Making Sense: The sixth sense

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All knowledge must come through the senses.

All that we perceive.

and all of the awareness of our daily existence.

Light.

Double rainbow.

Oh my God.

Sound?

Listen to me.

Listen to me.

Touch.

Squishes.

Odors.

You.

And tastes.

What are your thoughts concerning the human senses?

As meat and wine are nourishment to the body,

the senses provide nutriment to the soul.

All that we perceive, seeing, all the awareness.

Hearing, all knowledge must come through the senses.

I have an incredible sense of touch.

All that we perceive, tasting, all the awareness.

Smelling, all knowledge must come through the senses.

Does it make sense?

Somehow we lawful.

It's Unexplainable.

I'm Noam Hassenfeld here with science editor Brian Resnick.

Hi.

So, Brian, we've spent five episodes at this point talking about all the incredible things that we can sense and how much we still don't know about how our senses work.

But, you know, this whole series, I keep listening to our theme song and I keep hearing this line, you know, that was originally from this 50s documentary I pulled.

All knowledge must come through the senses.

And there's this list of, you know, smell, sight, sound, touch, taste.

And I don't know, it seems like maybe obvious on its face, but the more I listen to it, the more I keep wondering.

Does all my knowledge come through these five senses?

Like, I must know things that aren't through

these five, right?

Yeah, yeah.

So I think the concept of there being five senses and it being like a very finite number comes from Aristotle, who's, you know, very old at this point.

You know, there is definitely a view that we have more sensations than just these five broad categories.

What other kinds of senses could there be beyond the five?

So you have a sense of like stuff going on inside your body.

So sometimes this is called interoception.

And maybe each component of this is a different sense.

Like you definitely have a sense of hunger, a sense of thirst.

You also know, you know, when your bladder is full.

Does this include like feeling your own heartbeat?

Yeah, yeah.

Just being aware of, you know, how stressed your body is.

I think it's like whenever you like kind of think about any sensation, you can think about like all the components of it too.

Like, you know, can we sense gravity or do we just sense like the pressure on the bottom of our feet?

Right.

These are all kind of like gray areas.

How many senses are there?

Some would say like, oh, it reduces all down to three, light, chemicals, and mechanical sensations.

Or then like there's an argument that there are nine senses and then 21 or 33.

Right.

Okay.

There's even senses where.

It's kind of hard to sense them, if that makes sense.

What do you mean?

Like, so, for example, proprioception, which is your sense of where your body is in space.

Yeah, we did a whole thing on proprioception for Today Explained a couple years ago.

And I remember when you tried to explain it to me, I just couldn't wrap my head around it.

All right, do you have anything near you that you can put out in front of you, like a glass or a cup, or I want to just like a can of La Croix over here?

Okay, touch your finger to your nose,

and then touch the can.

Got it.

Okay, now I want you to do the same thing, but keep your eyes closed.

Yeah, I got it.

You got it.

That felt really easy.

Yeah, it's pretty easy.

This is the question.

When you close your eyes, how do you know where the can is?

I

saw it when my eyes were open and I took a, I remembered.

So you have an image in your head.

You have a depiction of space in your head.

Okay.

How do you know where your finger was?

I just know.

You just know, right?

Yeah.

It's actually really hard to think about.

When you close your eyes, it's not like the world disappears.

There's an impression of it.

That's proprioception.

It's the sense of where our bodies are in space.

Okay, so that explanation of proprioception, we had that chat a couple years ago, but I remember you went to see an experiment with people that don't have proprioception, right?

Yeah, so I went to the NIH in Bethesda, Maryland.

That's, you know, National Institutes of Health, and was led into this room that looked a little bit like a gym.

There was like a lot of like padded areas, a lot of

kind of like harness type equipment.

There was this woman named named Sana, and she was sitting at a desk, and there was a cylinder in front of her on the desk.

And on the top of the cylinder, there was this ball.

And the test was really quite simple.

She was asked to start with your finger on her sternum, and you're going to touch the ball.

Touch her sternum, you know, her chest, and then touch the ball on the top of the cylinder.

And this trial started with her eyes open, so she was directing her hand, and she did this fine.

Exactly, just like that.

But then...

So the next test would be a little more tricky.

We did the same thing, but eyes closed.

