When you smell a new scent that you have never come across before, you never have to stop and ponder “hmmm is this smell unpleasant?” On a daily basis, we take 24,000 breaths, each one full of molecules that interact with our olfactory receptors, allowing us to smell the chemical world we live in (Agapakis and Tolaas, 2012). While it has been shown that we are able to perceive several million colours and less than half a million different tones, new research suggests that we can smell so much more. In 1927, it was estimated humans could differentiate 6561 unique smells. Later on the value rose to 10,000 discriminable odors.
Our society has never valued smell as we have with artwork and music for the other senses. It follows that we would far underestimate the trillions of olfactory stimuli we can resolve (Figure 1). Where does this vast range of smells originate? The physical boundaries of the olfactory system are not known as it is composed of endless combinations of each component. For example a rose’s distinct smell is actually 275 molecules combined (Bushdid, Magnasco, Vosshall and Keller, 2014).
Figure 1: The number of smells compared to colours or tones that can be discriminated by the average human. The smells are shown grouped by the number of different molecules in the scents when experimentally tested. Mixtures of 30 molecules can overlap by 15 and still be discriminated, greatly increasing the number of distinct odours (Bushdid et al., 2014).
This sense of smell is vital for many aspects of life across the animal kingdom including maternal bonding, territorial defense, regulating emotional responses and mate selection (Auffarth, 2013). For something so integral to our lives, it comes naturally that we have tried over time to classify what we perceive. The first theory on the topic, from Lucretius in the first century BCE, hypothesized that good smells were smooth atoms and that bad smells were spiky atoms. Today, the differentiation between good and bad still depends primarily on physical characteristics including size, functional groups, and compactness (Agapakis and Tolaas, 2012; Haddad, Sobel and Harel, 2014).
We recognize odours as the molecules pass through our nasal cavities and are objectively classified, a process technology can replicate. eNoses can predict the pleasantness people perceive when introduced to novel smells (Haddad, Sobel and Harel, 2014). Similarly to the process in organisms, except with electronic sensors instead of olfactory receptor neurons, odorants activate sensors whose signals denote the odour footprint. Humans’ main axis for perceiving smells, pleasantness, is based primarily on the physiochemical molecular structure. Pleasant smells are typically loosely packed molecules, that depolarize more around charge or dipoles, and play a larger role in Van der Waals interactions. Bad smells are made of molecules less likely to engage in these weak interactions. Therefore eNoses can “smell” based on atomic volume, polarizability, atomic connectivity, how tightly packed molecules are, and their available surface area (Haddad, Sobel and Harel, 2014). Pleasant smells and unpleasant smells also activate different regions in the brain. Good smells are associated with the sensory network in the piriform cortex and amygdala while bad smells increase activity in the regions used for attention and working memory in the right parietal (Djordjevic, Boyle and Jones-Gotman, 2012).
For isolated smells, the perception of pleasantness based on the physiochemical structure and neural activity exists across cultures. People universally agree on bad smells out of context. In context, however, our perceptions can change across cultures. The smell of peppermint will be more pleasant in a culture that eats it more (Haddad, Sobel and Harel, 2014) and the same isovaleric acid that gives Swiss cheese its desired flavour is present in human body odor (Agapakis and Tolaas, 2012). There is minimal discussion of smell in general society and, compared to other senses, there is also less research in the scientific community. We still know so little about one of our most powerful senses, which underlies so many of our subconscious actions. We may know what properties our minds perceive as smelling bad, but we sure haven’t figured out why that perception came to be.
References
Agapakis, C.M. and Tolaas, S., 2012. Smelling in multiple dimensions. Current Opinion in Chemical Biology, 16(5–6), pp.569–575.
Auffarth, B., 2013. Understanding smell—The olfactory stimulus problem. Neuroscience & Biobehavioral Reviews, 37(8), pp.1667–1679.
Bushdid, C., Magnasco, M.O., Vosshall, L.B. and Keller, A., 2014. Humans Can Discriminate More than 1 Trillion Olfactory Stimuli. Science, 343(6177), pp.1370–1372.
Djordjevic, J., Boyle, J.A. and Jones-Gotman, M., 2012. Pleasant or Unpleasant: Attentional Modulation of Odor Perception. Chemosensory Perception, 5(1), pp.11–21.
Haddad, R., Sobel, N. and Harel, D., 2014. Predicting odor pleasantness with an electronic nose. [online] US8880448 B2. Available at: <http://www.google.com/patents/US8880448> [Accessed 1 Oct. 2015].