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What Your Nose Knows: Smelling is Believing...or Deceiving?

Jackie Marano's picture

When asked to define ‘reality’ or to consider the ways by which one concludes that a particular physical reality ‘exists,’ the average human will tend to reference their visual, tactile, and auditory abilities, and probably in that order too. These modes through which the majority of humans assess and process their surroundings are three major and essential contributors to how we, as humans, define the existence of a physical reality. However, a fourth mode of perception that is also extensively used and trusted, but whose scientific, historical, and practical significance is severely underestimated is our capacity to smell. Our ability to distinguish the reality of a scent and to link this reality with other perceptions (visual, tactile, or auditory) is perhaps so widely undervalued because much of this processing occurs without our immediate knowledge and is not realized until some later point in time in the form of a memory. In a very general sense, the extent to which all of these methods of identifying reality are shared, trusted, and employed among individuals and populations of humans seems to suggest that they convey to us the existence of something that is ultimately real. However, in the context of such a proposal, can we reasonably assert that all senses equally provide or contribute to our acknowledged physical realities? Contrary to popular belief, this is not likely the case; in fact, if we had to choose the one sense that stands out among the rest, it would unarguably be our sense of smell. Perhaps the most fundamentally, scientifically, and historically significant, our sense of smell may play a larger role in conveying reality to us than the other modes of perception. A brief consideration of the scientific, neurobiological, and evolutionary implications associated with our sense of smell, and its correlation to behavior and memory will allow us to better understand the true power of this sense, which may indeed be our most trusty detector (and creator) of physical reality.


What is it that potentially makes our sense of smell more significant than all of our other senses? The uniqueness of this scent is identifiable at the small scale of olfactory receptor cells (neurons) of the olfactory membrane, where all olfactory signal transduction processes begin; olfactory receptor cells are the only neurons in the entire nervous system that are regularly regenerated and replaced (1,3). It is also noteworthy that olfactory receptors are the only sensory receptors that are exposed to the outside world without intermediary (5). The continuous maintenance of fresh and functioning neurons that directly interact with the environment and send signals to the brain indicates that there must be some underlying and possibly evolutionary importance correlated with this sense. Additionally, the nature of the pathway by which a scent is relayed to the brain is also different in that the message travels via a direct route to many parts of the brain (3). This occurrence can potentially be explained through studies that have proposed that our sense of smell is evolutionarily older than those associated with sight and hearing, whose messages are relayed via a less direct route (they are deciphered at a relay center in the cerebral hemisphere before they reach other areas of the brain) (3). Furthermore, our sense of smell is, in the neurobiological sense, very close to several critical processing centers of the brain; there are no more than two synapses that separate the olfactory nerve from key components of the limbic system, which is responsible for our capacity to experience emotions, various forms of memory, and also emotional memory (5). Despite the proximity of these systems, the olfactory process is actually one of the slowest of all sensing processes; the lack of myelin surrounding olfactory neurons retards the rate by which an otherwise well-insulated charge would travel to the brain. (5). It seems reasonable to believe, then, that the balance of proximity and directness with measured transmission of messages in our olfactory system attributes it with increased purpose and reliability with respect to the reality that it ultimately establishes. But even beyond its characteristic molecular and biological uniqueness, it also plays a somewhat authoritative role in influencing of behavior and memory, which further legitimizes the potentially more genuine and trustworthy nature of smells.


While the visual and auditory senses are commonly linked to higher functioning, olfactory abilities are broadly associated with emotions and other behaviors (3). Studies of olfactory neurons and related phenomena in salamanders suggest that there are, indeed, correlations between the olfactory system and behavior; researchers have been able to train salamanders to alter the chemical composition of their skin upon perception of a specific odor (1). Additionally, olfaction has proven significance in the learning processes of animals such as rats and chipmunks, which been trained in labs to avoid/find objects or to perform certain tasks with remarkable precision based solely on their ability to distinguish particular smells (5). Even at the level of humans, behavioral responses to particular scents are displayed through everyday smell preferences, or even at specific moments or periods of one’s life; for example, women develop increased sensitivity to scents during pregnancy, individuals prone to epileptic seizures sometimes report experiencing olfactory hallucinations before an epileptic event, and certain odors are have been known to prevent the onset of seizures (4). Also, it is well-known that as humans age, the precision of their visual and auditory functions declines to such an extent that only manmade devices can reestablishe the perceived realities of their younger years. However, is there evidence that our capacity to smell diminishes significantly as we age? I am not aware of the existence of ‘smelling-aids,’ so does this make the reality that we perceive through our olfactory system more credible? Is our sense of smell (which ultimately establishes our sense of taste) more credible in the long run, since it may be the only sense with which the realities of our earlier life match those of our later life, with no ‘corrections’ necessary? While the link between olfactory sensations and physical behavior is certainly noteworthy, credible, and apparent in humans and more sophisticated life forms, a much more mysterious, yet intriguing, form of a relationship is that which exists between smell and various forms of memory.


