0:05

Why does a cold pool feel

0:08

warmer this second time you put your

0:10

toes in. Why does a person

0:12

who's trying to break into a safe run

0:15

his fingers over sandpaper?

0:17

Would it be great or not so great

0:20

if you couldn't feel any physical

0:23

pain? Why does stubbing your

0:25

toe have different sensations

0:28

through time? Why do Mediterranean

0:31

cultures touch each other more while

0:33

they're talking than Scandinavian

0:35

cultures? And what does this have to do with

0:37

cuddle puddles, or why NBA

0:40

players bump chests or why puppies

0:42

sleep in dogpiles. Welcome

0:48

to Inner Cosmos with me David Eagleman.

0:50

I'm a neuroscientist and author at

0:52

Stanford and in these episodes we

0:55

sail deeply into our three pound

0:57

universe to understand why

0:59

and how our lives look the way

1:01

they do. Today's

1:11

episode is a love story

1:14

about our sense of touch,

1:17

what it is, how it works, and

1:19

why it plays such a critical

1:21

role in our lives. So let

1:23

me start with an acquaintance of

1:25

mine, who many years ago worked

1:28

as a midnight phone operator

1:30

at AT and T. He would get

1:33

calls from people in the middle

1:35

of the night and he'd say, Hello, this is

1:37

AT and t how can I help you? And they

1:39

would say hi, and he'd say

1:41

what can I help you with? And they'd say, I

1:44

just need someone to talk with. So

1:46

there's a lot of loneliness in the world

1:49

now. I tend to be suspicious when people talk

1:51

about problems as modern

1:53

phenomenon, given that most things have a deep

1:56

history, but an increase in

1:58

loneliness might actually be something

2:01

new.

2:01

If you look back.

2:02

Even just a century ago, people

2:05

lived in boarding houses

2:07

with lots of other people, and there was lots

2:09

of intergenerational living where

2:11

you had at least three generations

2:13

in the same household. Now that was

2:16

probably crazy making, but whatever

2:18

the case, this situation has

2:20

changed a lot.

2:21

As the middle class grew.

2:23

The aspiration was for nuclear

2:26

families to each get their own

2:28

house with their own dishwasher and their own

2:31

washing machine and their own television set. And

2:33

in most neighborhoods, yards

2:35

became more and more private,

2:37

and at some point people stopped hanging

2:40

out in their front yards and playing on the streets,

2:42

but instead hung out in their backyards

2:44

behind their fences. And

2:47

by the year two thousand we got books

2:49

like Bowling Alone, which

2:51

was written by the political scientist Robert

2:54

Putnam, and this book was about

2:56

the decline of civic engagement

2:58

since the nineteen sixty people

3:01

used to be in bowling clubs

3:03

and belong to their church or synagogue or

3:05

mosque and see a community of

3:07

people every day and every weekend.

3:10

And they were in rotary clubs

3:12

and Shriner's clubs and Kawana's

3:15

clubs, and people who were recent

3:18

immigrants often had clubs from

3:20

the towns they originally came from. And these

3:22

clubs lasted for decades

3:24

until the wave of immigrants finally

3:27

died out and their children born in

3:29

America didn't really need to keep this

3:31

up anymore. So because of lots

3:33

of factors like suburbanization

3:36

and the rise of television,

3:38

and the decline of unions and

3:40

the disappearance of clubs, we

3:43

ended up with this decline of

3:45

social capital.

3:46

Now.

3:47

Of note here is that Putnam's

3:49

book was published before the

3:51

invention of the smartphone,

3:54

and while the Internet existed and video

3:56

games existed, they were nothing like

3:58

what they quickly became in the first

4:00

decade after that book was published.

4:03

And so society has changed much

4:06

further in the past quarter century.

4:08

And the question is, are we now

4:11

not just bowling alone but not

4:13

even bowling at all. Has society

4:16

actually gotten lonelier? Now,

4:19

I'm going to address this from both sides,

4:22

arguing that there's more opportunity

4:24

for relationship and less.

4:27

Now on the side of more opportunity,

4:30

one of the consequences that has always

4:32

amazed me about the Internet is

4:34

that now every person has

4:36

the chance to find their tribe.

4:39

If you are a stamp collector, or

4:41

a mathematical poet, or a

4:44

corporate mystic or a punk

4:46

rocker historian or whatever, you can

4:48

find your people. There

4:51

will be no end to the websites

4:53

and subreddits and substacks and whatever

4:55

else you need to make you feel like

4:58

you have your group where if

5:00

you grew up pre Internet, you would

5:02

never know that so many others

5:04

like you were out there. And related

5:07

to this, a lot of parents lament

5:09

that kids are spending all their time on video

5:11

games. But most video games have built in

5:14

social mechanisms so that you can

5:16

chat with other people. Now, chatting

5:18

is not the same as spending time

5:20

in real life, but with a million

5:23

games to choose from, you end up finding

5:25

people who share your interest

5:27

and there's a lower barrier to

5:29

entry from making acquaintances, and

5:31

sometimes these become friends. So

5:34

with multiplayer games, we have gone

5:36

from bowling alone to

5:38

bowling virtually virtually but

5:41

not alone with other people's

5:43

avatars who can chat with us and become

5:46

friends of a sort. And there's another

5:48

important way in which I think technology

5:50

will have a shot at reducing

5:53

loneliness, and this is with AI.

