Doctor 1: So, how did it get  to be this way? Doctor 2: Well, it’s my professional  opinion that the large gaping bite mark on its shoulder might have something  to do with it. D1: Thanks. I mean, what causes  its behavioral abnormalities? D2: Well, we know all behaviors  are rooted in the brain, so I’d expect that something’s gone  terribly wrong, probably in there. D1: Thanks again, Doctor Obvious. Let me be more specific. What changes in the brain  would have to occur in order to cause this kind of behavior? D2: Hmm. Well, let's see. The first thing I notice is how it moves. Stiff legged, with long, lumbering steps,  very slow and awkward. Almost like what you’d see  in Parkinson’s disease. Maybe something’s wrong  with his basal ganglia? Those are a collection of deep brain  regions that regulate movement, through a neurochemical called dopamine. Although most people think of dopamine  as the “happy” chemical of the brain, the dopamine-containing neurons  in the basal ganglia die off in Parkinson’s disease— that’s what causes it. Makes it more and more difficult  to initiate actions. D1: What? Look again at how it moves.  Stiff legs, long stance. These aren’t Parkinsonian movements; Parkinson’s patients take  short, shuffling steps. And the posture’s all wrong. This looks to me like what happens  when the cerebellum is damaged. The cerebellum’s a little cauliflower shaped area in the back of your head, but don’t let its size fool you. That little guy contains almost half  of the neurons in the entire brain. Patients who suffer degeneration  from this region, something called spino-cerebellar ataxia, show a lack of coordination that results in stiff legs, wide stance, and a lumbering walk. My money's on the cerebellum. D2: Touché. OK.  So we’ve nailed its motor problems. Now what about that whole groaning, lack of talking thing? D1: Hmm. You know, it sounds kind of like  expressive aphasia, or Broca’s aphasia, which makes producing words difficult. This is caused by damage  to the inferior frontal gyrus, or possibly the anterior insula, both regions behind your temple on the left side of your head. D2: I think you’re only half right. Zombies definitely can’t communicate,  that’s for sure. But they don’t seem to do a good job  of understanding things either. Watch this. Hey, Walker!  Your father smelt of elderberries! See? No reaction. Either it’s not a Monty Python fan,  or it can’t understand me. I’d say this is like spot-on fluent  Wernicke’s aphasia: damage to an area at the junction  of two of the brain’s lobes, temporal and parietal, typically on the left side of the brain,  is the culprit. This area is physically connected  to Broca’s area, that you mentioned, by a massive bundle of neurofibers  called the arcuate fasciculus. I hypothesize that this massive bundle  of connections is completely wiped out in a zombie. It would be like taking  out the superhighway between two cities. One city that manufactures a product, and the other that ships it  out to the rest of the world. Without that highway,  the product distribution just shuts down. D1: So, basically it’s a moot point  to reason with a zombie, since they can’t understand you,  let alone talk back. D2: I mean, you could try, man, but I’m going to stay  on this side of the glass.