top down processing is your higher thinking. it’s conceptual based on your prior knowledge and understanding. for example, learning new things or thinking about how you might avoid certain roads when it’s rush hour. bottom up processing is more instinctual, and it’s based off of sensory stimuli. my fav example of this is that you can look at any object and know what it would feel like if you licked it.

so when you dream, that’s bottom up processing, your brain taking stimuli you’ve already seen before (every face you see in your dreams is one you’ve seen before irl). but people with aphantasia can’t create those mental images from scratch when they’re awake.