Chapter 2: Vision Basics: Light to Neural Signals

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Welcome back to the Deep Dive.

Today, we are doing something a little bit different, something I'm honestly very excited about.

We're stripping away the noise, the op -eds, the conflicting headlines, and we're going straight to the source for what we're calling a sort of last -minute lecture series.

I like that.

We're tackling the absolute bedrock of how we experience reality.

We're talking about chapter two of Sensation to Perception, the sixth edition.

It's a foundational chapter.

I mean, the title really says it all.

The first steps in vision from light to neural signals.

And I don't think people realize just how much heavy lifting happens before your brain even gets a say in the matter.

This is basically the hardware manual for your eyes.

Exactly.

So if you're a student and you're maybe panicking a little because you have a midterm tomorrow and you still don't totally get the difference between a rod and a cone.

We've all been there.

Or if you're just someone who wants to understand how you are actually seeing, like physically seeing, this deep guide is for you.

And we're going to be pretty rigorous today.

We're following the text, tracking the concepts, page by page.

We're really going to walk through the chapter chronologically, just like you would in a lecture.

But hopefully with a bit more life to it.

Hopefully.

The goal is to demystify some of that really dense terminology.

I love that.

And I want to set the scene with the imagery the book uses right at the very start because it really grounds the physics in something, you know, human.

Okay.

Imagine you're standing outside on a clear, dark winter night.

You look up and you see a star.

It's a classic starting point for a reason.

That little twinkle of light has traveled an unbelievably long way.

It really has.

I mean, if you're looking at a star in our own galaxy, that light might be 2000 light years old.

And if you manage to spot the faint smudge of the Andromeda galaxy, the closest one to us.

Yeah.

That light has been traveling for over 2 million years, 2 million years, just to hit your eyeball.

It's mind boggling.

And that's our mission today, isn't it?

It is.

We're going to track the journey of that specific little photon of starlight.

From the moment it hits the Earth's atmosphere,

passes through the window of your eye, gets focused, and finally, and this is the crucial part, gets converted into an electrical signal.

Right.

And just to be clear on the scope for everyone listening,

we are stopping before we get to the brain.

Good point.

We aren't talking about the visual cortex, the occipital lobe, none of that today.

We are strictly in the eye.

It's all about the physics of light, the anatomy of the eye, and the wild neural processing that happens in the retina itself.

It's the capture and the initial processing, the raw data entry.

That's a perfect way to put it.

It's all about how the eye captures light and transduces it.

That word transduce is going to be key.

It means changing one form of energy into another.

Precisely.

In this case, light energy into the electrical energy that your nervous system understands.

Okay.

Before we dive into the nitty gritty, the chapter kicks off with some questions to contemplate.

And I love these because they're basically the learning objectives.

They frame the whole chapter.

If you can answer these, you've got it.

So first,

how are images actually formed on the retina?

What are the optics?

Second, how does that raw light energy turn into an electrical spark inside a cell?

That's the transduction part.

Third, what does it actually mean when a doctor says you have 20 -20 vision?

Is that perfect?

Is it just average?

And finally, and this is the big one for me, how on earth do we manage to see in a dark movie theater and then walk out onto a sunny beach?

The difference in brightness is astronomical.

That one really does blow my mind.

The range is just incredible.

But let's start at the beginning.

Part one, a little light physics.

I think most people have a vague idea that light is energy, but the book gets very specific.

It does.

Light is a form of electromagnetic radiation.

And this is where it gets a little tricky because light has a dual nature.

A wave and a particle at the same time.

Yeah, which sounds like a paradox.

The book suggests a really good rule of thumb to avoid getting bogged down in quantum mechanics.

Okay, I think we need that.

Just treat light as a wave when it's moving through the world, you know, bouncing off objects or passing through the air.

But when it's being absorbed by something, like a photoreceptor in your eye, it's better to think of it as a stream of tiny particles called photons.

So a wave for travel, a particle for impact.

That's a helpful distinction.

Exactly.

Now let's talk about the spectrum.

We hear about the visible spectrum all the time, but figure 2 .1a in the text really puts it in perspective.

Oh, it's a humbling chart.

I mean, the full electromagnetic spectrum is massive.

It goes all the way from gamma rays, which have incredibly short wavelengths.

High energy, the dangerous stuff.

Right.

All the way up to radio and television waves, which can be meters or even kilometers long and visible light.

The stuff we can actually see is just this tiny, tiny little slice of that whole pie.

Specifically, it says between 400 and 700 nanometers.

