ORIGINAL RESEARCH published: 24 November 2015 doi: 10.3389/fpsyg.2015.01776
Viewing Olfactory Affective Responses Through the Sniff Prism: Effect of Perceptual Dimensions and Age on Olfactomotor Responses to Odors Camille Ferdenzi 1*, Arnaud Fournel 1 , Marc Thévenet 1 , Géraldine Coppin 2 and Moustafa Bensafi 1 1 Centre National de la Recherche Scientifique UMR5292, Institut National de la Santé et de la Recherche Médicale U1028, Centre de Recherche en Neurosciences de Lyon, Université Claude Bernard Lyon 1, Lyon, France, 2 Max Planck Institute for Metabolism Research, Cologne, Germany
Edited by: Gesualdo M. Zucco, University of Padova, Italy Reviewed by: Asifa Majid, Radboud University Nijmegen, Netherlands M. Luisa Demattè, University of Padova, Italy *Correspondence: Camille Ferdenzi [email protected] Specialty section: This article was submitted to Cognitive Science, a section of the journal Frontiers in Psychology Received: 17 June 2015 Accepted: 05 November 2015 Published: 24 November 2015 Citation: Ferdenzi C, Fournel A, Thévenet M, Coppin G and Bensafi M (2015) Viewing Olfactory Affective Responses Through the Sniff Prism: Effect of Perceptual Dimensions and Age on Olfactomotor Responses to Odors. Front. Psychol. 6:1776. doi: 10.3389/fpsyg.2015.01776
Frontiers in Psychology | www.frontiersin.org
Sniffing, which is the active sampling of olfactory information through the nasal cavity, is part of the olfactory percept. It is influenced by stimulus properties, affects how an odor is perceived, and is sufficient (without an odor being present) to activate the olfactory cortex. However, many aspects of the affective correlates of sniffing behavior remain unclear, in particular the modulation of volume and duration as a function of odor hedonics. The present study used a wide range of odorants with contrasted hedonic valence to test: (1) which psychophysical function best describes the relationship between sniffing characteristics and odor hedonics (e.g., linear, or polynomial); (2) whether sniffing characteristics are sensitive to more subtle variations in pleasantness than simple pleasant-unpleasant contrast; (3) how sensitive sniffing is to other perceptual dimensions of odors such as odor familiarity or edibility; and (4) whether the sniffing/hedonic valence relationship is valid in other populations than young adults, such as the elderly. Four experiments were conducted, using 16–48 odorants each, and recruiting a total of 102 participants, including a group of elderly people. Results of the four experiments were very consistent in showing that sniffing was sensitive to subtle variations in unpleasantness but not to subtle variations in pleasantness, and that, the more unpleasant the odor, the more limited the spontaneous sampling of olfactory information through the nasal cavity (smaller volume, shorter duration). This also applied, although to a lesser extent, to elderly participants. Relationships between sniffing and other perceptual dimensions (familiarity, edibility) were less clear. It was concluded that sniffing behavior might be involved in adaptive responses protecting the subject from possibly harmful substances. Keywords: olfaction, motor response, affect, aging, hedonics
1
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
INTRODUCTION
response to valeric acid compared with phenylethanol (Johnson et al., 2006), and to isointense odors of rotten egg (ammonium sulfide, unpleasant) compared with rose (phenylethanol; Bensafi et al., 2003) or strawberry (Bensafi et al., 2007), either perceived or imagined. In the latter comparison, differences extended to sniff duration, and notably, proved resilient, persisting in spite of instructions to maintain each sniff for a specific, constant duration. A pairwise comparison of groups of pleasant vs. unpleasant odorants provided similar conclusions (Prescott et al., 2010). It is now clear that sniffing is part of the olfactory percept, since it (i) is influenced by stimulus properties, (ii) affects how an odor is perceived, and (iii) is sufficient in itself (with no odor present) to generate an olfactory percept and activate the olfactory cortex (Mainland and Sobel, 2006). However, the affective correlates of sniffing behavior, and in particular modulation of volume and duration as a function of odor hedonics, merit further investigation. Interpreting the motor expression of odor perception could, for example, be particularly informative in specific populations that are cognitively immature (children) or cognitively impaired (e.g., Alzheimer, Parkinson patients) and whose ability to verbally describe odor-related feeling is limited. However, to date many aspects of the relationship between sniffing behavior and odor hedonic valence remain unclear, in both these specific populations and the general population. In this regard, several questions arise. Firstly, which psychophysical function best describes this relationship (e.g., linear, polynomial)? To date, only pairwise comparisons have been performed (between a pleasant and an unpleasant odor: (Warren et al., 1994; Bensafi et al., 2003, 2007; Johnson