Science Questions

Why does a bump on the head make you see stars?

Sun, 16th Jan 2011

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Question

George Pope asked:

In the 8th grade I played full back for my football team. The play was a simple off tackle pitch with me as a lead blocker. I came around the corner, and went face-to-face with their middle linebacker, who I can best describe as a 6 foot fridge with legs. It was a beautiful hit, the coaches said they could hear the smack of the pads from over a 100 yards away. All I heard was the sound of my buddies trying to wake me up after the hit. When I opened my eyes, all I could see were stars.

 

My question is simply this, why is it whenever a person hits their head on something with enough force, that they see stars afterwards?

 

PS: I got a concussion from it, and we lost the game by 14 points.

Answer

Chris -   Yes.  Itís a cartoon clichť, isnít it?  A cartoon character get socked around the head and then you see the stars going around their head.  Itís probably because you get two things happening:

One is that when you slam your head down against the ground or some hard object very quickly, you get whatís called a contre-coup injury, because the brain bounces around inside your skull - because although itís cushioned by the cerebrospinal fluid, if itís travelling sufficiently fast there will still be a degree of shock in the brain.  And if that impacts on the visual areas, then it triggers a bit of pressure to be applied to the visual nerves, or the nerves that present to consciousness the fact you're seeing something, and as a result they become active.  If they're active, they're going to make you think you're seeing things that are not there, but they're not going to be built images because that involves whole assemblages in nerve cells.  Itíll just be random cells here and there and as a result, you see these little spots of light which we interpret as stars because that's the most sensible thing to say that they are.  So itís probably a trauma thing. 

The other time you occasionally see stars is when you stand up too quick in the bath.  This is a blood pressure hyperperfusion thing because when you're hot in the bath, all of your blood vessels dilate out to get as much blood as close to the skin surface as possible to cool you down.  So when you stand up, you've got lots of blood in your peripheries, very little is actually returning to the heart and so, thereís a drop in blood pressure, just transiently, and that impacts on the retina which has a very high metabolic rate.  If you interrupt the blood flow, just for a fraction of a second to the retina, all of the nerve receptors in the eye that turn light waves into brainwaves just start firing off all kinds of visual signals and you see those starbursts, and that's probably the reason why.  

Sarah -    Is that also a little bit like if you rub your eyes too hard and you start seeing little lights popping all over because the pressure in your eye is activating the photoreceptors?

Chris -   Absolutely.  Thatís another entoptic phenomenon.  If you press on the eye, you're deforming and distorting the retina, applying pressure to the photoreceptors.  This could do two things.  One, stimulate them in exactly the same way as you say.  The other is it could just move them a fraction away from the blood for a fraction of a moment.  That is enough to make them a little tiny bit oxygen-deprived and so they start also firing off these funny signals.  So yes, you're on the right lines.

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