She was instructed to close her eyes.

And then

when she closed her eyes, it's like that cylinder, that ball disappeared.

She would grope around to try to find the cylinder and smack into it.

And then, like, in trying to find her nose, like she would miss and like touch her ear.

She was there with her sister Sausen, who actually has the same condition as her.

And like the test was just to have her walk in a straight line.

From about right here,

and I'm going to have you walk along the yellow.

They actually hooked her up to a harness system so she wouldn't like fall and hurt herself.

But the test was similar in that it had an eyes open component and then an eyes closed.

So when her eyes were open, she was asked to walk in a straight line and she could.

But then

when her eyes were closed, she did not look very capable of walking in any direction very well.

She looked very kind of scared and shaky and wasn't really moving much at all.

And later, I was talking to her, I was talking to her through an interpreter, she's a French speaker, just about like her experiences at home.

And she was telling me,

I'm not in my

if she closes her eyes or if the lights go out in the house, she'll just fall.

And then I also asked her her sister, Sana, what this whole experience feels like.

And she said, like, when she closes her eyes,

it's like she's lost.

She feels lost.

It's so strange to try to explain it to you in words because, you know, we just have this implicit sense of where our bodies are at all times.

Like, imagine laying in bed and like not being sure if you were like in bed or like, you know, upside down in a swimming pool.

Yeah, it's hard to even wrap my head around how that could possibly work.

Like, I can imagine turning off my sense of vision.

I can imagine closing my ears.

I can imagine plugging my nose.

Yeah.

I can't imagine plugging my sense of space.

Yeah, because like the quote-unquote eyes of the proprioceptive system like aren't closable

so what what happens is that in in almost every muscle of your body there's this component called the muscle spindle okay and this muscle spindle is the thing that senses how far a muscle is stretched so how you know elongated or short it is you know how far away your arm is from your face and then within that muscle spindle there's a special receptor called piezo2 Actually, the discovery of this actually won the most recent Nobel Prize.

I think of it as like the doorway through which mechanical forces enter our nervous system.

So, this piezo receptor is also used in touch.

And basically, when a mechanical force pressing on your skin or a stretch is in the presence of this piezo receptor, that receptor actually opens and it starts the signal.

It's like telling the next nerve to send the signal on.

What was interesting about these people I met at the NIH, they're missing this receptor.

So, they don't have this like the eyes, or really is like the kind of like retina or like rods and cones of, of your proprioceptive system is like where all this information is starting.

You know, you, you keep talking about the sense of touch.

And I know we're sort of talking about senses beyond the five.

And I'm just wondering, proprioception, the sense of your body in space, is that technically a part of touch?

It gets really confusing here because

proprioception uses the same receptor, the same doorway as touch.

These women who I met who lack the piezo receptor, they can't feel a lot of light touch.

So one was telling me, like, when she goes into her purse, she's like constantly like going into her purse with her hand and coming out with like nothing in her hand because she can't feel the light contours of her phone or something like that.

But what's interesting here is that they can feel like sharp things.

They can feel like piercing pain.

And that totally like left the scientists who study this like baffled because they're like, we don't know how sharp pain enters our bodies.

Wait, so those kinds of touch are different?

Yeah.

What the researchers were doing here is kind of studying what are the smallest components of touch or even like weirder things like itch or this sensation called tactile elodynia.

That's like when you have a sunburn.

I know that's a too big word.

So that's like when you have a sunburn, normally things that feel good feel bad.

They're trying to like really separate out all of these components and just finding that there's like all these amazing overlaps and it's really hard to separate out one of these senses completely from others.

And is that to say because,

you know, these women don't have a sense of proprioception and they have a limited sense of gentle touch, but they do have a sense of sharp pain that what we think of generally as a sense of touch may be

many senses that we just sort of group together for convenience.

Yeah.

And isn't this the case of like all of the senses that we've heard of on this series too?

So like in Meredith's story about taste, like just listening to how hard it is to isolate like what taste is from scent too, from flavor or from texture, right?

Like talking about.

the mouthfeel of a cookie.

Or even technically like spicy foods, I believe the reason like you feel that spice, that heat is is like it's activating heat receptors.

And I was like, is that taste?

Right.

Or is that touch?

