The Smell Of Christmas Is Tree Screams

Summary

The Rest Is Science
Episode: "The Smell of Christmas Is Tree Screams" (December 25, 2025)
Hosts: Professor Hannah Fry & Michael Stevens (Vsauce)

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Episode Overview

In this festive episode of The Rest Is Science's "Field Notes," Professor Hannah Fry and Michael Stevens (filling in for Michael Blastland) dive into the surprisingly twisted science behind some beloved holiday traditions, tactile bodily illusions, and the quirky potential of elemental baby names. Recorded as a special for Christmas Day, the conversation gleefully balances playful banter with deep dives into evolutionary biology, neuroscience, and social perception. The main throughline centers on what Christmas trees are actually "saying" when they perfume our homes, using their scent as a jumping-off point to explore evolutionary strategies, human senses, and the difference between mental maps and reality.

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Key Discussion Points & Insights

1. Why Christmas Trees Smell the Way They Do

(Starts ~04:07)

• Rowan, a listener, asks: "Why do Christmas trees smell the way they do? Is there an evolutionary reason for producing those chemicals?"
• Hannah Fry explains:
  • The iconic scent from Christmas trees comes from a group of molecules called terpenes, specifically pinene.
  • The smell is not intended to make humans feel festive:

    "Basically the smell of fear, Michael, that's essentially what we are, what we’re smelling." (06:21)

  • For trees, this smell acts as a defense mechanism:
    • It is toxic or deeply unpleasant to insects and fungi—for pine beetles and other pests, it’s “chemical warfare.”
    • The scent is a damage response: when branches are snapped or needles crushed, more pinene is released.
    • There is evidence that trees may use these chemicals to communicate warning signals to other trees.
  • Ultimately:

    "What you’re doing then is you’re...sort of smelling the tree screaming, you know?” (07:01)

• Michael Stevens (Vsauce) responds:

  "It's a wonderful, festive smell, but it's the scream of a tree." (08:00)

Festive Evolution & Human Resonance

• Fry elaborates that humans may be evolutionarily primed to find pine scent appealing, as ancient humans would associate it with shelter, safety, and lack of insects.
• Michael:

  "It really is literally the smell of safety." (11:25)

• Hannah adds:

  "Yeah, and screams!" (11:29)

Flammability: An Evolutionary Fire Strategy

• Fry reveals that the same chemicals making pine “scented” also make them highly flammable:

  "It feels really counterintuitive...if you have a low-level fire in a forest, one that is very quick and runs through, then it will just clear out, like, competition...So in evolutionary terms, being a little bit explodey is actually quite a good long-term strategy." (09:54)

• Michael sums up:

  "That's a good life lesson." (10:34)

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2. Elemental Baby Names

(Starts ~11:34)

• Inspired by a listener's encounter with someone named Xenon, Fry and Michael riff on which elements from the periodic table would make the best human names.
• Michael’s take:
  • Names like Xenon, Neon, Tungsten, and Indium are fun, but remarks some might be "a bit much."
  • Suggests an element named after John Quincy Adams to finally integrate "J" and "Q" into the periodic table:

    "If we...could fix that in one fell swoop by naming an element after John Quincy Adams...Let’s get some J's and Q's in there and then the whole alphabet will be represented." (13:20)

• Hannah’s playful British alternative:
  • Using regional accents, elements like “Helium” become “Hey, Liam,” and “Argon” as “Are Gone.”

    "Periodic table as British regional accent pet names. I think we're already there." (14:21)

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3. Tactile Illusions & The Brain’s Map of the Body

(Starts ~18:36)

The Upside-down Tongue Illusion

• Michael leads listeners through a "hands-on" experiment:
  1. Twist your tongue upside down (manually or with fingers).
  2. Touch the tip from left to right.
  3. You can't accurately sense which side is being touched.

  "There's actually an illusion happening here which is that you can't locate which side of your tongue is being touched." (20:08)

  • Brain is unaccustomed to this configuration and projects touch sensations inaccurately.

Aristotle’s Finger Illusion

• Cross your pointer and middle finger and touch an object between the tips.
  • Feels as if two objects are present due to brain misinterpreting the now-adjacent "outer" edges.

    "It's called Aristotle's Illusion. And it's very fun to cross your fingers and just touch things...It feels like two very separate things when it’s really just one." (22:48)

The Warped Map of Sensation

• Hannah shares a British playground trick: tracing a finger up someone’s arm with their eyes closed, they’ll think you reach their elbow much sooner because the brain “stretches” sensation at different rates across the body.

  "Your mental map is warped. It's a bit like a London tube map in a way. You know, very good for navigation but terrible for accuracy." (25:54)

• Michael likens these moments to the jolt of an unmoving escalator: the brain’s “prediction” clashes with experience.

