Optional details in frames are easily displaced by reality. But perception of reality requires these default assumptions. "But why use default assumptions at all, instead of simply seeing what's really there? Because unless we make assumptions, the world would simply make no sense. It would be as useless to perceive how things 'actually look' as it would be to watch the random dots on untuned television screens."
NB: How does this relate to natural constraints on vision? Does this agency approach treat bottom-up constraints and top-down expectations with essentially the same mechanism?