Chapter 13: Touch & Somatosensory Perception

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Welcome back to the Deep Dive.

Today, we are going to get extremely close and personal.

I want you to take a second and just feel, feel the device you're holding or maybe the chair you're sitting in.

We are talking about the proximal sense.

I mean, it's the one sense you literally cannot turn off no matter how hard you try.

We are diving deep into chapter 13 of Sensation and Perception, the sixth edition, and our topic, as you've probably guessed, is touch.

It's a really fascinating chapter because, you know, we so often neglect touch.

We think of ourselves as visual creatures, right?

We love our high -definition screens.

Oh, absolutely.

We're touch.

Touch is, it's like the background operating system of reality.

That is such a good way to put it.

I mean, without touch, you don't really know if something is truly real.

You can see a hologram, you can hear a recording, but if you can touch it.

If you can touch it, it exists, period.

And the mission for this Deep Dive is really to decode the machinery that makes that reality possible.

We're going to look at how, you know, simple mechanical pressure or thermal energy or chemical signals, how they hit your skin and somehow transform into the sensation of a warm breeze, a sharp pinprick,

or a lover's hand.

Before we jump into the biology, and believe me, there is a lot of biology, I want to start with this thought experiment that the text opens with.

It really, really sets the stage for why touch is so different from our other senses.

Right.

This is a morning routine experiment.

It's a great one.

Okay.

So, listener, I want you to imagine you wake up morning, but you're blindfolded.

You can't see a single thing.

And you stumble into the kitchen to make a peanut butter and jelly sandwich.

Okay.

You reach into the cabinet.

You're feeling around for two jars.

Peanut butter and jelly.

Now, without your vision, could you tell them apart?

Oh, yeah, I think so.

The peanut butter jar is usually plastic, right?

It feels a little warmer to the touch, or maybe just not as cold as glass.

It's probably wider too.

And the jelly jar?

Glass.

Definitely heavier.

Probably feels colder.

The shape is different.

Exactly.

The text points out that you would have almost zero trouble telling them apart.

You'd find the lid, you'd feel the little ridges on it, the resistance as you twist it open.

You'd feel the nice scoop into the peanut butter, which feels, I don't know, thick and resistant,

versus the jelly, which would feel more gelatinous, more giving.

You could do the whole thing.

You could probably even pour a glass of milk, just listening for the sound of the liquid filling the cup and feeling that change in your hand.

You absolutely could.

But here's the critical distinction the authors make, and it's a big one.

Think about your eyes for a second.

You can see a mountain that's, what, 10 miles away?

Sure.

Or think about your ears.

You can hear a siren that's three, four blocks away.

Right.

Vision and hearing are our distance senses.

But touch.

Touch is exclusively proximal.

To perceive an object by touch, you can't just sit back in your chair and observe it from afar.

You have to.

You have to engage with it.

You have to make contact.

It's even more than just contact.

You have to act.

It's a crucial point.

If you just rest your hand on that peanut butter jar and don't move it at all, your sensation will actually start to fade.

To really know the jar, you have to run your fingers over it.

You have to squeeze it.

You have to lift it up and feel its weight.

That is a really cool framing.

To learn by touch, you must act.

It's the only truly It really is.

And that action triggers this incredible cascade of information that travels from your fingertips, up your arm, up your spine, and into the most complex and interesting maps in your brain.

So let's start at the beginning of that journey.

Let's talk about the hardware, the skin itself.

The skin is impressive just by the numbers.

It is the heaviest and also the largest sense organ you possess.

The text mentioned it's something like 1 .8 square meters on an average That's right, yeah.

And it weighs about 4 kilograms, which is roughly 9 pounds.

Just think about that.

Imagine carrying around a 9 -pound weight all day.

That is just your skin.

And it's not just one uniform sheet covering us, is it?

Not at all.

In the world of sensation and perception, we divide the skin into two really distinct types.

You have hairy skin, which is most of your body.

And glabrous skin.

That's a great word.

It basically means hairless.

So think of your palms, your fingertips, the soles of your feet, and your lips.

These are the high sensitivity, high traffic areas.

Exactly.

This is where we do our heavy lifting when it comes to exploring and manipulating the world.

And to understand how we feel things, we really have to look at what's embedded deep inside that glabrous skin.

The mechanoreceptors.

Right.

Now the text introduces four main types of mechanoreceptors that are found in glabrous skin.

But instead of just listing them like a vocabulary test, let's think about this like an engineering problem.

Okay, I like that.

Let's engineer a hand.

So if you were building a robot hand to feel the world, your first instinct might be to just put one type of sensor all over it.

A generic pressure sensor.

Simple, efficient, cheaper to manufacture.

But it would be terrible for navigating reality.

Because touch isn't just one thing, right?

Touch is texture.

Touch is vibration.

Touch is stretching.

Touch is detecting if something is slipping.