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4. The Bayesian Brain & Perceptual Plasticity

(~26:34 onward)

• Fry and Michael discuss the “Bayesian brain”—the idea we are always predicting and modeling the world based on incoming sensory and prior information, never accessing “reality” directly.

  "We're not actually a body. We're sort of...processing engine that's locked inside a dark, noiseless void inside our skulls." (26:43)

• Michael:

  "At night...close your eyes in the dark and think about how much darker it is in your head. Everything you're hearing is all just your brain putting that show on for you." (27:19)

Phantom Limbs, Remapping, and Technological “Vision”

• Hannah describes her cousin’s and Admiral Nelson’s experiences with phantom limbs—they felt sensations in limbs no longer present.
  • Nelson considered this evidence of the soul.
• Fry recounts an experiment where the brains of newly blind individuals "remap" visual input delivered via tactile signals on the tongue. Over time, they report “seeing” through their tongues.

  "You could, for example, throw a ball at somebody who no longer had vision but was wearing this tongue plate. And they could...see the ball coming towards them." (34:19)

• Takeaway: The brain can adapt its maps, given new input.

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5. Memorable Quotes & Moments

• Hannah Fry: "Basically the smell of fear, Michael, that's essentially what we are, what we're smelling." (06:21)
• Michael Stevens: "It's a wonderful, festive smell, but it's the scream of a tree." (08:00)
• Hannah Fry: "Periodic table as British regional accent pet names. I think we're already there." (14:21)
• Michael Stevens: "Our mental map in our head is the correct one. The problem is everything else." (31:06)
• Hannah Fry: "Reality is just the hallucination we all agree on." (32:04)

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Important Timestamps

• 04:07 — Introduction of the Christmas tree smell question
• 06:21 — The smell of Christmas trees as “tree screams"
• 09:54 — Evolutionary strategy: explosive, flammable trees
• 11:34 — Elemental baby names discussion
• 18:36 — Tactile illusions: The upside-down tongue
• 22:21 — Aristotle’s Illusion (crossed fingers)
• 25:54 — How the brain warps the body map
• 26:34 — Discussion on “the Bayesian brain”
• 34:19 — Remapping vision via the tongue in blind individuals
• 35:02 — The adaptability and plasticity of the brain

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Tone & Style

• Playful and witty, with a penchant for deadpan British humor and Vsauce-style curiosity.
• Rich metaphors (e.g., London tube map for the body’s perception).
• Inclusive: Both hosts encourage listeners to participate in the tactile experiments and invite audience questions for future episodes.
• Seriously silly: Deep dives into neuroscience follow jokes about “tree screams” and “explosive personalities.”

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Conclusion

This episode connects nostalgic Christmas scents to the realities of evolutionary warfare, uses bodily illusions to unpick the relationship between our brains and the world, and rounds things off with joyful speculation about baby names from the periodic table. The big message: reality is more fragile, weird, and wonderful than we think—and it all starts with questioning the things we take for granted, even on Christmas Day.

Transcript

[Hannah Fry]

This episode is brought to you by Cancer Research UK.

[Michael Blastland]

Dinosaurs walked the Earth 180 million years ago. But did you know cancer was part of their story too? Scientists have found tumors in ancient fossils.

[Hannah Fry]

Well, that is part of the reason why cancer is a big, big part of our story, right? It's the other side of evolution. It's the most complex disease that we face. There are more than 200 types of cancer in total, each with distinct characteristics, challenges and mysteries.

[Michael Blastland]

And that complexity demands scale. Cancer Research UK is the world's largest charitable funder of cancer research, with more than 4,000 scientists, doctors and nurses working across more than 20 countries in the search for answers and then sharing their discoveries beyond borders.

[Hannah Fry]

And the impact of this collaboration is clear because over the last 50 years, the charity's pioneering work has helped to double cancer survival in the uk. That, that is more people who are living longer, better lives.

[Michael Blastland]

Fossils can show us the past, but research is shaping the future. And for more information about Cancer Research uk, their research breakthroughs and how you can support them, visit cancerresearchuk.org REST ISscience.

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[Michael Blastland]

Hello and welcome to the Rest is Science. Merry Christmas. This is Field Notes, a show where we do some little exploring of objects, of ideas, of questions.

[Hannah Fry]

I never felt more festive in my life than when you just said merry Christmas then Michael. That was. That was jingling all the way.

[Michael Blastland]

That was, I'm slowly becoming Santa Claus if you can't tell.

[Hannah Fry]

Slowly, slowly. Year by year. 1 1. 1 gray hair at a time. Yeah. This is the show where, you know, every week we're gonna bring in something, we're gonna chat about it. So it's sort of like the rest of science's very own version of show and Tell. That's, that's what we bring you once a week, every Thursday.

[Michael Blastland]

Yeah, that's right. We're sort of building up a nice little library of curiosities, both mental and physical.

