Blink: A Horizon Guide to the Senses

Touch...sight...smell...

hearing...and taste.

The rich sounds of a symphony orchestra.

The visual splendour of the natural world.

The subtle notes of a fine wine.

Our senses help define what it means to be human.

Ugh!

In the animal kingdom,

we're alone in seeking out pure sensory pleasure.

Lovely day, innit? Beautiful.

But they give us far more than just an appreciation of beauty.

METRONOME TICKS

Every second our senses gather

millions of details about the world around us,

feeding our brains a constant stream of information.

RINGING, EXPLOSION

By interpreting those signals, we can feel the lightest touch,

hear the quietest sound.

They're our only link with the outside world.

Every experience we have is entirely shaped by our senses.

For over 40 years, Horizon and the BBC

have followed science's bid to learn how our senses decipher the world...

Bar. Bar. Bar.

I've seen stimuli thousands and thousands of times

'but the effect still works on me. I can't help it.'

Bar. Bar.

..discovering how they equip us for survival...

The eyes are moving about all the time.

They're darting around three times a second.

..and charting the advance of pioneering technology

that could step in if our senses fail.

BUZZER SOUNDS

-Good boy.

-Well done.

Now we're going back into the archives...

This is BBC Two.

..to reveal how our understanding of the senses has changed...

As you look at me now,

what I really look like is this on the back of your retina.

..so we can begin to answer the disarmingly simple question -

how DO we sense the world around us?

Have you ever wondered how you'd cope

without the steady flow of information coming from your senses?

In 2008, a bold experiment investigated sensory deprivation.

Six volunteers were taken deep inside a nuclear bunker...

Oh, wow. That's bleak.

..and kept in dark rooms for 48 hours with nothing to see or hear.

OK, Mickey, so, going to turn the light off now.

-We'll see you a bit later on.

-OK, thanks. See you later.

-Take care.

The lack of sensory information had a dramatic effect.

Oh, it's getting tough now.

This is harder than I thought. I don't know if I can do this.

HE LAUGHS

HE MUTTERS TO HIMSELF

After 30 hours with no external stimuli,

they began to invent their own.

Oh, God, I'm losing it now. Tim, I'm hallucinating.

By the end of the two-day experiment,

the volunteers had become anxious.

I think I'm hitting a wall now in my mind.

And tests showed that their reasoning skills

and short-term memory were temporarily impaired.

Sensory deprivation had robbed them of their ability to make sense of the world.

-Hello.

-Oooh!

In fact, it can have such a severe effect that it has been used as a form of torture.

God, I never thought a nuclear bunker would look so beautiful.

We're obviously not alone in being so reliant on our senses.

All animals share the ability to sense the world outside

but how they do that and the information that they gather varies wildly between species.

Butterflies have special cells in their feet

that allow them to taste everything that they land on,

while bees are attracted to the ultraviolet patterns of light that they see on flowers,

and birds can navigate by detecting the magnetic field of the Earth.

But all of those senses share a common purpose.

And that's survival.

Over millions of years, the senses of every animal have evolved

to solve life's most vital problems -

how to reproduce and avoid danger.

For many animals, their primary survival weapon

is their powerful sense of smell.

One of the hamster's babies is taken out of the nest

and as soon as she realises

he is missing, she hurries out to bring him home.

On the other hand, if a baby mouse is taken from its home

and you put it near the hamster's nest,

her reaction is quite different.

To her, it's food.

While mother hamster is away,

a tiny mouse is put into the nest along with her own offspring.

The purpose is to dress the baby mouse in hamster clothing, or,

in this case, hamster smell.

If, after only a quarter of an hour in the nest

along with the hamster babies,

the mouse is taken away and stranded like the other was

on the far side of the cage,

what will happen when mother hamster is returned to the scene?

Her reaction is blindly automatic. It looks nothing like her babies.

It is pink and naked. But now it has the right smell.

This time, the baby mouse is safe

because smell signals are so critical to the hamster

they override all other signs that the mouse doesn't belong.

Smell is a key communication tool

and many animals emit chemical smell messages called pheromones

to signal that they're ready to mate.

He's a big heavy animal

and successful mating is no light task for his lady friend.