If you had one sensor trying to do all of that, the signal would be a complete noisy mess.

So evolution was a better engineer.

It built distinct channels for different kinds of information.

Precisely.

We have four main channels, and they are defined by two main criteria.

First up, the receptive field.

How big is the patch of skin that a single sensor listens to?

Is it a tiny pinpoint dot?

Or is it a large broad area?

So small field versus large field.

Got it.

The second criterion is adaptation rate.

This is really crucial.

If I poke your hand and just hold my finger there, does the sensor keep firing the whole time I'm touching you?

That's called slow adaptation.

Okay.

Or does it fire once when I first touch you, then go completely silent, and then fire again only when I lift my finger off?

That's fast adaptation.

So it only cares about change.

Exactly.

So we've got this two by two grid, small versus large receptive fields, and fast versus adaptation rates.

This gives us our four channels.

Let's walk through them.

Let's do it.

First, let's take the small field, slow adaptation channel.

These are the SAI fibers, and they end in structures called Merkle disks.

Merkle disks.

What's their job?

What are they optimized for?

Well, because they have a tiny receptive field and they keep firing as long as they're being stimulated, they are your high resolution scanners.

They are all about detecting fine spatial details.

So the example from the text is reading Braille.

That's the perfect example.

When you're reading Braille, you're running your Merkle disks over those little bumps.

They're the ones that tell you the specific pattern.

They're the detail channel.

Okay.

So that's one, the detail channel.

What's next?

Next, let's look at the opposite in terms of field size.

Large field, slow adaptation.

These are the SAI fibers, and they're connected to Ruffini endings.

So large field means they aren't going to be good for fine detail, right?

Correct.

You couldn't read Braille with your Ruffini endings.

But because they adapt slowly, they are constantly firing whenever the skin is deformed or stretched.

They are the stretch sensors.

And why do I need to feel my skin stretch?

What's the point of that?

To know what your hand is doing.

When you grasp a baseball, your skin stretches around the ball.

The Ruffini endings are what tell your brain exactly how your grasp on an object.

Okay.

So Merkle is for texture and patterns.

Ruffini is for grip, shape, and finger position.

That makes sense.

Now let's get to the fast adapting ones, the ones that only fire when the stimulus changes.

Right.

So we have small field, fast adaptation.

These are the FAI fibers, and they connect to Meissner corpuscles.

Meissner.

Okay.

So what's the engineering use case for a sensor that stops firing if I just hold still?

This is one of my favorites.

It's slip detection.

Imagine you're holding a heavy wet glass of water.

Suddenly it starts to slide just a tiny, tiny millimeter through your fingers.

Okay.

Panic sets in.

That micro slip creates a wobble or a low frequency vibration on the skin.

And since the Meissner corpuscles only respond to change, they fire immediately at that first hint of a wobble.

They scream a signal to your spinal cord that says object slipping, and you instinctively tighten your grip before you're even consciously aware of it.

That is incredibly cool.

It's like traction control for your hand.

It's exactly like traction control.

Yeah.

And that brings us to the fourth and final channel.

Large field, fast adaptation.

These are the FA2 fibers, which end in the famous Piscinean corpuscles.

Piscinean corpuscles.

I've heard of these.

They're exquisitely sensitive to high frequency vibration.

So like when my phone buzzes in my pocket.

That's one example, but they can detect things that are even subtler.

The text gives a great one.

When a mosquito lands on your arm, that tiny impact is a vibration.

I feel that.

You feel that because of your Piscinean corpuscles.

Or when you tap a pencil on a desk, the shockwave of that tap hitting the skin on your hand is detected by them.

They're your event detectors.

They tell you that something has made contact with your body.

So to recap, we've got pattern detection with Merkle, stretch with Rafini, slip with Meissner, and vibration or impact with Piscinean.

And the text uses this beautiful analogy to show how they all work together in concert.

The simple everyday act of opening a door with a key.

I really like this breakdown.

It slows down a two second action into this whole sensory symphony.

Let's walk through it.

You reach into your pocket to find your key.

You're feeling around among coins and lint.

You feel the sharp edges of the key's head, the little teeth.

That's your Merkle discs giving the shape and pattern.

Okay, that's the identification step.

Then you grab the key.

You position your fingers around it just right.

Your skin stretches.

That's your Rafini endings confirming that you have a solid, stable grip.

Stability.

Got it.

Now you insert the key into the lock.

As you slide it in, you are feeling the friction.

If your hand wobbles a bit or the key starts to slip, your Meissner, your corpuscles fire, helping you adjust your force so you don't drop it.

So that's the correction phase.

And then you turn it.

You feel the tumblers move and then click.

The bolt hits the back of the mechanism.

That sharp impact, that thud vibration travels up the key into your skin and triggers your Pasinian corpuscles.

Confirmation.

The door is unlocked.

