Chapter 14: Olfaction: The Sense of Smell

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You know, we spend so much time obsessing over our screens.

We talk endlessly about vision, about high resolution, about frame rates.

Oh, absolutely.

We talk about audio fidelity, noise cancelling headphones, the whole immersive soundscape thing.

Right.

But there's this whole other data stream flooding into your brain every single second of the day.

And for the most part, I mean, unless something goes wrong, you are completely ignoring it.

Until you smell smoke.

Yeah.

Or a gas leak.

Or perhaps more pleasantly, that specific perfume an ex -partner used to wear that suddenly hits you on a crowded street.

Right.

And then in an instant, it's the only thing in your brain.

It feels like it bypasses everything else.

It's immediate, almost intrusive.

It really is.

Today, we are deep diving into that exact sense.

We're tackling olfaction.

We're working off a serious stack of research here, specifically focusing on a comprehensive chapter by chapter breakdown of chapter 14 from Sensation Reception Sixth Edition.

A fantastic tech.

It is.

And I have to be honest, I went into this reading thinking, okay, smell, that nice to have sense.

It makes wine taste good.

It make flowers nice.

But is it, you know, essential?

A lot of people fall into that trap.

I mean, even Freud thought it was a sense we needed to repress to become civilized, you know, moving away from the ground and up toward the stars.

I had no idea he thought that.

Yeah.

But the research paints a totally different picture.

This isn't just about enjoying a glass of Pinot Noir.

This is ancient, ancient hardware.

We're talking about the very first way life interacted with the universe.

That's really the mission for this deep dive, isn't it?

We need to strip away the idea that smell is just some background app running on our operating system.

That's a great way to put it.

If you go back to the primordial soup,

I mean, way back, before eyes, before ears, there was this simple chemical imperative.

You're a single cell.

You're floating there.

Right.

You need to answer two fundamental questions.

Is that blob over there going to feed me or is that blob going to kill me?

Approach or avoid.

That is the binary code for survival.

And here we are billions of years later, sophisticated mammals with smartphones and 401ks.

But that ancient software, it's still running for sure.

Every time you recoil from a trash can or lean in to smell a freshly baked loaf of bread, you are reenacting that single cell drama.

You are chemically sampling the environment to decide your next move.

So before we get into the, you know, the brain surgery part of this, and we will because the anatomy here is absolutely wild.

We need to clear up the terms.

Yes.

This is so important.

Because I feel like I use taste and smell interchangeably in my daily life.

And the authors of this text are very, very strict about this distinction.

They have to be.

It's arguably the biggest misconception in all of sensory science.

We misuse the word taste constantly.

Okay.

So lay it out for us.

Olfaction versus gustation.

Olfaction is the technical term for the sense of smell.

So the detection of molecules floating in the air.

Gustation is the sense of taste detecting molecules that are actually entering your mouth.

But here is the catch.

And it's a big one.

Gustation is incredibly limited.

We're talking just the basics, right?

Like what you learn in elementary school.

Exactly.

Sweet,

sour, salty, bitter, and umami.

That's it.

That is all your tongue, your taste buds can really tell you.

Everything else you think of as taste is actually smell.

So if I'm eating a strawberry jelly bean.

The sweetness.

That's your tongue.

That's gustation.

But the strawberry -ness, the fruity, floral, almost red flavor you perceive.

That is all specific molecules traveling up the back of your throat and hitting your nose from the inside.

Ah, this is the concept of retronasal olfaction.

Right.

Most people think of smell as orthonasal, sniffing through the nostrils.

You see a flower, you lean in, you inhale, the air goes up.

That's orthonasal.

Stated sniffing.

But retronasal is when you are chewing, swallowing, and breathing out through your nose.

The volatile chemicals from the food go up the, let's call it the back door, into the nasal cavity.

Which explains the hold your nose trick.

Precisely.

If you hold your nose and eat that jelly bean, you cut off the airflow, you lose the retronasal olfaction.

And what happens?

You don't taste strawberry anymore, you just get sweet blob.

It's basically just sugar texture at that point.

And this explains so much.

Like why food tastes like cardboard when you have a bad head cold.

You haven't lost your sense of taste, your tongue is fine.

You've lost your retronasal smell because your nose is all blocked up.

But there's a third player here too, right?

Because when I eat a chili pepper, or if I smell like strong ammonia, it doesn't feel like a smell or a taste.

It hurts.

It has a physical quality to it.

That is the trigeminal system.

It's mediated by the fifth cranial nerve.

It's not smell or taste in the traditional sense, it's the feel of a scent.

The feel.

Like texture.

Sort of, but more like a physical sensation.

Think about the cool snap of menthol or peppermint.

That cold feeling isn't a temperature drop, it's a chemical tricking your trigeminal nerves into signaling cold.

Okay, that makes sense.

Or the burn of chopping onions that makes your eyes water.

That isn't your olfactory nerve, that's the trigeminal nerve screaming irritation or pain or temperature change.

