It's called the Proust effect after a story in the author's "Remembrance of Things Past: Swann's Way." When a character dipped a madeleine, a sweet, buttery French cake, into some lime-blossom tea, the scent suddenly transported him back in time to the moment his aunt had served him that same combination:
"Immediately the old grey house upon the street, where her room was, rose up like the scenery of a theatre to attach itself to the little pavilion, opening on to the garden, which had been built out behind it for my parents… and with the house the town, from morning to night and in all weathers, the Square where I was sent before luncheon, the streets along which I used to run errands, the country roads we took when it was fine."
Nothing conjures up a memory so viscerally as the scent with which you associate it. While it's been understood for some time that our olfactory system has a unique ability to vividly summon memories, the mechanism behind the phenomenon has net been well-understood. Now a study by researchers from Northwestern University's Feinberg School of Medicine may have solved the puzzle. The olfactory system has an unusually direct connection to the brain's hippocampus, believed to play an important role in memory.
The study's published in the journal Progress in Neurobiology.
A lasting connection
Credit: schankz/Adobe Stock
Previous neuroimaging and intracranial electrophysiology investigations have revealed that our senses are functionally connected to the hippocampus, if not directly. However, the new research, for which the principle investigator is Christina Zelano, is the first rigorous comparison of the strength of those connections.
It turns out that our primary olfactory cortex is a sense that's still directly connected to the hippocampus.
"This has been an enduring mystery of human experience," Zelano tells Medical Xpress. "Nearly everyone has been transported by a whiff of an odor to another time and place, an experience that sights or sounds rarely evoke. Yet, we haven't known why. The study found the olfactory parts of the brain connect more strongly to the memory parts than other senses. This is a major piece of the puzzle, a striking finding in humans. We believe our results will help future research solve this mystery."
It's believed that during evolution, the hippocampus' role shifted away from its original strong relationship to the sensory cortexes and toward connections with higher association cortexes. (In rodents, for example, the hippocampus maintains a powerful connection to all sensory cortexes.) It now appears that as this occurred, the olfactory cortex alone continued to be directly wired to the hippocampus.
"Humans experienced a profound expansion of the neocortex that re-organized access to memory networks," explains Zelano. "Vision, hearing and touch all re-routed in the brain as the neocortex expanded, connecting with the hippocampus through an intermediary-association cortex-rather than directly. Our data suggests olfaction did not undergo this re-routing, and instead retained direct access to the hippocampus."
The importance of smell
It's known that people who experience a loss of smell, or "anosmia," often develop depression. "Loss of the sense of smell is underestimated in its impact," says Zelano. "It has profound negative effects of quality of life, and many people underestimate that until they experience it. Smell loss is highly correlated with depression and poor quality of life."
Anosmia is also associated with COVID-19. "The COVID-19 epidemic," says Zelano, "has brought a renewed focus and urgency to olfactory research." Lead author Guangyu Zhou agrees: "There is an urgent need to better understand the olfactory system in order to better understand the reason for COVID-related smell loss, diagnose the severity of the loss and to develop treatments."
"Most people who lose their smell to COVID regain it," notes Zelano, "but the time frame varies widely, and some have had what appears to be permanent loss. Understanding smell loss, in turn, requires research into the basic neural operations of this under-studied sensory system."
She notes that, "While our study doesn't address COVID smell loss directly, it does speak to an important aspect of why olfaction is important to our lives: Smells are a profound part of memory, and odors connect us to especially important memories in our lives, often connected to loved ones."
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The image of a mouse running aimlessly in a wheel is a common motif in scientific studies. Put the same wheel in nature and a feral mouse will still hop in and spin it around, just as any cardio junkie will jump on a treadmill.
Humans have invented a number of triggers to help them get to the gym to jump on that treadmill (or run in nature). Put your running shoes next to your bed so you see them upon waking up. Glue a toned athlete on your vision board. Set a mileage goal in Strava and remember you're being tracked by peers.
