Vision: Crash Course Anatomy & Physiology #18

Summary

This script delves into the intricacies of human vision, starting with an optical illusion to illustrate the disconnect between what the eyes see and what the brain perceives. It explains how light interacts with the eye, detailing the anatomy of the eyeball and the function of its three layers. The explanation highlights the roles of rods and cones in detecting light and color, and how they contribute to the illusion's afterimage effect. The video aims to educate viewers on the complex process of vision, from light waves to the brain's interpretation, and how our visual system can sometimes be deceived.

Takeaways

• 👀 Optical illusions exploit the disconnect between what your eyes see and what your brain understands, sometimes using physiological glitches in human vision.
• 🏳️‍🌈 Staring at a flag for an extended period can produce an afterimage with colors opposite to the original, demonstrating the physiological response of the eye's photoreceptors.
• 🧠 Vision is a complex process involving nearly 70% of all sensory receptors in the body and nearly half of the entire cerebral cortex.
• 🌈 Light is electromagnetic radiation with frequency determining hue and amplitude determining brightness, with visible light being just a small part of the full spectrum.
• 👁️ The eye's photoreceptors convert light energy into nerve impulses, similar to how other mechanoreceptors and chemoreceptors function.
• 🛡️ Eye protection features like eyebrows, eyelashes, and eyelids help shield the delicate eyeball from damage.
• 🏀 The eyeball is irregularly spherical, filled with fluids, and mostly hidden within the skull, with only the anterior part visible.
• 🌀 The eye wall consists of three layers: the fibrous layer (including the sclera and cornea), the vascular layer (including the iris and choroid), and the inner neural layer (the retina).
• 🔍 Rods and cones are two types of photoreceptors in the retina, with cones responsible for color and fine detail in bright light, and rods for peripheral vision and low-light conditions.
• 🔄 The retina's structure allows for the conversion of light into electrical signals, with multiple rods connecting to a single ganglion cell and each cone having its own dedicated ganglion cell.
• 🎨 The flag illusion occurs due to photoreceptor fatigue, where staring at certain colors can cause specific cones to stop responding, leading to an afterimage in complementary colors.

Q & A

What is the purpose of optical illusions in explaining human vision?

Optical illusions serve as a great way to illustrate the complex sense of human vision by demonstrating how our eyes and brain can sometimes misinterpret visual information.

How does the visible light spectrum relate to the electromagnetic spectrum?

The visible light spectrum is just a small part of the full electromagnetic spectrum, which extends from short gamma and X-rays to long radio waves.

What are the two main types of photoreceptors in the human eye, and what do they detect?

The two main types of photoreceptors are rods and cones. Cones, located near the retina's center, detect fine detail and color, while rods, more numerous and light-sensitive, detect grayscale and are responsible for peripheral vision.

What is the role of the iris in the eye?

The iris is the colored part of the eye that controls the amount of light entering the eye by adjusting the size of the pupil through the contraction and expansion of its muscles.

How does the structure of the eye contribute to its function?

The eye's structure, consisting of the fibrous, vascular, and inner layers, each with specific components like the cornea, iris, lens, and retina, is designed to protect the eye, focus light, and convert light energy into electrical signals for the brain to interpret.

What causes the afterimage effect seen when staring at a brightly colored flag and then looking at a blank screen?

The afterimage effect is caused by the photoreceptors in the eye continuing to fire action potentials even after the stimulus is removed, and by the temporary fatigue of the cones, which stop responding to certain colors after being exposed to them for too long.

How do rods and cones differ in their connection to the brain?

Rods are less detailed and connect to multiple ganglion cells, providing general shape and light/dark information, while each cone is connected to its own ganglion cell, allowing for detailed color vision in bright conditions.

What is the function of the lens in the eye?

The lens focuses light and projects it onto the retina, which is crucial for converting light energy into electrical signals that the brain can understand.

How does the brain interpret the information received from the eyes?

The brain interprets the information from the eyes through the optic nerve, which carries impulses from the retina's photoreceptors to the thalamus and then to the visual cortex.

