Received: 21 December 2016
Revised: 4 July 2017
Accepted: 7 September 2017
DOI: 10.1002/ffj.3429
RESEARCH ARTICLE
Key odorants or key associations? Insights into elemental and configural odour processing Sébastien Romagny
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Gérard Coureaud
Centre des Sciences du Goût et de l'Alimentation, INRA, CNRS, AgroSup Dijon, Université Bourgogne Franche‐Comté, F‐ 21000 Dijon, France
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Thierry Thomas‐Danguin
Abstract Determining whether odorants can be perceived in an odour mixture or whether the mixture smells different from its components remains challenging. Even in highly complex mixtures, the
Correspondence Thierry Thomas‐Danguin, Centre des Sciences du Goût et de l'Alimentation, INRA; 17 rue Sully, 21000 Dijon, France. Email: thierry.thomas‐
[email protected]
odour qualities of some elements can be perceived; thus, their identity is conserved within the
Funding information French Ministry of Higher Education and Research; FEDER (European Regional Development Fund); Regional Council of Burgundy
when they are omitted. In this case, mixture perception often relies on configural processing.
mixture. Such elements are considered key components and support the elemental perception of the mixture. The concept of key components is also related to elements that do not necessarily carry the odour quality of the mixture but that induce a change in overall mixture perception To disentangle these multiple aspects of these so‐called key odorants, we sought to study the perceptual role of odorants included in mixtures that are elementally or configurally perceived. Two mixtures, known to be processed configurally and elementally and containing the same 6 odorants in different proportions, were used as references in 4 similarity‐rating experiments. A total of 246 participants evaluated the similarity between the references and single odorants or mixtures of 2 to 6 odorants. This procedure aimed to evaluate whether single odorants or combinations of odorants can evoke the odour quality of the mixtures. Overall, the results highlighted that elemental perception depended primarily on the odour quality and concentration ratio of many of the mixed odorants, whereas configural perception depended on specific associations of odorants in strict concentration ratios. These results led us to reconsider the impact of key elements in odour mixtures within the framework of a perceptual model stating that elemental perception of odour mixtures relies on perception of key odorants, the perceptual features of which are still perceived within the mixture, whereas configural perception relies on key associations of odorants that lose their individual identity when mixed at specific concentration ratios. KEY W ORDS
human, perception, odour mixture, odour quality, complexity, concentration ratio
1
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I N T RO D U CT I O N
are also driven by the composition of the stimulus, especially the odour quality of each of the odorants and their relative concentrations.4
Our everyday living environment is composed of a myriad of volatile
The chemical and perceptual natures of an odorant, which lead to
chemicals, many of which provide odour cues, from food, plants and
its odour quality, are among the most important factors in determining
anthropic activities. To process such complexity, organisms can per-
whether an odour mixture is elementally or configurally perceived.
ceive mixtures of odorants through two non‐exclusive strategies: ele-
These factors are thus prevalent in discrimination and generalisation
mental or configural processing.
1-4
Elemental perception enables an
experiments with animals, where the response to odour mixtures dif-
organism to identify the different components of a mixture. Con-
fers from the response to the individual odorants.3,10,11,15-20
versely, the odorants of a mixture can blend and give rise to the
Configural processing was thus found to depend on the specific odor-
configural perception of a novel odour that is distinct from the odour
ants included in the mixture. For instance, a series of experiments with
5
of each individual odorant. These two processing strategies can be
newborn rabbits have shown that animals were not able to generalise
influenced by individual‐related factors such as learning,6-11 adapta-
their behavioural responsiveness to the odour of ethyl isobutyrate or
tion,12 and psychological state.13,14 However, these processing paths
the odour of ethyl maltol in a binary mixture (30:70 v/v ratio), whereas
Flavour Fragr J. 2018;33:97–105.
wileyonlinelibrary.com/journal/ffj
Copyright © 2017 John Wiley & Sons, Ltd.
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they were perfectly able to display such a generalisation to the odour
mixture, called RC, has been shown to trigger configural perception
of either odorant when separately mixed with guaiacol.21 Results in
and to evoke the specific odour of grenadine (Red Cordial) at a spe-
humans are consistent with these findings since the binary mixture
cific concentration ratio of the 6 odorants. The second 6‐component
of ethyl isobutyrate and ethyl maltol was found to carry an odour sig-
mixture (RCmod), which includes the same 6 odorants but in a
nificantly different from each single odorant, whereas this was not the
modified concentration ratio, has been found to be elementally
case for another binary mixture including odorants with similar odour
processed.36-38 Mixture processing has been evaluated through a
notes (ethyl caproate and furaneol).
