Retronasal smell

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Retronasal smell, retronasal olfaction , is the ability to perceive flavor dimensions of foods and drinks. Retronasal smell is a sensory modality that produces flavor. It is best described as a combination of traditional smell (orthonasal smell) and taste modalities. [1] Retronasal smell creates flavor from smell molecules in foods or drinks shunting up through the nasal passages as one is chewing. When people use the term "smell", they are usually referring to "orthonasal smell", or the perception of smell molecules that enter directly through the nose and up the nasal passages. Retronasal smell is critical for experiencing the flavor of foods and drinks. Flavor should be contrasted with taste , which refers to five specific dimensions: (1) sweet, (2) salty, (3) bitter, (4) sour, and (5) umami. Perceiving anything beyond these five dimensions, such as distinguishing the flavor of an apple from a pear for example, requires the sense of retronasal smell.

Contents

History

Evolutionarily, smell has long been presumed to be a less-important sense for humans, especially compared to vision. Vision appears to dominate human stimuli perception, but researchers now argue that smell cues are highly informative to humans despite being less obviously so. Before his death in 1826, French gastronome Brillat-Savarin published his book, The Physiology of Taste; Or, Meditations on Transcendental Gastronomy: Theoretical, Historical, and Practical Work, in which he makes the first mention of the importance of smell in the “combined sense” of taste. He defines taste in terms of the five taste dimensions in addition to flavor created with the nasal apparatus. [1] Avery Gilbert, in his book The Nose Knows, reviews the work of Henry T. Finck, an American philosopher from the late 1800s who published a groundbreaking essay titled “The Gastronomic Value of Odours.” Flink called flavor a “second way of smelling,” and much subsequent scientific investigation in the early 1900s focused on attempting to break down smell dimensions into basic categories, a feat that has proven too complicated due to the vast number and complexity of odors.

Food connoisseurs and chefs are increasingly capitalizing on the newly ascertained understanding of the role smell plays in flavor. Food scientists Nicholas Kurti and Hervé This expanded upon the physiology of flavor and its importance in the culinary arts. In 2006, This published his book, Molecular Gastronomy: Exploring the Science of Flavor, in which he explores the physical mechanisms that bring about flavor perception. Kurti and This influenced others, such as Harold McGee, whose 1984 book, On Food and Cooking: The Science and Lore of the Kitchen, has been extensively revised in 2004 and remains a key reference on the scientific understanding of food preparation. His book has been described by television personality Alton Brown as “the Rosetta stone of the culinary world.” Such a breakthrough in the understanding of the mechanisms behind experiencing the flavor of different foods is likely to continue inspiring those in the culinary arts to create novel combinations and recipes.

Today, one of the most active food psychologists, Paul Rozin has been the first to successfully map the role of retronasal smell in flavor. In 1982, he explained that smell is a “dual-sense” and made the explicit differentiation between retronasal smell and orthonasal smell. [1] Rozin describes orthonasal smell as “breathing in” and retronasal smell as “breathing out.” In 1982, he devised an experiment in which he trained participants to accurately recognize smells orthonasally before introducing them to the back of the mouth, at which point the success rate fell drastically, demonstrating that smell operates through two distinct mechanisms. [2] His favored example of this duality is Limburger cheese, which is known for its repulsiveness to the nose yet pleasantness to the mouth.

Originally published in 2012, Neurogastronomy by Gordon M. Shepherd provides an overview of the way smell is perceived in humans. The book comprises a detailed review of how retronasal smell, in combination with taste, creates flavor. Shepherd describes the neural basis for identification, recognition, and preference for certain flavors, and explores potential political and social implications of a deeper understanding of flavor perception, such as causes of obesity and concerns of loss of smell sensitivity in old age. [1]

Overview of the smell pathway

To better understand this mechanism, a simple breakdown of smell pathway is provided below. When humans chew, volatile flavor compounds are pushed through the nasopharynx and smell receptors.