Correct.

And a nanometer is one billionth of a meter.

So 400 nanometers looks violet to us and 700 nanometers looks red.

Everything else we see, all the colors of the rainbow falls in that narrow little band.

There's a quote in the source that I just love.

It lays out an analogy for the scale.

It says, if you laid the spectrum out linearly,

and the distance from radio waves to x -rays was the distance from New York to Los Angeles.

I love this one.

It's a great visualization.

And the part we can see, the visible light, would be less than an eighth of an inch wide.

An eighth of an inch across the entire United States.

That's all we get.

That is just wild.

It really highlights how much of the universe is technically invisible to us without instruments.

It's true.

We are effectively blind to the vast majority of reality.

We just happen to be tuned into this specific frequency because it's what's abundant from our sun and it interacts usefully with matter on our planet.

Speaking of interacting with matter, the book brings up Rayleigh Scattering.

And this is the answer to that classic kid question.

Why is the sky blue?

It is.

So when sunlight hits our atmosphere, it's not empty space.

It's full of dust, water vapor, air molecules, and the particles.

Exactly.

And Rayleigh Scattering describes how those particles scatter light.

The physics says that shorter wavelengths, the blue end of the spectrum, get scattered much more easily and in all directions compared to the long wavelengths.

So the blue light just gets bounced around all over the place, filling the sky.

Precisely.

It's ricocheting off all those particles and coming at your eyes from every direction so the whole dome of the sky looks blue.

But then you have to think about a sunset.

Right.

When the sun is low on the horizon.

Exactly.

The sunlight has to pass through a much, much thicker slice of atmosphere to get to you.

By the time that light gets to your eye, most of the short wavelength blue light has been scattered away completely.

It's just gone.

So what's left?

The longer wavelengths, the reds, the oranges, the yellows, they can punch through the atmosphere more directly without getting scattered as much.

So a red sunset is basically just sunlight with all the blue stripped out of it.

That is a perfect way to think about it.

It's filtered sunlight.

Okay.

So that's light moving through the air, but eventually light hits stuff.

The text lists four things that can happen to light when it hits a surface.

Reflection, absorption, transmission, and refraction.

Let's run through those quickly because these terms are going to come up again when we talk For sure.

Reflection is the easy one.

Light bounces off a surface.

This is why things look light or bright.

A piece of white paper reflects most of the light that hits it.

That absorption is the opposite.

Yeah.

The surface takes in the light energy, usually converting it to heat.

A dark surface, like black asphalt, absorbs most of the light.

That's why it gets so hot in the summer and, well, why it looks dark.

Transmission.

That's just light passing through something like a clean window or a glass of water, or very importantly for us, passing through the cornea of your eye.

And the last one, refraction.

This one is critical.

Absolutely critical.

Refraction is the bending of light.

It happens whenever light passes from one medium to another, like from air into water.

Like when you stick a straw in a glass of water and it looks like it's broken at the surface.

That's a perfect example.

Light changes speed when it changes mediums, and that change in speed causes it to bend.

And this is the fundamental principle of how a lens works.

Or how an eyeball works.

Exactly.

In fact, when you go to the eye doctor for new glasses, that whole test where they flip the lenses in front of your eye.

Better one or better two.

Yeah.

It's literally called a refraction.

They're measuring how much they need to artificially bend the light to get it to focus perfectly on your retina.

Which brings us perfectly to part two.

Eyes that capture light.

We're moving into the anatomy and the optics.

Figure 2 .2 shows a cross section of the eye.

So let's follow that starlight.

First up,

the cornea.

The cornea.

It's the transparent window at the very front of the eye.

And it's fascinating because to be transparent, it has to be unique.

It has absolutely no blood vessels.

None.

But cells need blood to live, right?

For oxygen and nutrients.

They do.

So the cornea has to scavenge.

It gets its oxygen directly from the air and from the tear film on the front.

And it gets its nutrients from the fluid right behind it, which is called the oqueous humor.

So it's living off the land, so to speak.

Pretty much.

The text also mentions that even though it has no blood, it has a ton of nerve endings.

Oh yeah.

Anyone who's had a tiny eyelash in their eye knows that.

It's one of the most sensitive tissues in the body.

And that's a defense mechanism, right?

The intense pain makes you close your eye, tear up, and protect that vital window.

The good news is, it heals incredibly fast.

Usually a minor scratch is gone in 24 hours.

Okay, so the light passes through the cornea, then the oqueous humor, and then it hits the iris and the pupil.

Right.

The iris is the muscle.

