Zombies. They are the lurking, groaning reminder of our own mortality. They embody our deepest fears and provide the perfect allegory for the self-destructive nature of man. They are frightening, they are fascinating, and they have infected our imaginations since the very first story ever written – the Epic of Gilgamesh (circa 2100 BC).
“I will knock down the Gates of the Netherworld… and will let the dead go up to eat the living! And the dead will outnumber the living!” – Ishtar to Anu, Epic of Gilgamesh VI. 94-100
This is the first documented reference to what we call zombies, and it also provides the first explanation for what zombies are (i.e., dead residing in Netherworld) and how they come to be (i.e., goddess sets them free). This passage would set the tone for zombies in story telling for the next two millennia; that is, depicting them as something demonic or magical unleashed by something divine or mystical. However, all that changed with the release of George Romero’s Night of the Living Dead (1968), when **SPOILER ALERT** Dr. Keller and a man accredited simply as “Scientist” state in a breaking news report that the zombie outbreak was “definitely connected” to high levels of radiation in Earth’s atmosphere. An exorcist didn’t say demon, a witch doctor didn’t say spell, a scientist said radiation. In that moment, during that fake black and white news report, the nature of “zombies” changed. They were now, more than ever, subject to scientific theory.
Since then — viruses, bio-chemical weapons, cancer treatment, fungi – these are what dominate the origin of the modern zombie. Each implicitly adheres to some scientific law (e.g., biology) instead of “magic”, and although a full explanation is rarely provided, there is usually enough information for geeky scientists to play with. This is especially true for neuropsychologists because they can easily observe the zombies’ behaviors as a way to better understand the structure and function of their brain (which we will do in a moment).
Now, as a note, zombies do not behave the same across movies, TV shows and comics. For instance, in 28 Days Later (my favorite zombie movie), the zombies are insanely high-functioning. In one scene, they are sprinting through a house, effectively navigating their way through a mansion systematically to find potential victims. This would require advanced motor abilities, problem-solving skills and visual-spatial processing that the derpie NOTLD zombies simply do not have. In fact, because of this “higher-cortical activity” (i.e., use fancier parts of the brain), some experts exclude Cillian Murphy’s red-eyed foes from the zombie club entirely! That being said, the following information applies primarily (if not exclusively) to The Walking Dead.
Here are the 10 reasons that Walkers are more neuro-capable than you might think:
1) Walkers are “brain alive”
When someone is diagnosed as “brain dead”, this means that they have suffered a catastrophic, irreversible loss of brain function, including at least some involuntary, life-sustaining functions. Unlike those in comas, brain dead patients will never wake up, and their body can only survive on life supports (e.g., ventilator). In these cases, brain activity has ceased at the cellular level, while some bodily functions continue.
Walkers are the exact opposite.
Their bodies are dead, but their brains are essentially alive, artificially, thanks to the pathogen within their central nervous system. In other words, their bodies no longer replace dead cells with new living cells (hence the corpse-like appearance), but their brains likely do, given their neural activity and behavior. In this way, they may be considered “brain alive”.
2) Walkers experience a complex, medically defined state of consciousness
Before 2010, perhaps the best way to describe the mental state of a Walker was a “severely minimal conscious state”. Clearly, they seem to have some primitive forms of awareness and decisive instincts, but such a state is not possible or relevant to humans – why bother even defining it, right? That’s when psychologists Timothy Verstynen and Brad Voytek kicked down the door of the medical community, locked and loaded with science, grunting these words past the cigars clenched in their teeth: “Consciousness Deficit Hypoactivity Disorder (CDHD)”.
Yes, Walkers have their very own classification of a conscious state (CDHD), referring to a “loss of rational voluntary and conscious behavior, replaced by delusional, impulsive aggression; stimulus-driven attention and the inability to coordinate motor and linguistic behavior”. Treatment? Severe trauma to the brain.
3) Walkers demonstrate brain functioning beyond the brainstem.
*** First, a note on the brain death that occurs during zombification: When the pathogen is introduced directly into the bloodstream (i.e., zombie bite), it quickly breaches the blood brain barrier and proceeds to shut the nervous system down. The heart and lungs stop, blood circulation and airflow stop, and ALL of the neural tissue becomes ischemic, meaning the brain becomes absolutely useless – just a graveyard of dead neurons. Then, after a time of zero electrical activity, neurocellular reanimation occurs – dead brain cells may be repaired and/or new cells are created (a basic definition of “life”). ***
OK, so in the season finale of the first season (episode “TS-19”), the initial Walking Dead crew that learned to “stick together!” found their way into a relatively safe CDC research lab manned by a in-no-way-shady-scientist named Dr. Jenner. We discover that his last test subject was his dead wife whom he observed reanimate into a Walker via some sort of streaming fMRI machine. Based on the imaging, he concludes that Walkers demonstrate high activity (red circuitry in picture) exclusively in their brain stem – a fair interpretation of the data, given regional illumination in the medulla, pons and maybe lower midbrain. But here’s the problem:
He shoots his dearly beloved in the face – right in the kisser – like, FIVE seconds after she reanimates! (EEEK! *BANG*… sorry guys – totally lost my composure).