So the spicy burn of a jalapeno is actually a pain response.

My brain is interpreting that chemical as pain.

Technically, yes.

The trigeminal system fuses with the olfactory experience to give us a holistic sensation.

When you say a curry smells hot, you are referencing the trigeminal sensation, not just the aroma.

It's fascinating how the brain just stitches all these inputs, the tongue, the nose, the pain receptors, into one seamless experience we call eating.

It's an incredible integration.

Okay, so we have our definitions, we know what we're talking about.

Today, we are going to follow the roadmap of the chapter exactly.

We're going to start with the hardware, the physiology of the nose.

The nuts and bolts.

Then we'll look at how humans stack up against other animals.

We'll trace the wiring into the brain, look at the genetics, and finally get into the really weird stuff behavior, memory, and language.

It's a fascinating journey, moving from a simple molecule floating in the air to a complex emotional memory in the brain.

So let's start with that stuff floating in the air, the source code of smell.

We call them odors, but the chapter makes a pretty sharp distinction between odor and odorant.

It's the classic sensation versus stimulus distinction you see in all of sensory science.

So what's the difference here?

An odor is the psychological experience, like the smell of fresh -cut grass.

That is what happens in your head.

The odorant is the actual chemical compound, the specific molecule itself that's floating in the room.

But not everything is an odorant, right?

I can't smell my laptop.

I can't smell the oxygen in this room.

I can't smell the glass of water on the table.

Exactly.

The air is basically a blank canvas filled with things we can't detect.

To be an odorant, to be something the human nose can register, a molecule has to meet three very specific physical criteria.

Okay, what are they?

First, it must be volatile.

Meaning it floats?

Essentially, yes.

It has to be able to turn into a gas at room temperature and float in the air.

If it stays solid or liquid and can't fly, you can't sniff it.

It has to physically enter your nose.

Makes sense.

Yes.

If it's stuck to the table, it's not getting at my nostrils.

Second, it has to be small.

We're talking atomic weight here.

The textbook says it needs to be between about 25 and 300 daltons.

And if it's too big?

If it's too heavy, like a protein, it's just too big and clunky to interact with the receptors.

It won't fit the lock.

Okay.

And the third one?

And third, it must be hydrophobic.

Hydrophobic meaning water -fearing.

Right.

It repels water.

This is crucial because the mucus in your nose is mostly watery.

The odorant molecule needs to be able to partition out of the air and be willing to interact with the lipid -heavy or fatty cell membranes of the receptors.

It can't just dissolve in the mucus and disappear.

So volatile, small, and hydrophobic.

That's the checklist.

That's the price of admission to your nose.

Okay.

But here is where it gets really interesting.

Yeah.

There are things that meet all those criteria.

Small, volatile, hydrophobic, but we can't smell them.

The text brings up the methane exception.

This is a classic example of evolutionary redundancy.

Methane and carbon monoxide are small, volatile, and hydrophobic.

By all the laws of physics and chemistry, we should act like they smell.

We should have receptors for them.

We should.

But we don't.

We don't.

And it's terrifying because they can kill you.

Carbon monoxide is the silent killer for this very reason.

So why?

Why the blind spot?

It is.

But think back to that single -celled organism or our early ancestors living on the savanna.

In nature, finding high concentrations of pure methane or carbon monoxide was incredibly rare.

It almost never happened.

So there was no pressure to learn to smell it?

There was no advantage to it?

Exactly.

There was no evolutionary pressure to develop and maintain receptors for them because they weren't a common threat in the wild.

You don't evolve a lock for a key you never encounter.

So because our ancestors didn't need to avoid gas leaks in their caves, we have a biological blind spot.

Precisely.

That's why gas companies add a chemical called tertiary butyl mercaptan to natural gas.

That's that distinct rotten egg smell.

That's the one.

We are naturally very, very sensitive to sulfur compounds because in nature, sulfur usually means rotting biological matter.

Which we definitely need to avoid.

Right.

It's a strong avoid signal.

So we had to hack our own sensory system to stay safe in the modern world.

We piggybacked a safety warning onto an ancient disgust response.

I love that.

It's biohacking at a societal level.

Yeah.

Okay.

Let's talk about the nose itself.

Because when I look in the mirror, I just see two holes.

But the text describes the nose as this incredibly complex air conditioning unit.

It's an engineering marvel, truly.

And its primary function isn't actually smelling.

It's protecting your lungs.

That's job one.

How does it do that?

It filters, it warms, and it humidifies the air you breathe.

Inside, you have these bony ridges called turbinates.

Which sound like a part of a jet engine.

They function similarly.

They act like spoilers or baffles.

They create turbulence in the air you breathe.

If the air just shot straight back to the lungs, it wouldn't hit the sensors effectively.

So it swirls the air around?

The turbinates spin the air around so that a small puff of it rises upward to the very top of the nasal cavity.

And that little space is called the olfactory cleft.

And that's where the good stuff is.

