I see nothing. I just select a scope and do a mental step-by-step simulation. I do one pass for expected inputs, and then a lot of passes for unexpected inputs. If anything I hold a stack of intermediate values in short term memory, but it's formless. It feels like a story. Since I stare at the code while doing it, you could say that some characters in the story are the relevant slices of code on the screen. If there's no code to stare at, like a planning thing, I write pseudo code or ascii diagrams regardless.