All four channels firing in a precise sequence to perform a task that you don't even have to think about.

It's incredible.

But there's a missing piece here, isn't there?

The text mentions that feeling the key isn't enough.

I also need to know where my hand is in space to find the keyhole in the first place.

Ah, yes.

You can have most sensitive fingertips in the world, but if you don't know where your arm is, you'll never find the keyhole.

This brings us to kinesthesia.

Okay.

I always get kinesthesia and proprioception mixed up.

Are they the same thing or not?

They're cousins.

Proprioception is the big umbrella term.

It basically means sensing the self.

It includes all the signals from your vestibular system for balance,

sensory neurons in your internal organs, and of course your limbs.

Okay.

Kinesthesia is more specific.

It's the perception of the position and the movement of our limbs in space.

So where are the sensors for that?

Are they in the skin along with the other four?

No, these are internal.

These are mechanoreceptors that are embedded deep within our muscles and tendons.

They're deep undercover agents.

Exactly.

You have things called muscle spindles.

These are tiny receptors coiled around the muscle fibers themselves.

When the muscle stretches, the spindle stretches with it.

They send signals about the rate at which the muscle's length is changing.

So they're measuring the velocity of the movement.

Precisely.

And then you have Golgi tendon organs.

These are located in the tendons, which attach the muscle to the bone.

They measure the tension or the force that's being exerted by the muscle.

So between the rate of length change from the spindles and the tension from the Golgi tendon organs, the brain builds this constantly updating 3D model of where the arm is and what it's doing.

Exactly.

And most of us take this completely utterly for granted until we hear the story of Ian Waterman.

This story from the text, honestly, it's the stuff of nightmares.

It's just terrifying to contemplate.

It is a profound neurological tragedy, but it taught us so, so much about the importance of touch.

Ian Waterman was a healthy 19 year old.

He contracted a viral infection and this virus was incredibly cruelly specific.

It attacked and destroyed only the sensory neurons for touch and kinesthesia below his neck.

But his motor nerves were fine.

He could still tell his muscles to move.

That's the kicker.

His brain could still send the signal, move arm.

The nerves carrying that command were totally intact, but the nerves carrying the feedback, the information that says the arm has moved to this position, were dead.

So he's a dashboard with new gauges, a car with no speedometer.

It's an even better analogy from the text.

Imagine driving a car with a blacked out windshield and no speedometer, no sensation of speed, no sound of the engine.

You turn the wheel, but you have no feedback to know if the car actually turned.

So what does that look like in daily life for him?

At first, he was completely helpless.

He just collapsed in a heap.

The text describes this terrifying scenario.

Imagine he's standing in an elevator and the lights go out.

He just falls.

Instantly, like a puppet with his strings cut, because without vision, he has absolutely no idea where his legs are.

He doesn't know if his knees are locked or bent.

He doesn't feel his feet on the floor.

He has to wait on the floor in the dark until the lights come back on so he can see his limbs and consciously command them to stand up again.

He has to fly entirely by instrument.

He has to watch his own feet to walk.

Every single step requires conscious visual confirmation.

If he looks away for a second, he might fall.

He has to watch his hands to button his shirt.

The cognitive load is immense.

It just shows us that we don't just walk with our muscles, we walk with our sense of touch.

That is a very humbling realization.

Okay, let's pivot from the let's talk about temperature.

Thermal reception.

Yes.

So we have specific receptors for hot and for cold.

We do.

We have warmth fibers and cold fibers.

And interestingly, they're not distributed equally across the skin.

Cold fibers outnumber warmth fibers by about 30 to 1.

30 to 1.

Why such an imbalance?

From an evolutionary perspective,

extreme cold is often a more immediate threat to body's chemistry than moderate warmth.

We need to know if we're losing heat fast because that can lead to hypothermia and death very quickly.

Now, I've always been confused by the physics of this.

The book gives a great example.

If I walk into a room and I touch a steel table, it feels freezing cold.

But if I touch a wooden table right next to it in the same room, it feels neutral, maybe even warmish.

But they're both room temperature, right?

They're exactly the same

but here's the secret.

Your thermal receptors do not measure the temperature of objects.

They don't.

What do they measure them?

They measure the temperature of your skin.

Ah, okay.

That makes sense.

Steel is a very high thermal conductor.

When you touch it, it sucks the heat out of your fingertips incredibly fast.

Your skin temperature plummets.

Your cold fibers screen cold.

And the wood is the opposite.

Right.

Wood is a poor conductor.

It's an insulator.

When you touch it, not much heat transfers from your skin to the wood.

Your skin temperature stays roughly the same, so the cold fibers stay quiet.

So I'm not feeling the object.

I'm feeling the rate of my own heat loss to the object.

Exactly.

You are a thermometer that only measures itself.

That clears up so much.

But speaking of heat, let's talk about the chemical hack.

