Abstract
Olfaction, or our sense of smell, is one of the least explored human senses. Despite this, many scents seem universal: burnt popcorn, spoiled milk, or a freshly-baked cookie. When we smell something, small chemicals reach the olfactory neurons in our nasal cavities from one of two paths: either directly through our nose or from our mouth. Each scent chemical activates a specific set of neurons that will then send signals to the brain for scent recognition. Scents are tied to our memories, and our odor memory ensures that we remember each unique scent combination as a whole, as well as the experiences around it. Smells can help us detect dangerous substances, but they can also be used to influence our behaviors. Namely, companies use scent technologies to influence our purchasing behavior or to enhance an environment. Researchers are currently taking this smell technology to the next level; synthetic light-based smells and bionic noses are being developed today. As our understanding of the olfactory system expands, such scent technologies will become even more complex and accurate.
Keywords: scent, smells, olfaction, consumer marketing, immersive experiences
Introduction
Growing up, my family’s special weekend activity was a trip to the mall – the Spotsylvania Towne Centre, to be precise. While I have not returned to this mall in a long time, there are random moments when memories from these trips come flooding back to me. Whenever I smell a pretzel at a Los Angeles Kings game, I instantly remember the Auntie Anne’s pretzels that I begged my parents to buy. Any time I smell the rubber of a running track, I remember the countless pairs of shoes my dad tried on in the shoe section of our Dick’s Sporting Goods store. When I smell sweat and taquitos together, I think of the hours I spent in our Nike outlet. While each memory is different, they are all connected to specific smells that can be found in other environments.
Olfaction, or our sense of smell, is often the most overlooked out of our five senses. Other animals – like sharks and some dogs – have incredibly strong senses of smell compared to humans. However, the human sense of smell is still impressive and, more importantly, useful. With just a quick sniff, a person can determine whether milk has spoiled, if something is burning in the kitchen, or if their beloved pet has had an accident nearby. Our sense of smell can help us quickly assess a situation through its connection to our memories. Olfaction is a reliable sense that can enhance any situation, whether you are baking cookies or walking through a park. The most interesting part of our sense of smell is, perhaps, how easily it can be influenced. Smell manipulation can happen for a variety of reasons, and those reasons are worth exploring; there is so much potential for growth in scent-related technology.
How Does the Nose Know?
Our sense of smell influences everyday life, but to understand how, we must first understand the biological processes behind smelling. All sensory inputs – taste, touch, smell, sight, and hearing – involve the use of sensory neurons, special cells that link the outside world to the inner workings of the brain. Olfactory sensory neurons are able to recognize scents high up in the nasal cavity in the head. When a small airborne chemical from something we can smell – say, chocolate – is inhaled, that chemical mixes with mucus in the nose [1]. This chemical-mucus mixture then excites olfactory sensory receptors – the receiving ends of the sensory neurons – that then send a complex signal up to the brain. Once these signals reach the brain – specifically, the olfactory cortex – the brain can process the signals to recognize the scent [1]. What makes our sense of smell unique is its secondary detection pathway. As shown in Figure 1, the nasal cavity is connected through a small pathway to our mouth. Because of this, when we chew food, small chemicals are released, and they can travel through this pathway to excite olfactory neurons [2]. So, not only can we smell what we sniff, but we can smell what we chew as well.
Figure 1: The olfactory sensory system. Adapted from https://www.quora.com/Does-nasal-cavity-lead-to-the-brain
Each smell is a result of the excitation of a specific combination of sensory neurons by scent chemicals. For example, chemicals from a pretzel may activate five sensory neurons while chemicals from a strawberry may activate seven totally different sensory neurons. Different foods, such as a pretzel and croissant, can produce different chemicals that activate the same neurons. Humans have about 20 million total olfactory sensory neurons with over 400 different types of receptors, so, considering the specificity and potential overlap of scent activation, you can imagine just how complex these scent signals can be [3,4]. Luckily, our brains do a great job of sifting through and identifying signals, which is why we can smell the difference between a pretzel, strawberry, and croissant. Regardless of which neurons are excited, the olfactory sensory pathway is simple: chemicals enter the nasal cavity, olfactory sensory neurons are excited, and those excited signals are recognized by the brain as distinct smells.
