It varies. Smarter (?!) horses that rear do it once, smash their head on the ceiling once, and figure out that's probably a bad idea. Stupider horses that rear and smash their head are too thick/don't really care about their own safety, and will keep doing it regardless.
Whether they consciously think "Do I have enough room to rear, or will I smash my head?", I highly doubt it, as if a horse was going to rear, it's generally not a calculated thing.
The reason I ask is because I'm doing a project at uni where I have to come up with an innovation in horse training and I was think of a kind of cap that attaches to the head collar, so that the horse can't judge what's space it has above it's head, a prevent it from rearing. e.g. when loading?
What do you think?
Rearing is a million times more complex than just the (final) act of rearing. Sitting behind that act is a whole raft of training, management, feeding, schooling, pain, god knows what stuff that needs to be addressed in order to resolve a rearing problem. Even if a sort of portable ceiling worked (which it won't) a desperately unhappy horse will look for another way of avoiding whatever it's rearing for. And that could be a whole lot worse!
sorry doubt it my friend tried a poll guard that was like a little cap just to see but made absolutely no difference... i think its so individual quite hard to generalise if you have luck let us know!!!