I have no real basis for this, but I always suspected that the majority of differences in ability to picture things is actually just a difference in semantics about terms like "visualizing", "picturing", etc. I don't think anybody is "literally" envisioning things, as in hallucination. On the other end, I don't think anybody is actually unable to "think of" what a thing looks like. But it's really difficult to objectively describe what it's like to picture something in your head - so difficult, in fact, that I can see some people calling it "literally summoning an image" and others calling it "not seeing anything at all", while both talking about the exact same thing.
Not that there isn't a difference in ability, just that it might not be as dramatic/binary as we seem to think.
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