, for example, placing a stimulus

at different points relative to the receptive field.

And if we do that and measure the number of

spikes that are produced by a neurone, we would see

something like this profile, like a Gaussian, usually a normal

distribution whereby the same stimulus is capable of finding many

spikes when space in the centre of the receptive field,

but only a few in space on the periphery.

Now, the observant of you there will think, well, hang

on a sec.

If if there is any field means that the number

of spikes in your produces for the same stimulus depends

on the location with respect to this centre of the

receptive field.

And the number of spikes produced depends on the intensive

stimulus.

Isn't there a confound there?

Isn't there something?

Couldn't I trade off the intensity of stimulus for the

spatial position and get the same number of action cancels

from this nerve?

So.

And if you were thinking that that's precisely correct, that's

one of the major compounds in sensory pathways, trying to

extract the important things from what is often multiple different

types of causes that could give rise to the same

number of action potentials.

And hopefully we'll be able to get through that a

little bit later in this lecture.

So I said that receptive fields at his low clients,

discreet places in your skin or in your eyes or

wherever with something is responsive to this is most nicely

exhibited in the snout essentially system because the body the

skin provides his surface, which he can explicitly think about

or simply feels.

And there's actually been some recordings from humans here where

you put a small electrode into one of the nerves

in the arm and you can, in five circumstances, pick

up the activity of sensory nerve cells and those nerves.

And you can map out the respective fields of both

sensory nerve cells.

And it turns out that they look a little bit

like this, honey, to form circles on the hand here.

And you see that they have different sizes such that

they're a bit larger.

If you're on the palm of the hand and it's

quite a bit smaller from the fingertip.

And you can think of that here.

For example, having many small receptive fields on the thumb

of the finger and large ones on the palm of

the arm.

Fewer of them.

So it turns out this is, as I said, as

a general principle, since we systems that distribution of receptors

on the body or in the eye is not the

same across the whole body.

Some parts of the body have much higher density of

receptors and some have much lower density.

However, the whole surface is what we call tile or

antisense receptors.

That is, every part of the skin or every part

of the eye has at least one sense for detecting

stuff in there.

So the combination of these two things, the timing and

the sizes, has profound consequences on our sensory abilities.

You can actually do this yourself, or rather you can

do this with a partner.

If you get a paper clip and expose the two

ends, you can do what's called a two point discrimination.

The idea there is that if you move the ends

of the pits further and closer together, you can change

the distance on the other parts of the skin that

you're going to stimulate when you press that paperclip onto

the skin.

Now, if those two points are close enough together and

you put it on, say your arm here, you detect,

you experience the sensation of just having a single pinprick

on your on your arm.

If, on the other hand, you stuck that on the

finger, you will detect or experience two different distinct pinpricks

in your fingertip.

So what's the reason for that?

Why is it that the same stimulus feels like one

thing?

One's on your arm or on your palm for two

things.

One is on your fingertips.

So the reason for that can be is logical from

the structure of these receptive fields on your fingertips or

on your thumb.

The receptive fields are very small, so that seems distance

apart of those two pinpricks will activate two distinct sensory

receptors and evacuating two distinct sensory receptors, you will sense

two distinct objects that are touching your skin.

On the other hand, if those same interests are made

on your arm, you're activating only a single sensory receptor.

And if you activate only a single sensory receptor, you'll

feel only one object on your skin.

So the size of these receptors means that you can

distinguish between two objects or one object being present.

I'm not going to talk much about the subcortical pathways

for presentation.

The key point here in the Spanish sensory system, and

as we'll discover in a second in the visual system,

is that these signals are taken from the skin, in

this case through the spinal cord, taken up through the

thalamus, everything goes through the thalamus of cerebral cortex, and

they project upon what is called the primary somatic sensory

cortex.

It's not a sensory to touch primary because that's the

major source of input from the thalamus.

So the primary cortices are those part of the cerebral

cortex, the input from the thalamic relay cells for each

different modality.

And they form this, they project in this case onto

the present.

So we'll see in a second in the cerebral cortex.

By the way, a lot of what we know about

this is actually from work, from Walter Penfield and his

colleagues in McGill.

