Mister Volition is one of Greg Egan's short stories. It's stand-alone, meaning it isn't set in any particular universe.
It centres around the modularity of the mind by following a psychopath as he explores his notion of free will with a black market eye-patch (branded Pandemonium) that takes cues from his neural activity and displays it as visual feedback.
After he's confident that he's identified the branches responsible for conscious thought amongst the overall image, he sets out to prove his control over the rest of the 'pandemonium' - only to discover that in reality, this control is no more than an illusion.