My boy is a rescue who came to me with this light/shadow thing. I tried training him out of it too but wasn't successful. I settled for making it a controlled game, at certain times, in certain places and nowhere else. This has worked. We play shadow pounce at the dog beach every day (when its sunny) for awhile and then its time to play w other dogs. Never while on leash, ever, period. Its comfortable now. He loves it and gets to do it during his free/play time, for a awhile but not the whole time. So far this summer, he's been getting bored with it on his own after a bit and gone off w the other digs on his own instead of on my command.
After reading everything I could find about it and getting all worried I just came to a point where I realized there's gotta be a better way than getting freaked out that my dog was going to be deranged/seizing and I tried to work with it rather than eradicate the behavior. Hope this helps.