1
00:00:06,830 --> 00:00:10,424
Have you ever noticed something swimming
in your field of vision?

2
00:00:10,448 --> 00:00:14,061
It may look like a tiny worm
or a transparent blob,

3
00:00:14,085 --> 00:00:18,052
and whenever you try to get
a closer look, it disappears,

4
00:00:18,076 --> 00:00:20,763
only to reappear
as soon as you shift your glance.

5
00:00:21,727 --> 00:00:23,792
But don't go rinsing out your eyes!

6
00:00:23,816 --> 00:00:26,393
What you are seeing is a common phenomenon

7
00:00:26,417 --> 00:00:27,449
known as a floater.

8
00:00:28,451 --> 00:00:32,593
The scientific name for these objects
is Muscae volitantes,

9
00:00:32,616 --> 00:00:34,755
Latin for "flying flies,"

10
00:00:34,779 --> 00:00:37,909
and true to their name,
they can be somewhat annoying.

11
00:00:37,933 --> 00:00:42,045
But they're not actually bugs
or any kind of external objects at all.

12
00:00:42,069 --> 00:00:44,331
Rather, they exist inside your eyeball.

13
00:00:45,209 --> 00:00:48,742
Floaters may seem to be alive,
since they move and change shape,

14
00:00:48,766 --> 00:00:50,013
but they are not alive.

15
00:00:50,832 --> 00:00:54,649
Floaters are tiny objects
that cast shadows on the retina,

16
00:00:54,673 --> 00:00:57,228
the light-sensitive tissue
at the back of your eye.

17
00:00:58,077 --> 00:00:59,590
They might be bits of tissue,

18
00:00:59,614 --> 00:01:00,722
red blood cells,

19
00:01:00,746 --> 00:01:02,681
or clumps of protein.

20
00:01:02,705 --> 00:01:06,206
And because they're suspended
within the vitreous humor,

21
00:01:06,230 --> 00:01:09,277
the gel-like liquid
that fills the inside of your eye,

22
00:01:09,301 --> 00:01:11,580
floaters drift along
with your eye movements,

23
00:01:11,604 --> 00:01:14,331
and seem to bounce a little
when your eye stops.

24
00:01:15,291 --> 00:01:18,532
Floaters may be only
barely distinguishable most of the time.

25
00:01:19,460 --> 00:01:22,711
They become more visible
the closer they are to the retina,

26
00:01:22,735 --> 00:01:26,217
just as holding your hand closer
to a table with an overhead light

27
00:01:26,241 --> 00:01:28,727
will result in a more
sharply defined shadow.

28
00:01:29,555 --> 00:01:31,580
And floaters are particularly noticeable

29
00:01:31,604 --> 00:01:34,582
when you are looking
at a uniform bright surface,

30
00:01:34,606 --> 00:01:36,219
like a blank computer screen,

31
00:01:36,243 --> 00:01:37,446
snow,

32
00:01:37,470 --> 00:01:38,814
or a clear sky,

33
00:01:38,838 --> 00:01:43,083
where the consistency of the background
makes them easier to distinguish.

34
00:01:43,107 --> 00:01:46,761
The brighter the light is,
the more your pupil contracts.

35
00:01:46,785 --> 00:01:50,698
This has an effect similar
to replacing a large diffuse light fixture

36
00:01:50,722 --> 00:01:53,018
with a single overhead light bulb,

37
00:01:53,042 --> 00:01:55,323
which also makes
the shadow appear clearer.

38
00:01:56,189 --> 00:01:59,744
There is another visual phenomenon
that looks similar to floaters

39
00:01:59,768 --> 00:02:01,319
but is in fact unrelated.

40
00:02:02,382 --> 00:02:04,760
If you've seen tiny dots of light
darting about

41
00:02:04,784 --> 00:02:06,977
when looking at a bright blue sky,

42
00:02:07,001 --> 00:02:11,098
you've experienced what is known
as the blue field entoptic phenomenon.

43
00:02:12,273 --> 00:02:15,140
In some ways,
this is the opposite of seeing floaters.

44
00:02:15,947 --> 00:02:17,578
Here, you are not seeing shadows

45
00:02:17,602 --> 00:02:21,028
but little moving windows
letting light through to your retina.

46
00:02:21,982 --> 00:02:24,796
The windows are actually caused
by white blood cells

47
00:02:24,820 --> 00:02:27,886
moving through the capillaries
along your retina's surface.

48
00:02:28,696 --> 00:02:32,753
These leukocytes can be so large
that they nearly fill a capillary

49
00:02:32,777 --> 00:02:35,907
causing a plasma space
to open up in front of them.

50
00:02:37,132 --> 00:02:39,142
Because the space
and the white blood cells

51
00:02:39,166 --> 00:02:41,696
are both more transparent to blue light

52
00:02:41,720 --> 00:02:44,961
than the red blood cells
normally present in capillaries,

53
00:02:44,985 --> 00:02:48,041
we see a moving dot of light
wherever this happens,

54
00:02:48,065 --> 00:02:51,924
following the paths of your capillaries
and moving in time with your pulse.

55
00:02:52,829 --> 00:02:54,786
Under ideal viewing conditions,

56
00:02:54,810 --> 00:02:58,912
you might even see what looks
like a dark tail following the dot.

57
00:02:58,936 --> 00:03:02,578
This is the red blood cells
that have bunched up behind the leukocyte.

58
00:03:03,428 --> 00:03:07,679
Some science museums have an exhibit
which consists of a screen of blue light,

59
00:03:07,703 --> 00:03:12,242
allowing you to see these blue sky sprites
much more clearly than you normally would.

60
00:03:13,195 --> 00:03:16,195
While everybody's eyes experience
these sort of effects,

61
00:03:16,219 --> 00:03:18,799
the number and type vary greatly.

62
00:03:18,823 --> 00:03:20,315
In the case of floaters,

63
00:03:20,339 --> 00:03:23,487
they often go unnoticed,
as our brain learns to ignore them.

64
00:03:24,311 --> 00:03:26,335
However, abnormally numerous

65
00:03:26,359 --> 00:03:29,045
or large floaters
that interfere with vision

66
00:03:29,069 --> 00:03:33,664
may be a sign of a more serious condition,
requiring immediate medical treatment.

67
00:03:34,569 --> 00:03:37,597
But the majority of the time
entoptic phenomena,

68
00:03:37,621 --> 00:03:40,210
such as floaters and blue sky sprites,

69
00:03:40,234 --> 00:03:43,623
are just a gentle reminder
that what we think we see

70
00:03:43,647 --> 00:03:46,766
depends just as much
on our biology and minds

71
00:03:46,790 --> 00:03:49,044
as it does on the external world.