It is a matter of common experience that we see spots of light when hit on the head or eyes. This is referred to as seeing the stars. In fact these are flashes of light produced when our brain gets confusing signals from our eyes, after being hit on the head or eye.
When a nerve carries a signal, it has an electric current flowing through it. And thus every signal is in the form of an electric current. But its meaning depends upon its place of origin and destination.
When we are hit on the eye, the pressure on the eye ball upsets the nerve cells. These nerves are connected with parts of the brain that receive eye signals. Normally these signals mean light. In this case they are caused by pressure, but our brain cannot identify them. It interprets the signals as light and we see the flashes or stars.
If you want to confuse your brain and see lights or stars, close an eyelid and press your finger gently in the corner of your eye. You would see a patch of light on the side opposite to the finger. Similarly by pressing the eyelid we can see stars. Same is the case when we are hit on the head.