One of the most confounding things about zombies is how, after death, they are able to retain at least a few of their basic functions, even though they are technically dead. But, what exactly is death? While that may sound like a strange question, it is not. For the most part, death is defined as the period when, collectively, brain activity has ceased, the damage caused by a lack of oxygen to the brain has irreversibly destroyed what is termed consciousness (that which defines us as unique entities possessing self-awareness), and respiration has come to a complete halt. But, it's not quite as simple as that. "Brain death" is described as occurring when the brain is starved of oxygen, which then leads to the necrosis, or the destruction, of the billions of neurons in the brain. A person who is brain dead, however, may still exhibit a functioning heart and lungs and can be kept alive via artificial means. Then there is the matter of "clinical death," which is when the individual exhibits a complete lack of respiration and blood circulation. Problematic, here, is the fact that the clinically dead can be kept from the clutches of the Grim Reaper via a respirator.
On top of that, under carefully controlled circumstances, the human blood-supply can be brought to a complete halt in those parts of the body, specifically below the heart for around half an hour - with little sign of appreciable damage occurring. Since neurons play vital roles in the function of the human sensory organs - those responsible for our ability to speak, hear, touch, smell and taste - how is it that the reanimated dead are able to make use of their senses if brain death, and a lack of organs to the neurons, destroyed them when death took them in their original human form? Granted, the average zombie does not speak, but it does growl or grunt. And the creature certainly exhibits the ability to smell, hear, touch and taste.
This gives rise to an important question: since, in most zombie-themed scenarios, the infected person dies and reanimates relatively quickly, should we consider them to be literally dead? Perhaps, instead, the dead are actually returning from a very deep, but brief, state that is not literal death, but one that only mimics the appearance of death. That would be a far more likely scenario in the real world. There is some evidence that suggests we should look at the zombie in a whole new light. In September 2013, it was revealed that doctors in Romania had discovered something remarkable: on occasion, when a patient flat-lined, and both clinical death and brain death were declared, a small amount of what is termed electroencephalogram - or EEG - activity in the brain could still be detected. The patient was, for all intents and purposes, gone; yet, in both the hippocampus and the upper-cortical areas something was still going on. Interestingly, the hippocampus plays a significant role in memory.
Perhaps, when the infected die - or descend into a brief, death-like state - a portion of the hippocampus survives, albeit in a somewhat damaged state, but which allows for just about the most basic memories to survive - namely, those which are based around self-preservation. After all, no one can say that the zombie does not do all it can to preserve its "life." More intriguingly, scientific tests have demonstrated that people whose hippocampus are damaged display clear signs of hyperactivity - a state into which each and every zombie descends when it spots a tasty human meal on the horizon. That particularly applies to the "fast-running" variety of the deadish.
Finally, there is a near-consensus that the hippocampus performs a significant function in the development of what are termed "cognitive maps." Essentially, and in simple technology, a cognitive map allows us to store data to recognize, and to remember our immediate locale. And what does this have to do with the not-quite-entirely dead? Very often, the zombie, while searching for food, will continue to roam around its own, original neighborhood, rather than venturing far and wide. A classic example is the situation in the 2004 remake of Dawn of the Dead, in which hordes of the infected head for the local mall - a place they would have been familiar with in their previous lives. In view of the above, perhaps we should rename the dead as the "almost dead."