Speaker 1: Why does a cold pool feel warmer this second time
Speaker 1: you put your toes in. Why does a person who's
Speaker 1: trying to break into a safe run his fingers over sandpaper?
Speaker 1: Would it be great or not so great if you
Speaker 1: couldn't feel any physical pain? Why does stubbing your toe
Speaker 1: have different sensations through time? Why do Mediterranean cultures touch
Speaker 1: each other more while they're talking than Scandinavian cultures? And
Speaker 1: what does this have to do with cuddle puddles, or
Speaker 1: why NBA players bump chests or why puppies sleep in dogpiles.
Speaker 1: Welcome to Inner Cosmos with me David Eagleman. I'm a
Speaker 1: neuroscientist and author at Stanford and in these episodes we
Speaker 1: sail deeply into our three pound universe to understand why
Speaker 1: and how our lives look the way they do. Today's
Speaker 1: episode is a love story about our sense of touch,
Speaker 1: what it is, how it works, and why it plays
Speaker 1: such a critical role in our lives. So let me
Speaker 1: start with an acquaintance of mine, who many years ago
Speaker 1: worked as a midnight phone operator at AT and T.
Speaker 1: He would get calls from people in the middle of
Speaker 1: the night and he'd say, Hello, this is AT and
Speaker 1: t how can I help you? And they would say hi,
Speaker 1: and he'd say what can I help you with? And
Speaker 1: they'd say, I just need someone to talk with. So
Speaker 1: there's a lot of loneliness in the world now. I
Speaker 1: tend to be suspicious when people talk about problems as
Speaker 1: modern phenomenon, given that most things have a deep history,
Speaker 1: but an increase in loneliness might actually be something new.
Speaker 2: If you look back.
Speaker 1: Even just a century ago, people lived in boarding houses
Speaker 1: with lots of other people, and there was lots of
Speaker 1: intergenerational living where you had at least three generations in
Speaker 1: the same household. Now that was probably crazy making, but
Speaker 1: whatever the case, this situation has changed a lot.
Speaker 2: As the middle class grew.
Speaker 1: The aspiration was for nuclear families to each get their
Speaker 1: own house with their own dishwasher and their own washing
Speaker 1: machine and their own television set. And in most neighborhoods,
Speaker 1: yards became more and more private, and at some point
Speaker 1: people stopped hanging out in their front yards and playing
Speaker 1: on the streets, but instead hung out in their backyards
Speaker 1: behind their fences. And by the year two thousand we
Speaker 1: got books like Bowling Alone, which was written by the
Speaker 1: political scientist Robert Putnam, and this book was about the
Speaker 1: decline of civic engagement since the nineteen sixty people used
Speaker 1: to be in bowling clubs and belong to their church
Speaker 1: or synagogue or mosque and see a community of people
Speaker 1: every day and every weekend. And they were in rotary
Speaker 1: clubs and Shriner's clubs and Kawana's clubs, and people who
Speaker 1: were recent immigrants often had clubs from the towns they
Speaker 1: originally came from. And these clubs lasted for decades until
Speaker 1: the wave of immigrants finally died out and their children
Speaker 1: born in America didn't really need to keep this up anymore.
Speaker 1: So because of lots of factors like suburbanization and the
Speaker 1: rise of television, and the decline of unions and the
Speaker 1: disappearance of clubs, we ended up with this decline of
Speaker 1: social capital.
Speaker 2: Now.
Speaker 1: Of note here is that Putnam's book was published before
Speaker 1: the invention of the smartphone, and while the Internet existed
Speaker 1: and video games existed, they were nothing like what they
Speaker 1: quickly became in the first decade after that book was published.
Speaker 1: And so society has changed much further in the past
Speaker 1: quarter century. And the question is, are we now not
Speaker 1: just bowling alone but not even bowling at all. Has
Speaker 1: society actually gotten lonelier? Now, I'm going to address this
Speaker 1: from both sides, arguing that there's more opportunity for relationship
Speaker 1: and less. Now on the side of more opportunity, one
Speaker 1: of the consequences that has always amazed me about the
Speaker 1: Internet is that now every person has the chance to
Speaker 1: find their tribe. If you are a stamp collector, or
Speaker 1: a mathematical poet, or a corporate mystic or a punk
Speaker 1: rocker historian or whatever, you can find your people. There
Speaker 1: will be no end to the websites and subreddits and
Speaker 1: substacks and whatever else you need to make you feel
Speaker 1: like you have your group where if you grew up
Speaker 1: pre Internet, you would never know that so many others
Speaker 1: like you were out there. And related to this, a
Speaker 1: lot of parents lament that kids are spending all their
Speaker 1: time on video games. But most video games have built
Speaker 1: in social mechanisms so that you can chat with other people. Now,
Speaker 1: chatting is not the same as spending time in real life,
Speaker 1: but with a million games to choose from, you end
Speaker 1: up finding people who share your interest and there's a
Speaker 1: lower barrier to entry from making acquaintances, and sometimes these
Speaker 1: become friends. So with multiplayer games, we have gone from
Speaker 1: bowling alone to bowling virtually virtually but not alone with
Speaker 1: other people's avatars who can chat with us and become
Speaker 1: friends of a sort. And there's another important way in
Speaker 1: which I think technology will have a shot at reducing loneliness,
Speaker 1: and this is with AI. Already we've seen an explosion
Speaker 1: of companies selling AI relationships, and if you're interested in
Speaker 1: the complexities of this, please check out episode thirty nine,
Speaker 1: which is on the future of AI relationships. But beyond
Speaker 1: AI girlfriends or boyfriends, I suspect one of the places
Speaker 1: will see the most immediate impact with AI is in
Speaker 1: areas like mental health therapy. The fact that you can
Speaker 1: have a therapist twenty four to seven, someone you can
Speaker 1: talk to in the middle of the night, someone who
Speaker 1: remembers everything you've said, someone who is not at all
Speaker 1: interested in talking about themselves but just about listening to
Speaker 1: you and giving you good advice and being there and
Speaker 1: truly has your best interests in mind. And so the
Speaker 1: question is does new technology like the Internet and AI
Speaker 1: scratch the itch of loneliness. Because I'm generally a cyber optimist,
Speaker 1: I feel like there are many ways that can be correct. However,
Speaker 1: while this hits on many cylinders, what it misses is
Speaker 1: a big one. Touch And we really care about touch.
