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Do Zombies Dream of Undead Sheep?
A Neuroscientific View of the Zombie Brain
By Timothy Verstynen, Bradley Voytek PRINCETON UNIVERSITY PRESS
Copyright © 2014 Princeton University Press
All rights reserved.
ISBN: 978-1-4008-5192-8
CHAPTER 1
GRAY'S (UNDEAD) ANATOMY
With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health.
—Charles Darwin, The Descent of Man
You are about to read a book about the zombie brain. Just think about that for a minute. Let the thought really soak in. Reflect on the decisions you've made in your life that led you to this point.
Now let's get a bit meta for a moment and think about all of that thinking and reflecting you just did. First, you read some words that we wrote via a semi-creative process. You understood those words and they changed your behavior. You reflected on your life by some internal memory recollection process. Perhaps you even thought about what decisions led us to the point of writing this book in the first place.
This amalgamation of thoughts, memories, and emotions that you just experienced, and will keep experiencing while reading this book, are all the result of a never-ending symphony of electrochemical processes in your brain. Each step of thinking that you just performed, from seeing the printed letters on the page to following the linguistic requests that we asked of you by pulling up the memories of the past, is performed by little networks of neurons distributed throughout that gray matter sandwiched inside your skull.
As neuroscientists, the fact that we can do all of that "thinking" is completely amazing. But what if you couldn't do any of that? Or what if you could do some of those things, but could feel no emotions about them? Or what if you could feel emotion, but had no memory?
The study of neuroscience isn't just about tissues and neurons and signals; it also has strong philosophical, computational, and psychological roots. It is a very difficult, sometimes wonderful, but often frustrating, problem.
Which is how we got to this point. As we said in the introduction, this is a book by a couple of scientists who also happen to be zombie movie nerds.
Our goal for this little thought experiment is to understand what has happened to the walking dead that has changed them from normal human beings to so-called "mindless walking corpses." To do this we need to understand how the brain gives rise to behavior, in both humans and zombies. Which means we first have to understand exactly what the brain is.
But before we can get knee-deep in zombie gray matter, let us take a step back and look at the little three-pound piece of tissue sandwiched between your ears.
NEUROSCIENCE WITHOUT BRAIN SCANNERS
In this chapter and those that follow, we will attempt to link features of zombie behaviors to the various parts of the brain by adopting a classical forensic neurology approach.
What do we mean by this?
Classic neurology was the original scientific method for studying the brain before we had big machines to take pictures inside the living skull. Neurology is mainly focused on understanding why certain things go wrong in the brain to cause a patient's symptoms, but along the way it has learned a lot about how the healthy brain works too. When neurology began in the mid-1800s, doctors had to deduce how the brain works by simply observing the behaviors of people and animals. This is a delicate art that involves making deductions about the brain by carefully detailing your subject's behavior. But it didn't just start with the advent of neurology in the nineteenth century. In fact, this form of investigation has been going on for centuries.
Indeed, while we tend to think of neuroscience (the empirical study of the healthy brain, as opposed to neurology, which is the medical branch dealing with brain disorders) as a "modern" scientific endeavor, some of the first experimental research linking the brain and nerves to behavior came from experiments and demonstrations by the Roman physician Claudius Galen, sometime between 150 and 190 CE.
Keep in mind that we're talking about a time nearly 2000 years before brain imaging, well before Dr. House could just send his patients to an MRI to see how healthy their brains were. Back then, physicians and scientists had to do a lot with very little information. They had to get creative. This meant that they tried a lot of things; some worked and some didn't. But sometimes they learned something new that would add just a bit more to what little was known about the brain.
For example, in a famous experiment on a living pig, Galen was trying to trace out the nerves involved in breath control when he accidentally cut the recurrent laryngeal nerve, which controls the muscles of the larynx (aka the vocal cords). The live pig immediately stopped squealing, but was still moving and breathing. Thus, like many great scientific discoveries, he found out how vocal cords are controlled, purely by accident.
Galen was also the doctor to the Roman gladiators, a group of folks that were highly susceptible to injury. In the process of treating these often brutally injured men, he observed how cuts to the spinal cord affected behavior, notably causing paralysis below the level of the cut. He continued this work by experimenting on animals and noticed that cutting the spinal cord very high up, in the brainstem, would kill the animal. This observation gave us the first glimpse into how our limbs are controlled by different outputs along the spine.