Back in the 1940s and fifties, where as they were

preparing patients for surgery for epilepsy, actually made in different

parts of the cerebral cortex.

And that's what the patients felt.

These experiments are no longer very often conducted, but they

were incredibly illuminating at the time.

They were able to map out in humans the structure

of this amount of sensory cortex, for example, by asking

people what they felt when you stimulated different parts of

the cortex.

So this is the central stool because these are going

to be the potential supertankers get some fees.

This is a central focus as part of the major

Suvi in the brain and before pre and post after

the simple superset is post mains behind towards the back

of the brain premiums in front was a frontal brain

and if you looked along this potential suitcase, you find

a structure which is incredibly beautiful.

What happens there is that if you stimulate different parts

of this personal sulcus, some person will report that they

feel sensations at different parts of their body.

So, for example, if you down on the lateral side

and you stimulate it, you might find that some reports

that they felt a sensation on their face.

Whereas if you're up on the top medial side, that

report, instead of sensation on that front or on the

foot or leg.

So depending on whereabouts alone, this person was suicidal whereabouts

and cerebral sucres are you are encoding.

You are representing activity on different parts of your skin.

I think this gives rise to the concept, which is

the homunculus in terms of the touch among us means

little man.

And if you look at the representation of the body

on this first episode, the touch representation, you find that

this continuous representation of different parts of the body and

different parts of the body that are closer together are

generally speaking, represented closer together on the cortex.

So if example, the foot is represented a similar location

to the leg or the trunk, whereas if face the

lips and the nose are represented close to each other

and in between these of that hand in the arm.

So you get this map of the body that's formed

on this matter sensory cortex.

It's the different types of touch that you can get

in different parts of your body as represented on the

similar sensory cortex.

And you find that actually some parts of the body

seem to be overrepresented.

For example, the face is a large part of the

sensory cortex, whereas the foot is quite a small part,

even though its size is relatively speaking, even not in

many cases in the face or the trunk, for example,

occupies almost a minuscule part of the cerebral cortex.

And the reason for that is fairly obvious if you

think of just one simple principle.

Every sensory receptor has about the same amount of cerebral

cortex devoted to it.

It follows then that we have more sensor receptors, for

example, on your finger, on your thumb, on your face,

you'll have more cerebral cortex devoted to that part of

the body.

And when you have less sensory receptors, it shrunk your

arm, your leg, you have less part of the body

devoted to that is what we would call cortical magnification.

The idea that the cerebral cortex is like a magnifying

glass onto your body.

That magnifying glass, how much it magnifies depend on the

density of the sensory receptors at that part of your

body.

This is what the locomotive looks like if you're trying

to represent it as an intact human being.

This cortical magnification has some implications.

It means that your ability, as I said, to detect

small changes in the position of objects or the presence

of two objects instead of one depends on the density

of sensory receptors and therefore the amount of cerebral cortex

that's actually devoted to that part of the skin.

This graph here shows you compares is perceptual, the psychophysical

acuity that you have the different parts of your body

and that low numbers in mean high acuity.

That means you're very sensitive to small distances and large

numbers being low acuity.

That means you in much larger distances between objects to

determine tunes at one there and can see that the

areas of the body with the largest or the lowest

acuity at a lower arm of the arm, shoulder, belly,

back breast, thigh, half all those areas where you know

yourself that when you touch those things, you're very less

tense, very much less sensitive to the structure of the

things that are touching that part of the body.

Whereas for example, the fingers and the upper lip, the

cheek, the nose have much higher acuity and much more

sensitive to different the structure of the things that are

touching the body.

So this density of receptors determines the amount of cerebral

cortex that is actually devoted to that part of the

sensory apparatus.

And that amount of cortex is devoted to that.

Sensory in turn dictates how sensitive or how how much

acuity you have, the sensory stimuli that impinge in that

part of the body.

I just briefly wanted to show you the structure, the

visual pathways, very similar.

I'm not going to go through all these lines.

I just want to illustrate to you from a paper

that we produced many years ago now, that in the

eye there are retinal nerve cells which include photoreceptors that

signal they're actually communicated by a little network of cells

in the retina, which you learn about later stages via

ganglion cells, whose axons make up the optic nerve.