Mice have triggers for exercise, too, and this one might teach us a bit about our own fitness inspiration: scent.
Researchers at the University of California, Riverside wanted to understand how influential scent was to racing rodents. A team led by Sachiko Haga-Yamanaka, assistant professor in Department of Molecular, Cell and Systems Biology, found out, and the answer is quite a bit. That's according to their new study published in PLOS One.How do we smell? - Rose Eveleth
Many animals utilize olfaction to navigate their terrain. Comparatively, humans have a pretty weak sense of smell. For this study, the researchers looked at the vomeronasal organ (VNO), a feature of a number of amphibians and mammals, and its influence on volunteer wheel running (VMR) in mice.
"Although the role of the vomeronasal chemosensory receptors in VWR activity remains to be determined, the current results suggest that these vomeronasal chemosensory receptors are important quantitative trait loci for voluntary exercise in mice. We propose that olfaction may play an important role in motivation for voluntary exercise in mammals."
The team chose fanatical runners that are more intrinsically motivated to get on the wheel than their peers. (The lab that produced this study even has a High Runner Mice website.) Apparently, these mice have strong vomeronasal sensory receptor neurons, which pick up the scent of pheromones (among others) as a form of motivation.
A link between these neurons and sexual behavior already exists; this study appears to expand the olfactory sense to another physical activity. The chemosensory signals received by VNO activation sets off a chain reaction in their nervous system. Just like humans can't help but dance to a good beat, mice crave the rush of running when the right scent hits them.
Could this apply to humans as well?
Credit: BillionPhotos.com / Adobe Stock
Christopher Bergland thinks so. The elite athlete knows all about treadmills. He holds the world record for the longest treadmill run over a 24-hour period. In a recent column, he claims that scents have been motivating him to exercise for decades.
"Even as a middle-aged person with a middle-of-the-road libido, smells from my adolescence—such as classic Coppertone sunscreen mixed with a spritz of vintage Polo Green cologne—still give me a "Vroom!" feeling that gets my juices going. The same smells that I used to run five back-to-back marathons through Death Valley in near 130º heat and to break a Guinness World Record by running 153.76 miles on a treadmill decades ago, still motivate me to go for daily jogs at a 'conversational pace.'"
He still uses smells to inspire his workout regimen. In his 2007 book, "The Athlete's Way," Bergland discusses aromatherapy as a performance enhancement and motivational tool. This makes sense: we might have devolved in our olfactory senses a bit, but smells still heavily influence our world. Flavor, for example, is just as much about smell as taste.
"Acquiring information related to scent through the back of the mouth is called retronasal olfaction—via the nostrils it is called orthonasal olfaction. Both methods influence flavor; aromas such as vanilla, for example, can cause something perceived as sweet to taste sweeter. Once an odor is experienced along with a flavor, the two become associated; thus, smell influences taste and taste influences smell."
We're certainly motivated to eat thanks to the scent of our favorite foods. The idea that smell would get us out of bed and onto a bike is not far-fetched, whether we realize it or not.
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Stay in touch with Derek on Twitter and Facebook. His new book is "Hero's Dose: The Case For Psychedelics in Ritual and Therapy."
Science can tell us quite a bit about how the brain converts light and soundwaves to our sense of sight and hearing. But our sense of smell is less understood.
Scientists know that smell is based on the chemical makeup of things we encounter in the world. The nose communicates information about odor molecules to the brain's olfactory bulb, which then sends signals to the piriform cortex. This brain region then processes that information to produce our perception of smell. What's remained mysterious, however, is how the brain encodes and categorizes information about various types of scents.
A new study, published in Nature by researchers from Harvard Medical School, sheds light on the inner workings of the olfactory process.
"All of us share a common frame of reference with smells," senior study author Sandeep Robert Datta, associate professor of neurobiology in the Blavatnik Institute at HMS, told Harvard Medical School News. "You and I both think lemon and lime smell similar and agree that they smell different from pizza, but until now, we didn't know how the brain organizes that kind of information."
