We answered this question on the show...
I think 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 – 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 burns off about 20% of the energy that you consume in any given moment in time and it contributes only a fraction of body mass. So it’s very metabolically hungry.
And if you stand up too quickly then you have what's called a postural drop. The blood that's coming up from your legs into your heart to then get pumped around the body, the perfusion pressure drops just briefly when you're standing up and before your heart compensates, and that causes there to be a momentary reduction in perfusion to your retina. That slightly reduces the supply of oxygen and sugar to the retina from the blood, and that causes the retina to start to fire off abnormal signals. When you deprive the retina of the right blood flow then it starts to fire off these abnormal sparkly light signals, fooling the brain 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 wishy-washy consistency, a bit like blancmange. If you have a sudden interruption of movement to your head – so you hit your head very hard against the wall or pavement, the brain then cannons inside the head into the front of your skull and 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 you see funny stars, it’s possible that you can also affect the brain cells directly at the back of the head. I think probably that is responsible for triggering these funny stars that you see when you suffer a head injury. But it’s an excellent question and thank you very much for asking it.