She'll only stand in the arched posture that will support him

if she's both in a receptive state and receiving the pheromone.

If you remove the boar's scent gland, she won't stand.

It seems to be an automatic response on her part to his chemical message.

On a farm where there are no boars

and they use artificial insemination,

a pig man can broadcast fake messages.

A squirt of factory-made canned boar taint.

Now she's getting both signals -

the weight on her back and the chemical.

She does stand so it means everything is exactly right to inseminate her.

Sexual signals...

..finding your way by following a smell trail...

..alarms, the recognition of friends or enemies...

..all these are functions that smell messages perform.

Smell and taste are closely linked.

Together they act as a warning system

and can help an animal decide what's safe to eat and what isn't.

This footage of wolves eating a dead sheep was filmed in the 1980s

by a team of American scientists

trying to prevent wolf attacks on sheep.

The scientists put a pill

containing a mild poison into a piece of mutton wrapped in sheep hide.

Within half an hour of eating the bait, the wolves started to vomit.

Several days later, a live sheep was put into the pen with the wolves.

You'd expect it to be pulled apart.

But after just one nip, the wolves backed off,

having learnt that the smell and taste of sheep now represented danger.

Ugh.

Humans are less reliant on their sense of smell than most animals.

But we still use it instinctively to recognise potential danger.

By the time it smells off,

rotting meat contains such high levels of bacteria

that eating it could be fatal.

HE GAGS

So we find the smell disgusting

to stop us putting rotting flesh anywhere near our mouths.

Ugh.

Oh. Hideous.

Smell became less important to us once we evolved to stand upright.

The best noses stay close to the ground.

Nothing at all.

-You can't smell them?

-Not at all, no.

So if not smell, which of our senses do we depend upon the most?

BABY CRIES

At birth, many of our senses are still fuzzy.

Our eyes can't focus properly

and our ears just hear a roar of meaningless sounds.

So the sense we rely upon when we enter the world

is the one that offers the most direct link to it.

Touch.

BABY CRIES

At this stage, our capacity to feel

seems far more advanced than our capacity to use our other senses.

It provides the basis for our first understanding of someone else outside ourselves.

Without touch cells in our lips, moreover,

we would be denied the very source of life.

As we grow, touch remains an essential part of our development.

To understand its importance, scientists in the '60s and '70s

examined the effect of depriving other young primates of physical contact.

Individuals who do not have a reasonable background,

in terms of tactile stimulation,

often fail to form meaningful relationships.

The interactions tend to be unco-ordinated

and by and large they tend to be exceedingly aggressive.

They discovered that a lack of touch in the early years

could have a heavy impact on their emotional development.

For us, even as adults,

the briefest touch can have a surprisingly powerful effect.

At this library at Purdue University, Indiana,

a most curious experiment is being conducted.

When a book is checked out, the clerical procedure is boring,

mechanical and quite unremarkable.

The library clerk in this case is instructed to conduct this operation identically with everybody.

Her posture, gaze, vocabulary, voice tone and so on must be held constant.

The next encounter has just one difference.

The clerk touches the other person when handing back the identity card,

and the touch must be as fleeting and insignificant as possible.

-There you go.

-Thanks.

Every reader is approached after the check-out

and asked to fill in a questionnaire about the library in general.

A brief interview then follows.

-Have you finished?

-Yes.

Thank you.

We're particularly interested in how the clerk behaved.

-Did you notice whether the clerk smiled at you?

-Yes, she did.

Did you notice whether she touched you?

I don't think she touched me.

OK.

Well, the librarian didn't smile, but she did touch.

The questionnaire was about the reader's feelings that day

about the library, librarians and other readers.

Typically, these are perceived as unremarkable

unless the person was touched.

Then perceptions become enthusiastic and quite vivid.

Professor Dick Heslin.

That kind of a touch has such potency as to actually change a person's mood

and even change their feeling towards something like a library, where it occurred.

The addition of one fleeting touch transformed this encounter.

-There you go.

-Thanks.

So how might an experience be affected by removing sense?

In a fast ball game like squash,

you'd think the main thing was to keep your eye on the ball.

But a surprising amount of information comes through the ear,

as Suzanne is about to find out.

Suzanne Burgess is ranked 10th in the country for women's squash,

but even she will find it difficult playing under these conditions.