It's the colored part of your eye, brown, blue, green.

That's your iris.

And the pupil isn't actually a thing.

It's just the hole in the middle of the iris.

An absence of a thing.

And the iris controls the size of that hole.

Exactly.

That's the pupillary light reflex.

When you walk into bright light, the iris constricts, making the pupil smaller to let less light in.

In a dark room, it dilates, opening the pupil up to let more light in.

But the book mentions there's a trade -off here, which is interesting for anyone into photography.

Yes.

The depth of focus.

When your pupil is huge, fully dilated in low light, your depth of focus is very shallow.

The image quality actually gets a little worse, a little blurrier.

Like using a wide open aperture on a camera.

It's the exact same principle.

When the pupil is small and constricted in bright light, your depth of focus is much greater.

Everything is sharper, from near to far.

Okay.

Next up is the lens.

Also, no blood supply.

Same reason, it has to be transparent.

But the key difference here is that the lens is adjustable.

The cornea does most of the heavy lifting for bending light, about two -thirds of the total refractive power, but it's fixed.

It can't change its shape.

So the lens does the fine -tuning.

The lens does all the fine -tuning.

And then after the lens, we have the biggest part of the eye,

the vitreous humor.

Yeah.

This is the big gel -like substance that fills up about 80 % of the eye's volume.

Its main job is just to keep the eyeball spherical.

But it's also where you get floaters.

It is.

Have you ever looked at a bright blue sky or a white computer screen and seen little specks or squiggly lines drifting around?

Oh yeah.

I have a few of those.

That's literally bio -debris.

Little bits of protein or cell strands that have broken off and are just drifting around in that vitreous gel, casting shadows on your retina.

And because the gel doesn't really cycle out, they just kind of stay there forever.

Great.

So the light has made it through all those structures.

Now we need to actually focus it.

The text makes a really big deal about accommodation.

This is a crucial concept.

Figure 2 .3 in the book has a great diagram of this.

So as we said, the cornea has a fixed focusing power.

Accommodation is the process where the lens changes its shape to adjust the focus, specifically for near objects.

And this involves my favorite anatomical name in the entire book.

I think I know what you're going to say.

The Zonials of Zin.

It sounds like a sci -fi villain or a prog rock band, doesn't it?

And now the Zonials of Zin.

It really does.

Yeah.

But they're just tiny fibers holding the lens in place.

Right.

And here's the mechanism, and it's totally counterintuitive, so you have to listen closely.

Okay, I'm ready.

When you look at something far away, the ciliary muscles, this ring of muscle around the lens, are relaxed.

When they relax, they pull the Zonials of Zin tight.

Wait, relaxing the muscle makes the fibers tight?

Yes.

Think of the muscle as a big ring.

When it relaxes, it gets wider.

That pulls on the fibers attached to its inner edge, which stretches the lens out, making it flat and thin.

A flat lens has less power, which is perfect for focusing distant light.

Okay, so far away equals relaxed muscle, tight fibers, flat lens.

Exactly.

But now you want to look at something close up, like your phone.

To do that, your ciliary muscles have to contract.

Okay, so they flex.

They flex.

The ring of muscle gets smaller.

This releases the tension on the Zonials of Zin.

It creates slack in those little ropes.

So contracting the muscle relaxes the fibers.

That's so weird.

It is weird.

And because the lens is naturally elastic, it wants to ball up.

So when those fibers go slack, the lens bulges out, gets fatter and more curved.

A fatter, curvier lens bends light more, which is what you need to focus on that close -up text message.

Checking your phone is literally a muscular workout for your eye.

Constant muscular effort.

That's why you get eye strain from reading for hours.

Your ciliary muscles are flexed the whole time.

But this ability to accommodate?

It doesn't last forever.

Sadly, no.

And that brings us to presbyopia, which literally means old sight.

Figure 2 .4 in the book shows a graph of this.

And whoa, it's a little depressing if you're over 30.

It's a cliff.

The ability to accommodate starts dropping from about age 8, and then it just falls off a cliff.

By age 50, it's almost completely gone.

Why?

What happens?

Two things.

The lens itself hardens and becomes less elastic, and the capsule it's in also loses its elasticity.

It just can't bulge out anymore, even when the ciliary muscles contract and the Zonials relax.

And this is why you see older people holding a menu way out at arm's length to read it.

Exactly.

Their near point, the closest point they can focus on, has moved further away than their arms are long.

And the solution, invented by Ben Franklin, no less, Is bifocals.

Or reading glasses.