I love spicy food.

I put hot sauce on everything.

Why does a chili pepper feel hot?

It's not actually burning my tongue, is it?

It's not burning you in the sense of causing thermal tissue damage.

But chemically, your brain cannot tell the difference.

This brings us to the nociceptors, the pain receptor.

The body's alarm bells.

Nociceptors are basically bare nerve endings that fire when there is potential or actual tissue damage.

And on these nerve endings, there are these little ion channels called thermo TRP channels.

TRP.

It stands for transient receptor potential.

Just think of them as little molecular gates.

There is a specific gate that is designed to swing open only when the temperature hits a dangerous level like if you touch boiling water.

Okay, so that's the heat alarm.

Right.

Well, it turns out that the chemical capsaicin, which is the active ingredient in chili peppers, has the exact right molecular shape to unlock that very same gate.

So the key for boiling water and the key for jalapeno are the same key as far as the nerve is concerned.

To the nerve, yes.

The gate opens.

The ions rush in.

The signal shoots to the brain.

Heat.

Burning.

The brain has no way of knowing if it's thermal energy or chemical energy that opened the gate.

It just knows the burning phone line is ringing.

And I assume the same thing happens in reverse with mint.

Correct.

The chemical menthol triggers the specific channel that normally responds to cool temperatures.

So mint feels chemically cold even at room temperature.

It's amazing how easily our basic hardware can be tricked.

But while we are on the subject of pain, we need to break down the different types of pain.

Because, you know, a paper cut feels very different from a sprained ankle.

That's a great point.

Pain is not a monolith.

We generally talk about two stages of pain, and they're carried by two very different types of nerve fibers.

Let's say I stub my toe on the coffee table.

Okay.

The absolute instant it happens, you feel a sharp, bright, shocking burst of pain.

That OWU moment.

That is the adulta fibers.

These nerve fibers are

myelinated.

Myelin is a fatty sheath that insulates the nerve and makes the electrical signal travel super, super fast.

These fibers are the getaway signal.

They trigger the reflex to pull your foot back immediately.

And then about a second or two later, the throb sets in the dull, aching burning sensation that lingers.

That is the C fibers for crappy C for unmyelinated.

They don't have that fatty insulation.

So the signal travels much more slowly.

It takes longer to get to the brain.

But this pain is persistent.

It has a strong emotional component.

It's the body saying, okay, the initial damage is done.

Now you need to guard this area.

Don't walk on it.

Let it heal.

So one is an alarm bell and the other is a sentry guard.

That's a perfect analogy.

But we're missing a huge, huge part of the touch experience here.

We talk about mechanics, temperature, pain, all very functional, very utilitarian.

What about a hug?

Or petting a cat.

Right.

For a very long time, scientists thought touch was just discriminative.

It was all about telling us what is there.

But relatively recently, we've identified a very special fifth type of fiber called the C -tactile afferent, or CT afferent.

And these are different from the Merkel and Ruffini guys we talked about earlier.

Very, very different.

First, get this.

They are found only in hairy skin.

Wait, really?

So my palms and fingertips don't have these.

No.

Your glabrous skin, your palms, they're for work.

For identifying objects for manipulation, your hairy skin, your arms, your back, your legs, that seems to be for social connection.

How do they work?

What activates them?

These CT fibers are tuned to a very specific kind of stimulation,

slow light stroking, specifically movement that's between one and 10 centimeters per second.

That seems incredibly specific.

It is the exact speed that people all over the world naturally use to caress a romantic partner or to stroke a baby to calm them down or to pet a dog.

If you move too fast, the CT fibers don't fire.

If you press too hard, they don't fire.

They're exquisitely tuned to gentle care.

That's amazing.

And where do these signals go in the brain?

This is the really fascinating part.

All the other touch signals we've talked about go to this metasensory cortex, the part of the brain that says there is an object touching my arm.

These CT signals, however, project to the insula and the

The emotional centers of the brain.

Yes.

This is the social wire.

Stimulation of these fibers doesn't just feel nice.

It triggers real hormonal responses.

It lowers the heart rate in babies.

It increases oxytocin, the bonding hormone.

It even reduces the perception of pain.

It's a natural analgesic.

So when a parent rubs a kid's back after they fall down, they aren't just comforting them psychologically.

They are literally activating a dedicated pain circuit in their nervous system.

Exactly.

We are biologically wired to be soothed by touch.

It is a fundamental imperative for social animals like us.

That is beautiful.

Okay.

So we've stimulated the skin.

We have all these signals, sharp pain, throbbing pain, vibration, stretch, pleasant caresses.

Now they have to get to the brain.

And it's a long journey.

If you stub your toe, that signal has to travel up to two meters to get from your foot to your brain.

And the entry point is the spinal cord.

Specifically,

the dorsal horn of the spinal cord.

You can think of this as the main entry gate.

