What is the subject of Usher's research?

Roderick spends time in intellectual pursuit. He has become fixated on the idea of the sentience of all "vegetable," as well as even inanimate, things. He pores over books in his vast library, speaks of a living "atmosphere" about the waters and walls of the house.

He is convinced that the inanimate universe is full of "sentience," that seemingly dead objects or matter, such as the "atmosphere" he describes encircling his home, are endowed with senses and perhaps even life of their own. When Poe introduces this concept, it seems almost a digression. The principal arc of the narrative has been Usher's madness, his fear of what he regards as his own inevitable doom. Rather than a window into his tortured psyche, as provided by the bizarre painting of the vault or the improvised song of the "Haunted Palace," the intellectual pursuit of "sentience" seems a projection into the outer world, as though Usher is trying to occupy his mind with something other than himself.