Perhaps the best way to approach this topic is with the simple analogy that each smell is a ‘key’ to a highly specified ‘lock’ in the brain, and that the brain is merely a medium through which a bank of smell-memories operates (2). It is widely accepted that many forms of memory are stored in the left dorsal prefrontal cortex of the brain, and that they are retrieved in the right prefrontal cortex region, where both the processing of smells and the recollection of emotional events have also been identified (2). Such observations may account for the well-known phenomenon of Proustian Memory, in which a smell from one’s past can lead to a sudden realistic or graphic memory that is often correlated to particular emotions (2). However, there is more to the relationship between smells and memory than simply accessing the ‘locks’ of our memory bank with the proper ‘key,’ and subsequently reacting to them with emotion. The exact nature of establishing, loading, and accessing this memory bank with the proper ‘keys’ and the influence of such these processes in exciting or initiating emotions as a function of memory remains a mystery, but is worth some thought. How do smells actually generate or excite other forms of memory that become so ‘real’ to us? Can emotions effect what we believe we smell, or is it always a scent initiates the feeling of emotion? Does the environment in which the smell is perceived affects our perception of the smell?


It is important to distinguish that there are memories for odors and memories that are elicited by odors. One way in which we likely establish our memory bank is through ‘conditioned responses,’ by which a newly-introduced smell is immediately (and perhaps unconsciously) associated with an event, a person, an object, or a specific instant in time (6). But how does the perception of the smell itself relate to the memory that is established? For example, is it possible to actually dislike a smell, but to ultimately like the smell because of a positive memory that it conjures, or vice versa? Or is it possible to prefer one smell to another because our other senses influence what we actually smell, and, in turn, invoke additional memories associated with this smell? Is it necessary, that a specific environment nurtures every perception of a smell and the memory that it summons, or can preferences for particular scents and memory associations develop as a part of human nature? Consider the fact that humans in a tropical rainforest avoid consuming poisonous plants that have a toxic scent and also avoid eating decaying organisms because of a certain unpleasant scent, while humans in more industrialized countries generally avoid consumption of household cleaning solutions that have a toxic/chemical smell (like Windex) and readily dispose of food that smells rotten. In this case, we note that very distant populations of humans use their sense of smell to establish similar physical realities that lead to the same fundamental actions or behaviors. But why then, do we also observe such immense differences in smell preferences among individuals and populations of humans worldwide? For example, how would a member of an African tribe react to the smell of freshly baked cookies or homemade macaroni and cheese? The smell will likely not initiate visions of a caring mother or grandmother bringing steaming hot and delicious food to the table at a family holiday gathering. Should we reasonably expect these tribal members to associate these smells with a form of happiness or nourishment at all? Would such smells make their stomachs growl? That is, would they perceive the same physical reality or associated images as the average American upon encountering such scents? Perhaps the lack of shared memories and shared similar experiences would account for the expected difference in behaviors (whether learned or not) or as a consequence of the input of scent into the nervous system.


The results of an experiment conducted at the University of Oxford suggest that there is, in fact, a link between cognition, our other senses, and the nature of our smell perceptions (6). In this experiment, an “ambiguous Brie-like scent” was labeled as either ‘cheddar cheese’ or ‘body odor’ (6). The results of the test indicated that the subjects had increased preference for this scent when they saw that it was labeled ‘cheddar cheese’ over when it was presented to them as body odor (6). Even more interesting is that MRI scans of the subjects indicated greater activity in the olfactory region of the brain when the subjects believed they smelled the ‘cheddar cheese’ version of this scent (6). If such altered perceptions, reactions, and behaviors are distinguishable within an individual, it seems highly probable that one smell can lead to differing behavioral outputs among individuals of different upbringing, ethnic background, or exposure to different experiences that generate or contribute to a sort of ‘scent package’ that is composed of linked memories.