[Hannah Fry]

So to add to those curiosities, we would also like your questions, your, your theories, your, your thought experiments, whatever you want to send us, really. You can send them in to our email therestiscienceolehanger.com and. Yeah, and we'll, we'll dust off a shelf in our, in our infinite libra.

[Michael Blastland]

Of curiosities and we'll add you to it later in the episode. I am going to be showing off the object that I have brought. And today my object is your body, Hannah.

[Hannah Fry]

How dare you.

[Michael Blastland]

And it's also you, the listener's body. It's all of our bodies. We're going to do some, some tactile stuff, but first I want to start with questions from you, all the listeners out there. I want to begin with a question from Rowan. Because it's just so perfect, perfectly like festive.

[Hannah Fry]

Because it is Christmas Day. This is coming out on Christmas Day, right? I mean, we were recording this a couple of days ago.

[Michael Blastland]

I mean, it is Christmas Day.

[Hannah Fry]

We're cheating. We're warping time.

[Michael Blastland]

Is the show not live?

[Hannah Fry]

You are listening to a future version of yourself, Michael. That's, that's, that's what you're doing.

[Michael Blastland]

I wonder what I'll be like. Rowan asks, why do Christmas trees smell the way they do? Is there an evolutionary reason for producing those chemicals?

[Hannah Fry]

Absolutely. There is, Rowan. And it's not to make you feel more festive. I should just say in advance of this, I decided finally last year to switch to an artificial tree. Are you an artificial tree or a.

[Michael Blastland]

Yeah, I'm an artificial tree guy. I just, I'm not against cutting down trees, but I feel bad personally, like, it's on my conscience. I know we've got a lot of trees. I know that they can be sustainably grown. You know, I'm not anti live trees.

[Hannah Fry]

There's a trillion of them, Michael. There's a lot. That's okay.

[Michael Blastland]

There are more trees on Earth than there are stars in the Milky Way galaxy.

[Hannah Fry]

There genuinely are. There's so many trees in London that technically it counts as a forest. Really. That's absolutely true. Yeah.

[Michael Blastland]

And that's not to say let's all go like cull the trees, but for me, the artificial tree we get them with the lights already installed and like a whole remote. And you can choose different colors based on your mood and they're much easier to put up. Less mess. And you can customize them so much. We have one that's covered in fake snow.

[Hannah Fry]

Do you. That's very 1980s of you.

[Michael Blastland]

I think that if you're going to go artificial, you should go really far. Artificial like pink tinsel. We've got a. We've got a gold tinsel tree and that's that. Actually we still haven't moved here, so we had to pick up this snow covered one.

[Hannah Fry]

The more fake the better. I. So I've actually. I really resisted getting a fake tree out because there's something about the smell of Christmas trees that just really sort of brings it home for me. And actually last year I did the calculations and the environmental impact essentially of having a plastic tree as opposed to cutting one down every year. And essentially if you have it for your whole life, if you keep the plastic tree forever, then, then you know, environmentally you're doing all right. It's better than cutting them down every year. But I miss the smell of trees so much that I've now. I now also have artificial tree scent that I spray on the tree.

[Michael Blastland]

Where to spray for that? You put on the tree?

[Hannah Fry]

Yeah.

[Michael Blastland]

Is it built for that purpose? Like, is that why they sell it?

[Hannah Fry]

It is. That's. It's a tree spray and it's good. I mean it's very sickly. It's not nowhere near as good as the real thing, but it sort of like scratches the little itch. The little itch because this, this, this particular smell that trees are producing. It's a. It's a terpene. It's particular type of molecule that is released by plants. In particular, the one you get from Christmas trees is called pinene. And as. As warm and cozy and comforting as it feels, basically the smell of fear, Michael, that's, that's essentially what we are, what we're smelling.

[Michael Blastland]

It's a tree experiencing some analog of fear and releasing that smell.

[Hannah Fry]

Some tree version of fear. That's. That's what's all warm and cozy.

[Michael Blastland]

Oh no. Why do they. Do they release it? To communicate to other trees? Or is it an accidental emission while they're being harmed or something?

[Hannah Fry]

Well, I don't think it's accidental. I mean, I think it's. It serves a very distinct purpose. So, so terpenes. They're these, this, this molecule is toxic. Or at the very least it' sort of very deeply Unpleasant to lots of insects and fungi. So, you know, if you're a bark beetle or, you know, a pine tree doesn't smell like Christmas. It smells like chemical warfare. You don't want to be anywhere near that. It's very, very nasty. The other thing that it does is it's a damage response. So if you, you know, snap a branch or, you know, you crush some needles, then the. The tree releases more of those chemicals. And there is an idea that actually trees are able to communicate using these. These types of. Of chemical. But. But essentially what you're doing then is you're. You're sort of smelling the tree screaming. You know, that's.