Those ganglion cells in turn go to this little structure

in the thalamus the lateral clinic is, and from there

their signals projected the primary visual cortex or V1.

So very similar in structure to this, not a sensory

pathway, except that one went through the spinal cord to

get the thalamus and then the cortex or this one

from the eye or straight through the optic nerve, through

the thalamus and the visual cortex.

There are different pathways from the eye to the thalamus,

and that's to visual cortex.

We often call these the P of the M or

the PARTICELLE, and the Minnesota pathways is quite a bit

in the reading that I suggested you do that discusses

how the signals of these nerve cells differ.

I just want to introduce you to the idea that

they actually have different structure and different types of signals.

Now, I said on the cement essentially surface, you can

tell quite easily that the finger, for example, is what

you used to touch on things, and you can feel

the fine gradations in the texture, for example, the surface.

Similarly, if you look in the eye, if you take

a photo through the eye and this is what a

photo through the eye looks like, you see an object.

This is the bit where the optic nerve starts with

the axons of the ganglion cells come out of the

eye and go into the optic nerve.

It's also the place where the blood vessels come into

the eye from the optic nerve.

This is the picture of the eye through a fan

scope and in the middle of the eye.

See, the structure is called the phobia.

It's an incredible structure.

And this structure, you've got no blood vessels.

This is Photoshopped as a smaller than anywhere else in

the body.

And then the other apparatus in the retina has been

pushed away.

So the photos have to have direct access to the

light that comes through the lens and hits the retina.

And in this location, in this part of the eye,

the small part of the eye is about three or

four millimetres in size.

You have this incredibly dense population of nerve cells called

cone photoreceptors, and that's represented by this paper down here.

You can see the density of these cones peaks in

that area.

And that means that there's because there's so many different,

so many more photoreceptors in this particular part of the

eye that you can distinguish between an object that's slightly

displaced.

So when I wanted to see the structure of the

visual world, what I need to do is I need

to move my eyes around so that part of the

world that I'm interested in falls on the phobia.

Because then I can distinguish the difference between what might

be, for example, a happy face, a sad face, a

bald face.

So if I want to see that fine special detail,

someone at the back of the room who's sitting there

looking at me, that is about one quarter of my

thumb.

And is it slightly less than one degree of ice?

If I want to be able to distinguish the difference

between someone's eyes or their face to recognise their face,

I have to bring my phobia onto that object.

I have to move my eyes so that that part

of the visual falls onto my phobia where I have

this really dense array of photoreceptors.

I won't talk about them at all.

But the gods, the ones which are important for night

vision actually more dense just outside of the phobia.

And so it turns out if you're ever out there

in the dark night looking at the stars, if you

want to see a star, you don't look directly at

it, you slightly side of it.

And that's because of what photoreceptors are actually absent from

the photograph from the centre of your gaze.

And instead you need to bring that light onto the

side of your phobia with regard to actually most dense.

It turns out that this structure, this density photo receptors

in the in the phobia, which is so much greater

in the phobia than elsewhere in the eye, is paralleled

by changes in the structure of the subsequent nerve cells

in the eye.

And the consequence is that we can't see fine spatial

detail in the periphery of our vision.

This again is the idea of cortical magnification.

We can see fine spatial detail when we're looking directly

at something, but not when away from that centre of

our guys.

Our cortex has magnified that small part of the visual

field, which is occupied by the phobia.

I think that's about the size of your thumb.

That part of the visual field where all these thousands

of kind photoreceptors are sitting waiting for light to come.

The primary visual cortex is actually totally magnifying that part

of the visual field.

Some estimates would put it to be something like 20

or 30% of primary visual cortex is devoted to this

tiny little part of the visual field, and the rest

of the visual field is consequently represented by many fewer

ganglion cells, cells in the cortex.

So therefore we're much less capable of seeing the finer

spatial detail away from the centre of gaze because the

cortical magnification is so pronounced for us.

For real.

You can see this yourself.

If you have a look at this slight demonstration here.

