The Olfactory Process and its Effect on Human Behavior
Early childhood memories can be evoked by many triggers, of which one of the most powerful is a particular smell. A couple of years ago, I was unpacking boxes of Christmas decorations from the attic. One of the boxes contained old, partially melted candles that were to be put on the fireplace mantle and lit on Christmas Eve. Unrolling each uniquely fragrant candle from the yellowed newspaper, I suddenly had a vivid recollection of a childhood experience. I was between the ages of two and three, wandering through a candle store with my parents in the Greek section of Detroit, Michigan. I gazed wide-eyed at the seemingly endless shelves of wax figurines, reaching through the restraining arms of my father in attempts to feel their smooth contours. After slowly returning to reality, I realized that the smell of the candles being used to decorate for the holidays triggered my earliest memory of childhood. I thought about the memory frequently after that, and longed to return to the store to see if my physical presence there would evoke other memories. When I visited Detroit a few months ago, I was disappointed to discover that the store had long been out of business and only my new memory would remain.
I found it somewhat disturbing that my earliest memory was of an insignificant retail store
that would have no bearing on my adult life
. Why did I not remember a more significant event, such as an early Christmas, or my second birthday? The answer is that the sense of smell, that is, olfaction, has a powerful command over many behaviors, including memory. Intrigued with this connection as an example of sensory input influencing behavior, it is my goal to examine the neurobiology of the olfaction process in humans and to investigate the ways in which odors elicit particular behaviors.
For humans, olfaction is a primitive sense, whereas other mammals, birds and insects rely predominately on their sense of smell for survival. The approach of a charging bear would be recognized by a human within seconds of its attack, while a dog would have certainly caught his scent long before the human companion had any knowledge of the bear's presence. Although smell seems far less meaningful to humans, there is an important link between olfaction and behavior. For example, in my memory described above, the simple visual cues provided by the sight of any ordinary candle does not evoke my memory of the sweet smelling store. Rather, I have to rely on my olfactory
processing of a particular waxy scent in order to recall the detail of the store, its hundreds of wax figures and my childhood perspective of the event.
The world as we know it is filled with fragrances, from sweet smelling foods and beauty products to the unpleasant smells of pollution and chemical supplies. How is it possible for humans to distinguish and discriminate between the millions of odors present? To answer this question, one must start at the beginning, with an individual odor molecule. With such a diverse array of fragrances in the world, the shape of an individual odor molecule is unique to the emitting substance (1). When inhaled, the odor molecule is absorbed in the nasal passage and binds to chemoreceptors in the olfactory epithelium, which are specific to certain odor molecules (2). This binding initiates a change in the permeability of the sensory neuron, which creates a slow electric potential that travels to the olfactory bulb (3). From the olfactory bulb, the transmitted signal is sent to the limbic system of the brain for further processing (2). Recognition of the odor occurs in the limbic system when the signal is interpreted through a comparison to past experiences with the odor and relation of the smell to the emitting substance (4).
A couple of prominent features of the olfactory system deserve some close attention. First, it is interesting to note that olfactory neurons, unlike most other types of neurons, are produced in constant supply with the regeneration of new cells approximately every 60 days to replace the dead olfactory neurons (5). Researchers are currently studying the possibility that insulin or other hormones may be responsible for the capability of olfactory cell growth, which is unique to the mammalian central nervous system (3). Second, the limbic system of the brain, which receives information from the chemoreceptors about a particular odor, not only mediates mood and emotion, but also serves as a memory storage area (4). This common junction, where memories, emotions and odors meet, explains why smell is often an intense trigger for distinct memories and potent emotions (4). When perceiving a particular aroma that is associated with a past memory, the recognition of the odor in the olfactory process will simultaneously evoke the correlated memory. Third, in just one square inch of the brain, humans have the capacity to process about 10,000 different odors (6). The majority of aromas perceived involve a complex organization of hundreds of odor molecules, and through the simultaneous recognition of the individual odor molecules, a complex scent is recognized as a whole equal to the sum of its parts (1).
Determining the links between specific odors and a produced behavior is a technique that scientists are still trying to perfect. However, in one documented case, conducted by neuroscientists at Tufts Medical School and New England Medical Center, a salamander's perception of a particular odor will evoke a change in a its skin potential (5). Elsewhere, other scientists attempt to learn about the details of emotional response generated from odors in the new science of aromachology, which is often mislabeled aromatherapy. The practice of aromatherapy has been documented since the days of Cleopatra, in which she allegedly used perfumes and aromatic oils to seduce Julius Caesar and Mark Antony (7). This practice involves the therapeutic use of herbal extracts as natural, "essential" oils to soothe the body, spirit and mind (8). Aromachology, on the other hand, is a newly developing science in which the effect of odors on behavior is scientifically measured through carefully controlled experiments (9). Developed in 1982 by the Olfactory Research Fund, aromachology was designed "to scientifically study the interrelationship of psychology and fragrance technology" by analyzing the emotions produced when odors activate the olfactory pathways that lead to the limbic system of the brain (9). In this region, odors initiate the release of neurotransmitters, which can affect the brain and mental state of an individual in a variety of ways (10). Serotonin produces a relaxing, soothing effect while endorphins inhibit pain and increase sexual arousal (10). Studies have shown that the use of certain aromas can benefit mood, behavior and productivity by determining what neurotransmitters will be released. For example, peppermint is perceived to be a mental stimulant that can increase a traveler's alertness while on a long journey, while vanilla, a mental relaxant, relieves stress, especially for uneasy MRI patients (11). The field of aromachology is relatively new and thus continues to develop through continual advances in science and technology.
My research on the olfactory processing has left me with two final thoughts. First, if the sense of smell is so vital to other animals, why is it a less prominent feature in humans? The reduction of olfaction appears to be an evolutionary disadvantage that violates the laws of natural selection. Clearly, the ability of any animal to sense a predator before its attack is a vital survival tactic. If it is such a critical characteristic, why then is olfaction suppressed in humans? Second, we often take for granted the importance of our sense of smell. But can anyone really imagine a world without odors? Without the sweet fragrances of spring or smell of freshly baked cookies, life would be less enjoyable. As primitive as it is, to lose our ability to stop and smell the roses would be a terrible tragedy.
1)Scent of a Market
2)The Physiological Principle of the Olfactory Method
3)Monell's Neuroscience of Olfaction
4)How Do You Smell?
5) The Mystery of Smell
6)Cleopatra's Secret: Aromatherapy Massage
7)Aroma-Chology vs. Aromatherapy
8)Distinctions Between Aroma-Chology and Aromatherapy
10)Aroma-Chology: The Psychology of Scent