He got tired of switching between his distance glasses and his reading glasses, so the story goes he just cut the lenses in half and glued them together in one frame.

Brilliant.

Okay, so that's a problem with the lens.

What about when the shape of the eye itself is the problem?

The refractive errors.

Right.

So the ideal situation is called emetropia.

That's perfect vision.

It means the optical power of your cornea and lens is a perfect match for the physical length of your eyeball.

The light comes to a perfect focus right on the retina.

But for most of us, that's not the case.

No.

The most common issue, especially for students doing a lot of reading, is myopia, or nearsightedness.

Nearsightedness means you can see near, but not far.

Correct.

And this usually happens because the eyeball is too long.

Think of it like a movie projector where the screen is too far back.

The lens focuses the image, but the focal point happens in front of the retina.

So by the time the light actually hits the retinal screen, the rays have crossed and started to spread out again, making the image blurry.

And the textbook has a little sidebar about a correlation between education, screen time, and myopia.

It does.

There's a lot of research suggesting that all the near work we do, reading, computers, phones, might be causing the eyeball to grow longer during development, essentially locking kids into myopia.

Wow.

Okay, then there's the opposite problem, hyperopia.

Farsightedness.

You can see far, but near objects are blurry.

In this case, the eyeball is too short.

The focal point would theoretically be behind the retina.

So the light hits the retina before it's had a chance to fully converge into a sharp point.

And the last one, astigmatism.

This one always seems to confuse people.

It's all about the shape of the cornea.

For a perfect eye, the cornea should be spherical, like a basketball.

With astigmatism, the cornea is shaped more like a football.

It's elliptical.

It has different curves and different directions.

So what does that do to the image?

It means that, say, vertical lines might be in perfect focus, but horizontal lines are blurry, or vice versa.

Figure 2 .7 has that fan chart test for it.

If you look at it and some of the radiating lines look darker or sharper than others, you probably have some astigmatism.

Okay.

We have navigated the optics.

The light has been bent and focused, and it's about to hit the back of the eye.

Part 3.

The retina.

This is where the magic really starts.

The retina.

And it's so much more than just the screen.

It's a mini -computer.

It's literally a piece of the brain that gets pushed out into the eye during development.

But first, let's look at it like a doctor would.

Figure 2 .8 shows the fundus, which is the view of the back surface of the eye.

Yeah.

What the doctor sees with an ophthalmoscope.

And you can see a few key landmarks.

The most obvious one is a bright spot.

The optic disc.

The optic disc.

This is where all the arteries and veins enter and exit the eye, and it's also where the axons of the optic nerve all bundle together to leave the eye and go to the brain.

And crucially, there are no photoreceptors here.

None whatsoever.

It is a blind spot.

Figure 2 .9 is the blind spot test.

I think we should walk the listener through this because it is so weird when it works.

Let's do it.

If you're listening, try this.

Close your left eye, just use your right eye, and look at a fixed point straight ahead of you.

Now, hold up your right thumb at arm's length, off to the right.

Now, while keeping your eye locked on that point straight ahead,

slowly move your thumb from the far right back toward the center.

At a certain point,

your thumbnail just vanishes.

Gone.

Completely erased.

It's gone.

It's falling on your optic disc.

But here's the amazing part, and this is what freaks people out.

You don't see a black hole in your vision.

No.

You see whatever is in the background.

If it's a white wall, you just see more white wall.

Exactly.

The brain fills it in.

It's a constructive process.

It takes the visual texture from the area around the blind spot and just paints over the hole.

It makes an assumption that the world is continuous.

It's like the Content -Aware Fill tool in Photoshop, but it happens automatically in your head.

It's a perfect analogy.

Okay.

So moving away from the blind spot on the fundus, we have the macula and the fovea.

The macula is the central part of the retina, responsible for your sharp, detailed central vision.

And the fovea is a tiny pit in the very center of the macula.

And that's the sweet spot.

That is the sweet spot.

Highest acuity.

When you're reading these words, you are pointing your fovea at each one in succession.

It's the only part of your eye that can truly see fine detail.

Now, here's a really strange fact from the text.

The retina is built inside out.

It is so counterintuitive.

Figure 2 .0 shows a cross -section of the retinal layers.

Light comes into the eye.

And before it hits the photoreceptors, the rods and cones that actually detect the light, it has to pass through all the other layers of neurons.

Through the ganglion cells, the amicrine cells, the bipolar cells?

By all of them, the wiring is in front of the sensors?

It seems like a terrible design.

Yeah.

