Lindsay Gray
Nov 9 · 7 min read

Describe your favorite smell. Try not to describe it in terms of other smells, in terms of objects or situations. You might find this, as I found it, extremely difficult to do. The sense of smell is our oldest sense, our most indefinable, arguably the one that relies most heavily on an implicit understanding of a more or less objective consensus — a rose smells like a rose, and we agree that it’s pleasant. A dead rat or a garbage truck, not so much. How heavily we rely on a specific sense to build up a map of the world around us informs the character of its topography and the kinds of landmarks we need to place in it, and so our descriptive capabilities may vary dramatically depending on how and where we live. The Jahai hunter-gatherers, for example, have a precise linguistic shorthand for various important categories of smells, discrimination among which is critical for survival. A recent study asked two groups of Jahai and Dutch participants to describe and categorize an array of chemical odors. The Jahai participants all responded quickly, in succinct, abstract terms that were used in a remarkably consistent way across all Jahai speakers. The Dutch speakers’ responses were much longer and more variable, tending toward detailed contextual descriptions that referred to scenarios associated with these odors, or to other things they smelled like. Interestingly, the study also suggested that while the Jahai participants existed in a more coordinated and complexly articulated olfactory landscape, immediate emotional reactions to these odors were the same across groups and reactions were stereotyped and uncomplicated: pleasant smells elicited a slight smile, unpleasant smells elicited a grimace.

It’s easy to imagine how essential our other senses are to the shape of our cultures. We leave museums or concert halls with experiences that stay with us; we buy prints or records to revisit them, and we discuss our opinions of those experiences with our friends. It’s such second nature to articulate these things verbally that we can make careers of writing about them. A museum of smells doesn’t really exist in the galvanized institutional way that other museums do (although this otherworldly synesthetic gallery and fragrance shop perched on the outer edges of the Arctic Circle is probably the world’s best approximation). Still, we’re defined as individuals by the subtlest variations in our perceptions. And while our sense of smell might be our hardest to articulate, it’s our most deeply ingrained sense, and we’re beholden to it in ways we’re barely conscious of.

In October of 2017, I was at an academic conference when an unprecedentedly devastating wildfire flattened most of where I spent a large portion of my young adulthood and displaced a lot of people I loved. I remember that I gave a talk that evening but I can’t account for its contents. I remember standing in the lecture hall, and then a short time later standing at a ticketing counter at LAX, saying a lot of not-very-coherent things in the general direction of someone who eventually found me a seat on an earlier flight home. Most of all, though, I remember the way the air felt when I got off the bus in Santa Rosa. A smell that in the immediacy of recollection I would describe now as:

Orange.

Stringent.

Heavy.

Chemical.

Organic, like dry wood.

Soft, like paper, like loose charcoal.

I could spend every day for a year enumerating the individual volatile aromatics that comprised the air for those couple of weeks because each of them returns vividly, and in the moment slowed time to an appropriate pace, rather than the jarring fragments wrought by a strong desire but total inability not only to do anything useful, but to do anything.

We couldn’t fathom at the time that this would become a grim tradition; every October, a local news station would release an ominous but hopefully innocuous video of a stand of trees exploding orange and electric against a clear pale sky and then hours later, more news coverage of blackened acres, the same sky now oddly linen-colored in a way that would feel nostalgic in another context. So today, a little over two years on, it started again and I drove back home to see my family and the smell rolled over me immediately in a nauseating wave. I think, we must think about smell the way we think about pain: hard to grasp in memory but easy and excruciating when we’re in it, or when we’re dropped back into it without warning.

The basis of this unique ability of certain tastes or smells to pull us out of time is their privileged access to the deep structures and circuits in our brains that govern emotion, fear, and memory. We take in overwhelming amounts of sensory data that need to be sorted and passed along through successively finer filters before we can decide where we are, what we see, and how we’re supposed to feel about it. Ordinarily, any incoming sensory information first needs to check in with the thalamus, which then decides how far up the cortical chain of command to send it. If this information signals imminent danger, it gets shuttled to the amygdala, a tiny almond-shaped alarm bell deep in your brain that engages your autonomic nervous system to quickly mobilize you to safety. Right next door is your hippocampus, waiting to take notes on all of the circumstances surrounding this situation to keep you from finding yourself in it again.

Smells, though, sneak past your thalamus and talk more or less directly to your amygdala and, by extension, your hippocampus. So while I may not be able to verbally recount any number of days I spent at my grandmother’s house as a very small child, sometimes I’ll catch the scent of her perfume or fabric softener and vivid flashbulb memories come back, accompanied by an instant sense of calm. Of course, this works the same way for unpleasant memories, especially traumatic ones. Classical Pavlovian fear conditioning experiments in mice have shown that odors associated with stressful events physically restructure sensory circuitry from the nose up, increasing the number of receptors tuned to respond to that specific odor in the olfactory epithelium, the soft bed of tissue in your nasal cavity that serves as the first checkpoint between your nose and your brain. More receptors means greater sensitivity to this smell in the future — and a strong emotional link between it and the stressor it was paired with.

Anyone who has lived in Northern California during wildfire season is probably painfully tethered to the smell of a particular kind of smoke. For me, seeing pictures or videos on social media or in the news stirred a lot of complicated and deeply buried feelings, but nothing so acutely summoned the immobilizing panic I felt during the 2017 fires as the smell. This makes sense, neurologically, based on what we’ve learned from mice. And it’s common for sufferers of PTSD to identify odors as the most potent emotional triggers in studies that examine the ability of sensory cues to retrieve traumatic memories.

It turns out, though, that the same kind of remodeling that our olfactory neurons use to put us on alert for smells that signal danger also works in the opposite direction. In fact, our olfactory system is unique in that the birth and dynamic rearrangement of new neurons persists into adulthood; the only other place where we see this consistently is in the hippocampus. So, mice who are trained to associate a certain odor with something unpleasant happening to them will continue to behave fearfully when presented with that odor for several weeks following conditioning, even in the absence of the stimulus. However, if the mice are presented with the odor multiple times after conditioning but without the accompanying unpleasant stimulus, their fearful behavior will gradually subside. The structural changes to their olfactory neurons, which helped them learn this negative association in the first place, are also reversible, adaptively remodeling themselves when it’s clear that the odor no longer comes with harmful consequences.

This is a remarkable phenomenon, and provides a neurobiological basis for the concept of exposure therapy, which may help people manage extreme phobias or anxiety by allowing them to experience sensory cues or physical sensations they associate with feared situations in a context that’s safe. Eventually, like our stressed-out mice, your brain can help de-escalate the dizzying fight-or-flight response that these powerful sensory informants recruit automatically. At one time, maybe, these responses helped us avoid catastrophe; long after we don’t need them anymore, our brains will continue to hold on to them until we tell them it’s OK to let them go. This is the elegant thing about neuroplasticity: we might think of these reflexes as hard-wired and evolutionarily ancient, but in reality our brains are spectacularly adaptive. We rely on our senses to reassure us that we’re safe, so it makes sense that our emotional reactions to sounds or smells that we experienced when we weren’t are strong and instantaneous. In time, though, and by paying close enough attention to those seemingly beyond-our-control reflexes, we can push back on the kind of panic that makes our hearts race when the wind brings even a suggestion of burning leaves or ash. Inevitably, the smoke will clear for long enough that we can teach ourselves new ways to breathe.

Lindsay Gray
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