Only participants in the negative same-odor group spent less time on the tests. Moreover, the decreased time spent by participants in the negative same-odor group was restricted to difficult test problems that were ultimately left blank and thus not efficiency. In other words, lowered motivation on the puzzle tests was produced by the emotional association acquired to the ambient odor.
In sum, past studies and current empirical work provide strong experimental evidence to support the hypothesis that odor hedonic perception and odor-related behavior result from a learned association between an odor and the emotional context in which the odor was first encountered. Further research exploring the full range of emotional associations, the situations in which they can be expressed and a detailed analysis of the classical conditioning mechanisms involved in odor-associative learning are now needed.
Herz et al. Ayabe-Kanamuura, S. Senses , 23 , 31— Cain, W. Perception , 7 , — Davis, M. Psychiatry , 6 , 13— Dilks, D. Senses , 24 , Engen, T. Praeger, New York. Epple, G. Herz, R. Cerebrum , 3 , 31— Emotion , 29 in press. Mennella, J. New Engl. Senses , 20 , — Moncreiff, R.
Wiley, New York. Robin, O. Dental Res. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Skip Nav Destination Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Introduction. Current experimental evidence.
Article Navigation. Herz Rachel S. Correspondence to be sent to: Rachel S. Oxford Academic. Google Scholar. Cite Cite Rachel S. Select Format Select format. Permissions Icon Permissions. Chemical Senses Vol. Issue Section:. Download all slides. View Metrics. Email alerts Article activity alert. Advance article alerts. New issue alert. In progress issue alert. Receive exclusive offers and updates from Oxford Academic. Related articles in Web of Science Google Scholar. In mammals, mitral cells project to the anterior olfactory nucleus, anterior and posterior piriform cortex aPC and pPC , olfactory tract, lateral entorhinal cortex, and part of the amygdala, among other regions Mori and Sakano, The piriform cortex PC has been the center of several investigations related to odor discrimination and representation, some of which have used mixtures of odorants Haberly and Bower, ; Granger and Lynch, ; Litaudon et al.
The processing of olfactory information in the OB and the PC is highly contrasted. From a functional point of view, it has been shown in rats that the PC can rapidly discriminate a mixture from its components, thereby producing a minimal cross-habituation to components after habituation to the mixture, while the OB still computes the mixture like the sum of odorants Wilson, , Because, the aPC and pPC are quite different in their anatomical organization, they likely have distinct roles in odor encoding: encoding of odorant identity may occur in the aPC while encoding of odor similarity or odor quality occurs in the pPC Kadohisa and Wilson, ; Yoshida and Mori, These dissociated roles of the aPC and pPC were confirmed by a functional magnetic resonance imaging fMRI study performed in humans with single odorants Gottfried et al.
When taken together, these results suggest that the pPC is a key structure for the perception of odor mixtures since it may contribute to their configural processing, namely their putative coding as odor objects, each carrying a specific odor quality. Higher-order cortices are also involved in olfactory information integration.
However, the contrast between the two types of stimuli revealed activation in the middle cingulate cortex, superior frontal gyrus, and angular gyrus Boyle et al. The anterior portion of the OFC acted such as an on-off detector for odor mixtures because it was activated in response to odor mixtures and deactivated in response to single odorants; the lateral portion of the OFC responded in a graded fashion to relatively small differences in intensity ratios of the two mixed odors Boyle et al.
This cortex is known to encode odor identity quality but also odor valence Anderson et al. Therefore, this structure probably plays a major, but still unknown, role in the configural processing of complex odor stimuli. A contrasted processing of binary odor mixtures and their single odorants was also observed by fMRI in higher-order brain areas but not primary olfactory cortices Grabenhorst et al.
In this study, different parts of the OFC simultaneously and independently represented the positive and negative hedonic value of an odor mixture that contains pleasant and unpleasant components. This response may reflect the perceptual synergy or pleasantness enhancement of the pleasant odor sometimes observed when mixed with an unpleasant one.
