How Your Nose Knows: The Science of Smell and Memory

by Eion Plenn

The smell of funnel cake wafts through the air. Suddenly, you remember summer vacations spent on the Ocean City boardwalk. Or perhaps it's the whiff of fresh pine in HomeGoods that reminds you of Christmas, or the scent of Axe body spray in CVS that brings you back to those awkward middle school days. The human nose has a unique way of picking up the scents of our surroundings and communicating these scents to the brain. The brain can then respond with a cocktail of emotion — and action — from avoiding food that has spoiled to sensing a fire from the smell of smoke. 

But how is the brain able to detect cues from our environments? And more so, why is it that certain scents bring back vivid memories, some of which may date back to childhood? 

Behold the olfactory system. Taking its name from the Latin word olfacere, which means "to smell,” the olfactory system is an amusement park of neurons contained in the brain which allow it to identify and differentiate the smells of our environment. While the olfactory system is often outshined by the neuroscience of its cousins, such as how the brain sees or how the brain hears, how the brain smells is imperative to our quality of life. Without a sense of smell, we would not be able to protect ourselves from toxic odorants, such as propane leaking from a kitchen stove, or enjoy pleasurable smells, such as fresh baked apple pie.

The science of scent is not as simple as it sounds, as it starts with the brain converting sensory stimuli in our environment. Luckily, biology is good at this, and with the help of proteins known as G-protein coupled receptors (GPCRs), we are able translate the chemistry of odors in the air into electrical signals in the brain. Located in the nasal epithelium of the nostrils, GPCRs are diverse and abundant in the olfactory system, with each latching on to many different scents and triggering a unique response. Human nasal cells contain roughly 400 types of olfactory GPCRs, whereas those of some mammals such as dogs contain more than 1,000, making them keen at distinguishing odors — this also means that there are scents in human environments that we cannot even smell. 

Still, how do these infamous GPCRs recognize odor molecules so specifically? This aspect of the olfactory system is still not completely known. However, the basic pathway can be illustrated by the following: It is winter break, and you pull into your parents’ driveway. The first thing you smell when you walk in the door is a Christmas tree. You don’t see a Christmas tree, but you smell one. Your brain has primed you to expect a Christmas tree, and low and behold, you turn the corner and a bushy, green Christmas tree is standing in the family room. So, what’s happening here? 

For a start, the “Christmas tree” scent is actually pinene, a volatile chemical which comes from pine trees. Our nostrils have mucus in them which are able to “trap” this pinene like a fly in honey. Once “trapped,” the odor molecule binds to a specific GPCR in the olfactory receptor neuron — like a “shape” fitting into a “mold.” From there, a cascade of chemical reactions will lead to the opening of ion channels in the olfactory receptor neurons. (Let's call these ion channels “flood-gates.”) Once the flood-gates have opened, positively charged ions will rush into the olfactory receptors’ neurons like water down a reservoir, creating an electrical signal which is then processed as a “smell” in the brain.

Recognizing the smell of Christmas sounds easy enough, right? Except, what if you aren’t home for the holidays? What if you are reading this article about the scent of fresh pine and are suddenly smelling Christmas, but there’s no Christmas tree on site? Your parents’ family room is 300 hundred miles away. How is it possible to “smell” Christmas without any pinene around to trigger the olfactory receptor neuron? 

When you first “smelled” that Christmas tree in childhood, these olfactory receptor neurons sent a spark to higher regions of the brain — distinct from the olfactory cortex responsible for processing smells. This region is called the hippocampus, a complex structure which is the brain’s “black box '' for storing memories. Every time we smell, our neurons “zap” our hippocampus, creating memory associations with certain odors. This process was highlighted in a 2004 study published in “Learning and Memory.” The study showed that patients with lesions in the hippocampal regions of their brains had impaired odor recognition recall. In other words, they were unable to associate certain smells with memories.

This “bridge” between smell and memory is nothing new. In the early 20th century, French essayist Marcel Proust brought this phenomenon to the spotlight in his novel “Remembrance of Time Past.” Here, Proust recounts an episode where he dips a Madeline cookie in tea and is suddenly flooded with childhood memories. Psychologists deemed this as the “Proust Effect,” occurrences in which sensory stimuli such as taste, touch and smell can involuntarily evoke memories in our brains. While the neuroscience behind the “Proust Effect” is still not completely understood, it’s relationship with our sense of smell may have novel applications — especially in the lives of a college student.

For example, in 2012 the “Journal of Consumer Behaviour” published a study testing the impact of scent and pictures on advertisement recall. The subjects were placed in a movie theater with an ambient scent and showed various advertisements. Results showed that while visual and olfactory cues enhanced ad recall — the participants’ ability to remember the advertisements they were shown— it was the olfactory cues (the “smell” of the movie theater) that actually enhanced recall to a greater extent. Even after a long time delay, subjects were able recall the advertisements to a greater extent after re-experiencing the scent of the movie theater.

While a variety of factors are involved in memory recall, this study shows that the olfactory system is a formidable force in triggering past memories. Imagine harnessing the power of “scent” to study for midterms and standardized tests. For instance, if you crammed for your American history exam in a Starbucks, the same venti dark roast on test day could perhaps give you that extra “boost” in memory recall. 

While this may reek of pseudoscience to some, studies have shown that smell has a great impact on enhancing one’s object and spatial memory. So perhaps the neuroscience and psychology of the olfactory system is not only beneficial to understanding the world around us, but a useful tool for self-improvement. You just have to know your nose.