I'm like you, which I had assumed is the norm: I can visualize things, but that "visualization space" is completely separate from my normal vision. When I'm imaging an image (or, more often, a short "clip" of something) I can't also focus my attention on what's in front of me at the same time.

You put it into words better than I could. That's the exact same for me.

Yes, that's the source, sorry.

Math, CS, physics and engineering tend to attract people with visual/spatial aptitude. This may be because sighted people first encounter the subject matter of those disciplines through their visual perception. Whatever the explanation is, HN readers are likely to be an atypical population.

My interpretation of the "superimposed" comment is that we are not talking about images appearing as if literally overlaid on what we are seeing. What people experience is that an object that they are visualizing seems to be spatially located in the same place as their visual field—hence Elmer Sperry's comment that he could just "draw around it". Would you agree that your visual imagination and your visual field occupy the same space "in front of" your head?

If it's any help, what I'm trying to say about the relationship between real and imaginary is close to the mathematical idea of a covering space: https://en.wikipedia.org/wiki/Covering_space