Name: 視覺。速成班A&P #18 (Vision: Crash Course A&P #18)
Uploaded: 2021-01-14T05:32:15.000Z
Duration: 9 min 39 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

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

This is the first stage of an optical illusion.

關係子句

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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,

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.