That's where the retina of the nose lives.

The olfactory epithelium.

The olfactory epithelium.

That sounds like a kingdom in a fantasy novel.

It might as well be.

It's a yellowish patch of mucous membrane, only about the size of a postage stamp, tucked away about two and a half inches up from the nostril.

This is where the magic happens.

It's the interface between the outside world and the brain.

And the text says it contains three specific types of cells.

Right.

Let's break them down.

Go for it.

First, you have the sustentacular cells.

These are the supporting cast.

They provide metabolic and physical support for the main neurons.

They're like the roadies for the rock band.

Okay, keeping everything running.

Then you have basal cells, which are incredibly important.

They're sem cells.

Their job is to create new neurons throughout your life.

And finally, the stars of the show.

The olfactory sensory neurons, or OSNs.

These are the actual detectors.

They are small neurons located right in the mucous layer.

And they have little hairs on them.

They do.

They have these hair -like protrusions called cilia on their dendrites.

And on those cilia are the receptor sites, the locks waiting for the chemical keys.

And the mechanism here is pretty complex, right?

It's not just a button being pressed.

It's not a simple mechanical thing.

No, it's a very elegant biochemical cascade.

The receptors are what we call G -protein coupled receptors, or GPCRs.

And that's a huge family of receptors in the body, right?

The biggest.

Imagine the receptor on the neuron is a lock with a very specific, intricate shape.

The odorant molecule, the thing floating in the air, is the key.

It floats in, clicks into the lock, and boom.

What is that boom?

What happens inside the cell?

It triggers a G -protein inside the cell, which then sets off a chain reaction, an amplification cascade.

This opens up ion channels, letting charged particles like sodium and calcium flow into the cell.

And that changes the electrical charge.

Exactly.

It depolarizes the cell, and if it reaches a certain threshold, it fires an action potential, an electrical signal that travels up the axon to the brain.

And there's a rule mentioned in the text that I found really helpful for visualizing this whole wiring diagram, the one -to -one -to -one rule.

It's the golden rule of olfactory wiring.

It simplifies what could be an impossibly complex system.

So what's the first part of the rule?

Rule part one.

Each OSN expresses only one type of odorant receptor.

It's a specialist.

It only looks for one kind of chemical feature.

OK, so a vanilla detector only cares about vanilla -like molecules.

It's not also looking for lemon.

Roughly, yes.

It's highly specialized.

And rule part two.

All the thousands of OSNs that have that same receptor type, no matter where they are in the epithelium, project their axons to the same specific spot in the brain.

A spot called a glomerulus.

A glomerulus, yes.

It's a spherical bundle of nerve endings in the olfactory bulb.

So if I have, say, a thousand different neurons all over my nasal epithelium that are detecting minty molecules, they all wire to the single minty collection point in my brain.

Exactly.

Think of it like a massive male sorting room.

You have thousands of letters coming in from all over the city, which is the nose.

But every letter addressed to Minty Avenue gets routed to the exact same bin, which is the glomerulus.

It creates a physical spatial map of smells in the brain.

But to get from the nose to that sorting room, those nerves have to pass through a very dangerous bottleneck.

The text calls it the cribriform plate.

This part always makes me wince a little bit.

The cribriform plate is a thin perforated piece of bone right between your eyes, behind the bridge of your nose.

It separates the nasal cavity from the brain.

And the book says it looks like a sieve or a cheese grater.

That's a great description.

It's full of these tiny little holes.

And the axons of the OSNs have to thread their way through these holes to get to the olfactory bulb on the other side.

They do.

Which is fine under normal circumstances.

But it becomes a huge liability if you get hit in the head.

Imagine the brain shifting inside the skull during a hard blow like in a car accident or a boxing match.

The bone stays still, but the soft brain moves.

And those delicate little nerves that are threaded through the bone.

They act like they're in a guillotine.

They get sheared right off the plate.

And that causes an osmia.

Smell blindness.

A complete and total loss of the sense of smell.

Now the good news is that those OSNs, because of the basal cells,

usually regenerate.

They are one of the few neurons in the body that do.

But there's a catch.

There's a catch.

If the cribriform plate is fractured or damaged, scar tissue can form over the holes.

So the door is nailed shut.

The new neurons grow back.

They're ready to connect.

They knock on the door, but they can't get through to the brain.

The smell is gone permanently.

That is a sobering thought.

Yeah.

It really highlights how vulnerable this system is, despite being so ancient and essential.

Now, moving to section two.

Let's talk about capabilities.

OK.

Because I think we have this cultural idea, this myth, that humans are terrible smellers compared to, say, a dog.

That is a very persistent myth.

And the text does a great job of pushing back on it.

It likely dates back to 19th century religious and philosophical writings that wanted to distance humans from base animal instincts.

We wanted to be rational creatures of sight, not sniffing animal.

But the data tells a different story.

A very different story.

Let's look at the numbers.

How many smells can we actually detect?

The text cites a really fascinating, though somewhat controversial estimate, over one trillion distinct odors.