[Michael Blastland]

I was going to say it's. It's a wonderful, festive smell, but it's the scream of a tree.

[Hannah Fry]

Scream of a tree.

[Michael Blastland]

This reminds us of our discussion of AI and how if they become suitably advanced, like way beyond us, they might also love our screams and feel that they're festive. And on their Christmas, they'll torture us and they'll say, ooh, it just feels like Christmas when you've got Michael in the corner going, oh, please, no, let's.

[Hannah Fry]

Crush it into a bottle and spritz it around the room, shall we?

[Michael Blastland]

This also makes me wonder if trees will, given enough time and no intervention from us, evolve away from emitting those because we. We won't be cutting down the trees that don't have the smell. And so through natural selection, they will lose their wonderful, festive, Christmas evergreen scent.

[Hannah Fry]

Yeah, I mean, it would take. It would take a very long time to have that sort of evolutionary pressure. There is one more reason why trees release that. That particular molecule, and I wonder if you can guess it. So I'll give you a little clue.

[Michael Blastland]

Wait, wait, don't give me a clue. Let's see what I come up with. There's another reason.

[Hannah Fry]

Yeah.

[Michael Blastland]

Okay. I guess it could.

[Hannah Fry]

It's not about the smell of it. It's about something.

[Michael Blastland]

It's not about the smell. Sure. So is it. Is it about, like, healing the wound, like covering it up and making it waterproof or something?

[Hannah Fry]

No, no, but it is oily. You're right. It's got one other property. It's. It's very flammable.

[Michael Blastland]

Ah. Why would they want to become flammable when scared? Okay, my. Here's my guess.

[Hannah Fry]

Yeah, go on.

[Michael Blastland]

It's flammable in a way that causes the flame to go fast and not as hot as if it burned the wood.

[Hannah Fry]

Ooh, interesting.

[Michael Blastland]

So the fire spreads and doesn't like destroy the tree more deeply.

[Hannah Fry]

I mean, you're totally correct. Cause this is the thing, right? Why would you want a forest to be flammable? It feels really counterintuitive. But the thing is that actually if you have a low level fire in a forest, one that is very quick and runs through, then it will just clear out, like competition. All the saplings are gone. It will also clear out loads of parasites. But then you've got these kind of these pine trees which have really got thick barks. They've got, you know, they're conifers, right? They're there to survive. And they can survive a low level fire and then essentially reclaim the land. So in evolutionary terms, being a little bit explode is actually quite a good long term strategy.

[Michael Blastland]

I think that's a good life lesson.

[Hannah Fry]

Hey, my life motto be a little bit fiery.

[Michael Blastland]

That's really cool because, yeah, we've all seen after conflagrations in cities that many times the trees are still standing.

[Hannah Fry]

The other thing I like about this is that not only are the trees evolved to have that particular scent, but I think that humans are also evolved to find it appealing. Because if you think about, you know, hunter gatherer humans, right? Really ancient humans, the smell of pine, it means shelter, it means food, it means that you haven't got these biting insects around. And okay, maybe it's sort of gone on to mean something much more festive, but actually I think it does ignite something in us too, which is like this deep evolutionary need for our basic survival to be met, which you can do in a forest very easily.

[Michael Blastland]

Yeah, it really is literally the smell of safety.

[Hannah Fry]

Yeah, and screams.

[Michael Blastland]

That's wonderful.

[Hannah Fry]

Good, huh? Yeah. Merry Christmas. Okay, another question. One for you, Michael. This one, this one came in from said, I once met a boy called Xenon. Amazing name. What element do you think is the best name to call a person?

[Michael Blastland]

Ooh, well, xenon. Or as I would say, xenon. That's a great one. It's like sci fi. You get to use the letter X, which is underutilized by our language and our naming systems. Okay, so first of all, there are some elements that are already named after people. Einsteinium, curium, there's that. There's some cool names. Like, okay, if I met someone named Tungsten, I might roll my eyes. I shouldn't be judgmental though, because there's probably someone named Tungsten listening who's gonna be either an angry commenter or just to have a really sad day by what I just said.

[Hannah Fry]

If you were called tungsten, though, you'd have to be really careful not to be too heavy, wouldn't you?

[Michael Blastland]

Yeah, right. Because people would. Oh, geez, Tungsten is so dense. Like, dude, open your mind. Carbon, hydrogen, Tin tin. Like ren tin tin.

[Hannah Fry]

Yeah, that would be quite good.

[Michael Blastland]

We need to name some more elements. I know there are some that are still called, like, they don't have final names. And I'm going to suggest that we name another one after a person. Specifically, I want a new element to be called John Quincium.

[Hannah Fry]

Go on.

[Michael Blastland]

Because the letters J and Q are not used in the periodic table.

