Why do you see speckles when you sneeze?

19 April 2009

Question

Why if you sneeze in the dark when you open your eyes again (apparently it’s impossible for you to sneeze with your eyes shut) why do I see white speckles in front of my eyes?

Answer

Chris - That's a fantastic question. Have you guys seen this?

Helen - Hmm. Yeah. Dave - Quite often see white speckles in front of my eyes just when it's dark. Chris - This is what's called an entoptic phenomenon. In other words, it's a visual hallucination, effectively which is arising from within your eyeball. There's a number of possibilities here. I think that we could be seeing one of two things. One is that if you stimulate your eye: because the retina's very sensitive to pressure if you just push on the side of your eye you'll see an hallucinogenic light display. You can do it yourself just by pressing the side of your eye. Try that. If you press on the side of your eye you can see some interesting colours. I'm not saying poke your eye out. Just apply gentle pressure to the side of your eye. This is because the retina is sensitive to pressure and I think it causes discharges of different bits of the retina which I think causes those visual hallucinations you see. They're just photo-receptors firing off as though they've seen light they haven't really seen. When you sneeze you've got air jetting out of your nose, ricocheting around at 100 miles an hour. I suspect that the vibration of the air coming out of your nose probably impacts on air that's in your sinuses which are like cul de sacs. You pressurise the air in your sinuses. This causes a shockwave, probably, to reverberate through your skull. I suspect some of that shockwave might go into your eyes and it could therefore cause the retina to jolt a little tiny bit. The fact that not everyone gets that and everyone does sneeze a lot says that it's unlikely. I was poking round and I found a very interesting comment on the internet about someone called Scheerer and Scheerer's phenomenon. This is amazing because it's used as a way of measuring blood flow through the retina. What you see when you look at a lovely blue sky and nothing else in the visual world. You see these little white speckles coming in they follow a very stereotype path. If you keep watching them you'll see that they all tend to take the same pathway: a wiggly line running in from one part of your retina to another. They're not random, they're coming in and flowing. These are white blood cells and what's happening is because they're very infrequent compared with red blood cells they only crop up now and then as these white blobs. When you look at the blue sky the red blood cells which your blood vessels are full of absorb the blue light quite well. The white blood cells which are infrequent don't. They reflect it and you see a white sparkle on your retina. That's why you see these white blobs when there's nothing else in the visual world to distract you.

I wonder if what's going on with that question is that maybe the sneeze provoked him to look at the sky and he then saw what's called Scheerer's Phenomenon. People can be trained to look at a computer screen and track the number of white blood cells they see come across. This can be used as a measure of retinal blood flow if you don't have another fancy way of doing it. Isn't that amazing?

Helen - That is crazy.

Dave - The other kind of speckle I've noticed occasionally is if it's very dark everything seems to be covered with a very fine speckle - a bit like noise on a TV screen.

Chris - That's called prisoner's or captives' hallucinations. This is where your retina is tuning itself to the ambient conditions. You know how you go from a bright room to a dark room and your retina (over the course of about half an hour) changes its sensitivity. You couldn't see in the dark but now you can. That's your retina tuning up the sensitivity of photoreceptors, making them more sensitive. You use rods rather than cones. You increase the connections between cells to increase the signal. It's a bit like if you take your radio and the signal coming out is not very good you turn the volume up. What your eyes are effectively doing is turning their volume up to become more sensitive to the light that's coming in. Because there's almost no light coming in you start to even see the noise that's being made in your retina, even though there's no signal there. People who go in caves will say if you turn all the lights out in a cave so it's genuinely jet black you see this a lot. Prisoners who are kept in solitary as a form of torture - nasty stories from that.

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