Physics of the Senses 101: Olfactory Perception or Smell

Foreword

Human beings are provided with a variety of senses that help them navigate the world around them, including five basic ones: sight, hearing, touch, smell, and taste. The organs associated with each of the latter report on certain sensations to the brain, which then translates them into understandable information, via a complex, yet fascinating process. Bright colours, a loud thud, an acute pain, a familiar flavour on the tongue or a sweet smell that tickles the nose – all these stimuli are put together into one big picture for us to identify our surroundings. However, while these systems are remarkably sophisticated ones in humans, some animals have super sensors. Felines are well-known for their nocturnal vision, elephants have the most powerful nose of the animal world, while bats rely on sound waves to hunt. Magnetoreception – the ability to detect the Earth’s magnetic field, is even considered a sixth sense that birds, along with certain mammals, reptiles, and fish, are gifted with.

The Physics of the Senses 101 series offers to explore the physical processes that make up each sense, including the so-called sixth sense, and to explain the extent to which they grant certain species 'super-capacities'.

1. Physics of the Senses 101: Visual Perception or Sight

2. Physics of the Senses 101: Auditory Perception or Hearing

3. Physics of the Senses 101: Haptic Perception or Touch

4. Physics of the Senses 101: Olfaction or Smell

5. Physics of the Senses 101: Gustatory Perception or Taste

6. Physics of the Senses 101: Magnetoreception or a Sense without a Receptor

A survey by the McCann Workgroup titled The Truth About Youth found that, out of 7,000 young people around the world, 53% of those aged 16-22 and 48% of those aged 23-30 would rather give up their sense of smell over their phone or laptop (2011). Yet olfactory perception, i.e., ‘a process that starts in the nose with the stimulation of olfactory sensory neurons in higher cerebral centers, which when activated, makes us consciously aware of an odor’ (Slotnick & Weiler, 2009, p.3007), is essential for survival. Natural environments are indeed filled with odors that allow animals to locate food sources, identify mates, or avoid predators: ‘[i]ts sensory basis is chemosensory receptor cells which show astonishing similarities in all species from snails to higher vertebrates’ (Shulten & Schiltz, 1992, p.13). ‘From an evolutionary perspective’, Purves explains in Neuroscience (2004), ‘the chemical senses —particularly olfaction—are deemed the “oldest” sensory systems; nevertheless, they remain in many ways the least understood of the sensory modalities’ (p.337). This article will therefore examine the mechanisms involved in smell, then explore its links to memory and even disease detection, before detailing the abilities that a super nose provides animals such as dogs, bears and elephants with.

Figure 1: Olfactory Discrimination (Armitage, 2022)

To begin with, ‘sensory systems must map accurate representations of the external world in the brain’, explains neuroscientist Imai (2010, p.1). However, while ‘touch and vision build topographic representations of the spatial coordinates of the body and the field of view, the chemical sense of olfaction maps discontinuous features of chemical space, comprising an extremely large number of possible odor stimuli’ (Imai et al., 2010, p.1). Further, smells cannot be classified by a simple parameter, unlike light or sound with wavelength or frequency, respectively (Su et al., 2009). Olfaction, in humans and mammals in general, originates in the nose. The primary role of the nasal cavity is to humidify and warm up the air inspired through the nostrils, then to remove minute airborne particles or debris before they reach the lower airways. Orthonosal or retronasal airflow then transport odorant particles up to the olfactory epithelium at the apex of the cavity (Sobiesk & Munakomi, 2021). The olfactory epithelium is a type of pseudostratified columnar epithelium, that is a sheet of neurons and supporting cells lining the nasal cavities (Purves et al., 2001).

The particles subsequently attach to receptors on cilia, which transmit specific signals up through the cribriform plate. The axons arising synapse directly onto neurons in the olfactory bulb, which in turn ‘sends signals through the olfactory nerve (CNI) into the secondary neurons for higher processing before entering the brain’ (Sobiesk & Munakomi, 2021, p.2). A single receptor cell can detect only one odorant type, and can’t regenerate, which is a unique feature of the olfactory system. Projections are then sent to the pyriform cortex in the temporal lobe along with other structures in the forebrain. Another specificity of the olfactory system is that it does not include a thalamic relay from primary receptors to the neocortical region, which usually processes the sensory information, making the pyriform cortex a specialized processing center dedicated to olfaction (Purves et al., 2001). Moreover, the pyriform cortex is considered to be phylogenetically older than the neocortex, and the olfactory tract also projects to the hypothalamus and amygdala, which proceed to identify the odorants and initiate ‘appropriate motor, visceral, and emotional reactions to olfactory stimuli’ (Purves et al., 2001, p.339).