Speaker 1: So that's what I'm going to talk about in today's episode,
Speaker 1: what touch is all about? Because our world has become
Speaker 1: a lot of screens and less about holding hands or
Speaker 1: hugging with strangers or friends. And the more I surveyed
Speaker 1: friends of mine on this point, I was surprised to
Speaker 1: realize how much touch is lacking in a lot of
Speaker 1: people's lives. One of my friends goes to Burning Man
Speaker 1: every year, and the thing he loves the most is
Speaker 1: that people hug people. Total strangers will say, give me
Speaker 1: a hug, and it's innocent. People can do this without sexuality,
Speaker 1: and people clearly appreciate it. And there get togethers all
Speaker 1: over the world called cuddle puddles or cuddle parties where
Speaker 1: dozens of people lie on mats and cuddle one another
Speaker 1: and it's not sexual. Instead, it's fulfilling the very primitive
Speaker 1: desire we all have for touch. And by the way,
Speaker 1: the same desire is seen to cross the animal kingdom,
Speaker 1: where you constantly see non sexual physical contact among social animals.
Speaker 1: The way you'll see puppies collect into a big dog
Speaker 1: pile when they're sleeping, And so from the point of
Speaker 1: view of the brain, I've become really interested in what
Speaker 1: we might be losing here. We're in an era with
Speaker 1: infinite hyperlinks between people, but fewer physical links. So for
Speaker 1: today's episode, I want to take us on a journey
Speaker 1: about touch, what it's all about, and how it colors
Speaker 1: our lives. When we look at touch across the animal kingdom,
Speaker 1: we see that it plays a critical role in everything
Speaker 1: from child rearing to finding food to getting mates. But
Speaker 1: not everyone thinks highly of it. Plato thought that vision
Speaker 1: was hot stuff and considered touch to be the most carnal,
Speaker 1: the least noble, of the senses. But his student Aristotle
Speaker 1: developed a different view. He felt, we learn the world
Speaker 1: by going out and touching it. That's how we figure
Speaker 1: things out. Not everything can be achieved purely by looking
Speaker 1: around and reasoning. So Aristotle said, quote, while in respect
Speaker 1: of all other senses we fall below many species of animals,
Speaker 1: in respect of touch, we far excel all other species
Speaker 1: in exactness of discrimination. That is why man is the
Speaker 1: most intelligent of all animals. So why did Aristotle privilege touch.
Speaker 1: What is special about touch and how does it all work?
Speaker 1: So let's start at the beginning. The largest organ in
Speaker 1: your body is actually on your body. It's your skin,
Speaker 1: and it is a miraculous sort of material. Unlike your
Speaker 1: other senses, which are focused at your eyes and your
Speaker 1: ears and so on, touch is spread throughout the body,
Speaker 1: and touch is the only sense that puts you in
Speaker 1: direct con tact with your subject, instead of assessing it
Speaker 1: from a distance, as you do with photons with seeing,
Speaker 1: or air compression waves with hearing. In contrast, you have
Speaker 1: to be right up against something. You can't touch something
Speaker 1: without being touched yourself. So the really amazing part is
Speaker 1: how it works. You have a whole zoo of touch
Speaker 1: receptors in your skin. These are specialized sensors for pressure,
Speaker 1: for itch, for stretch, for temperature.
Speaker 2: Some receptors are specialized for pain.