5:55

Already we've seen an explosion

5:57

of companies selling AI

6:00

relationships, and if you're interested in the

6:02

complexities of this, please check

6:04

out episode thirty nine, which is

6:06

on the future of AI relationships.

6:08

But beyond AI girlfriends

6:10

or boyfriends, I suspect one of the places

6:13

will see the most immediate impact

6:15

with AI is in areas like

6:18

mental health therapy. The fact that you

6:20

can have a therapist twenty

6:22

four to seven, someone you can talk to in

6:24

the middle of the night, someone who remembers

6:27

everything you've said, someone who is

6:29

not at all interested in talking about themselves

6:31

but just about listening to you and

6:33

giving you good advice and being

6:35

there and truly has your best

6:38

interests in mind. And so the question

6:40

is does new technology like the

6:42

Internet and AI scratch

6:44

the itch of loneliness. Because

6:47

I'm generally a cyber optimist, I feel

6:49

like there are many ways that can be correct.

6:52

However, while this hits

6:54

on many cylinders, what it misses is

6:57

a big one. Touch

6:59

And we really care about touch. So that's

7:01

what I'm going to talk about in today's episode, what

7:04

touch is all about? Because

7:06

our world has become a lot of screens

7:08

and less about holding hands

7:11

or hugging with strangers or friends. And

7:13

the more I surveyed friends of mine

7:15

on this point, I was surprised to realize

7:18

how much touch is lacking

7:20

in a lot of people's lives. One of

7:22

my friends goes to Burning Man every year, and

7:25

the thing he loves the most is that people

7:27

hug people. Total strangers will say,

7:29

give me a hug, and it's innocent. People can

7:31

do this without sexuality, and

7:33

people clearly appreciate it. And

7:36

there get togethers all over the world

7:38

called cuddle puddles or cuddle

7:40

parties where dozens of people lie

7:42

on mats and cuddle one another

7:45

and it's not sexual. Instead, it's

7:47

fulfilling the very primitive

7:49

desire we all have for touch.

7:52

And by the way, the same desire

7:54

is seen to cross the animal kingdom, where

7:56

you constantly see non

7:58

sexual physical contact among

8:01

social animals. The way you'll see

8:03

puppies collect into a big

8:05

dog pile when they're sleeping, And

8:08

so from the point of view of the brain, I've

8:10

become really interested in what

8:12

we might be losing here. We're

8:15

in an era with infinite

8:17

hyperlinks between people, but

8:20

fewer physical links.

8:23

So for today's episode, I want to take

8:25

us on a journey about touch,

8:28

what it's all about, and how it colors

8:31

our lives. When we look at

8:33

touch across the animal kingdom, we see

8:35

that it plays a critical role in everything

8:37

from child rearing to finding food

8:40

to getting mates. But not everyone

8:43

thinks highly of it. Plato thought

8:45

that vision was hot stuff and

8:48

considered touch to be the most carnal,

8:50

the least noble, of the

8:52

senses. But his student

8:55

Aristotle developed a different

8:57

view. He felt, we learn the

8:59

world by going out and touching

9:01

it. That's how we figure things out. Not

9:04

everything can be achieved purely by

9:06

looking around and reasoning. So

9:08

Aristotle said, quote, while in

9:11

respect of all other senses

9:13

we fall below many species of

9:15

animals, in respect of touch,

9:18

we far excel all

9:20

other species in exactness

9:23

of discrimination. That is why

9:25

man is the most intelligent

9:27

of all animals. So

9:30

why did Aristotle privilege touch.

9:33

What is special about touch and how does

9:35

it all work? So let's start at

9:37

the beginning. The largest

9:39

organ in your body is actually

9:42

on your body. It's your skin, and

9:45

it is a miraculous sort of material.

9:48

Unlike your other senses, which are focused

9:50

at your eyes and your ears and so on,

9:53

touch is spread throughout the body,

9:56

and touch is the only sense that puts you

9:58

in direct con tact

10:00

with your subject, instead of assessing

10:02

it from a distance, as you do with

10:04

photons with seeing, or air

10:07

compression waves with hearing. In contrast,

10:09

you have to be right up against

10:11

something. You can't touch something without

10:14

being touched yourself. So

10:16

the really amazing part is how it works.

10:19

You have a whole zoo of touch

10:22

receptors in your skin. These are

10:24

specialized sensors for

10:26

pressure, for itch, for stretch,

10:29

for temperature.

10:30

Some receptors are specialized for pain.

