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Scent-imental

by Sophia Virkar

art by Hailey Koppart by Hailey Kopp


Introduction

Have you ever smelled a box of crayons and been transported to your second-grade classroom? Or maybe you caught a whiff of your mother’s perfume and were reminded of how much you miss her? Our sense of smell, a crucial but often overlooked element of human perception, has the unique power to transport us back in time, connecting our emotions and memories. Beyond evoking nostalgia, scents also impact our social interactions and choices, from selecting romantic partners to influencing everyday decisions. They influence the taste of our food and the ambiance of our surroundings. Our sense of smell allows us to do more than just perceive odors. It shapes how we connect with the world and our past, impacting our emotions, memories, and social behavior.


Olfaction

Our sense of smell, also known as olfaction, results from the detection of airborne chemical odorants [1]. The ends of olfactory neurons in the nose contain hairlike extensions called cilia, where the nose’s specialized odorant receptors are found. From the nose, these signals are transmitted to the olfactory cortex and other parts of the brain [1]. Similar to how natural light is created from a mixture of wavelengths, natural odors are the result of a combination of different odorant molecules [2]. For example, the odorant benzaldehyde produces an almond smell and vanillin produces a vanilla smell. However, scents often arrive in combination with one another. Think of a freshly baked almond croissant. You would smell a sweet mixture of vanilla and almond scents paired with the butter and dough, deciphering a unique combination of odorant molecules. The olfactory epithelium, located in the nasal cavity, has about 10 million olfactory receptor neurons (ORNs), each specialized to bind to a specific odorant molecule [2]. These ORNs transduce odorant identity into olfactory information that the brain then uses to determine the identity of a given scent. With about 350 types of ORNs, humans can discriminate up to one trillion odors [2].

Along with olfactory memory, there are several other categorizations of sensory memory, such as muscular, visual, and auditory [3]. Olfactory memory is unique, as it is retained for longer compared to other senses [4]. You may not exactly remember what your mom’s oven looked like, but you could probably pinpoint the scent of her freshly baked cookies. In fact, studies demonstrate that most odor-cued memories are linked with the first decade of life, whereas memories associated with verbal and visual cues peak in early adulthood [5].

Olfaction connects directly to the brain’s emotional and memory centers. All other senses are first processed at the thalamus, a brain structure involved in sensory and motor processing, before being relayed to higher brain structures for further processing [6]. Because our sense of smell goes straight to other structures such as the hippocampus and amygdala, olfaction is directly linked to emotions, social behavior, and memory [7]. This connection is facilitated by the olfactory nerve’s connectivity to the amygdala and hippocampus, which are both largely involved in emotional and long-term memory [3]. The olfactory nerve is closely connected to these structures: only two synapses, or neuron connections, away from the amygdala and only three away from the hippocampus [3].


Emotion

Because olfaction both bypasses the thalamus and has close neuronal connections to the amygdala and hippocampus, it is heavily intertwined with emotion. Some suggest the connection between smell and emotion is a product of learning and memory. For example, food preferences could be influenced by a mother’s diet during pregnancy, or one might form a negative association with eugenol, a substance used in dental fillings, after treating a cavity. Human and animal studies show that odors can evoke autonomic responses, such as the fight or flight response, along with emotions via pathways to the amygdala [8]. These signals travel to the hippocampus and become part of memory [8].

One instance in which odor elicits emotional distress is the case of post-traumatic stress disorder (PTSD) patients [9]. It is clinically recognized that unexpected intrusive smells can significantly impact individuals with PTSD, leading to the involuntary recall of odor memories. The olfactory cortex is crucial in emotional processing, and clinical observations confirm that odor-evoked memories are influential in PTSD symptomatoogy. Specific trauma-related smells, such as blood, napalm, and diesel, have been identified as triggers for anxiety and fear-related memories in PTSD patients. Intrusive reliving, a core symptom of PTSD, is traditionally linked to classical fear conditioning mechanisms, in which a person or animal negatively associates a stimulus with an event. In fear conditioning, a life-threatening situation activates the limbic system, which is a network of brain structures involved in processing memories and emotions. Then, these external cues become associated with arousal or anxiety. When these cues are later encountered, they can re-trigger that fear response. For instance, a study on 100 refugees in a psychiatric clinic revealed that 45% had experienced olfactory-triggered panic attacks in the previous month, with 58% reporting instances of intrusive reliving during such attacks [9].