Smelling to Remember, Remembering to Smell
Because the olfactory sensory pathway is simple, smells can be tied back to many unique and specific memories. When asked to identify a single smell from a mixture of smells, people have a difficult time recognizing more than three individual scents [5]. In fact, as more smells are added to the mixture, people are less likely to recognize individual smells [5]. As a result, people are assumed to have odor memory – an ability to classify and remember mixtures of smells as one whole smell [6]. At the same time, the experience around a smell can be just as important as how the smell is recognized biologically. Recent studies have shown that neurons in the brain responsible for interpreting smells are also linked to recognizing locations [7]. Plus, smells have been found to improve memories of pictures and activate specific regions of the brain associated with stress and fear [8,9]. Odor memory is clearly a powerful thing. After all, the olfactory bulb – where all the olfactory neurons cluster in the nasal cavity – is directly connected to the amygdala and the hippocampus, two parts of the brain associated with memories and emotions [10]. Because of odor memory, one whiff of a smell can instantly remind us of a specific time and place, whether it be sunscreen and salt water from a beach vacation or funnel cakes and fireworks from the local fair.
Scientists do not agree, however, on the origins of human odor memory. Some researchers believe that odor memory gave evolutionary advantages to early humans as they migrated to new areas [4]. Other researchers believe that odor memory guided humans to food [11]. While these are two of many theories, the fact that our sense of smell has lasted this long shows how important it is to our survival. Evolution likely gave us our present-day ability to sniff out danger. Think about it: when we sit in a calm, safe room, we do not smell anything out of the ordinary, but the moment a fire starts, we often smell the fire before we see smoke. Because of this, we add scents to potentially dangerous substances. For example, sulfur, a stinky chemical that reeks of rotten eggs, is added to natural gas to help identify gas leaks [12]. Our sense of smell can be quite useful, and this sulfur addition to natural gas is one of many ways in which smells are used to help us today.
Controlling the Experience with Scents
While scents can help protect us from danger, companies can play with our sense of smell for other reasons, too. The most prominent example of this is seen in retail spaces. Returning back to my childhood mall, I know the scents of the pretzel stands and the Dick’s Sporting Goods store. While you read those names, you likely recognized those scents as well. This is not a coincidence; this is a clever use of “atmospherics,” a marketing concept that emphasizes the entire consumer experience, not just the product the consumer buys [13]. Essentially, a store or brand tries to create an image beyond their products; they will change aspects of their stores, like scents and lighting, to create a pleasurable experience for their customers. Brands control smells at three different levels. First, they can use a marketing scent – a widely-known, appealing scent – to attract new customers; for example, a realtor might bake cookies during an open house [14]. Next, the product scent is the smell of the actual product; this could be anything from a Target candle to a new pair of Nike shoes [14]. Lastly, the ambient scent is the smell of the shopping environment, like when a furniture store smells of leather and wood [14]. All three levels can be, and often are, controlled by brands to convince you, the shopper, to buy their product.
Not all company-controlled scents are meant to increase purchases, though. By changing the scent of an environment, a company can also enhance how immersive an experience is for a customer. One of the most noticeable examples of this comes from Disney Imagineering. While Disney theme parks certainly use a marketing scent to boost candy sales, they also use patented scent cannons called “smellitizers,” pictured in Figure 2, in their rides. These smellitizers can spray scented air up to 200 feet away [15]. Smellitizer scent options include orange groves on the Living with the Land ride, rosewater at the Taj Mahal on the Soarin’ ride, and cannon smoke on the Pirates of the Caribbean ride [15]. All of these scents enhance the ride experience, turning a slow boat ride into a pirate adventure and a floorless movie theater into a hang-gliding ride around the world.