A trillion.

That number seems impossibly large.

It is.

And whether we can actually perceive a trillion distinct sense is debatable because we don't live long enough to smell them all.

I mean, a trillion seconds is about 32 ,000 years.

I don't have that kind of time.

No one does.

But the point is, physically, we have the hardware, the variety of receptors to detect almost any volatile molecule that comes our way.

We have somewhere between 10 and 20 million OSMs.

Which sounds like a lot until you compare it to a dog.

Right.

And that's where the comparison gets tricky.

A dog has at least 100 times more OSMs.

Some breeds have many more than that.

So it's a numbers game.

It's a density and sensitivity game.

Because of that sheer density of receptors, they can detect odor concentrations nearly 100 million times lower than we can.

So while we can smell the same types of things, they can smell a single drop of it in an Olympic swimming pool.

We need a whole bucketful.

But the text mentions an animal that puts even the bloodhound to shame.

The elephant.

The extreme sniffer of the animal kingdom.

It's not even close.

Elephants have approximately 2 ,000 functional olfactory receptor genes.

And how does that compare?

It's five times more than humans have.

It's twice as many as dogs have.

And they use it in these incredible ways.

The example about the ethnic groups was wild.

It's a matter of life and death for them.

In their habitat in Kenya, they encounter the Maasai people, who traditionally hunt elephants and demonstrate virility by spearing them.

So a major threat.

A major threat.

They also encounter the Kamba people, who are primarily farmers and pose no threat.

Elephants can distinguish between a Maasai man and a Kamba man just by scent.

They can smell their clothes, their diet, their genetics, and know which one is dangerous.

That is unbelievable differentiation.

Yeah.

It's not just human.

It's dangerous human versus safe human based purely on smell.

It gets even better.

Researchers did this other study with Asian elephants.

They had these buckets of sunflower seeds, but the buckets were covered with just tiny holes for the scent to escape.

The elephants could smell the difference in quantity.

They could consistently tell which bucket had more seeds.

Just by the amount of scent coming out, they were performing better than dogs on similar tasks.

They're doing math with their noses.

That's incredible.

It really is.

Okay.

So elephants win.

But the chapter also goes to great lengths to debunk the idea that humans are useless.

There was this chocolate tracking study.

This is one of my favorite experiments because it sounds like a prank on the participants.

Researchers went out into an open grass field and dragged a pheasant carcass to create a scent trail for dogs to follow.

Classic science behavior.

And then they did the same thing with a string dipped in chocolate essential oil to create a human -friendly trail.

And they got people to track it.

They blindfolded human participants,

put them on their hands and knees, and told them to follow the chocolate smell across the field.

Did it work?

They did it.

And not only that, the tracking pattern, the way the humans moved their heads back and forth, casting across the trail to find it again when they lost it,

was strikingly similar to how dogs track pheasants.

And with practice, they got faster and more accurate.

So we can do it.

We just usually don't have to hunt for chocolate on all fours.

Exactly.

We have the latent ability.

We also have stereo smell.

The tiny time difference in an odor hitting your left versus your right nostril gives you directional cues.

We can navigate a room using odors, almost like a homing pigeon.

And what about our brains?

Is our olfactory bulb just tiny and atrophied?

Not at all.

Structurally, the human olfactory bulb is actually quite large in absolute terms.

Physically larger than a mouse's, right?

The text made that point.

Yes.

Even though it takes up a smaller percentage of our total brain volume than a mouse's does, because our visual cortex is so enormous,

the sheer number of neurons in the bulb is relatively conserved across mammals.

We aren't biologically deficient.

We just allocate our brain real estate differently.

So we're not bad smellers.

We're just distracted smellers.

That's a very fair way to put it.

We prioritize vision, but the olfactory capability is sitting there under the hood waiting to be used.

Let's move to section three then and talk about how this wiring actually works in the brain.

We mentioned the axons go through the cribriform plate.

Where do they go next?

They bundle together to form the olfactory nerve, that's cranial nerve eyes, and they enter the olfactory bulb, which sits right on the underside of the frontal lobe.

And you said something interesting earlier about how it's wired differently from other senses.

Yes, this is a key difference.

Vision, hearing, touch, they're all primarily contralateral.

The right eye maps to the left brain hemisphere.

The right hand maps to the left motor cortex.

But smell.

Smell is ipsilateral.

The right nostril connects to the right olfactory bulb.

The left nostril connects to the left olfactory bulb.

There's no crossing over, at least not initially.

That's a strange evolutionary quirk.

And once inside the bulb, we hit those glomeruli again.

Those spherical conglomerates of axons.

And here's where the humans are actually pretty complex.

Argument gets more ammo.

How so?

Mice are the gold standard for smell research, right?

They live and die by their noses.

A mouse has about 3 ,600 glomeruli in its olfactory bulb.

And humans, how many do we have?

About 5 ,500.

Wait, wait, we have fewer functional receptor genes than mice, but more glomeruli.