22-24
All these results support the
similarity rating task, during which subjects rated the similarity of
idea that configural and elemental odour processing depends on the
odour samples containing 1 to 6 components to either the RC or
perceptual quality of the mixed odorants.
the RCmod reference mixtures. As a direct comparison paradigm,
The relative concentrations of the components (or ratio) also con-
similarity rating has several advantages with regard to our hypothe-
tribute to the overall quality of an odour mixture. In rats, it was shown
sis: it limits cognitive bias; it does not engage panellists in an elemen-
that mixtures of citral and octanal or citronellal and octanal could lead
tal strategy, which may impair the perception of mixture‐specific
to elemental or configural perception depending on the proportions of
odour quality22; and it is primarily based on the evaluation of odour
5,17
the elements in the mixture.
In rabbit neonates, modifying the com-
quality.24
ponent concentration ratio in a binary mixture completely broke its
Four experiments were conducted in this study. In Experiment 1,
configural processing.20,25 In humans, the odour quality of several mix-
we tested whether specific associations of odorants in sub‐mixtures
tures has been shown to be a function of the concentration ratio of the
or single odorants carried the configural odour quality of the RC mix-
odorants, namely their respective intensity.26,27 Moreover, very small
ture. To do so, participants had to rate the odour similarity between
changes in the odorant concentration ratio were shown to be suffi-
RC (constant reference) and a selection of RC sub‐mixtures and the
cient to decrease the configural perception of a binary or a ternary
individual components of RC. In Experiment 2, we investigated
28
mixture.
Therefore, the component concentration ratio is thought
to be a driving factor of odour mixture quality.29 Depending on the odour quality of each of the odorants and their
whether the simplest associations of odorants, namely binary mixtures, could carry the configural odour quality of the RC mixture. Hence, participants were asked to rate the odour similarity between RC (constant
concentration ratio in the mixture, several odorants may be considered
reference) and the 15 possible binary sub‐mixtures of RC. Experiments
as impacting elements, insomuch as their omission changes the overall
3 and 4 were designed to control for the specificity of the results
perception of the mixture.30-33 These elements are often defined as
obtained in the two first experiments with regard to elemental and
key/impact components. However, two distinct situations have been
configural mixture processing. Thus, Experiment 3 aimed to assess
reported in the literature. The first one was described by Bult and col-
whether specific associations of odorants in sub‐mixtures or single
leagues (2002)34: in a 10‐component mixture smelling like apple, at
odorants carried an odour quality close to the elementally processed
least two odorants already carried the apple quality of the whole mix-
mixture RCmod. Participants had to rate the odour similarity between
ture; therefore, these odorants were considered key. The second
RCmod (constant reference) and a selection of RC sub‐mixtures, the
example situation occurred in a study exploring the odour of caramel,
individual components of RC, and the RC mixture. Finally, Experiment
in which the omission of nutty notes drastically decreased the caramel
4 replicated Experiment 3 while using the odorant concentrations that
typicality of the whole odour mixture.35 In this case, the nutty com-
were used in RCmod instead of RC.
pounds were believed to be key for the mixture perception, even if their odour quality was different from the caramel odour. These results led us to consider that the impact of the so‐called key elements actu-
2
MATERIALS AND METHODS
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ally differs between these two situations. For the apple mixture, the key odorants could have led to the elemental perception of the mixture, whereas for the caramel mixture, the nutty notes may have con-
2.1
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Subjects
tributed to the configural perception of the mixture. Thence, we
The 246 participants were randomly assigned to only one of the four
hypothesised that for some mixtures, the “key odorants” carry, at least
experiments. Experiments 1 to 4 included, respectively, 61 (32 women,
in part, the odour quality of the mixture and can be perceived through
29 men; mean age 36.7 ± 12.6 years old), 63 (37 w, 26 m; 34.6 ± 11.9
its elemental perceptual processing, whereas for other mixtures, the
y/o), 59 (32 w, 27 m; 43.9 ± 12.2 y/o) and 63 subjects (32 w, 31 m;
“key odorants” do not smell like the mixture but are essential to its
40.4 ± 13.9 y/o). Panellists self‐reported having neither problems with
odour and thus may contribute to its configural processing.
their sense of smell nor allergies. They were asked to avoid smoking,
Testing this hypothesis, namely disentangling the multiple
drinking and eating for at least 1 hour before each session and to avoid
aspects of key odorants in mixtures, requires studying the perceptual
the use of perfume the day of the sensory test. All participants signed
roles of the odorants that are included in mixtures elementally or
an informed consent form, but the aim of the experiment was not
configurally perceived. Such knowledge may in turn contribute to
revealed. Subjects were paid for their participation (10 €/hour). The
our understanding of odour perception in real‐world situations and
experiments were conducted in accordance with the Declaration of
to the efficiency of the aroma analysis process. Thus, we set out to
Helsinki for Medical Research involving Human Subjects and with eth-
examine the perceptual impact of the nature and concentration ratio
ical rules enforced by French law; the protocol was reviewed and
of the odorants included in two 6‐component mixtures and their sub‐
approved by the human research ethics committee (Dijon, France;
mixtures containing 2 to 5 components. The first 6‐component
# 2012‐A00959‐34).
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ET AL.