What is the significance of the different layers of the retina in vision?

The retina's layers, including the outer pigmented layer and the inner neural layer with photoreceptors, bipolar cells, and ganglion neurons, are essential for absorbing light, processing visual information, and transmitting it to the brain.

Why might the flag illusion show different colors when looking at a blank screen after staring at the flag?

The flag illusion shows different colors due to the fatigue of the blue and green cones after staring at the flag, leaving only the red cones to respond to the white light on the blank screen, creating a red afterimage.

Outlines

00:00

🎨 Understanding Optical Illusions and Vision

This paragraph introduces the concept of optical illusions, explaining how they manipulate the disconnect between what the eyes see and what the brain perceives. It uses an example of a flag illusion to demonstrate how afterimages can be created through prolonged staring, resulting in a color shift. The paragraph also delves into the complexity of human vision, highlighting that nearly 70% of all sensory receptors are in the eyes and that vision involves nearly half of the cerebral cortex. It sets the stage for a deeper exploration of light, color perception, and the anatomy of the eye.

05:01

👀 Anatomy of the Eye and Photoreceptor Function

The second paragraph provides a detailed overview of the eye's anatomy, starting with the outer protective features like eyebrows, eyelashes, and eyelids. It describes the eye's structure as an irregularly spherical shape filled with fluids and protected by muscles and fat. The paragraph then explains the three layers of the eye: the fibrous layer with the sclera and cornea, the vascular layer with the choroid, ciliary body, and iris, and the inner layer which includes the retina. The retina is highlighted as being crucial, containing millions of photoreceptors that convert light energy into electrical signals. The paragraph also explains the function of rods and cones, the two types of photoreceptors responsible for vision in different lighting conditions and color perception. It concludes with an explanation of how the eye's photoreceptors can create afterimages, tying back to the flag illusion introduced earlier.

Keywords

💡Optical Illusion

Optical illusions are visual phenomena where images appear different from the way they are in reality, often due to the way our brain interprets sensory information. In the video, optical illusions are used to demonstrate the disconnect between what the eyes see and what the brain understands, highlighting the complexity of human vision. The example of the flag illusion is used to illustrate how afterimages can occur due to the physiology of human vision.

💡Sensation and Perception

Sensation refers to the process by which our sensory organs receive and respond to stimuli, while perception is the brain's interpretation of those stimuli. The video explains that optical illusions exploit the disconnect between sensation and perception, showing how our eyes can see something but our brain may interpret it differently, as evidenced by the flag illusion where an afterimage appears in a different color scheme.

💡Photoreceptors

Photoreceptors are specialized cells in the retina that convert light into electrical signals. The video discusses two types of photoreceptors: rods and cones. Rods are more sensitive to light and allow us to see in low-light conditions, but they do not detect color. Cones, on the other hand, are responsible for color vision and work best in bright light. The video uses the concept of photoreceptors to explain the afterimage effect seen in the flag illusion.

💡Cone Cells

Cone cells are a type of photoreceptor in the retina that are responsible for color vision and detail detection. They are sensitive to different wavelengths of light and can be categorized as red, green, and blue-sensitive types. The video explains that staring at a brightly colored image, like the flag, can cause the cones to become fatigued, leading to the appearance of an afterimage in complementary colors when looking at a blank screen.

💡Rod Cells

Rod cells are photoreceptors that are more numerous in the retina and are responsible for vision in low-light conditions. Unlike cone cells, rods do not detect color and only provide information about the general shape of objects and the presence of light or dark. The video uses rods to explain the black and white aspect of the afterimage seen after staring at the flag.

💡Retina

The retina is the light-sensitive inner layer at the back of the eye where photoreceptors are located. It plays a crucial role in converting light energy into electrical signals that can be interpreted by the brain. The video describes the retina's structure, including its photoreceptors, bipolar cells, and ganglion cells, and how it contributes to the process of vision.

