WEBVTT

00:00:06.830 --> 00:00:10.423
Have you ever noticed something swimming
in your field of vision?

00:00:10.448 --> 00:00:14.060
It may look like a tiny worm
or a transparent blob,

00:00:14.085 --> 00:00:18.051
and whenever you try to get
a closer look, it disappears,

00:00:18.076 --> 00:00:20.763
only to reappear
as soon as you shift your glance.

00:00:21.727 --> 00:00:23.792
But don't go rinsing out your eyes!

00:00:23.815 --> 00:00:26.393
What you are seeing is a common phenomenon

00:00:26.417 --> 00:00:27.449
known as a floater.

00:00:28.451 --> 00:00:32.593
The scientific name for these objects
is Muscae volitantes,

00:00:32.615 --> 00:00:34.755
Latin for "flying flies,"

00:00:34.779 --> 00:00:37.909
and true to their name,
they can be somewhat annoying.

00:00:37.932 --> 00:00:42.045
But they're not actually bugs
or any kind of external objects at all.

00:00:42.069 --> 00:00:44.331
Rather, they exist inside your eyeball.

00:00:45.209 --> 00:00:48.742
Floaters may seem to be alive,
since they move and change shape,

00:00:48.765 --> 00:00:50.012
but they are not alive.

00:00:50.832 --> 00:00:54.649
Floaters are tiny objects
that cast shadows on the retina,

00:00:54.673 --> 00:00:57.228
the light-sensitive tissue
at the back of your eye.

00:00:58.076 --> 00:00:59.589
They might be bits of tissue,

00:00:59.613 --> 00:01:00.721
red blood cells,

00:01:00.746 --> 00:01:02.681
or clumps of protein.

00:01:02.704 --> 00:01:06.206
And because they're suspended
within the vitreous humor,

00:01:06.230 --> 00:01:09.277
the gel-like liquid
that fills the inside of your eye,

00:01:09.301 --> 00:01:11.579
floaters drift along
with your eye movements,

00:01:11.603 --> 00:01:14.331
and seem to bounce a little
when your eye stops.

00:01:15.290 --> 00:01:18.531
Floaters may be only
barely distinguishable most of the time.

00:01:19.459 --> 00:01:22.710
They become more visible
the closer they are to the retina,

00:01:22.734 --> 00:01:26.216
just as holding your hand closer
to a table with an overhead light

00:01:26.240 --> 00:01:28.727
will result in a more
sharply defined shadow.

00:01:29.555 --> 00:01:31.580
And floaters are particularly noticeable

00:01:31.603 --> 00:01:34.581
when you are looking
at a uniform bright surface,

00:01:34.605 --> 00:01:36.218
like a blank computer screen,

00:01:36.242 --> 00:01:37.445
snow,

00:01:37.469 --> 00:01:38.813
or a clear sky,

00:01:38.837 --> 00:01:43.082
where the consistency of the background
makes them easier to distinguish.

00:01:43.106 --> 00:01:46.760
The brighter the light is,
the more your pupil contracts.

00:01:46.784 --> 00:01:50.697
This has an effect similar
to replacing a large diffuse light fixture

00:01:50.721 --> 00:01:53.018
with a single overhead light bulb,

00:01:53.042 --> 00:01:55.323
which also makes
the shadow appear clearer.

00:01:56.188 --> 00:01:59.743
There is another visual phenomenon
that looks similar to floaters

00:01:59.768 --> 00:02:01.319
but is in fact unrelated.

00:02:02.382 --> 00:02:04.760
If you've seen tiny dots of light
darting about

00:02:04.784 --> 00:02:06.977
when looking at a bright blue sky,

00:02:07.001 --> 00:02:11.098
you've experienced what is known
as the blue field entoptic phenomenon.

00:02:12.272 --> 00:02:15.139
In some ways,
this is the opposite of seeing floaters.

00:02:15.947 --> 00:02:17.578
Here, you are not seeing shadows

00:02:17.602 --> 00:02:21.027
but little moving windows
letting light through to your retina.

00:02:21.981 --> 00:02:24.795
The windows are actually caused
by white blood cells

00:02:24.819 --> 00:02:27.885
moving through the capillaries
along your retina's surface.

00:02:28.695 --> 00:02:32.752
These leukocytes can be so large
that they nearly fill a capillary

00:02:32.776 --> 00:02:35.906
causing a plasma space
to open up in front of them.

00:02:37.132 --> 00:02:39.141
Because the space
and the white blood cells

00:02:39.165 --> 00:02:41.695
are both more transparent to blue light

00:02:41.719 --> 00:02:44.961
than the red blood cells
normally present in capillaries,

00:02:44.985 --> 00:02:48.041
we see a moving dot of light
wherever this happens,

00:02:48.064 --> 00:02:51.924
following the paths of your capillaries
and moving in time with your pulse.

00:02:52.829 --> 00:02:54.786
Under ideal viewing conditions,

00:02:54.810 --> 00:02:58.912
you might even see what looks
like a dark tail following the dot.

00:02:58.936 --> 00:03:02.578
This is the red blood cells
that have bunched up behind the leukocyte.

00:03:03.427 --> 00:03:07.679
Some science museums have an exhibit
which consists of a screen of blue light,

00:03:07.703 --> 00:03:12.241
allowing you to see these blue sky sprites
much more clearly than you normally would.

00:03:13.194 --> 00:03:16.194
While everybody's eyes experience
these sort of effects,

00:03:16.218 --> 00:03:18.799
the number and type vary greatly.

00:03:18.823 --> 00:03:20.314
In the case of floaters,

00:03:20.338 --> 00:03:23.486
they often go unnoticed,
as our brain learns to ignore them.

00:03:24.311 --> 00:03:26.335
However, abnormally numerous

00:03:26.359 --> 00:03:29.045
or large floaters
that interfere with vision

00:03:29.068 --> 00:03:33.663
may be a sign of a more serious condition,
requiring immediate medical treatment.

00:03:34.568 --> 00:03:37.596
But the majority of the time
entoptic phenomena,

00:03:37.621 --> 00:03:40.210
such as floaters and blue sky sprites,

00:03:40.234 --> 00:03:43.623
are just a gentle reminder
that what we think we see

00:03:43.646 --> 00:03:46.765
depends just as much
on our biology and minds

00:03:46.789 --> 00:03:49.043
as it does on the external world.