[Hannah Fry]

There's no Q at all.

[Michael Blastland]

There's no Q and there's no J at all.

[Hannah Fry]

That feels like a great oversight.

[Michael Blastland]

And so if we. We could fix that in one fell swoop by naming an element after John Quincy Adams. Now he's. This is a very, like, American centric answer, But I'm sure we could find a JQ person, you know, somewhere else in the world. But the point is, let's get some J's and Q's in there and then the whole Alphabet will be represented.

[Hannah Fry]

I am. I'm going to give you a more British centric answer because I think actually some of the names in the periodic table work very well already as names. Just as long as you're willing to include a regional accent. He. Liam, right?

[Michael Blastland]

Yeah. Oh, wait, like Liam?

[Hannah Fry]

Yeah, like, hey, Liam, if you're very northern.

[Michael Blastland]

You know what I mean? Hey, Liam.

[Hannah Fry]

There you go. It already works. Also in Yorkshire, people say like, oh, you know, our Jim or our Karen or are gone.

[Michael Blastland]

Are gone. Not your gon. Our gone.

[Hannah Fry]

It still works. I think. Periodic table as British regional accent pet names. I think we're already there.

[Michael Blastland]

I think we've already there. I don't think we even need the regional accents. Like, indium is kind of a beautiful name.

[Hannah Fry]

Yeah. Didn't appear on the list when it came to naming your daughter, though.

[Michael Blastland]

No, no, it didn't. I think you gotta choose something that almost doesn't sound like an element. There's so many of those in the early stages of the periodic table. Eventually they all just become eons. But carbon. No, not a great name.

[Hannah Fry]

Chlorine. Little baby chlorine.

[Michael Blastland]

Chlorine, like Carolyn, but more elemental.

[Hannah Fry]

Boron.

[Michael Blastland]

Boron. Oh, neon. I think neon is like, gonna be up there. Xenon as well. Those are both great. Yeah, Krypton too. Too like kryptonite sounding.

[Hannah Fry]

Thorium.

[Michael Blastland]

Oh, that's like a big, strong name, isn't it?

[Hannah Fry]

Very Nordic.

[Michael Blastland]

Yeah, it's Nordic, it's, it's Thor, but chemical. Thorium.

[Hannah Fry]

Thorium. That could be a Marvel villain. That's, that's what I want to see. That is a Marvel villain.

[Michael Blastland]

Zinc. Zinc is like Zeke. All right, I'm tapped out. I think we've, we've touched on the ones that we recommend to parents.

[Hannah Fry]

That's fine. I actually, I mean, I'm not sure I recommend any of those to parents, but. But nonetheless, parents of baby xenon. Congratulations on, congratulations on extending your family.

[Michael Blastland]

Yeah, you guys, yeah, you guys chose one of the best.

[Hannah Fry]

Right. Well, I think what we've decided there is that our naming abilities expired after the birth of our own separate children. Yeah. But, hey, we've got more coming up for you. It's Christmas, you know, pour yourself another sherry, have another mince pie, and come back after the break when Michael's got a gift for us. This episode is brought to you by Cancer Research uk. In the uk, nearly one in two people will face cancer in their lifetime. Wow. Tell you what, though, I've already had it. So between us, we're fine now.

[Michael Blastland]

I'm safe.

[Hannah Fry]

That's not how statistics works.

[Michael Blastland]

Shoot.

[Hannah Fry]

The question is, could science stop cancer before it begins?

[Michael Blastland]

And over the past 50 years, Cancer Research UK has helped double cancer survival in the UK, and that's proof of what research can achieve. Like take cervical cancer. Almost every case is caused by hpv, the human papillomavirus. And when scientists uncovered that link, prevention became possible.

[Hannah Fry]

Indeed it did, by vaccine and it's protection that works way before the cancer itself can actually grow. After the vaccine was introduced, cervical cancer rates in England were nearly 90% lower than expected in women in their 20s.

[Michael Blastland]

And knowing about HPV improves screening, and that's, you know, vital for diagnosing cervical cancer early.

[Hannah Fry]

I mean, we're now genuinely at a point where this is a disease that is disappearing in younger women in the uk. This is something that I really hope my daughters will never, never have to deal with.

[Michael Blastland]

For more information about Cancer Research uk, their research breakthroughs and how you can support them, visit cancerresearchuk.org restiscience.

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[Michael Blastland]

Welcome back in the Christmas spirit. I'm feeling very generous and I wanted to bestow upon everyone some tactile illusions. These should be good for, for those who are only listening. I think we've done a lot of visual stuff in some of these episodes and so I wanted to do one where we can all follow along using our own bodies. So I think, I think we should start with the tongue. Are you ready, Hannah?

[Hannah Fry]

Absolutely.

[Michael Blastland]

Can you, can you twist your tongue upside down?

