Mark, even you said in one of your older posts that light hits can cause a concussion.

http://neurotalk.psychcentral.com/thread82429.html

Nonetheless this is not the only source I found. I googled for a couple of minutes and saw quite a number of posts, where people experience concussion symptoms after a light bump(even without prior head injuries)

I don't excactly know what you mean with 'dinger'