That's what sensory deprivation tanks are for. As the brain is a constant pattern-matcher and input-filter, depriving it of fresh inputs seems to turn it a bit

That's what sensory deprivation tanks are for. As the brain is a constant pattern-matcher and input-filter, depriving it of fresh inputs seems to turn it a bit hallucinatory, it starts to make up stuff (could be described as boredom, perhaps?). Feynman seemed to enjoy the experience somewhat (described in "Surely you're joking, mr Feynman"). Of course, every person's experience could be unique and different.