That is hardly enough time to properly study the neural-reanimation process and begs the question: what does Jenner suck harder at, science or marriage? I submit that Dr. McTriggerfinger is alarmingly poor at both. The fact is, given what we know about Walker behavior, it’s likely that more of the brain is reanimated. For example, Walkers demonstrate gross motor abilities, balance, hand-eye coordination, some vocal expression (groaning that becomes louder when aggressive), sensory interpretation, an understanding of object permanence (will get into later), and some types of memory, just to list a few. That being said, the following superior areas of the brain are likely active (not exhaustive):
- Dorsolateral prefrontal cortex (to understand object permanence)
- Thalamus (interpret senses)
- Cerebellum (motor control, muscle memory)
- Striatum (procedural memory retrieval)
- Subiculum (short term spatial memory)
- Amygdala (experience rage)
An article could be written on the functional neuroanatomy of Walkers alone, but suffice to say that mid- to high-cortical functioning is present to some degree (not just the brainstem). Looking at you, Jenner – maybe spend less time planning explosive murder traps and more time improving your experimental methods.
4) Walkers can remember and learn
Perhaps the most respected and empirical supported model of cognitive intelligence is based on the Cattell-Horn-Carroll theory (CHC theory), which basically states there are nine primary cognitive abilities. Most of these are not possessed by Walkers (beyond very lower-level computations), but at least two are: long-term memory and short-term memory.
- Long-term memory: Walkers do not remember their first kiss, NES game codes, or the capital of Thailand. These are conscious memories — what we might call “episodic” (i.e., autobiographical) or “semantic” (i.e., basic facts) – and they are long since gone. They do, however, demonstrate another form of long-term memory called “procedural memory” – memories which allow them to perform relatively complex motor abilities like walking, banging on windows, reaching and grabbing, and eating with their hands.
- Short-term memory: Although extremely limited, Walkers do demonstrate some short-term memory. For instance, frequently in TWD, we see people peek out of a door, get seen by a Walker, and zoop back on in. If Walkers did not have some capacity for short-term memory, they would not continue walking towards the door once their prey left their sight. Furthermore, they would not continue to try and get through the door as if something unseen exists behind it. When not in a herd, the Walker will eventually forget what they saw, stop growling, and go back to loitering, but it usually takes a minute or two (thanks to their short-term memory).
Walkers also demonstrate behaviors that suggest they can learn. Consider Michonne’s armless, jawless Walker companions that disguise her presence. In episode 303, she mentions that after she removed their ability to eat, they eventually stopped trying to attack and became docile. This “learned helplessness” indicates a capacity to learn from behavioral consequences (i.e., instrumental learning). Furthermore, we have seen Walkers navigate through relatively complex spatial environments without having to slam into every obstacle to proceed. Now, part of this may simply be low-level spatial awareness and proprioception, but consider this TWD scene: A man in the attic makes a noise. Walkers in the kitchen hear this – they navigate through the house, around the railings, and up the stairs. They know they can’t go directly towards the sound; they know they have to take the stairs. This suggests that they’ve achieved some “model-based learning”, in that they’ve successfully created a visual map of the house and can navigate through it successfully.
5) Walkers can play hide-and-seek (object permanence)
As mentioned earlier, if a Walker sees you walk into a building, they will know that you are still there, even if you shut the door. They know this because they grasp “object permanence” — the understanding that objects continue to exist even when they cannot be observed. Why is this notable? Because during your first year of life, you lacked this ability; instead, you thought things simply vanished horrifically from “this cold world of scary feels”. This is because developing object permanence requires maturation of the dorsolateral prefrontal cortex, an area that is only present in the most advanced species of life (e.g., apes, dogs, cats, some birds), and doesn’t occur in humans until about 8 months of age. So, when you’re hiding behind the curtains, and Walkers comes to tag/eat you, take a moment to appreciate their unique conceptual ability before they tear your face off (just like in Face Off).
6) Walkers can feel pleasure
Walkers cannot feel pain. This is why they continue to attack even when you’re stabbing, hacking, shooting or bo-staffing them. It may have something to do with an absence of nerve-endings or deficient hypothalamic functioning, but in any case, pain will not affect their behavior. Pleasure, on the other hand, is a different matter. Consider the following:
“Zombies in a herd are a force of nature”. – Eugene Porter
The keyword here is “herd”. Zombies are rarely found in isolation – they are usually found in groups, standing or moving together. This is not by accident; rather, it is a reliable pattern of behavior that research has found (in humans and animals) to be directly associated with a pleasure-inducing neurochemical called “oxcytocin”. Depending on the context, this chemical may trigger feelings of attachment, bliss and/or euphoria. It is the same chemical that is released when you orgasm. However, the “bliss response” that Walkers feel is probably not social in nature like in humans (i.e., YAY I have friends!). It is more likely a “dude, I’m about to get my noms on!” kind of feeling – an indicator that delicious baby meat is on the menu. In any case, it suggests that their hypothalamus and posterior pituitary still function at least in some ways. It’s also suggests that pleasure, in this sense, can be used to modify Walker behavior.