et al., 2006); or between a group of pleasant and a group of unpleasant odors: Prescott et al., 2010), which could not address this question. Secondly, does sniffing differentiate only clearly pleasant from clearly unpleasant smells, or can it discriminate between more subtle hedonic variations (e.g., slightly from strongly pleasant)? Thirdly, how sensitive is sniffing to other perceptual dimensions of odors such as familiarity or edibility? Fourthly, is the sniffing/hedonic valence relationship valid in other populations than young adults (e.g., in the elderly)? With regard to the possible use of sniffing measurement in the specific populations mentioned above, these four questions are essential and were addressed through four distinct experiments involving, for the first time, a very wide range of odorants. These aims were achieved through the use of an experimental sniffing measurement system developed in our laboratory.
One important characteristic of the human sense of smell is that it is a highly emotional sense. Affective responses to odors, and especially the most obvious ones such as attraction and disgust, serve important adaptive functions (Stevenson, 2010). They are involved in the regulation of behavioral response to events in the surrounding environment. Some particular smells can warn against toxic or dangerous substances (e.g., spoiled food, fire), enabling us to avoid serious environmental hazards. Other types of odor play a major role in sensory pleasure, modulating the ingestion of food, or contributing to social communication through attraction toward mates or attachment to kin. Such emotional responses to odors are expressed at different levels, from conscious and possibly verbalized subjective feelings to physiological changes and motor expression (e.g., Scherer, 2000). Measuring them thus requires differing methodological approaches, at the verbal (Churchill and Behan, 2010; Ferdenzi et al., 2013a), autonomic (e.g., Alaoui-Ismaïli et al., 1997; Bensafi et al., 2002a) and motor levels (such as sniffing behavior: Bensafi et al., 2003, 2007). Research in animals and in humans has shown that sniffing, which is the active sampling of olfactory information through the nasal cavity, is of considerable importance in odor perception. The mere act of sniffing (whether or not an odorant is present) induces activation in the piriform cortex (Sobel et al., 1998), thus probably preparing the primary olfactory cortex for the arrival of olfactory information and detection of odors by the olfactory system. Laing, who was one of the first to investigate sniffing in humans, wrote (Laing, 1983, p. 99–102): “Perception of an [odor] in the environment usually initiates a sniffing episode [. . . ]. Each sniff appears to be of shorter duration and to have a greater inhalation velocity than a normal breath” and “this [behavior] may enhance [odor] perception by increasing the amount and rate at which [odor] molecules reach the olfactory receptor epithelium.” He also reported that sniff volume, duration and number during a sniffing episode decreased with increasing odor concentration, thus reducing the amount of inhaled odor when strong. Sniff volume and duration were also found to be inversely related to odor concentration in later studies (Warren et al., 1994; Walker et al., 2001; Johnson et al., 2003) and topdown accommodation to stimulus properties seems to occur very rapidly (160–260 ms) after onset of the first sniff (Johnson et al., 2003). This “concentration-dependent” characteristic of sniffing behavior was later exploited to set up a simple test of olfactory sensitivity based on the reduction in sniff volume and duration in presence of an odor compared to non-odorized air (Frank et al., 2003). Although some authors have argued that other perceptual dimensions of odors such as hedonics occur too late in the neural cascade to have an influence on the nearly reflexive sniffing behavior (Johnson et al., 2003), there is now psychophysiological evidence that sniffing is modulated not only by odor intensity but also by subjective pleasantness. For example, breathed volume was visibly lower for the unpleasant odor of acetic acid than for the pleasant rose-like odor of phenylethanol (Warren et al., 1994). Similar findings were obtained comparing sniff volume in
Frontiers in Psychology | www.frontiersin.org
MATERIALS AND METHODS Participants A total of 102 volunteers participated in 4 experiments (Experiment 1: 14 females, 6 males, mean age ± standard deviation = 24.45 ± 1.63 years; Experiment 2: 16 females, 6 males, 23 ± 2.71 years; Experiment 3: 14 females, 16 males, 29.40 ± 1.05 years; Experiment 4: 16 females, 14 males, 67.37 ± 0.77 years). Participants were tested individually and
2
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
paid e16 for their participation. Exclusion criteria included self-reported olfactory impairment and/or neurological disease. All participants claimed normal sense of smell. The study was conducted in accordance with the Declaration of Helsinki and experimental procedures were approved by the local Lyon SudEst II review board.