Speaker 1: Some are for pain caused by chemicals like an acid,
Speaker 1: some are for mechanical pain like a cut, and some
Speaker 1: are for thermal pain like a burn. And all of
Speaker 1: these things work in collaboration to read different sensory aspects
Speaker 1: of the world. So when you pass your finger over
Speaker 1: something like the petal of a flower or the sleeve
Speaker 1: of your jacket or the back of your dog. Different
Speaker 1: receptor types read the object in different ways. And what's
Speaker 1: interesting is that feeling doesn't happen in the top layer
Speaker 1: of your skin, but in the layer underneath. And this
Speaker 1: is why safe crackers sand their fingertips to expose the
Speaker 1: sensitive skin in the layer underneath, so they can be
Speaker 1: really sensitive to the tiny, tiny shifts in the locking
Speaker 1: mechanism of the safe. So collectively, this whole zoo of
Speaker 1: receptors inside your skin allows you to tell a lot
Speaker 1: about what's happening in the outside world in terms of
Speaker 1: texture and temperature and pressure and roughness and hardness, and
Speaker 1: your sense of what's out there is totally dependent on
Speaker 1: these sensors.
Speaker 2: If you block their operation with a local.
Speaker 1: Anesthetic, then you don't sense anything out there at all.
Speaker 1: So these sensors work together to give you exquisite sensitivity.
Speaker 1: This is why you can reach into your pocket and
Speaker 1: you can tell a quarter from a nickel without looking.
Speaker 1: Your sensitive fingertips detect the tiny ridges around the quarter's edge,
Speaker 1: and those are very small, but that's nothing. Your fingers
Speaker 1: can actually detect textures that are way finer. You can
Speaker 1: feel a texture seventy five nanometers high. That's one one
Speaker 1: thousandth of a human hair. If you run your fingers
Speaker 1: over that, you can tell it's there. And you know
Speaker 1: all those ridges and grooves on your fingertips they serve
Speaker 1: as sensation amplifiers. They expand the skin's surface area when
Speaker 1: they encounter pressure. So evolution has sculpted your fingertips into
Speaker 1: incredibly fine instruments of touch. They traffic in information we
Speaker 1: can't otherwise perceive with our other senses. So I'm gonna
Speaker 1: come back to skin in just a second, but first
Speaker 1: I want to make it clear that your system of
Speaker 1: sensation from the body picks up information both from the
Speaker 1: outside but also from the inside, and collectively this is
Speaker 1: grouped into what we call the somatosensory system, which just
Speaker 1: means sensing from the body. As I said a minute ago.
Speaker 1: Unlike vision or hearing, somata sensation doesn't happen in a
Speaker 1: discrete sensory organ like eyes or ears. Instead, the receptors
Speaker 1: are widely distributed in the skin, and the muscles and
Speaker 1: the bones and the joints, in internal organs, in the
Speaker 1: cardiovascular system. So even though we call it one system,
Speaker 1: it's giant, and it's distributed, and every second of your life,
Speaker 1: these millions of receptors are streaming information to your brain,
Speaker 1: where the information then steers your behavior. Now, in today's episode,
Speaker 1: I'm not going to talk too much about the internal monitoring,
Speaker 1: but just for sketching out the landscape, I'll tell you
Speaker 1: two of the main things you're doing on the inside.
Speaker 1: The first is constantly determining where your limbs exist in space,
Speaker 1: your body position, and your limbs movement. This is known
Speaker 1: as proprioception. Now, knowing how your body is positioned this
Speaker 1: is something we just take for granted. But if you
Speaker 1: lose this information, as sometimes happens, for example, from a
Speaker 1: viral infection, you can no longer do things like walk
Speaker 1: because now you need to visually look at your feet
Speaker 1: and your hands to have a sense of where they
Speaker 1: even are. So normally, your brain knows where your limbs
Speaker 1: are by constantly drinking in this fat stream of information.
Speaker 1: But if that goes offline, your only way to estimate
Speaker 1: where your limbs are is to look at them visually.
Speaker 1: And if the lights in the room turn off, you
Speaker 1: are going to fall to the floor. You can't maintain
Speaker 1: posture much less make any kind of fine movements without
Speaker 1: having visual feedback. So even though you may or may
Speaker 1: not have even been aware of this crazy thing called proprioception,
Speaker 1: you cannot function without it. And the other internal monitoring
Speaker 1: that I'll mention is called interoception, which is this amazing
Speaker 1: system by which you monitor the inside of your body,
Speaker 1: like movements of your gut or muscle stretch or changes
Speaker 1: in heart rate or stretch receptors in your lungs that
Speaker 1: modulate your breathing rate, or stretch receptors in your stomach
Speaker 1: and guts that tell you when you're full. You also
Speaker 1: have chemical receptors and the brain which monitor carbon dioxide
Speaker 1: levels to tell you if you're suffocating, and you have
Speaker 1: these in the circulatory system to monitor blood levels and
Speaker 1: trigger thirst. You have receptors in the throat that are
Speaker 1: close cousins of the touch receptors in your skin, and
Speaker 1: these are what detect whether there's an object in there
Speaker 1: to trigger gag. And again, all the stuff we tend
Speaker 1: to take for granted, but every single one of these
Speaker 1: receptors is critical for you to have the experience in
Speaker 1: the world that you do. These underpin your ability to
Speaker 1: be a body moving around in the world. But for
Speaker 1: today we're gonna talk about the other side of somatic sensation,
Speaker 1: not monitoring the inside but touching the outside world.
Speaker 2: So for this your.