10:33

Some are for pain caused by chemicals

10:36

like an acid, some are for mechanical

10:38

pain like a cut, and some are for thermal

10:41

pain like a burn. And all of

10:43

these things work in collaboration

10:46

to read different sensory

10:48

aspects of the world. So

10:51

when you pass your finger over

10:53

something like the petal of a flower

10:56

or the sleeve of your jacket or

10:58

the back of your dog. Different

11:00

receptor types read the object

11:03

in different ways. And

11:05

what's interesting is that feeling doesn't

11:07

happen in the top layer of your skin, but in the

11:09

layer underneath. And this is why safe

11:12

crackers sand their fingertips

11:14

to expose the sensitive skin in

11:17

the layer underneath, so they can be really sensitive

11:19

to the tiny, tiny shifts

11:22

in the locking mechanism of the safe.

11:24

So collectively, this whole zoo

11:26

of receptors inside your skin allows

11:28

you to tell a lot about what's happening in the outside

11:31

world in terms of texture and

11:33

temperature and pressure and roughness and hardness,

11:36

and your sense of what's out there

11:38

is totally dependent on these sensors.

11:40

If you block their operation

11:43

with a local.

11:44

Anesthetic, then you don't sense

11:46

anything out there at all. So these

11:48

sensors work together to give you exquisite

11:51

sensitivity. This is why

11:54

you can reach into your pocket and you can tell

11:56

a quarter from a nickel without looking.

11:59

Your sensitive fingertips

12:01

detect the tiny ridges

12:04

around the quarter's edge, and

12:06

those are very small, but that's nothing. Your

12:08

fingers can actually detect textures

12:10

that are way finer. You can

12:12

feel a texture seventy

12:15

five nanometers high. That's one

12:17

one thousandth of a human hair. If

12:19

you run your fingers over that, you can

12:21

tell it's there. And you know all those

12:24

ridges and grooves on your fingertips

12:26

they serve as sensation amplifiers.

12:29

They expand the skin's surface

12:32

area when they encounter pressure.

12:34

So evolution has sculpted your

12:36

fingertips into incredibly

12:39

fine instruments of touch. They

12:41

traffic in information we can't

12:44

otherwise perceive with our

12:46

other senses. So

12:58

I'm gonna come back to skin in just a second, but

13:00

first I want to make it clear that your system

13:02

of sensation from the body

13:04

picks up information both from

13:07

the outside but also from the inside,

13:09

and collectively this is grouped into what we

13:11

call the somatosensory

13:14

system, which just means sensing from

13:17

the body. As I said a minute

13:19

ago. Unlike vision or hearing,

13:21

somata sensation doesn't

13:23

happen in a discrete

13:26

sensory organ like eyes or ears.

13:28

Instead, the receptors are widely

13:30

distributed in the skin, and the muscles

13:33

and the bones and the joints, in

13:35

internal organs, in the cardiovascular

13:38

system. So even though we call it one system,

13:40

it's giant, and it's distributed,

13:43

and every second of your life,

13:45

these millions of receptors are

13:48

streaming information to your brain,

13:51

where the information then steers your

13:53

behavior. Now, in today's

13:55

episode, I'm not going to talk too much about the internal

13:58

monitoring, but just for sketching

14:00

out the landscape, I'll tell you two of the main

14:02

things you're doing on the inside. The first

14:05

is constantly determining where your

14:08

limbs exist in space,

14:10

your body position, and your

14:12

limbs movement. This is known

14:14

as proprioception. Now,

14:16

knowing how your body is positioned

14:19

this is something we just take for granted. But

14:22

if you lose this information, as

14:25

sometimes happens, for example, from a viral

14:27

infection, you can no longer

14:29

do things like walk because

14:32

now you need to visually

14:34

look at your feet and your hands

14:37

to have a sense of where they even are. So

14:39

normally, your brain knows where

14:41

your limbs are by constantly

14:44

drinking in this fat stream of

14:46

information. But if that

14:48

goes offline, your only way

14:50

to estimate where your limbs are is

14:52

to look at them visually. And

14:55

if the lights in the room turn off, you

14:57

are going to fall to the floor. You can't

14:59

maintain posture much

15:01

less make any kind of fine movements without

15:04

having visual feedback. So even

15:07

though you may or may not have even been aware

15:09

of this crazy thing called proprioception,

15:12

you cannot function without it. And

15:15

the other internal monitoring that I'll

15:17

mention is called interoception,

15:19

which is this amazing system by

15:21

which you monitor the inside

15:24

of your body, like movements of your

15:26

gut or muscle stretch or changes

15:28

in heart rate or stretch

15:30

receptors in your lungs that modulate

15:33

your breathing rate, or stretch receptors

15:35

in your stomach and guts that tell you when you're

15:37

full. You also have chemical

15:40

receptors and the brain which monitor

15:42

carbon dioxide levels to tell you if you're

15:44

suffocating, and you have these in the circulatory

15:47

system to monitor blood levels

15:49

and trigger thirst. You have receptors

15:51

in the throat that are close cousins

15:54

of the touch receptors in your skin, and

15:56

these are what detect whether there's an object

15:58

in there to trigger gag. And again,

16:01

all the stuff we tend to

16:03

take for granted, but

16:06

every single one of these receptors

16:08

is critical for you to have the

16:10

experience in the world that

16:12

you do. These underpin your

16:15

ability to be a body moving

16:18

around in the world. But for today

16:20

we're gonna talk about the other side

16:23

of somatic sensation, not monitoring

16:25

the inside but touching

16:27

the outside world.

