I'm on my way back from the Arctic, but there is time for one more guest video! And it's from Inés at Draw Curiosity, who's talking about those weird floaty things that you get in your eye sometimes. So Tom's taken his eyes off his channel and he's invited me to talk to you about 4 visual phenomena and disturbances that we all experience, and you don't have to go particularly far for any of them. For the first one, all you need to do is to look at a blank wall, or up at the sky, and you'll probably notice some dark grey spots floating around. These are floaters and what you're seeing are the shadows cast by bits of debris which have come loose from the back of the eye. They come in all shapes and sizes because the proteins and the cells can clump together in different manners and you can even estimate whereabouts in the eye they are based on their appearance. The sharper the shadows appear to be, the closer they are to the retina, and those that are blurrier and less defined are closer to the centre of the eye.
Although the can be a bit annoying, as long as they haven't appeared suddenly, they're completely harmless. Whilst you're still up looking at the sky you'll probably also notice some white spots floating around. This is known as the blue entoptic phenomenon, and what you're seeing are your white blood cells being pumped through a network of blood vessels that's in front of the retina. Our brain actually does a great job of correcting the shadows cast by these blood vessels onto the retina, but because they're composed of a majority of red blood cells, it means that the white blood cells which are a different colour, shape and size don't get filtered out as well. But much like the floaters, they don't really affect everyday vision. Now so far, the two things I've told you about are real objects found inside our eyes, but the next two are artefacts that aren't there at all. Visual snow is a phenomenon that everyone reports experiencing to some degree, and it's the appearance of a static or grainy like texture over all or parts of the visual field.
Personally, I find it to be the most noticeable over dark colours or in the dark, and it reminds me of footage shot on a camera with a high ISO. High ISO refers to a high sensitivity setting on the sensor, which means that is a lot more sensitive to light, but also a lot more prone to noise, which you can see in the grainy footage around me. Now whilst we can explain grainy texture in cameras, we don't actually know what causes visual snow but we believe it to be neural, which means it has a lot more to do with the neurons and the way nerves process these signals, as opposed to the eye structure itself. And last, but not least, are phosphenes. I'm sure you've heard the expression "to see stars", and if you've ever received a blow to the head, or you've coughed particularly hard, you know what that expression means, those are phosphenes. Likewise, if you've ever rubbed your eyes, or tapped the sides of them, you've probably seen some pretty geometric shapes appear before them, and these are also phosphenes. Generally, the images that we see are the result of light stimulating our photoreceptors at the back of the eye, but they can also be triggered in other manners, for instance, mechanically.
If I tap the side of my eye, I'm actually changing the pressure inside it, and this causes some of the photoreceptors to fire and cause those sparks and pretty geometric shapes that we see. We also know that phosphenes can arise in the brain. People who take psychedelic drugs also report seeing phosphenes, and some scientists found that if you pass a gentle electric current through the brain's visual cortex, you will also report seeing some pretty geometric shapes. These were the 4 disturbances that I wanted to talk to you about today, and I think it just goes to show how complex and fascinating our visual system really is, and adds a whole new layer of meaning to the expression, "there's plenty more, than meets the eye" Inés, thank you very much! Go subscribe to Draw Curiosity, there are links on screen or in the description now. Also in the description is a link to Inés' blog post with the full research notes and bibliography for the video you've just seen.
That is it for the guest videos! Unless I've been eaten by a polar bear, in which case this will be a very ironic end tag, I am back next week, and I'll see you then..