Hm, I don't think it's likely a function of basic differences in visual perception - I have normal vision as far as I'm concerned, but I have very vivid mental imagery. I also have very vivid dreamscapes, and every dream I have is a new scape - I've never had the same one twice. (Unrelatedly or relatedly, I dream A LOT, even when I doze off for 5-10 minutes.) In any case, I can be physically looking at something in the real world, but be "looking" at something completely different in my mind's eye, but there is a definite shift in attention that facilitates how much information I can get from either the current sensory input or the mental image.