It sounds like you're talking more about memory than ability to imagine? As a child I used to watch cartoons in my head when lying in bed at night waiting to go

It sounds like you're talking more about memory than ability to imagine? As a child I used to watch cartoons in my head when lying in bed at night waiting to go to sleep - they weren't remembered though; excepting they were abstracted from actual cartoons. I have very poor ability to imagine/remember music. Though curiously I'm good at "intros" (guessing songs from the first few notes); I couldn't hum you the first few notes of anything.