Scents and sensibility.

Adam Spencer
11 minutes ago
5 min read

A French team has gone looking for the brain cells that hold our earliest smell memories. But do these memories lay dormant for decades only to burst back to life? Or are we subtly topping them up across the years?

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

("Proust's Madeleine": created by the author via midjourney)

Inhale!

We’ve all been there: a smell hits your nostrils and you’re transported back to your seventh birthday and a special pie your nan made.

In 2006 two Swedish researchers released the awesomely titled paper "Smell your way back to childhood: autobiographical odor memory", which noted that while words and photos tend to dredge up memories from our teens or twenties, smells transport us to younger days.

In Marcel Proust’s 1913 novel Swann’s Way, the narrator dips a madeleine pastry in a cup of linden tea and has a childhood memory come flooding back. That’s why psychologists call this phenomenon Proust’s Madeleine.

“But when from a long-distant past nothing subsists, after the people are dead, after the things are broken and scattered … the smell and taste of things remain poised a long time, like souls, ready to remind us” — Marcel Proust, Swann's Way, 1913, translated by C. K. Scott Moncrieff.

A new paper in PLOS Biology, released a couple of weeks ago, from Nathalie Mandairon's group in Lyon might just explain how this all happens.

You smell ... but how?

A quick recap on the science of smelling.

Up the top of your nasal cavity sits a patch of smell cells, usually quoted at around five square centimetres. Think of a postage stamp. A dog's is a more handkerchief-like 170.

These cells are unique in being your only ones directly exposed to the outside air. Also uniquely among your sensory neurons, they get replaced across your life.

These neurons send messages upwards through the cribriform plate, a sieve of bone above the nose perforated with dozens of tiny holes, to the olfactory bulb.

That is a pair of small structures sitting directly above the roof of the nose, tucked under the front of the brain. It is the closest your brain comes to the open air, and it is where this story takes place.

This is where the fun happens. See your olfactory bulb wires straight into your amygdala, the part of your brain which handles emotion and which you might have heard of. It also hits your entorhinal cortex which, like me until I wrote this NerdNews, you have NEVER heard of. Think of it as a gateway to the memory processing part of your brain, your hippocampus.

So while every other sense goes through a relay station first, smell takes a mere two steps before it reaches your brain’s memory and emotion centres.

And if you’ve laboured under the assumption that we humans suck at smelling, a 2017 survey compared the neurons in the olfactory bulbs of two dozen mammals and found humans sitting comfortably mid-pack.

And another thing to know. Every time you’ve had a blocked nose and dinner has tasted bland is proof that most of what we ‘taste’ is actually smell arriving at the back of our throats.

("Into the Ento(rhinal)": created by the author via Dall-E)

Of mice and madeleines.

Researchers asked 647 French volunteers to describe and rate their earliest smell memory.

The memories were overwhelmingly happy, and they came from repeated events: 73 per cent said the episode occurred more than five times.

The team then turned their attention to mice aged 23 to 33 days — the mouse equivalent of ‘childhood’. They entered a large cage pimped out with all the trimmings. We’re talking a wheel, tunnels, ladders, shelters and cornflakes. On each of five visits a particular smell wafted through the cage.

This was a smell that previous research had shown mice enjoy. Turns out mice are bandits for citrus, rose and camphor!

Now as the mice navigated the funhouse, ultrasonic recording devices picked up the high-pitched squealing researchers think indicates mice are having a good time. Importantly, more of that squealing than mice in boring old plain cages.

In theory this means the enjoyable smell of, say, citrus had been associated with a pleasant experience, running the funhouse.

Then, one month after their whiffy cage run, those mice still sniffed that particular smell long after they should have become bored with it.

Mice that got the smell in a boring cage formed no such preference, which is how we know it's the association acting here and not just familiarity.

This is as close as the researchers could get to thinking they had established a Proust madeleine mouse moment. The recollection of a pleasurable activity, connected to a smell. So what could they tell about this experience for the mouse?

Who knows a mouse’s nose?

Turns out, the mouse olfactory bulb, like ours, is packed with tiny neurons called granule cells. Production of these cells peaks literally at day one of a mouse’s life. The team tagged those cells and found they lit up when adult mice met their childhood smell.

Switch those cells off with an implanted laser and they lost interest in those nostalgic childhood smells the same way they would with any odour.

This will take more work, but perhaps we have found the place in the olfactory system where childhood smell-memories are stored?

Take a long hard sniff at this.

While interesting and headline grabbing, we need to inhale gently here. There are a few important things to note:

* A mouse sniffing a smell for longer than other mice do hints at a preference. That's not the same as remembering a childhood incident.

* By six months the preference had faded, and the day-one granule cells were no longer picking out the childhood smell. Mice given regular top-ups of the smell kept the memory, despite their day-one cells having checked out. Something else was doing the work.

* We are yet to investigate unpleasant smells and memories.

* In general the stats here sit more in the realm of “trend” and “imply” rather than inarguable proof.

* A 2021 study found that smell cued less emotionally intense childhood memories than sight or sound.

"The only peculiarity of smell [is] that it is like a password and needs to be exact." — Luca Turin, biophysicist, University of Buckingham, quoted in Scientific American.

A final whiff.

So the next time a smell hauls you into a kitchen you have not stood in for forty years, is your brain accessing a mothballed memory that has sat sealed for decades. Or have you been subconsciously topping it up over the years?

At this stage we just don’t know.

What I do know, is this phenomenon is a gorgeous connection between the magic of memories and the science of smell.

I support anything we can do to sniff out what is really going on.