Saturday, November 01, 2014

Why Do Zombies Lumber?

In the spirit of the Halloween weekend, here is a cool article from Slate on why zombies lumber when they walk. The article is an excerpt from the new book Timothy Verstynen and Bradley Voytek, Do Zombies Dream of Undead Sheep?: A Neuroscientific View of the Zombie Brain (2014). Not only is it a good explainer for zombie walking patterns, but it also introduces some basic neuroscience.

Why Do Zombies Lumber?

Two neuroscientists explain why zombies have so much trouble walking.

By Timothy Verstynen and Bradley Voytek
October 30, 2014 

Drawing by Anne Karetnikov

Excerpted from Do Zombies Dream of Undead Sheep?: A Neuroscientific View of the Zombie Brain by Timothy Verstynen and Bradley Voytek. Out now from Princeton University Press.

In the movie Dawn of the Dead (1978) there is a scene when anarchist outlaws break into the mall that the movie’s heroes had secured and lived in for weeks. This invasion subsequently allows the horde of zombies—which had been aggregating outside—free range of the place. The humans are zipping around playing games while the zombies lumber along slowly and clumsily. The humans easily dispatch the threat of individual zombies because the undead are just too darn slow; there’s no real threat until the humans are outnumbered.

The slow and uncoordinated movements of zombies are perhaps the most identifiable feature of their behavior (next to the whole biting and flesh-eating thing of course). Ask anyone to impersonate a zombie and the first thing she’ll do is hold her arms out, widen her stance, stiffen her legs, and utter a low, guttural moan. That’s because, in the movies, as soon as zombies rise from the dead, they begin walking. Well not walking ... more like lumbering. Each step is slow and arduous. Their stance is wide and stiff. This presents us with a very important clue about what’s happened to their brains.

So what does it take to turn the normally smooth, fast, coordinated movements of a healthy person into the traditional zombie lumber? First, let’s consider the pathways in the brain that give rise to our movements.

While “higher” cognitive functions (aka thinking) tend to get all the glory in neuroscience, before the brain did a lot of deep thinking it did a lot of moving. In fact, some scientists have argued that the entire reason we have a brain is to get us moving around in the environment.

The logic for this argument arises from observations of a little ocean creature called the sea squirt. Seriously, that’s its name. The sea squirt is a small and evolutionarily old animal of the phylum Chordata (when scientists say “evolutionarily old,” we mean that the life form has been in a relatively unchanged state for millions and millions of years). In its young life, the sea squirt is a little larval creature that has a very primitive brain and sensory organs. Its goal during its larval stage of development is to swim around and find a rock to perch on. Once it has found a suitable home, like, say, a nice secure rock with plenty of organic food just flowing by, the sea squirt will attach itself with its head facing out. Then it basically just sits there catching food as it floats by. As it matures into a full-grown adult creature the sea squirt does something quite strange: It digests its own brain.

Yeah, you read that right. Let that sink in. It digests its own brain.

Biologists and neuroscientists have argued that this is evolutionarily advantageous. See, the brain is really expensive, from a metabolic standpoint. Meaning that it takes a lot of energy to keep the brain going, and energy (food) is pretty hard to come by when you’re little more than a stick on a rock with a mouth attached. So when you no longer need a metabolically expensive organ like the brain, it is better to just get rid of it. Thus, no longer needing to navigate around its environment, the sea squirt simply has lost the need for its brain and does away with it. But waste not, want not, in nature. So “doing away with it” means “eating it.” And thus the sea squirt digests its own brain.

Now luckily for us, we humans are more than mouth-sticks attached to rocks. We need to keep moving. We can’t just sit around and digest away our own brain, because food doesn’t just come to us. No, we still have to go out and get our food, even if it is just by driving to the local fast-food chain down the block. This means we get to keep our brains because, for the brain, movement is life.

Unfortunately, the same is true for zombies. Because humans rarely run to zombies, the walking dead have to go to their food source. Which means the zombies also still need their brains. Well, at least part of their brains.

If we presume that the primary function of the brain is to get us moving around in the world, then it’s not surprising that a lot of neural real estate is devoted to the planning and execution of actions. In fact, the computations required to simply move around our environment are distributed across vast swaths of both cortical and subcortical areas. So let’s take a walk through the multitude of brain systems that move us around, shall we?

Most of our voluntary movements start in the neocortex, in two of the four major lobes: the frontal and parietal lobes. Neurons in the parietal lobe that primarily maintain spatial awareness, and those in the frontal lobe that control decision making, are constantly negotiating with one another as to what action we should do next. We might imagine the dialogue going something like this:
  • PARIETAL LOBE: “Hey, there’s a tasty piece of broccoli 30 degrees to the left.”
  • FRONTAL LOBE: “Broccoli??? No way! I want something more awesome!”
  • PARIETAL LOBE: [sigh] “OK, how about that doughnut 10 degrees to the right?”
  • FRONTAL LOBE: “Now you’re talking. Hey! Right arm! Attention, right arm! Prepare the triceps, deltoids, and hand muscles for action. We’re going to make a reach.”
  • MOTOR CORTEX: “Jawohl, Lord Frontal Cortex!”
In our silly little sketch here, the parietal lobe tells us where to attend to things that are in the environment while the frontal cortex in the front of the head decides what to do. Then the motor areas, in the back part of the frontal cortex, make the movement happen.