MUSIC PLAYS

The music conflicts with the sounds Suzanne would normally hear in the squash court

so she keeps missing the ball.

It was much more difficult because you couldn't tell how hard you were hitting the ball

and it slowed down your reaction time because you take a lot of signals from your hearing

that you don't really realise until you haven't got them there.

That experiment shows how our senses work together,

even when we're unaware of it, to give us a full picture of the world.

But of all the senses,

the one we've evolved to rely upon the most is vision.

The visual system is incredibly sophisticated.

With the briefest glance, it can extract the finest detail from a scene

and then process that information in a split second.

It's capable of distinguishing between millions of different colour hues,

assessing depth and distance,

of recognising faces and tracking moving objects.

Man is primarily a visual animal.

The eyes alone account for 70% of the total information reaching our brains.

Your brain seems to take visual information much more seriously

than information from your muscles and balancing mechanisms.

Here, the floor isn't moving at all,

only the walls are moved just a little.

That's quite enough to make the toddler fall down.

We use our eyes for more than just the obvious task of navigating the world.

Eye tracking software has shown how well our eyes scout out

potential mates without us even realising.

In this nightclub,

scientists have set up an experiment to see what people's eyes get up to.

These volunteers have agreed to wear a headset that will record every move their eyes make.

An eye tracker gives us the opportunity

to see where we're actually looking

and this is kind of important to record

because eyes are moving all the time. They're darting around three times a second

and we're certainly not aware of this

so we need some piece of equipment that will show us what's happening.

The volunteers think they're waiting to be tested by the psychologists,

but the real test is happening now.

The software will track their eye movements as they wait

next to a table of attractive models who have been planted there.

Afterwards, they're shown the results.

Here we are,

you check this guy's face as you walk by.

Did you realise you were looking at him?

No, I noticed someone over there laughing at me. That's all I did.

But not that particular person.

The volunteers thought they were just chatting with their friends

but the software reveals where their eyes were actually looking.

I'm really embarrassed!

I don't even know there was a guy in the bar with a camel coat on,

and yet I must have been sat looking at him.

Oh, my God! Oh, no!

So what's really happening as our eyes are checking out the world?

How are photons of light collected, then turned into moving pictures?

To understand its secrets,

scientists first probed the eye's physical properties.

How does an eye work?

Well, here's a model of an eye.

Supposing I turn this round,

you'd be able to see what the eye is like inside.

So light comes through this transparent skin,

through the hole in the iris, through the lens,

and right across the eye to the far side there,

which is called the retina -

a whole area of light-sensitive cells.

Studying the retina revealed that it's made up of millions of specialist cells

that respond to light and individual colours.

Keep staring at the screen at the dot in the centre of the flag.

In about 20 seconds, we'll take the pattern away

and the screen will go white.

But you should still see an image.

At this moment, we're not transmitting any image to you

but you should be seeing the Union Jack in its proper colours.

What's happening is that as you stare at the green area,

your green and blue cones get tired out and respond less than normally to the white screen,

but the red cones fire normally giving a red after-image,

and the yellow areas tire out the red and green cones, leaving blue.

As we learned more,

we became increasingly aware that the eye alone

couldn't explain the remarkable abilities of our visual system.

Being able to receive an image through the eye

is only the very beginning of the process.

For a long time, people thought simply that light came into the eye,

you've got the picture at the back of the eye,

and somehow the mind accepts that picture.

Almost like Brighton Rock.

You have a picture at one end, a picture at the other, that's all there is to it.

In fact, this is entirely wrong.

The difference is that you have to read the picture in the eye,

rather like reading the words in a book.

Steve, a player in an amateur basketball team.

Steve's noticed the ball and recognised it as a ball,

and now, like us, he's following the action with his eyes.

At the back of the eye, on the retina,

it's upside down, but that's the least of the problems.

Light levels vary wildly and it's also ambiguous.

There seems to be nothing there to tell you if the ball is enormous and far away

or small and close, or whether it's you, your eye,

or something in the scene which is moving.

When you look around the world, usually it looks nothing like this.

It looks so real.

It's hard to realise that what you see is not a picture at all,

but a description of the world in brain language.

