(slide whistle)
- As a kid, I was obsessed with Magic Eye pictures.
I just look at them all the time.
And I became really good at seeing the 3D image quickly.
In other words, through practice,
I gained extra voluntary control
over the movement of my eyes.
And I retain that ability to this day,
despite the fact that I don't really look at
Magic Eye pictures much anymore.
I mean literally, one a day, max.
And it's this extra voluntary control that means I'm good
at going cross-eyed and it means
that I can move one eye independently of the other.
Which is what I'm gonna show you how to do in this video,
just in a few seconds, actually.
But also as I was researching the video,
I discovered all these amazing things
about the way our eyes move.
It's really fascinating stuff. So I'm gonna talk about that
as well, stick around for it.
But first of all, here is how you move one eye
independently of the other.
The first thing you need to do is go cross-eyed.
And I think that's something that
most people can do probably, or at least to some degree.
But often what people do when they go cross-eyed
is that they go like this, they look down.
And there's a good reason for that.
It makes it easier 'cause you got this stimulus here.
You've got something for your eyes
to converge on, your nose.
But it's better if you can go cross-eyed
whilst looking straight ahead, like that.
And you can practice that, you can gain that ability.
If you put your finger right in front of your face
like that, so you're going cross-eyed.
When you remove your finger,
your eyes will suddenly converge on something
in the distance.
In my case, the camera that I'm filming with.
What you wanna try and do is when you remove your finger,
just try and slow that conversion.
So when you remove your finger, you'll see a double image,
and that double image will snap into a single image.
Try and slow that down so the two
just come together more slowly.
Eventually, you'll be able to get those two images to stop.
So they don't snap together.
And then with a bit more practice,
you'll be able to pull them back again and separate them.
So practice that for a bit so that you can go cross-eyed
whilst looking straight ahead.
The next step for moving one eye on its own is really easy.
So when you go across-eyed like that, you see double.
One image drifts to the left, the other image
drifts to the right.
And all you have to do is decide to yourself,
I am going to follow one of those images.
I'm not gonna have one drifter that I've wanted to like.
I'm just gonna track one of them as it moves.
And it's really not hard.
It's not like, you know, entering the "Heart of Stone,"
for Rothfusss fans.
It's like, you just think it and it happens.
So that's how you do it.
I'm genuinely interested to know
if those instructions are helpful.
So let me know in the comments, if you've been able
to move one eye on its own.
It might take a bit of practice, so come back
and leave a comment if that's you.
So let's talk about how eyes move.
It's really interesting.
There's the mechanics of it and there's the neurology of it.
Like what's going on inside your brain.
So the mechanics first is really cool.
Each eyeball has six muscles attached to it.
And they kind of splay out as they reach the eyeball.
So they sort of look like hands.
I've built a model of how it looks.
So here we go, shut up, it's a good model.
So think about this every time your eyes move.
It's like little hands pulling on your eyeballs
inside your skull.
It's a nice thought, isn't it?
I'm only showing four of the muscles on here.
These are the muscles that pull the eyeballs this way
and this way.
The other two, cause your eyeball to move
in this twisting motion.
It's not something you do very much.
It's very subtle, but it is very important.
What's really interesting is these modes of eye movement
that you have.
There are four of them, four main ones.
They are saccades.
They are smooth pursuit vestibulo-ocular, and vergence.
And I wanna talk about all of them,
'cause they're all interesting.
Actually, we've already talked about vergence.
That's the movement that your eyes make to converge
on near and distant objects.
The one that you can gain voluntary control of
to go cross-eyed.
So saccades, let's look at saccades.
So a saccade is the basic eye movement.
So if you're looking at one thing and you decide that
you're gonna look at something else,
your eye will saccade from one to the other.
It's a rapid movement from one place to the next.
And, okay, two interesting things about saccades,
the first is that during a saccade, you are blind.
So like, imagine if you saw what was happening
during a saccade.
It would just be a blur, would be a sickening motion blur.
So instead, the communication pathway between your eyes
and your brain is shut down.
Like a little bit of information gets through,
but not much. You're essentially blind during that movement.
It's like tens of milliseconds,
but you don't see anything during that period.
What's interesting is your brain.
So your brain is hiding the saccade,
but it's also hiding the fact that it's hiding it.
Like surely if no information is getting from your eyes
to your brain, you would see just black during the saccade.
You kind of have a mini blackout every time your eyes
did that sudden movement, but you don't.
Instead, your brain just kind of fudges.
It fudges your perception of time.
And it just kind of tries to fill in the gaps for you
so that you don't notice the blackout.
And you can experience this,
look in a mirror and look at your left eye.
Then look at your right eye, then back to your left.
Keep going back and forth between your eyes.
And obviously, your eyes are moving
because you're moving from one to the other,
but you will never see your eyes move in the mirror,
because of saccadic masking.
That moment of movement masks the actual movement
in the mirror.
If you want to see your eyes move when you dart
from one to the other.
Try the experiment again, but using the front-facing camera
of your smartphone, there's a slight delay between
what the camera sees and what's displayed on the screen.
So your eyes will saccade and then you will see it
on the screen.
The second interesting thing about saccades
is that they're described as ballistic.
So the way the eyeball moves,
it's more like a bullet and less like a guided missile.
