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Sometimes our eyes do weird things. One of the things that it sometimes does is get floaters. What are they? Where do they come from? Join us today on SciShow as Hank explores the science behind these little specks.
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Sources:
http://www.nei.nih.gov/health/floaters/floaters.asp
http://www.health.harvard.edu/blog/what-you-can-do-about-floaters-and-flashes-in-the-eye-201306106336
http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Eye_floaters
http://www.todayifoundout.com/index.php/2010/06/what-causes-eye-floaters/
Do not adjust your video viewing device. The screen is intentionally white. It is white so that the light coming off of it will pass through clumps of collagen in the gooey liquid of your eyeballs, and cast a shadow on your retina. It's white so that you can better see your eye floaters.

Eye floaters are those cobwebs, or speckles, or little crystal worms that sometimes drift across your field of vision. You probably notice them when you're looking at a bright, blank surface like a piece of paper, or a clear blue sky. Everybody has them, though everybody notices them. And the weird thing is, if you've ever seen a floater in your life, it is probably still there right now.

See, the middle of your eye, between the pupil, where the light enters, and the retina, where the light is detected, is your vitreous humour, sometimes just called the vitreous. It's the gooey stuff inside your eyeball that holds everything in place. 

Your vitreous is incredibly clear - it's clearer than the water that you get outta your sink. And in fact, it's about 99% water. The other 1% is collagen. That's the protein your body uses to make connective tissue, like your tendons and ligaments. It's that 1% collagen that gives your eyeball its jelly consistency.
I don't know if you've ever seen the inside of an eyeball... watch The Brain Scoop!

Collagen is spread throughout the vitreous in microscopically thin strands, which are supposed to be evenly spaced apart. But, these strands are so thin, and so close together that sometimes they experience molecular attraction.
When that happens, the collagen forms clumps. Normally, these clumps are way too small to see, unless they form right in front of your retina, just a few millimeters or less. If that happens, then the light, as it passes through them, will cast a shadow on your retina. That shadow is what you see as a floater.

And even though we call them floaters, they don't actually float. Otherwise, they'd rise to the top of your eyeball, and you wouldn't be able to see them. They are, instead, neutrally buoyant - meaning they don't sink to the bottom of your eye, or rise to the top, but drift around in the currents, just enough to make the shadow on your retina move. You can't focus on them, because when your eye moves, so does the floater, and they actually don't ever go away. 

So why can't you see them all the time? Well, unless you're looking at a bright, blank surface, that shadow won't be strong enough and won't stand out enough for you to be able to pick it out.

If you still can't see them, that probably means that they've stopped moving, and your brain is doing this amazing thing where it stops registering anything that it's been looking at for too long. It's called sensory adaptation - it's the same phenomenon that makes you forget that you're wearing a wristwatch, or stop hearing a ticking clock. That stimulus has stopped triggering neural activity, so as far as your brain is concerned, it's just not there anymore. So when your floaters stop moving, they become literally invisible to your brain. 

There are other types of floaters though - as you age the collagen in your eyeball jelly starts to break down and your vitreous starts to liquefy. When that happens, little chunks of the vitreous that are still jelly-like start to break off and float around in your eyeball's liquid center.

These floaters tend to be a lot blurrier than the ones that you get when you're younger, because the obstruction is further away from your retina. Also, they're non-neutrally buoyant, so over time they gradually sink in the liquid below your line of sight, and stop being such a distraction.

Technically, anything that might cause stuff to get into your eyes can make you see floaters, and if you start seeing lots of new floaters all at once, along with bright flashes of light, that might be a sign that your retina's about to detach, and you should go to a doctor like, right now. Like, stop watching. 

But for the most part, floaters are just a side-effect of your vitreous going about its business, providing a clear medium for light to pass through on its way to your retina, and holding all your eyeball's fancy hardware in place.
And it does that while being sealed off from the metabolic processes of the rest of your body - it's encased in its own membrane, and it doesn't get replenished from the outside. That's why floaters never go away - the same vitreous humour that you were born with is what you'll be seeing through when you're 90.

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