Investigating the role of the brain

and how it processed the eye's signals was the next big challenge.

In the 1980s, the BBC replicated a classic experiment

that turned the world upside down

to see if the brain could still make sense of the images it received.

Susannah Fiennes is trained to look and to see.

What happens if the image in the eye is turned upright?

A pair of spectacles were made for her by an optician.

She was to wear them for a whole week, whenever her eyes were open.

They're obviously going to feel very strange at first,

that's only to be expected.

I can't quite see your face so it must... Oh, perhaps...

If I come down to your level, that might be easier.

How does that feel? OK?

Heavens, it's really peculiar! Now, just let me...

The image entering her eyes has been inverted.

I've just got to see if I can walk around a bit.

I'm being rational about this.

All right? Oh, no!

Erm...where is it?

Because I'm sure I'm holding the cup up. Oh, God.

Now I'm going to try and write my name normally

while I'm looking at it.

I can't think how I do it

because I don't really even know where to begin.

By the end of the experiment, Susannah's brain had adapted to the glasses

and she could once again see the world the right way up.

She'd learnt to re-interpret the world and the alien information

she was getting through her eyes in just one week,

and that shows you how hard the brain works to process visual messages.

Information from your eyes finds its way to the back of the brain

here, to the visual cortex, and it's at this location

that we form an image of the world to give us a picture that makes sense.

Scientists still didn't understand exactly how that image forms.

So they began to investigate the way individual sections of the cortex respond in monkeys.

It used to be thought

that the brain uses a building-block strategy to analyse vision,

that, as information passed through different areas of the brain,

it was analysed in greater and greater complexity,

but the building-block model was challenged by studies of single cells in monkey brains.

In this area, called V5, cells responded only to moving objects.

In this area, V3, cells were mainly responsive to the form and depth of an object.

And V4, the colour-coded cells,

making decisions about colour on the basis of everything in the scene.

So a new theory emerged which suggests that different areas of the brain perform specialised tasks.

The discovery that different parts of the brain had evolved

to respond to specific parts of an image was a major breakthrough.

And scientists were keen to devise more experiments.

They soon found that even when an image is pared down to just one element such as movement,

the brain can still work out what it's seeing.

The points of light mean very little...

until they move.

These are fixed to the joints of people's limbs.

Using rules about relative motion derived from prospective geometry,

you can recognise absolute form in spite of changes in size or shape in the image.

That allows you to find out an enormous amount about things in the world

from minimal information about just how they're moving.

We've learned a lot more about where in the brain signals from the eye were being processed,

but we were still grappling with how it made sense of that information.

New insight would come from an unexpected source.

It was the 1980s and the computer revolution was making itself felt in every branch of science,

and it would inspire a radical new theory of vision.

The theory was developed by David Marr.

He combined computer science

with existing knowledge of the eye and brain.

The principle behind his theory is that vision extracts the essential features from images.

Seeing is rather like weather forecasting.

The weather data are collected in several forms -

temperature, pressure, wind speed and wind direction.

Similarly, the eye responds to light, colour and motion.

Just as there is no real wind, pressure or temperature in the collected weather data,

so there is no colour, light or motion in the brain.

Instead there is a code representing the incoming information.

'Sule Skerry,

'south southeast 3, 11 miles, 981...'

This is a symbol for the weather.

There are symbols which represent objects in the brain.

But in what form are the brain's equivalent of isobars?

How might an object such as a deer be encoded and symbolised?

The problem is considerable

because as a deer turns its shape changes totally,

yet it is still seen as a deer.

Before David Marr began his work,

a popular theory was that the brain analysed and refined the signals

until finally a single deer cell responded.

Whitman Richards.

You're going to run out of cells very quickly.

I mean, think of all the objects, possible objects,

and size and shape and orientations that you could encounter.

Even just take one of them, like the deer.

You can put that in many different orientations and positions and distances,

and each of those, the single cell would have to respond to - deer.

And you're just going to run out of cells.

There are an infinite number, unlimited number of possible combinations.

So David Marr discarded the single cell idea

and instead asked what was the simplest recognisable symbol of an animal.

The stick figure is simple to store in non-picture form in the brain's computer

as the length, angle and junctions of the lines.

It can also be used to represent any angle at which the deer is seen.