Stimuli
Each vial was coded with a random three‐digit number, and the
The RC mixture was formulated by a flavourist to produce the specific odour quality of “Red Cordial” (“grenadine” in French). The RCmod mixture contained the same 6 food‐grade odorants than RC but in a different ratio (Table 1). Adequate volumes of the stock solutions were mixed to obtain the target proportions for the two mixtures. The final concentrations of each odorant in the RC and RCmod mixtures were selected to ensure that the overall perceived odour intensity was similar for the two mixtures, but the overall odour quality of the mixtures was different (see below experimental procedure and Sinding et al. 201336). The six odorants were also used alone or in sub‐mixtures (2 to 5 components). In those cases, the relative proportion of the odorants in each sub‐mixture was kept the same as in the RC mixture, except in Experiment 4, in which the proportions of the odorants in each
order of stimulus presentation followed a Williams' Latin square design for each experiment. Following this design, each vial was opened no more than 10 times during an experiment, and an interval of at least 1 hour was imposed between two participants to ensure that the gas phase had re‐equilibrated. A marginal decrease in odour intensity occurred in one of the 8 sensory sessions of Experiment 1 (F(7, 45) = 3.23, p = 0.007 vs F(7, 50) < 1.37, p > 0.2 for all other experiments). In Experiment 1, this effect was related to the 6th testing session (see experimental procedure below), in which the mean perceived intensity was rated as slightly lower (11%, p < 0.01) than in the other sessions. Nevertheless, no significant between‐stimuli statistical interactions were observed (F(14, 630) = 0.89, p = 0.57), indicating that slight intensity differences could not have contributed to the similarity results.
sub‐mixture corresponded to the proportions in RCmod. A series of well‐selected sub‐mixtures were tested; they were specifically chosen depending on the aim of each experiment (see experimental
2.3
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Experimental procedure
procedure below). While maintaining constant odorant proportions,
All of the experiments were conducted in an air‐conditioned odourless
the overall odorant concentration was occasionally increased (up to
room (21 ± 1°C) dedicated to human sensory studies (ChemoSens plat-
4×), particularly for the individual odorants and the binary sub‐
form, CSGA). For each experiment, the tests took place on two consec-
mixtures, to equalise the overall intensity of the stimuli. This
utive days in sessions that each lasted 1 hour and were spread
approach avoided the possibility that differences between stimuli
throughout the day, with 8 participants seated in individual booths.
could be attributed to samples having different intensities. Iso‐
In Experiment 1, subjects had to rate the similarity between the
intensity was checked by a panel of 18 volunteers in preliminary
odour of the reference RC mixture and the odour of a sample including
ranking task experiments (data not shown) and after each experi-
either a single odorant, a sub‐mixture or the RC mixture (control). The
ment (see below). The results indicated that only the single odorants
sub‐mixtures were selected based on the results reported in Sinding
F and EA were perceived as less intense than the other stimuli, and
et al. (2013)36 regarding the odour quality of the single odorants.
these slight differences did not affect the results or conclusions of
Indeed, the odour qualities of IA and V have been shown to be closer
the experiments reported here (no significant correlation (p > 0.1)
than those of the four other odorants to the odour of RC. Therefore,
between similarity and intensity data).
we selected 4 sub‐mixtures of increasing chemical complexity that
To deliver the odour stimuli to the participants, 20 μL of the mix-
contained these two odorants (IA‐V, IA‐V‐F, IA‐V‐F‐D, IA‐V‐F‐D‐B)
ture, sub‐mixture or single odorant solution was poured onto four
and 4 others that did not (EA‐B, EA‐B‐D, EA‐B‐D‐F, EA‐B‐D‐F‐IA;
strips of filter paper (40 x 5 mm). Ethanol was evaporated for 2 minutes
see also Romagny et al. 201437). Each participant received monadically
under the constant airflow of an extractor hood, and the strips were
15 pairs of coded vials, one containing the reference (RC) and the other
immediately put in a brown glass vial (60 mL). Vials were kept at room
containing the tested sample. The order of presentation of RC in each
temperature (21 ± 1°C) for 36 hours before testing to reach gas phase
pair was balanced, and participants were instructed to successively
equilibrium. Gas Chromatography (GC‐FID) analyses showed that eth-
smell each vial just once. Subjects rated the odour similarity using a
anol was still present in the vial but was not detectable by participants.