16:29

So for this your.

16:30

Entire body is covered by that one

16:33

giant sensory organ, the skin. Now,

16:36

this large, seamless

16:38

sensory sheet is completely

16:41

packed with receptors that allow you to feel

16:44

when something or someone makes contact

16:46

with you, when a nearby object

16:48

is vibrating, or when something is hot or cold.

16:51

Now, touch seems pretty

16:53

simple. You just push your skin up against

16:56

something. You say, oh, that's there, and it's

16:58

hot or it's cold, or it's or

17:00

it's smooth. But this apparent simplicity

17:03

masks the incredible variety

17:06

and complexity of how your

17:08

brain and body do this. So let's

17:11

surface the details. So first,

17:14

to detect pressure and vibration,

17:17

you have a specific flavor

17:19

of sensory receptors in your skin called

17:22

mechano receptors, and these are

17:24

just built of special structures that

17:27

make them sensitive to being physically

17:29

stretched or bent. Now

17:31

you have different types of mechanic receptors

17:34

that are different structures and locations, and

17:36

that gives you the ability to sense and

17:38

interpret a huge range

17:41

of things.

17:41

In the world.

17:42

I won't go into too much detail, but i'll mention

17:44

you have some receptors which

17:46

respond to fine light touch

17:49

if you're interested. These are called Meisner's

17:51

corpuscles or mercle discs. And

17:53

you have other types of receptors found

17:56

in the deeper layers of the skin that respond

17:58

to stronger and cruder

18:01

pressure if you're interested. These are

18:03

called Piscinian corpuscles or Ruffini's

18:05

endings. Now, some of these receptor

18:07

types respond quickly right when

18:10

something gets touched, but then they stop responding

18:12

if the touch remains there, and

18:15

others of these are slowly

18:17

adapting, which means they keep firing

18:19

if the touch stays there. So the

18:22

rapidly adapting ones are well

18:24

suited to respond to something that's going

18:26

on and off like a vibration, because

18:28

they stop responding right after the initial

18:31

thing, and therefore they're ready to respond

18:33

again a moment later. But others

18:35

stay firing, so you can know when

18:38

you touch your coffee cup that you're still touching

18:40

it. So the thing I want to illustrate here

18:42

is that you have this interplay of different

18:44

receptor types that gives you a very rich,

18:48

multilayered sensation of touch,

18:51

fine and coarse, fast and

18:53

slow. Now it's not just mechanicy

18:55

receptors that you have in your skin, but you've got these other

18:57

things called thermo receptor

19:00

which tell you about temperature, and you

19:02

have separate thermo receptors

19:04

for cool and warm.

19:07

Now, if this wasn't something you already knew,

19:09

it's kind of amazing. Right, You're like this sophisticated

19:13

meat robot with all these sensors

19:15

packed into your skin to detect

19:17

different things in the world. Now,

19:19

what these thermoreceptors do is they

19:21

detect changes in temperature on the

19:23

skin surface relative to body

19:26

temperature. So a cold thermo

19:28

receptor in your finger, it's

19:30

going to signal only a little tiny bit of change.

19:33

When you test the water coming out of the sink. There's

19:35

going to trigger a really strong signal when

19:37

you dunk your hand into a bucket of ice water.

19:40

Now, speaking of cold water, you've

19:42

probably noticed that a cold lake

19:45

feels warmer the.

19:46

Second time you dip your foot in.

19:48

Why, obviously the temperature the water

19:51

hasn't changed, but the relative

19:53

temperature of the water compared to

19:55

your now chilled skin, that

19:58

difference is decreased, and that changes.

20:00

Your perception of the water temperature.

20:03

They have a separate set of receptors

20:06

that carry information about.

20:07

Warmth, but these cool

20:09

and.

20:10

Warm receptors, they track temperature

20:12

changes only to a point, and

20:14

after that there's signaling drops off and

20:16

information about things that are very

20:18

cold or very hot that

20:21

gets taken over by separate

20:23

receptors pain receptors,

20:25

and I'll return to those in a moment, but before

20:27

we get to pain, I just want to mention that these

20:30

cool and warm thermoreceptors

20:33

can get activated by other things.

20:35

Like certain shapes of molecules.

20:36

For example, you have molecules

20:39

that happen to bind to these cold

20:41

receptors, like menthol.

20:44

Menthol is just a shape that happens

20:46

to activate these cool thermoreceptors,

20:49

giving the pleasant illusion

20:51

of coolness. And because we generally

20:54

like that, we dump that molecule

20:56

into our toothpastes and our shaving

20:58

creams to activate these cool

21:00

thermoreceptors. Similarly,

21:03

the active ingredient in chili peppers

21:06

is called capsaicin, and this is

21:08

a molecule of a certain shape that happens

21:10

to bind to warm thermoreceptors,

21:13

and therefore it creates the illusion of

21:16

heat. So capsaicin is used

21:18

in ointments that you rub on your skin,

21:21

like icy hot. Okay, so now

21:23

we've talked about how receptors in your skin detect

21:26

touch and detect temperature, and now

21:28

let's come back to pain. The

21:30

sensation of pain is

21:33

one that most of us would probably rather

21:35

not have, but it's actually a gift

21:38

from nature that's critical for our

21:40

survival. Feeling pain is how

21:42

we detect damage to our cells

21:44

and our tissues.

