You don't have to see it clearly, just see it. And you can be fooled, too, by level lines that look like horizons: the corner between wall and ceiling in a room with slight contrast in paint colours can light the reflex and calm the spin. This is stone-age lizard-brain stuff so it doesn't have to be perfect.

I think it's the same thing that lets a chicken or an eagle track a spot while turning.

I caught a virus at college that was going around. They didn't try to trace it but like a dozen of us who met during treatment all suffered damage to our vestibular network in one or both ears and were staggering about until this secondary thing took over full-time. The damage is permanent but this reflex thing keeps us vertical. It's really astounding.