Yeah, I'm aware. Can we frame the concept differently?

Why the projection of "alone with thoughts"? Why seed the concept of existential dread in the mind of the reader of the study results?

How do the researchers know that it was assuredly the terror of the subject's own mind, their internal monologue or what have you, which certainly provoked them to preoccupy themselves with pain?

Answer: they cannot know why the individuals shocked themselves, but only that the shock happened.