That seems backwards.

It is.

Almost double the number of these processing hubs.

This was a huge surprise to researchers like Charles Greer at Yale.

It suggests our wiring is much more complex and convergent.

We might be doing more sophisticated processing with less raw input.

So it's a cautionary tale about

assuming human biology is just mouse biology, but worse.

Absolutely.

It's a different strategy.

So inside the bulb, it's not just a path through station.

The text describes several layers of neurons.

It's a sophisticated processing center, not just a relay.

You have the juxtaglomerular neurons on the surface.

They're broad responders, kind of noisy.

Then you go deeper to the tufted cells, which have sharper responses.

Deeper still are the metral cells, which are very, very specific responders.

And there's an inhibitory layer.

Yes, the granular cells form this massive inhibitory network.

They're responsible for lateral inhibition, which is a process that sharpens the contrast between different smells.

So what's the point of all those layers?

What's the end result?

It's all about sharpening the signal.

It's about feature detection and contrast enhancement.

The bulb is filtering and organizing the chaos of the chemical world into a clean, coherent signal that the rest of the brain can understand.

And where does that clean signal go?

This is the so -what of the anatomy section.

It goes primarily to a region called the primary olfactory cortex, also known as the piriform cortex.

But, and this is absolutely critical, it also has a direct one -stop shop connection to the amygdala hippocampal complex.

The limbic system.

The emotional and memory center of the brain.

This is totally unique.

Every other sense vision, hearing, touch has to go to the thalamus first.

The thalamus is like the brain's central switchboard operator.

It directs traffic.

But smell gets to bypass the operator.

Smell has a VIK pass straight to the amygdala and hippocampus.

It doesn't have to check in.

This explains why a smell can trigger an emotional memory so instantly and powerfully before you even know what you're smelling.

Exactly.

The emotional response comes online before the conscious identification.

Evolutionarily, this makes perfect sense.

The night hunting theory suggests that early mammals needed massive olfactory capacity to survive the age of dinosaurs by hunting at night.

So our brains evolved around smell.

This need to process complex smells precipitated the evolution of the mammalian brain.

Our big brains, in many ways, are basically built on top of a smell processor.

We think we're these rational thinking creatures, but we're just smelling creatures with a giant neocortex bolted on top.

In many ways, that's not far from the truth.

And after that initial processing, the signal also goes to the orbitofrontal cortex, or OFC.

That's the secondary olfactory cortex.

And what happens there?

That's where the conscious perception happens.

The moment you think, oh, that is strawberry.

It's also where we judge pleasure and displeasure.

The OFC decides if a smell is good or bad.

Now, despite this direct connection to the limbic system, the text says smell is a slow sense.

Glacially slow compared to vision or hearing.

It takes about 400 milliseconds, almost half a second, for a smell to register and be processed in the brain.

Vision takes about 45 milliseconds.

Why the huge lag?

It's a few things.

The axons of the OSNs are very thin and they're unmyelinated, which means they don't have the fatty insulation that helps signals travel fast.

Plus, there is this sensation versus perception lag.

You might physically register the chemical quickly, but for it to bubble up into your conscious awareness takes time.

Smells tend to emerge into consciousness rather than flashing like a strobe light.

Let's pivot to section four,

genetics.

Because if we're talking about how we smell, we have to talk about Buck and Axel.

The rock stars of all faction, Linda Buck and Richard Axel.

They won the Nobel Prize in 2004 for their work.

What did they discover?

They discovered the massive gene family that codes for all our odorant receptors.

It is the largest single gene family in the entire mammalian genome.

It's a huge chunk of our DNA.

But not all of them work, right?

The book mentions pseudogenes.

No, not all of them work.

We have something called pseudogenes.

These are genes that are present on the chromosome.

You can see them there.

But they have mutations that mean they don't produce a functional protein.

They're broken.

So how broken are we?

Well, dogs have about 25 % of their olfactory genes as pseudogenes.

Humans.

It's about 52%.

So more than half our smell genes are just junk.

Junk might be a harsh word.

Some research suggests they might play a regulatory role.

But functionally, yes, we have fewer working receptor types than many other mammals.

But here is the kicker.

The specific pattern of which genes are broken and which ones are working varies dramatically from person to person.

Everyone has a unique nose.

Literally, at the genetic level, two people can sniff the exact same molecule and have a completely different sensory experience because of their genetic makeup.

This leads to what we call specific anosmia.

The text gave some great examples here.

Let's start with androstenone.

Ah, the classic.

This is a steroid molecule found in things like armpit sweat and famously in male pigs, so it's in pork.

It's a perfect example of genetic variability.

So what are the different reactions?

To about a third of the population, it smells like nothing.

They are genetically anosmic to it.

To another third, it smells like a sweet musk or sandalwood, something pleasant.

And to that final third?

Urine.

An intense, unpleasant urine smell.

Imagine cooking a pork chop and smelling nothing but a dirty bathroom.