Odor can also elicit positive emotions such as reductions in anxiety and depression levels. A study by Ballanger et al. showed that odors such as lavender, citrus scents, and green leaf odors possess anxiety-reducing properties [10]. These effects are thought to be mediated through mechanisms similar to those of pharmacological treatments. For instance, in studies with animals, exposure to the scent of lavender has been shown to alter the signaling of GABA receptors, which ultimately inhibits brain activity. This exposure has also been shown to alter serotonergic signaling, playing a crucial role in mood regulation. Both of these signaling pathways are targets of pharmacological medications aimed at reducing anxiety. Additionally, green leaf odor has been found to produce anti-depressive effects in mice, partly by elevating serotonin levels. This mechanism is similar to that of certain antidepressants like selective serotonin reuptake inhibitors [10]. However, it is important to note that many of these studies use a no-odor control, which limits the ability to directly compare the effectiveness of different odors in inducing these effects [11].

Lastly, aromatherapy, the use of essential oils for therapeutic benefit, has been shown to reduce pain in various contexts. A meta-analysis conducted by Lakhan et al. found that aromatherapy significantly reduced pain, particularly in post-surgery and gynecological settings [12]. The majority of studies indicated that with the use of aromatherapy, patient satisfaction was increased, while patient anxiety and depression were decreased [12]. Furthermore, a randomized trial performed by Tanvisut et al. reported significant reductions in labor pain and duration along with a decreased need for painkillers following the use of aromatherapy [13]. Despite several studies in favor of aromatherapy, research is still limited. In a study done by Tang and Tse, there was only a slight reduction in pain among older adults treated with aromatherapy [14]. This suggests that the effectiveness of aromatherapy varies across populations and contexts, and more research needs to be done to truly determine its effectiveness as a pain reduction tool [14].


Reminiscence

I associate many scents with childhood, and chances are, you do too! When I think back to the scents of my childhood, several examples come to mind. I can easily recall the Scholastic book fair, where they had chocolate-scented calculators and banana scratch-and-sniff stickers. Another distinct memory is of my dad’s green, minty deodorant, which my sister accidentally licked when we were kids. While it is unlikely that everyone shares these particular scent memories, many people can re - member specific scents of their childhood.

Nostalgia is a complex emotional state that combines elements of both happiness and sadness, often triggered by sensory experiences that remind individuals of significant moments in their past [15]. Among these sensory triggers, scents have a powerful ability to evoke nostalgia. This phenomenon was named the “Proust Phenomenon” after the French writer Marcel Proust, who documented that he was vividly reminded of his childhood memories upon smelling the aroma of a tea-soaked cake [15].

A study by Petratou et al. revealed that certain scents, particularly those from childhood, are strongly linked to emotions and memories [5]. In that study, bubblegum was found to be the most familiar and nostalgic scent, leading to increased self-esteem, social connection, optimism, and inspiration. They found that nostalgic scents can elicit strong feelings. For example, sweet odors are often associated with positive feelings, and heavier or stinkier odors cause negative reactions. This research supports the idea that childhood scents can trigger nostalgia, with some scents, such as bubblegum, being more effective than others [5].

This evidence aligns with the findings of Barrett et al., who observed that scents are remarkably effective nostalgia inducers compared to other sensory experiences, such as listening to music [16]. In their research, over half of the scents in both of their studies received nostalgia ratings at or above 2 on their scale (1 = not at all; 4 = very much). This is a stark contrast to studies using musical excerpts, where only about 26% of stimuli achieved similar nostalgia ratings. Moreover, scent-evoked nostalgia not only mirrors the emotional profile of music-evoked nostalgia but also generates greater positive emotion [15, 16].

Furthermore, research conducted by Rachel S. Herz demonstrated that scent-cued memories also contain more relevant details compared to memories triggered by visual or auditory cues [17]. In experiments where participants recalled memories using different sensory cues, those triggered by scents using oilbased beads were found to be more emotionally intense and detailed than those cued by visual or auditory stimuli. This suggests that scent-cued memories have a stronger connection to emotional and relevant details, and are more effective in bringing individuals “back in time” [15, 17].

As mentioned earlier, the olfactory nerve is closely connected to the amygdala.


Studies using positron emission tomography (PET), a useful non-invasive neuroimaging technique, have provided insight into why olfactory memories elicit strong emotional responses [5]. Imaging shows that compared to auditory or visual stimuli, olfactory cues achieve greater activation of the amygdala, a center for emotional processing. Additionally, when memories are cued by scents as opposed to words, there is a notable increase in activity within the limbic and temporal lobes, which are regions associated with positive memory processing [5].