And from there, the traffic splits into two major highways heading north to the brain.

Why two separate highways?

Why not just send everything up one big pipe?

Because of priority and speed.

We have the spinothalamic pathway.

This is the evolutionarily older of the two.

It carries pain and temperature information.

The fibers are smaller.

It's a bit slower.

That's the danger and weather report lane.

You could call it that.

And then there's the DCML pathway, the dorsal column medial lemniscle pathway.

This is the superhighway.

Big fat myelinated fibers,

extremely fast transmission speeds.

This carries all that critical tactile and kinesthetic information.

Wait, why does knowing where my arm is need to be faster than knowing my arm is on fire?

That seems counterintuitive.

It's all about movement control.

If you are running or catching a ball or fighting off a predator, you need to know exactly where your limbs are and what you are touching right now in order to adjust your motor commands in real time.

If that info was slow, you'd be clumsy, you'd trip, you'd miss the ball.

Right.

Pain is just a warning, but proprioception is the active guidance system for movement.

Exactly.

So these two highways travel up the spinal cord, they pass through the thalamus, which acts as a relay station, and they finally land in the vital lobe in a region called the somatosensory cortex, or S1.

And this is where we meet the famous homunculus.

The little man inside the brain.

I've seen the drawings of this in the textbook.

It's a map of the body laid out on the surface of the brain, but it looks really distorted.

It is highly, highly distorted.

He has these giant hands and massive lips and then this tiny little torso and legs.

Why?

Think of the brain's cortex like a parking lot.

You only have finite number of parking spaces for neurons.

The skin on your back.

It's a huge piece of real estate in terms of surface area, but it's not very sensitive.

You don't use your back to weed braille or thread a needle.

So it doesn't need that many neurons to process its relatively simple data.

It gets one compact little parking spot way in the back of the lot.

Sorry, back.

But your thumb,

your lips, these are your primary tools for exploring the world.

They have a massive density of receptors.

They are sending terabytes of data to the brain constantly.

They need a VIP section right up front.

The thumb gets 50 parking spots.

The lips get even more.

So the map isn't a picture of your body's physical size.

It's a picture of your body's importance to the brain for sensory processing.

That's the perfect way to put it.

We call it cortical magnification.

And this map isn't just some static picture.

It fundamentally dictates our body image, how we perceive ourselves in space.

And the text highlights some wild ways this body image can get warped and extended.

The Pinocchio Effect.

Yes.

But even more interesting is what happens with tool use.

When you use a tool, like a rake or a long stick, to reach for something,

your brain actually extends your internal body schema.

So the tool literally becomes part of me in the brain's map.

In a very real sense, yes.

They did a study where people used a mechanical tool to retrieve objects.

Afterwards, when they asked the people to point to where their own elbow and wrist were, the participants perceived their arm as being physically longer than it actually was.

The brain had incorporated the length of the tool into the length of the arm.

That explains why, when you drive a car for a long time, you start to feel exactly where the bumper is when you're parking.

The car becomes a kind of body suit.

Exactly.

And then there is the even stranger rubber hand illusion.

Have you ever seen this done?

I have.

It is genuinely spooky.

For the listener.

Imagine you sit down at a table, you put your right hand on the table, and put a little screen up so you can't see your own hand.

Then we place a fake rubber hand in front of you where your hand should be.

Okay.

So I'm looking at the fake rubber hand, but my real hand is hidden from my view.

And then I take two small paint brushes,

and I start stroking your real hidden hand and the rubber hand at the exact same time, in the exact same rhythm, on the exact same spot.

So my eyes see the brush touching the rubber hand, and my skin feels the brush touching my real hand.

Your brain hates a conflict.

It sees touch happening on the rubber hand.

It feels touch happening at the same time.

After a few seconds, it comes to a conclusion.

Okay, that rubber hand must be my hand.

It just adopts the plastic.

It happens almost immediately.

And here is the truly crazy part.

If I suddenly take a hammer and smash the rubber hand, you will flinch.

Your heart rate will spike.

You will feel a phantom shock of pain because your brain has extended your sense of self into that inanimate object.

It really shows how constructed our sense of self is.

It's just the brain's best guess correlation of all the incoming inputs.

And that construction is nowhere more evident and more critical for our wellbeing than in how we process pain.

We hinted at this earlier, but pain isn't just a simple signal.

It's a negotiation.

This is the gate control theory.

And this is something everyone should understand because it explains so much about how we handle or fail to handle injury.

Right.

The old view of pain was a direct line.

You hit your thumb with a hammer, a bell rings in the brain.

Simple.

But the gate control theory, proposed by Melzack and Wall, posits that there is a gate in the spinal cord, specifically in an area called the substantia gelatinosa.

And who is the gatekeeper?

What opens or closes this gate?

Two main things.

First, other touch signals.

Remember those fast, myelinated fibers that carry information about pressure and vibration?