If you look in the centre of this thing here

on the projector, you should be able to read.

If you look where the arrow is, you should be

able to define or distinguish what each of the different

letters is.

Does everyone agree with that?

Approximately.

If you look at the centre, you can still see

this on the left, the K on the right and

left from bottom on the top.

It's like going to.

I then if, on the other hand, you look at

the air over here, you should not be able to

define many of the letters.

They are present there.

So you may be able to see that the ace

in the S is there and you could see the

K in the R, But many of the things like

T.P. y are actually indecipherable to you.

And the reason for that is that what I've done

in making this diagram here is scale the size of

the letters so that they occupy approximately the same amount

of cerebral cortex when you're looking at the centre of

the diagram.

And because they'll find the same amount of cerebral cortex,

you're equally able to see all those letters.

But when we look at the eye and this is

no longer matched, sometimes that is a much smaller and

occupying enough of the cerebral cortex.

The magnification is wrong.

So this viewpoint.

So you are less able to be able to detect

what these different letters are.

Hmm.

I said that there were parallel pathways that take the

signals from the eye to the visual cortex, and these

are quite pronounced if you look in the thalamus.

You can see in this little structure overlap, which is

a beautiful structure.

I spent most of my life studying it.

So I think it's beautiful.

It's called that unique because it looks a bit like

a me.

If you're younger than me, actually bend your knee, then

you would look a little bit like me.

And in this little genic nucleus, which you can see

by the naked eye is these different layers or parts

of magnet.

So it layers smaller and larger cell bodies.

And it is these layers.

Now we know that these cells would communicate different things

to the cerebral cortex.

These different layers get different inputs from the eye, in

particular these ganglion cells, the cells that form apple to

the eye, to the thalamus.

The particular ones are much smaller.

They're going to pass through the magnet.

So everyone's a much larger.

They're going to be sterilised.

And then these signals of these thalamic neurones then go

to primary visual cortex.

Now, turns out there's very few ways to test this.

But the only in fact, the only way to really

test is supply small lesions, as we discussed last week.

You can place small lesions in the brain, in animals,

in a small controlled lesions, and you can destroy some

of the nerve cells in the palm.

So all the magnet, some of the layers.

And if you try an animal farm to report things

about the outside world and therefore ask whether or not

these nerve cells that come from the right things in

the retina and send them to the cerebral cortex with

these nerve cells saying different things, the cerebral cortex.

And it turns out they do.

So, for example, in this set of beautiful work is

Bill Murray and to conduct in the late 1980s and

90.

Summarised in this review that I cited here, the idea

is that there's a lesion this place in the palm

of the layers of the macaque monkey.

Or in the magnesium layers of the of a different

macaque monkey with monkey before the length made.

This monkey is trained to report simple things about the

outside world.

What the colour they trying to simply report.

What is something they will not.

In a particular location.

Hmm.

If there was something that I went to make one

important.

If there wasn't a to make another report.

These graphs here, the solid lines show the capacity of

the animal to detect something which varies either in the

kind of striping this of the patterns that are present

or in the flicker.

That is the kind of amount of times per second

something flickers, a light flickers.

Africa is defined by the tempo frequency.

That is how many times a second something, because the

spatial frequency here is just how many of these fine

bars you have in one degree of visual angle, the

solid limestone, what the monkey does in the normal case

without a lesion and the different points.

So the monkeys performance when you've after you've made a

lesion, it turns out that if you make a lesion

at the end pathway, you have almost no impact on

the monkey's capacity to detect the spatial form of an

object.

Whereas if you make a leap into the pathway, this

is almost most.

On the other hand, if you make a lesion at

the pathway you detect, you abolish the animal's capacities, the

black lines here to detect very rapidly flickering things.

Whereas if you abolish the pathway, we preserve the capacity

to detect those rapidly flickering things.

And finally, if you lesion the pathway, you kind of

the monkey cannot see coloured objects, whereas if you lesion

the pathway, the monkey can.

So these things, these different perceptual abilities in the presence

and absence of different pathways suggest that the different signals

that come from the eye to the cortex carry qualitatively

different and qualitatively different signals about the outside world that

carried along the parallel pathways to the cerebral cortex where

they are.