The various elements are brought together with colour, texture

and movement to see the animal in all its detail.

This work revolutionised the field of visual theory.

Marr showed that the human brain works rather like a computer,

storing visual information in code form and paring it down

to its most basic elements to be rapidly processed and understood.

In fact, we read information from the eye so fast that we can even predict the future.

Scientists have recently discovered

that baseball hitters do something quite extraordinary.

The ball is moving so fast that the hitter can't simply watch the ball

as it comes towards him and then adjust his swing,

so how does his bat end up in the right position?

Top baseball stars like Gabe Kapler don't even try to watch the ball.

I don't think that you actually have time to see the ball

out of the pitcher's hand and then make conscious decisions.

I think it's much more instinctive

in that you just have to let your body take over

and allow the physical training that you've done over a long period of time to completely take over.

After facing thousands of throws,

Gabe has learned to see into the future.

Scientists have discovered that within a few thousandths of a second

of the ball leaving the pitcher's hand,

Gabe's brain analyses the speed, spin and angle of the ball

and then he predicts where it's likely to end up.

So he's swinging at the ball

before any normal human would know where it's going.

And he does it with astonishing accuracy.

Our brains process information incredibly quickly

but they still have to decide what's important and what isn't.

Right now I'm focusing on this camera,

even though I'm bombarded by sensory inputs.

I have to filter out the sound of voices in the corridor,

the feel of these clothes on my skin,

even the sight of this microphone just hovering here above my head.

And there's a very famous experiment that shows just how much

our brains can miss if they get distracted by too much information.

Watch carefully.

This is an experiment, a battle of the sexes, men against women.

There'll be a difference but I won't say which way round.

It's just a simple observation test.

All you need to do, you'll see there are three guys in yellow here.

They have a basketball and it's your job to count

the number of times they throw the basketball to each other.

To make things slightly harder,

there's also three guys in blue tracksuits.

Ignore them, ignore their basketball, and just concentrate on this one.

So, if we can run the tape?

OK, so that's number one.

OK, if we can stop the tape there for the moment. OK, be honest here.

Anybody notice anything unusual? Be honest.

OK, about four or five of you. Excellent.

The rest of you didn't notice anything strange? OK.

Right, for you guys, enjoy this moment.

The first time I saw this, it completely threw me.

I want you to watch the tape again but this time as you would a normal piece of television,

no counting the basketball. If we can have the tape?

He's going to make his entrance this time.

Here he comes.

LAUGHTER

No way! That didn't happen. That did not happen!

It totally did. It did.

I'm absolutely shocked.

I thought I'd spot a monkey walking across the screen.

If you were fooled, it's because your brain was distracted by counting

and your eyes weren't expecting the gorilla.

But can you be tricked even if you know the truth?

This is simply a face, actually two faces.

This face on the left is an ordinary face.

Plaster of Paris, but it's a face.

This one however is not.

It is actually hollow but look what happens when you move it.

They're fixed together rigidly

in this box.

You'll see them moving in opposite directions.

That's because you assume that this nose is sticking out

when it's really in, so all the motions get reversed.

This is even more dramatic if we make them nod.

If we make them nod, it really looks weird.

If I make it nod a great deal, you'll see it really is hollow.

The face simply disappears inside the box.

Now you'll see it popping out again.

As soon as the features become visible, these outlines,

all your knowledge of faces comes back into your brain.

It has to be a face because it's got these features.

You see it incorrectly, even when you know the answer.

The probability is so low that that's hollow,

you just don't see it as hollow.

The brain has an uncanny ability to make things up,

filling in the gaps to complete patterns and recognise objects

with very few visual cues.

It's a skill that scientists think may have developed

as part of our survival strategy,

allowing us to see predators and prey even when they're well hidden.

In the course of evolution, the ability to discover prey despite camouflage

has certainly played an important role for survival.

So there may be an evolutionary advantage

behind our eyes' tendency to be fooled.

But what's the explanation for our ears being tricked?

Bar. Bar. Bar.

Have a look at this. What do you hear?

Bar. Bar. Bar.

Bar. Bar. Bar.

But look what happens when we change the picture.

SOUNDS LIKE: Far. Far. Far.

Far. Far. Far.

-Far...

