Timothy Verstynen and Bradley Voytek take on the neuroscience of the zombie brain in Do Zombies Dream of Undead Sheep? (Princeton University Press, 2014) with witty analyses of the characteristic lumbering gait, insatiable hunger for brains and memory-less existence of the undead. Through this exploration they shed light on fundamental neuroscientific questions while bringing in examples from zombie popular culture. The following excerpt from Chapter 6, “There’s No Crying in the Zombie Apocalypse!” deals with how olfaction works in the healthy brain—and how it applies to evading zombies.
How do you smell? Not, like, how stinky are you right now, but how does your sense of smell work and why does that connect so strongly with certain emotions? The smell of your grandmother’s house when she’s baking cookies; the smell of the perfume or cologne of the person you love; the scent of rot and decay of the undead beasts hunting you down: why do these smells move you?
First, we must understand how olfaction, our sense of smell, works. Our senses include vision, touch, hearing, balance, taste, and smell, all of which somehow combine into a unified percept that informs us about ourselves and the world around us; but of these, only taste (gustation) and smell (olfaction) require us to directly, chemically sample our world. This can be a dangerous bargain given the number of toxic and noxious chemicals out there.
Ultimately we only become aware of a sensation when it enters our explicit consciousness. From a neuroscientific point of view this translates into a stimulus activating a set of neurons in the nose that in turn send signals to an early sensory area called the olfactory bulb that in turn send signals to the more cognitive parts of the neocortex where they are registered into awareness. But often there are many steps that allow for processing of these sensory signals even before they reach the neocortex. This fact means that, in rare cases of brain damage, someone can react to stimuli without even being aware of them. The classic example is blindsight, where people who are technically blind can respond to certain visual inputs without being aware of doing so. Individuals with blindsight will swear they don’t see objects in the room, but when asked to walk around can somehow avoid obstacles on the floor that would otherwise make them trip.
This ability to use sensory information (without necessarily being consciously aware of it) happens because every sense, except for one, passes first through a neural gatekeeper just before entering our neocortex. This neural gatekeeper is the thalamus, which sits atop our brainstem and helps modulate the sensory input before the input enters our awareness. The one outlier sense that doesn’t pass through the thalamus is smell. Instead, olfactory input goes directly into our neocortex, particularly to the cortical areas that process both emotion and memory. This fact is thought to underlie the strong link between smells and memories, which they can trigger even after many years have passed—for example, when the smell of freshly baked cookies reminds you of your grandmother, or when the rancid smell of rotting flesh reminds you of your first run-in with an undead walker.
While we may have a direct link between the sense of smell and the more cognitive areas of the brain, it doesn’t mean that we in fact use olfaction as a primary sense when interacting with the world. That privilege appears to be restricted to our visual and auditory senses. In fact, humans are often thought to have a relatively terrible sense of smell, especially compared to our canine friends. This is why we’ve spent millennia training and domesticating dogs to help us hunt; us humans seem to have a terrible time finding a buffalo based on its smell!
But are we really that bad at finding things with our noses? In a famous experiment published in the journal Nature Neuroscience in 2007, researchers in Berkeley, California found something quite surprising: people who are forced to rely only on their sense of smell (by being blindfolded, earplugged, and having mittens over their hands) were able to track weak scent trails sprayed along a grass lawn in much the same way as a bloodhound. As the lead author of that study, Jess Porter, said in an interview about the study (Sanders 2006), “Our sense of smell is less keen partly because we put less demand on it…. But if people practice sniffing smells, they can get really good at it.” The results of this study don’t mean that we are all really bloodhounds at heart (our canine friends are still much better at this than us), but perhaps we’re not so bad at olfactory localization as is popularly believed.
Regardless, even if you or I can’t easily pick out a person in a crowd just by sniffing around, it may be because we rely so heavily on our other senses first. We don’t need to sniff anyone out when we can just look for them. But clearly the walking dead don’t operate like this. The zombies of the TV series The Walking Dead apparently tell the living from the dead based not only on how they look, but also on how they smell.
As wild as this might seem, it is not really that unusual to do…even for humans.
Sure, dogs and other animals may smell each other all the time, whereas most of us don’t go walking around sniffing each other’s butts to tell friends from strangers. But think how many times you’ve been to a new place, or visited a friend’s house, and thought to yourself, “This doesn’t smell like home.” We may not realize it, but our sense of smell plays a major role in our comfort and familiarity, and The Walking Dead scene with Rick and Glenn covering themselves in rotting flesh demonstrated this in one of the most brutal ways possible.
While the link between sociality and olfaction in humans is disputed and still unresolved, there is a clearer link in other animals. In most mammals there is a little cavity inside the nostril where there are the receptors of what is known as the vomeronasal organ. This tiny set of molecule-sensing receptors is highly responsive to pheromones, the chemical messengers used by plants or animals to communicate or alter behavior. Without going off on too much of a tangent, pheromones are used all over the animal and plant kingdom, from modulating the endocrine system in rats during mating, to being used by ants to mark a scent trail.
There are many types of pheromones that can affect sexual selection and mating, increase aggression, or even modify social interactions in animals. The extent to which humans are affected by pheromones remains uncertain; however, there are several lines of research that show that our trust and sociability can be manipulated using certain neuropeptides (small, protein-like molecules that modify neural activity). For example, research has shown social effects in rats of the neuropeptide vasopressin. If vasopressin is blocked from reaching the rats’ olfactory receptors, they exhibit an impaired ability to recognize each other, meaning they become socially blind to knowing who is who in the rest of the colony. Imagine if blocking just one chemical made it impossible for you to tell your mother from a stranger).
We humans may also be susceptible to the influences of neuropeptides on our social behaviors. A lot of popular press these days has been focused on the chemical oxytocin, a hormone that is released in the brain during childbirth. (In fact, oxytocin—from the Greek for “quick birth”—can be given to mothers during childbirth to speed the process along.) While the role of oxytocin in social behaviors is still somewhat controversial, there is plenty of tantalizing evidence that intranasal administration of an oxytocin spray can increase trust, which is considered a “prosocial” behavior. But wait, there’s more! While oxytocin does seem to lead to increased bonding and social behaviors within your social group, it also increases aggression toward those outside of your social group. So the next time someone calls oxytocin the “love hormone” (as it is often annoyingly referred in the media), you can smack them and say that oxytocin made you do it.
Reprinted with permission from Do Zombies Dream of Undead Sheep? A Neuroscientific View of the Zombie Brain by Timothy Verstynen and Bradley Voytek and published by Princeton University Press, 2014.