Dr. David Berson

of encoding, just like in the cones of the retina.

Yeah, the puppy head tilt.

That's the other one.

So the point is that your brain is eventually going to be able to unpack what these sensors are telling you about how you just rotated your head.

Now you can tell if you're rotating your head left or right, up or down.

That's the sensory signal coming back into your brain

confirming that you just made a movement that you will.

A lot of this is happening under the surface of what you're thinking.

These are reflexes.

Maybe the best way to think about how these two systems work together is to think about what happens when you suddenly rotate your head to the left.

When you suddenly rotate your head to the left, your eyes are actually rotating to the right.

So there are cells called ganglion cells.

Automatically, you do this in complete darkness.

If you had an infrared camera and watched yourself in complete darkness, you can't see anything.

Rotating your head to the left, your eyes would rotate to the right.

That's your vestibular system saying, I'm going to try to compensate for the head rotation so my eyes are still looking in the same place.

These are neurons that are the key cells for communicating between eye and brain.

So the brain works really hard to mostly stabilize the image of the world on your retina.

Now, of course, you're moving through the world, so you can't stabilize everything.

But the more you can stabilize most of the time, the better you can see.

And that's why when we're scanning a scene, looking around at things, we're making very rapid eye movements for very short periods of time, and then we just rest.

But we're not the only ones that do that.

If you ever watch a pigeon walking on the sidewalk, it does this funny head bobbing thing.

But what it's really doing is racking its head back on its neck while its body goes forward so that the image of the visual world stays static.

Really?

And you've seen the funny chicken videos on YouTube, right?

You take a chicken, move it up and down, and the head stays in one place.

It's all the same thing.

All of these animals are trying hard

to keep the image of the world stable on their retina as much of the time as they possibly can.

And then when they've got to move, make it fast, make it quick, and then stabilize again.

The eye is like the camera.

It's detecting the initial image, doing some initial processing, and then that signal gets sent back to the brain proper.

And of course, it's there at the level of the cortex that we have this conscious visual experience.

I mean, I think the fundamental problem typically when you get motion sick is what they call visual vestibular conflict.

That is, you have two sensory systems that are talking to your brain about how you're moving through the world.

There are many other places in the brain that get visual input as well, doing other things with that kind of information.

And as long as they agree, you're fine.

So if you're driving, your body senses that you're moving forward.

Your vestibular systems is picking up this acceleration of the car and your visual system is seeing the consequences of forward motion in the sweeping of the scene past you.

Everything is honky dory, right?

No problem.

But when you are headed forward, but you're looking at your cell phone, what is your retina seeing?

Your retina is seeing the stable image of the screen.

There's absolutely no motion.

in that screen.

Or if you're playing a game or you're watching a video, a football game, you know, the motion is uncoupled with what's actually happening to your body.

Your brain doesn't like that.

Your brain likes everything to be, you know, aligned.

And if it's not, it's going to complain to you.

By making me feel nauseous.

By making you feel nauseous and maybe you'll change your behavior.

So you're getting... I'm getting punished.

Yeah, for setting it up so your signals don't conflict.

Yeah.

So, you know, the way I try to describe the cerebellum to my students is,

is that it serves sort of like the air traffic control system functions in air travel.

It's a system that's very complicated and it's really dependent on great information.

So it's taking in the information about everything that's happening everywhere, not only through your sensory systems, but it's listening into all the little centers elsewhere in your brain that are computing what you're going to be doing next and so forth.

And it really has an important role in coordinating and shaping movements.

But it's not that you would be paralyzed if your cerebellum was gone, because you still have motor neurons, you still have ways to talk to your muscles, you still have reflex centers, but you wouldn't be...

coordinating things so well anymore.

The timing between input and output might be off.

Or if you were trying to practice a new athletic move, like an overhead serve in tennis, you'd be just terrible at learning all the sequences of muscle movements and the feedback from your sensory apparatus that would let you really hit that ball exactly where you wanted to after the nth rep, right?

