The sad reality is that human brains are incredibly fragile, and they do not recover from injury. This is why concussions are no joke because once you get a concussion, it means reduced brain function that you can never get back. This loss can be subtle or not so subtle, but our memories are too poor to make a proper comparison between past performance vs. current performance without a recorded benchmark.
When hit in the proper spot, it only takes a small amount of force to rock the brain. This means that even young kids can knock you out if they have decent technique, and also women.