Yeah, the DCML highway.

If you rub a stubbed toe,

you are flooding the spinal cord with non -painful rubbing signals.

These signals actually activate an interneuron in the spinal cord that closes the gate on the slow, throbbing pain signals from the C fibers.

That's why we instinctively rub a shin when we bang it.

We aren't just comforting ourselves psychologically.

We are actively jamming the pain signal at the source.

We are mechanically hacking our own spinal cord.

But the second gatekeeper is even more powerful.

It's top -down control.

Signals coming from the brain itself.

So your thoughts can control the gate.

Your thoughts, your emotions, your expectations, your tension.

The brain can send signals down the spinal cord to release enkephalins, the body's natural opiates that effectively lock the gate shut.

So if I'm distracted by something else or I'm really confident or I'm pumped up on adrenaline,

it literally hurts less.

Significantly less.

This brings us to the crucial distinction between the sensation of pain and the unpleasantness of pain.

The text makes it clear these are handled by different parts of the brain.

Wait, isn't pain inherently unpleasant?

That seems like the very definition of pain.

You would think so.

But the pure sensation, the part that says there is a sharp pressure of X intensity on my left arm, is handled by the somatosensory cortex, S1.

But the emotional component, the I hate this, make it stop, I am suffering, is handled primarily by the anterior cingulate cortex, or ACC.

And you can actually separate those two experiences.

They've done it in the lab.

There was a hypnosis study cited in the Texas, just remarkable.

They put people in an fMRI scanner and applied a painfully hot stimulus to their hands.

Then they gave them a hypnotic suggestion.

The pain will not bother you, you will feel the sensation, but it will not be unpleasant.

And did it work?

The activity in the ACC, the suffering center, dropped dramatically.

And the people reported, yeah, I can feel the heat, but I honestly don't care.

But what about the sensation part?

The S1 activity.

The activity in S1 stayed exactly the same.

The fact of the pain stimulus remained, but the opinion of the pain vanished.

That is absolutely mind -blowing.

It perfectly explains how soldiers in the heat of battle sometimes don't even notice they've been severely wounded until the fighting stops.

That's a classic example of analgesia.

The body releases endogenous opiates, its own internal painkillers, during extreme stress or excitement.

It blocks the signal at the gate so you can keep fighting or running to safety.

But expectation works both ways, doesn't it?

We have to talk about the placebo effect and its evil twin.

The placebo effect is so often misunderstood.

People think it means it's all in your head, as if you're just imagining the relief.

Right, like you're faking it or something.

But brain imaging shows that when you take a sugar pill that you believe is a powerful painkiller, your brain actually sends real physical signals down to the dorsal horn in the spinal cord to inhibit the incoming pain neurons.

It creates a genuine physiological pain relief.

Your belief becomes your biology.

And then there is the dark side.

Yeah.

The placebo effect.

I expect this to hurt, so it does affect.

Precisely.

The text describes a study with a pain cream.

They brought participants in and put a generic, completely inert cream on their skin.

No medicine in it at all, just lotion.

None.

But they told one group, this is a cheap, generic cream.

They told the other group, this is a new, expensive, high -end pharmaceutical.

Okay, yeah.

Then they applied a painful heat stimulus.

The people who thought they had the expensive cream on their skin reported feeling more pain.

Wait, more?

Why on earth would that happen?

The researchers speculated that maybe they expected the expensive medicine to be more potent and perhaps have side effects like skinning or increased sensitivity.

The point is, the narrative you are told about a sensation can change the physical reality of how you experience that sensation.

We are so, so suggestible.

I want to touch on something related to pain,

but a bit weirder.

Itch.

Ah, itch.

It's distinct from pain, but they interact in a fascinating way.

In fact, pain actually inhibits itch.

Which is exactly why we scratch in it.

Exactly.

When you scratch, you're causing minor damage to the skin.

You're creating a mild pain signal.

That pain signal travels up to fast fibers and closes the gate on the slower itch signal.

You are literally hurting yourself just a little bit to stop the itching.

The text mentions something really interesting about aging and itching.

Why do older people tend to suffer from dry, itty skin more often?

It's a really cool finding.

It turns out that Merkel cells, remember those little texture sensors from the beginning?

They normally play a role in inhibiting itch.

They tend to kind of keep quiet signal to the itch -specific neurons.

As we age, we naturally lose Merkel cells.

That population of sensors diminishes, so the inhibition fails.

The brakes are gone, and the itch signals get through much more easily.

That is so annoying.

And what about the ultimate weird sensation?

Tickling.

The great mystery of neuroscience.

We know that the somatosensory cortex is involved.

But for the longest time, we couldn't study it because you can't tickle yourself.

But we finally found an animal model for it.

Rats.

Rats are ticklish.

Seriously.

They are.