Why on earth would evolution do that?

It would be like putting the camera's wiring in front of the lens.

You'd think so, but the text explains it's a biological necessity.

The photoreceptors are the most metabolically active cells in the entire body.

They need a huge amount of energy and constant maintenance.

They need to be in direct physical contact with a layer at the very back called the pigment epithelium.

This layer is like their life support system.

It provides nutrients.

And very importantly, it recycles the waste products of vision.

So the sensors need to be plugged into the wall, basically.

Exactly.

And the wall is at the very back of the eye.

So the light has to punch through the mostly transparent wiring to get to the sensors, because the sensors can't live without their life support.

That makes sense.

Okay, speaking of the sensors, let's get into part four.

Photoreceptors, rods and cones.

The duplex retina.

We have two completely different parallel systems for seeing packaged into one eye.

Let's break them down.

Rods first.

Rods.

We have about 90 million of them.

They are built for night vision, which is called scotopic vision.

Scotopic.

They are incredibly sensitive to light.

A single rod can respond to a single photon of light.

They have downsides.

Right.

There's always a trade -off.

They don't see color at all.

They're color blind.

And they have very low acuity.

They give you a fuzzy black and white picture of the world.

And then we have the cones.

Cones.

We only have about four to five million of them.

Way fewer than rods.

They're built for daylight or photopic vision.

Photopic.

They are the opposite of rods in almost every way.

They give us our rich color vision because we have three types of them.

And they provide high acuity, our ability to see sharp details.

But they're downside.

They are divas.

They need a lot of light to work.

In the dark, they're completely useless.

And the distribution of these two types of cells across the retina is key.

Figure 2 .12 in the book shows this really clearly.

It's a dramatic graph.

Right in the very center of the fovea, there are only cones.

Densely packed together for maximum detail.

There are zero rods in the center of your gaze.

Zero.

Zero.

Then as you move away from the center into the periphery, the cone density drops like a rock and the rod density shoots up.

It actually peaks about 20 degrees out from the center.

The book suggests a rule of thumb to visualize this in Figure 2 .14.

This is great.

If you hold your thumb out at arm's length, your thumbnail covers an area of about two degrees of visual angle.

Okay.

That's roughly the size of your fovea.

That tiny patch is the only part of your visual field that you're seeing in high resolution, full color detail.

Everything else.

The entire rest of the world you see.

Is being picked up by the rod -rich, low acuity periphery.

So we think we see the whole world in high def, but we absolutely do not.

Not at all.

We have this tiny little window of high definition that we move around the room really, really fast.

They're called saccades.

And our brain stitches all those little snapshots together into what feels like a complete seamless picture.

It's an illusion of completeness.

This distribution also explains a classic astronomy trick.

Why, if you look directly at a very dim star at night, it seems to disappear.

Right.

It's the perfect demonstration of this.

If you look directly at a dim star, you're putting its image right on your fovea.

Where there are no rods.

Exactly.

And the cones in your fovea aren't sensitive enough to see that faint little star.

So poof, it vanishes.

So what do you have to do?

You have to use your averted vision.

You look slightly to the side of the star.

So its image falls on the periphery.

Yep.

Its image moves off your fovea and onto that part of the retina that's packed with super sensitive rods.

And suddenly the star pops back into view.

That is so cool.

Okay, part five is about adaptation.

The text says the visual system has to handle a range of illuminations spanning 12 log units.

That's a number that's almost impossible to comprehend.

12 log units is a trillion.

A one with 12 zeros after it.

That's the difference in brightness between a starlit night and a sunny day on a ski slope.

And a camera would just be completely overwhelmed by that.

The pictures would be either solid black or solid white.

Totally blown out.

But our eyes handle it and they do it using four main mechanisms.

Okay, mechanism number one.

People size.

We talked about this.

It constricts and dilates.

But honestly, this is the weakest mechanism of the four.

It only accounts for about a 16 -fold change in light entry.

And we need to cover a trillion -fold change.

Right.

So the pupil is just the first quick but limited adjustment.

Mechanism two.

Photopigment regeneration.

This is a really clever one.

So in dim light, your photoreceptors are packed full of photopigment molecules ready and waiting to catch any stray photon that comes along.

Maximum sensitivity.

Right.

But in bright light, photons are hitting the pigment so fast that it gets bleached.

Bleached sounds bad.

It's actually a protective mechanism.

Bleaching means the photopigment molecule has absorbed a photon and changed shape and it can't absorb another one until it gets recycled.

