An Intro to the Perception of Smell

Smell, also known as olfaction, is the general term for sensing odor molecules, whereas smell perception is the ability to experience a smell. While this distinction may sound minimal, it plays an important role in distinguishing different lines of research.


What is smell perception?

Smell perception is the ability to identify and interpret scent signals as they appear in the direct environment.


Smell is not something that arises from odors alone, but rather is a perceptual feat that relies on the inner biological workings that are found in humans or other species.[1]


Since inner workings, such as genes and experiences, vary from individual to individual, making smell perception differ as well. Despite this variation, researchers have identified that humans use ‘pleasantness’ as the primary dimension when describing how they perceive odors.


Smell Perception is Plastic



Perceiving an odor is a plastic, or an easily shaped, phenomenon which highly depends on your past experiences, but also the current sensory context.


Thus, smell perception is not fixed and can change overtime based on many factors. For example, the more you are exposed to a particular scent, the more sensitive you become to it and the faster you become at identifying it.[2]


When anxious, odors that were previously considered neutral may be perceived as irritating,[3] just one of the many examples of how scent and emotions are connected. Thus, smell perception is an intricate experience that is not easily predicted.


Smell is also intertwined with other senses, such as taste or even sound. Did you know that people that smell a fragrant tree scent while hearing noise traffic will rate the traffic as being less obnoxious compared to people that are in an odor-free environment?[4] Such sensory feats are being considered by urban planning authorities as another reason to increase green areas in cities in order to improve public health and well-being.


Nature vs. Nurture for Smell Perception

The nature vs. nurture debate stems from psychology and questions whether nature (i.e. genetics) or nurture (a person’s environment or external factors) are responsible for human behavior. The same framework can be applied for understanding smell perception.


Individual differences influence smell perception, such as age or cognitive skills, which are essentially a complex interaction between one’s nature and environment.


External or Environmental Determinants of Smell Perception

Factors that are beyond an individual's immediate genetic profile have been found to significantly influence smell perception, this includes treatments like chemotherapy,[5-6] habits like smoking,[7] and even advertising.[8]


Smell perception is also affected by the way that a smell is taken in by the organism, including the volume, duration, and velocity of the sniff.[9] Collectively, these factors influence the stimulating effectiveness of odors entering the olfactory system.


Your current physical state also plays a role in your perception of smell. For example, if you are fatigued, smell perception becomes more blunt.[9]

How you feel about a smell is also a highly subjective experience that varies from person to person. For example, you might rate the smell of oranges more positively than someone else because it reminds you of a happy childhood experience of playing in an orchard.

Genetic Determinants of Smell

Genes give rise to the structure and content of the olfactory system and receptors. Subtle differences or mutations in genes can thus steer the perceptual experience of smell on an individual level.


Recent findings have shown that just changes to a single receptor can influence odor perception. A study by Trimmer et al. established that people that have a less-functional OR11A1 receptor perceive the 2-ethylfenchol odor molecule less intensely than those with the fully functional version of the OR11A1 receptor.[10]



Genetic conditions can also alter the ability to perceive odors through abnormalities to the olfactory system.


For example, smell is compromised in neurodegenerative disorders, including Alzheimer’s disease and Parkinson’s disease. Both diseases are are typified during their early stages by observable olfactory deficits.[11]


Examples of Altered Smell Perception in Health-Related Conditions

Smell perception can be compromised by many health-related conditions, including conditions that are commonly encountered by the general population such as diabetes, COVID-19, and even mental health.



Numerous instances have been reported where mental health conditions such as depression are correlated with decreased smell perception.[12] This decreased olfactory perception may even be observed sometimes in sad but otherwise healthy individuals. While a direct link has yet to be established, researchers have hypothesized that the reduction in olfactory sensitivity may be due to a reduction of volume of the olfactory bulb or the ability to encode odor-related information.


In other conditions, such as COVID-19, possible smell loss is one of the key defining symptoms of the disease. While the mechanism is still under investigation, smell loss in COVID-19 patients has been highly prevalent and observed across all regions of the world during the course of the pandemic.[13]


Type 2 diabetes patients have been observed to have altered smell and taste perception when compared to healthy controls. Decreased smell perception is even more common about diabetes patients that are also hypertensive. While the mechanism is unknown, glycemic levels may be implicated in the pathophysiology of distorted smell perception in type 2 diabetes patients.[14]


Measuring Smell Perception

Since smell is a subjective experience, researchers have developed systematic approaches for categorizing smell in order to study smell perception in greater depth. The most commonly measured factors of smell perception are: familiarity, intensity, and emotional valence toward the scent.[12]