Human olfactory system. 1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus 6: Olfactory receptor neurons Olfactory system.svg
Human olfactory system. 1:  Olfactory bulb 2:  Mitral cells 3:  Bone 4: Nasal epithelium 5:  Glomerulus 6:  Olfactory receptor neurons

Olfactory epithelium

The first stop in the olfactory system is the olfactory epithelium, or tissue resting on the roof of the nasal cavity which houses smell receptors. Smell receptors are bipolar neurons that bind odorants from the air and congregate at the olfactory nerve before passing axons to the dendrites of mitral cells in the olfactory bulb. [3] Sensory receptors in the mouth and nose are polarized at resting state, and they depolarize in response to some change in environment, such as coming in contact with odor molecules. Odor molecules, consisting of hydrocarbon chains with functional groups, bind to sensory receptors in the nose and mouth. Properties of functional groups include: (1) length of carbon chain, (2) terminal group, which concord with differences associated with different smells, (3) side group, (4) chirality, (5) shape, and (6) size. When odor molecules bind to sensory receptors, they do so in according to these properties. Each olfactory cell has a single type of receptor, but that receptor can be “broadly tuned” and odor molecules further interact at the receptor level, meaning that, in certain cases, an odor molecule alone may not bind to a receptor, but in the presence of another odor molecule, the original would bind and thus create a sensation of smell only in the presence of the second molecule. [4]

Olfactory bulb

In the olfactory bulb, smell molecules are mapped spatially. These spatial representations are known as “smell images." [1] Spatial representation permits lateral inhibition, or contrast enhancement and gain compression. Contrast enhancement is sensitive to change and highlights stimuli in the brain that are changing rather than at rest. Gain compression heightens sensitivity to low-intensity stimuli while lessening sensitivity to high-intensity stimuli. The olfactory bulb, while still in the primary stages of its understanding by researchers, distinguishes smell from other senses because it marks a deviation in the sensory pathway from what is characteristic of all other senses. Namely, all non-olfactory sensory information passes through the thalamus after the receptor level, but the fact that odor information instead enters its own specialized area could suggest the primitive history of smell and/or a distinct type of processing of odor information on its way to the cortex. The olfactory bulb houses glomeruli, or cell junctures, on which thousands of receptors of the same type, in addition to mitral cells, converge. This organization allows a vast amount of information to be concisely represented without requiring an equally large number of receptor types. The resulting combination of odor information is dubbed an odor image at the level of the olfactory bulb. [5]

Imaging in the olfactory bulb

2DG method

In 1977, biochemist Lou Sokoloff, Seymour Kety, and Floyd E. Bloom developed a way of mapping activity in the brain by tracking the rat brain's metabolization of oxygen. Nerve cells require oxygen and glucose for energy. 2-deoxyglucose (2DG) is a radioactive glucose isotope that can be tracked in the brain since it leaves a trace in the cell where it would normally be metabolized for energy if it were glucose. After stimulation of a certain region of cells, X-ray photographs can be sliced to reveal which cells were active, particularly at synaptic junctures. [6]

Functional magnetic resonance imaging (fMRI) can also be used to measure metabolism of an odor. This method is not terminal as is the 2-deoxyglucose method, so one animal can be measured with many odors, and the resulted images can be compared.

Green fluorescent protein method

Finally, the green fluorescent protein method genetically engineers mice to express a protein in active neurons, and a camera can then be placed inside the skull of the mouse to measure activity. [7]

Findings

These methods reveal, most notably, that the organization of smell information in the olfactory bulb is spatial. Similar molecular patterns result in similar activation patterns with regard to glomeruli, and glomeruli that are closer together encode similar features of smell information. [1] [6] [7]