TABLE 1 | List of the odorants used in Experiments 1, 2, 3, and 4. Odorant
CAS number
Concentration
Experiments
(volume/volume)
Odorants Forty-eight odorants were used in Experiment 1, and 20 in Experiment 3 and 4 (19 of which were also used in Experiment 1; see Table 1). These stimuli were chosen to represent a wide range of perceived pleasantness. All odorants (molecules provided by Sigma-Aldrich) were diluted in mineral oil and presented in 15 ml flasks (opening diameter: 1.7 cm; height: 5.8 cm; filled with 5 ml solution). Stimuli were absorbed on a scentless polypropylene fabric (3 × 7 cm; 3 M, Valley, NE, USA) to optimize evaporation and air/oil partitioning. In Experiment 2, 16 complex aromas were used (see Table 1). These stimuli were chosen because they represent subtle variations within the positive pole of the pleasantness scale. They were used to further investigate (after Experiment 1) the link between sniffing and pleasantness with a different, more evocative, set of odorants. All odorants (provided by Firmenich SA) were diluted in odorless dipropyleneglycol to obtain similar subjective intensities (see Delplanque et al., 2008). Solutions (4 ml) were injected into the absorbent core of cylindrical felt-tip pens (14 cm long, inner diameter 1.3 cm, Burghart, Germany).
(−)-Fenchone
7787-20-4
0.67
1
(+)-Fenchone
4695-62-9
0.67
1
1,8-Cineol
470-82-6
0.17
1
1-Butanol
71-36-3
0.04
1
1-Propanol
71-23-8
0.07
1
2,3-Butanedione
431-03-8
0.110), whereas they were for the unpleasant odors (R2 s > 0.71, ps < 0.0001 for sniff volume, and for duration with or without outlier).
relationship was significant for prediction of sniff parameters by familiarity and edibility (both of which correlated strongly with pleasantness: R = 0.76 and R = 0.84, respectively, p < 0.001): increasing familiarity was linearly associated with increasing sniff volume (Table 2). The partial correlation between pleasantness and sniff volume revealed a slight decrease in Rvalue and significance level (R = 0.67 instead of 0.73 and p < 0.01 instead of 0.001) when familiarity was a covariate, suggesting that familiarity is involved, although moderately, in the relationship. Again in this experiment pleasantness and intensity were independent (R = −0.11, p = 0.653), but this time intensity predicted sniff duration (significant linear and quadratic relationships, with sniffing duration decreasing with increasing intensity; Table 2).
Experiment 2 (Relationship between Pleasantness and Sniffing for Odors Ranging from Neutral to Pleasant) In the second experiment, odors were rated as rather pleasant on average (5.8 ± 1.4, ranging from 3.8 for Leather to 7.6 for Shampoo). No outliers were found for any of the analyzed variables. In agreement with the results obtained in Experiment 1 on the pleasant sub-group of odorants, Experiment 2 found no significant relationships (linear or quadratic) between pleasantness and any of the sniff parameters (R2 s < 0.03, ps > 0.110; see Figure 3 for all R2 s and ps). Pleasantness was unrelated to perceived intensity (Pearson Correlation: R = 0.27, p = 0.319) and positively correlated with familiarity (R = 0.85, p < 0.0001). No significant linear or quadratic relationships were found between perceived intensity or familiarity and the sniff parameters (Table 2).