-And yet the sound hasn't changed.

In every clip, you are only ever hearing "bar" with a B.

Bar. Bar.

Bar.

It's an illusion known as the McGurk Effect. Take another look.

Bar...

Concentrate first on the right of the screen.

Now to the left of the screen.

..bar...

The illusion occurs because what you are seeing

clashes with what you are hearing.

In the illusion, what we see overrides what we hear

so the mouth movements we see as we look at a face

can actually influence what we believe we're hearing.

If we close our eyes, we actually hear the sound as it is.

If we open our eyes, we actually see how the mouth movements

can influence what we're hearing.

Bar. Bar. Bar.

It's a bizarre effect.

Remember, the only sound you're hearing is "bar" with a B.

SOUNDS LIKE: Far. Far..

Bar. Bar. Bar.

What's remarkable about this illusion

is even knowing how it's done doesn't seem to make a difference.

Far.

The effect works no matter how much you know about the effect.

I've been studying the McGurk Effect for 25 years now

and I've been the face in the stimuli,

I've seen stimuli thousands and thousands of times,

but the effect still works on me. I can't help it.

Bar. Bar. Bar.

Bar. Bar. Bar.

The McGurk experiment shows us that even when our senses are working normally,

we can still make mistakes in interpreting their signals.

But in some people, the senses merge.

Sight and sound intermingle.

Touch and taste run together.

It's called synaesthesia.

The oboe for me makes a rich, creamy-yellow sound.

It's not a disease or a psychological problem...

..more a difference in perception.

Denny Simon has a complicated form of synaesthesia

where music will cause her to see shapes.

I'm listening to Pat Metheny now. It's a really wonderful piece.

It makes me feel really good.

I'm seeing it on a screen in front of my face.

It's about this big and wide.

It's a very horizontal piece on the screen

and it's got points of light and colour.

It's golden, just golden.

When I listen to it, I feel kind of like

there's warm water inside of my body.

It's a very relaxing piece, which is why I like to listen to it when I'm skating.

These bands are seen, not imagined.

In the same way we see the blue sea,

the red towel on the lifeguard station, Denny sees golden bands.

Unlike Denny, most synaesthetes see words and numbers as colours and shapes.

But there are some relatively rare variations.

Running this pub can get very confusing for James Wannerton.

He has an unusual form of a condition

which means that he doesn't just hear words, he also tastes them.

The problems I have are, somebody will come in and then order,

say, a pint of that.

I get the bacon rind taste.

They then pay with a fiver,

from which I get a taste of strawberry jam sandwiches, very, very specific.

I then have to give them their change.

Change invariably tastes of... processed cheese, a cheesy taste.

John Fullwood sees colours

when he hears certain words, even though he's blind.

It's a nice thing to have because it enables you to be able

to distinguish things, one from another.

You can distinguish Saturday from Sunday because they've got different colours.

Researchers were so intrigued by John's case,

they invited him for an MRI scan.

Megan Steven of Oxford University is conducting an experiment

to discover what is happening inside John's brain

when he sees his synaesthetic colours.

First, she studies his brain activity when he listens to words that don't give him colours.

Master.

Like.

Exquisite.

As expected, John's brain scan shows activity

in his sound processing areas when he listens to these ordinary words.

OK, here we go.

Megan Steven then reads John a list of words that do trigger his colours.

February.

April.

When John hears words like Monday or January,

he sees a specific colour and you can see here

the area of his brain that lights up when he sees that colour,

an area of the brain we call V4.

It's a visual area, an area that processes information about colour.

Brain scans reveal that synaesthesia is caused by the creation of special connections

between areas of the brain that are normally quite separate.

But rather than finding it to be a curse,

those who have it often consider it more of a blessing.

When I kiss my husband, I see...purple frosting!

-Purple frosting?

-Purple frosting.

Synaesthesia is the result of faulty wiring in the brain.

Some sensory conditions can be caused by damage to the brain.

Horizon followed the case of John Alderson,

a man whose eyes work perfectly

but who finds it difficult to make sense of what he sees.

Um, I think that I'm not doing too well.

'John, I understand that five years ago you had a stroke'

and you've been having some trouble with your vision since.

Yes, that's true.