Speaker 1: Entire body is covered by that one giant sensory organ,
Speaker 1: the skin. Now, this large, seamless sensory sheet is completely
Speaker 1: packed with receptors that allow you to feel when something
Speaker 1: or someone makes contact with you, when a nearby object
Speaker 1: is vibrating, or when something is hot or cold. Now,
Speaker 1: touch seems pretty simple. You just push your skin up
Speaker 1: against something. You say, oh, that's there, and it's hot
Speaker 1: or it's cold, or it's or it's smooth. But this
Speaker 1: apparent simplicity masks the incredible variety and complexity of how
Speaker 1: your brain and body do this. So let's surface the details.
Speaker 1: So first, to detect pressure and vibration, you have a
Speaker 1: specific flavor of sensory receptors in your skin called mechano receptors,
Speaker 1: and these are just built of special structures that make
Speaker 1: them sensitive to being physically stretched or bent. Now you
Speaker 1: have different types of mechanic receptors that are different structures
Speaker 1: and locations, and that gives you the ability to sense
Speaker 1: and interpret a huge range of things.
Speaker 2: In the world.
Speaker 1: I won't go into too much detail, but i'll mention
Speaker 1: you have some receptors which respond to fine light touch
Speaker 1: if you're interested. These are called Meisner's corpuscles or mercle discs.
Speaker 1: And you have other types of receptors found in the
Speaker 1: deeper layers of the skin that respond to stronger and
Speaker 1: cruder pressure if you're interested. These are called Piscinian corpuscles
Speaker 1: or Ruffini's endings. Now, some of these receptor types respond
Speaker 1: quickly right when something gets touched, but then they stop
Speaker 1: responding if the touch remains there, and others of these
Speaker 1: are slowly adapting, which means they keep firing if the
Speaker 1: touch stays there. So the rapidly adapting ones are well
Speaker 1: suited to respond to something that's going on and off
Speaker 1: like a vibration, because they stop responding right after the
Speaker 1: initial thing, and therefore they're ready to respond again a
Speaker 1: moment later. But others stay firing, so you can know
Speaker 1: when you touch your coffee cup that you're still touching it.
Speaker 1: So the thing I want to illustrate here is that
Speaker 1: you have this interplay of different receptor types that gives
Speaker 1: you a very rich, multilayered sensation of touch, fine and coarse,
Speaker 1: fast and slow. Now it's not just mechanicy receptors that
Speaker 1: you have in your skin, but you've got these other
Speaker 1: things called thermo receptor which tell you about temperature, and
Speaker 1: you have separate thermo receptors for cool and warm. Now,
Speaker 1: if this wasn't something you already knew, it's kind of amazing. Right,
Speaker 1: You're like this sophisticated meat robot with all these sensors
Speaker 1: packed into your skin to detect different things in the world. Now,
Speaker 1: what these thermoreceptors do is they detect changes in temperature
Speaker 1: on the skin surface relative to body temperature. So a
Speaker 1: cold thermo receptor in your finger, it's going to signal
Speaker 1: only a little tiny bit of change. When you test
Speaker 1: the water coming out of the sink. There's going to
Speaker 1: trigger a really strong signal when you dunk your hand
Speaker 1: into a bucket of ice water. Now, speaking of cold water,
Speaker 1: you've probably noticed that a cold lake feels warmer the.
Speaker 2: Second time you dip your foot in.
Speaker 1: Why, obviously the temperature the water hasn't changed, but the
Speaker 1: relative temperature of the water compared to your now chilled skin,
Speaker 1: that difference is decreased, and that changes.
Speaker 2: Your perception of the water temperature.
Speaker 1: They have a separate set of receptors that carry information about.
Speaker 2: Warmth, but these cool and.
Speaker 1: Warm receptors, they track temperature changes only to a point,
Speaker 1: and after that there's signaling drops off and information about
Speaker 1: things that are very cold or very hot that gets
Speaker 1: taken over by separate receptors pain receptors, and I'll return
Speaker 1: to those in a moment, but before we get to pain,
Speaker 1: I just want to mention that these cool and warm
Speaker 1: thermoreceptors can get activated by other things.
Speaker 2: Like certain shapes of molecules.
Speaker 1: For example, you have molecules that happen to bind to
Speaker 1: these cold receptors, like menthol. Menthol is just a shape
Speaker 1: that happens to activate these cool thermoreceptors, giving the pleasant
Speaker 1: illusion of coolness. And because we generally like that, we
Speaker 1: dump that molecule into our toothpastes and our shaving creams
Speaker 1: to activate these cool thermoreceptors. Similarly, the active ingredient in
Speaker 1: chili peppers is called capsaicin, and this is a molecule
Speaker 1: of a certain shape that happens to bind to warm thermoreceptors,
Speaker 1: and therefore it creates the illusion of heat. So capsaicin
Speaker 1: is used in ointments that you rub on your skin,
Speaker 1: like icy hot. Okay, so now we've talked about how
Speaker 1: receptors in your skin detect touch and detect temperature, and
Speaker 1: now let's come back to pain. The sensation of pain
Speaker 1: is one that most of us would probably rather not have,
Speaker 1: but it's actually a gift from nature that's critical for
Speaker 1: our survival. Feeling pain is how we detect damage to
Speaker 1: our cells and our tissues.