The researchers wanted to better understand how the brain is able to discern between related but distinct scents, such as that of a lemon and lime.
"The fact that we all think a lemon and lime smell similar means that their chemical makeup must somehow evoke similar or related neural representations in our brains," Datta said.
Illustration of multiphoton microscopy
Pashkovski et al.
To investigate, the researchers created a database of thousands of odorous chemical structures, and they used machine learning to categorize them by features such as number of atoms, molecular weight and electrochemical properties. These odors were separated into three categories: high diversity, intermediate diversity and low diversity.
Then the researchers exposed different odors to mice, and used multiphoton microscopy to record neural activity in the piriform cortex and olfactory bulb. The results showed that when odors are chemically similar, so too is neural activity. In other words, the cortex emphasizes relationships between chemically similar odors, and it creates groupings for similar odors, which helps us distinguish between objects in the world.
Smell and neuroplasticity
The results also suggest that perception of smell is flexible. For example, the team repeatedly exposed mice to a combination of two chemically dissimilar odors. Over time, images showed that the neural patterns produced by the pair of odors become more strongly correlated.
"We presented two odors as if they're from the same source and observed that the brain can rearrange itself to reflect passive olfactory experiences," Datta said. "The plasticity of the cortex may help explain why smell is on one hand invariant between individuals, and yet customizable depending on our unique experiences."
Pixabay
The study provides some of the first information on how the olfactory cortex maps different odors. And the results also suggest that, by better understanding the chemical structure of different odors and how that mapping process works, scientists may someday be able to better control our sense of smell.
"We don't fully understand how chemistries translate to perception yet," Datta said. "There's no computer algorithm or machine that will take a chemical structure and tell us what that chemical will smell like. To actually build that machine and to be able to someday create a controllable, virtual olfactory world for a person, we need to understand how the brain encodes information about smells. We hope our findings are a step down that path."
Have you ever watched a sci-fi film and thought, "I wonder what everything smells like in this scene?"
If that sounds like you, then you're in luck. A Kickstarter campaign is raising funds to produce a perfume that smells like outer space. Now you can go about your day smelling like the inside of the Lunar Lander, Millennium Falcon, or Discovery One.
In Space, nobody can figure out what that smell is.
NASA has been concerned about what space smells like for years, primarily to reduce the surprise to astronauts who go up for the first time. According to Eau de Space's Kickstarter video, the space agency has been using a reproduction of the smell of space for decades.
In 2008, they asked Steve Pearce, a chemist who founded Omega Ingredients, to help them create the smell for an exhibition, presumably a more difficult task than giving new astronauts a spritz. Now, thanks to what they dub "sheer determination, grit, a lot of luck, and a couple of Freedom of Information Act (FOIA) requests," the team behind the Kickstarter hopes to bring the scent to the public.
Descriptions of what it smells like are all over the place, and include raspberries, rum, "spent gunpowder," hot metal, fried steak, and "ozone."
For those wondering when you'd get a chance to notice the smell with a helmet on, as is required for spacewalks or moonwalks, the scent follows astronauts as they return from spacewalks. According to a researcher who spoke to The Atlantic, the odor is created by "high-energy vibrations in particles brought back inside which mix with the air."
As to why it smells like the various things mentioned above, the jury is still out. One suggestion is that at least some of the particles are hydrocarbons, which can also be found in things like tobacco smoke and car exhaust here on Earth. NASA argues that at least some of the smell is caused by oxidation of these particles, whatever they may be, as they enter the oxygen-rich environment of the spacecraft.
The plan is for the fragrance to be used primarily as an educational tool, sparking conversations about outer space in the classroom. To this end, each purchase includes a one bottle donation to a K-12 school. According to Engadget, there are currently no plans to mass-produce the fragrance after the Kickstarter ends.
If you want some, you might want to make that move before the countdown hits zero.
