Sensing Scents: Olfactory (Smell) Perception

Dr. Renu Heir

Sensing Scents: Olfactory (Smell) Perception

Animals are incredibly dependent on their sense of smell (olfaction) as it can help them locate food, find a mate, and avoid predators, among other things. It is one of the oldest of the senses and has evolved to accomplish impressive feats. It can be exquisitely sensitive: polar bears, for example, can detect the odor of seal breath through their breathing holes in the Arctic ice from up to 3 km away.1 Humans, long thought to have poorer olfaction than other animals, are theoretically capable of discriminating 1 trillion different scents even then.2 Despite its importance, we have only recently begun to understand the intricacies of how the sense of smell actually works, transmitting signals from the nose, to the brain, and into our perception.

Transforming odors from scent to signal

What we smell are chemicals in the air, so they must go from volatile compounds in our environment to a scent we experience. The first step is for these scented compounds, called odorants, to enter the nose and find their way to a small part of the roof of the nasal cavity called the olfactory epithelium. There, they interact with a specialized group of cells, olfactory sensory neurons, to turn a chemical signal into an electrical signal that is propagated within the brain.

In the 1990’s Linda B. Buck and Richard Axel discovered that this signal transformation occurs through the binding of scented compounds to odorant receptors on olfactory sensory neurons, and later went on to win the a Nobel prize for this work.3, 4 The complexity of all the scents we smell is conveyed by these receptors, but there are fewer receptor subtypes than discernable odors, hinting that perceiving a smell isn’t just the result of one compound activating one type of receptor. Instead, every scent is made up of a unique combination of odorants that then goes on to activate a particular subset of odorant receptors and olfactory sensory neurons, generating electrical impulses that can travel elsewhere in the brain. The identity of the activated neurons forms a sort of signature, representing the intricacy of the scent that is being coded, and this is passed on through connections with other neurons that translate the signature.

Once electrical signals are generated by sensory neurons in the olfactory epithelium, they are relayed to other parts of the brain for processing.5 The first stop in this process is the olfactory bulb, a small structure on the bottom of the human brain, close to the nasal cavity where interpretation of sensory neuron activation patterns begins. From there, neurons connect with components of the limbic system—the amygdala and hippocampus—which are key to emotion and memory, as well as with the cortex, a higher brain area that conducts further processing and integration of sensory information into our perception of odors.

Does that smell ring a bell? Olfaction is tied to memory and emotion

Scents seem to have a unique ability to evoke vivid memories that are steeped in emotion. Take the smell of cloves, cinnamon, and pine: for many, this doesn’t merely remind them of cooking and trees. Rather, it is the smell of the holidays, and can bring memories of joyous dinners with extended family, the warmth of spending time around a fireplace in the wintertime, or feelings of comfort. In addition to anecdotal accounts of this phenomenon, the ability of olfaction to conjure feelings and trigger recollections has been the subject of numerous experiments. For instance, in a study where subjects were presented pictures to remember at the same time as stimuli from different senses, odor-induced memories were more emotional compared to those triggered by verbal, visual, tactile, and musical stimuli.6 In another experiment, using smells as cues when asking subjects to retrieve memories improves their performance in doing so.7 There are clear ties between olfaction, memories, and emotions, and a potential reasons for this lies in the path of connections that make up olfactory perception.

Information from the olfactory bulb goes to the amygdala and hippocampus, both parts of the limbic system responsible for emotions. The sense of smell is unique in these connections: it is the only sense that proceeds to limbic areas without first passing through the thalamus, a relay structure believed to play a role in consciousness. Neuroscientists postulate that it’s this directness to the limbic system that is responsible for the distinctively emotional nature of olfactory memories. In addition to its role in emotions, the hippocampus is also the main center for memory formation and consolidation. A close and direct route from the olfactory bulb to the hippocampus, then, may result in the ability of smells to trigger particularly vivid memories.

Scent perception is neither static nor universal

Odors have a powerful influence on human emotion, memories, and behavior, but there is an added layer of complexity in that the perception of odors can be variable even within a single person. As an anecdotal example, most people will say that the same food smells better when they are hungry compared to when they are full. Indeed this was borne out in a research setting as well: in one study, subjects who were hungry were more sensitive to food-related odors and found them more pleasant than those who had just eaten to satiety.8 This diversity of perception to a given odor within the same individual extends beyond hunger. Disgust and fear can increase sensitivity to scents, which is intuitive considering that they are both emotions that generally lead to avoidance behaviors that may benefit from increased smell detection.9 While these are just two examples in a large and complex body of literature, it is evident that physiological and emotional state can affect olfactory perception within a given individual.

Cultural background is key to how fragrances are perceived. One of the first studies to address this looked at the responses to a panel of odors across participants in 16 different countries, and found that perceptions were regional, with more positive impressions for scents that were more familiar.10 A larger survey was conducted by National Geographic by including scratch-and-sniff inserts in an issue of the magazine, and received responses from 1.4 million participants over 5 continents that were subsequently interpreted by scientists. One analysis showed clear geographical differences in perception of odors, confirming results from the smaller study.11 A specific example of this regionality is the scent of wintergreen, which was much more positively rated in the United States as compared to Europe. The authors of that report speculate that this is a result of the fact that in the US, wintergreen is often the scent of candy, whereas in Europe, it is found more often in medicines.

A picture of odor perception as intensely personal emerges from these experiments. Because the sense of smell is so intimately tied with memory, impressions are formed based on a person’s unique set of experiences. Some of these may come from individual life events, some may also come from a more collective origin through cultural factors, but the combination of all of them will be distinct for each individual.

Smells can sell through sensory marketing

Sensory marketing seeks to engage consumers’ senses in order to affect their perceptions and behaviours. Given the close ties between smell, emotion, and memories, olfaction is a good fit for this type of marketing. As a result, fragrance has become an important part of the manufacturing and branding of products.

The scent of a product can increase the ability of a consumer to recall details about it. This is key, as a customer must remember a product in order to decide to purchase (or re-purchase) it. This idea was clearly demonstrated in an experiment conducted with pencils: subjects were given pencils that were either unscented, scented like pine, or scented like tea tree oil, as well as an ad with the product name and some advertising claims. Subjects remembered more details when they had been presented scented pencils, and furthermore those who had received pencils with a more distinctive fragrance (tea tree oil) remembered details for longer.12 This highlights the importance of fragrance in the product experience, and suggested that a scented product is a more memorable product.

Another interesting application of these ideas is in the retail spaces that products are sold in. In the 1990s, Alan Hirsch, a neurologist, postulated that a space with a pleasant ambient aroma may encourage shoppers to stay longer and increase their likelihood to make a purchase. To test this idea, he and his team set up two identical rooms containing sneakers that differed only in the fact that one was filled with a floral scent, and found that customers were over 80% more likely to purchase the shoes in the scented room.13 Though this initial report has been criticized for its methodology14, subsequent research led to this type of marketing exploding into an industry worth hundreds of millions of dollars annually.15

Fragrance in the future

Research into olfaction and smell perception has taken off in recent years, gaining momentum since Buck and Axel’s ground-breaking work on odorant receptors in the 1990s. Olfaction was once thought to be relatively insignificant, and was therefore neglected as an area of study.16 However, we are now realizing that through its effects on memory and emotion, smells have a powerful position in our lives. As such, the roles of scent specialists of all kinds—perfumers formulating new fragrances, sensory marketing experts, wine connoisseurs describing the “nose” of a particular vintage, olfactory neuroscientists and more—will grow in years to come.


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