13 cm linear scale that ranged from “Not similar at all” to “Totally sim-
The overall concentration of Volatile Organic Compounds (VOCs) was
ilar”, and was displayed on a computer screen (Fizz software,
checked with a photoionization detector (PID) after each sensory ses-
Couternon, France). Ratings were then converted into similarity scores
sion to ensure that similarity results were never due to slight variations
ranging from 0 to 10. A 30‐second pause was imposed between each
in the concentration of VOCs (r(158) < 0.02, p > 0.8).
pair in order to limit sensory adaptation.
Odorants (Sigma‐Aldrich, France) included in the 6‐component RC and RCmod mixtures. Stock solutions were prepared in ethanol (anhydrous, 99.9%, Carlo Erba, France)
TABLE 1
Odorant
Acronym
CAS #
Concentration of stock solutions (w/w)
Concentration in RC (mg/ml)
Concentration in RCmod (mg/ml)
Vanillin
V
8014‐42‐4
10°%
16.2
0.40
Frambinone
F
5471‐51‐2
10°%
16.2
0.35
Isoamyl acetate
IA
123‐92‐2
1°%
1.9
0.22
Damascenone
D
23696‐85‐7
1°%
1.1
3.3
β‐ionone
B
79‐77‐6
1°%
1.6
3.4
Ethyl acetate
EA
141‐78‐6
10°%
1.6
0.33
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In Experiment 2, the procedure was the same as in Experiment 1,
same chemical complexity, the nature of the odorants influenced the
except that the samples compared to RC were the 15 possible binary
perceptual similarity of that sub‐mixture to the RC odour. More pre-
sub‐mixtures, as well as RC as a control. Experiments 3 and 4 were
cisely, sub‐mixtures containing the IA and V odorants were rated as
identical to Experiment 1 with two exceptions. First, the reference
more similar to RC, regardless of their chemical complexity, a result
for the similarity rating was RCmod instead of RC; second, the propor-
shown in Figure 1. With the exception of the binary IA‐V mixture,
tions of the odorants in the sub‐mixtures of Experiment 4 were consis-
the sub‐mixtures containing both IA and V received mean similarity
tent with those in the RCmod mixture, whereas the proportions of
scores that were not significantly different from the control RC sample.
odorants used in Experiments 1, 2, and 3 were those of RC.
In the case of the sub‐mixtures that did not include both IA and V, the
Control pairs, namely RC vs RC in Experiments 1 and 2 and RCmod mod
odour similarity clearly increased with chemical complexity (see sub‐
in Experiments 3 and 4, were included to assess the consis-
mixtures from EA‐B to EA‐B‐D‐F‐IA in Figure 1) but remained lower
tency of similarity ratings. No significant difference between the two
than the similarity score of the IA‐V binary mixture. Moreover, the
control samples was detected (p > 0.5; Dunnett's tests).
results of this experiment confirm the configural processing of the
vs RC
RC mixture because the odour similarity of each single odorant was
2.4
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Statistics
All statistical analyses were conducted with R software.39 Similarity scores were subjected to analyses of variance (ANOVA) using linear mixed‐effects models with participants as random factor (nlme package40). The fixed effects tested were the odour stimulus (i.e., the tested
significantly lower than that of the control RC sample. Taken together, the results of Experiment 1 support the assumption of an influence of components' odour quality on the perceived similarity between the sub‐mixtures and the RC configural mixture even if the single components did not evoke the RC odour quality.
sample) and the stimulus chemical complexity (1 to 6 odorants included in a given stimulus, not considered for Experiment 2). We also tested the effect of the sensory session (i.e., the time rank of a given sensory session within an experiment) as well as the age and sex of the participants. The factor age was never significant (p > 0.5), and the factors sex and session were significant in Experiment 2, but the interaction between these factors and the factor stimulus was never significant (p > 0.8). Thus, they were not considered further. For each experiment, RC (or RCmod in Experiments 3 and 4) was set as the reference sample; consequently, post hoc tests were carried out using Dunnett's test. Except for Experiment 2, correlations between the similarity ratings and complexity of the stimuli were assessed with Spearman's correlations (or Kendall's in the case of equally ranked samples) because complexity was considered as a discrete factor. For all data analyses, the effects were considered significant when p < 0.05.
