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  • Take a good long look at this -- were 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, youll

  • 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 complexnearly 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 theyre 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 were 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 eyesphotoreceptors 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 youll 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 youre

  • 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 youve 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, youve 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 youve 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 theyre 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 theyre 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 weve 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

Take a good long look at this -- were gonna mess with your brain.

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Vision: Crash Course A&P #18

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    bsofade posted on 2015/05/27
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