Yeah, it also just makes me think of what I learned when I was reporting on hearing.

You know, that so much of what we think we hear is actually constructed by our brain.

And in a sense, there can sometimes even be these fuzzy distinctions between hearing and expectation and memory.

Yeah, I think overall, the more I report on the senses and the more I learn about it, like, I think this whole construct of like, you know, drawing a circle around five senses, six senses is just, it's just so fuzzy.

I appreciate it so much more, like understanding how complicated these systems are and how much they do potentially overlap.

Yeah, that makes me think of the sisters from the NIH.

Like, they couldn't touch the ball or walk in a straight line when their eyes were closed, but they could do it when their eyes were open.

Is it almost like they do have proprioception, but only when they can see?

Yeah, yeah.

So the problem isn't with their brain or even with their nerves or spinal cord or anything.

They're just missing that first switch that tells the body like where it is.

But they do have ability to compensate based on using their vision.

And this is just an amazing thing the brain does with a lot of different injuries to our different senses.

Like if you lose a sense, you try to compensate.

And your brain can sometimes like recreate that sense in really intriguing ways.

So some people who are blind have gotten these implants of electrodes on their tongue.

Yeah.

Yeah.

And so the electrodes are attached to a camera and will basically paint a picture for the user of this implant on their tongue like by zapping it with like a little electricity.

With enough training, that kind of becomes a little bit like vision.

And in this case with these sisters, like sight almost becomes part of their proprioceptive system.

So that's another way that all the senses and the categories of the senses really dissolve when you really study them, because one sense can influence and sometimes even compensate for another.

Yeah, it's like we don't experience the world in five buckets.

We don't experience the world in 21 buckets.

We experience the world in one bucket.

with all of our senses coming together in this way that kind of blows my mind.

But I don't experience the world as like, okay, I'm tasting now, and I'm seeing now, and I'm hearing now.

Yeah, that's the most amazing and arguably the most mysterious thing the brain does.

Like, how does it take all these various inputs and create a singular consciousness?

Like, you're right, you don't feel like you are many things, you feel like you are one thing.

Like, I've described it in stories before as like this wizard that's stirring a potion, and the wizard like takes all these various inputs from like vision and and hearing and touch and kind of stirs it all together into one magical thing,

which is not a good explanation necessary.

But maybe an accurate one, right?

I think it feels like magic.

Yeah.

Bringing all these components together and creating like one singular sense of self.

Like, that's probably the most amazing thing our brain does.

Even with our more than five senses, what humans can sense is just a tiny sliver of what's actually out there.

Up next, how far we might be able to push into that unknown.

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i want to tell you my secret now

i see unexplainable

okay brian we're back hey so we're nearing the end of this sense series here and this whole time we've really been talking about what we can can sense.

So, what about what we can't?

Like, how much is out there that we're not sensing?

Yeah, it's like you can't sense most of it, right?

Right.

You, you, you have access to very little.

Like, right now, we're on Wi-Fi, which is an electromagnetic wave around us, and you don't see that.

Yeah, I mean, I saw this TikTok from this guy, Dr.

Karan Raj, that was like, If you're okay with existential panic, keep watching.

Our eyes can only see visible light, and that's just 0.0035%

of the entire electromagnetic spectrum.

So he's basically like, we're blind to over 99% of the world.

Yeah, but I think that's okay.

Cause if you did see like all electromagnetic radiation, so if you saw microwaves, if you saw radio, if you saw ultraviolet, you wouldn't be able to see anything because it'd just be opaque.

Like, we need darkness.

We need voids.

We need blank spaces to make contrasts to see like what's in front of us.

But do we need the voids to be like 99%

of everything out there?

Couldn't we have asked for a smaller void?

Yeah.

Well, I don't know if it works like that because like what we're evolved to do is like to see what's useful for us on planet Earth.

So, you know, it's really useful on Earth to see, you know, blues and greens and like all the colors of the electromagnetic spectrum that we can see.

You know, if we were evolved on Mars, maybe we would see more like detail and beauty there, but you know, we just don't.

It's interesting to think about the fact that our senses are attuned to what's useful to us, because in this whole conversation, I keep circling back to dark matter in my head and this knowledge that like dark matter and dark energy are what it's like 95% of the stuff in the universe, something like that.