21:45

Now, our perception of.

21:47

Pain is mediated by another

21:50

type of little receptors called no

21:52

susceptors, and what activates

21:55

these is damage to the tissue.

21:57

And even though pain just seems like pain,

22:00

you actually have very different types

22:02

of no susceptors. First, you have

22:04

no susceptors that are activated

22:06

by physical damage to the tissue,

22:09

like caused by pressure, a

22:11

needlestick, a broken bone. These

22:13

are called mechanical no susceptors.

22:16

Then you have thermal no susceptors,

22:19

which respond to things that are extremely

22:21

hot or extremely cold. Then you have

22:24

chemical no susceptors, which are activated

22:26

by things like spider toxins

22:28

when you get bit, or by certain cooking

22:31

spices, or by poisonous gases

22:33

like the kind used as chemical weapons

22:36

in World War One. Now again,

22:38

you might think that feeling no pain

22:40

would be a blessing, but how would we know whether

22:42

that's true or not. The answer is

22:45

this sometimes happens because of genetic

22:48

mutations, and the inability

22:50

to feel pain is a

22:52

dangerous curse. So take as

22:54

an example a man who I'll call

22:56

Paul. When Paul was a child,

22:59

he would push swing and let it smash

23:01

into his face. He didn't mind the broken

23:04

nose and chipped teeth because he

23:06

has never felt physical

23:08

pain. He has a rare disorder called

23:11

congenital analgesia and inability

23:14

to register painful sensations.

23:17

By the way, congenital just means you're born

23:19

with it, and algesia

23:21

means pain, and analgesia

23:23

means no pain. So congenital

23:26

analgesia just means you

23:29

can't feel any pain and you were born that way.

23:31

So Paul can feel a knife

23:34

cutting his finger, but only the touch is

23:36

registered, not the pain

23:38

of the injury. And because

23:40

his thermoreceptors are intact,

23:42

he can easily tell the difference between warm

23:45

and cool water, but he doesn't feel

23:47

the pain of things that are really

23:49

hot or really cold.

23:51

So feeling no.

23:53

Pain is a curse for someone like Paul,

23:55

because pain is critical to

23:58

avoiding bodily damage. It's

24:00

what tells you about harms and threats.

24:02

When Paul interacts with extreme

24:05

temperatures, he doesn't get any immediate

24:07

feedback telling him to pull

24:10

his hand away or to avoid the situation.

24:13

One of his most frequent injuries

24:15

as a child was burning himself. He

24:17

was interested in listening

24:19

to the sizzling sound that his skin

24:22

made, so his parents were constantly

24:24

forced to take creative and

24:26

desperate measures to keep their son

24:28

safe, like putting socks over both

24:31

his hands and goggles over his eyes.

24:33

Often they put a helmet on him to

24:35

protect him from his regular risk of

24:37

hen injury. They constantly checked

24:40

him for swelling and bruising

24:42

and burns. Because of his lack of

24:44

sensation, this kind of outside

24:46

inspection was the only way that they or

24:48

he could tell whether tissue

24:51

had been damaged. So much of Paul's

24:53

childhood was spent in hospitals,

24:56

whether from jumping off extreme heights or

24:58

banging his head against the wall and his forehead

25:00

swelled up. So Paul is

25:03

lucky to be alive because not everyone

25:06

with congenital analgesia makes

25:08

it through childhood. The moral

25:10

of Paul's story is that pain

25:13

is two faced. It hurts, but

25:16

it also protects, So

25:18

back to normal pain perception.

25:22

Fascinatingly, you have different

25:24

types of fibers that carry these

25:26

signals to the brain, and they carry information

25:28

at different speeds.

25:31

So let's say you stub your toe.

25:32

The next time you do this, try to pay

25:34

attention to the way the pain is

25:36

registered, because thanks

25:39

to some fast fibers

25:41

called a delta fibers, you'll

25:43

register a fast, sharp pain,

25:46

and then you have these other much

25:48

slower fibers called sea fibers,

25:51

and when their signals finally catch up, you'll

25:53

feel the more prolonged but slightly

25:55

less intense pain. So

25:57

if you start paying attention to this, you can start

26:00

unmasking the mechanisms of your body.

26:02

The last thing I just want to note about these

26:04

no suceptors, these pain receptors, is

26:07

that they're distributed throughout

26:09

your skin and your body, and your

26:11

internal organs and in your joints.