Olfactory cortex

The three-layered olfactory cortex, containing pyramidal cells is the next benchmark on the smell pathway. One pyramidal cell receives information from a multiplicity of mitral cells from the olfactory bulb, making the previously organized glomerular pattern distributed in the olfactory cortex. This dispersion of mitral cell information allows for self-excitatory feedback connections, lateral excitation, and self- and lateral-inhibition. These processes contribute to Hebbian learning, named after Donald O. Hebb, and is often simplified by the saying “neurons that fire together wire together.” Long-term potentiation, the neural mechanism for Hebbian learning, allows for memory formation at the pyramidal cell level. Hebbian learning is thus essentially the phenomenon by which the olfactory cortex “remembers” the output of combinations of smell molecules and allows for recognition of previously sensed combinations faster than novel ones by matching them to stored input. The resulting smells that were previously called odor images are stored in the olfactory cortex for recognition are referred to now as odor objects. [5] Experience therefore strengthens signal-to-noise ratio in that a previously sensed odor object can be more easily distinguished against greater background noise. [8]

Orbitofrontal cortex

The orbitofrontal cortex (OFC) is the final destination of the odor information and is where conscious smell perception arises. Smell information enters directly after passing through the olfactory cortex, which marks the distinction from other sensory information that first pass through the thalamus. The OFC is located dorsal to the prefrontal cortex, allowing smell information direct input to the prefrontal cortex, or the major decision-making area of the brain. There are three sets of neurons that process smell information before it reaches the OFC: the olfactory receptor cells in the olfactory epithelium, mitral cells, and olfactory pyramidal neurons. [1]

At the level of the OFC, associations with other brain areas are made, including input from the mouth (somatosensation), emotional input (amygdala), visual information, and evaluative information (prefrontal cortex). The OFC is responsible for selective odor tuning, fusing of sensory domains, and hedonic evaluations of smells.

At-home evidence of the role smell plays in flavor

The experience of eating favored foods with a cold often disappoints. This is because congestion blocks nasal passageways through which air and flavor molecules enter and exit, thus temporarily reducing retronasal smell capacity.

Another way to isolate the sense of retronasal smell is to use the “nose pinch test.” When eating while pinching the nostrils closed, the flavor of food appears to dissipate, namely because the pathway for air exiting the nose that creates the flavor image is blocked.

Some commercial products rely on retronasal smell, such as a water bottle whose scented pods create the illusion of flavor when drinking plain water. [9]

Speculative evolutionary significance

Deeper understanding of the role of retronasal smell in flavor has led many to rethink smell's evolutionary significance in humans. To dispel the notion that vision is wholly superior in humans and higher primates to olfaction, Gordon M. Shepherd contrasts the anatomy of the human nose to that of a canine. [1] In canines, smell receptors reside in the back of the nasal cavity. They have a unique cartridge-like organ that serves as an air filter. During quiet breathing, this cartridge directs the stream of air normally, but during active smelling, the rate of direction of information increases, allowing a canine to sniff as much as six to eight times faster than a human. [1]

This suggests that canines are adapted for stronger orthonasal smell capabilities. By contrast, humans seem to be selected to have superior retronasal smell capacities. The bipedal posture of humans reduces the need[ how? ] for a cartridge that functions in canines to mainly clean air entering. The short nasopharynx for retronasal smell in humans is what allows the volatiles from foods and drinks to travel from the mouth to the smell receptors in the nasal cavity. What remains less clear is the fact that canines still have a strong ability to discriminate foods.

Other speculations include the idea that the short route from the mouth to the nasal cavity resulted from selection from long-distance running when humans migrated out of Africa 2 million years ago. [10] The idea is that a shorter nasal apparatus would aid in balancing the head to facilitate distance running. Lieberman cites other evolutionary changes that could have resulted from selection for running such as wider joint cartilages and longer bones in the legs. [11]

Related Research Articles

<span class="mw-page-title-main">Olfactory nerve</span> Cranial nerve I, for smelling

The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell.

<span class="mw-page-title-main">Vomeronasal organ</span> Smell sense organ above the roof of the mouth

The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth in various tetrapods. The name is derived from the fact that it lies adjacent to the unpaired vomer bone in the nasal septum. It is present and functional in all snakes and lizards, and in many mammals, including cats, dogs, cattle, pigs, and some primates. Some humans may have physical remnants of a VNO, but it is vestigial and non-functional.

<span class="mw-page-title-main">Sensory nervous system</span> Part of the nervous system

The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception and interoception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, balance and visceral sensation. Sense organs are transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.