💡Pupil

The pupil is the opening in the iris of the eye that controls the amount of light entering the eye. It expands and contracts to regulate light intake. The video mentions how the pupil reacts to different light conditions, such as dilating in the dark and constricting in bright light, to protect the eye from excessive light exposure.

💡Visual Cortex

The visual cortex is the area of the brain responsible for processing visual information received from the eyes. Nearly half of the cerebral cortex is involved in vision, indicating its importance. The video explains that the optic nerve carries the electrical signals from the retina to the thalamus and then to the visual cortex, where these signals are interpreted as visual images.

💡Electromagnetic Spectrum

The electromagnetic spectrum is the range of all types of electromagnetic radiation, from short gamma rays and X-rays to long radio waves. The video mentions that visible light is just a small part of this spectrum, with different frequencies corresponding to different colors. This concept is used to explain how our eyes perceive different colors based on the frequency of light waves.

💡Afterimage

An afterimage is a visual phenomenon where an image continues to appear even after the original stimulus has been removed. The video uses the flag illusion to demonstrate how afterimages occur due to the fatigue of photoreceptors, which continue to fire action potentials even after looking away from the stimulus, resulting in the perception of a complementary color image.

💡Globe Luxation

Globe luxation is a rare condition where the eyeball pops out of its socket, often due to trauma or intense physical activity like a sneeze. The video humorously mentions this condition as a cautionary note about the fragility of the eyeball, emphasizing the importance of the protective structures around the eye.

Highlights

Optical illusions exploit the disconnect between sensation and perception, using patterns of light or perspective.

Some illusions produce afterimages by taking advantage of glitches in human vision physiology.

Nearly 70% of all sensory receptors in the human body are in the eyes.

Half of the entire cerebral cortex is involved in seeing, perceiving, and recognizing.

Light is electromagnetic radiation with frequency determining hue and amplitude relating to brightness.

The visible light spectrum is a small part of the full electromagnetic spectrum.

Eye photoreceptors convert light energy into nerve impulses for the brain.

The eye's outermost layer is made of connective tissue, including the sclera and cornea.

The iris, a part of the middle vascular layer, controls the size of the pupil.

The lens focuses light onto the retina, which is composed of millions of photoreceptors.

The retina has two layers: a pigmented layer and a neural layer containing various neurons.

The optic nerve carries visual impulses from the retina to the brain.

Cones detect fine detail and color, while rods are more sensitive and detect grayscale.

Cones and rods are wired differently to the retina, affecting the detail and color perception.

Afterimages occur when photoreceptors continue firing action potentials after looking away from a stimulus.

Cones can get tired from staring at a brightly colored image, affecting color perception in afterimages.

Rods, being less sensitive to color, create a black and white afterimage in certain illusions.

The complex system of human vision can be understood through the study of optical illusions.

Transcripts

Take a good long look at this -- we’re gonna mess with your brain.

This is the first stage of an optical illusion.

Many illusions use patterns of light or perspective to exploit the disconnect that exists between

sensation and perception -- between what your eyes see and what your brain understands.

But not all illusions work that way. Some produce ghost effects, or afterimages, that

take advantage of glitches in the physiology of human vision.

Like this flag.

I’m not trying to make a political statement here. And I’m not going ask you to swear

allegiance to the Republic of Hank or anything. I mean, if I was gonna start my own country,

my flag would be way cooler than that -- not that I’ve thought about that a lot.

And now, look at this white screen.

If you looked at that flag for at least 30 seconds without moving your eyes, you’ll

see something, even though this screen is blank -- an afterimage of the flag. But instead

of being turquoise, and black, and yellow, it’s red, white, and blue.

OK so that’s pretty cool, but I’m not here just to entertain you. This kind of illusion

is actually a great way to explain your very complex sense of vision.

And I do mean complex… nearly 70 percent of all the sensory receptors in your whole

body are in the eyes!

Not only that, but in order for you to see, perceive, and recognize something -- whether

it’s a flag or a handsome guy in glasses and a sport coat sitting behind a desk -- nearly

half of your entire cerebral cortex has to get involved.