If you tickle a rat's belly, it emits these ultrasonic 50 kilohertz chirps, which are basically rat laughter.

They will even chase the researcher's hand around the cage to get tickled more.

It's clearly a form of social play.

That's adorable.

But here is the key finding.

If the rat is stressed, if you shine a bright light on it or put a cat scent nearby, it is not ticklish.

It won't laugh.

Just like humans.

If a stranger runs up and starts tickling you on the street, you don't laugh.

You punch them.

Context is everything.

Now earlier, we talked about how the brain maps the body with that homunculus.

But that map isn't set in stone.

It's incredibly plastic.

Neuroplasticity.

The brain's ability to rewire itself based on experience and input.

The text mentions a blindfold experiment that really demonstrates how unbelievably fast this rewiring can happen.

Yes.

This is a classic study.

They took sighted people and blindfolded them for just five days.

Total darkness.

And in that time, they taught them how to read Braille.

Five days isn't very long to learn Braille.

It's not.

But after just those few days, they put them in an fMRI scanner.

And when these newly trained people touched the Braille dots, their visual cortex, V1, started lighting up.

The part of the brain that's supposed to be used for seeing was hijacked for touch?

In less than a week.

The brain does not like unused real estate.

If the eyes aren't sending any data, the highly practiced touch system from the fingertips moves in and sets up shop.

And interestingly, when the blindfold came off, the effect disappeared within about a day.

It just snapped back to normal.

It's reversible plasticity.

But sometimes, the changes are not reversible.

And this brings us to phantom limbs.

This is one of the most famous and counterintuitive phenomena in all of neuroscience.

It is.

When an arm is amputated, the hand area of the somatosensory cortex

suddenly goes silent.

It's getting no input at all.

It's empty cortical real estate.

Right.

But nature abhors a vacuum.

And if you look at the homunculus map, right next to the hand area is the face area.

Over time, the neurons from the face area begin to invade the now empty hand territory.

So the wires literally get crossed in the brain.

The map gets smudged.

This means that if you now touch the amputee patient's cheek, the signal stimulates the face area, but it spills over into what used to be the hand area.

And the patient might feel you touching their cheek and their missing thumb at the same time.

That is just wild.

And understanding this plasticity is the absolute key to the future of bionics.

Right.

Because the goal isn't just to give someone a mechanical hand.

It's to give them a hand that they can actually feel with.

It's called biomimetic feedback.

We are getting to a point where sensors on a prosthetic hand can send electrical signals to the residual nerves in the amputated arm.

If we can make those signals mimic the natural firing patterns of mechanoreceptors, making the nerve think it's feeling the vibration of a key or the pressure of a cup, the brain will accept those signals and incorporate the bionic hand into its body image.

So the user feels the plastic and metal hand as their own.

It's like the rubber hand illusion, but functional and permanent.

Exactly.

It closes the sensory motor loop.

Let's shift gears for a bit to performance.

We've talked about how the system works, but how good is it?

How do we actually measure the sensitivity of our touch?

The classic method, which the text explains, is using von Frey hairs.

Like from a horse?

Originally, yes.

Now we use precisely calibrated nylon filaments.

You take a filament that is designed to bend at a specific known force, you press it against the skin until it just starts to bend, and you ask the subject, did you feel that?

You just keep using thinner and thinner hairs until they can't feel it anymore.

Right.

That gives you the absolute threshold of detection.

We find the face is the most sensitive.

You feel an absolute ghost of a touch there.

The lower extremities, like the calf or the sole of the foot, are the least sensitive.

Okay, that's sensitivity.

What about resolution, like pixels on a computer screen?

How much fine detail can I actually feel?

We test that with the two -point threshold test.

I take a device with two points, like a compass, and I touch your skin with both points simultaneously.

Can you tell if it's two distinct points or does it just feel like one big poke?

And on the fingertips, the resolution is really high, right?

Extremely high.

You can resolve two points that are only about one millimeter apart.

That's high definition touch.

And on your back?

It's much, much larger.

You might need several centimeters of separation to reliably feel two distinct points on your back.

It's very low resolution skin.

This goes right back to that receptor density and cortical magnification we talked about earlier.

Does this acuity fade with age?

I mean, my eyes are getting worse every year.

Is my touch getting worse too?

For most people, yes.

The text says that sighted people lose about 1 % of their tactile acuity per year after a certain age.

Ouch.

That's a depressing statistic.

However, and this is a big however, studies have shown that blind, braille readers do not lose this acuity with age.

Really?

There stays sharp.

And neither do concert pianists.

So it's a use it or lose it scenario.

It appears to be.

High levels of focused attention and daily practice seem to keep the sensory representation in the cortex sharp and distinct.

It's not just inevitable biological decay.

It's about cognitive maintenance and use.

Speaking of biology and genetics, the text mentioned a potential link between autism and touch sensitivity.

Yes, there is a strong correlation.