[Hannah Fry]

Allow me to try. Not with ease, but yes.

[Michael Blastland]

Can you see what I'm doing? I'm literally just twisting my tongue over so that the bottom of my tongue is pointing as up as I can get it.

[Hannah Fry]

Yeah, I'm there. It's difficult to. What I've noticed is that even though this is an audio only illusion, it's. It's quite difficult to narrate it.

[Michael Blastland]

Yeah, I mean, I think, I think that twisting your tongue over like that's, that's literally what we're doing. And if you can do it with your tongue muscle, great. If not, you can reach into your mouth, grab your tongue and just twist it 180 degrees. It won't hurt. It's a safe thing to do. I find that once I twist it, I need to like bite down a little bit with my teeth or even lips to just keep it there. But once you've done that, reach out your finger and touch the tip of your tongue and slide along the tip left to right.

[Hannah Fry]

Oh, that feels weird.

[Michael Blastland]

Uh huh.

[Hannah Fry]

Not weird from my finger's perspective, but from the perspective of my tongue.

[Michael Blastland]

Yeah, no, it's very normal for my finger. I think I'm touching tongue like stuff all the time, but my tongue feels strange. There's actually an illusion happening here which is that you can't locate which side of your tongue is being touched.

[Hannah Fry]

Sorry, you're just. Everyone just now has to sit quietly for a moment while I just try and poke.

[Michael Blastland]

Yeah, we're both touching our tongue so we can't speak, but you should be doing this too.

[Hannah Fry]

That's so strange though, because if you hold your tongue in an ordinary position and lift it up and touch underneath, then it's like, okay, yeah, that's the underside of my Tongue, but turn your tongue over so that it sort of physically appears where the top of your tongue previously would. And then you can't tell.

[Michael Blastland]

You can't tell. And specifically what's happening, what you should be feeling is that when you touch the left side of your tongue, it feels like your finger is over on the right side of your mouth, touching your tongue from the other side. It's quite bewildering because you know where your finger is. But yet your brain is so unaccustomed to upside down tongue touching that it has not learned, it hasn't retained plasticity there. So it just goes, okay, look, when, when those cells are touched, it's coming from the right side. But if I put the right side on the left by flipping my tongue over, it's all, it all goes wrong.

[Hannah Fry]

Like you have a map of your body in your head and this is off the map.

[Michael Blastland]

It's off the map. And it's, it's, it's, it's almost more weird than that because it's, it's a map of sensation. So it shows that we feel things and then our brain receives that and then it projects it onto an experience of the world. And it says, yep, there's a finger on the right side, even if you're on the left. So there's like this stage play going on in your head that is everything out there. And you can actually see the difference between the two by doing this with your tongue. You can also do it with your fingers if you take your middle and pointer finger and just cross them, cross them like as much as you can. For me, that means putting the pointer under the middle finger. I've just crossed them as much as I can, right? Oh, yeah.

[Hannah Fry]

Wow.

[Michael Blastland]

Look at your finger flexibility.

[Hannah Fry]

I mean, thanks so much.

[Michael Blastland]

Once you've done this, touch an object. A marble is great, but a pen, a pencil, anything that's kind of rounded, put it right in between where the fingers cross. So what you're touching with is actually what used to be the outside edges of both fingers, the edges that are away from the center of the fingers. But now you've put that in the middle and they're touching one object.

[Hannah Fry]

Okay, I'm going for a coin.

[Michael Blastland]

You've got a coin might do it. Yeah. Touch the curved side of the coin and you might want to try crossing them less or more. But it should feel like there's actually two objects there. It works best if you don't look even, even closing your eyes. Touching the edge of a table can do it as Normally if you put your finger, your two fingers together, your middle and pointer together, their outside edges are going to be touching different objects almost all the time. But when you cross them so their outsides are together, your brain goes, oh, wait, they're both getting a sensation. It must be two different objects. And this was discovered thousands of years ago. Aristotle was the first known person to write about it. So it's called Aristotle's Illusion. And it's very fun to cross your fingers and just touch things, different textures, different sized objects, and do it with your eyes closed and picture what you're feeling. It feels like two very separate things when it's really just one.

