They recently screened TV series The Walking Dead here. I enjoyed it. In the final episode a scientist shows what is supposedly a brain scan of someone dying from a zombie bite.
All the major frontal lobe activity (personality, intelligence and reasoning etc) shuts down first and then the lower brain stem finally stops and they are dead.
Then, as the Zombie infection takes hold the brain stem begins to reactivate and they come back to "life"
The lower brain contains the autonomic system. All the stuff we do without consciously thinking about it is controlled by this autopilot. It tells our hearts to beat and our lungs to breathe, it tells us to eat and excrete.
So far so good. You got these mindless automata satisfying their basic needs. They aint bad guys, they're just maintaining themselves by eating us. Can't blame em.
However, in the first episode, hero Rick is startled by a crawling female torso cut off at the waist. How can this half creature eat and gain energy when it doesn't have a digestive system?