26:14

But the one notable exception

26:17

is the brain, which has no

26:19

suception, and that's why

26:22

patients can be kept conscious

26:25

during brain surgery. I've

26:27

done experiments before while a patient

26:30

is undergoing brain surgery, and

26:32

I'm talking with them and having them watch

26:34

things on a screen and asking them questions,

26:37

and the patient can just sit there and talk and provide

26:40

feedback about their perceptions without feeling

26:42

any pain from the surgery

26:45

itself. It's one of nature's

26:47

funny ironies that the

26:50

organ that perceives pain for

26:52

your entire body is the

26:54

one organ that registers

26:56

no pain itself. So

27:11

we've been talking about these receptors

27:13

all over the body, and from the

27:16

body, these signals come screaming

27:18

up the spinal cord to the brain,

27:20

and there these signals come slamming

27:22

into a strip of real estate that we

27:25

call the primary somatosensory

27:27

cortex, which is just underwhere

27:29

you would wear headphones.

27:32

If you were to measure brain.

27:33

Activity along the strip, what you find is that

27:36

every little bit of it corresponds

27:38

to some part of your body. And

27:40

what you find is that not all body

27:43

parts are represented equally.

27:45

Some parts have more real

27:47

estate devoted.

27:48

To them, like the hands and the

27:50

mouth and the lips and the tongue, and

27:52

this corresponds to their importance.

27:55

Not surprisingly, as you know from experience,

27:58

you're able to discern much more subtly

28:01

touch stimuli on these parts of your body

28:03

then, say on your thigh or

28:05

your back, which have smaller

28:08

representations in the cortex.

28:10

Now, it's not surprising that we devote so

28:12

much brain territory to our hands.

28:15

Humans began to walk upright several

28:18

million years ago, and that freed

28:20

up our hands to reach out

28:22

and examine objects,

28:25

and over time that changed the importance

28:27

of hands, making them some of the most important

28:30

body parts that we have for examining

28:33

the world, and so our brain changed

28:35

accordingly. The situation now is

28:37

that each of our fingertips has two thousand

28:40

touch receptors in it, and they're streaming

28:42

continuous, detailed information

28:44

to the brain.

28:46

The philosopher A.

28:47

Manual Kant once said, quote,

28:50

the hand is the visible part

28:52

of the brain. Now, what's cool is

28:54

that in different animals, the

28:56

sizes of different body parts

28:59

represented in the sensory cortex.

29:01

This differs across species because

29:03

different animals rely on different

29:06

sensations to survive

29:08

in their niche.

29:09

So when you look at the rat.

29:11

Sensory cortex and the enormous

29:13

part of the real estate is devoted to the whiskers,

29:15

because that's a huge part of what lets

29:18

the rat navigate dark spaces

29:20

to find food and avoid obstacles.

29:23

Or in an elephant, there's an enormous

29:25

amount of real estate devoted to its trunk,

29:28

or in the star nosed mole, it

29:30

has a lot of territory devoted to its snout,

29:32

which has these fingers on it that

29:35

feel around to construct

29:37

its three D model of its tunnels.

29:40

Okay, so back to humans. We have this

29:42

strip of cortex that receives

29:44

the information from the body. That's

29:47

the primary somatosensory cortex,

29:49

and that neighbors these other regions

29:51

that we call the sensory somatosensory

29:54

cortex and the tertiary somatisentury

29:56

cortex, and these are involved in

29:58

more comp integration

30:02

of sense, like what you need to recognize

30:05

objects based on touch. This sort

30:07

of higher order processing

30:09

lets you translate round

30:11

and smooth into.

30:12

The recognition of an apple.

30:15

And if you get damage to these

30:17

higher areas, then you get

30:19

disorders like tactile agnosia,

30:22

which means not knowing what

30:25

you are touching. So if you have tactile

30:27

agnosia and I have

30:29

you close your eyes and I put a book

30:32

in your hands, you feel it with your fingers

30:34

and you run your hands over it, and I'll say what

30:36

is that object? And you'll say, I

30:38

don't know. Let's say I put a cell

30:41

phone in your hands or a shoe. You have no

30:43

idea what they are

30:45

when you feel them. But

30:47

now I ask you to open your eyes

30:49

and look at those things sitting on the table, and you

30:51

have no trouble saying, oh, that's a book,

30:54

that's a cell phone, that's a shoe. So it's not

30:56

that you don't know what objects

30:58

are, it's that you no longer have the

31:00

ability to tell what something is simply

31:03

by feeling it, simply by touch.

31:05

What I always find so.

31:07

Fascinating about neuroscience is seeing

31:10

how the self breaks down.

31:12

Just think about that example.

31:13

It seems so easy and obvious

31:15

that you can close your eyes and feel something

31:17

with your fingers to determine what it is.

31:20

Of course you can do that.

31:21

What we learn from life's

31:24

cruel natural experiments is

31:26

that easy, obvious things are

31:29

underpinned by very complex

31:31

brain networks. They don't come for

31:33

free, and when these delicate

31:36

pink brain areas get damaged,

31:38

we see that the ease of

31:41

doing the task was just an

31:43

illusion. The task in fact,

31:45

is massively complex, and it does

31:48

not come for free.

31:50

So many of the things in our.

31:52

Life result from hundreds

31:54

of millions of years of evolution and

31:56

super complicated mechanisms running in the

31:58

brain that you didn't even

32:01

realize you had. Remember

32:03

that the brain sits in chambered

32:06

in a dark, silent skull, and

32:08

it doesn't have direct access to any of the

32:10

stuff out there. So your skin

32:13

is a highly specialized

32:15

machine that converts different types

32:18

of energy mechanical

32:20

and thermal and chemical into electrical

32:23

energy and sends information racing

32:25

up the spine to the cerebral

32:27

cortex.

