I've had three concussions skiing (additional on my bike) where I have forgotten several days of my life. The last one caused me to forget the week prior to my accident, and it took three weeks to feel normal again. All of them have occurred with a helmet on. I'm fairly certain I would have cognitive damage if it wasn't for my helmet.
All of these happened when I was cruising and well within my comfort zone. Take from it what you will.