Why are odors important

For instance, one conditioning experience to the previously mentioned mixture of ethyl isobutyrate and ethyl maltol which smells like pineapple to human adults allowed rabbit pups to generalize their response to both odorants, something they cannot do when tested with the mixture after single conditioning to one odorant only Coureaud et al.

However, repeated conditioning to this binary mixture led to a drastic decrease of generalization and the pups became more responsive to the mixture than to the elements. This result suggests an improved configural perception of the mixture. Conversely, after repeated conditioning to a single component, the pups responded to the mixture, which suggests improved elemental perception. Interestingly, these perceptual changes greatly depend on the mixture and its components.

These results suggest that the initial status of the mixture, either purely elementally processed or akin to configural perception i. Perceptual experience can also be acquired by passive exposure to odors Rabin, When the olfactory environment of rats was enriched, their ability to discriminate odorants in binary mixtures increased regardless of the odorant to which the rat was exposed during the enrichment period; Mandairon et al.

This effect was linked to neurogenesis in the rat OB Mandairon et al. In human adults, the mixture of ethyl isobutyrate and ethyl maltol was less configurally processed by a group of subjects after passive exposure to the single elements compared to non-exposed subjects.

Perceptual learning would then favor the elemental perception of the mixture Le Berre et al. Expertise is also a cognitive factor that can influence odor mixture perception. Thus, experts would be less sensitive to the configuration induced by the mixture. The ability to focus on the elements may be linked to their familiarity with the odorants, insomuch that the identification ability increases when the target is familiar Rabin, ; Rabin et al.

Nevertheless, learning, considered as perceptual training in experts, increases the absolute ability to identify odors in low but not highly complex mixtures. Expertise can also rely on semantic knowledge Rabin, ; de Wijk and Cain, ; Stevenson, b , which is another cognitive factor that influences odor mixture processing in humans.

In a dedicated experiment assessing the impact of semantic learning on the perception of odor mixtures, it was found that exposure to the mixture target odor label semantic learning facilitated the perception of the configural odor of blending mixtures Le Berre et al. Thus, verbal labels could have provided perceptually expected and reliable information regarding the frame of reference for odors Herz and von Clef, ; Rouby et al.

A similar cognitive top-down effect, even if not directly related to semantic knowledge, could explain the results obtained in a study exploring the influence of odor context on odor mixture perception Arao et al.

Using colors that are congruent with the odor of each element of a binary mixture, it has been shown that participants judged the odor of the element congruent with the color to be more dominant in the mixture. In line with attentional processes, perceptual processing strategies may also modify odor mixture perception.

According to the high complexity of the environment, it is likely that learning and attention can fine-tune the perception by highlighting the meaningful elemental features or configural shapes from the background Wilson and Stevenson, b.

In the real life situation, odors are important vectors of information that elicit behavioral decisions from animals in their natural environment. For instance, odors are involved in the interaction between conspecifics, with competitors and predators, and in the selection of habitats, preys and food. Odors are never perceived alone, but among other odors, and chemical mixtures are usually the global stimuli that drive chemically mediated patterns of animal behaviors.

Therefore, animals have no choice but to simplify the surrounding amount of information, which constantly varies over time. They must adapt to the chemical complexity of the environment by extracting information from this mass of molecules, especially in mixtures, by discriminating and assigning meaning to some of them and responding in a way adapted to their needs. One strategy to reduce this complexity is to respond to certain odorants among others present in the same mixture, i. This occurs when organisms respond to key odorants in complex odorous substrates, e.

A second strategy consists of attributing additional or unique information to the odorants forming a mixture as a whole, which carries a behavioral value that is distinct from the individual value of each component, i. This configural strategy is functional both in aquatic and terrestrial organisms. For instance, after food-rewarded exposures, catfish differentially modify their swimming activity in response to mixtures of amino-acids and to their elements Valentincic et al. In a double-choice test, a mollusk, the terrestrial slug, displays a strong aversion to a binary mixture while the odor of each component remains strongly attractive Hopfield and Gelperin, In insects, the configural perception of odor mixtures is involved in flower-foraging behaviors.