32:28

This is how we read detailed.

32:30

Information from the outside

32:32

world. Okay, so we've

32:34

surveyed how touch works,

32:37

and all of the incredible detail of it

32:39

might make you suspect that touch means

32:41

a lot to the brain, and you'd be right. It's

32:43

fundamental. Now how do we know

32:46

that?

32:46

Well.

32:46

For example, in the nineteen fifties, a

32:49

scientist named Harry Harlow asked

32:51

questions about the importance of touch

32:54

to baby monkeys. He separated

32:57

an infant from its mother and he

32:59

raised it in a cage with two

33:01

substitute wire frame

33:03

monkeys. These were adult monkey shapes

33:06

that were just built out of wire. Now

33:08

one was bear wire but had a

33:10

bottle of milk for the infant,

33:12

and the other had no milk but was

33:15

covered in terry cloth. So

33:17

what happened was the baby monkeys would

33:19

drink some milk and then they would immediately

33:21

steal over to the terry cloth

33:24

mother and clutch at it for the rest

33:26

of the time. And if the babies

33:28

got frightened, they ran only for

33:30

the terry cloth mother. So these

33:32

studies confirmed earlier suspicions

33:35

that there was more to the mother

33:37

infant relationship than nourishment.

33:41

Harlow realized that this contact

33:43

comfort was essential

33:45

to the normal brain development of monkeys,

33:47

and by extension, other studies have shown

33:50

how critical this is for human

33:52

children. Touch is a massively

33:55

important part of the mother child interaction.

33:57

So Harlow did another experiment to under

34:00

stand this.

34:00

He had infant monkeys raised

34:03

in cages, and they could see other monkeys

34:05

and could smell them and hear them, but they couldn't

34:08

touch them. And these monkeys

34:10

were devastated. They cried and they paced

34:13

frantically. But then when the screen

34:15

between the monkeys had holes in it, so

34:17

the mothers and babies could touch. That

34:20

was enough to keep the youngsters

34:22

from developing behavioral problems.

34:25

So proper brain development

34:27

in monkeys and humans requires

34:31

touch. It's not optional for

34:33

a baby having loving adults

34:36

interact with you and play with you, and tickle

34:38

you and comfort you in an early age.

34:40

These are all requisite for shaping

34:43

the brain. Cuddling a child

34:45

isn't just something that's nice for the parents.

34:47

It's actually necessary for

34:49

the child's normal brain development.

34:52

So touch shapes the brain

34:54

and is the basis for a healthy

34:56

emotional life. But of course it's not just about

34:58

emotion. It's also about information.

35:01

It's how we come to understand

35:03

our world and exert our own

35:05

influence over it. In fact, the way

35:07

that we understand and interpret

35:09

the world, our cognition, is fundamentally

35:12

rooted in our physical bodies.

35:15

How things feel to us, rough

35:18

or warm or heavy. This interacts

35:20

with our thoughts and our behaviors. We build

35:23

metaphors on top of our interactions

35:26

with the physical world. We say things like that

35:29

exam was rough, we

35:32

say she has a warm

35:34

personality, or we say, boy,

35:37

that was a heavy movie.

35:40

At the root of our language is what we

35:42

can touch and feel. This is called

35:44

embodied cognition. And I'm going

35:46

to talk about this more in a later episode,

35:49

but for now, I just want to say that

35:51

not only our language, but many other sorts

35:53

of judgments we make are rooted in

35:56

our bodies, even things like making

35:58

judgments about other people. Is that

36:01

person warm? Is that company

36:03

competent and trustworthy? These

36:06

use some of the same brain machinery

36:09

as feeling warmth or solidness

36:11

and texture and all the things

36:14

our touching exploration of the world

36:16

gives us and in our daily lives.

36:18

Touch serves as a high bandwidth

36:21

channel that continually moves information

36:23

back and forth between people.

36:25

It's a really powerful communication tool.

36:28

We give assurance

36:30

by laying a hand on someone's shoulder.

36:33

We give kudos with a slap on the

36:35

back in an aggressive situation,

36:38

we poke with a finger. In

36:40

an affectionate relationship, we move

36:43

a hair out of the way, or we nuzzle somebody's

36:45

cheek. In a first

36:47

meeting, we shake hands, and sometimes

36:49

we make judgments about the other person by the

36:51

firmness of their hold. In

36:54

fact, people communicate much

36:56

more information through touch than we are typically

36:58

aware of. One study asked

37:01

volunteers to communicate

37:03

any motion to a stranger just

37:05

using touch. They were both blindfolded,

37:08

so they would try to communicate things like anger

37:11

or fear or disgust, or

37:14

love or gratitude or

37:17

sympathy, or happiness or

37:19

sadness. Think about how you would try to communicate

37:22

these with touch. So most people

37:24

were pretty sure they weren't going to be able to

37:26

do this accurately. But the result

37:28

turned out that the other person was

37:30

able to understand the social

37:33

emotion about seventy five percent of the

37:35

time. That's surprisingly

37:37

good, and it underscores how

37:40

nuanced and sophisticated.