Vision is considered the dominant sense of humans and while we can get along without

it and it can be tricked, what you are about to learn is NOT an illusion.

When we talked about your sense of hearing, we began with the mechanics of sound. So before

we get to how your eyeballs work, it makes sense to talk about what they’re actually

seeing -- light bouncing off of stuff.

Light is electromagnetic radiation traveling in waves.

Remember how the pitch and loudness of a sound is determined by the frequency and amplitude of its wave?

Well, it’s kind of similar with light, except that the frequency of a light wave determines

its hue, while the amplitude relates to its brightness.

We register short waves at high frequencies as bluish colors, while long, low frequencies

look reddish to us.

Meanwhile, that red might appear dull and muted if the wave is moving at a lower amplitude,

but super bright if the wave has greater amplitude and thus higher intensity.

But the visible light we’re able to see is only a tiny chunk of the full electromagnetic

spectrum, which ranges from short gamma and X rays all the way to long radio waves.

Just as the ear’s mechanoreceptors or the tongue’s chemoreceptors convert sounds and

chemicals into action potentials, so too do your eyes’ photoreceptors convert light

energy into nerve impulses that the brain can understand.

To figure out how all this works, let’s start with understanding some eye anatomy.

Some of the first things you’ll notice around your average pair of eyes are all the outer

accessories -- like the eyebrows that help keep the sweat away if you forgot your headband

at raquetball, and the super-sensitive eyelashes that trigger reflexive blinking, like if you’re

on a sandy beach in a windstorm.

These features, along with the eyelids and tear-producing lacrimal apparatus are there

to help protect your fragile eyeballs.

The eyeball itself is irregularly spherical, with an adult diameter of about 2.5 centimeters.

It’s essentially hollow -- full of fluids that help it keep its shape -- and you can

really only see about the anterior sixth of the whole ball. The rest of it is tucked into

a pocket of protective fat, tethered down by six straplike extrinsic eye muscles, and

jammed into the bony orbit of your skull.

While all this gear generally does a fantastic job of keeping your eyeballs inside of your

head (which is good), on very rare occasions, perhaps after head trauma or -- or even a

really intense sneeze! -- those suckers can pop right out -- a condition called globe

luxation, which you really do not want to google.

I’ll just sit here while you Google it.

Now, you don’t need to pop out an eyeball in order to learn how it’s structured. I’ll

save you the trouble and tell you that its wall is made up of three distinct layers -- the

fibrous, vascular, and inner layers.

The outermost fibrous layer is made of connective tissue. Most of it is that white stuff called

the sclera, while the most anterior part is the transparent cornea.

The cornea is like the window that lets light into the eye, and if you’ve ever experienced

the excruciating pain of a scratched one, you know how terrible it can be to damage

something so loaded with pain receptors.

Going down a little deeper, the wall’s middle vascular layer contains the posterior choroid,

a membrane that supplies all of the layers with blood.

In the anterior, there’s also the ciliary body, a ring of muscle tissue that surrounds

the lens; but the most famous part of this middle layer is the iris.

The iris is that distinctive colored part of the eye that is uniquely yours. It’s

made up of smooth muscle tissue, shaped liked a flattened donut, and sandwiched between

the cornea and the lens.

Those circular sphincter muscles -- yeah, that’s right, you’ve got sphincters everywhere!

-- contract and expand, changing the size of the dark dot of your pupil.

The pupil itself is just the opening in the iris that allows light to travel into the

eye. You can see how an iris protects the eye from taking too much light in if you shine

a flashlight in your friend’s eye in a dark room. Their pupils will go from dilated to

pinpoints in a couple of seconds.

Light comes in through the cornea and pupil and hits the lens -- the convex, transparent

disc that focuses that light and projects it onto the retina, which makes up the inner

layer of the back of the eyeball.

Your retinas are loaded with millions of photoreceptors which do the crucial work of converting light

energy into the electrical signals that your brain will receive. These receptor cells come

in two flavors -- rods and cones -- which I’ll come back to in a minute.

But the retina itself has two layers, the outer pigmented layer that helps absorb light

so it doesn't scatter around the eyeball, and the inner neural layer.