That isn't you being a difficult eater.

That is your genetics.

You can't learn to change that perception easily.

It's hardware.

And the cilantro debate.

I feel like relationships have ended over this.

This is the taste like soap thing.

It's a specific genetic mutation in an olfactory receptor gene.

If you have it, you can't detect the pleasant, herbal, floral note of cilantro.

All you get is the aldehyde component, which normally is hidden, but on its own, smells distinctly soupy.

So when someone says cilantro tastes like soap, they aren't being picky.

No, they are accurately reporting their sensory reality.

For them, it literally tastes like soap.

And then there's asparagus urine, which the book explains is a double whammy of genetics.

It is.

It's so interesting.

First, there is a genetic variation whether your body produces the sulfur compound, the metabolite, in your urine after eating asparagus.

Not everyone's digestive system does that.

So that's variable what?

Then there is a completely separate genetic variation in whether you have the receptor to be able to smell it.

So you could be a producer but not a smeller, or a smeller but not a producer.

Or both.

Or neither.

If you don't think asparagus makes peas smell funny, you might just be anosmic to that specific metabolite, even if you're producing it.

These genetic differences also affect our behavior, right?

Yeah.

Specifically with food intake.

Yes.

The text mentions that gene copy number matters.

If you have more copies of a receptor for, say, the banana scent in a cream pie, you will perceive that banana flavor as more intense.

And what's the consequence of that?

Intense smells often act as a satiety signal.

They help tell your brain you've had enough.

If you perceive the smell as weak because you have fewer gene copies, you might eat more of the pie because your brain isn't getting that I'm full signal as quickly or as strongly.

So my genetics might be making me eat more dessert.

Let me use that excuse.

It's a valid contributing factor.

Moving on to section five.

We've got the chemicals, we've got the receptors.

But how does the chemistry turn into perception,

into the feeling of rows?

There are theories.

The dominant one, the one with the most evidence, is shape pattern theory.

It's the lock and key model we mentioned earlier.

The molecule's 3D shape determines which receptor it can bind to.

But it's not just one key for one lock, you said.

No, it's combinatorial, and that's the key to its power.

One odorant molecule, because of its different parts, can activate multiple different types of receptors.

And one type of receptor can bind with multiple different odorants that share a similar feature.

So it's the overall pattern.

It's the pattern of activation across the whole array of hundreds of receptor types that tells the brain, this is a rose or this is a skunk.

It's like a chord on a piano, not a single note.

But there's a challenger theory, vibration theory.

The alternative, championed by Luca Turin.

The idea is that the nose isn't checking the shape of the molecule at all.

He thinks the nose is actually a tiny biological spectroscope.

A spectroscope, what does that mean?

He thinks it's listening to the vibrational frequency of the atomic bonds within the molecule.

That sounds like science fiction.

Like my nose is listening to the hum of a molecule.

It's a wild idea.

The argument is something like this.

Hey, these two molecules have totally different shapes, but they both smell like citrus.

Why?

Oh, look, they both have bonds that vibrate at the same frequency.

It's a compelling correlation.

It is.

But the shape theorists have a trump card, a piece of evidence that vibration theory just completely breaks its teeth on.

And that would be stereoisomers.

Stereoisomers, these are mirror image molecules.

Take a molecule like carvone.

It has a right -handed version and a left -handed version.

They're called decarvone and l -carvone.

Like looking at your hands.

Same fingers, same bones, just flipped.

They're non -superimposable.

Exactly.

And because they are made of the exact same atoms connected in the same way, their bonds vibrate at the exact same frequency.

If vibration theory were true, they should smell identical.

No.

Not even close.

It's the killer evidence.

One, decarvone smells like caraway seeds like rye bread.

The other, l -carvone, smells like spearmint.

That's a huge undeniable difference.

It's undeniable.

And shape theory explains it perfectly.

Try to put your right hand into a left -handed glove.

It doesn't fit.

The overall shape is wrong.

So the spearmint receptor accepts the left -handed molecule, but it rejects the right -handed one.

That feels like the nail in the coffin for vibration theory.

For now, it seems to be.

But it shows just how mysterious this sense still is.

We are still debating the basic physics of how the signal even starts.

And the text also says timing matters too, right?

It's not just which keys are pressed, but the order.

Exactly.

The melody of the smell.

A molecule might hit receptor A, then receptor B a millisecond later as it tumbles through the mucus.

The brain uses that timing sequence as another piece of the combinatorial code.

It's a dynamic system.

Which brings us to section six, mixtures.

Because in the real world, we rarely smell pure single chemicals.

Almost never.

The text says a rose is over 1 ,000 different volatile molecules.

Bacon, one of the most distinct smells in the world, is a mixture of about 150 different volatiles.

And we don't perceive a list of 150 ingredients.

We perceive bacon.

Precisely.

This brings us to the analysis versus synthesis debate.

Can you break that down?

Right.

Analysis is what your ears do.

It's like hearing a chord on a piano.