Many individuals on the autism spectrum report heightened sensory sensitivity.

Things like clothes feeling unbearably scratchy or certain textures being overwhelming.

Mouse studies have started to suggest this might be linked to genetic changes in how the tactile afferent nerves develop.

If the sensory input is too intense or too noisy from the very beginning, the brain might withdraw socially as a protective measure.

That connects the purely sensory world to the complex social world in a really profound way.

It absolutely does.

And if we want to talk about the ultimate sensory specialist, we have to bow down to the champion of touch.

The starnose mole.

The undisputed star of the animal kingdom.

You have to see a picture of this thing to believe it.

It has a nose with 22 fleshy pink tentacle -like rays.

It looks like an alien hand exploding out of its face.

And it's not for smelling, right?

It's a touch organ.

It's purely a touch organ.

It has over 25 ,000 mechanoreceptors packed into that nose.

That is five times more than in the entire human hand, all crammed into a space smaller than your fingertip.

That's insane efficiency.

And it has what's called a tactile fovea.

One of the rays, ray number 11.

Just like the fovea in your eye is your area of high -resolution vision, ray 11 is for high -res touch.

The mole scans the dark, muddy environment with the other rays.

And if it finds something potentially edible, it brings ray 11 over to inspect it in detail.

The text says it can identify and eat its prey faster than the human eye can even follow the action.

And it does it purely by touch.

It can find, identify, and consume a piece of prey in 120 milliseconds.

It is the sensory speed champion of the world.

I'm very glad I don't have to eat that way.

But it brings us to the final crucial section of the chapter,

haptics.

This is how we actually use touch in the real world.

Haptic perception.

It's the seamless combination of touch from the skin and kinesthesia from the muscles and joints.

As we said right at the start,

action for perception.

And we have these very specific, almost pre -programmed movements that the text calls exploratory procedures.

Right.

If I hand you an object behind your back and I ask you, is it rough or smooth?

Right.

What do you do with your hand?

I rub my finger back and forth across it, a lateral motion.

And if I ask, is it hard or soft?

I press on it.

I apply pressure.

Is it warm or cold?

I just hold it still, static contact.

What is its overall shape?

I wrap my whole hand around it.

I try to enclose it.

These are universal, unconscious strategies.

We switch between them instantly to extract the specific information we need about an object.

But touch definitely has its limitations.

The text brings up the failure of the Susan B.

Anthony dollar.

A classic design failure from the annals of haptics.

The US Mint released this new dollar coin back in 1979.

The problem was, it was very similar in size and weight to the common quarter.

And people absolutely hated it.

They did because haptically, it was too confusing.

It turns out that we are very, very good at judging the material properties of an object by touch, its roughness, its weight, its temperature.

But we are actually quite poor at judging precise geometric properties, like the exact size or raised line drawings, without the help of vision.

The coin just felt too much like a quarter.

It didn't have a distinct enough tactile identity.

We prioritize material over geometry.

That's really interesting.

The text also says we have a special algorithm for perceiving curvature, right?

We do.

It's not as simple as just feeling a bump.

When you run your finger over a curved surface, your brain uses two pieces of information.

The changing slope of the skin that's in contact with the surface and the changing height of your finger in space.

It combines these to calculate the curve.

It's a complex computation that happens instantly and without any thought.

Okay, we have covered a massive amount of ground here.

From the four tiny mechanoreceptors in the skin of the finger, all the way up the spinal highways, into the bizarre, distorted maps of the cortex, and then back out into the world of active exploration.

It really highlights how deeply integrated touch is with everything we do.

It's not just a passive receiver of information.

It's a way of actively asking the world questions.

Is this hot?

Is this heavy?

Is this sharp?

Is this safe?

Is this someone who loves me?

That's the part that really sticks with me.

The connection between those CT fibers, the pleasant touch channel, and the emotional centers of the brain.

We are fundamentally wired to be touched.

It's not a luxury.

It's a survival need.

Absolutely.

Ian Waterman showed us we need touch to move.

The CT fibers show us we need touch to bond and feel safe.

And the phantom limb studies show us that our brain is constantly looking for that input.

So much so that it will create it if it has to.

It really makes you rethink where you end and the world begins.

That is the ultimate question, isn't it?

If I can incorporate a tool into my body image, or a simple rubber hand, or a complex bionic limb, and if I can project my presence and my sense of self into a car, or even a video game controller,

well then the boundary of the self is much more fluid and strange than just our skin.

A fluid self in a very tactile world.

I love that.

It's a powerful thought to leave on.

Thank you so much for breaking all of this down with us.

This has been a truly fascinating journey into the proximal sense.

My pleasure.

Touch is really the anchor of our reality.

And thank you, listener, for tuning in.

This has been the Deep Dive.

Thank you from the Last Minute Lecture team.

We'll catch you on the next one.