No, sir.

[Crew] Alright, I'm gonna go turn the hose on.

Ahhh.

Ahhhh God.

Oh, Goodbye feet.

They're done.

Icicles.

This is .

.

.

ha I'm not looking forward to this.

Let's get it over with.

Three, two, one.

(breathing heavily) Why am I in an ice bath, you're asking yourself?

Well, we will get to that.

But first, remember when you were a kid, you'd walk into a friend's house and you  know it had a smell, maybe not a bad smell, but there's like a distinct detectable aroma.

It might be the leftover smells from cooking  food.

Maybe granny's sitting in the corner or the actual physical house itself, you know, houses, just, they sometimes smell, okay?

Well, as you consider the unique  smell of your friends' humble abode, a realization flashes through your  head.

Maybe my house smells too.

Maybe I'm the stinky kid.

And now that's why everybody talks about my house smelling.

And then they don't invite me to the party on Friday.

And sorry, let's get back to what we were talking about.

Well, I'm here to assure you in the name of science that yes, your house definitely has a smell, and you just can't smell it.

Why?

Well it's thanks to an absolutely weird but surprisingly common phenomenon called sensory adaptation that you experience every day in countless ways without even realizing.

In fact, I'm experiencing it right now.

This ice water bath is actually no longer extremely painful, just a little bit painful, because the receptors in my skin that detect cold temperatures have stopped firing in response to this ice  water.

They've adapted to this new steady condition, and they're no longer screaming  at my brain.

We're gonna look at a bunch of surprising ways that sensory  adaptation impacts your life in different sensory systems.

And what you will soon realize is without this very strange phenomenon, you would be completely lost, overwhelmed, unable to  navigate the external world.

Okay.

God, get me out of here.

Woo!

(light music) Ah, that's better.

Hey smart people, Joe here.

Sensory adaptation is happening  basically all the time in at least one of your sensory systems.

I mean consider just your sense of touch.

When you place your hand on a table, you immediately feel the surface and then you just suddenly don't.

You don't feel the constant downward push on your body from gravity until you do something like jump and land and experience a sudden and new gravitational deceleration.

And when you put on your clothes, I'm gonna assume you're wearing clothes right now because that makes this much less weird you feel their texture and pressure on your skin but within seconds and for the rest of the day, you don't even know that they're  there.

Just checking.

Good.

To understand why this happens, we need to step back for a moment and consider what it means to sense the world around you and how you actually  do that.

Here's the problem.

Your brain is sitting in a silent, dark, squishy inner world, trying to makes sense of  everything outside.

How do we build a picture of the external universe?

How does all of this out here get in here?

The answer is our senses.

So what does it mean to sense something?

Well your body is covered in specialized neurons that each respond to specific types of  signals.

When sensory information is detected by one of these sensory  receptors, that's a sensation.

For example, the light that enters your eye causes chemical changes in cells at the back of the eye.

These chemical changes set off  a chain reaction of messages in the form of nerve impulses that reach the central nervous system.

That's a sense, make sense?

Consider this.

Every moment of every day, your body is being bombarded by countless signals containing information about the external world.

And these signals arrive at your body as different forms of energy, light and heat, sound wave pressure, even molecular vibrations in  the chemicals that we smell.

The job of a sensory cell is to convert these various kinds of energy into a form that the brain can  interpret into tiny electrical nerve impulses.

But if our brains are overwhelmed with signals, with too much sensory input, then interpreting those  signals becomes impossible.

We can't efficiently make sense of our world.

So to avoid being overwhelmed by  information, we set different limits of detection for our senses to make sure that the only information that arrives in our brain is the information that we need.

For many of your senses, the absolute lowest limit of detection is ridiculously  impressive.

I mean, for example, on a completely dark night, most of us could see a candle flame on top of a mountain 30 miles away.

We could feel a bee's wing fall on our cheek.

We could hear the tick of  a small clock 20 feet away or smell a single drop of  perfume in an entire house.

In the real world though, we're  rarely presented with conditions that test the absolute  limits of our sensory systems More often, the question  your brain has to figure out is how different do two stimuli have to be in order to detect just one of them?

This is known as the just noticeable  difference.

Musicians need to be able to detect minute differences in the tuning of their instruments.

Parents are able to pick out the sound of their own children's voices among the chaotic tornado of screaming children at the playground.

Trust me, I've been there.

Imagine you're in a dark movie theater and someone nearby opens their phone to read a text.

Not cool.

Chances are you'd notice that sudden light in the dark environment and  not be very happy about it.

But if someone opens their  phone on a brightly lit bus or something, then you  probably wouldn't even notice.

