Science Questions

Why do you see speckles when you sneeze?

Sun, 19th Apr 2009

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Andrew Norton asked:

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



Chris - Thatís a fantastic question. Have you guys seen this?

Helen - Hmm. Yeah.

Dave - Quite often see white speckles in front of my eyes just when itís dark.

Chris - This is whatís called an entoptic phenomenon.  In other words, itís a visual hallucination, effectively which is arising from within your eyeball.  Thereís 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 retinaís very sensitive to pressure if you just push on the side of your eye youíll 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.  Iím 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.  Theyíre just photo-receptors firing off as though theyíve seen light they havenít really seen.

When you sneeze youíve 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 thatís 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 itís unlikely. 

I was poking round and I found a very interesting comment on the internet about someone called Scheerer and Scheererís phenomenon.  This is amazing because itís 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 youíll see that they all tend to take the same pathway: a wiggly line running in from one part of your retina to another.  Theyíre not random, theyíre coming in and flowing.  These are white blood cells and whatís happening is because theyíre 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 donít.  They reflect it and you see a white sparkle on your retina.  Thatís why you see these white blobs when thereís nothing else in the visual world to distract you.

I wonder if whatís going on with that question is that maybe the sneeze provoked him to look at the sky and he then saw whatís called Scheererís 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 donít have another fancy way of doing it.  Isnít that amazing?

Helen - That is crazy.

Dave - The other kind of speckle Iíve noticed occasionally is if itís very dark everything seems to be covered with a very fine speckle Ė a bit like noise on a TV screen.

Chris - Thatís called prisonerís 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 couldnít see in the dark but now you can.  Thatís 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.  Itís 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 thatís coming in.  Because thereís almost no light coming in you start to even see the noise thatís being made in your retina, even though thereís no signal there.  People who go in caves will say if you turn all the lights out in a cave so itís genuinely jet black you see this a lot.  Prisoners who are kept in solitary as a form of torture Ė nasty stories from that.


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Andrew Norton asked the Naked Scientists: Hi Chris Why when you sneeze in the dark(after opening your eyes) as it is nigh impossible to sneeze with them open do I see white speckles in front of my eyes? Kind regards ANDREW NORTON† What do you think? Andrew Norton , Wed, 15th Apr 2009

I think the speckles are appearing because the preassure the sneeze creates in the sinuses.

A sneeze accelerate the air in your lungs up to 200 km/h, and that creates preassure to your blood vessels in your eyes, forcing the blood out of your eyes, and "black-out" the light-sensitive cells for a split-second.

You will get the same effect if you are boxing and gets a hard face/head hit from your opponent. Eldar, Wed, 15th Apr 2009

I think yor_on has the answer from this thread:

Like Eldar said, the pressure is caused in this case by the sneeze.
Chemistry4me, Thu, 16th Apr 2009

so interesting! I see little dots or shapes like music notes when I sneeze. started recently. its scattered and fades towards the right side..... so glad I read this article jen, Fri, 4th Oct 2013


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