7) Walkers may need to eat living to tissue for mobility, but not neural activity
** A 5’10’’ man at 170lb who spends his day primarily walking has a daily caloric cost (DCC) of 2400cal. If he is a Walker, than he doesn’t need to fuel his vital systems, so his DCC may be closer to 800cal. Without food, he’ll need to digest his own body for fuel. As a standard male, he has 71 pounds of muscle (42%) and 36 pounds of fat (21%) to burn, which equates to 42600 cal of muscle and 129600 cal of fat – a total of 172200 calories. Not accounting for changes in weight, he may have some mobility for up to 215.25 days without eating. **
Whether or not Walker’s need to eat living tissue for sustenance is debatable. In the comics, Eugene observes a starving Walker who is still animate, but no longer mobile. A similar situation appears in episode 4 of the game, when a hungry teen Walker is found to be too weak to crawl. In these cases, it seems that Walkers do rely on eating for energy in a nontraditional sense; they do not digest food, as their intestines are either entirely necrotic or absent (also, I don’t want to live in a world that has zombie poop), but something about the act of eating exclusively living tissue keeps them mobile. Working on this assumption, how long will it take for a Walker to starve? One estimate based on a standard metabolic rate is around 200 days (see italics above), but here’s the deal: although their bodies may starve, their brains do not seem to require ANY known form of energy (e.g., food, light) to operate, which brings us to our next fact.
8) Walker brains don’t need blood, oxygen, or a body
In addition to a known form of energy (as mentioned above), Walker brains do not seem to need blood, oxygen or a body. How do we know this? Consider the following:
- (No oxygen) In episode 408, a Walker crawls out of deep mud to attack a girl. This walker was there for days without oxygen and remained functionally in tact.
- (No body) In episode 303, we see the Governor’s “wall of heads”, where we see severed Walker heads in tanks of water, still moving their jaws and facial muscles.
- (No blood) In episode 203, we see an active Walker hanging by its neck – the brain cut-off from whatever blood circulation there may be.
9) Walker brains are virtually immune to decay, hydrocephalus, and heat.
Let’s consider traditional decay of a human body. Within the first few days of decomposition, the body becomes discolored and is wrecked by gas production, causing it to look green, blistered and bloated. From there, the gas causes the cavity to burst and is proceeded by tissue liquefaction, starting with the least muscular organs (e.g., eyes). After a month, the body is basically thick liquid over a pile of bones. This is not the case for Walkers. Thanks to the Walker pathogen, after a few days of regular decomposition, their bodies show a supernatural resistance to decay. Statistically, most Walkers are older than 1 year (daaww), with the most aged ones only appearing to have severe decay in the softest tissue (e.g., eyes). This suggests that their bodies decay at a rate >6000% slower than a traditional corpse. Even more impressive than that, it seems their brains are virtually immune to decay, as even skeletal Walkers exhibit identical neuro-functioning as fresh-from-the-oven Walkers.
Furthermore, their brains demonstrate great resistance to some forms of trauma like heat and hydrocephalus (too much fluid on the brain), as demonstrated by the following:
- (Hydrocephalus) In episode 204, a super bloated Walker is pulled from the bottom of a well — its body useless from having absorbed so much water – but is still moving and trying to attack, even with its big, water-soaked head.
- (Heat) In episode 501, a few Walkers are burnt to a crisp after being set aflame – their brains having experienced roughly 450°C heat (over 10 times the heat a human brain can withstand), but are still writhing on the ground and moving their heads.
That being said, what is more plausible regarding Walkers: (1) that they’re the result of a biological weapon created to kill people, OR (2) that they’re the by-product of a pursuit of immortality?
10) Walkers have a superhuman sensory profile similar to a rat
As mentioned above, the Walker brain is virtually immune to decay; however, sensory interpretation still relies on the quality of the body’s hardware (i.e., eyes, ears, olfactory bulb). Even with great resistance to body decomposition, most Walkers have notably decaying eyes. As their eyes decay, their vision starts to lost sharpness, then contrast, then perception of light and movement. Ultimately, they will become blind to some degree, like a rat.
But also like a rat, they seem to make up for this vision-loss with an insanely good sense of smell. How do we know this? Because they can determine the difference between humans and other Walkers by smell alone — from dozens of yards away!
When Rick and the Super Friends cover themselves in dead entrails, they are essentially undetectable to Walkers. Granted, if they go around saying “Mom! Mom!” they’re screwed, but otherwise they’re in the clear! This is because Walkers rely more on smell than sight or hearing. When they are standing in a group, they do not pursue one another even if they see or hear each other. It is only when they smell humans that they attack, meaning they can differentiate between living and dead tissue from, say, the length of a prison courtyard – a feat impossible to humans, but not for a rat. This suggests that the Walker pathogen either generates millions of olfactory receptors within a new, secondary odor-detecting organ (as the nose is usually gone; rats also have a secondary organ) and/or amplifies the power of the olfactory bulb (i.e., the neural structure in charge of smell).
In other words, Walkers can smell as far (if not farther) than they can see.