Then we combine.

So when we get to Cortex, we've got all these

parallel pathways doing stuff in the sensory periphery, whether in

visual cortex and other sensory cortex, wherever it is.

We have these parallel pathways from a sensory final thalamus,

bringing all this information up to the cortex.

And somehow the cortex has to rearrange these interesting judgements

about the outside world.

For many years.

Before that, the brain was basically you what you're born

with.

You had that there was no plasticity in the brain.

It took several decades of experiments to actually reject that

hypothesis.

I want to show you a couple of those experiments

in the substance lines on this line.

These are some of the very early experiments that were

able to reject the hypothesis that the brain, the cerebral

cortex in particular, was indifferent to experience.

It was the same when you were born as when

you were dying.

These two experiments relied again on work on monkeys because

these monkeys were easy enough to be trained to report

what had happened to to the outside world.

In the first experiment here, I want to say is

the monkey was trained to hold the fingers against a

little rotating disc, and I had to make judgements about

that rotating this, I think the direction of motion, that

rotating disk using the fingers only.

The question was, would the experience, the long term experience

of making this judgement with your fingers change the representation

of the fingers in the cerebral cortex?

And the way that the researchers went about trying to

address that question is that they made electrophysiological recordings from

this ninth century cortex of these monkeys before, during and

after training to do this simple task.

And what they find is summarised in this slide here,

which is a little bit complicated, but the end result

is very straightforward.

So this is recording from this mid-century area of an

al monkey, people that monkey, and they're recording from the

region that is most important in representing the hand associated

before it's a monkey.

Listen, the sensory cortex that's there now monkeys as it

is in humans.

What they did was they made recordings from these part

of the cortex before experience, and they found a particular

representation of the digits, the final four digits, five, which

of the hand in this part of the cortex.

And that's described over here.

So is the fifth, fourth, second or third digit.

And the normal here.

In the normal case, these different digits have approximately equal

parts of the cortex devoted to them.

However, following this experience, following this training, you find a

substantial overrepresentation of the second and the third digit, and

those are the two digit, the two fingers that the

animal is using to make this judgement.

So prolonged exposure to these kind of tasks has changed

how the cerebral cortex is organised.

It has increased the amount of cerebral cortex that's important,

that's used for extending information from those two digits.

The animals used to do the task.

So the brain is plastic.

The organisation, the cerebral cortex is plastic.

It can adapt to the structure of experience and tasks

that we would need to accomplish.

Although a substantial amount of work as shown this many

different systems since, and this was the original work to

show that this plasticity was there.

I really encourage you to read it with a beautiful

set of experiments.

The converse is it can also be studied.

That is what happens when you lose a digit or

lose some part of your sensory periphery.

In this case, if the animal, for example, loses a

third digit.

This case is surgically removed and the seizure.

Again, you can make the recordings before and after that

surgery.

And again, you can measure in this case again from

one case in the parts of the brain that represent

the hand.

And you find, at least in some cases and this

is still controversial, that when the third digit is removed,

a bit of the cerebral cortex that was responsible for

including things that happened on that anger now include things

that happened on the second or the fourth or the

ninth and fingers.

It's like this piece of cerebral cortex wants to do

something, wants to do anything.

And in the absence of any input from the third

digit, it's asking for input from the second and the

fourth digits help the brain represent things that are going

on there.

So the brain is the cerebral cortex is plastic.

It can adapt to changes in the input from the

outside world.

Say this is not uncontroversial.

In some systems, this seems to be less the case,

and some systems seem to be more the case.

It is certainly dependent on what kind of life injury

happened early.

Younger people who suffer injuries have more cortical plasticity.

Older people have less.

One of the things that this leads to is phantom

limb.

And I just want to spend a couple of seconds

showing you a really effective video from one of the

leaders in this field.

Ramachandran.

I find this quite an effective video, so just spend

a couple minutes on.

Twins First patients this Derek Steen.

All right.

One of Ramachandran first patients was Derek Steen.

13 years ago, he was involved in a motorcycle accident,

and I pulled the nerves out of my spinal cord

up in my neck.