Yeah.

And it only reacts as far as we know with gravity.

That's how we know it's there, but we can't see it.

We can't touch it.

And then I think that, like, okay, it's kind of sad, sort of, that we can't sense 95% of the universe.

But almost even sadder is it's like 95% of the universe isn't useful to us.

Like

our senses have only evolved to experience the tiniest little fraction of the universe.

And it's almost like someone out there saying like, this is not for you.

Well, dark matter is still useful.

It's this thing that seems to hold galaxies together.

And it's there, even though we can't detect.

Right, right.

And, you know, there are a lot of people who are trying to figure out what dark matter is.

And it's in the realm of possibility that we will be able to sense it one day.

But, like, what we need to do is expand our senses with technology.

So, I think about something like the Webb Telescope and how it's this great big eye into things that you and I or any human on earth could never see alone.

But collectively, you know, scientists have built this telescope to see an infrared, to see the beginnings of like our organized universe.

And that is like an extension of our senses, you know, like studying the stars just started with our eyes.

And now we can sense anything that the well, not anything.

Not dark matter.

No, not dark matter, but like we can sense like a lot of what the universe throws at us.

Like we can even sense like little waves of gravity moving across space-time and ripples.

Yeah, I wonder, you know, bringing this back to all knowledge must come from the senses.

You know, where we started this episode.

If we're kind of extending our senses, is that to say that there is knowledge out there

that we could get not through our senses?

Oh, yeah, that quote feels incomplete.

Like, I know we can extend our senses with technology and, you know, we can analyze what we see and sense with logic, but I just keep thinking about knowledge beyond humans and it gets into a weird space.

Like, you can imagine an artificial intelligence or like an alien intelligence life that had its own like way of sensing things and acquired knowledge about the universe.

Like

we would have to respect that because, you know, it's not coming through our senses, but there are potentially other ways of knowing things in the universe than just like through the human brain.

Yeah.

When I think about what we can sense, you know, even with the technology that we've invented.

I almost picture it like there's this universe out there and we think we can see a lot of it, but we're just looking at it or sensing the whole thing through this tiny little keyhole.

But look what we get from just looking through that.

Well, yeah, I mean, it's nuts, right?

Like we are just looking through a keyhole and we can see all these like jaw-dropping things like galaxies and black holes and we can see back in time like billions of years through a keyhole.

And like we understand like how to send wireless communications.

We understand,

you know, how to use sonar to map the bottom of the sea.

We understand, like,

you know, we can sense chemicals in the air or radiation that would never, you know, be detected by us immediately as human beings.

That's amazing.

You know, we started off with these senses and we've just kind of keep expanding and expanding in the circle.

And will we become like the masters of every kind of force and

possible sensation in the universe?

Like, probably not, but I think what what we have is pretty cool.

Yeah, I just love this tension between

what

we are able to know and what's just inaccessible to us, at least for now.

It's like the main way we get knowledge is by using our senses.

And our senses are just these constant reminders that our knowledge is limited, that our experience of reality is limited.

Yeah, it's just having that intellectual humility to know there's always going to be something that you don't have access to, you know, and this goes outside of senses too.

This goes out, you know, into the realm of just like information.

But I think the lesson is to like, just be curious about it.

What is that thing that we don't currently see right now or taste or smell, but it's there?

Sometimes people talk about like intellectual humility as like a kind of a bashfulness, but I think it's a motivator.

I think it's like the ultimate curiosity of like, what's out there hiding in the dark?

This was the final episode of our Making Sense series.

If you missed some of the other episodes, go back and check them out.

We have a home for all of them, along with some more articles about our series on our site at vox.com slash unexplainable.

This episode was reported and produced by Brian Resnick and Noam Hasenfeld.

It was edited by Catherine Wells and Meredith Hodnott.

We had mixing from Christian Ayala, music from Noam, and fact-checking from Richard Seema.

The rest of the Unexplainable team includes Dory Dominguez, Mandy Nguyen, and me, Bird Pinkerton.

As always, if you have thoughts about the show, you can email us at unexplainable at vox.com, or you could leave us a a review or a rating, which we would love to.

Unexplainable is part of the Vox Media Podcast Network, and we will be back with some nonsense next week.