<span class="mw-page-title-main">Olfactory bulb</span> Neural structure

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.

<span class="mw-page-title-main">Olfactory system</span> Sensory system used for smelling

The olfactory system or sense of smell is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an accessory olfactory system. The main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli.

<span class="mw-page-title-main">Glomerulus (olfaction)</span>

The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons and glial cells.

<span class="mw-page-title-main">Sensory neuron</span> Nerve cell that converts environmental stimuli into corresponding internal stimuli

Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them:

<span class="mw-page-title-main">Mitral cell</span> Neurons that are part of the olfactory system

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish.

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

Olfactory fatigue, also known as odor fatigue, olfactory adaptation, and noseblindness, is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the odor normally fades to the point where the smell is not perceptible or is much weaker. After leaving the area of high odor, the sensitivity is restored with time. Anosmia is the permanent loss of the sense of smell, and is different from olfactory fatigue.

Dysosmia is a disorder described as any qualitative alteration or distortion of the perception of smell. Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. Dysosmia can be classified as either parosmia or phantosmia. Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. Phantosmia is the perception of an odor when no odorant is present. The cause of dysosmia still remains a theory. It is typically considered a neurological disorder and clinical associations with the disorder have been made. Most cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. Dysosmia tends to go away on its own but there are options for treatment for patients that want immediate relief.

<span class="mw-page-title-main">Sense of smell</span> Sense that detects smells

The sense of smell, or olfaction, is the special sense through which smells are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

Olfactory memory refers to the recollection of odors. Studies have found various characteristics of common memories of odor memory including persistence and high resistance to interference. Explicit memory is typically the form focused on in the studies of olfactory memory, though implicit forms of memory certainly supply distinct contributions to the understanding of odors and memories of them. Research has demonstrated that the changes to the olfactory bulb and main olfactory system following birth are extremely important and influential for maternal behavior. Mammalian olfactory cues play an important role in the coordination of the mother infant bond, and the following normal development of the offspring. Maternal breast odors are individually distinctive, and provide a basis for recognition of the mother by her offspring.

A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of stimuli. Although, in some cultures, five human senses were traditionally identified as such, many more are now recognized. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.

Gordon Murray Shepherd was an American neuroscientist who carried out basic experimental and computational research on how neurons are organized into microcircuits to carry out the functional operations of the nervous system. Using the olfactory system as a model that spans multiple levels of space, time and disciplines, his studies ranged from molecular to behavioral, recognized by an annual lecture at Yale University on "integrative neuroscience". At the time of his death, he was professor of neuroscience emeritus at the Yale School of Medicine. He graduated from Iowa State University with a BA, Harvard Medical School with an MD, and the University of Oxford with a DPhill.

<span class="mw-page-title-main">Sniffing (behavior)</span> Nasal inhalation to sample odors

Sniffing is a perceptually-relevant behavior, defined as the active sampling of odors through the nasal cavity for the purpose of information acquisition. This behavior, displayed by all terrestrial vertebrates, is typically identified based upon changes in respiratory frequency and/or amplitude, and is often studied in the context of odor guided behaviors and olfactory perceptual tasks. Sniffing is quantified by measuring intra-nasal pressure or flow or air or, while less accurate, through a strain gauge on the chest to measure total respiratory volume. Strategies for sniffing behavior vary depending upon the animal, with small animals displaying sniffing frequencies ranging from 4 to 12 Hz but larger animals (humans) sniffing at much lower frequencies, usually less than 2 Hz. Subserving sniffing behaviors, evidence for an "olfactomotor" circuit in the brain exists, wherein perception or expectation of an odor can trigger brain respiratory center to allow for the modulation of sniffing frequency and amplitude and thus acquisition of odor information. Sniffing is analogous to other stimulus sampling behaviors, including visual saccades, active touch, and whisker movements in small animals. Atypical sniffing has been reported in cases of neurological disorders, especially those disorders characterized by impaired motor function and olfactory perception.

