Take a look at this picture. What do you see? When you look at it again, do you see something else?
Image courtesy Steve Stuart Williams and Tim Urban
Most people see a man, off balance, running into a snowy forest. Then after looking again, they see a dog running toward us. Some people see the dog first and have a hard time seeing the human.
What's going on is that there are two opposite streams of information processing going on in your brain. One stream is like a camera. Light enters your eye and resolves into shapes and patterns that move to the back of the brain and up through the cerebral cortex to higher level processing.
But while this is going on, the brain is constantly generating theories of what it's seeing and delivering those theories down the pipeline, optimizing what you're actually seeing to fit its dominant conception.
All along you're reality-checking the top-down theory against the information coming up the pipeline from the eyes.
If the first top-down reading doesn't continue to fit the bottom-up facts, you start generating new interpretations.
A similar process happens with auditory processing when you hear a gunshot...or was that a firecracker?...or was someone popping a paper bag? You can feel your adrenaline surge when you think it's a gunshot, and all that changes when you realize it isn't.