You hear the C major chord as a whole, but a trained musician can also pick out the individual notes.

The C, the E, the G.

Synthesis is what your eyes do with color.

Red light plus green light looks yellow.

You can't see the red or the green components anymore.

They've been synthesized into a new whole percept.

So a smell, a chord, or a color?

Primarily synthetic light color.

You smell bacon, not 150 chemicals.

But with training, it can become more analytical.

Perfumers and sommeliers can train themselves to pick out individual notes in a complex mixture.

But for most of us, it's a synthetic blob.

And then there's this concept called olfactory white.

This blew my mind when I first read about it.

If you take about 30 or more different odorants of roughly equal intensity, as long as they are chemically diverse and span the smell space.

What happens?

The result is a neutral, generic smell.

And it doesn't matter which 30 you pick.

As long as they are diverse, they all converge on smelling like this olfactory white.

It's the white noise of the nose.

There's also this weird thing called binaural rivalry.

Yes, this is another great example of the brain processing, not just sensing.

If you use a special device to pipe a rose smell into your left nostril and say a marker pen smell into your right nostril.

You don't get a blend.

You don't smell a blend.

You don't smell rosy marker.

You smell rose.

And then after a few seconds, it completely switches to marker, then back to rose.

Your brain alternates attention between the nostrils, just like retinal rivalry in vision.

That is wild.

It shows how much the brain is actively constructing and editing the experience.

Let's get into section seven, psychophysics.

How we measure this stuff.

You mentioned training earlier.

Can we actually get better at smelling?

Yes, absolutely.

The brain is plastic.

The text shows that smell training can even cure some specific anosmias.

If you can't smell endrostinone repeated,

intentional exposure can actually flip a switch in your brain and allow you to perceive it.

So you can train your nose like a muscle.

You're training the brain, really.

The connections become stronger.

But attention is a huge factor in whether you smell something at all.

The text discusses inattentional anosmia.

This is the scary one.

It is.

They did a study where people were doing a very difficult, engrossing visual task on a computer.

While they were completely focused, the researchers pumped the strong smell of coffee into the room.

Most of them didn't smell it at all.

And even worse, after the distraction was over and they were asked if they smelled anything, they still couldn't smell it because they had already adapted to it while ignoring it.

So if I'm watching a really intense movie, the house could be filling with smoke and I might not notice.

It's a real possibility.

Your brain prioritizes the visual load and filters out the olfactory information.

We also have to talk about memory.

The Proustian effect is famous,

where a smell unlocks a memory, but the data actually backs it up in a very specific way.

The memory curve for smell is unique among the senses.

With visual memory, you forget a lot of details very quickly.

The curve drops steeply.

But with smell?

With smell, recognition memory drops a little bit after maybe 30 seconds, but then it flatlines.

It stays nearly horizontal for weeks, months, even a year.

So if I smell a specific perfume today, I'll likely recognize it a year from now, just as well as I would tomorrow.

You'll have that feeling of, I know I've smelled this before.

The recognition is incredibly resilient to the passage of time.

Section 8 connects smell to language, or rather the striking lack of connection.

The tip -of -the -nose phenomenon.

We've all had it, and you smell something incredibly familiar, you know it, you can almost feel the memory attached to it, but you cannot, for the life of you, name it.

It's different from tip -of -the -tongue, though, isn't it?

It is.

With tip -of -the -tongue, when you're trying to remember an actor's name, you often know the first letter, or you know it has three syllables, or it rhymes with cat.

You have access to the words, shaped its phonology, but with tip -of -the -nose.

You have nothing, just the raw feeling of familiarity, no partial information about the word at all.

Why is that?

Why are we so bad at naming smells?

The text suggests it's a brain architecture issue.

As we said, olfaction is processed primarily in the right hemisphere.

Language is heavily dominant in the left hemisphere.

They are physically and neurally disconnected.

So they don't talk to each other well.

In fact, they compete.

Brain imaging shows that the act of trying to name a smell actually suppresses activity in the piriform cortex.

Naming a smell is harder than just smelling it because the language center interferes with the sensory processing.

But the text points out this isn't a human universal.

The Jahai people.

This is a crucial anthropological counterpoint.

The Jahai are a hunter -gatherer group in Malaysia.

And unlike English speakers, they have a rich abstract vocabulary for smells, much like we have for colors.

So they have a word for musty.

More specific than that, they have a single abstract word for the smell of dead branches and the fur of certain animals.

It's not a comparison.

It's a fundamental category.

Just like we have a word for red.

Exactly.

And the reason is cultural necessity.

For them, smell is a survival tool.

They need to smell a tiger or a food source or a coming rainstorm.

We live in a sanitized, deodorized world.

We don't need to smell our enemies, so we've lost the language for it.

It's a use it or lose it cultural trait.

And speaking of weird brain connections, there was the visual time warp experiment.

This one sounds like magic, but it's real.

If you smell an apple while you're looking at a picture of an apple, that picture seems to last on the screen longer than it actually does.