Experiment 4 (Relationship between Several Perceptual Dimensions and Sniffing in Older Adults) As in Experiment 3, the 20 odorants received relatively varied pleasantness ratings in a group of elderly participants: mean pleasantness was 5.1 ± 1.0, ranging from 2.5 (for Hexanoic acid) to 6.7 (for L-Carvone). No outliers were found for any of the analyzed variables. The detailed results of the linear and quadratic regressions between pleasantness and sniff parameters are shown in Figure 4 (right column) and are in line with the conclusions of Experiments 1 and 3 on the prediction of sniff volume and sniff duration by odor pleasantness. Although the predictions appeared to be more moderate and had lower levels of significance than in Experiment 3 with younger adults (maximum level of significance: p < 0.05), computation of the difference between the two age-groups’ Rs using the rto-Fisher-z transformation revealed no significant difference (ps > 0.276 for the linear predictions, and ps > 0.104 for the quadratic predictions). When considering the prediction
Experiment 3 (Relationship between Several Perceptual Dimensions and Sniffing in Young Adults) The 20 odorants used in this experiment were relatively varied in pleasantness: mean pleasantness was 4.9 ± 1.4, ranging from 2.1 (for Hexanoic acid) to 7.0 (for Isoamyl acetate). No outliers were found for any of the analyzed variables. The detailed results of the linear and quadratic regressions between pleasantness and sniff parameters are shown in Figure 4 (left column) and are fully in line with the conclusions of Experiment 1 on prediction of sniff volume and sniff duration by odor pleasantness. In contrast with Experiment 1, however, maximum sniff flow rate linearly increased with increasing pleasantness (p < 0.05). Only one
Frontiers in Psychology | www.frontiersin.org
5
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
FIGURE 2 | Sniff characteristics (A: volume, B: duration, and C: maximum flow rate) as a function of odor pleasantness for 48 odorants in Experiment 1. Linear and quadratic relationships are represented by trend curves, R2 and level of significance (***p < 0.001; ns: non-significant or p > 0.05; linear: dashed line and regular font; quadratic: continuous line and bold font).
Frontiers in Psychology | www.frontiersin.org
FIGURE 3 | Sniff characteristics (A: volume, B: duration, and C: maximum flow rate) as a function of odor pleasantness for 16 odorants in Experiment 2. Linear and quadratic relationships are represented by trend curves, R2 and level of significance (ns: non-significant or p > 0.05; linear: dashed line and regular font; quadratic: continuous line and bold font).
6
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
TABLE 2 | Results of the linear and quadratic regressions illustrating the prediction of sniff parameters (volume, duration, and maximum flow rate) by perceptual variables other than pleasantness (familiarity, intensity, edibility) in Experiments 2, 3, and 4. Sniff volume R2
Experiment 2
Intensity Familiarity
Experiment 3
Intensity Familiarity Edibility
Experiment 4
Intensity Familiarity Edibility
Sniff duration
p
R2
p
Maximum sniff flow rate R2
p
Linear
0.06
0.343
0.00
0.925
0.13
0.174
Quadratic
0.08
0.565
0.00
0.993
0.15
0.338
Linear
0.01
0.670
0.00
0.903
0.00
0.861
Quadratic
0.04
0.751
0.00
0.990
0.00
0.985
Linear
0.18
0.060
0.36
Viewing Olfactory Affective Responses Through the Sniff Prism: Effect of Perceptual Dimensions and Age on Olfactomotor Responses to Odors Camille Ferdenzi 1*, Arnaud Fournel 1 , Marc Thévenet 1 , Géraldine Coppin 2 and Moustafa Bensafi 1 1 Centre National de la Recherche Scientifique UMR5292, Institut National de la Santé et de la Recherche Médicale U1028, Centre de Recherche en Neurosciences de Lyon, Université Claude Bernard Lyon 1, Lyon, France, 2 Max Planck Institute for Metabolism Research, Cologne, Germany
Edited by: Gesualdo M. Zucco, University of Padova, Italy Reviewed by: Asifa Majid, Radboud University Nijmegen, Netherlands M. Luisa Demattè, University of Padova, Italy *Correspondence: Camille Ferdenzi [email protected] Specialty section: This article was submitted to Cognitive Science, a section of the journal Frontiers in Psychology Received: 17 June 2015 Accepted: 05 November 2015 Published: 24 November 2015 Citation: Ferdenzi C, Fournel A, Thévenet M, Coppin G and Bensafi M (2015) Viewing Olfactory Affective Responses Through the Sniff Prism: Effect of Perceptual Dimensions and Age on Olfactomotor Responses to Odors. Front. Psychol. 6:1776. doi: 10.3389/fpsyg.2015.01776
