What causes us to "see stars"?
What causes us to "see stars" when we bash our heads? I also notice the same phenomenon when I stand up suddenly and feel light-headed when blood pools in my legs. This is especially confusing, since the two "stimuli" operate by different mechanisms: a blow to the head versus a brief episode of hypotension. I'm embarrassed to admit that I don't know the answer, even though I've been a professor of ophthalmology for 35 years! It would be better to ask a neurologist or neuro-scientist.
Both of [these situations] are basically artificial phenomena in a sense that in one case - the standing up too quickly - this is an entoptic phenomenon, which means "something going on inside the eyeball". The retina, as you'll know as an ophthalmologist, has one of the highest metabolic rates of any tissue in the body. The brain and central nervous system tissue burn off about 20% of the energy that you consume in any given moment in time and yet they contribute only a small fraction [about 2%] of body mass. So the retina is very metabolically hungry. And if you stand up too quickly you can have what's called a "postural drop" in blood pressure. Blood comes up from your legs into your heart to get pumped around the body. When you stand up, and before your heart compensates, the return of blood drops slightly which causes the perfusion pressure to drop briefly. That causes a momentary reduction in perfusion of your retina. That slightly reduces the supply of oxygen and sugar to the retina from the blood, which causes the retina to start to fire off abnormal signals, which we experience as "sparkly" light signals; the brain is fooled into thinking you're seeing light when it's not there.
Now, conversely, when you bash your head, what's probably going on there is that because the brain is bobbing around inside your head in a fluid - the cerebrospinal fluid - and has a very soft, blancmange-like consistency, if you have a sudden interruption of movement to your head - so you hit your head very hard against the wall or pavement for instance - the brain cannons into the inside surface of your skull; it then can rebound and hit the back of your skull as well. And if you irritate the part of the brain that decodes what you're seeing - the visual cortex which is right at the back of your head - then it's possible that, in the same way that irritating the nerve cells in the retina by not having enough blood flow makes them fire abnormal signals which you see as stars, you can also affect the brain cells in the visual system similarly. So I think, probably, this is responsible for triggering the experience of "seeing stars" when we suffer a head injury.
It's an excellent question and thank you very much for asking it!