The absolute brightness of the phone screen is the same in both cases, but your ability to detect it depends on the context and how strong the new stimulus is compared to the background  stimulus.

Now it turns out that whether or not we perceive a  difference between two stimuli depends on the percentage difference  between them, not an absolute difference.

And that percentage varies depending  on the sense.

Two sources of illumination must differ by  between 5 to 10% to be detected.

That means we can tell the  difference between two moonlit nights with only a small change in the amount of light, but it takes a bigger absolute change to tell the difference in two brightly lit rooms.

We can sense differences in weight of just 2% and we can detect differences in sound frequencies of less that one half of a percent.

(sound wave beeping) Did you hear a difference?

As you read along with this passage, gradually following the  words with your eyes across and down the page, are you able to notice the difference in the size of text between the lines?

Even when you see this as a large block of text, and attempt to read  it normally from top to bottom, are you able to notice the difference in how the size of the words change from line to line?

And did  you notice that the background color behind the text slowly changed throughout that entire last scene?

Go ahead, rewind it and check it out.

I'll wait right here.

These examples show you how  detecting the differences between stimuli can be a really huge challenge for your sensory systems and your brain.

So it's crucial that your body tunes your senses to block out everything that isn't important and focus in on those differences.

And this is where sensory adaptation comes in.

It's how your body changes the strength of a sensation up or down to adapt to your current environment.

Let's see how that works in  some different sensory systems.

And you might have been taught in elementary school that we have five senses, vision, hearing, smell, taste, and touch.

In reality, we have far more than just five.

We have senses for balance, our body position, movement, pain, and temperature just to name some.

Sensory adaptation happens in many ways, in your visual system,  particularly in the sensory cells at the back of your eye  that actually detect light.

And it even happens in the cells that process that information in the brain.

This is an American flag with the normal colors replaced.

Stare at that mark in the center of the screen and try not to blink.

Don't move your eyes.

Just continue to stare at that little mark.

I know this feels weird, but just a few more seconds.

And now when you stare at the white, what most people see is a faint afterimage of the flag in its normal colors.

Pretty weird.

So what happened?

Well, when the color sensitive cone photoreceptors at the back of your eye are exposed to constant stimulus, they become less sensitive and they're tuned down so that they don't fire.

So where you saw yellow, your red and green sensitive cones were tuned down so that only blue cones fired in those areas of your retina when white light hit.

Where you saw this turquoise color, your blue and green cones were tuned down and only your red cones fired in response to the white light.

That's why you saw an afterimage for several seconds until all your cones were turned back up to full sensitivity.

Just because I'm in the mood  to really mess with your brain, let's look at another one that may make you question reality itself.

Stare at the dot the center of this image.

Again, try not to blink your eyes.

Just relax, keep your eyes centered right there.

Just continue looking at this strangely colored picture just a few more seconds.

Almost there.

And now when the image changes,  you should see normal color.

Except if you move your eyes and look at a different area of the image, you see that you're actually looking  at a black and white picture.

Again, if you wanna rewind and do that again to make sure, I will wait right here.

It's bonkers.

Afterimages occur because of sensory adaptation.

When certain sensory cells  are constantly turned on, they're quickly tuned down  so that they stop firing.

But if that's true, then when we stare at something like an object without flinching, why does the image not just disappear as our photo receptors become fatigued?

Well because, unnoticed by you or me, our eyes are always moving in these microscopic little flutters called saccades to make sure that the stimulation on our visual receptors is always changing.

Here is a really cool way that you can demonstrate sensory adaptation involving temperature  that you can do yourself at home.

But warning, this show will  not be held responsible for any freak outs you or others experience because this is pretty freaking weird.

Here's what you're gonna  need.

Three bowls of water.

One's ice water, one's room temperature water, and one is hot water, but not too hot to touch of course.

We're gonna put one hand in the ice water and one hand  in warm water for one minute.

(fast forward sounds) Then put them both in the room-temperature water.

What do you feel?

The room temperature water feels warm to the hand that had been in the cold and cold to the hand that  had been in the warm water.

Even though both hands are in the same room temperature water, you perceive a different sensory experience.

It's because your body stopped listening to the cold receptors in the hand that came out of the ice water.

So only the warm receptors are firing there.

And the hand that came out of the hot water, it tuned down its warm receptors so only the cold one fired in this room temperature water.

This is so weird.

That's because your senses  are tuned to detect changes, not absolute measurements.

Interestingly, we can also see adaptation when it comes to sound.

Your hearing can adapt extremely  quickly in certain cases.