So in bright light, most of your pigment is in this bleached, unusable state.

Which means the eye becomes less sensitive.

Exactly.

It's automatically turning down its own sensitivity to avoid being overwhelmed and damaged.

It's essentially throwing away excess photons it doesn't need.

And when you walk back into a dark room,

that pigment has to slowly regenerate to get the sensitivity back up.

Precisely.

That's a big part of dark adaptation.

Which leads us to mechanism three.

The duplex retina itself.

Figure 2 .15 shows the dark adaptation curve and it's a classic graph in psychology.

It shows two distinct phases.

Okay.

What are we seeing?

When you first turn off the lights, your cones adapt pretty quickly.

Their sensitivity increases for about five to seven minutes, but then they hit a floor.

They're just not built for the dark and they give up.

And then what happens?

There's a kink in the curve.

That's the rod -cone break.

At that point around the seven to ten minute mark, the rods, which have been adapting much more slowly in the background, finally become more sensitive than the cones.

They take over the relay race.

So it's a handoff.

It's a handoff from the fast but weak cones to the slow but powerful rods.

And it takes a full 20 to 30 minutes for the rods to become fully dark adapted and reach their maximum sensitivity.

This is why pirates wore eye patches, right?

That is the theory.

Keep one eye dark adapted under the patch.

So when you go from the bright deck to below deck where it's dark, you just flip the patch to the other eye and you can see immediately.

That's brilliant.

Okay.

So before we move on, there's a sidebar about when good retina goes bad.

Yeah.

This covers two major diseases, AMD and RP.

Let's start with age -related macular degeneration.

Or AMD.

This disease, as the name implies, attacks the macula.

That's the central part of the retina, the part that's full of cones.

So you lose your central high detail vision.

What would that be like?

It's devastating.

Imagine looking at a loved one's face and seeing a gray, blurry splotch where their nose and eyes should be.

You can't read, you can't drive, you can't recognize faces.

And the other one is retinitis pigmentosa, RP.

This is often a genetic disease and it does the opposite.

It kills the rods first.

So the first symptom is usually night blindness.

Then you start to lose your peripheral vision.

Sounds like tunnel vision.

Exactly.

It's like looking through a tunnel that slowly closes in over years until you're completely blind.

But the book says there's some hope.

Figure 2 .17 shows some new technologies.

There is.

Things like retinal prosthetics where they implant an electrode array on the retina to stimulate the surviving cells with a signal from a camera.

It's very low resolution right now, just spots of light.

But it can allow someone who is blind to navigate a room.

And gene therapy.

Yeah, that's really cutting edge.

Using a harmless virus to deliver new genes into the surviving retinal cells to make them light sensitive again.

It's a long way off, but the research is promising.

Okay, part six.

This is the heavy lifting.

Retinal information processing.

We're finally talking about transduction.

The actual mechanism of seeing.

This is where physics becomes biology.

And it all happens in the outer segment of the photoreceptor.

Inside these photoreceptors, we have visual pigments.

Right.

And these pigment molecules have two parts.

A big protein called an opsin and a little molecule attached to it called a chromophore.

And the chromophore is retinal.

Which comes from vitamin A.

Which comes from vitamin A.

This is the direct link.

This is why they tell you to eat your carrots.

Right.

Your body turns the beta carotene in carrots into vitamin A, which your eye uses to make retinal.

No carrots, no retinal, no seeing.

So a photon of starlight comes in and hits one of these visual pigment molecules.

What happens?

Photo activation.

The energy from the single photon is absorbed by the retinal molecule.

And it causes the retinal to instantly change its shape.

It straightens out.

Okay.

And this change in shape kicks off a massive biochemical cascade inside the cell.

But here's the big twist.

And this is the part that confuses every single student who learns this for the first time.

Usually when you stimulate a neuron, it gets excited, right?

Depolarizes.

It fires an action potential.

That's the standard model.

But in photoreceptors, light causes hyperpolarization.

It makes the electrical charge inside the cell more negative.

Wait, so light turns the cell off?

In a way, yes.

It's completely backward from what you'd expect.

In total darkness, your photoreceptors are actually very active.

Their ion channels are open.

Positive ions are flowing in.

And they are constantly steadily releasing a neurotransmitter called glutamate.

So they're shouting, dark, dark, dark, into the synapse.

That's a great way to put it.

They're active in the dark.

When a photon of light hits them, it triggers that cascade which closes those ion channels.

The cell becomes more negative.

It hyperpolarizes and it stops releasing glutamate.

So the signal is the silence.

