WEBVTT Kind: captions Language: en 1 00:00:00.070 --> 00:00:01.530 I'm on my way back from the Arctic, 2 00:00:01.530 --> 00:00:03.439 but there is time for one more guest video! 3 00:00:03.439 --> 00:00:05.690 And it's from Inés at Draw Curiosity, 4 00:00:05.690 --> 00:00:07.840 who's talking about those weird floaty things 5 00:00:07.840 --> 00:00:08.840 that you get in your eye sometimes. 6 00:00:08.840 --> 00:00:12.540 So Tom's taken his eyes off his channel and he's invited me to talk to you 7 00:00:12.540 --> 00:00:15.990 about 4 visual phenomena and disturbances that we all experience, 8 00:00:15.990 --> 00:00:19.310 and you don't have to go particularly far for any of them. 9 00:00:19.310 --> 00:00:23.599 For the first one, all you need to do is to look at a blank wall, or up at the sky, 10 00:00:23.599 --> 00:00:27.400 and you'll probably notice some dark grey spots floating around. 11 00:00:27.400 --> 00:00:30.009 These are floaters and what you're seeing are the shadows cast 12 00:00:30.009 --> 00:00:33.489 by bits of debris which have come loose from the back of the eye. 13 00:00:33.489 --> 00:00:34.940 They come in all shapes and sizes 14 00:00:34.940 --> 00:00:38.250 because the proteins and the cells can clump together in different manners 15 00:00:38.250 --> 00:00:43.020 and you can even estimate whereabouts in the eye they are based on their appearance. 16 00:00:43.020 --> 00:00:44.579 The sharper the shadows appear to be, 17 00:00:44.579 --> 00:00:46.239 the closer they are to the retina, 18 00:00:46.239 --> 00:00:50.370 and those that are blurrier and less defined are closer to the centre of the eye. 19 00:00:50.370 --> 00:00:51.460 Although the can be a bit annoying, 20 00:00:51.460 --> 00:00:54.489 as long as they haven't appeared suddenly, they're completely harmless. 21 00:00:54.489 --> 00:00:59.100 Whilst you're still up looking at the sky you'll probably also notice some white spots 22 00:00:59.100 --> 00:01:00.630 floating around. 23 00:01:00.630 --> 00:01:02.809 This is known as the blue entoptic phenomenon, 24 00:01:02.809 --> 00:01:05.430 and what you're seeing are your white blood cells being pumped through 25 00:01:05.430 --> 00:01:08.360 a network of blood vessels that's in front of the retina. 26 00:01:08.360 --> 00:01:12.930 Our brain actually does a great job of correcting the shadows cast by these blood vessels onto the retina, 27 00:01:13.930 --> 00:01:16.450 but because they're composed of a majority of red blood cells, 28 00:01:16.450 --> 00:01:19.340 it means that the white blood cells which are a different colour, 29 00:01:19.340 --> 00:01:22.079 shape and size don't get filtered out as well. 30 00:01:22.079 --> 00:01:25.880 But much like the floaters, they don't really affect everyday vision. 31 00:01:25.880 --> 00:01:30.579 Now so far, the two things I've told you about are real objects found inside our eyes, 32 00:01:30.579 --> 00:01:34.320 but the next two are artefacts that aren't there at all. 33 00:01:34.320 --> 00:01:37.540 Visual snow is a phenomenon that everyone reports experiencing to some degree, 34 00:01:37.540 --> 00:01:42.860 and it's the appearance of a static or grainy like texture over all or parts of the visual field. 