Back at the lab, Walter is chipping away at the back of Officer Williams's skull with a bone saw while trying to have a discussion with Astrid about what human brains might taste like. Prying away gory sections of bone, Walter explains that normal hypnosis only boosts alpha waves, but the kind of forced suggestions Williams was subjected to might cause physical damage to the brain. Well, yes, but so might a bone saw.
By the time he's gotten that brain dug out, Olivia and Peter have joined them in the lab, all the way back from New York. Holding it up on a tray for their inspection, Walter announces, "This is not hypnosis, this is mind control." Look, he can make William's mind go up, and he can make it go down! He can move it side to side! Oh, that's not what he means. He says Peter was right about the limits of hypnosis, and points out the brain hematomas that he says are "the result of conflicting neural impulses. A conflict of mind and body." Olivia rather stupidly asks if that's what killed her, and Walter, chuckling, points out the big black bullet track that did that. "But mind control could cause brain damage." His theory is that it travels through the cochlear nerve, i.e. via sound. "An auditory trance is a far more effective path to manipulation than the other senses." Astrid points out that even if they find them, it's not safe to approach them, so Olivia tasks Walter with figuring out how to block it. "The ransom call could come at any minute and we need to be ready." Ready to go off half-cocked on an untested theory, apparently.