Right! I felt like no one was quite pointing this out. Your brain is expecting certain input in given situations, and you have layers and layers of habits bas

Right! I felt like no one was quite pointing this out. Your brain is expecting certain input in given situations, and you have layers and layers of habits based on those expectations. If you are accustomed to NOT having that input, you don't expect it. You don't fake it (imagining a black space of the size a visual field would be...), you just don't have it, and it's not a part of your interactions with the world. If the shape of your visual field changes suddenly -- you're about to pass out and you get "tunnel vision" -- it's obvious and startling. If it changes gradually, you don't even notice; you just start hitting your head on low beams more often, or bumping into table edges as you go by, until you adjust (this is from experience...).