For example, when exposed to flower-scents containing dozens of components, bees perceive certain mixtures of volatile molecular constituents as configurations, an ability that certainly contributes to the discrimination of flowers and expression of preferences for those offering higher quality or quantity of nectar Deisig et al.

In rats, the configural perception of odor mixtures influences their spatial performance, localization of reward, and digging activity related to foraging Staubli et al. In dogs, and especially military dogs, the discrimination between complex mixtures of volatiles and their elements may be critical in the detection of explosives Lazarowski and Dorman, In humans, odor mixture processing may support the categorization of food while simultaneously keeping the ability to differentiate between different products that belong to the same category due to the perception of inconstant elements in addition to invariant configurations Gottfried, The chemical environment is complex not only for adult organisms but also for young, neonates, fetuses, and embryos, even if it is more limited during earlier periods of development e.

Indeed, maternal fluids such as amniotic fluid, colostrum, or milk in mammals, and more generally the maternal body itself, generate or carry a large number of odorants Antoshechkin et al. Very young organisms have an urgent need to respond to some of these odors to rapidly interact with the mother; to localize the nipples and suck; and to expand their knowledge about the surroundings. Interestingly, although this remains to be more generally investigated, both elemental and configural processing appear functional early in life.

Thus, newborn rabbits respond to the monomolecular mammary pheromone 2-methylbutenal carried in milk among other odorants Coureaud, ; Schaal et al. They are also able to perceive configurations in some binary and senary mixtures Coureaud et al. As in adults, the ability of very young organisms to process odor mixtures both configurally or elementally may contribute to decision making and to the discrimination between a peculiar conspecific, the mother, which carries peculiar odor elements or definite configurations, and another category of conspecifics, the lactating females, which emit the same or at least overlapping elements and configurations Coureaud et al.

Because of the partial overlap between the brain structures involved in affective disorders, olfaction and emotion, olfactory impairments can be observed in several psychiatric diseases: major depression Pause et al.

These impairments affect different aspects of olfactory function i. The majority of olfactory studies and mood disorders have focused on the perception of single odorants.

To date, only a few studies have investigated olfactory perception in major depression using odor mixtures Atanasova et al. However, studies using odor mixtures are of specific interest because complex olfactory stimuli reflect daily life situations, which is important in the study of anhedonia failure to gain pleasure from normal pleasant experiences.

Anhedonia is considered as a core symptom of major depression in an objective way. Depressed subjects also had low performance in correctly identifying the odor of the odorants within the binary iso-intense mixture, and they more readily perceived the unpleasant compound compared to control subjects. These observations were confirmed and generalized in a study using an iso-intense mixture of another pleasant 2-phenylethanol and unpleasant isovaleric acid odorant Naudin et al.

Since the same results were obtained in patients during a depressive episode and in remission, the authors suggested that these olfactory impairments may constitute potential trait markers of depression. These results could be explained by the cognitive bias for emotionally negative stimuli observed in depression that could persist in the remitted state Bhalla et al. All of the observations revealed that anhedonia can be advantageously observed in depressed patients at the olfactory level with complex olfactory stimuli.

They also suggest that the loss of food cravings often described in depression could be partly explained by a modification in olfactory perception, ending in a better perception of unpleasant sensory components in food. This finding emphasizes the importance of using complex mixtures of odorants, which are more ecologically relevant stimuli, to better understand the modulation of olfactory perception in mood disorders. Future psychophysical, neurophysiological, and neuroimaging investigations are needed in this field to increase our knowledge of the etiology of the diseases and to develop the appropriate tools to better care for patients with affective disorders.

Odors orthonasal smell and retronasal aroma are key perceptual characteristics to formulate in foods and in home and personal care products. It is the first chemical sense involved when a consumer is using such a product. Consumers base their opinion on the quality of a product, i.

Therefore, formulating the right olfactory experience cannot be taken lightly. Most food and beverage companies employ the services of flavor companies to create the flavors or aromas that will enter the formulation of the end product.

Indeed, food and beverage companies may require flavors for their new products or for compensating changes in the formulation of their existing products. Focusing on olfactory perception, which is largely involved in flavor Hornung and Enns, ; Thomas-Danguin, , we explained in the previous sections of this review that odors arise from perceptual representations of mixtures of odorants, whose construction is far from being fully understood and remains mostly impossible to predict on the basis of chemical composition.