3
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3.2 | Experiment 2 – Odour similarity of the RC mixture with all its binary sub‐mixtures The 15 possible binary RC sub‐mixtures were rated by the panel for their odour similarity with the 6‐component RC mixture. ANOVA on similarity scores showed a highly significant effect of the factor stimulus. This finding provided clear evidence for differences in odour similarity to RC among the binary mixtures (F(15, 915) = 28.4, p < 0.0001; Figure 2). The comparison of means revealed that all binary mixtures had significantly lower similarity than the control RC sample, except for the IA‐V sub‐mixture, which received a lower but not significantly different score (p = 0.2). While the majority of the binary mixtures did not obtain a mean similarity higher than 4 (the mean for the control RC sample was 7.7), all the mixtures that contained IA received scores that were the closest to RC (Figure 2). The binary mixtures that
RESULTS
3.1 | Experiment 1 – Odour similarity of the RC mixture with its binary to quinary sub‐mixtures In this experiment, participants rated the odour similarity of a selection of binary, ternary, quaternary and quinary RC sub‐mixtures to the target 6‐component RC mixture. The sample set also included the 6 single odorants and the RC mixture as a control. Among the 15 tested samples, the RC control sample (comparing RC vs RC) received the highest similarity score (mean similarity ± standard error: 7.69 ± 2.57). An ANOVA on odour similarity scores highlighted that the complexity factor was highly significant (F(5, 849) = 94.3, p < 0.0001). It must be noted that, even if single odorants likely contributed to the complexity effect, the correlation between odour similarity and complexity was still significant when only the sub‐mixtures were considered (r(549) = 0.24, p < 0.0001). This result suggested that all six odorants contribute to the RC mixture odour quality. ANOVA results provided evidence that the stimulus factor was also highly significant (F(14, 728) = 59.2, p < 0.0001), which indicated that for two given sub‐mixtures of the
FIGURE 1 Evaluation of the similarity (Experiment 1; n = 61 participants) between single odorants, mixtures of 2 to 5 odorants, the 6‐odorant mixture RC (control) and the reference configural mixture, RC. The concentration ratio of odorants in sub‐mixtures corresponds to their concentration in RC. Bars display mean (± 95% CI) similarity scores, and mixture complexity is indicated by the grey shading. The horizontal line on the top of the bars indicates stimuli whose mean similarity scores were not significantly different from the control RC (p > 0.05)
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FIGURE 2 Evaluation of the similarity (Experiment 2; n = 63 participants) between each possible binary mixture or the 6‐odorant mixture RC (control) and the reference configural mixture, RC. The concentration ratio of odorants in sub‐mixtures corresponds to their concentrations in RC. Bars display mean (± 95% CI) similarity scores, and the presence of the two odorants IA and V is indicated by the grey shading. The horizontal line on the top of the bars indicates stimuli whose mean similarity scores were not significantly different from the control RC (p > 0.05)
FIGURE 3 Evaluation of the similarity (Experiment 3; n = 59 participants) between single odorants, mixtures of 2 to 5 odorants, the 6‐odorant mixtures RC or RCmod (control) and the reference elemental mixture, RCmod. The concentration ratio of odorants in sub‐ mixtures corresponds to their concentrations in RC. Bars display mean (± 95% CI) similarity scores, and mixture complexity is indicated by the grey shading. The horizontal line on the top of the bars indicates stimuli whose mean similarity scores were not significantly different from the control RCmod (p > 0.05)
included V but not IA obtained higher mean similarity scores than the
in Experiment 1, more complex mixtures (i.e., 4 and 5 odorants) did not
remaining mixtures. Therefore, the results of Experiment 2 highlighted
receive the highest similarity scores. Indeed, the significant influence
that, among the six odorants, the qualities of IA and, to a lesser extent,
of the factor complexity was due to the individual odorants; when only
V are major in‐mixture contributors to the odour quality of the
the sub‐mixtures were considered, the correlation between complexity
configural RC mixture. Moreover, the IA‐V association elicited an
and similarity was marginally significant, but the correlation was very
odour quality very similar to the full RC mixture.
low and negative (r(472) = ‐0.06, p = 0.075). Thus, the results obtained in this experiment support a key role for the concentration ratio of the
3.3 | Experiment 3 – Odour similarity of binary to quinary sub‐mixtures of RC with the RCmod mixture
odorants in the odour quality of complex mixtures. They also suggest
In this experiment, participants rated the odour similarity of the RC
association in the quality of RC.