Speaker 2: Now, our perception of.
Speaker 1: Pain is mediated by another type of little receptors called
Speaker 1: no susceptors, and what activates these is damage to the tissue.
Speaker 1: And even though pain just seems like pain, you actually
Speaker 1: have very different types of no susceptors. First, you have
Speaker 1: no susceptors that are activated by physical damage to the tissue,
Speaker 1: like caused by pressure, a needlestick, a broken bone. These
Speaker 1: are called mechanical no susceptors. Then you have thermal no susceptors,
Speaker 1: which respond to things that are extremely hot or extremely cold.
Speaker 1: Then you have chemical no susceptors, which are activated by
Speaker 1: things like spider toxins when you get bit, or by
Speaker 1: certain cooking spices, or by poisonous gases like the kind
Speaker 1: used as chemical weapons in World War One. Now again,
Speaker 1: you might think that feeling no pain would be a blessing,
Speaker 1: but how would we know whether that's true or not.
Speaker 1: The answer is this sometimes happens because of genetic mutations,
Speaker 1: and the inability to feel pain is a dangerous curse.
Speaker 1: So take as an example a man who I'll call Paul.
Speaker 1: When Paul was a child, he would push swing and
Speaker 1: let it smash into his face. He didn't mind the
Speaker 1: broken nose and chipped teeth because he has never felt
Speaker 1: physical pain. He has a rare disorder called congenital analgesia
Speaker 1: and inability to register painful sensations. By the way, congenital
Speaker 1: just means you're born with it, and algesia means pain,
Speaker 1: and analgesia means no pain. So congenital analgesia just means
Speaker 1: you can't feel any pain and you were born that way.
Speaker 1: So Paul can feel a knife cutting his finger, but
Speaker 1: only the touch is registered, not the pain of the injury.
Speaker 1: And because his thermoreceptors are intact, he can easily tell
Speaker 1: the difference between warm and cool water, but he doesn't
Speaker 1: feel the pain of things that are really hot or
Speaker 1: really cold.
Speaker 2: So feeling no.
Speaker 1: Pain is a curse for someone like Paul, because pain
Speaker 1: is critical to avoiding bodily damage. It's what tells you
Speaker 1: about harms and threats. When Paul interacts with extreme temperatures,
Speaker 1: he doesn't get any immediate feedback telling him to pull
Speaker 1: his hand away or to avoid the situation. One of
Speaker 1: his most frequent injuries as a child was burning himself.
Speaker 1: He was interested in listening to the sizzling sound that
Speaker 1: his skin made, so his parents were constantly forced to
Speaker 1: take creative and desperate measures to keep their son safe,
Speaker 1: like putting socks over both his hands and goggles over
Speaker 1: his eyes. Often they put a helmet on him to
Speaker 1: protect him from his regular risk of hen injury. They
Speaker 1: constantly checked him for swelling and bruising and burns. Because
Speaker 1: of his lack of sensation, this kind of outside inspection
Speaker 1: was the only way that they or he could tell
Speaker 1: whether tissue had been damaged. So much of Paul's childhood
Speaker 1: was spent in hospitals, whether from jumping off extreme heights
Speaker 1: or banging his head against the wall and his forehead
Speaker 1: swelled up. So Paul is lucky to be alive because
Speaker 1: not everyone with congenital analgesia makes it through childhood. The
Speaker 1: moral of Paul's story is that pain is two faced.
Speaker 1: It hurts, but it also protects, So back to normal
Speaker 1: pain perception. Fascinatingly, you have different types of fibers that
Speaker 1: carry these signals to the brain, and they carry information
Speaker 1: at different speeds.
Speaker 2: So let's say you stub your toe.
Speaker 1: The next time you do this, try to pay attention
Speaker 1: to the way the pain is registered, because thanks to
Speaker 1: some fast fibers called a delta fibers, you'll register a fast,
Speaker 1: sharp pain, and then you have these other much slower
Speaker 1: fibers called sea fibers, and when their signals finally catch up,
Speaker 1: you'll feel the more prolonged but slightly less intense pain.
Speaker 1: So if you start paying attention to this, you can
Speaker 1: start unmasking the mechanisms of your body. The last thing
Speaker 1: I just want to note about these no suceptors, these
Speaker 1: pain receptors, is that they're distributed throughout your skin and
Speaker 1: your body, and your internal organs and in your joints.
Speaker 1: But the one notable exception is the brain, which has
Speaker 1: no suception, and that's why patients can be kept conscious
Speaker 1: during brain surgery. I've done experiments before while a patient
Speaker 1: is undergoing brain surgery, and I'm talking with them and
Speaker 1: having them watch things on a screen and asking them questions,
Speaker 1: and the patient can just sit there and talk and
Speaker 1: provide feedback about their perceptions without feeling any pain from
Speaker 1: the surgery itself. It's one of nature's funny ironies that
Speaker 1: the organ that perceives pain for your entire body is
Speaker 1: the one organ that registers no pain itself. So we've
Speaker 1: been talking about these receptors all over the body, and
Speaker 1: from the body, these signals come screaming up the spinal
Speaker 1: cord to the brain, and there these signals come slamming
Speaker 1: into a strip of real estate that we call the
Speaker 1: primary somatosensory cortex, which is just underwhere you would wear headphones.