The signal is the silence.

The absence of glutamate is what tells the next cell in line, the bipolar cell.

Hey, we found some light over here.

It's like a dead man's switch on a train.

You have to hold the pedal down to keep things going.

Light comes along and knocks your foot off the pedal and that's the signal to stop.

That is a crucial distinction and another key point from the text.

Photoreceptors do not fire action potentials.

No spikes.

No, they use graded potentials.

This means they're analog, not digital.

The amount of hyperpolarization is directly proportional to the intensity of the light.

A little light causes a little change.

A lot of light causes a big change.

This allows for incredibly fine distinctions in brightness.

Okay, so the signal has started.

The photoreceptor has stopped shouting glutamate.

Now that signal has to move through the retina.

Part seven,

the vertical and lateral pathways.

Right, because the signal doesn't just go straight from the photoreceptor back to the brain.

It goes sideways too.

This is where lateral inhibition comes in using the horizontal and amicrine cells.

Exactly.

These cells run perpendicular,

or laterally, across the retina.

Their job is to create lateral inhibition.

And what that means is, if one photoreceptor gets very excited by bright light, it signals a horizontal cell, which then sends an inhibitory signal to all the neighboring photoreceptors.

It tells them to shut up.

It basically tells them to be quiet.

But why would it do that?

Why suppress the neighbors?

To create contrast.

To sharpen edges.

It means the signal going forward isn't just how bright is this spot.

It's how bright is this spot compared to its immediate neighbors.

If I'm bright and my neighbors are also bright, the signal is weak.

But if I'm bright and my neighbors are dark, the signal is incredibly strong.

This highlights boundaries, which is what the brain needs to see shapes.

Okay, so that's the sideways path.

Then we have the vertical pathway.

This involves the bipolar cells.

The middlemen.

They get the signal from the photoreceptors and pass it on to the next layer, the ganglion cells.

And this is where the wiring for foveal vision and peripheral vision really diverges.

Let's talk about the periphery first with the rods.

In the periphery, you have what are called diffuse bipolar cells.

They practice a high degree of convergence.

That means many, many photoreceptors all connect to one single bipolar cell.

As many as 50 rods to one bipolar cell.

Right.

So that bipolar cell is pooling all the light signals from a large area of the retina.

That makes it super sensitive.

Incredibly sensitive.

If 50 different rods each catch just one photon, the bipolar cell gets a big, summated signal.

This is why your peripheral vision is so good at detecting faint light, but...

There's always a but.

It has terrible acuity.

The bipolar cell knows that some light hate that group of 50 rods, but it has no idea exactly which one of the 50 got hit.

The signal is spatially blurry.

High sensitivity, low acuity.

And in the fovea, with the cones.

It's the complete opposite.

You have midget bipolar cells, and here there is a one -to -one connection.

One cone connects to one midget bipolar cell, which connects to one ganglion cell.

No pooling of information at all.

Zero pooling.

It's a direct private line from that single cone all the way to the brain.

This preserves every tiny detail.

The acuity is incredibly high, but the sensitivity is low because it's not gathering light from a wide area.

High acuity, low sensitivity.

It's the perfect trade -off.

It is.

It's why we have two systems.

Okay, we're at the final output layer of the retina, part eight.

Ganglion cells and receptive fields.

The ganglion cells.

These are the neurons whose axons bundle together to form the optic nerve.

They're the only cells that send signals out of the eye to the rest of the brain.

If the retina is a computer, these are the ethernet cables leaving the building.

And the text says there are two main types.

P cells and M cells.

Right.

P ganglion cells for parbocellular, which means small cell.

These make up about 70 % of the total.

They get their input from the midget bipolar cell, so the cone pathway.

So they're all about detail.

All about detail.

They have a sustained firing pattern and they're sensitive to high contrast, color, and fine details.

These are your what -is -it cells.

And the M ganglion cell.

Magnocellular large cell.

They have these big sprawling dendrites that look like umbrellas.

They get input from the diffuse bipolar cells, the raw pathway.

They respond with quick transient bursts of firing.

They don't care much about color, but they are fantastic at detecting motion and temporal changes.

These are your where -is -it -going cells.

Now, to really understand how these ganglion cells see the world,

we have to talk about Stephen Kuffler's experiments from the 1950s.

Figure 2 .20 in the book.

This is historic stuff.

It really is.

Kuffler was recording from single ganglion cells in a cat's retina while shining little spots of light on a screen in front of the cat.

He was mapping out the receptive field for each cell.

