What's the distinction between vivid mental imagery and a closed-eye hallucination? I have aphantasia too (what I used to call "the opposite of a photographic memory") and it always struck me as odd that seeing imagery with closed eyes is considered normal. We put people in mental institutions for hearing voices in their head, but we get a pass if we're hallucinating a visual scene.
My theory is that it's a spectrum from "I only see images when photons hit my eyes" to "I see god right here in front of me." and somewhere on that line is the optimal amount of visual hallucination. It certainly serves some evolutionary advantage.
There's a distinction between delirium, being in a delusional state, where you can't tell the difference between fantasy and reality, which is pathological, vs the normal everyday action of replaying a sensory memory in your head, or imagining sensory input in order to explore an idea. Delusional states may or may not involve a sensory component. It's not a spectrum from one to the other.
The term "hallucination" is tricky because the usual connotation is that it is part of a delusion, whereas some people can generate "closed-eye hallucinations" (or whatever you want to call it) as an alternative kind of mental imagery.
For me personally, the distinction between ordinary "mental imagery" vs "closed-eye hallucinations" that I generate under conscious control is that they are subjectively quite different. The former feel like they are in a different "mental domain" than visual input, and I can perceive this imagery with my eyes open. Another commenter in this thread called this a third frame buffer in their head, separate from their eyes. The latter kind of imagery feels like it is coming from my retina, but it's very dim, and I can only see it with my eyes closed, or in a completely dark room. Apparently, visual input from my retina interferes with this second kind of imagery. And it's this kind of imagery that the OP discusses. You can train yourself to experience it. I've managed to improve both kinds of mental imagery by practicing.
My theory is that it's a spectrum from "I only see images when photons hit my eyes" to "I see god right here in front of me." and somewhere on that line is the optimal amount of visual hallucination. It certainly serves some evolutionary advantage.