Speaker 2: If you were to measure brain.
Speaker 1: Activity along the strip, what you find is that every
Speaker 1: little bit of it corresponds to some part of your body.
Speaker 1: And what you find is that not all body parts
Speaker 1: are represented equally.
Speaker 2: Some parts have more real estate devoted.
Speaker 1: To them, like the hands and the mouth and the
Speaker 1: lips and the tongue, and this corresponds to their importance.
Speaker 1: Not surprisingly, as you know from experience, you're able to
Speaker 1: discern much more subtly touch stimuli on these parts of
Speaker 1: your body then, say on your thigh or your back,
Speaker 1: which have smaller representations in the cortex. Now, it's not
Speaker 1: surprising that we devote so much brain territory to our hands.
Speaker 1: Humans began to walk upright several million years ago, and
Speaker 1: that freed up our hands to reach out and examine objects,
Speaker 1: and over time that changed the importance of hands, making
Speaker 1: them some of the most important body parts that we
Speaker 1: have for examining the world, and so our brain changed accordingly.
Speaker 1: The situation now is that each of our fingertips has
Speaker 1: two thousand touch receptors in it, and they're streaming continuous,
Speaker 1: detailed information to the brain.
Speaker 2: The philosopher A.
Speaker 1: Manual Kant once said, quote, the hand is the visible
Speaker 1: part of the brain. Now, what's cool is that in
Speaker 1: different animals, the sizes of different body parts represented in
Speaker 1: the sensory cortex. This differs across species because different animals
Speaker 1: rely on different sensations to survive in their niche.
Speaker 2: So when you look at the rat.
Speaker 1: Sensory cortex and the enormous part of the real estate
Speaker 1: is devoted to the whiskers, because that's a huge part
Speaker 1: of what lets the rat navigate dark spaces to find
Speaker 1: food and avoid obstacles. Or in an elephant, there's an
Speaker 1: enormous amount of real estate devoted to its trunk, or
Speaker 1: in the star nosed mole, it has a lot of
Speaker 1: territory devoted to its snout, which has these fingers on
Speaker 1: it that feel around to construct its three D model
Speaker 1: of its tunnels. Okay, so back to humans. We have
Speaker 1: this strip of cortex that receives the information from the body.
Speaker 1: That's the primary somatosensory cortex, and that neighbors these other
Speaker 1: regions that we call the sensory somatosensory cortex and the
Speaker 1: tertiary somatisentury cortex, and these are involved in more comp
Speaker 1: integration of sense, like what you need to recognize objects
Speaker 1: based on touch. This sort of higher order processing lets
Speaker 1: you translate round and smooth into.
Speaker 2: The recognition of an apple.
Speaker 1: And if you get damage to these higher areas, then
Speaker 1: you get disorders like tactile agnosia, which means not knowing
Speaker 1: what you are touching. So if you have tactile agnosia
Speaker 1: and I have you close your eyes and I put
Speaker 1: a book in your hands, you feel it with your
Speaker 1: fingers and you run your hands over it, and I'll
Speaker 1: say what is that object? And you'll say, I don't know.
Speaker 1: Let's say I put a cell phone in your hands
Speaker 1: or a shoe. You have no idea what they are
Speaker 1: when you feel them. But now I ask you to
Speaker 1: open your eyes and look at those things sitting on
Speaker 1: the table, and you have no trouble saying, oh, that's
Speaker 1: a book, that's a cell phone, that's a shoe. So
Speaker 1: it's not that you don't know what objects are, it's
Speaker 1: that you no longer have the ability to tell what
Speaker 1: something is simply by feeling it, simply by touch.
Speaker 2: What I always find so.
Speaker 1: Fascinating about neuroscience is seeing how the self breaks down.
Speaker 2: Just think about that example.
Speaker 1: It seems so easy and obvious that you can close
Speaker 1: your eyes and feel something with your fingers to determine
Speaker 1: what it is.
Speaker 2: Of course you can do that.
Speaker 1: What we learn from life's cruel natural experiments is that easy,
Speaker 1: obvious things are underpinned by very complex brain networks. They
Speaker 1: don't come for free, and when these delicate pink brain
Speaker 1: areas get damaged, we see that the ease of doing
Speaker 1: the task was just an illusion. The task in fact,
Speaker 1: is massively complex, and it does not come for free.
Speaker 2: So many of the things in our.
Speaker 1: Life result from hundreds of millions of years of evolution
Speaker 1: and super complicated mechanisms running in the brain that you
Speaker 1: didn't even realize you had. Remember that the brain sits
Speaker 1: in chambered in a dark, silent skull, and it doesn't
Speaker 1: have direct access to any of the stuff out there.