They told my parents directly that I would never use

my arm again.

About seven years ago, I was reading through the classifieds

and I saw an ad in there.

Amputees wanted that.

It was a joke like that.

It's just basically connecting the club to the ball.

So I called the number and it was Dr. Ramachandran.

Go relax.

Today, Derek is teaching Ramachandran how to play golf.

But several years ago, Derek made a crucial contribution to

Ramachandran pioneering work in brain science.

Yes, I was amazing.

After my surgery, I sat up in the bed and

still felt the arm there.

Still felt everything there.

And I'm looking down and I'm seeing nothing.

It was pretty bizarre.

The more I thought about it, the more it hurt.

The more it hurt, the more I thought about it.

So it was it was like it was never ending.

I mean, I'd break out in a cold sweat and

turn pale.

Just standing here talking to you because the pain would

hit so bad.

If there is any one thing about our existence that

we take for granted.

It's the fact that we have a body.

Each of us has a body.

And, you know, you give it a name, it has

a bank account and so on and so forth.

But it turns out even your body is something that

you construct in your mind.

And this is what we call your body image.

Now, of course, in my case, it's substantiated by the

fact that I really use a body with bone and

tissue.

But the sense I have, the internal sense I have

of the presence of a body and arms and all

of that is, of course, constructed in my brain and

it's in my mind.

And the most striking evidence for this comes from these

patients who have had an amputation and continue to feel

the presence of the missing.

How?

It was the beginning of an important relationship.

Important for Derek, because not only would he finally understand

his phantom pain, he would also get to the bottom

of a mysterious sensation he felt while shaving.

When I first started shaving after my surgery, I would

feel my absent hand start to hurt and tingle whenever

I shaved this left side of my face.

Meeting Derrick was important for Ramachandran because the explanation he

came up with would rock the world of neuroscience.

Photograph.

That's just my arm.

The first thing Ramachandran did was to invite Derek to

his lab for a simple test that I want to

touch different parts of your body.

And I just want you to tell me what you

feel and where you experience the sensation.

Close your eyes.

I could feel that on my forehead.

Anything anywhere else?

No.

So my nose.

Okay.

My chest.

Your chest.

Okay.

I can feel that on my cheek and I can

feel rubbing on the phantom left hand.

On the phantom left hand in addition to your cheek,

I'm going to run the Q-Tip across your jaw and

see what happens.

I can feel like you did by my cheek and

I can feel a stroking sensation across the phantom hand.

You actually feel that stroking across your phantom hand.

Okay, so that small visual video and you can just

it goes on for a while.

I encourage you to watch it.

That shows the fact that this person has lost their

arm, that some part of their representation of their body

has distorted not just the inside of the brain, but

also perceptually cognitively.

And the likely explanation for this is that one part

of the likely explanation for this is that the representation

of the face is actually quite close to the representation

of the hand.

And as we saw with the monkey who is missing

the third digit, when you lose inputs to certain parts

of the cerebral cortex, that cortex seems to want to

do something anyway, stop to draw input from neighbouring cortical

areas.

So that part of the body which was representing the

arm is now no longer there.

Now he's also drawing input from the face as clearly

more complicated than just simply saying that because this person's

body image is constructed, image is something that is not

simply explained just by the amount of sensory cortex, but

that distortion in the cortical representation is going to contribute

to the fact that this person feels something, even though

there is no longer there is plasticity is important in

helping this, in helping the brain effectively try to reconstruct

or to do what it would like to do, even

in the absence of inputs.

I just want to spend a couple of minutes facing

what will be spending most of the next lecture on

or the next one.

And I'd really like you to do some reading in

the next section, which is this review that I've put

up online from Colby and go back to leaders in

the field reviews to look at old.

Now that is probably the best conceptualisation of the ideas

we'll go through in the next lecture.

We discussed that, that sometimes that is kind of a

frame of reference in which you understand these sensations is

is depends on how you want to think about things.

I just want to explain to you what I mean

by frames of reference for a few slides.

So when we look at the cerebral cortex, we see

this translated into many distinct areas and visual cortex, for

example, as primary visual cortex.