35 00:01:43.860 --> 00:01:47.250 Personally, I find it to be the most noticeable over dark colours or in the dark, 36 00:01:47.250 --> 00:01:50.520 and it reminds me of footage shot on a camera with a high ISO. 37 00:01:50.520 --> 00:01:54.820 High ISO refers to a high sensitivity setting on the sensor, 38 00:01:54.820 --> 00:01:56.940 which means that is a lot more sensitive to light, 39 00:01:56.940 --> 00:01:58.880 but also a lot more prone to noise, 40 00:01:58.880 --> 00:02:01.060 which you can see in the grainy footage around me. 41 00:02:01.060 --> 00:02:03.600 Now whilst we can explain grainy texture in cameras, 42 00:02:03.600 --> 00:02:06.060 we don't actually know what causes visual snow 43 00:02:06.060 --> 00:02:07.680 but we believe it to be neural, 44 00:02:07.680 --> 00:02:12.010 which means it has a lot more to do with the neurons and the way nerves process these signals, 45 00:02:12.010 --> 00:02:13.790 as opposed to the eye structure itself. 46 00:02:13.790 --> 00:02:16.900 And last, but not least, are phosphenes. 47 00:02:16.900 --> 00:02:19.510 I'm sure you've heard the expression "to see stars", 48 00:02:19.510 --> 00:02:21.500 and if you've ever received a blow to the head, 49 00:02:21.500 --> 00:02:22.880 or you've coughed particularly hard, 50 00:02:22.880 --> 00:02:27.120 you know what that expression means, those are phosphenes. 51 00:02:27.120 --> 00:02:30.830 Likewise, if you've ever rubbed your eyes, or tapped the sides of them, 52 00:02:30.830 --> 00:02:34.300 you've probably seen some pretty geometric shapes appear before them, 53 00:02:34.300 --> 00:02:36.470 and these are also phosphenes. 54 00:02:36.470 --> 00:02:38.090 Generally, the images that we see 55 00:02:38.090 --> 00:02:41.480 are the result of light stimulating our photoreceptors at the back of the eye, 56 00:02:41.480 --> 00:02:45.530 but they can also be triggered in other manners, for instance, mechanically. 57 00:02:45.530 --> 00:02:49.340 If I tap the side of my eye, I'm actually changing the pressure inside it, 58 00:02:49.340 --> 00:02:51.760 and this causes some of the photoreceptors to fire 59 00:02:51.760 --> 00:02:55.870 and cause those sparks and pretty geometric shapes that we see. 60 00:02:55.870 --> 00:02:58.360 We also know that phosphenes can arise in the brain. 61 00:02:58.360 --> 00:03:01.920 People who take psychedelic drugs also report seeing phosphenes, 62 00:03:01.920 --> 00:03:04.920 and some scientists found that if you pass a gentle electric current 63 00:03:04.920 --> 00:03:06.930 through the brain's visual cortex, 64 00:03:06.930 --> 00:03:10.780 you will also report seeing some pretty geometric shapes. 65 00:03:10.780 --> 00:03:13.171 These were the 4 disturbances that I wanted to talk to you about today, 66 00:03:13.171 --> 00:03:14.870 and I think it just goes to show 67 00:03:14.870 --> 00:03:18.670 how complex and fascinating our visual system really is, 68 00:03:18.670 --> 00:03:21.180 and adds a whole new layer of meaning to the expression, 69 00:03:21.180 --> 00:03:23.490 "there's plenty more, than meets the eye" 70 00:03:23.490 --> 00:03:25.070 Inés, thank you very much! 71 00:03:25.070 --> 00:03:26.410 Go subscribe to Draw Curiosity, 72 00:03:26.410 --> 00:03:29.210 there are links on screen or in the description now. 73 00:03:29.210 --> 00:03:31.130 Also in the description is a link to Inés' blog post 74 00:03:31.130 --> 00:03:34.940 with the full research notes and bibliography for the video you've just seen. 75 00:03:34.940 --> 00:03:36.230 That is it for the guest videos! 76 00:03:36.230 --> 00:03:37.960 Unless I've been eaten by a polar bear, 77 00:03:37.960 --> 00:03:40.430 in which case this will be a very ironic end tag, 78 00:03:40.430 --> 00:03:42.900 I am back next week, and I'll see you then.