Unfortunately, after Galen there was a long hiatus in the development of our knowledge of the brain, until the Enlightenment brought a resurgence in the idea of the scientific method. In the early 1800s, Marie Jean Pierre Flourens conducted experiments similar to those done by Galen, but mainly on rabbits and pigeons. He removed different parts of their brains and observed their behaviors in order to understand how different brain areas related to behavior. He found that depending on the specific region that was removed, the animals lost their ability to coordinate their muscles, or control their breathing, or perform certain cognitive functions. These results provided early, but valuable, insights into how the brain keeps us alive.
From the Industrial Revolution until the adoption of the first brain imaging technologies by the medical community in the 1940s and '50s, these classical observations represented that main body of the neurological literature, and was all that doctors had to go on.
Now imagine the year is 1916 and you're a military doctor. You have a soldier who has just survived an explosion resulting in a sharp blow to the head. The victim was knocked out for a while, but recovered—except now that he is awake, the soldier has some trouble writing and using a fork to eat.
How do you diagnose this behavior? Remember, you don't have brain imaging tools. You can't just take a picture of your patient's brain and say, "I'm sorry, but it looks like your cerebellum is damaged, and that's why you're having trouble writing, but here's what we can do."
To do your work you've got to rely on previous research, mostly on animals like Flourens's rabbits and pigeons, to inform your diagnosis. Therefore, if you want to understand what area of a soldier's brain might be damaged to cause him to no longer know how to use everyday objects like a toothbrush, you have to combine a keen investigative wit with an extensive knowledge of the previous neurological literature, all with much less technology than what we have today. We are very much in the same boat when it comes to understanding what has happened to zombie brains. Since we can't get our hands on a real-life zombie to throw into an MRI scanner, we'll have to resort to this classic method of diagnosis by observation. Our first step on this journey to diagnosing the zombie brain is to provide a basic roadmap of the brain and its different parts. This will become useful when we try and break down what's gone wrong in zombie brains.
A VAST BIOLOGICAL COMMUNICATION NETWORK
The brain is the organ that drives all voluntary behavior. It is what gets you out of bed in the morning. It is what allows for you to see a sunset, to smell a rose, to taste chocolate, to kick a soccer ball, and to swing a battle-axe at the head of an oncoming zombie.
Essentially the brain is nothing more than a collection of billions of tiny cells, called neurons and glia. Neurons act like little input-output operators, sort of like the transistors in computers, but a little more complicated. They have little branches at the top, called dendrites, that allow them to listen to other cells. The information from these branches then travels through the main part of the cell, called the cell body or soma. This is what gives gray matter, the part of the brain that contains your neurons, its name. The dense cell bodies make it look a little darker than tissue without cell bodies. The information from the dendrites is integrated in the cell body and a decision to "fire" is made. It doesn't really fire, but it does start an electrochemical signal that is transmitted away from the cell through a long tendril called the axon. The axon is sometimes called white matter because it looks, well, white. Basically, axons can be considered the biological wires of the computer that is our brain. At its end, each axon contains many little offshoot arms, called axon terminals, that connect with the dendrites of other cells. If the dendrites are the branches of a tree, then the axon is the trunk and the axon terminals are the roots.
Each neuron communicates with other neurons by building up an electrical charge that causes a cell's axon to shoot chemicals across the small gap between itself and a downstream cell's dendrites. This gap is called the synaptic cleft. These chemicals (known as neurotransmitters and neuromodulators) change the voltage of the downstream cell, making it more or less likely to fire its own action potential. This transmission process is the fundamental computation of the brain: one cell decides to fire (or not) based on the signals that the cells connected to it send (or don't). We'll discuss this a bit more in the next chapter.
But what about those other cells that we mentioned, the glia? Well, for a long time most neuroscientists thought that they were sort of like the support staff for neurons. They clean up the messes that happen when neurons shoot those neurotransmitters all over the place. They also help keep neurons healthy and foster communication between cells. While this support staff model of glia seems accurate as far as it goes, it is becoming increasingly apparent that glia are so much more than that. Each year, more studies come out showing that glia are also doing a little bit of computing on their own. However, what this computing is and how it relates to behavior is still a big mystery.
But how does all of this make the brain work?
We've known for some time that the brain is a massive interconnected network. Of course, early estimates of how massive this network is were a bit overstated. Take, for example, the headline of an article that ran in the New York Times on June 25, 1933, "Brain Phone Lines Counted as 1 Plus 15 Million Zeros: Scientists Told of Figures So Stupendous That Astronomy Fades in Comparison." Assuming what we know about the size of neurons and their axons, this would require that your brain take up an area slightly larger than the solar system. But while this number was just a little bit inflated, there are in fact a lot of neurons: somewhere between 80 and 100 billion cells with anywhere from a hundred to tens of thousands of connections from each. So basically, the brain functions as a massively connected computer network, one with trillions (with a "t") of connected parts.