When you detect loud sustained  noise, like say a jet engine nearby, there's a small muscle in  your inner ear that contracts and dampens the vibrations  reaching your sensory cells in order to protect your eardrum  and inner ear from damage.

However, this muscular mute button takes a few seconds to engage.

So it can't react to an extremely sudden stimulus like a gunshot, which can still  cause damage to your ears.

Now obviously, in a quiet room, you can easily hear a friend speaking next to you, but you've probably also been in a situation where you can  hear your friends speaking next to you in the middle of loud concert or something.

This is because the sensory cells in your inner ear tune down the sound from the concert and stop firing so that your sensory input can focus on the new thing that is not the background, your friend's voice.

If you live next to train tracks or a loud highway, you've probably experienced this too.

After repeated exposure to  the same stimulus of trains or vehicles going by, you  just no longer hear them.

But if someone visits you  that's not used to this noise, they might ask you how you're able to stand it.

The answer, sensory adaptation.

One of the weirdest types of  sensory adaptation happens with your sense of proprioception, knowing where you are in physical space.

Back in the 1890s, this psychologist wore these special glasses that flipped, reflected his  whole world top to bottom.

And after wearing these for five days, suddenly he wasn't seeing upside down anymore.

His brain had adapted and  flipped what he was seeing so that the world just looked rightside up.

This tells us something important, that sensory adaptation doesn't just occur at the level of the sensory cells themselves.

It also happens in the higher processing areas of your brain.

Now this next one kind of freaks me out and makes me question if  anything I see is even real.

Psychologists put people in this simple machine where you push a button and it makes a light go on.

At first, the button and flash are totally in sync.

Push button, light comes on immediately.

Now what's weird is the sensation of your finger pushing the button and the sensation of light hitting your eyes, they actually arrive at slightly different times in your brain because different parts of the nervous system move at different speeds.

But your brain is constantly doing these cool motor sensory feedback tests.

You know that when you're the one who did something, all your senses should be synchronized with that action.

When you push the button, your brain says, I'm going to adjust the timing of all these signals arriving at my brain at different times so that the light hitting my eyes and the sensation in my finger  are in sync because that's what I expect should happen.

But then the researchers make a little tweak and the flash comes on a hundred milliseconds after you push the button.

What does your brain do?

It adjusts your perception of the timing to make it still feel like the light is coming on right when you push the button, even though there's 100 millisecond delay, it adapts your senses and  your perception to make these out of sync things match up.

And it gets weirder.

Finally, after you've adapted to that 100 millisecond delay, they get rid of the delay and the light is actually in sync with the button push again.

And what people say is that they see the flash happening before the push.

Cause and effect get completely reversed thanks to sensory adaptation.

And then of course there's smell.

Each of the oodles of odor molecules that you are capable of detecting fits into a specific smell sensory receptor, just like a key fits in a lock.

And these smell receptors firing alone or in combination is what gives your brain the perception of a particular smell.

But if you're around a smell for a few minutes, those sensory smell cells get tuned down and stop firing.

They stop sending  signals to your brain.

Because again, what your body craves is new stimuli.

It wants to turn down the volume on whatever the basic background level is so that it can detect something novel when it arrives.

That's why when you cook bacon or some delicious Indian food, those wonderful smells are so intense for a few minutes, but then they fade.

But if a  friend walks in your house, they're like, wow, what are you eating, that smells amazing?

This is why you can't smell your own house, even though it definitely has a smell.

And it's why you can't smell yourself.

Even though you definitely  have a natural smell too.

It's the same if you wear perfume or if you smoke.

You can't smell it.

But literally everyone around you  can, at least for a few minutes.

You tune out all of these all of the time sensory experiences.

These are examples of adaptation in just a few of your senses, but literally every sensory system in your body does this in some way.

And more often than you realize.

Sensory adaptation makes you less sensitive to a constant stimulation, which might seem like a bad thing.

But this actually provides a benefit.

You're free to focus on changes in your environment while  ignoring all the background noise.

Without it, you'd be absolutely overwhelmed.

In fact, many people with sensory processing disorders have trouble tuning their senses to keep the volume down to a tolerable level.

Our sensory systems constantly  crave what's new and interesting.

They get bored with repetition.

They tune it out so they can wait for something important to happen.

This is why YouTubers use jump cuts in their videos in order to maintain your attention and constantly give you novelty.

Okay guys, let's just tone it down a little bit.

We get it, okay?

This is a good reminder that what you sense is only a fraction of what's out there in the world around you.

And what you perceive is only  a fraction of what's possible.

At least your brain's trying to  focus on what's important though,