Odor molecules are detected by the olfactory receptors in the olfactory epithelium of the nasal cavity. Each receptor type is expressed within a subset of neurons, from which they directly connect to the olfactory bulb in the brain. Olfaction is essential for survival in most vertebrates; however, the degree to which an animal depends on smell is highly varied. Great variation exists in the number of OR genes among vertebrate species, as shown through bioinformatic analyses. This diversity exists by virtue of the wide-ranging environments that they inhabit. For instance, dolphins that are secondarily adapted to an aquatic niche possess a considerably smaller subset of genes than most mammals. OR gene repertoires have also evolved in relation to other senses, as higher primates with well-developed vision systems tend to have a smaller number of OR genes. As such, investigating the evolutionary changes of OR genes can provide useful information on how genomes respond to environmental changes. Differences in smell sensitivity are also dependent on the anatomy of the olfactory apparatus, such as the size of the olfactory bulb and epithelium.

Neurogastronomy is the study of flavor perception and the ways it affects cognition and memory. This interdisciplinary field is influenced by the psychology and neuroscience of sensation, learning, satiety, and decision making. Areas of interest include how olfaction contributes to flavor, food addiction and obesity, taste preferences, and the linguistics of communicating and identifying flavor. The term neurogastronomy was coined by neuroscientist Gordon M. Shepherd.

References

  1. 1 2 3 4 5 6 7 8 9 Shepherd, Gordon M. (2012). Neurogastronomy . New York: Columbia University Press. ISBN   9780231159104.
  2. ↑ Rozin, Paul (1982-07-01). ""Taste-smell confusions" and the duality of the olfactory sense". Perception & Psychophysics. 31 (4): 397–401. doi: 10.3758/BF03202667 . ISSN   0031-5117. PMID   7110896.
  3. ↑ Moran, D. T.; Rowley, J. C.; Jafek, B. W.; Lovell, M. A. (1982-10-01). "The fine structure of the olfactory mucosa in man". Journal of Neurocytology. 11 (5): 721–746. doi:10.1007/bf01153516. ISSN   0300-4864. PMID   7143026. S2CID   25263022.
  4. ↑ Duchamp-Viret, P.; Chaput, M. A.; Duchamp, A. (1999-06-25). "Odor Response Properties of Rat Olfactory Receptor Neurons" (PDF). Science. 284 (5423): 2171–2174. doi:10.1126/science.284.5423.2171. ISSN   0036-8075. PMID   10381881.
  5. 1 2 Evolution of Nervous Systems. Academic Press. 2016-11-23. p. 17. ISBN   9780128040966.
  6. 1 2 Stewart, William B.; Kauer, John S.; Shepherd, Gordon M. (1979-06-15). "Functional organization of rat olfactory bulb analysed by the 2-deoxyglucose method". The Journal of Comparative Neurology. 185 (4): 715–734. doi:10.1002/cne.901850407. ISSN   1096-9861. PMID   447878. S2CID   41466291.
  7. 1 2 Brazelton, T. R.; Rossi, F. M.; Keshet, G. I.; Blau, H. M. (2000-12-01). "From marrow to brain: expression of neuronal phenotypes in adult mice". Science. 290 (5497): 1775–1779. doi:10.1126/science.290.5497.1775. ISSN   0036-8075. PMID   11099418. S2CID   16216476.
  8. ↑ Wilson, Donald A.; Stevenson, Richard J. (2006-05-11). Learning to Smell: Olfactory Perception from Neurobiology to Behavior. JHU Press. ISBN   9780801883682.
  9. ↑ "We Tested the Water Bottle that "Flavors" Your Water Through Scent". Good Housekeeping. 2023-03-28. Retrieved 2023-10-16.
  10. ↑ Lieberman, Daniel E., and Dennis M. Bramble. 2007. The evolution of marathon running: Capabilities in humans. Sports Medicine 37(4-5): 288- 290.
  11. ↑ Lieberman, Daniel. The Evolution of the Human Head. Cambridge, MA: Belknap of Harvard UP, 2011. Print.
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