Speaker 1: So your skin is a highly specialized machine that converts
Speaker 1: different types of energy mechanical and thermal and chemical into
Speaker 1: electrical energy and sends information racing up the spine to
Speaker 1: the cerebral cortex.
Speaker 2: This is how we read detailed.
Speaker 1: Information from the outside world. Okay, so we've surveyed how
Speaker 1: touch works, and all of the incredible detail of it
Speaker 1: might make you suspect that touch means a lot to
Speaker 1: the brain, and you'd be right. It's fundamental. Now how
Speaker 1: do we know that?
Speaker 2: Well.
Speaker 1: For example, in the nineteen fifties, a scientist named Harry
Speaker 1: Harlow asked questions about the importance of touch to baby monkeys.
Speaker 1: He separated an infant from its mother and he raised
Speaker 1: it in a cage with two substitute wire frame monkeys.
Speaker 1: These were adult monkey shapes that were just built out
Speaker 1: of wire. Now one was bear wire but had a
Speaker 1: bottle of milk for the infant, and the other had
Speaker 1: no milk but was covered in terry cloth. So what
Speaker 1: happened was the baby monkeys would drink some milk and
Speaker 1: then they would immediately steal over to the terry cloth
Speaker 1: mother and clutch at it for the rest of the time.
Speaker 1: And if the babies got frightened, they ran only for
Speaker 1: the terry cloth mother. So these studies confirmed earlier suspicions
Speaker 1: that there was more to the mother infant relationship than nourishment.
Speaker 1: Harlow realized that this contact comfort was essential to the
Speaker 1: normal brain development of monkeys, and by extension, other studies
Speaker 1: have shown how critical this is for human children. Touch
Speaker 1: is a massively important part of the mother child interaction.
Speaker 2: So Harlow did another experiment to under stand this.
Speaker 1: He had infant monkeys raised in cages, and they could
Speaker 1: see other monkeys and could smell them and hear them,
Speaker 1: but they couldn't touch them. And these monkeys were devastated.
Speaker 1: They cried and they paced frantically. But then when the
Speaker 1: screen between the monkeys had holes in it, so the
Speaker 1: mothers and babies could touch. That was enough to keep
Speaker 1: the youngsters from developing behavioral problems. So proper brain development
Speaker 1: in monkeys and humans requires touch. It's not optional for
Speaker 1: a baby having loving adults interact with you and play
Speaker 1: with you, and tickle you and comfort you in an
Speaker 1: early age. These are all requisite for shaping the brain.
Speaker 1: Cuddling a child isn't just something that's nice for the parents.
Speaker 1: It's actually necessary for the child's normal brain development. So
Speaker 1: touch shapes the brain and is the basis for a
Speaker 1: healthy emotional life. But of course it's not just about emotion.
Speaker 1: It's also about information. It's how we come to understand
Speaker 1: our world and exert our own influence over it. In fact,
Speaker 1: the way that we understand and interpret the world, our cognition,
Speaker 1: is fundamentally rooted in our physical bodies. How things feel
Speaker 1: to us, rough or warm or heavy. This interacts with
Speaker 1: our thoughts and our behaviors. We build metaphors on top
Speaker 1: of our interactions with the physical world. We say things
Speaker 1: like that exam was rough, we say she has a
Speaker 1: warm personality, or we say, boy, that was a heavy movie.
Speaker 1: At the root of our language is what we can
Speaker 1: touch and feel. This is called embodied cognition. And I'm
Speaker 1: going to talk about this more in a later episode,
Speaker 1: but for now, I just want to say that not
Speaker 1: only our language, but many other sorts of judgments we
Speaker 1: make are rooted in our bodies, even things like making
Speaker 1: judgments about other people. Is that person warm? Is that
Speaker 1: company competent and trustworthy? These use some of the same
Speaker 1: brain machinery as feeling warmth or solidness and texture and
Speaker 1: all the things our touching exploration of the world gives
Speaker 1: us and in our daily lives. Touch serves as a
Speaker 1: high bandwidth channel that continually moves information back and forth
Speaker 1: between people.
Speaker 2: It's a really powerful communication tool.
Speaker 1: We give assurance by laying a hand on someone's shoulder.
Speaker 1: We give kudos with a slap on the back in
Speaker 1: an aggressive situation, we poke with a finger. In an
Speaker 1: affectionate relationship, we move a hair out of the way,
Speaker 1: or we nuzzle somebody's cheek. In a first meeting, we
Speaker 1: shake hands, and sometimes we make judgments about the other
Speaker 1: person by the firmness of their hold. In fact, people
Speaker 1: communicate much more information through touch than we are typically
Speaker 1: aware of. One study asked volunteers to communicate any motion
Speaker 1: to a stranger just using touch. They were both blindfolded,
Speaker 1: so they would try to communicate things like anger or
Speaker 1: fear or disgust, or love or gratitude or sympathy, or
Speaker 1: happiness or sadness. Think about how you would try to
Speaker 1: communicate these with touch. So most people were pretty sure
Speaker 1: they weren't going to be able to do this accurately.
Speaker 1: But the result turned out that the other person was
Speaker 1: able to understand the social emotion about seventy five percent
Speaker 1: of the time. That's surprisingly good, and it underscores how
Speaker 1: nuanced and sophisticated.
