What our sense of smell can tell us about language, memory, and how we handle trauma

Describe  your favourite 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 odours. 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  odours, 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 odours 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-coloured 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, a structure whose major function is to relay external sensory  information to the cortex, where more complex processing takes place. 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, an important structure in learning  and memory, 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 too, especially traumatic ones. Classical Pavlovian fear conditioning experiments in mice have shown that odours associated with stressful events physically  restructure sensory circuitry from the nose up, increasing the number of  receptors tuned to respond to that specific odour 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 odours 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 remodelling 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 odour with something  unpleasant happening to them will continue to behave fearfully when  presented with that odour for several weeks following conditioning, even  in the absence of the stimulus. However, if the mice are presented with  the odour multiple times after conditioning but without the  accompanying unpleasant stimulus, their fearful behaviour 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 remodelling themselves when it’s clear that the odour 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  phenomenon known as neuroplasticity, the tendency of our neurons to  strengthen or weaken connections between each other in response to  patterns in our daily experiences.

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.