What that means is, once the eyeball has started to move,
its trajectory cannot be updated with new information.
What that means is, if I show you a dot
on this side of the screen, and then I move the dot
over to here, your eye will saccade to the new location.
But if I move the dots again,
while your eyeball is still moving,
it will continue to move until it reaches that spot
where the dot was.
It's only once it gets there,
that your brain can calculate a new saccade
to try and catch up to the new location of the dot.
So if I move dots around on the screen,
like, you know, once every hundred milliseconds or so
then you won't get to see any of these dots directly.
They'll all be in the periphery of your vision.
Saccades, but what about smooth pursuit?
Well, it's how it sounds really like if you try and track
my finger, as it moves on the screen,
your eyes will smoothly pursue my finger
in your field of view.
And what's really interesting about smooth pursuit
is that it's completely involuntary.
Like you cannot voluntarily move your eyes smoothly.
Like if I ask you to move your eyes from this finger
to this finger, you won't be able to do it
slowly and smoothly.
It will always be a sudden saccade
from this finger to this figure.
Or a series of short saccades that gets you from
one place to the other.
The final type of movement is the
vestibulo-ocular movement.
And this is the movement that tries to compensate
for the movement of your head and body.
Like if I go like this, I'm moving my head around,
but I'm able to keep my eyes fixated
on the lens of the camera.
And you might think that's just another example
of smooth pursuit.
Like everything in my field of vision
is moving around like this.
And so my eyes are just pursuing smoothly
one point in that moving image,
but it's not they're separate systems and you can prove it.
So what you can do is if you close your eyes,
do this now, close your eyes and rest your fingers
just lightly on your eyelids.
And then slowly turn around like this,
and when you do that you will feel your eyeballs moving
underneath your fingers.
So humans stabilize their field of view
against head movement using muscles in their eyes,
but birds do it differently.
They use muscles in their necks
and that's, this looks like thanks to Peter Macintosh.
This relationship between voluntary and involuntary motion
is really interesting.
Like what's happening inside the brain.
And it's not just to do with your eyes,
like any kind of motion that is voluntary or involuntary
take breathing, for example.
So breathing is voluntary. You can choose to breathe.
(exhales heavily)
You can choose to not breathe, you can hold your breath.
(exhales heavily)
Oh, you don't have to think about it at all
and you'll just breathe.
And you basically have a junction in the brain.
So you've got neurons going to the set of muscles
that make you breathe.
And coming into that junction, you've got
the involuntary neurons.
So, you know, you've got neurons coming from your
kind of ancient lizard brain if you like.
And you've got neurons coming from the voluntary part
of your brain and they can override each other
and things like that.
So generally speaking, the voluntary neurons
can override the involuntary neurons,
which is why you can hold your breath.
But there's some really cool chemistry
that goes on around these junctions.
Breathing's a great example.
Basically, if there's a drop in oxygen in the brain,
then the involuntary neurons can override
the voluntary neurons at that junction.
And you suddenly can't hold your breath anymore.
You're forced to breathe.
(exhales heavily)
I want to finish this video with a story
about a professor of neurobiology, Susan R. Berry.
And she was born with cross-eyes so she was stereo blind.
Which means that she doesn't perceive depth
in the same way that we do.
She doesn't have a stereo depth perception.
Like if she goes to see a 3D movie,
it doesn't work, for example.
Her eyes don't converge on objects in the same way
that most people's do.
And then received wisdom, the wisdom that she was teaching
in her classes is that if you don't correct that
within the first year,
then that's it, you'll be like that
for the rest of your life.
But then she consulted
a developmental-behavioral optometrist,
and she showed her how to take voluntary control
of that vergence reflex that we were talking about.
And she managed to do something that most scientists
thought were impossible.
She gained stereo depth perception for the first time,
and she writes about the experience really eloquently.
So I'm gonna read this out,
but there's a link to the article in the description
and you can jump off and read her other writings
from there as well.
She says, "My new view of the world was magical, enchanting
"and beyond anything I could've imagined.
"Tree branches loomed out toward me or grew up forward
"and closing and capturing palpable volumes space
"that I had never seen before."
This video was made possible by brilliant.org.
If you don't know what that is yet,
it's a website full of really fun puzzles and problems
that help you to think like a scientist
and think like a mathematician.
I'm really grateful that they continue
to sponsor this channel
'cause I think it's a really good match.
Like the way they think about education really clicks
with what I'm thinking about in terms of,
you know, how people learn.
So I stumbled across the eight principles recently.
They've laid them all out and I wanna talk about one
in particular, which says that great science
and mass education should allow for failure.
And I really believe that.
I think that if you wanna teach someone to love learning,
you need to teach them to be comfortable with failure.
To not fear failure, 'cause it's such an important part
of the process.
And it's interesting to see how brilliant
have implemented that principle on the website.
Like if you get a question wrong,
the language is a really positive,
and the interface kind of nudges you towards a discussion
of the solution where you can engage
with other members of the community to talk about
how they reached the right answer.
It's really good.
Like, I feel like school should be like that, you know.
So anyway, check it out for free by clicking on the link
in the description today.
And as a bonus, the first 76 people to use that special link
will get 20% of annual premium membership.
Should they choose to upgrade.
I hope you enjoyed this video, if you did don't forget
to hit subscribe and I'll see you next time.
(lively cheerful music)