mod
sub‐mixtures (identical to those used in Experiment 1) with RC
the importance of the individual qualities of odorants B and D in the odour quality of the mixture RCmod and confirm the role of the IA‐V as
the target mixture. In addition to the 6 individual odorants and the RCmod mixture (control), the RC mixture was included in the sample set for comparison with RCmod. The mean similarity scores are
3.4 | Experiment 4 – Odour similarity of the RCmod mixture with its binary to quinary sub‐mixtures
reported in Figure 3. We first compared the similarity score between
In this last experiment, the panel rated the odour similarity of a selec-
RC and RCmod and found that the odour of RC was highly dissimilar
tion of binary, ternary, quaternary and quinary RCmod sub‐mixtures
mod
(p < 0.0001). This result indicated that the mod-
with the entire RCmod mixture. The sample set also included the 6
ification of the concentration ratio of the odorants within the mixtures
single odorants and the RCmod mixture itself as a control. The RCmod
led to a large change in the odour quality. ANOVA results showed that
sub‐mixtures contained the same odorants as those used in
the factor stimulus significantly affected the similarity scores (F(15,
Experiments 1 and 3, but at the concentrations used in the RCmod
750) = 14.2, p < 0.0001). According to post hoc tests, all samples
mixture. The mean similarity scores are reported in Figure 4.
to the odour of RC
were judged as significantly different from the control RCmod sample
The results of ANOVA on similarity scores showed that the factors
(p < 0.001), with the exception of the EA‐B‐D mixture (p > 0.5). In con-
stimulus and complexity were highly significant (F(14, 756) = 54.1 and
trast to the results of Experiment 1, two single odorants (B and D)
F(5, 877) = 81.1, p < 0.0001). The correlation between complexity
obtained quite high scores, suggesting that their odours were similar
and similarity was still significant when single odorants were not con-
to that of the RCmod mixture. Interestingly, all of the mixtures contain-
sidered (r(504) = 0.27, p < 0.0001), suggesting that all the odorants
ing both odorants IA and V, even if mixed with odorants B or D, had
contribute to the odour quality of the whole mixture. In line with
low similarity scores; these scores were comparable to the similarity
Experiment 3, odorants B and D specifically obtained a high similarity
score of RC vs RCmod (Figure 3B). This result was in line with the data
score when compared to the RCmod mixture, reaffirming the major role
obtained in Experiment 1, which supported the influence of an associ-
of their qualities in the odour of RCmod. This result was also supported
ation between the two odorants IA and V in the RC mixture odour
by post hoc tests. All of the tested sub‐mixtures that included one of
quality. The factor complexity was also significant in the ANOVA
these two odorants, regardless of their complexity, did not differ signif-
(F(5, 880) = 10.2, p < 0.0001). However, in contrast to what was found
icantly in similarity score from the RCmod target sample. Thus, results
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ET AL.
results obtained in Experiment 3 confirmed that these two mixtures have dissimilar odour qualities. The RC mixture was used here as a reference for configural processing because it was previously found to be configurally processed.22,36-38,41 The similarity data obtained in Experiment 1 supported the configural perception of this mixture. Indeed, compared with RC, the average odour similarity of each single component was judged to be very low (< 3.2), demonstrating that none of the components had an odour quality similar to that of the mixture. The RCmod mixture was chosen as a reference for elemental processing because it has been shown to be elementally processed.36,38 Indeed, in a previous sorting study, RCmod was grouped with two of its components (B and D), which suggested that the odour quality of the mixture FIGURE 4
Evaluation of the similarity (Experiment 4; n = 63 participants) between single odorants, mixtures of 2 to 5 odorants, the 6‐odorant RCmod mixture (control) and the reference elemental mixture, RCmod. The concentration ratio of odorants in sub‐mixtures corresponds to their concentration in RCmod. Bars display mean (± 95% CI) similarity scores, and mixture complexity is indicated by the grey shading. The horizontal line on the top of the bars indicates stimuli whose mean similarity score was not significantly different from the control RCmod (p > 0.05)
was similar to that of the components.42 The results of Experiment 3 and 4 were fully in line with these previous findings, since both odorants B and D were rated as similar to the mixture RCmod, demonstrating that the mixture's odour resembled the odours of these two components. Nevertheless, the similarity scores for the single odorants B and D were significantly lower than the score obtained for the reference pair (RCmod vs RCmod), suggesting that the characteristic odour qualities of both components were perceived within the RCmod mixture and produced its composite odour. Even if not directly tested
of Experiment 4 showed that the nature of the odorants contributed to
through the comparison between the binary BD mixture and the
the odour quality of the RCmod mixture and confirmed that the individ-
RCmod mixture, the assumption that the quality of RCmod was the
ual qualities of B and D largely accounted for the odour quality of the
sum of the odour qualities of B and D is supported by several observa-
overall mixture.