If we return to the idea of the bank of locked smell-memory boxes, we could objectively state that this one ambiguous-cheese scent belongs in one box that is only accessible with the proper key. However, it appears that the test subjects placed this one particular scent into two locked boxes, and that they preferred one box over the other. Does a different ‘key’ open each of these boxes? Or is there still only one box, but multiple keys? Whatever the case, it appears that the mechanism that allows us to access or unlock these memory banks is one that is entirely separate that which enables us to characterize what we have actually locked in this box. Perhaps it is the greater nervous system perceives and detects the smells, and that decides which key it will give to the another portion of the nervous system, which will not only unlock the box, but enhance or degrade the nature of what is locked in the box. This more subjective role in this nervous system relay is likely performed by the I-function, a hypothetical region of the brain that we can use to identify the sense of the ‘I’ or the ‘me’ that is the main character in our own individual story of reality. So then, to what degree to auditory, visual, and tactile senses influence the contents of a box that we open so squarely with our sense of smell? Is it possible that our sense of smell could never actually function by itself, but that it must rather share or compromise its seemingly trusty and precise contribution to reality with that of the other senses? Does the I-function complicate or overexploit its privileged access to the blend of realities stored in this bank that is created and maintained by the rest of the nervous system? Does the I-function’s role in complicating and intertwining our perceptions of reality benefit us, in that it provides us with an averaged mixture of realities that is cumulatively more trustworthy in the end? Or, is it perhaps blending our perceptions so much, that the greater blend is actually less reliable of a reality than all of its parts? Would its parts even function practically or realistically in isolation? One way to test this hypothesis might be to investigate the magnitude or potency of emotional memory and memory in general on subjects who lack one or more senses, and to compare their perspectives on reality, their preferences, and their psychological/emotional status to those who possess all of their senses. However, if it is agreed that there does exist a certain form of reality outside of the nervous system, such experiments may only identify differences in behavior that result from varied precision with which the I-function manipulates perceived physical realities; these experiments would not in any way allow us to judge how accurately a different blend of realities might correspond to what really exists outside of the nervous system.


Evidently, beyond scientific reasons, there are also social and emotional factors contribute to associated memories, which likely account for some of the different scent preferences among individuals. Not only do such factors influence the reality that is ultimately convey to us, but they force us to question whether it is possible to perceive the greater reality in its purest form. Observed relationships between numerous biological, physiological, neurological, and evolutionary phenomena and the olfactory mode of perception suggest that this method of sensing reality has greater immediate purpose than the other senses, and that it potentially brings us closest to a greater ultimate reality. We have concluded that, while our olfactory capacities may be the most trustworthy modes by which we perceive reality, the inability to examine this mode of perception in complete isolation from the other senses makes us question the pureness of the realities we perceive. And since we cannot isolate this mode of perception, we can only argue that it usefully operates off of a memory bank, and that it works with this bank to define physical realities in a continuous way. And although we have contemplated the existence of a memory bank, what is it that is ultimately doing the remembering? Does the nervous system remember, while the I-function forgets, that is, until it is handed the ‘key’? While we cannot be certain of how ‘real’ our reality actually is, it appears that enjoy having such experiences. We all want to be able to see, to hear, to smell, to taste, and to touch; if we lack one or more of these abilities, we find that our overall quality of life declines, unless we can fix or habituate to these losses. Perhaps what is real is that there is no such thing as fake. Even if we constantly manipulate some greater genuine reality to the extent that it does not exactly match was may truly be outside the nervous system, is our fundamental ability to manipulate not quite real?



1. “The Mystery of Smell: The Memory of Smells” Howard Hughes Medical Institute (accessed 26 April 2008).

2. “Great Moments in Science: Smell and Memory 2” ABC News (accessed 20 April 2008).

3. “The Olfactory System: Anatomy and Physiology” Macalester College Neuroscience (accessed 26 April 2008).

4. “Disorders of Smell” Macalester College Neuroscience (accessed 26 April 2008).

5. “Olfaction and Memory” Macalester College Neuroscience (accessed 26 April 2008).

6. “Smell and Memory” (accessed 23 April 2008).




Paul Grobstein's picture

smell as a special sense ...

Maybe the strongest evidence is how much money we spend on perfumes, toilet water, deoderants, and the like?