Within flavor houses, flavor formulation is thus performed by specially trained scientists called flavorists, who have empirical knowledge about the perception of chemicals in mixtures. Usually, they follow a brief delivered by the client.

This brief must specify the direction of flavor to be formulated e. The flavor house may also seek the assistance of an application specialist to ensure that the newly formulated flavor will deliver its expected quality in the application for which it is intended.

Indeed, when formulated in a complex matrix, such as a food matrix e. In perfume composition, creation also relies on empirical knowledge. For instance, it is known that adding sulfur components, which are often unpleasant e. Indeed, we have presented several examples of the impact of an unpleasant odor mixed with a pleasant one.

Synergistic effects are also extensively used in perfume design. For instance, fatty aldehydes are known to enhance many floral odors at low concentrations, even if their own odor is very different from the target one. These synthetic odorants have been used in floral-aldehydic perfumes such as the famous Chanel no. Perfume chords are also very well empirically used in this industry. The concept of perfume chords is reconcilable with configural processing of odor mixtures.

Indeed, chords usually rely on mixtures of three or four odors which are sometimes linked to pure chemicals that are included in larger formulae. Moreover, as explained by the famous perfumer Edmond Roudnitska quoted by Chastrette, , a perfume composition includes not only one chord but an unknown number that are not smelled one after the other but can overlap, be enhanced, or be canceled.

Therefore, the perceptual interactions that result from smelling a perfume are likely the playground of the artist and allowed him to create esthetic odor objects. Besides the complexity of formulating a flavor or a perfume based on product properties, top-down influences also play a role in the way consumers perceive a product. Finally, the above examples demonstrate the empirical knowledge and methods used in the formulation of aromas and fragrances but also describe how recent insights into odor processing and perception impact the development of new products.

The study of odor mixtures is an original window to investigate olfactory processes in a manner that may be more relevant to ecological perceptual contexts, which is crucial to understanding how organisms, including humans, represent and adapt to their chemical complex environment.

It is also an original path to identify, characterize and further treat adaptation disorders in humans. Yet, a better understanding of the underlying biological processes involved when organisms manage to identify an odor object based on hundreds of chemicals in a few milliseconds would likely impact many scientific fields.

Moreover, extending our investigations on the odor processing of natural mixtures would shed light on the ability of organisms, including humans, to code complex information in the olfactory brain and how, through development, learning, or evolution, the resulting odors are stored as perceptual objects and reused by individuals. It appears from this review that the appropriate description of the stimulus representations is likely the most critical factor in odor mixture perception.

This is fundamental and should not be overlooked since a mixture is not a simple addition of each of its component and because it is the starting point of every following process. This requires for a large part to clearly pinpoint the peripheral spatiotemporal coding processes of odorants in mixtures, which is the only way to decipher the role of mixture composition and to predict accurately odor perception on the basis of chemical composition. Nevertheless, the incoming information is highly subjected to modulations at all stages of integration.

If we highlighted in this review that the processing is contrasted at each stage, the specific role of these distinct stages remains largely to be discovered. To take up these research challenges, one should favor a systemic approach that would combine several investigation levels thus gathering cellular, neurobiological and psychological aspects both in human and other animal species.

That was the guideline of this review to put together the results obtained in various models in order to underline similitude and differences in perception mechanisms. Indeed multidisciplinary studies may help to tackle specific questions regarding both odor mixture coding and perception, plasticity of perception and behavioral consequences, and thus would likely bring the field forward.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The most significant role of olfactory signals in humans appears to be the modulation of their behavior and interpersonal relationships, of their affiliation to certain groups or social classes, having a major influence in their tastes and personality.

Olfactory sense is, in terms of evolution, one of the oldest senses, allowing the organisms with receptors for the odorant to identify food, potential mating partners, dangers and enemies.

For most living creatures and for mankind smell is one of the most important ways of interaction with the environment. The olfactory area in humans is about 2. Only volatile substances, soluble in mucus, can reach the receptors and interact with them and finally produce sensations. The olfactory epithelium contains a variable number of basal cells, which are capable of mitotic division giving rise to mature receptor neurons. The olfactory neuron's turn over is about 40 to 60 days [ 1 ].

