Several incidents, the most serious of which were:
* I was sent backwards through the windscreen of a car which drove into me from behind at speed - drunk driver. Impacted the windscreen with the back of my head at the equator. Result: Mild concussion and scarring. Wheelchair for two months from injuries to my legs.
* Cut up by a car, fell off sidey ways - impacted the ground with the side of my head just above my right ear. Result: Lump on head.
* Cut up by a taxi at speed, launched into a junction and landed flat on my face at about 25 mph. Result: Serious oncussion.
I am quite sure that the last crash caused slight but permanent brain damage, memory loss. Though I've always been a bit vague. So. You know.
In none of the above collisions was I wearing any head protection. Had I been wearing a helmet I am sure that the health professionals who cared for me would have said that my helmet saved my life. In any case it wasn't the lack of a helmet which hurt me but the danger posed to me by the idiocy of the morons we are obliged to share road space with. I cannot see how a helmet - at the sort of speeds I banged my head - could have helped. Actually, I'm convinced that it would have exacerbated the issue - bending my neck and twisting my brain inside my skull.
Hope this helps.