Such perceptual outcome could be due to an attention-capturing effect of hedonically complex mixtures that operate unconsciously and involve the superior frontal gyrus Grabenhorst et al. Perceptual interactions induced by the previously reviewed neurobiological mechanisms can be considered as an effectiveness of the olfactory system to capture the complex chemical information as a whole or as elements pertaining to the whole.
Indeed, in both mammals and insects, these perceptual interactions are the basis of configural and elemental processing of mixtures of odorants, which may lead to the perception of mixtures as odor objects configurations or not. This section of our review focuses on the results that support the notion of odor objects perception. This is most likely one of the most investigated question in the human perceptual analysis of odor mixtures Laing and Francis, ; Laska and Hudson, ; Jinks and Laing, a , b ; Laing and Jinks, A series of studies have shown that humans are hardly able to identify more than three odorants in a mixture that contains up to eight odorants Laing and Francis, ; Laing and Livermore, This limitation is not a function of the stimulus features.
Indeed, untrained subjects cannot correctly identify more than four familiar odors in a mixture containing up to eight odorants Livermore and Laing, b. Trained subjects reach the same odor identification limit when submitted to mixtures of familiar odors issued from a complex composition designed to evoke real odor sources e.
Cognitive factors play a minor role in the human in-mixture odor identification limit. Moreover, training or expertise does not enhance the identification performance since only three or four components of a mixture containing up to five odorants can be correctly identified by either a trained panel or an expert panel Livermore and Laing, Considering these results, the group of D.
Laing concluded that the human limit of identification of in-mixture odors may be imposed physiologically or by processing constraints. Similar findings were reported in animal studies. Adult rats have difficulty identifying components within mixtures with more than three or four components Staubli et al.
The interpretation of this compilation of more than 10 years of research appears to be in line with the hypothesis of configural functioning of olfaction, which is analogous to that for facial and object recognition Jinks and Laing, Odor object recognition would allow for the sense of smell to perform feature extraction and object synthesis that lead to the elaboration of a stable, background-detached representation of complex signals.
Due to interactions within the olfactory processing pathway, a stereotyped map could be elaborated; this map, where odor identity can be represented in spatiotemporal patterns, may be specific to a given complex stimulus and contain information about the elements of the mixture and likely about their association. The unique spatial and temporal signature could be recognized in the brain as an entity against a background of other odors and identified as an odor object Margot, To perform this complex task, the brain could rely on rapid and specific cortical adaptation to background odors and recognition of bulbar activation patterns Stevenson and Wilson, ; Frank et al.
When a stimulus activates the olfactory system, the activation pattern produced at the OB level, and further processed in cortical areas, would be compared to stored ones for details about the processing mechanisms see the previous sections on interactions at the periphery and interactions at higher levels. If there is a good match, we consciously experience a discrete odor that is distinct from the background and discriminable from other odors Stevenson and Wilson, If there is no match between the bulbar incoming pattern and a stored one, the novel pattern may be rapidly acquired Stevenson and Wilson, Even if alternative definitions of odor objects have been proposed Yeshurun and Sobel, , suggesting a critical role of hedonic features, the most commonly accepted definition relies on the integration of a specific blend of volatile molecules that can be separated from the surrounding clutter of volatiles to stand out as an entity reflecting a putatively unidentified specific source e.
In other words an AB binary mixture may be conceptualized as being composed of the individual A and B elements as well as a separate stimulus unique to the AB combination, usually noted U unique-cue; Figure 1A. However there is an unresolved debate in the literature regarding the unique-cue theory and its consequences in complex stimuli configural learning experiments Brandon et al.
The unique-cue stimulus is thought to occur at the level of memory representation rather than that of perceptual representation or spontaneous processing Rescorla et al. There are several lines of evidence showing that animals are able to perform configural processing of odor mixtures and thus differentiate between mixtures and their constituting monomolecular odors insects: Chandra and Smith, ; Lei and Vickers, ; Riffell et al.