Figure 2: Organization of the Human Olfactory System (Purves et al., 2004)

The olfactory system is similar to other sensory systems, however, in that it detects a wide range of stimuli and discriminate them (Su et al., 2009). The complexity resides in the fact that odorant identities are diverse, hard to define, and ‘[w]hether analogous maps of specific odorants (e.g., rose or pine) or odorant attributes (e.g., sweet or acrid) exist in the olfactory bulb or pyriform cortex’ is not yet certain (Purves et al., 2004, p.339). The most widely used classification to study since the 1950s thus divides odors into categories ‘based on their perceived quality, molecular structure, and the fact that some people, called anosmics, have difficulty smelling one or another group’ (Purves et al., 2004, p.339). This classification, including pungent, floral, musky, earthy, ethereal, camphor, peppermint, ether, and putrid odors, remains entirely empirical, although its longevity ‘makes clear that the olfactory system can identify odorant classes that have distinct perceptual qualities’ (Purves et al., 2004, p.339).

Humans indeed have superb olfactory discrimination, being able to tell more than 1 trillion stimuli apart thanks to the combined output of around 400 subtypes of receptors (Bushdid et al., 2014). Such discrimination depends on ‘combinatorial coding and on circuit-level interactions at multiple steps of olfactory processing’, which can be enhanced by learning (Su et al., 2009, p.1). Moreover, ‘any two individuals differ by ∼30% of their olfactory receptor subtype genome’, suggesting the potential for a distinct nose for everyone (Secundo et al., 2015, p.2). A 2015 study has indeed developed a highly sensitive perceptual test to capture the olfactory fingerprint that each person exhibits through a nearly unique olfactory genome. Such a fingerprint was found to be odor specific but descriptor independent and showed the potential to predict aspects of immune regulation, as well as revealing meaningful non-olfactory genetic information (Secundo et al., 2015).

Figure 3: Spring Scents (Lucid Eve, n.d.)

Lastly, the bond vibration-assisted olfactory theory argues that human smell perception is not influenced by the shape of the molecule, but by oscillations in which electrons quantum tunnel across energy gaps in the receptors (Hoehn et al., 2018). A recent study tested this hypothesis by exciting the molecular bond oscillation in odor molecules using infrared light and measuring differences in their perception thanks to the spin-residual information contained in the emitted infrared photon (Bandari, 2019). 23 subjects were given a citrusy and a musky smell illuminated under three different conditions (two infrared and one non-infrared wavelengths) and asked to rate the intensity of their smell perception for each inhale. The researchers found that the intensity of the smell was in each case affected by external infrared illumination, suggesting that quantum spin-residuals indeed play a role in olfactory perception (Bandari, 2019).

Heightened sense of smell or hereditary hyperosmia, granted Joy Milne, a retired nurse from Scotland, a peculiar faculty: she was able to identify progressive stages of Parkinson’s disease, the second most common neurodegenerative disorder (Sarkar et al., 2022) that afflicted her husband for years. She has worked with doctors and researchers since his passing in 2015, helping a team from the University of Manchester to develop a simple swab test to detect Parkinson’s within three minutes (Osborne, 2022). The scientists identified that certain lipids of high molecular weight are significantly more active in the sebum of people suffering from Parkinson’s disease, which Milne was able to scent on her husband’s upper back region (Sarkar et al., 2022). The skin swab test, which has shown 95% accuracy under laboratory conditions, could substantially improve the diagnosis and symptom management of Parkinson’s disease, for which there is currently no known cure (Lee & Yankee, 2022).

Figure 4: Olfactory Memory (Scent Fie, 2018)

Besides, recent research suggests that human hippocampal connectivity is stronger in olfaction, which may be explained by the fact that olfaction retained it’s a relative structural conservation throughout mammalian evolution, unlike the visual, auditory, and somatosensory systems. Such olfactory-hippocampal connectivity also oscillates with nasal breathing, suggesting that smell holds the potential to provide insight into the correlations between memory, cognition, and hippocampal interactions (Zhou et al., 2021). The detailed nature of the olfactory system along with its direct route to the limbic system, which is the part of the brain involved in behavioral and emotional responses, specifically the amygdala and hippocampus, explains the close relationship between smell, emotion, and memory (Zhou et al., 2021). While odor memories are not necessarily more accurate, they tend to be more emotionally evocative (Castellanos et al., 2010), having the potential to spark nostalgia, anguish or even PTSD, and a surge in study over the last decade has bettered our understanding of odor memories, as well as their ability to boost or heal our brains (Khamsi, 2022).

Such a connection was identified through rodent studies, which allowed gain insight into how smell and memory are coded together in the brain. As mentioned earlier, ‘the functional organization of the olfactory system is remarkably similar in organisms ranging from insects to mammals’ (Su et al., 2019, p.45), suggesting that many discoveries on the olfaction of other mammals could be applicable to humans.