Evolution and Attraction

Evolutionarily, olfaction has served to detect disease, injury, and unsafe foods. This ability has likely emerged as a protective mechanism. Humans have evolved to find sickness-related odors and rotted food pungent since being able to identify health-related issues in both ourselves and our food enhances our chances of survival [18].

Furthermore, olfaction has also played an evolutionary role in social relationships. A review done by Calvi et al. discusses humans’ ability to recognize negative emotions (ie. fear, stress, or anxiety) through body odors, suggesting an evolutionary basis for this response [19]. They also discuss subsequent studies in which similar results were obtained for positive emotions (ie. sexual arousal and happiness). This suggests that people can unconsciously send messages to others through chemical signals in body odors [19].

While the term “pheromone” may be familiar, the evidence for the role of pheromones in human behavior is unclear and somewhat inconclusive, with small-scale studies yielding mixed results. There is some evidence for a possible human pheromone from breastfeeding mothers. During lactation, a substance is secreted from the mother’s nipple. When put under the nose of a newborn, the baby responds with movements consistent with nursing behavior (eg. tongue protrusion, or lip pursing) [2]. This suggests that this substance may be a pheromone. Research is still being conducted on the existence of human pheromones. So, unfortunately, those pheromone perfumes you see on Amazon are not quite backed by science ... yet [2].

Instead of the commonly misused word pheromone, the concept of a chemosensory signal can be utilized in reference to smell communication in humans. Several studies have been conducted to test how these chemosensory signals influence behavior. One study

shows that heterosexual men prefer the scent of women while they are in the fertile phase of their menstrual cycle, and this effect dissipates when women use contraceptives [20]. This reveals that body odors shape physical attractiveness and can have an effect on the choice of partner.


Society

Smells influence every aspect of society, from our food to our clothes to the physical spaces we are in. The aroma of food connects us with our familial and cultural memories. This connection between smell and the body can also be applied in the context of clothing. Due to the close relationship between the body and clothing as well as the ability of clothing fibers to retain scents, odors linked to the body or infused into clothing can trigger strong reactions [21].

Our perception of visual stimuli is deeply intertwined with our feelings and past experiences towards them [22]. Palmer and Schloss’ ecological valence theory (EVT) suggests that color preferences are influenced by feelings toward associated items. If a color is linked to positive experiences, we tend to like it, whereas if it is associated with negative experiences/memories/ feelings, we are likely to dislike it. This theory suggests that our preferences guide us towards beneficial objects and away from harmful ones. Schloss et al. sought to determine whether this visual theory could be extended to smells. Recent findings indicate that preferences for familiar odors are shaped by collective feelings toward all associated items. For instance, the scent of apples is welcomed due to its connection with positive items like pie, soap, and candy, while fish odor is disliked because it is associated with negative things like dead fish and trash. This approach, known as odor WAVEs (weighted affective valence estimates), more accurately predicts smell preferences than just considering the namesake object. It suggests that preferences for smells are a summary of past experiences with those smells, influencing choices toward beneficial situations and away from harmful ones [22]. This could explain why some people may be drawn to smells that many find unpleasant, such as the scent of gasoline. Our individual experiences with different scents shape how we perceive them. While some people might associate the scent of gasoline with pleasant boat days, others might associate it with the anxiety of being on the road or traumatic experiences like car crashes.

Because smells are so intertwined with our environment and our emotions, they also often influence social dynamics. For example, a study by Qian Hui Tan found that the scent of cigarette smoke was polarizing, contributing to physical and social segregation between smokers and non-smokers in public places in Singapore [23]. However, scents of physical spaces can also be used advantageously. Companies use the connection between smell and memories to create more positive experiences for their consumers. A notable example of this was when the food company McCain used interactive advertising to emit the smell of baked potatoes at bus stops during the cold month of February. A more common example is hotel lobbies using certain fragrances to “scent brand” their hotels [24].


Conclusion

Olfaction weaves itself into many parts of our lives, shaping our experiences, memories, and social interactions. From its ability to trigger the nostalgic recall of childhood memories, like the unique scent of a childhood toy or a parent’s cologne, to its role in evoking emotions and influencing behaviors, our sense of smell does more than just perceive odors. Scents can spark positive neurological reactions such as pain relief during childbirth or for negative emotions as in the case of individuals afflicted with PTSD. The scientific study of olfaction is an ongoing pursuit, with a real potential to open doors for future therapeutic interventions.


REFERENCES:

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