Figure 2: The patented Smellitizer. Adapted from https://patentimages.storage.googleapis.com/b3/93/dc/88b81b0ba5937c/US4603030.pdf
But companies are not the only ones controlling scents. One of the more intriguing uses of scents is in PTSD exposure therapy where therapists expose their patients with PTSD to small amounts of scents that are connected to that patient’s traumatic event. Smells can be one of the most impactful triggers for someone with PTSD [16]. This could be for a variety of reasons, some of which have already been discussed: odors are closely linked to our memory, odors are processed in the same parts of the brain that are associated with PTSD, and invisible odor chemicals leave people with no time to prepare for exposure to a scent [9,10,16,17]. Scent-based exposure therapy has been used for Vietnam and Iraq War veterans in the United States using smells of smoke, rubber, and dirt [18, 19]. This technology could also be used on civilians, as is the case with survivors of the 9/11 World Trade Center attacks [20]. While this method of controlling scents may not be seemingly as fun as making a boat ride seem like a pirate adventure, scent-based PTSD treatment methods are arguably one of the most meaningful and useful examples of controlling scents.
Other companies are taking the scent experience even further by incorporating smells into virtual reality (VR). This year, researchers developed the HTC Vive VR system, which recognizes grasping and sniffing gestures on a handheld device so players can smell the virtual world around them [18]. This handheld device is depicted in Figure 3. From the handheld part of the VR system, scent chemicals are sprayed towards the user’s nose [18]. A system like this could be used for many things – therapy, education, or recreation – but so far, it has only been used for a VR wine cellar game where users smell eight different wines [18]. The user interacts with the handheld device as they would with a wine glass; they can swirl the virtual glass, move it away from the nose to “reset the smell,” or even make the smell more intense by moving the handheld device closer to the nose [18]. Virtual reality presents a great opportunity to further develop scent-enhanced experiences.
Figure 3: The HTC Vive VR system. Adapted from [18].
Pushing the Boundaries of Scent Technology
The aforementioned marketing scents, Smellitizers, exposure therapy, and VR games are wonderful examples of how new scent chemicals can be introduced to an environment and change an experience. But, could we make someone smell a chemical that is not there? As it turns out, this might be possible through olfactory neuron manipulation. Currently, many questions remain about how neurons translate scents into brain signals. However, scientists have made mice recognize “synthetic odors” by shining light patterns on their olfactory bulbs [21]. For comparison’s sake, imagine being able to walk into a room, sit down with a special light contraption, and smell an entirely new experience quite literally out of thin air; that is what these scientists have accomplished, just at a mouse-sized scale. These results could lead to improved VR scent-enhanced experiences, as light might be easier to incorporate into a VR setup. Needless to say, these kinds of olfactory neuron manipulation experiments are on the frontier of scent technology.
This technology also begs the question, what about people who cannot smell? As it happens, engineers are currently developing an olfactory system for an artificial nose. This artificial nose mimics the important parts of the olfactory system. Sensors replace the olfactory neuron receptors, a recognition system turns scents into signals, and a functional module replaces the nose’s mucus and cartilage structures [22]. A similar medical diagnostic tool called Deep Nose is being developed to smell compounds that our bodies produce as a result of diseases like cancer, tuberculosis, and Parkinson’s [23]. These artificial nose technologies could be used in any field that uses airborne chemical detection, including food production evaluation, environmental monitoring, and drug screening. – They could even be used to build a full bionic nose for humans [24]. The possibilities are incredibly exciting!
Conclusion
Regardless of our perception of its effects in daily life, olfaction has the power to shape our realities and help us in ways that other senses cannot. Smells are strongly tied to memories and can keep us safe. Despite this importance, scent technology has been highly underdeveloped until now. In effect, companies have been using our sense of smell – a wonderful technology already built into our bodies after thousands of years – to either maximize profits or manipulate consumer experiences. As our physical world becomes increasingly dependent on the virtual world, one of the critical barriers to fully incorporating the two worlds is olfactory stimulation. Some researchers are aware of this barrier and are developing scented virtual reality technologies. At the same time, even more advanced technologies are being developed to either mimic or replace the nose in a variety of settings. Engineers are starting to understand just how vast the field of scent technology is and we should expect to see new scent technologies in the near future. Ultimately, our sense of smell is more important than we think. Sure, we may not walk into a room and think, “I don’t smell anything,” in the way that we think, “I can’t see anything,” in a dark room. Nevertheless, it is clear that our olfactory sense affects our experiences, and a better understanding of that process will lead to countless new and interesting scent technologies.
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