They've told me it's called visual agnosia

which I, being a good, well-brought-up Greek scholar know

means I don't know what I'm looking at.

Um, I don't know who's with me

but somebody I hope is not too upset...

They're being terribly approachable. Oh, wait a minute.

Yes, I recognise my wife.

Because I know what shoes she's wearing.

When he's identifying objects,

he only seems to pick up bits at a time.

You can see when he tries to identify things,

he goes around picking out the very noticeable parts of objects.

He knows it's a pig because it's got a curly tail.

So what it looks like from this is that he's only getting fragments of the world.

In a way, he sees the bits

but he's unable to pick up a whole picture of what the world should look like.

So when John looks at what is for us an obvious scene,

he has to try to determine what it is from individual details.

-What's that building?

-The tall one?

-Yes.

-As it's got very large windows

and very extended corner pieces

and something right at the very top that looks like a flagpole or something equivalent...

I'm not awfully sure but it's not a domestic building.

I think it's more likely to be some sort of storehouse or storage building.

I don't think the owners of the building would be very flattered by your description.

Although John's failure to recognise the Houses of Parliament is bizarre,

it gives us an insight into how we must structure detail

in order to recognise this object as a tower at all.

Through John, we glimpse how, from a collection of lines,

we create order.

Studying agnosia helps scientists understand how the brain interprets visual signals,

but they still don't know enough to help sufferers like John.

-A church, it must be.

-I'm sorry, dear, you're wrong.

It's the Houses of Parliament. That's Big Ben.

Good girl.

Where's the house?

It's often said that people who lose one sense altogether...

HIGH-PITCHED WHINING

..can compensate because their other senses become extra keen.

Yes, I see.

However, in the 1970s, this was cast into doubt.

Part of the evidence for this belief stems from a series of experiments

in which children, sighted and blind,

were required for example to identify where a sound came from.

The sighted child is blindfolded and told to swivel her chair

so as to face directly the person calling her name.

Justine.

Justine.

Although blindfolded, she does well, as do other sighted children.

Blind children are also blindfolded to exclude any residual vision.

Ian.

Ian.

This blind boy, for example,

consistently faces about 30 degrees away from the source of the sound.

And the implication is that sight has been acting

as a kind of integrating sense,

allowing one to develop the non-visual senses more rapidly and more finely.

Far from compensating, by refining their other senses,

visually handicapped children may be hampered in doing so.

All along the line, their development is a struggle.

Ian.

To lose a sense is to lose a vital link with the world

but can science step in where biology fails?

Over the years, the BBC has documented some of the groundbreaking ways

that technology has enhanced, altered and even replaced the human senses.

That's fine.

In the 1970s and '80s, millions of pounds

were poured into research aimed at helping to restore sight.

Some scientists turned to the animal kingdom for inspiration.

This 1977 invention tried to create a human version

of the echo locators used by bats.

It looks like a pair of glasses but it's not.

The important bit is here.

This central spot has a tiny transmitter.

It sends out a very high-pitched sound.

When the sound hits an object, it bounces back, an echo.

These two little receivers hear that echo.

Then the sound is carried to your ears through these little earpieces.

THROBBING HUM

HIGH-PITCHED THROBBING

There's a lamppost.

Notice how the pitch goes from high to low as I get nearer.

THROBBING FADES AND LOWERS IN PITCH

REGULAR THROBBING

THROBBING BECOMES MORE HIGH-PITCHED

That, I think, is a car. It's parked by the side of the road.

And there's a bush. Yes.

REGULAR THROBBING

WHINEY THROBBING

This is a sound I rather like.

It reminds me very much of church bells

and it does in fact mean a basket-weave fence.

THROBBING FADES

So, with their other senses, the help of good ideas

and modern technology, people with no sense of sight

can get about almost as easily as the rest of us.

Despite this optimism,

the echo-locating glasses never quite took off.

But the idea of using one sense to replace another did.

Thank you very much. That's quite enough of that.

One 1970s device substituted touch for vision.

It allowed blind people to see things

that were beyond reach, for the first time.

The visual display represents approximately the image

Bill is receiving via his camera.

By controlling the camera,

he's picking up a two-inch model 15 feet away.

The image in the camera is transferred to the back of the chair

and then split up electronically.