The thousandth rep or something, you get much better at it.

So the cerebellum is all involved in things like motor learning and refining the precisions of movement so that they get you where you want to go.

Right.

If you reach for a glass of champagne, that you don't knock it over or stop short.

So the first thing to understand about light is that it's just a form of electromagnetic radiation.

Absolutely.

The typical thing would be...

a patient who has a cerebral stroke or a tumor, for example, might be not that steady on their feet.

If the dynamics of the situation, you're standing on a streetcar with no pole to hold onto, they might not be as good at adjusting all of the little movements of the car.

There's a kind of tremor that can occur as they're reaching for things.

because they reach a little too far and then they over-correct and come back, things like that.

So it's very common neurological phenomenon, actually.

Cerebellar ataxia is what the neurologists call it.

And it can happen not just with cerebellar damage, but damage to the tracts that feed the information into the cerebellum.

Exactly, or output from the cerebellum.

In a very key place in the cerebellum, which is, it's really one of the oldest parts in terms of evolution.

The flocculus, right.

It's vibrating, it's oscillating.

This is a, it's a critical place in the cerebellum where visual and vestibular information comes together for recording just the kinds of movements we were talking about.

This image stabilizing network, it's all happening there.

And there's learning happening there as well.

So that if,

Your vestibular apparatus is a little bit damaged somehow.

Your visual system is actually talking to your cerebellum saying there's a problem here, there's an error.

And your cerebellum is learning to do better by increasing the output of the vestibular system to compensate for whatever that loss was.

So it's a little error correction system that's sort of typical of cerebellar function.

And it can happen in many, many different domains.

This is just one of the domains of sensory motor integration that takes place there.

Well, in a sense, photons are certainly moving through space.

Yeah, so there's a lot of pieces there.

I think the first thing to say is if you imagine...

the nervous system in your mind's eye you see this big honking brain and then there's this little thin little uh wand that dangles down into your vertebral column the spinal cord and that's kind of your visual impression um

We think about photons as particles, and that's one way of thinking about light.

What you have to imagine is starting in the spinal cord and working your way up into this big, magnificent brain.

And what you would do as you enter the skull is get into a little place where the spinal cord kind of thickens out.

It still has that sort of long, skinny, trunk-like feeling.

Right.

It starts to spread out a little bit.

And that's because your evolution has packed more interesting goodies in there for processing information and generating movement.

So this midbrain you're talking about is the last bit of this enlarged sort of spinal cordy thing in your skull, which is really the brainstem is what we call it.

The last bit of that before you get to this relay up to the cortex is the midbrain.

And there's a really important visual center there.

But we can also think of it as a wave, like a radio wave.

It's called the superior colliculus, but this is where most of the action is in terms of interpreting visual input and organizing behavior around that.

You can sort of think about this region of the brainstem as a reflex center that can reorient the animal's gaze or body, or maybe even attention to particular regions of space

out there around the animal and that could be all for all kinds of reasons i mean it might be a predator just showed up in one corner of the forest and you pick that up and you're trying to avoid it or just any movement many movement right it might be you know that suddenly uh you know something splats on the page when you're reading a novel and and your eye

Either way is acceptable.

reflexly looks at it you don't have to think about that that's a reflex but these are centers that emerged early in the evolution of brains like ours to handle complicated visual events that have significance for the animal in terms of space where is it in space and in fact this same center actually gets input from all kinds of other sensory systems that take information from the external world from particular locations

And the radio waves have frequencies, like the frequencies on your radio dial.

and where you might want to either avoid or approach things according to their significance to you.

So you get input from the touch system.

You get input from the auditory system.

I worked for a while in rattlesnakes.

They get input from a part of their warm sensors on their face.

They're in these little pits.

They have a version of an extra receptive sensory system.

And certain frequencies in the electromagnetic spectrum can be detected by neurons in the retina.

That is, they're looking out into the world

using a completely different set of sensors.

