>I don't think anybody is "literally" envisioning things, as in hallucination

I think it's basically exactly like a hallucination for some people, except it's mentally tagged as originating "internally" instead of "externally" (which is what freaks people out about having a hallucination). I think it's basically the same thing with internal monologue vs. auditory hallucinations.

(for the record I have neither internal monologue nor visualization)