[Hannah Fry]

This makes no sense. This makes no sense whatsoever in terms of. It's like that feeling when you're, I don't know, stepping off an escalator that isn't very moving and you're sort of, your brain is making this prediction about what it's supposed to happen and then your actual experience doesn't match up with it. The one which I remember from, from being a child, I don't know whether you played this one in the playgrounds of, of Kansas, but is where you expose your forearm. What you do is you ask another person to expose their forearm, have their eyes closed, and then you start to just very, very gently tickle the sort of underside of their wrist and then very, very slowly move your finger up towards the crook of their arm, the kind of joint of their elbow, and ask them to tell you to stop when they hit the middle, when they hit the very, the crease in the middle of your elbow. And what happens is that almost every single time people will tell you to stop when you're basically halfway up their forearm so still like a great distance away from the crook. They believe you're at the crook of your arm well before you get there. And I mean, this sounds like a similar, there's a similar thing going on here, right? Your, your brain has this, this map of your, your body that actually is more like a model, you know, it's not, it's not the real. You're not sort of accessing really where things are. You're accessing where your brain thinks they are. And most of the time that map is correct. Right. But just every now and then it sort of lets you down. But also I think what this one does is it demonstrates that this map in your brain is not this, like distances are not accurately represented. You have way more nerve endings in certain parts of your body than others and they, your sort of skin sensitivity Drops up off as you go upwards in your arm. And so your mental map is warped. It's a bit like a sort of London tube map in a way. You know, very good for navigation but terrible for accuracy. And you can find. Find these little flaws in it.

[Michael Blastland]

I did this to myself with my eyes closed. I just wanted to add that I think instead of doing little short strokes as you go up from the wrist to the elbow, just a smooth pursuit, a smooth run. I did find myself going, oh, I must be there now. But I wasn't. I think, yeah, if you do it to someone else, it's probably a lot better. They. They'll get really confused. These are great little slumber party tricks. Mm. So, yeah, the elevator comparison is really good because it's. It's a moment where there's friction between our mental map of reality and reality as it's coming towards us. And it's a great reminder that they are different.

[Hannah Fry]

Yeah, they absolutely are. You know, people describe it as the Bayesian brain.

[Michael Blastland]

Oh, yeah. We're just kind of predicting as best we can. We aren't made of perfect sensors.

[Hannah Fry]

Exactly. Sometimes I get really caught on this idea that we're not actually a body. We're sort of. It's like processing engine that's locked inside of a. Of a dark, noiseless void inside of our skulls. And then we're just receiving these inputs, these sort of senses that we have around our body. We're receiving these inputs and making guesses about the world around us and our own physical body based on that information. And, yeah, the guesses that we're making are the best predictions that we can with the available information.

[Michael Blastland]

Yeah, yeah, I know. It is really trippy, especially at night, to just close your eyes in the dark and think about how much darker it is in your head that everything you're hearing is all just your brain putting that show on for you. And optical illusions show us that we are little Bayesian engines. But tactile illusions like the three we've done today don't get as much attention because they're not as easy to publish in a book. And yet they are. For that reason, I think all the more surprising.

[Hannah Fry]

What was the explanation from Aristotle? Like, what did the ancient Greeks think was going on?

[Michael Blastland]

Oh, I don't know. I don't know what Aristotle said about it. I don't think that he proposed, like, an important solution. I think it was just more of a curiosity he'd noticed when trying to investigate the priority of the senses and the difference between logic and Perception. To this day, we still can use it a lot to understand how people's nerves are working in their hands. And different conditions can cause the illusion to appear in different ways.

[Hannah Fry]

This reminded me, actually I did an episode for the BBC the other day about phantom limbs.

[Michael Blastland]

Oh yes.

[Hannah Fry]

Because I think there's a connection here to that, that you have this, this, this map in your mind of where your body parts are and if that map changes because perhaps you have an amputation, perhaps you know, something happens and you lose, lose part of your body, a limb or whatever it might be, that map doesn't get deleted. So I remember going to see my cousin, lost their arm in a, in a farming accident when they were 16. And I remember going to see him in hospital and he was saying how the thing that was really driving him crazy was that his arm that was no longer there was really itchy. I mean, this can be like a really serious problem for people with amputees, for example, is that you can end up having chronic pain in your phantom limbs. But there's a story about Nelson, who lost his right arm. This is In, I think 1790s or so in this attack on Santa Cruz in Tenerife. And he writes this letter where he lost his arm and yet he still feels as though the rest of his arm is still there. He's still sort of. The map hasn't updated in his head. And what he did was he believed that this was evidence of the soul, which is really lovely, right? There's a version of you. And even if certain parts of it change as you go through your life.

[Michael Blastland]

You'Re still a whole person in the form of a soul. And of course you would think that. I mean, look, I think that this whole mental map thing extends beyond the physical world. I think we also are stuck in an analog world in our heads of concepts. The way we think things should go, the way we want to be, the way we think life is. And we're navigating that and furnishing this analog world in our head. And it doesn't always fit with reality. And when it's a physical illusion, we find it really amusing. But when it's a social or cultural or, or knowledge based confrontation between the two, we have to resort to, you know, coping mechanisms and we try to like, reason our way back to. No, no, the analog map in my head is the correct one. And the problem is everything else. It's not, it's, it's, it's not me, it's kids these days, you know, stuff like that.

[Hannah Fry]

Go on, give me an example. Do you mean, like, I don't know, Copernicus, for example? Right. Like, give me. Give me an example.