Contrary to what you may have heard, there’s not just a single motor cortex. In fact, there are several “motor” areas that are spread out across the frontal lobe and provide the groundwork for planning your movements. You can think of these as the middle management of motor planning. They take the decisions handed down from frontal areas and turn them into plans that the heavy lifters in the arms, legs, and other muscles know what to do with. Which isn’t as easy as it sounds.

Let’s consider the following scenario: You’re a zombie sitting very patiently on the examination table, hand resting on your desiccated, disgusting lap. The nerdy scientists in their awkward lab coats then place a tasty chunk of human flesh right in front of you. What remains of your undead frontal lobes will immediately say “GO GET THAT!” because, hey, free thigh.

Before you can actually grab that tasty piece of meat, however, the motor planning areas in your undead brain, called premotor regions, have to figure out how to get your hand from your lap to the yummy flesh. Now remember, while you can see the tasty morsel, the process of getting your hand off of your lap and to the chunk of meat is pretty complicated. Somehow your brain has to convert a map of the world that’s being projected from the back of your eyeballs to a plan of muscle contractions that uses your bones as levers, much as a puppet master has to coordinate the strings of a marionette doll to make it dance—except here the puppet master is your own brain.

Let’s return our attention to that horde of walking dead outside. While zombie movements are slow, stiff, and uncoordinated, zombies do seem to be able to plan movements in the right direction. That is, when a zombie wants to lunge toward you, it mostly gets the direction right. Once it gets its hands on you, it has no problem grasping and holding on. Therefore it would appear that the cortical motor systems are all intact. So what could be wrong? The only real neural culprits left as plausible candidates for the motor dysfunction seen in zombies are the basal ganglia and the cerebellum.

Given this restriction, let’s consider what happens when the basal ganglia are malfunctioning and compare that with when something goes wrong with the cerebellum. In both cases, people have trouble walking and coordinating their movements, but in dramatically different ways. For example, in Parkinson’s disease, people develop a slouched posture and walk by taking short, shuffling steps. They also have difficulty generating actions without a very obvious goal (they tend to freeze up). In contrast, people with spinocerebellar ataxia develop a stiff, wide-legged stance and take big, lumbering steps. And unlike those afflicted with Parkinson’s disease, these patients have no problem initiating movements.

How can we use this information to diagnose a zombie’s brain? We know that the walking dead are shown in movies as having a stiff, wide-legged stance and a big, lumbering walk. They tend to move slowly (most of the time) and lack smooth, coordinated actions. Yet they don’t seem to have trouble initiating movements. In fact, zombies are almost constantly on the move, they never have problems starting a movement (say reaching for a new victim), and they don’t stall in the middle of movements. They also don’t shuffle or have curvature in their posture.

For these reasons we argue that the cluster of symptoms seen in zombies, the wide stance, lumbering walk, lack of freezing, ease in general planning and execution of actions, reflects a pattern of cerebellar degeneration. That is, cerebellar dysfunction would lead to many of the motor symptoms of the zombie infection. However, cortical motor areas and basal ganglia pathways should be relatively intact.

At about this point, the really astute zombie movie fan will ask, “What about fast zombies?” For those who haven’t seen movies like World War Z, 28 Days Later, or the 2004 remake of Dawn of the Dead, “fast zombies” don’t appear to have any motor dysfunctions. They can move quickly and don’t appear to have any coordination problems. Given the terrifyingly coordinated movements that “fast zombies” exhibit, it is our belief that their cerebellums are likely intact. Any difficulty fast zombies may have moving are likely more to do with the fact that their arms and legs are rotting than any sort of neural damage.

In fact, this difference in presentation may allow us to develop neurological classifications of different subtypes of the disorder that may give important clues to the etiology of the zombie epidemic.
  • Subtype I (slow-moving subtype): First observed variant of the disease. 

  • Subtype II (fast-moving subtype): Distinguished from Subtype I variant by healthy motor coordination.
Hey folks ... sometimes diseases mutate. Why wouldn’t zombism?

Truth be told, when we had the opportunity to ask George Romero why he made his ghouls walk the way they did in the Living Dead movies, he responded, “They’re supposed to be dead. They’re stiff. That’s how you’d walk if you were dead.” Not quite the answer that appeals to our neuroscience instincts, but a good alternative hypotheses to test in the next zombie apocalypse.

Excerpted from Do Zombies Dream of Undead Sheep?: A Neuroscientific View of the Zombie Brain by Timothy Verstynen and Bradley Voytek © 2014 by Princeton University Press. Reprinted by permission.

Timothy Verstynen is an assistant professor in the department of psychology and at the Center for the Neural Basis of Cognition at Carnegie Mellon University.

Bradley Voytek is an assistant professor of computational cognitive science and neuroscience at the University of California–San Diego.
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