They're using the same sensors that would feel the warmth on your face if you stood in front of a bonfire, except evolution has given them this very nice specialized system that lets them image where the heat's coming from.

You can sort of do that anyway, right?

If you walk around the fire, you can feel where the fire is from the heat hitting your face.

It's one of the major ways.

And in fact, they use vision as well.

And they bring these two systems together in the same place, in this tectum region, this brainstem.

Those are the things we see.

But there are still different wavelengths within the light that can be seen by the eye.

Now the brain is confused and that may be where your motion sickness comes from.

So it's great to have, as a brain, it's great to have as many sources of information as you can have.

Just like if you're a spy or a journalist, you don't want as much information as you can get about what's out there.

But if things conflict, that's problematic, right?

Your sources are giving you different information about what's going on.

And those different wavelengths are unpacked in a sense or decoded by the nervous system to lead to our experience of color.

Now you've got a problem on your hands.

What do you publish?

Essentially, different wavelengths give us the sensation of different colors.

Yeah, so I mean, the basal ganglia are sitting deep in what you would call the forebrain, so the highest levels of the brain, and it's deeply intertwined with cortical function.

The cortex can't really do what it needs to do without the help of the basal ganglia and vice versa.

And in a way, you can think about this logically as saying, if you have the ability to withhold behavior or to execute it, how do you decide which to do?

through the auspices of different neurons that are tuned to different wavelengths of light.

Well, the cortex is gonna have to do that thinking for you.

You have to be looking at all the contingencies of your situation to decide, is this a crazy move or is this a really smart investment right now or, you know, what?

I mean, another great example is that, you know, the marshmallow test for the little kids, you know, they can get two marshmallows if they hold off, you know, just 30 seconds initially, you know.

They can have one right away, but if they can wait 30 seconds, they got two.

So that's the no-go because their cortex is saying, I would really like to have two more than having one, but they're not going to get the two unless they can not reach for the one.

So they've got to hold off the action.

And that has to result from a cognitive process.

So the cortex is involved in this in a major way.

I mean, I think it's really just another, it's a special case of a very general phenomenon, which is brains are complicated.

And the brains we have are the result of genetics and experience.

And my genes are different from your genes and my experiences are different from your experiences.

So the things that would be easy or hard for us won't necessarily be aligned.

They might just happen to be just because they are.

But the point is that you're dealt a certain set of cards, you have certain set of genes, you are handed a brain.

You don't choose your brain, it's handed to you.

But then there's all this stuff you can do with it.

You can learn to have new skills or to act differently or to show more restraint, which is kind of relevant to what we're talking about here.

Right.

Sure.

So the point is that you all, those of us who see, have representations of the visual world in our visual cortex.

What happens to somebody when they become blind because of problems in the eye, the retina perhaps, right?

you have a big chunk of the cortex is really valuable real estate for neural processing that has come to expect input from the visual system and there isn't any anymore.

So you might think about that as fallow land, right?

It's just it's unused by the nervous system and that would be a pity but it turns out that it is in fact used.

And the case that you're talking about is of a woman who was blind from very early in her life and who had risen through the ranks to a very high level executive secretarial position in a major corporation.

And she was extremely good at Braille reading and she had a Braille typewriter and that's how everything was done.

Right.

So if you imagine that in the first layer of the retina where this transformation occurs from electromagnetic radiation into neural signals, that you have different kinds of sensitive cells.

And apparently she had a stroke and was discovered at work, collapsed and they brought her to the hospital.

And apparently the neurologist who saw her when she finally came to said, you know, I've got good news and bad news.

Bad news is you've had a stroke.

The good news is that it was in an area of your brain you're not even using.

It's your visual cortex.

And I know you're blind from birth, so there shouldn't be any issue here.

The problem was she lost her ability to read Braille.

So what appears to have been the case, and this has been confirmed in other ways by imaging experiments in humans, is that in people who are blind from very early in birth, the visual cortex gets repurposed as a center for processing tactile information.

And especially if you train to be a good braille reader, you're actually reallocating somehow that real estate experience.

to your fingertips, a part of the cortex that should be listening to the eyes.