This seems to be true even early in life. For instance, a binary mixture of ethyl isobutyrate and ethyl maltol is configurally processed, at least in part, by newborn rabbits. For the pups, this mixture spontaneously evokes an odor that is different from the one of its constituting odorants and provokes very contrasted behavior in a conditioning paradigm using the mammary pheromone Coureaud et al.
Similar results were obtained with a more complex mixture of six odorants RC mixture; Sinding et al. The results show that after conditioning to AB, the pups respond both to the odorants and the different mixtures. Therefore they perceive the elements A and B during the learning episode. These results from animal studies demonstrate the possibility of specific encoding for odor mixtures compared to their constituting elements.
However, it is worth noting that the nature of stimulus representation is inferred from experiments examining how the conditioned response to one odorant or a mixture of two or more odorants generalizes to another single odorant or mixture Harris, As a consequence, whether the mixture configuration is reconcilable with odor object encoding is not straightforward in animal studies. One way to circumvent this issue is to address the question in parallel in animals and humans. In humans, even if configural processing is difficult to demonstrate, it is advantageously possible to assess whether an odor mixture has a different quality from its single odorants Livermore and Laing, a ; Jinks and Laing, ; Bott and Chambers, ; Weiss et al.
Similar results were obtained with the RC mixture of six components, which is configurally perceived by newborn rabbits and specifically evokes a red cordial odor in human adults Le Berre et al. These findings, which resulted from the combined data obtained in rabbit pups and human adults, support the idea that mixtures of odorants can be perceived as odor objects in the sense that they can be configurally processed and can evoke new percepts, different from those of the elements, and which could be attributed to unique sources e.
Mean typicality ratings gray bars of the term pineapple obtained with a group of 20 untrained subjects for a binary mixture of ethyl isobutyrate and ethyl maltol, each single odorant and a control odorant allyl caproate carrying a typical pineapple odor. The error bars represent the standard error of the mean.
The results indicated that the binary mixture carried a pineapple odor that was significantly less present in the single odorants. This finding supports the idea that the odor quality of the mixture is different from those of its components adapted from Le Berre et al. Natural chemical signals frequently undergo concentration changes that produce differences in both the level and pattern of activation of ORs.
This variability makes the processing of complex stimuli even more difficult, since the olfactory system must extract perceptual constancy from inconstant input Gottfried, It has been argued that complex stimuli recognition might be concentration-invariant and mostly results from ratio-information extraction Cleland et al. For instance, rats can discriminate binary odor mixtures according to the molar ratios of their components, which further ensures mixture odor recognition at higher or lower concentrations Uchida and Mainen, The ratio of odorants in binary odor mixtures was also found to be the driving factor for odor processing and perception in insects e.
In rats, a binary mixture of the same two odorants can be processed elementally, configurally, or induce overshadowing Kay et al. The impact of mixed odorants ratios was clearly observed at the OSN level in rats Chaput et al. Supporting these findings, data obtained in both rabbit pups and human adults demonstrate the influence of in-mixture odorant ratios on processing and perception. In human adults, a barely detectable variation of one odorant concentration in the same mixture slight variation the ratio of the odorants , influenced its perception and particularly decreased its typicality toward pineapple Le Berre et al.
Therefore, the odorant concentration ratio in a mixture is clearly a key factor that can drive the configural versus elemental perception of the mixture. The chemical nature, or the odor quality, of the mixed odorants is another key factor of mixture processing Kay et al. Indeed, it is well-established from human studies dealing with food aroma analyses that there are key compounds in the complex chemical mixture of volatiles responsible for a given food aroma e.
Studies in animals have also demonstrated that certain odorants in mixtures can be more readily identifiable than others Staubli et al. Therefore these odorants can contribute more strongly to the overall perceptual quality of the whole odor mixture. For instance, in rats, the identity of the odorant removed from a complex component mixture affected the discrimination between the odorant mixture and the nine-odorant sub-mixtures. Nevertheless, rats had difficulty discriminating the whole mixture from the same mixture with one component missing.
