Hedonic treatment is a crucial level of processing sensory information. The sense of smell is of particular interest in this regard: in humans, odors induce attractive or repulsive reactions and may influence cognition and behavior in various contexts (Alaoui-Ismaili et al., 1997a,b; Yeshurun and Sobel, 2010; Croy et al., 2011). From a cognitive point of view, odor-grouping experiments showed that hedonics is the most salient dimension of olfaction (Harper, 1966; Berglund et al., 1973; Schiffman, 1977). In these studies, subjects were exposed to various pairs of olfactory stimuli and asked to judge their similarity. It was usually observed that two main clusters were formed: one grouping together pleasant and the other unpleasant odors (Schiffman, 1974; Godinot et al., 1995).

Whereas psychophysical investigations have shown that such hedonic processing of smells is influenced by physicochemical properties (Khan et al., 2007; Mandairon et al., 2009; Poncelet et al., 2010; Joussain et al., 2011; Kermen et al., 2011; Zarzo, 2011), many other experiments, however, showed that odor pleasantness can be modulated by physiological (Fernandez et al., 2013; Joussain et al., 2013a,b) or cognitive factors (Herz, 2003; Rolls, 2004; de Araujo et al., 2005; Barkat et al., 2008; Rinck et al., 2011). For example, it has been shown that pleasantness judgments are enhanced when subjects are able to identify the odorant source (Ayabe-Kanamura et al., 1998). When verbal information about an odor is available, subjects shift their pleasantness judgment in line with the affective connotation of the label (Herz, 2003). Such top-down modulation by verbal association has been found even in children (Bensafi et al., 2007; Rinck et al., 2011). In summary, it would seem that both bottom-up (molecular feature coding) and top-down (training and language) processes contribute to build our hedonic responses to smells, which may be thus very variable across individuals.

Another factor that may explain olfactory individual differences is expertise. Training and verbal associations are crucial in professional situations in which odorants have to be associated systematically to label in order to ensure a common vocabulary to enhance perceptual agreement between individuals. Past and more recent studies showed that experts in olfaction used more consistent, rich, and precise language to describe smells (Bende and Nordin, 1997; Valentin and Chollet, 2000; Parr et al., 2002). Moreover, it has been shown that wine experts use more specific and relevant wine descriptors (Zucco et al., 2011). Although experts are known not only to acquire a systematic knowledge of the chemistry of odorants but also to learn to describe olfactory qualities of odorants and odor sources in a shared language, very little is known about the importance of hedonic processing in both the ways: (i) they describe but also (ii) they perceive smells. On a descriptive level, the literature in the field suggests that whereas pleasantness is a prominent attribute that drives odor verbalizations (Dubois and Rouby, 1997; Dubois, 2000), experts may be inclined to avoid any reference to pleasantness (Yoshida, 1964; Ehrlichman and Bastone, 1992; Holley, 2002). In the present study, we aim to test experimentally this hypothesis on a verbal level and to further assess how expertise modulates hedonic perception of odors. To this end, experts and non-experts in olfaction were compared during two olfactory tasks: (i) a verbal description task whereby participants were asked to freely describe odors and (ii) a perceptual rating task whereby participants were asked to judge the pleasantness of odors. Practically, four groups of subjects, differing in their levels of expertise, were tested: (i) an untrained group, (ii) a group of apprentice cooks, who had no specific course on olfaction but were daily exposed to odors, (iii) a group of experts in aroma formulation, and (iv) a group of experts in perfume formulation.

Moreover, because there is evidence of the existence of two different systems dedicated to treating aversive and appetitive smells [unpleasant odors are processed faster than pleasant ones (Bensafi et al., 2002d; Jacob et al., 2003), induced specific patterns of autonomic (Miltner et al., 1994; Brauchli et al., 1995; Ehrlichman et al., 1995; Alaoui-Ismaili et al., 1997a,b; Ehrlichman et al., 1997; Bensafi et al., 2002a,c) and olfactomotor responses (Bensafi et al., 2003a; Rouby et al., 2009) and specific neural activations (Zald and Pardo, 1997; Gottfried et al., 2002b; Anderson et al., 2003; Rolls et al., 2003; Royet et al., 2003; Bensafi et al., 2012)], odor hedonic valence per se was included as a factor in the analysis. Here, all participants were thus presented with unpleasant and pleasant odorant molecules. Specific hypotheses were: (i) on a verbal and descriptive level, experts (flavorists and perfumers) should use precise terminology without reference to pleasantness, whereas non-experts (novices and trainee cooks) should use less precise terminology accompanied by references to pleasantness; (ii) on a perceptual level, experts should not consider pleasantness and thus should rate pleasant odors as less pleasant and unpleasant odors as less unpleasant than non-experts.

The main question addressed by the present investigation concerned the effect of expertise on verbal descriptions and perceptual assessments of pleasant and unpleasant odors. It was assumed that flavorists and perfumers should rate pleasant odors as less pleasant, and unpleasant odors as less unpleasant than non-experts. Moreover, on a descriptive level, whereas flavorists and perfumers were expected to use chemical and odor terminology without referring to odor hedonics, novices were expected to accompany their odor descriptions by references to pleasantness.

An important finding of the present study is that, in contrast to our expectations, hedonic perceptual ratings of unpleasant and pleasant odors was not affected by expertise: novices, trainee cooks, flavorists and perfumers rated similarly unpleasant odors on the one hand and pleasant odors on the other hand. As was shown in wine tasting (Valentin and Chollet, 2000) where experts and naïve subjects do not significantly differ in perceptual similarity judgment, the present study suggests that experts in olfaction are able to discriminate and/or categorize odors on the basis of their hedonic valence. However, although this is true at an evaluative or perceptual level (pleasantness ratings), verbal data suggest that experts describe and conceptualize odors with few references to pleasantness: a result of interest of our study was the low number of references to pleasantness in the verbal descriptions of experts, whereas novices used hedonic terms to describe odors (especially words with negative connotation). These results are in line with the literature in the field suggesting that experts in olfaction avoid references to odor hedonic valence (Yoshida, 1964; Ehrlichman and Bastone, 1992; Holley, 2002).