37:43

A communication channel we have in

37:45

touch.

37:46

And just look at how important

37:49

touches in the world of sports.

37:52

Watch professional basketball players.

37:54

They're constantly patting each other on

37:56

the back or chest, bumping or high fiving.

37:59

What is that all about.

38:00

Well, one possibility is it's just a

38:02

tradition with no particular meaning. But

38:05

some people started to wonder

38:07

whether all that human touch was

38:10

about a deeper form of bonding

38:12

and therefore good for their game. So scientists

38:15

at the University of Illinois measured

38:17

the amount of physical contact

38:20

each team had in the NBA. How

38:22

many times did players make

38:24

friendly contact with one another on the court.

38:27

They then followed the ranking of all the

38:29

teams through the seasons, and they

38:31

found that NBA teams who had

38:33

more contact did better

38:36

by the end of the season. In other words,

38:38

touch seemed to predict performance

38:41

across all the NBA teams. Why

38:44

well, One hypothesis is that

38:46

the contact has the effect of

38:48

increasing trust and affiliation

38:51

while also lowering stress hormones,

38:54

And that sheds light on a tradition

38:57

in long distance bike races. If

38:59

someone is really slowing down, really

39:01

out of energy, a fellow biker

39:04

will ride along beside them and simply

39:06

touch them with one finger, and their

39:08

riding speeds up. Apparently it's very

39:10

effective. Just a bit of human

39:13

touch is enough to reinvigorate

39:15

someone so they can find a second wind.

39:18

And it's for reasons like these that we see

39:21

bonding touch throughout the primate

39:23

world. When monkeys groom

39:25

one another, they're using touch

39:27

as a tool to communicate

39:30

trust and to strengthen bonds.

39:32

And across the world.

39:34

People want to keep their communication channel

39:37

of touch open, but different cultures

39:39

do it differently. You've probably

39:42

noticed that people in southern

39:44

climates like the Mediterranean touch

39:46

each other a lot more than people

39:48

farther north. Like the Scandinavians. Those

39:52

closer to the equator, they hug,

39:54

they kiss on the cheek, they slap each other

39:56

on the back, they hold hands. Why

39:58

Well, one hypoth this goes that people who

40:01

are closer to the equator, where it's warmer,

40:03

they wear fewer clothes, they have more

40:05

skin exposed, and it only

40:07

pays off to touch someone if

40:10

they're going to feel it. Up north, they

40:12

wear heavy clothes, and this communication

40:15

channel is essentially cut off, so

40:17

it doesn't.

40:17

Get used as much.

40:19

So let's wrap up what we've seen

40:22

is that touch is a system that

40:24

covers your body, and although we take

40:26

it for granted it's a massive

40:28

communication channel, that it's right at the

40:30

center of our perception and cognition.

40:33

That big, beautiful sensory organ

40:36

we come wrapped in tells us

40:38

so much about the physical world.

40:40

So let's come back to the effect of

40:42

technology.

40:43

I'm generally optimistic about the ways

40:46

that technology can connect people

40:48

across long distances, but

40:50

what is not clear is what this will mean

40:53

for the basic sense of touch.

40:56

Brains need touch all throughout development.

40:59

We drop into the world half baked, and

41:01

mother nature expects certain kinds

41:03

of input, and touch is a major

41:06

one of those, and we need.

41:07

It as we move throughout our lives.

41:10

So in other episodes, I've talked about hearing and

41:12

vision, but what we've seen today

41:15

is that the extremely dense

41:18

zoo of receptors in your skin, which

41:20

pick up touch and vibration and stretch

41:23

and temperature and paint. This zoo

41:25

of receptors is one of your

41:27

brain's main ways

41:30

of knowing its world. And as

41:32

we move into an era of more screens

41:35

and Internet protocols and zoom meetings

41:37

and VR, we will fill

41:39

our eyes and ears with the

41:42

joys of people on the other side of the planet.

41:45

But we should be careful about

41:47

what this means for touch. So,

41:50

returning to the old phone company

41:52

AT and T, their slogan

41:54

in the nineteen eighties was reach

41:57

out and touch someone, and

41:59

I love this sentiment, but I always found

42:01

the slogan ironic because the technology

42:04

of the telephone actually reduces

42:07

touch interaction. So I

42:09

hope that after you've heard this episode today,

42:12

you'll think about ways to make sure

42:15

you're getting enough touch, whether from

42:17

a dog or cat, a cuddle, puddle, a friend,

42:19

a lever, because our bodies

42:22

are built for this, so for real,

42:24

without technology reach

42:27

out and touch someone.

42:32

Go to eagleman dot com slash podcast

42:35

for more information and to find further reading.

42:37

Send me an email at podcast at

42:39

eagleman dot com with questions or discussion,

42:42

and check out and subscribe to Inner Cosmos

42:45

on YouTube for videos of each episode

42:48

and to leave comments.

42:50

Until next time.

42:51

I'm David Eagleman and this is Inner

42:54

Cosmos