In contrast, rats could reliably discriminate mixtures containing even small traces of contaminants from unadulterated complex mixtures; indeed, the replacement of an odorant by another was easily detected, and in a concentration-dependent manner Wilson and Sullivan, ; Lovitz et al. Data obtained in newborn rabbits have shown that once conditioned to one of the odorants, whatever the odorant, animals cannot generalize their behavioral response to a six-odorant RC mixture configurally processed.
This result supports the idea that the two stimuli are discriminated. Nevertheless, animals can generalize their response to the same mixture in which one odorant is missing five-component mixture , regardless of the odorant Sinding et al. These last results suggest that each odorant is a key odorant for rabbit pups. In contrast, data obtained using the same mixture in human subjects have shown that the red cordial odor quality carried by this six-odorant RC mixture is significantly different from the odor quality of some, but not all, sub-mixtures in which one odorant was missing Sinding et al.
Therefore, in human adults, many components would contribute more strongly to the overall perceptual quality of the odor mixture than do others. Interestingly, it has been recently reported that different mixtures made of 30 equally intense, non-overlapping components that span the physicochemical space of odorants, give rise to a similar odor quality for humans.
Even if such specific mixtures would be unlikely in ecological conditions, their processing is consistent with the concept of odor objects and might be of significant value as a model to decipher the mechanisms of odor mixture perception. Individuals from the same species do not necessarily perceive the same odor in a particular odorant, and more generally, they do not present the same sensibility to odor cues Amoore, ; Frumin et al.
This inter-individual variability may result from many factors, e. For example, anosmia to certain odorants is shared between identical twins and transmitted to offspring Wysocki et al. Conversely, some individuals have a better sensitivity for certain odorants compared to other individuals Keller et al. In this context, one may hypothesize that a contrasted sensitivity toward the components of a mixture can affect the ability to perceive odorants in mixtures and therefore directly influence the elemental vs.
One may suggest that the ratio of the component thresholds drives the perception of the mixture by the subjects, as occurs with the ratio of concentrations. Such questions remain to be explored in detail, but preliminary results in human adults indicate that some subjects perceive the pineapple AB mixture in a more robust configural way than do others; curiously, the more the subjects have a configural perception of AB, the lower their detection thresholds of the components Sinding, ; Sinding et al.
Regarding developmental aspects, one may consider that due to the maturation of the sensory systems and brain and the change in ecological niches encountered by the organism over the development, the processing of odor mixtures may also be modified over time. In particular, around birth, the urgent need for neonates to acquire knowledge about the novel, aerial environment, could result in higher elemental abilities than in adults.
Later in life, increased experience with a large variety of more or less complex odors and repeated exposure to some of the complex odors could promote their encoding as odor objects. While some results are in line with this developmental hypothesis Sinding et al.
This is consistent with the chemical complexity of early life environments perinatal niches from which organisms must rapidly extract salient information despite their immaturity, only relative see the section dedicated to behavioral aspects below. In addition to the previously discussed factors that clearly influence odor mixture processing, it is crucial to emphasize that the perception of odor mixtures is under cognitive control and that learning could shape this perception, but depending on the mixture.
In humans, odor-odor perceptual learning has been described and is likely comparable to odor-taste learning Wilson and Stevenson, a ; Case et al. This was demonstrated in a study in which an odorant, initially perceived with a cherry odor, smelled smokier after having been repeatedly experienced in mixture with guaiacol, another odorant perceived with a smoky odor. Furthermore, guaiacol smelled more like cherry after the co-exposure Stevenson, a. Odor-odor learning is not just stimulus -or quality- specific but is also a direct consequence of the learning procedure Stevenson, a.
Odors experienced in a mixture were judged to be more alike than were odors smelled an equal number of times but out of mixture. This exchange of perceptual qualities between mixed odorants is related to how similar the elements were judged Stevenson, a. These results support the idea that the representation of odor qualities can combine to form new configurations that carry their own odors. These results also indicate that cognitive processes are engaged to decrease the chemical complexity of the environment by building experience-dependent perceptual associations Wilson and Stevenson, a.
Results obtained in animal studies also demonstrate the impact of conditioning on odor mixture processing Livermore et al. 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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