The specific area of the visual world that makes that one neuron fire.

And he found it wasn't just a simple circle where light makes it fire?

No.

He found this incredible organization.

It was like a target or a bullseye.

We call it a center -surround receptive field.

Okay, so let's explain an Owen center cell.

Imagine a doughnut.

The little hole in the middle is the center.

The doughy part around it is the surround.

An Owen center cell loves light in the center.

If you shine a tiny spot of light that just fills that center hole, the cell fires like crazy.

What if you shine the light only on the doughy part, the surround?

It hates it.

Light in the surround sends an inhibitory signal.

It makes the cell stop firing.

Even below its normal spontaneous baseline rate, it goes quiet.

And what if you shine a big light that covers the whole thing, center and surround?

They cancel each other out.

The excitation from the center and the inhibition from the surround fight to a draw.

The cell barely changes its firing rate at all.

And this is why you said earlier that the retina doesn't really care about the overall brightness level.

This is exactly why.

If the whole room gets brighter, both the center and the surround get more light.

And the signal from the ganglion cell doesn't change much.

It ignores uniform illumination.

But if you put an edge right across the receptive field, where the center is light and the surround is dark.

Then the cell goes wild.

It screams.

It's a contrast detector.

It's an edge detector.

It filters the visual image to find the important stuff.

Boundaries, borders, changes.

That's the information the brain needs to start building a picture of the world.

And office center cells are just the opposite.

Just the mirror image.

They love darkness in the center and light in the surround.

They get excited by a dark spot on a light background.

This center surround mechanism actually explains some famous visual illusions, right?

The text shows mock bands in figure 2 .21.

Yes.

Mock bands are those illusory bright and dark stripes you see right at the edge of a gradient, like at the edge of a shadow.

The dark side of the edge looks extra dark and the light side looks extra light.

But those stripes aren't really there in the physical world.

They're not.

They are created inside your retina by lateral inhibition.

The cells on the bright side are inhibiting their neighbors on the dark side.

Making the dark side look even darker right at the border.

It's your visual system automatically enhancing the contrast at every edge to make it easier to see.

We are nearing the end of the chapter.

There's a fascinating scientist at work sidebar that asks a really fundamental question.

Is one photon enough?

It's the ultimate limit of sensitivity, right?

Can we detect a single quantum of light?

Hecht, Schler, and Purin did a classic study back in 1942.

They did the math and figured that to consciously say, I see a flash of light, we probably need about five to seven photons to be absorbed by five to seven different rods.

But a much newer study in 2016 pushed that even further.

It did.

Tinsley and his colleagues used a quantum light source.

Basically a gun that can reliably shoot single photons one at a time.

And they found that humans can, with above chance probability,

detect a single photon being absorbed by the retina.

One.

One single photon.

The smallest possible unit of energy in the universe.

And your brain can register that it happened.

It's just staggering.

Evolution has pushed our biological hardware to the absolute theoretical limit of physics.

We are quantum detectors.

Okay, let's bring it all home.

Let's do a final summary.

We've covered a ton of ground.

Let's trace that starlight one last time really quickly.

Okay.

Light travels for millions of years from a star.

It hits the cornea, gets bent,

passes through the pupil, which is regulated by the iris.

It goes through the lens, which fine tunes the focus using accommodation with help from the zonules of zin.

Travels through the vitreous humor.

It hits the retina, passes through the transparent ganglion and bipolar layers.

And finally, it strikes a single rod receptor.

Transduction happens.

The photopigment molecule bleaches.

This causes the cell to hyperpolarize.

To turn off, it stops releasing glutamate.

That silence is the signal.

It's picked up by bipolar cells, which can be ONN or OFF cells.

The signal is then processed sideways by horizontal and amicrine cells, creating lateral intubation to sharpen the edges.

And finally, it reaches the ganglion cells, the P cells for detail or the M cells for motion.

Their center surround receptive fields filter the image for contrast, not for absolute brightness.

And that signal, now converted into a train of digital spikes, shoots down the optic nerve.

And that is where we leave it.

The signal is on its way to the brain.

But that, as the book says, is a story for the next chapter.

I think the biggest takeaway is this.

The retina is not a camera.

A camera just passively records pixels.

The retina is an active computer.

It compresses information.

It enhances contrast.

It detects motion.

And it processes everything before the brain even gets a look.

The eye has a brain of its own.

That is the perfect takeaway.

Thank you so much for diving deep with us into the mechanics of vision.

This has been the Last Minute Lecture team.

Keep looking up.

We'll see you next time.