Vision: Crash Course A&P #18


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 possible through their monthly contributions.
If you like Crash Course and want to help us keep making great new videos, you can check out
Patreon.com/CrashCourse to see all of the 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

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