Speaker 2: A communication channel we have in touch.
Speaker 1: And just look at how important touches in the world
Speaker 1: of sports.
Speaker 2: Watch professional basketball players.
Speaker 1: They're constantly patting each other on the back or chest,
Speaker 1: bumping or high fiving.
Speaker 2: What is that all about.
Speaker 1: Well, one possibility is it's just a tradition with no
Speaker 1: particular meaning. But some people started to wonder whether all
Speaker 1: that human touch was about a deeper form of bonding
Speaker 1: and therefore good for their game. So scientists at the
Speaker 1: University of Illinois measured the amount of physical contact each
Speaker 1: team had in the NBA. How many times did players
Speaker 1: make friendly contact with one another on the court. They
Speaker 1: then followed the ranking of all the teams through the seasons,
Speaker 1: and they found that NBA teams who had more contact
Speaker 1: did better by the end of the season. In other words,
Speaker 1: touch seemed to predict performance across all the NBA teams.
Speaker 1: Why well, One hypothesis is that the contact has the
Speaker 1: effect of increasing trust and affiliation while also lowering stress hormones,
Speaker 1: And that sheds light on a tradition in long distance
Speaker 1: bike races. If someone is really slowing down, really out
Speaker 1: of energy, a fellow biker will ride along beside them
Speaker 1: and simply touch them with one finger, and their riding
Speaker 1: speeds up. Apparently it's very effective. Just a bit of
Speaker 1: human touch is enough to reinvigorate someone so they can
Speaker 1: find a second wind. And it's for reasons like these
Speaker 1: that we see bonding touch throughout the primate world. When
Speaker 1: monkeys groom one another, they're using touch as a tool
Speaker 1: to communicate trust and to strengthen bonds.
Speaker 2: And across the world.
Speaker 1: People want to keep their communication channel of touch open,
Speaker 1: but different cultures do it differently. You've probably noticed that
Speaker 1: people in southern climates like the Mediterranean touch each other
Speaker 1: a lot more than people farther north. Like the Scandinavians.
Speaker 1: Those closer to the equator, they hug, they kiss on
Speaker 1: the cheek, they slap each other on the back, they
Speaker 1: hold hands. Why Well, one hypoth this goes that people
Speaker 1: who are closer to the equator, where it's warmer, they
Speaker 1: wear fewer clothes, they have more skin exposed, and it
Speaker 1: only pays off to touch someone if they're going to
Speaker 1: feel it. Up north, they wear heavy clothes, and this
Speaker 1: communication channel is essentially cut off, so it doesn't.
Speaker 2: Get used as much.
Speaker 1: So let's wrap up what we've seen is that touch
Speaker 1: is a system that covers your body, and although we
Speaker 1: take it for granted it's a massive communication channel, that
Speaker 1: it's right at the center of our perception and cognition.
Speaker 1: That big, beautiful sensory organ we come wrapped in tells
Speaker 1: us so much about the physical world.
Speaker 2: So let's come back to the effect of technology.
Speaker 1: I'm generally optimistic about the ways that technology can connect
Speaker 1: people across long distances, but what is not clear is
Speaker 1: what this will mean for the basic sense of touch.
Speaker 1: Brains need touch all throughout development. We drop into the
Speaker 1: world half baked, and mother nature expects certain kinds of input,
Speaker 1: and touch is a major one of those, and we need.
Speaker 2: It as we move throughout our lives.
Speaker 1: So in other episodes, I've talked about hearing and vision,
Speaker 1: but what we've seen today is that the extremely dense
Speaker 1: zoo of receptors in your skin, which pick up touch
Speaker 1: and vibration and stretch and temperature and paint. This zoo
Speaker 1: of receptors is one of your brain's main ways of
Speaker 1: knowing its world. And as we move into an era
Speaker 1: of more screens and Internet protocols and zoom meetings and VR,
Speaker 1: we will fill our eyes and ears with the joys
Speaker 1: of people on the other side of the planet. But
Speaker 1: we should be careful about what this means for touch. So,
Speaker 1: returning to the old phone company AT and T, their
Speaker 1: slogan in the nineteen eighties was reach out and touch someone,
Speaker 1: and I love this sentiment, but I always found the
Speaker 1: slogan ironic because the technology of the telephone actually reduces
Speaker 1: touch interaction. So I hope that after you've heard this
Speaker 1: episode today, you'll think about ways to make sure you're
Speaker 1: getting enough touch, whether from a dog or cat, a cuddle, puddle,
Speaker 1: a friend, a lever, because our bodies are built for this,
Speaker 1: so for real, without technology reach out and touch someone.
Speaker 1: Go to eagleman dot com slash podcast for more information
Speaker 1: and to find further reading. Send me an email at
Speaker 1: podcast at eagleman dot com with questions or discussion, and
Speaker 1: check out and subscribe to Inner Cosmos on YouTube for
Speaker 1: videos of each episode and to leave comments.
Speaker 2: Until next time.
Speaker 1: I'm David Eagleman and this is Inner Cosmos