tions: 1) in both Experiment 3 and Experiment 4, the ternary mixture EA‐B‐D was highly similar to RCmod, while EA was very dissimilar to RCmod; 2) the binary mixture EA‐B was not more similar to RCmod than
4
|
DISCUSSION
was B alone, showing that EA was not a key odorant or a contributor to the odour of RCmod; 3) in Experiment 4, even without EA, the IA‐V‐F‐
The present study aimed to evaluate the influence of chemical compo-
B‐D quinary mixture received a very high score for similarity to RCmod;
sition on the perception of odour mixtures and especially to disentan-
and
gle the multiple aspects of “key odorants” regarding the elemental or
Experiments 1 and 3 (Figure 5) very clearly showed that B and D had
configural processing of a mixture. Our central hypothesis was that,
odour qualities very similar to that of the RCmod mixture, so would
in some mixtures, odorants may carry the odour quality of the mixture
be their binary mixture. Overall, these results converge with previous
and thus can be perceived through elemental perceptual processing of
data to support the idea that the RC mixture is configurally processed,
the mixture, whereas for other mixtures, odorants that do not smell
whereas the RCmod mixture is elementally processed.
4)
the
elementalness‐configuralness
scale
derived
from
like the mixture but are nevertheless essential to its odour may contribute to its configural processing. This would lead to reconsider the notion of “key” or “impact” components in an odour mixture.
4.1 | Configural perception of the RC and elemental perception the RCmod mixtures mod
4.2 | Mixture‐related factors leading to configural processing Our results also provide evidence that mixture complexity, in terms of number of components, is not the only composition‐related factor that contributed to the emergence of a configural odour.[43,44] For instance,
, which are considered very dif-
the components' concentration ratio plays a role in the emergence of
ferent perceptually despite sharing the same 6 components.36 The
the configural odour, as shown by the highly dissimilar odour qualities
We used two mixtures, RC and RC
Elementalness‐Configuralness scale derived from the combination of Experiments 1 and 3. RC and RCmod were the references for configural and elemental perception, respectively, so that an index of Elementalness‐Configuralness was calculated based on the means as follows: Similarity(stimulus vs RC) ‐ Similarity(stimulus vs RCmod), where each stimulus is either a single odorant, a sub‐mixture or a mixture. According to their index values, stimuli were plotted on the line ranging from elemental to configural. The line illustrates the contribution of each stimulus to elemental or configural perception.
FIGURE 5
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of the RC and RCmod 6‐component mixtures (Experiment 3). Moreover,
a specific role for the IA‐V combination in the configural odour percep-
in Experiment 2, where complexity was kept constant, the nature of
tion of RC.
the odorants played a critical role in the binary mixtures' odour quality. Thus, when odorants IA and V were mixed (even in combination with any other odorant of the RC mixture), their perceptual combination contributed to an increase in the perceived similarity between the
4.3 | Mixture‐related factors leading to elemental processing
sub‐mixtures' odour quality and that of the RC mixture. Furthermore,
Regarding elemental processing, we expected that the odour similarity
combining the results of Experiments 1 and 3, it appeared that all
between sub‐mixtures and the RCmod mixture would be driven by the
sub‐mixtures that contained both IA and V, regardless of the other
nature and the concentration ratio of the mixed odorants.
mod
components, had an odour that was dissimilar to RC
but similar to
First, it has been shown that the proportion of odorants' intensity
RC. The roles of these two odorants and their combination became
directly predicted the R Found. Stat Comput probability of identifying
obvious when using an index based on means of similarity extracted
each component; the higher a given odorant's proportion is, the higher
from Experiments 1 and 3. In these experiments, the mixtures RC
its identification probability.26,43 We also found that odorants' concen-
mod
and RC
were taken as the references for configural and elemental
tration ratio affected the perceptual qualities of mixtures, as shown by
perception, respectively, so that an index of “Elementalness‐
the contrasting results obtained in Experiments 3 and 4. For instance,
Configuralness” can be calculated on the basis of similarity scores:
the complex mixture IA‐V‐F‐D‐B was highly dissimilar to RCmod in
Similarity(stimulus vs RC) – Similarity(stimulus vs RCmod), where the
Experiment 3, whereas when the proportions of its components were
stimulus is either a single odorant, a sub‐mixture or a mixture. Accord-
changed (Experiment 4), its odour quality became very similar to
ing to this index, stimuli were plotted on a line ranging from elemental
RCmod.