An interpretation of the discrepancy between an expert’s ability to use less references to unpleasantness than controls vs. his actual perceptual hedonic appreciation of unpleasant (and pleasant) odors which remains the same, could be that on a perceptual level, hedonic valence and especially its negative side, represents the basic level of odor categorization for any perceiver, independent of his/her expertise. This affective perception would occur quickly and unwittingly. In accordance with the above, autonomic responses to unpleasant odors occur implicitly when subjects are not given any particular instruction (Bensafi et al., 2002b), and response times are significantly shorter for unpleasant than for pleasant odors (Bensafi et al., 2003b). These results seem to indicate a “quick and dirty” pathway, fast-tracking decision for bad odors. Brain imaging studies also show that pleasant and unpleasant odors activate different neural networks (Zald and Pardo, 1997; Gottfried et al., 2002a; Anderson et al., 2003; Rolls et al., 2003; Royet et al., 2003; de Araujo et al., 2005; Bensafi et al., 2008). Taken together, these results support the hypothesis that only a rudimentary level of processing is necessary to hedonically pre-process odors, and that this pre-processing takes place when perceivers do not attend to any other specific feature of the odorant stimulus, whatever the expertise level. However, when experts are engaged in a verbal task requiring subtle discrimination and description, they process the same odors more deeply on a lexico-semantic level, with few hedonic references.

On a lexical level, verbal descriptions in relation to smells were significantly longer in experts than untrained subjects, confirming expectations regarding experts’ explicit knowledge. Previous studies described the language of experts (perfumers and flavorists) as richer, more proficient, precise, expressive and/or consistent (Bende and Nordin, 1997; Parr et al., 2002). In line with this, the linguistic-based criteria used here showed that experts’ verbal skills were characterized by the use of chemical names and terms referring to odor qualities and sources. Moreover, trainee cooks used more odor source references than novices, suggesting that daily exposure to odor sources (food sources in this case, without explicit olfactory associative learning) can increase the verbal ability to describe smells.

Lack of verbal resources in odor processing is a characteristic of untrained subjects. Indeed, it is a common experience to like (or dislike) a specific odor, and to be quite sure of recognizing it even if no name can be put on it: this so-called ‘tip of the nose phenomenon’ highlights implicit knowledge of odors, despite failure to name them. This interaction between language and olfaction can be seen in the development of olfactory function: whereas a 3-year-old child learns to name colors, odor naming is mostly developed through autonomous learning (Rouby and Sicard, 1997) and expressed in terms of idiosyncratic experience (Engen, 1987). On the contrary, expert verbal skills in our study were characterized by the use of domain-specific terminology, with very few references to pleasantness. These differences between experts and novices reflect the effect of learning for flavorists and perfumers since their olfactory education includes learning of chemical names and olfactory qualities with adjectives (see the “the field of odors,” Jaubert et al., 1987; Jaubert, 1995). For example, in perfumers and flavorists particularly, the creation of fragrances and flavors involves recognizing hundreds of odorants and memorizing the effects of their combinations. Reports indicate that perfumers are better able to imagine odors (Gilbert et al., 1998; see also Rinck et al., 2009) and can routinely group odors in classes, from 18 (Rimmel, 1895) to 88 (Arctander, 1969; Chastrette et al., 1988). For perfumers, these classes usually contain further sub-classes (Roudnitska, 1991; Ellena, 2007). Moreover, notions such as “notes,” “faces,” and “sub-tones” are used in perfumery to represent odors (Ellena, 2007). Experts, through such continuous repetitive olfactory training, can communicate their perception using verbal supports which is of upmost importance in their professional practice.

Although the present study provides evidence for an influence of expertise on odor verbalization, some of the findings warrant discussion. Indeed, another particular feature of the present findings was the increased perceived intensity in perfumers. One potential explanation may be that perfumers have lower odor threshold leading to higher perceived intensity due to their past training. Unfortunately, very little information is available to confirm this hypothesis and one of the few studies that compared experts and novices on a sensory level was that of (Bende and Nordin, 1997) who showed no expertise effect on olfactory detection, rendering less likely this possibility. Another explanation may be that perceived intensity is higher for identified odors (Distel and Hudson, 2001). In this psychophysical study, the authors tested human participants with a large set of everyday odorants, and asked their subjects to rate odor pleasantness, familiarity and intensity. Results showed that all these ratings (including odor intensity) were enhanced when participants either were given the name by the experimenter or could identify the odorant source themselves. In the same line, the increase in odor intensity seen in experts of our study may be related to their better ability to describe, name and identified the odors used.

In conclusion, we showed here that expertise does not influence odor hedonic perception per se when the subject’s attention is focused on pleasantness: experts and novices appreciated similarly pleasant and unpleasant odors. On a verbal level, in contrast to experts, novices do not have rich lexical representations of smells, and they often use words referring to environmental odor sources accompanied by perceptually hedonic terms, often referring to unpleasantness. However, when attention is directed toward the lexical component of odor representations, experts seem to avoid references to pleasantness. These findings offer new insights into odor hedonic perception in untrained and expert populations, highlighting for the first time an influence of expertise at the verbal but not at the perceptual level of processing, providing new understanding on perceptual processing of pleasant and unpleasant odors.

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.