Why the White Flash during Nerve Attack
This isn't related to psychology it is pure neurology... I just wanted to make that clear to begin. When you get hit in the head, your brain sometimes "freaks out". One of two things happens:
1. (For a light, quick traumatic injury) Your brain sends out erratic pules or electricity which are carried through nerve cells, or neurons. Because there is a nerve in your eye, you get this sudden jolt sensation. But keep in mind, that this often happens in the occipital lobe of the brain, (where the two optic nerves go, and their nerve impulses are translated into sight) and it messes with our interpretation of nerve signals sent from the eye
2. (For a more serious trauma or heavy blow, possibly with injury) First of all, you have to know that our brain is kind of selfish. It will "shut off" oxygen supply to organs, so the body can keep enough energy to get out whatever mess your in. When you are bleeding, your brain looses out on oxygen. It tells the circulatory system to stop sending blood there, or for the organ to stop using blood so the brain can get it, and repair any damaged neurons while it has the chance. One of those unlucky organs to get shut off are our eyes. But this is just a flash, it usually persists for a while before it goes away.
The white flash people experience in a nerve attack is the combination of nerves in your head being stimulated all at once from the blow to your head or body. The result is what we commonly call "Seeing Stars" as it produces flashes from the nerves as they receive massive over stimulation. The part of the brain that handles what you see is called the occipital lobe, it's located at the back of your head. Its role is to take the information sent from your retina and turn it into something that makes sense to you (based on your experiences). So before you know or understand what is in front of you, your retina has to take in the observable light, convert it first into a chemical signal, and then into an electrical impulse, before sending back to you brain for interpretation. The occipital lobe will then coordinate with the eyes as they work together to understand what's in front of you.
A bang on the head (as example, but it can be in any part of the body), can shake or overload the brain, creating pressure on these tissues and causing the cells to discharge, scattering electrical impulses across the cerebral cortex in a random manner. The electric discharge is interpreted as light causing the flash and later the stars, but as you massage the area (sending more electrical stimulus), where the blow struck, the stars will disappear and everything returns to order, with no permanent damage or cause for concern.
One other factor that will also cause the sense of a light flash, can be when pupils dilate and allow in much more light... The sharp light is a result of some physical stimulus on the retina, the light sensitive part at the back of the eye (rather like the screen of a projector)... again not a call for concern.