Specialized categorization systems exist for certain items, such as the beer flavor wheel or the wine aroma wheel. There are also various categorization approaches for natural odors. Despite these options, there is no classification system that has been universally accepted for the purposes of odor classification.[1]


Advances in research methods and statistics are beginning to show that there are certain odors that are similar based on perceptual similarity, even across various cultures. These perceptual categories include fruity, sweet, earthy, resinous, smoky, camphoraceous, and herbaceous. Other studies have suggested that at least 32 odor categories should exist in order to classify smell perception, including fishy, malty, urinous, sweaty, and musky.[1]


Conclusion

Any ideas about how to tie smell perception to scent recognition technology? What is our message?


Smell perception is a subjective feat that guides and influences many parts of daily life. Since perception is highly specific to the individual, relating to emotions and memories, it is difficult for scientists to predict with full certainty how a smell may make a person feel.


In some cases, human smell perception does not work as well as it should and smells go undetected, like if you are sick or suffering from a health condition. In other instances, genetics may play a role in how a smell is perceived.

Because of the elusive world of smell, scent recognition technology plays an important role in quantifying smell and making odors more accessible for research. This is why NanoScent is committed to developing unique sensing elements, so that scent recognition technology can be applied to wellbeing and healthcare, ultimately helping people make better decisions

References

  1. Auffarth, B., 2013. Understanding smell—The olfactory stimulus problem. Neuroscience & Biobehavioral Reviews, 37(8), pp.1667-1679.

  2. Yeshurun, Y. and Sobel, N., 2010. An odor is not worth a thousand words: from multidimensional odors to unidimensional odor objects. Annual review of psychology, 61, pp.219-241.

  3. Krusemark, E.A., Novak, L.R., Gitelman, D.R. and Li, W., 2013. When the sense of smell meets emotion: anxiety-state-dependent olfactory processing and neural circuitry adaptation. Journal of Neuroscience, 33(39), pp.15324-15332.

  4. Ba, M. and Kang, J., 2019. Effect of a fragrant tree on the perception of traffic noise. Building and Environment, 156, pp.147-155.

  5. Spotten, L.E., Corish, C.A., Lorton, C.M., Dhuibhir, P.U., O’donoghue, N.C., O’connor, B. and Walsh, T.D., 2017. Subjective and objective taste and smell changes in cancer. Annals of Oncology, 28(5), pp.969-984.

  6. Drareni, K., Bensafi, M., Giboreau, A. and Dougkas, A., 2021. Chemotherapy-induced taste and smell changes influence food perception in cancer patients. Supportive Care in Cancer, 29(4), pp.2125-2132.

  7. Da Ré, A.F., Gurgel, L.G., Buffon, G., Moura, W.E.R., Vidor, D.C.G.M. and Maahs, M.A.P., 2018. Tobacco influence on taste and smell: systematic review of the literature. International archives of otorhinolaryngology, 22(1), pp.81-87.

  8. Koubaa, Y. and Eleuch, A., 2021. Multimodal perceptual processing of cues in food ads: Do you smell what you see?: Visual-induced olfactory imagery and its effects on taste perception and food consumption. Journal of Advertising Research, 61(1), pp.78-94.

  9. Ennis, M., Puche, A.C., Holy, T. and Shipley, M.T., 2015. The olfactory system. The rat nervous system, pp.761-803.

  10. Trimmer, C., Keller, A., Murphy, N.R., Snyder, L.L., Willer, J.R., Nagai, M.H., Katsanis, N., Vosshall, L.B., Matsunami, H. and Mainland, J.D., 2019. Genetic variation across the human olfactory receptor repertoire alters odor perception. Proceedings of the National Academy of Sciences, 116(19), pp.9475-9480.

  11. Gottfried, J.A., 2006. Smell: central nervous processing. Taste and smell, 63, pp.44-69.

  12. Schablitzky, S. and Pause, B.M., 2014. Sadness might isolate you in a non-smelling world: olfactory perception and depression. Frontiers in psychology, 5, p.45.

  13. Cetinkaya, E.A., 2020. Coincidence of COVID-19 Infection and Smell—Taste Perception Disorders. The Journal of craniofacial surgery.

  14. Catamo, E., Tornese, G., Concas, M.P., Gasparini, P. and Robino, A., 2021. Differences in taste and smell perception between type 2 diabetes mellitus patients and healthy controls. Nutrition, Metabolism and Cardiovascular Diseases, 31(1), pp.193-200.