Image: Wikimedia
Ever look up at the sky and see greyish objects floating around? Or what about those weird geometric figures and colors you see when you rub your eyes? Both these phenomena, and more, are caused either by particles in the eyes or originate in the brain. And this video by YouTube explainer Tom Scott describes just exactly what they are.The grey objects you see when looking at the sky or a wall are called floaters: shadows cast by debris that's come loose from the back of the eye. These floaters come in different shapes and sizes given that the proteins and cells clump together in different ways. And as long as they don't appear suddenly, they're harmless.When gazing up at the sky, you might also notice white spots in your vision. This is called the blue entoptic phenomenon: white blood cells being pumped through blood vessels in front of the retina. While the brain usually corrects these shadows blood vessels cast on the cast on the retina, the white blood cells don't get filtered out as well as the red ones.So while the white spots and the grey objects both derive from real things inside the eye, phenomena called visual snow and phosphenes are caused by the brain. Visual snow, or the appearance of a grainy texture throughout the visual field, is neuronal, having to do more with how neurons and nerves process signals, than with the structure of the eye itself.Lastly, phosphenes are the stars you might see if you get hit in the head, as well as the geometric shapes and colors you see when you rub your eyes or take a psychedelic drug. If a gentle electrical current passes through your brain, you'll see phosphenes, as well. While generally perceived images are caused by light stimulating photoreceptors at the back of the eye, they can also be triggered by applying pressure to the eye.So next time you see any or all of the above, know that they are commonly reported by just about everyone. No you're not hallucinating, it's just back-of-the-eye debris and neuronal mishaps.