[Michael Blastland]

Well, sure, yeah. Scientifically, there are paradigms that we just can't really cross because that's not the way we've furnished our analog minds and our scientific discourse. There's also social ones. Like, we cultivate an idea of how things are supposed to go and how things are supposed to feel from movies and from books and from the way people tell stories about their lives. And then when that doesn't happen for us, it causes a lot of internal anguish. Like, wait, no, Prom isn't supposed to be like this. Hey, wait, I'm not supposed to feel this way after I give birth. Like, I've learned how it's supposed to be from movies. And that's the way it is in my head in the analog world I've created, the terrain I've invented. But now that I'm actually here in reality, it's different. And. And there must be something wrong with me or with the world. And it's okay to just say, hey, man, they're two different places. They're not supposed to be the same.

[Hannah Fry]

Yeah, what's that really nice phrase? Reality is just the hallucination we all agree on.

[Michael Blastland]

Yeah, yeah.

[Hannah Fry]

The other thing about that, though, I mean, I guess the way that the body gets around it is that the body is able to adapt, or at least the brain is, anyway. I mean, we have plasticity in our brains that is, you know, extremely high when you're younger and decreases with age, but is always there all the way through your life that you can learn new things, learn new maps. There was a really amazing experiment a few years ago where they took people who had lost sight so previously had had vision, and then something had happened. Either a condition which had. Had caused them to become blind or damage to their. In some other way. And what they did is they placed on their tongue a little metal plate. Have you heard of this experiment? And basically what the metal plate could do is it had almost like pixels across it, but instead of being pixels of images, they would be little electric shocks that you would get on your tongue as you. As you encountered a scene. So this little plate was hooked up to a camera and essentially gave a very, very crude way, a crude visualization in the form of little miniature electric jolts on the tongue. And the idea being that your tongue is extremely sensitive. It's sort of extremely capable of picking up on these things that are physically distant from one another. You know, you've got a lot, a lot of. A lot of nerve endings in on your tongue. And the people that wore this sort of didn't really expect anything to happen, but when they wore it for a really prolonged period of time, after a while, brains literally remapped themselves. Right. The plasticity literally worked until there was a point where they suddenly felt that they could see the scene that they were in. So you could, for example, throw a ball at somebody who no longer had vision but was wearing this tongue plate. And they could inverted commas, see the ball coming towards them based on. On these tiny sensations, but not just see in sort of a loose sense, like they actually felt like they had this sensation of seeing it.

[Michael Blastland]

Yeah. If you asked them, why did you catch the ball? Or how did you know? They'd say, well, I saw it. And then they would probably correct themselves and say, well, I guess I tasted it. But I wouldn't be surprised if that data from their tongue started being processed in the visual part of their brain.

[Hannah Fry]

Absolutely, absolutely. I mean, there's lots of work that's still ongoing in this space. Right. Of using the plasticity of the brain along with technology to work with people who have perhaps lost vision or lost other senses in order to try and repair them to some degree or another. It's like a really fascinating area of research, but kind of just demonstrates how this. This map that you have in your brain is not fixed across the course of your lifetime.

[Michael Blastland]

That's right. I bet if you spent a few weeks with an inverted tongue and you licked a bunch of stuff, your brain would learn the difference between left and right on an upside down tongue. And then the illusion wouldn't work anymore until you put your tongue right side up again. And it might take a while for your brain to learn. Okay. Sometimes the tongue twists. It didn't used to, but now it does. And I can adapt.

[Hannah Fry]

So, yeah, maybe the concluding thought for this is if you want to turn your tongue upside down, wear it like that permanently and start tasting things like that. You can, but be very careful because you may lose your ordinary ability.

[Michael Blastland]

Yeah, it's one of those. Like, if you keep making that face, it'll become stuck that way. The brain is very plastic and it will adapt. So lick your candy canes the normal way unless you wanna have a flipped upside down New Year.

[Hannah Fry]

Okay, Well, I think that concludes our episode. Very merry Christmas to all of you. Enjoy. Enjoy your sherry. Michael's never tasted sherry. That's what we discovered in the break. So a bottle is winging its way to you right now. Michael, I'm gonna get you the sweetest sherry you can imagine. It'll be like drinking raisins. Oh.

[Michael Blastland]

Oh, great. Thank you.

[Hannah Fry]

Say it again, but with more sarcasm. Now, if you have any questions that you would like us to answer, please do send them in to us@the restofscienceolehanger.com.

[Michael Blastland]

And join our [e-mail] science.

[Hannah Fry]

And we're going to be back next Thursday with another edition of Field Notes, and on Tuesday with our normal episode. Merry Christmas.

[Michael Blastland]

See you guys later. Ho, ho, ho.

[Hannah Fry]

Sam.