So that's an extreme level of plasticity, but what it shows is the visual cortex is kind of a general purpose processing machine.

It's good at spatial,

and the skin of your fingers is just another spatial sense and deprived of any other input.

The brain seems smart enough, if you want to put it that way, to rewire itself, to use that real estate for something useful.

In this case, reading braille.

At least in that case, tragic.

Right.

that are expressing, they're making different molecules within themselves for this express purpose of absorbing photons, which is the first step in the process of seeing.

um the nervous system is doing things that i hadn't thought about i call you so i've uh please forgive me for the calls past present and future unless you change your number and even if you do i'll be calling it's been such a blast andy you know this has been a great uh session and it's always fun talking to you it always gets my brain racing so um thank you thank you

Now it turns out that altogether there are about five proteins like this that we need to think about in the typical retina.

But for seeing color, really it's three of them.

So there are three different proteins, each absorbs light with a different preferred frequency.

And then the nervous system keeps track of those signals, compares and contrasts them to extract some understanding of the wavelength composition of light.

So you can see just by looking at a landscape, oh, it must be late in the day because things are looking golden.

That's all a function of our absorbing the light that's coming from the world and interpreting that with our brain because of the different composition of the light that's reaching our eyes.

I'm just kidding.

It's a great question.

It's a deep philosophical question.

It's a question that really probably can't even ultimately be answered.

by the usual empirical scientific processes, because it's really about an individual's experience.

What we can say is that the biological mechanisms that we think are important for seeing color, for example, seem to be very highly similar from one individual to the next, whether it be human beings or other animals.

And so we think that the physiological process looks very similar on the front end, but once you're at the level of perception or understanding or experience, that's something that's a little bit tougher to nail down with the sorts of scientific approaches that we approach biological vision with, let's say.

It's not really five types of cones.

There are really three types of cones.

And if you look at the way that color vision is thought to work, you can sort of see that it has to be three different signals.

There are a couple of other types of pigments.

One is really mostly for dim light vision.

When you're walking around in a moonless night and you're seeing things with very low light, that's the rod cell and that uses its own pigment.

And then there's another class of pigments we'll probably talk about a little bit later, this melanopsin pigment.

Right.

So in the case of a typical, well, let's put it this way, in human beings, most of us have three cone types and we can see colors that stem from that.

In most mammals, including your dog,

um or your cat there really are only two cone types and that limits the kind of vision that they can have in the domain of wavelength or color as you would say let's talk about the that odd photopigment

Yeah, so this last pigment is a really peculiar one.

One can think about it as really the initial sensitive element in a system that's designed to tell your brain about how bright things are in your world.

And the thing that's really peculiar about this pigment is that it's in the wrong place in a sense.

When you think about the structure of the retina, you think about a layer cake, essentially.

You've got this thin membrane at the back of your eye, but it's actually a stack of thin layers.

How does that work?

And the outermost of those layers is where these photoreceptors you were talking about earlier are sitting.

That's where the film of your camera is, essentially.

That's where the photons do their magic with the photopigments and turn it into a neural signal.

Right.

So this is an old question, obviously.

It's the surface on which the light pattern is imaged by the optics of the eye.

And now you've got an array of sensors that's capturing that information and creating a bitmap, essentially.

but now it's in neural signals distributed across the surface of the retina.

And clearly in the end, the reason you have a visual experience is that your brain has got some pattern of activity that it associates with the input from the periphery.

But it turns out that this last photopigment is in the other end of the retina, the innermost part of the retina.

That's where the so-called ganglion cells are.

Those are the cells that talk to the brain, the ones that actually can communicate directly

what information comes to them from the photoreceptors.

And here you've got a case where actually some of the output neurons that we didn't think had any business being directly sensitive to light were actually making this photopigment.

absorbing light and converting that to neural signals and sending to the brain.

That's your circadian system.

It's keeping time and it's all built into our biology.

