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!

(Laughs) 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: (Laughs) I mean, you could try, man, but I'm going to stay on this side of the glass.