In Romero's zombie lore, destroying a zombie's brain kills the zombie. Merely severing a zombie's spine won't kill them. If you severed the head from a zombie, and the neck was still attatched to the zombie's head (leaving the brain stem undamaged), would it make the zombie's head dead, or would the head remain undead? Put another way: Would the severed zombie head with its neck still connected be a zombie lurker?