And this is actually one of the things that blind patients often complain about.

If they've got retinal blindness,

is insomnia and- Because the brain's awake in the middle of the night.

Exactly.

They're not synchronized.

Their clock is there, but they're drifting out of phase because their clock's only good to 24.2 hours or 23.8 hours.

Little by little, they're drifting.

So you need a synchronization signal because otherwise you have nothing to actually confirm when the rising and the setting of the sun is.

That's what you're trying to synchronize yourself to.

Right.

So the first thing to say is that, as you said, the clock is all over the place.

But you can have a visual experience with no input from the periphery as well.

Most of the tissues in your body have clocks.

The role of the central pacemaker for the circadian system is to coordinate all of these.

There's a little nucleus, a little collection of nerve cells in your brain.

It's called the suprachiasmatic nucleus, the SCN.

And it is sitting in a funny place for the rest of the structures in the nervous system that get direct retinal input.

It's sitting in the hypothalamus, which you can think about as sort of the great coordinator of

When you're dreaming, you're seeing things that aren't coming through your eyes.

drives and- The source of all our pleasures and all our problems.

Yes, it really is.

But it's sort of deep in your brain, things that drive you to do things.

If you're freezing cold, you put on a coat, you shiver, all these things are coordinated by the hypothalamus.

So this pathway that we're talking about from the retina and from these peculiar cells that are encoding light intensity are sending signals directly into

a center that's surrounded by all of these centers that control autonomic nervous system and your hormonal systems.

The hypothalamus uses everything to control the rest of the bodies.

And that's true of the suprachiasmatic nucleus, this circadian center as well.

It can get its fingers into the autonomic nervous system, the humeral system, and of course, up to the centers of the brain that organize coordinated rational behavior.

Are those memories?

I would say in a sense, they may reflect your visual experience.

Right.

The way this is seen is that if you were to measure your melatonin level over the course of the day, if you could do this, you know, hour by hour, you'd see that it's really low during the day, very high at night.

They're not necessarily specific visual memories, but of course they can be.

But if you get up in the middle of the night and go to the bathroom and turn on the bright fluorescent light, your melatonin level is slammed to the floor.

Light is directly impacting your hormonal levels through this mechanism that we just described.

So this is one of the routes by which light can act on your hormonal status through pathways that are completely beyond what you normally would think about, right?

But the point is that the experience of seeing is actually a brain phenomenon.

You're thinking about the things in the bathroom.

Oh, there's the toothbrush.

You know, there's the tube of toothpaste.

But meanwhile, this other system is just counting photons and saying, oh, wow, there's a lot of photons right now.

Let's shut down the melatonin release.

Cool.

So, I mean, the first thing to think about is that the vestibular system

is designed to allow you to sense how you're moving in the world, through the world.

Basically, the idea is that if we're just sitting in a car

But of course, under normal circumstances, we see the world because we're looking at it and we're using our eyes to look at it.

in the passenger seat and the driver hits the accelerator and you start moving forward, you sense that.

If your eyes were closed, you'd sense it.

If your ears were plugged and your eyes were closed, you'd still know it.

Anything that jostles you out of the current position you're in right now will be detected by

the vestibular system pretty much.

It's basically in your inner ear, hairy cells, they got little cilia sticking up off the surfaces.

And depending on which way you bend those, the cells will either be inhibited or excited.

But then they talk to neurons with a neuron-like process and off you go.

Now you've got an auditory signal if you're sensing things bouncing around in your cochlea, which is sympathetically the bouncing of your eardrum, which is sympathetically the sound waves in the world.

But in the case of the vestibular apparatus, evolution has built a system that detects the motion of, say, fluid going by those hairs.

And if you put a sensor like that in a tube that's fluid-filled, now you've got a sensor that will be activated when you rotate that tube around the axis that passes through the middle of it.

And fundamentally, when we're looking at the exterior world, it's what the retina is telling the brain that matters.

Right, three hula hoops.

Right, one the other way.

Three directions.

So three axes.