And this layer, as the name indicates, contains neurons -- not only the photoreceptors but

also bipolar neurons and ganglion neurons.

These two kinds of nerve cells combine to produce a sort of pathway for light, or at

least data about light.

Bipolar neurons have synapses at both ends, forming a kind of bridge -- on one end it

synapses with a photoreceptor, and at the other, it synapses with a ganglionic neuron,

which goes on to form the optic nerve.

So, say you’ve just been hit with a blinding flashlight beam. That light hits your posterior

retina and spreads from the photoreceptors to the bipolar cells just beneath them, to

the innermost ganglion cells, where they then generate action potentials.

The axons of all those ganglion cells weave together to create the thick, ropey optic

nerve -- your second cranial nerve -- which leaves the back of your eyeball and carries

those impulses up to the thalamus and then on to the brain’s visual cortex.

So that’s the basic anatomy and event sequencing of human vision, but what I really want to

talk about are those two types of photoreceptors -- your rods and your cones.

Cones sit near the retina’s center, and detect fine detail and color. They can be

divided into red, green, and blue-sensitive types, based on how they respond to different types of light.

But they’re not very sensitive, and they really only hit their activation thresholds in bright conditions.

Rods, on the other hand, are more numerous more light-sensitive. But they can’t pick

up real color. Instead they only register a grayscale of black and white. They hang

out around the edges of your retinas, and rule your peripheral vision.

Since these receptors function so differently, you might not be surprised to learn that your

rods and cones are also wired to your retinas in different ways.

As many as 100 different rods may connect to a single ganglion cell -- but because they

all send their information to the ganglion at once, the brain can’t tell which individual

rods have been activated. That’s why they’re not very good at providing detailed images

-- all they can really do is give you information about objects general shape, or whether it’s light or dark.

Each cone, by contrast, gets its own personal ganglion cell to hook up with, which allows

for very detailed color vision, at least if conditions are bright enough.

And all this brings us back to that weird flag.

Why does staring at this flag and then looking at an empty white space make us see a phantom

flag of different colors? Well, it begins with the fact that our photoreceptors can

make us see afterimages.

Some stimuli, like really brilliant colors or really bright lights, are so strong that

our photoreceptors will continue firing action potentials even after we close our eyes or look away.

The other part of the illusion has to do with another bug in our visual programming: And

it’s just that our cones can just get tired.

If you stare long enough at a brightly colored image, your cones will receive the same stimulus

for too long, and basically stop responding.

In the case of the flag, you looked at an image with bright turquoise stripes. Because

your retinas contain red, green, and blue-sensitive cones, the blue and green ones got tired after

a while, leaving only the red cones left to fire.

Then, you looked at the white screen. That white light included all of colors and wavelengths

of visible light. So, your eyes were still receiving red, green, and blue light -- but

only the red cones were able to respond. As a result, when the afterimage began to appear,

those stripes looked red.

The same thing happened to your rods. Except, since they only register black and white,

the afterimage was like looking at a negative of a photograph -- dark replaced with light.

That’s how those black stars and stripes turned white.

So, yes, human vision is fallible, but those mistakes that it makes can help us understand

that wonderfully complex system.

And that wonderfully complex system probably helped you learn about the anatomy and physiology

of vision today, starting with the structure of the eye and its three layers: the fibrous,

vascular, and inner layers. We spent most of our time exploring the inner layer, which

consists of the retina and its three kinds of neurons: photoreceptors, bipolar cells,

and ganglion neurons. And after learning how to tell our rods from our cones, we then dissected

how the weird flag illusion works.

Special thanks to our Headmaster of Learning, Thomas Frank for his support of Crash Course

and free education. And thank you to all of our Patreon patrons who make Crash Course

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cool things that we’ve made available to you.

Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode

was written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr. Brandon

Jackson. Our director is Nicholas Jenkins, the script supervisor and editor is Nicole

Sweeney, Michael Aranda is our sound designer, and the graphics team is Thought Café