‘By watching rodents navigate mazes guided by memories of odours, scientists are getting a sense of how neurons in the brain store this information. And there are also insights into the psychological elements of odour memories in humans’ (Khamsi, 2022, p.4).

For instance, in a 2014 experiment, rats were trained to choose the correct direction through a maze and succeeded more than 85% of the time after three weeks of training. An analysis of their brain recordings revealed that cells in the hippocampus and two other brain regions emitted electrical signals in sync as the animals learnt to respond to the odorant cues (Igarashi et al., 2014). A follow-up study later determined dopamine to be a key molecule in consolidating associative memory (Lee et al., 2021). Two other papers on mice established that in the short-term, exposure to familiar odors triggers an attenuation of their sensing to emphasize new scents instead, while in the long-term, ‘two radically different scents [can] map to a similar region of the cortex’, explaining why our unique personal smell memories can be a concoction of various odours’, and further confirming that experience shapes the association of smell memories (Khamsi, 2022, p.4).

Figure 5: I Smell A Rat Cartoon (toons, 2012)

To carry on, ‘[i]n humans, olfaction is often considered the least acute of the senses, and a number of animals are obviously superior to humans in their olfactory abilities’ (Purves et al., 2014, p.339). Many species indeed have a larger number of smell receptor neurons, odorant receptor molecules in the epithelium, as well as a proportionally larger forebrain area devoted to olfaction.

‘In a 70-kg human, the surface area of the olfactory epithelium is approximately 10. In contrast, a 3-kg cat has about 20 of olfactory epithelium. Similarly, the relative size of the olfactory bulb and related structures versus the cortical hemispheres in a rodent or carnivore is quite large compared to that in humans’ (Purves et al., 2014, p.339).

Canines are most well-known for their extraordinary olfaction, which seems to be their main sense, and it is very specialized, sensitive, and it has a ‘precision far exceeding the analytical capabilities of most modern instruments’ (Kokocińska-Kusiak et al., 2021, p.1). They have thus been used as ‘super-sensitive mobile area scanners, detecting specific chemical signals in real time in various environments’ (Kokocińska-Kusiak et al., 2021, p.1), and yielding outstanding results in a variety of fields including scent tracking, along with ‘the detection of drugs, explosives, and different illnesses, such as cancer, diabetes, or infectious diseases’ like Covid-19 (Kokocińska-Kusiak et al., 2021, p.1). The most recent development in dog studies even revealed that there are connections from the nose to the occipital lobe, where the visual cortex is located, suggesting that, although the specifics are not comprehended yet, vision and smell are intertwined in the brains of dogs – an ability not yet found in any other animal (Andrews et al., 2022).

However, with 2,000 receptors – double the amounts in dogs and five times as many as humans, and 1,948 olfactory receptor genes, African elephants were crowned best sniffers of the animal kingdom in a 2014 study led by Niimura at the University of Tokyo (Niimura et al., 2014). In comparison, orangutans and humans were found to respectively have only 296 and 396 such genes, while the common ancestor of the thirteen investigated species had 781, who lived about 100 million years ago. This points towards an evolutionary decrease in olfactory receptor genes in primates compared to an increase in elephants and rodents, a finding that shows promise towards the identification of the genetic mechanisms behind olfaction (Niimura et al., 2014). Moreover, elephants have been proved to be able to differentiate between ethnic groups in Kenya by odor and garment color (Bates et al., 2007), and to ‘detect differences between various quantities of food using only their sense of smell. Thus, elephants may be unique in their use of olfaction in cognitive tasks’ (Plotnik et al., 2019). The mastodon’s trunk is also extremely tactile, which seems to confirm Niimura’s that its nose is also its hand (Feltman, 2014).

Figure 6: Olfactory Receptor Genes by Species (Tate, 2014)

To summarize, smell detects, transduces and translates external stimuli through a variety of complex biophysical processes, but it majorly departs from other sensory systems in terms of its connections to the brain. By having a direct route to the hippocampus and amygdala, olfactory perception is indeed much more closely linked to emotions and memories. Therefore, while the picture that smell paints is more nuanced and less explicit than vision, hearing or touch, and while humans often underestimate their nose’s capacities, it remains a vital tool than can differentiate over a trillion odors, outperforming eyes and ears that only discriminate several million different colors and almost half a million different tones (Bushdid et al., 2014). Multiple species, however, have a much more developed olfaction, both with respect to their receptor, neural, and cortical areas and their genetics. Studies of rodents, dogs, and elephants have provided and will likely continue to provide promising insights into the intriguing sense known as smell. Besides, olfactory memory is currently a growing research field, that could yield life-changing results in medicine, particularly in understanding diseases such as Parkinson’s or even Alzheimer’s (Khamsi, 2022).

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