The congruent odor dilates your subjective perception of time for that object.

I wonder if I could use that to make my weekends feel longer.

Just bake apple pies constantly.

It might work.

Worth a shot.

Section 9 focuses on individual differences.

We talked about genetics, but what about age and sex?

The battle of the sexes has a clear winner here, according to the research.

Women, on average, consistently outperform men in tests of odor identification, detection, and discrimination.

Any idea why?

It could be hormonal, but the effect persists after menopause.

Postmortem studies have actually shown that women have, on average, more cells in their olfactory bulbs.

It might just be a baseline biological difference.

And age.

This feels like the bad news section.

It's the bad news section.

Olfactory decline starts to become noticeable in your 50s.

By age 85, a staggering 50 % of the population is effectively anosmic.

Wow.

Why does it drop off so hard?

It's a combination of things.

Cell death in the epithelium starts to exceed the rate of regeneration from the basal cells.

And those little holes in the kerberiform plate can start to close up with age.

But it's also cognitive.

We lose the semantic memory, the ability to name things, which makes identification harder.

This is where the medical application comes in.

Sudden smell loss isn't just about aging.

Right.

It's a canary in the coal mine for neurological health.

Sudden loss is a now famous marker for COVID -19.

But more subtly and for a longer time, olfactory identification deficits have been known to predict the onset of Alzheimer's and Parkinson's, sometimes years before the classic tremors or memory loss start.

The text mentions a new test of odor pleasantness for Parkinson's.

Yes, this is fascinating.

Patients with Parkinson's not only lose their sensitivity to smells, they lose the hedonic response.

They don't find pleasant smells as pleasant anymore.

It's a potential diagnostic tool that's non -invasive, cheap, and could provide a very early warning.

We also have circadian rhythms affecting our smell.

Don't trust your nose first thing in the morning.

Your sense of smell is at its worst between about 2 a .m.

and 10 a .m.

So the bakery actually smells better at noon.

Or you're just more awake and sensitive.

But sleep deprivation is particularly dangerous.

When you're tired, your brain disconnects the brakes, the connection from the orbit of frontal cortex to the insula.

And at the same time, it hypersensitizes the piriform cortex to food smells.

So when I'm tired, junk food smells better and I have less self -control.

That is the precise neurological recipe for the midnight snack and poor food choices.

Finally, let's cover section 10, adaptation,

the bakery effect.

This is something everyone has experienced.

It's called receptor adaptation.

When you first enter a bakery, the smell is overwhelming and amazing.

But after about 20 minutes, you barely notice it.

What's happening?

The receptors that are constantly being bombarded by the bread molecules literally retreat inside the cell body of the neuron to be recycled.

They hide.

So you physically cannot smell the bread anymore.

The hardware has been taken offline.

Temporarily, yes.

Not until you leave and give them time to reset and pop back out.

This is why you can't smell your own house or your own perfume.

It's a feature, not a bug.

It filters out the constant background noise so you can detect change.

So you can smell smoke in the bakery.

Exactly.

You need to be able to smell the new potentially dangerous thing, not the sourdough you've been around for an hour.

Inactive sniffing helps with this process.

Yes.

Sniffing is a motor action.

It's not passive breathing.

A sniff prepares the brain to analyze a new scene.

It changes the airflow and resets how the olfactory bulb processes the incoming information.

If you think you smell smoke, you sniff.

That sniff is an active investigation.

And lastly, cross -adaptation.

This is when smelling one thing makes you less sensitive to a different but similar thing.

If you smell several floral perfumes at a counter, eventually they all start to smell the same because the receptors they share have adapted.

But it's not always a two -way street.

Right.

The text notes that it's often non -reciprocal.

Smelling odor in A might block your ability to smell B, but smelling B first might not affect your ability to smell day at all.

It's messy and depends on which receptors overlap.

This has been an absolute marathon through the nose and into the brain.

It really has.

There's so much to cover.

So let's wrap this up.

We started with a single cell needing to find food or avoid poison.

We ended with a human brain that can time travel via scent,

diagnose diseases before they appear, and get completely confused by a bakery.

It's a journey from the purely molecular to the, you know, almost metaphysical.

We tend to think of ourselves as visual creatures.

Seeing is believing is the old phrase.

But our hardware is anciently fundamentally chemical.

Here's my final thought for the listener.

We live in a world of screens and earbuds.

It's all visual and audio.

But we have this incredibly sophisticated,

deeply emotional chemical detector sitting in the middle of our face and it's largely ignored.

And if we think about the Jahai people who live in a world of rich olfactory information that is critical to their survival, it really raises a question.

What invisible world are we walking through every day, completely blind to, simply because we haven't trained our attention or our language to notice it?

If we paid as much attention to our noses as we do to our screens,

what would we suddenly perceive?

What would we find?

That is the question.

Thanks for listening to this deep dive.

This has been the Last Minute Lecture Team.

Keep sniffing.