to configural in order to illustrate didactically the contribution of each
Second, in Experiment 4, the similarity scores were linked to the
stimulus to elemental or configural perception (Figure 5). From this
presence of odorants B and D in the mixtures. The role of these two
representation, it appears clearly that the odours of IA and V do not
odorants in the RCmod odour quality was also obvious when the results
contribute by themselves to the odour of RC. In contrast, when asso-
of Experiment 1 and 3 were combined. Indeed, based on the
ciated in the IA‐V binary mixture or in more complex mixtures, they
Elementalness‐Configuralness index (Figure 5), the odorants B and D
contribute highly to the RC odour quality. This finding thus suggests
were both close to RCmod, a result in stark contrast with those
FIGURE 6
Illustration of the perception of key elements and key association regarding elemental or configural mixtures, respectively, in a 6‐component mixture (A). Among the elements, several carry a specific object identity (boomerang, bird and cherry). With a specific ratio (represented here by the spatial relationship between the elements), their perception as individual elements can be still salient (B); they are key components. Therefore, the similarity between the whole mixture and the element is strong (similarity in A vs C); the perception is elemental. In contrast, when using another ratio (another spatial arrangement), the same elements may lead to a key association (D), in which the elements are no more perceived through their own object's identity but create another feature (a basic face). These elements can be considered as contributors to the key association. Adding other elements, which do not necessarily refer to specific objects (point, curved line), “polishes” the key association and provides an identity for the whole mixture (E); here, the face is sufficiently precise to be discriminable from another. Therefore, the similarity between the whole mixture and the element is weak (similarity in a vs e); the perception is configural.
104
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obtained for IA and V compared to RC. This finding indicates that the
Regional Development Fund). SR was funded by the French Ministry
odours of B and D were still perceived in RCmod and are thus both part
of Higher Education and Research.
of the mixture odour quality. Moreover, these data supported the idea that in odour mixtures of fewer than 10 components44,45 the qualities
ORCID
of some components can be still perceived, thus demonstrating at least
Sébastien Romagny
partial elemental processing in such mixtures. Nevertheless, this
Gérard Coureaud
hypothesis must be confirmed by further experiments.
Thierry Thomas‐Danguin
http://orcid.org/0000-0002-8111-6499 http://orcid.org/0000-0001-6754-3884 http://orcid.org/0000-0002-9756-355X
RE FE RE NC ES
4.4
|
A redefinition of the key odorant concept
When taken together, the results of our study showed that the chemical composition largely determined the human perception of odour mixtures containing 2 to 6 odorants. These results led us to consider that, in binary as well as in complex mixtures, some odorants may preserve their perceptual features such that the individual odour they carry as single molecules is still identifiable within the mixture.34,46 In that case, the mixture is perceived as a collection of a few individual odours carried by some of the odorants, which can be qualified as key odorants. This could be the case for the elementally perceived mixture RCmod, in which B and D appeared to act as key odorants. This could also be the case of several mixtures elementally processed (in humans31,34,47; in bees33). The mixture complexity likely has a rather low impact in these mixtures because key odorants might still be perceived even in very complex mixtures (e.g., natural odours) even though the number of key odorants that could be perceived in a mixture is probably constrained by limitations in the elemental perception abilities of humans.[43,44] However, in some binary and more complex mixtures, several odorants may lose some of their perceptual features and create meaningful associations that strongly contribute to the mixture odour quality. These associations can be considered as key associations. This would be the case for the configurally perceived mixture RC, in which the binary association of IA and V may be the key association that produces an odour that differed from the odours of the single components. The two odorants IA and V could be considered the contributors of this key association and therefore of the whole RC configuration. These two contributors lose their identity when they are simultaneously present in a mixture, likely at a specific concentration ratio. At this ratio, these two odorants could not be considered key odorants because their odours were no longer present in the mixture; only their association was contributing to the perception of the mixture. This assumption is consistent with studies dealing with real food odours.48,49 We suggest that when mixture complexity increases, the individual perceptual features of the contributors are perceived less and less, such that the presence of the other odorants favours the configural perception by “polishing” the key association. Such a perspective is summarised and illustrated in Figure 6, but the underlying assumptions must be tested in further experiments to evaluate, for example, the identification rates of contributors versus key odorants in configural and elemental mixtures, respectively. ACKNOWLEDGEMEN TS We thank the ChemoSens Platform (CSGA, Dijon, France) for providing the sensory evaluation facilities. The work was supported by grants from the Regional Council of Burgundy and the FEDER (European
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Thomas‐Danguin T. Key odorants or key associations? Insights into elemental and configural odour processing. Flavour Fragr J. 2018;33:97–105. https://doi.org/10.1002/ffj.3429