But then there's about three or four, maybe even ten

or 15 different visual areas that sit next to primary

visual cortex, the whole higher order cortical areas or association

cortex.

The same is the case in similar sensory cortex, the

same effects in the auditory cortex.

You have these primary areas, then you have these multiple

other satellite areas.

And the question arises, one that actually puzzled researchers for

many decades now is why do you have so many

cortical areas?

Why don't we just have one area that's responsible for

vision, one area that's responsible for autism?

And the hypothesis that I'd like to explore in the

next lecture is very much like these parallel pathways from

the sensory periphery to cerebral cortex.

These different cortical areas act as parallel representations or parallel

constructions of the outside world.

Each area is doing something, creating a slightly different interpretation

of the outside world.

This then raises the question of how can these different

the things that are arising in these different cortical areas

be brought back together?

How can the different maps of the outside, both the

construction of the outside world, be reconciled?

And the second thing that starts when I ask is

these topographical photographs and statements, these things that are maps

of your body, your maps of your eyes, that's fine

If we want to, you know, represent the precise location

in our body that something happens.

But it's not very useful if we want to move

around the world where I need to know where my

location is with respect to this table.

With respect to the microphone.

With respect to these chairs.

So the question that arises is how these topographic maps

of the sensory body of the body of the eye,

how these transformed into something that could be behaviourally useful,

could actually help us move around the world accomplish tasks.

It was not very useful.

Just simply know that this is a place of my

hand or my arm.

Sorry.

I would like to know where that place is with

respect.

For example, if my arm is moved with respect to

the rest of my body.

So I want to order frame a frame of reference

in which I can understand these different aspects of my

movement throughout the world.

And that's that is the majority past of what we

call the parietal cortex.

And that's what we're going to be spending the next

half, the next lecture on.

And as I said, I'd really like you to read

that coding review because that will help you understand what

it is that the cortex is trying to do, how

it's constructing maps of the outside world that we can

use to move around them.

And so we, we, we investigated that on Friday, and

I look forward to seeing you there.

Thanks.

Yeah.

I was.

Yes.

So it's and particularly in two volumes.

It was.

Particularly a good topic for the show.

But something about.

Are you actually doing.

It where it's kind of.

Surprising for you to feel particular?

I can't quite remember where where we are at with.

I think the general sensation of like, I'm going to

take on you, that's relatively easy enough to understand.

This is the migration and it's kind of been associated

with the reaction.

You know how it is that.

According to what's called a predictive coding framework in the

world.

Where you can predict quite well what the temptation is

that you.

Should get.

Right because you're doing.

Something and you can because you know what you should

be getting ready to.

Predict and therefore surprise what is predictable.

And think there's a framework of understanding brain function, which

Professor SEO has been particularly.

Important from guidance.

Which is that the job of the brain, you basically

find out things that are not.

And so a lot of the architecture, the brains that

Christians are predicting, that includes, you know, perception, suppressing, not

encoding things that, you know.

I mean, for example, I think that.

You can suppress your own sensations, especially during actions.

So that's predictive protein, which is really influential in.

On the questions regarding the object.

Yes.

So yes, in exactly the same way.

It's easier to see the things overlap altogether.

Like, yeah.

That's where you can.

Something.

I think, optimism.

And because here.

And in my.

You.

Yeah.

Yeah.

The overlapping.

For one thing, the idea of hiring people sometimes out.

I'm very.

Part of life, as you can imagine, is just a

single.

You would.

But even in this case.

Consequently.

That often.

That changes.

Central brain function.

Yes.

Like many.

And so even when you figure.

It was final.

Expect to really point and explain everything.

Thank you.

Thank you.

So.

And would the to.

Yeah, I look at that because it's so much easier

to like.

Yeah.

Okay.

Yeah.

You know, like in our.

I mean, the idea is to.

Always have an interactive story, but I don't know the

exact exact.

I like.

I felt.

I don't know.

I mean.

Yes.

Okay.

And I knew it would be like.

Well, this.

So people.

And.

We know.

Know.

I.

You know this.

Yeah.

Yeah.

Oh.

Yeah.

People.

I think.

And.