The axonal ramifications of these neurons go together in groups of 10 to fibers, they cross the ethmoid cribriform plate reaching the olfactory bulb where they converge and form synaptic structures called glomeruli and then they converge again to mitral cells. The total convergence ratio is and has the important role of increasing the sensibility of the olfactory signal that will be sent to the specialized areas in the brain [ 2 ].

The olfactory epithelium contains another sensitive system via sensitive branches of trigeminal nerve. Many odorants cam produce sensations transmitted by trigeminal nerve. For example levo—menthole. If placed in the nasal cavity it produces cold sensations in small amounts and hot sensations in much bigger quantities [ 3 ]. Just the same way camphor produces a cold sensation via trigeminal nerve.

Humans have only functional genes for olfactory receptors, compared with other mammals, e. The genes that encode the receptors are grouped in series of introns in the coding region. In mammals, these regions are organized in clusters of 10 or are often located on different chromosomes. In the human genome, there is a large amount of pseudo genes, which suggest that olfaction became less important during evolution.

Other studies are demonstrating that humans have a good sense of smell in spite of all genetic aspects that may tend to deny this theory. Oenologists or perfume creators are capable of distinguishing thousands of odors. Human olfaction can overdue tests like gaseous chromatography in detecting the odorant molecules. All these things are realized with a small number of receptors but with the aid of certain accessory functions gained during evolution.

In the process of achieving the bipedal position, the nose and olfactory receptors have risen above the ground level and the olfactory area became smaller to allow the orbits to come closer and provide stereoscopic view.

Being much further from the ground the odors received, stopped being contaminated and mixed with each other. All this, together with the air purification function of the nose, made the smells more easily to perceive and this means that the olfactory area grew smaller without many sacrifices for the olfactory sensations [ 1 ]. Once humans gained bipedal position, they began to cover large distances and to diversify their food.

Then, the discovery of fire about 2 million years ago, has diversified even more the odors and taste of food. This way, odorant molecules from food reached the olfactory area by retro nasal way while eating, being processed and integrated in specialized areas in the neocortex together with the taste sensations generating flavors. In time, during evolution, beginning with livestock, plant cultivation, and spice use, more and more information were received via retro nasal way creating many complex flavors.

No other species of mammals or primates could ever benefit from this kind of olfactory stimulation during their period of evolution. During the evolution of human race, the brain increased in size and volume. The classic approach considers that olfactory organs diminished in size and function leaving vision and hearing to be the most important senses, but the theory considers only the size of the receptor area.

Integration areas of olfactory sensations are extensive, including the olfactory cortex and the olfactory tubercle, some parts of cerebral tonsils, certain hypothalamic areas, medio—dorsal thalamus and medial and lateral orbito—frontal cortex [ 7 ]. The memory of discrimination and comparison between odors comes to contribute by activating areas from frontal and temporal lobes linked to the association areas introducing a new and very important cognitive component of olfaction, a component never found in other species.

Reduced repertoire of genes for olfactory receptors is compensated by the great capacity of human brain processing. But according to a biologist and an olfactory branding specialist Wednesday, it was the nose that was really at work. This should not be surprising, as neuroscience makes clear. Odors take a direct route to the limbic system, including the amygdala and the hippocampus, the regions related to emotion and memory.

But, as with Proust, taste plays a role, too, said Murthy, whose lab explores the neural and algorithmic basis of odor-guided behaviors in terrestrial animals. When you are eating all the beautiful, complicated flavors … they are all smell. For decades individuals and businesses have explored ways to harness the evocative power of smell. Think of the cologne or perfume worn by a former flame.

And then there was AromaRama or Smell-O-Vision, brainchildren of the film industry of the s that infused movie theaters with appropriate odors in an attempt pull viewers deeper into a story — and the most recent update, the decade-old 4DX system, which incorporates special effects into movie theaters, such as shaking seats, wind, rain, as well as smells.

Several years ago, Harvard scientist David Edwards worked on a new technology that would allow iPhones to share scents as well as photos and texts. Today, the aroma of a home or office is big business. Scent branding is in vogue across a range of industries, including hotels that often pump their signature scents into rooms and lobbies, noted the authors of Harvard Business Review article.