Each part of the image is then represented

by a tiny, vibrating, blunted needle.

It's these vibrations on the skin of his back

which Bill has learned to interpret visually.

The information's presented as touch, but Bill is actually perceiving it

as if it were vision, although he's totally blind.

OK. It's a telephone.

And the receiver is to the right.

The tactile chair offered its users a whole new outlook on the world.

All of a sudden I had acquired a new sense

because before I had always been restricted to feeling things.

Now I could perceive objects that were far away.

The most interesting thing about first seeing a candle

was that I realised the flame had a definite shape

and that it would change shape if someone moved it or blew on it.

Of course I had never been able to experience a flame directly

because when you touch a flame you only experience heat.

So I think that was the most interesting piece of it.

Very surprising, actually.

It never occurred to me that a flame would have a shape.

This innovative device opened the door to a new wave of research

and led to the development of ever smaller and more portable versions.

-I'm guessing this is going on my tongue.

-That's right.

What do you think the object is?

A spoon.

But it still couldn't come close to replacing

the complexity of the human eye.

Since the 1970s, scientists have tried a range of radical ideas

in an attempt to restore sight.

OK, that's fine. Put your head up.

From plugging directly into the brain's visual cortex...

Let's go. First word?

Eye?

..to retinal implants.

But advances take time.

There are no distinct edges.

It's just a subtle difference in the illumination.

Scientists working on another sense would have more success.

Now, you should feel that on the finger.

SHE SPEAKS INDISTINCTLY

In the 1970s, a range of devices were developed

to help deaf people communicate.

PHONE RINGS

Mrs Grant sees the phone ring and switches on.

She reads the message as it's written on her television.

Without even having to speak, Mrs Grant can now reply.

In the days before instant messaging,

this simple idea had an immediate impact.

But the greatest breakthrough in restoring hearing

came from the world of medical innovation.

Deep breath!

Four-year-old Nicholas was one of the first children in the country

to receive a cochlear implant,

a groundbreaking device placed directly into the ear.

It's this inner ear, or cochlea, that was damaged before Nicholas was born.

A cochlear implant will take its place,

processing sound electronically

and sending it directly to the hearing nerve

through 22 minute electrodes.

The electrodes are placed deep inside Nicholas's ear.

We're going to make a very small hole in the inner ear

and insert all those 22 electrodes, or wires,

right in the spiral of the inner ear itself.

Then his surgeon can test whether the implant is working.

In the middle ear we have a little muscle which is used to dampen sound.

If our implant system is working,

when we stimulate it to an uncomfortable level,

that muscle should contract rather dramatically. Let's go.

That's a massive response. You saw that?

For Nicholas, the crucial day has arrived.

During surgery at Nottingham, only half of the device was implanted,

the radio receiver.

Now Nicholas is due to be fitted with the rest of the system,

which he'll wear on the outside.

It's time for his implant to be switched on.

But will he hear and if he does, how will he react?

HARSH BUZZING

-Good boy!

-Well done.

That was a bit of a shock.

Can you hear the noise of the water?

SHE HISSES

Good boy.

Listen, Nicholas.

Good boy! Did you hear that?

Moo!

Good boy.

Cochlear implants are now commonplace

and have revolutionised life for hundreds of thousands of deaf people worldwide.

-Say goodbye. Bye-bye.

-Bye-bye.

-Bye-bye.

-Bye-bye. Bye-bye!

Through our senses, we experience the world.

CHILDREN SHRIEK

And their loss is keenly felt.

I've never yet discovered anybody that's been able

to convey in words the aesthetic beauty of vision.

To understand how and why our senses operate,

scientists have explored the inner workings of the human body...

These cells react to light.

..and looked deep inside the brain.

They've put our sensory abilities through their paces...

I thought I'd spot a monkey walking across the screen!

..and developed pioneering new techniques to help when they fail.

Our senses have evolved over millions of years to give us the best chance of survival

and that's something they do remarkably well.

But they are so much more than that.

Those complex systems come together to create everything that we know about the world.

Everything we see, everything we feel,

everything we love is a product of our senses.

# And I've got one, two, three, four, five

# Senses working overtime

# Trying to take this all in

# I've got one, two, three, four, five

# Senses working overtime... #

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