To put this into perspective, based on reports by the computer networking company Cisco, as of 2013 there were about 10 billion active connections on the entire internet. The entire internet will not even reach 50 billion connections until the year 2020. This means that your brain is nearly 100,000 times more densely connected than the entire internet is right now.
However, if you take a step back and look at a brain without a microscope, the first thing you notice is that it looks very wrinkly. The tissue folds over itself like the face of a Shar-Pei dog. That's because there's barely enough room in our skulls to fit all of those cells. So the tissue gets squished in there as compactly as possible. The mountains of the folds are called gyri (or gyrus if you're talking about just one) and the valleys are called sulci (or sulcus for just one). As neuroscientists, it is our job to navigate these mountains and valleys to understand which mountains allow us to see faces, which valleys allow us to move our arms, and what the neural code is that allows for communication across those gyri and sulci.
A ROADMAP OF THE BRAIN
In this book we'll mostly be focusing on these mountains and valleys of the brain, as well as all the curiously complex collections of neurons (called nuclei) buried deep inside. At first glance it might seem that the brain is just a random set of wrinkles and folds, but in fact it is quite consistently organized. Let's take a look at the various parts that make up the human brain.
THE REPTILE BRAIN
Our tour of the human brain starts in an area already linked to zombism. In the novel The Zombie Autopsies (2012), psychiatrist Steven Schlozman presents a case that the walking dead have had their brains destroyed in such a way that only the so-called "crocodile" or "reptile" brain is left functioning.
What is this crocodile brain and how is it different from other parts of the brain?
Neuroscientist Paul MacLean originally formalized the idea of a primitive "reptilian" brain that resides in each of us. This idea was then popularized by Carl Sagan, who borrowed heavily from the concept for his book The Dragons of Eden. MacLean's conceptual framework for the brain is referred to as the "triune brain model" because it is composed of three separate complexes (while the names aren't important, for the sake of completeness they're referred to as the reptilian, the paleomammalian, and the neomammalian complexes). These roughly map onto known anatomical distinctions that are still sort of used.
So far so good?
Unfortunately MacLean's hypothesis also conceives of these three complexes as representing different evolutionary stages (they don't) and as relatively independent of one another, allowing for distinct "consciousnesses" (they probably don't). What this means is that each animal is supposed to have a different type of consciousness, depending on its own evolutionary stage of development. While this idea is certainly an interesting one, modern neuroscientific evidence doesn't really support the argument that we normally have separate competing consciousnesses, given the heavy amount of communication that goes on between most brain regions.
All that preamble is just a way to say that we neuroscientists don't really like this term "reptile brain" because it gives the wrong impression of how the brain and evolution work together. Nevertheless, the name "reptile brain" has stuck, and we'll sarcastically use the term for shorthand. It's important to note that a reptile's brain is no less "evolved" than a human's brain. Why? Well, because all current species on Earth have been evolving for the same amount of time. Crocodiles and other reptiles or "less intelligent" animals have just evolved to meet different evolutionary pressures. Crocodiles don't need to be smart enough to build bridges or send witty Facebook status updates, because those things aren't necessary to eat a water buffalo or propagate the species.
Now, the reptile brain is made up of several large clusters of cells called nuclei. The most recognizable part of this circuit is a nucleus called the amygdala. The amygdala are almond-sized areas roughly behind the temple on each side of your skull. Far from being limited to any one function, the amygdala is associated with many different behaviors linked to basic survival, including "fight-or-flight" behaviors and emotional regulation. Buried deeper in the brain is another area called the hypothalamus. This little cluster of nuclei regulates things like hunger, sleep, and stress. The hypothalamus gets its name because it literally sits under another area called the thalamus. The thalamus is the main switchboard of the brain and talks to nearly every area in the neocortex (an area we'll talk about below) and many other regions that are found below the neocortex (called subcortical areas). Finally, the last critical part of the reptile brain consists of the nuclei of the basal ganglia. The basal ganglia are a set of different nuclei all wired together to form little computational loops with the neocortex. We'll discuss this more in chapter 4.
(Continues...)
Excerpted from Do Zombies Dream of Undead Sheep? by Timothy Verstynen, Bradley Voytek. Copyright © 2014 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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