Pain/pleasure metaphysics: a request

Lately I’ve been looking into causal connections between brain states and pain/pleasure.

I’m finding plenty of material on specifics such as nociceptors, gate circuits, correlative fMRI studies, and so forth, but there doesn’t appear to be a lot of research, or even much speculation, on the general question. I.e.,

What are pain and pleasure, in relation to systemic properties of the brain? E.g., what principles could be used to examine a brain and predict whether it’s experiencing pain or pleasure? If we knew someone was experiencing pain or pleasure, what principles could we apply to predict what’s going on in their brain?

Ditto for sadness and happiness.

If any readers have perspective on the literature or can put me in touch with someone who does, please let me know.