Frontiers in Psychology | www.frontiersin.org
Sniffing, which is the active sampling of olfactory information through the nasal cavity, is part of the olfactory percept. It is influenced by stimulus properties, affects how an odor is perceived, and is sufficient (without an odor being present) to activate the olfactory cortex. However, many aspects of the affective correlates of sniffing behavior remain unclear, in particular the modulation of volume and duration as a function of odor hedonics. The present study used a wide range of odorants with contrasted hedonic valence to test: (1) which psychophysical function best describes the relationship between sniffing characteristics and odor hedonics (e.g., linear, or polynomial); (2) whether sniffing characteristics are sensitive to more subtle variations in pleasantness than simple pleasant-unpleasant contrast; (3) how sensitive sniffing is to other perceptual dimensions of odors such as odor familiarity or edibility; and (4) whether the sniffing/hedonic valence relationship is valid in other populations than young adults, such as the elderly. Four experiments were conducted, using 16–48 odorants each, and recruiting a total of 102 participants, including a group of elderly people. Results of the four experiments were very consistent in showing that sniffing was sensitive to subtle variations in unpleasantness but not to subtle variations in pleasantness, and that, the more unpleasant the odor, the more limited the spontaneous sampling of olfactory information through the nasal cavity (smaller volume, shorter duration). This also applied, although to a lesser extent, to elderly participants. Relationships between sniffing and other perceptual dimensions (familiarity, edibility) were less clear. It was concluded that sniffing behavior might be involved in adaptive responses protecting the subject from possibly harmful substances. Keywords: olfaction, motor response, affect, aging, hedonics
1
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
INTRODUCTION
response to valeric acid compared with phenylethanol (Johnson et al., 2006), and to isointense odors of rotten egg (ammonium sulfide, unpleasant) compared with rose (phenylethanol; Bensafi et al., 2003) or strawberry (Bensafi et al., 2007), either perceived or imagined. In the latter comparison, differences extended to sniff duration, and notably, proved resilient, persisting in spite of instructions to maintain each sniff for a specific, constant duration. A pairwise comparison of groups of pleasant vs. unpleasant odorants provided similar conclusions (Prescott et al., 2010). It is now clear that sniffing is part of the olfactory percept, since it (i) is influenced by stimulus properties, (ii) affects how an odor is perceived, and (iii) is sufficient in itself (with no odor present) to generate an olfactory percept and activate the olfactory cortex (Mainland and Sobel, 2006). However, the affective correlates of sniffing behavior, and in particular modulation of volume and duration as a function of odor hedonics, merit further investigation. Interpreting the motor expression of odor perception could, for example, be particularly informative in specific populations that are cognitively immature (children) or cognitively impaired (e.g., Alzheimer, Parkinson patients) and whose ability to verbally describe odor-related feeling is limited. However, to date many aspects of the relationship between sniffing behavior and odor hedonic valence remain unclear, in both these specific populations and the general population. In this regard, several questions arise. Firstly, which psychophysical function best describes this relationship (e.g., linear, polynomial)? To date, only pairwise comparisons have been performed (between a pleasant and an unpleasant odor: (Warren et al., 1994; Bensafi et al., 2003, 2007; Johnson et al., 2006); or between a group of pleasant and a group of unpleasant odors: Prescott et al., 2010), which could not address this question. Secondly, does sniffing differentiate only clearly pleasant from clearly unpleasant smells, or can it discriminate between more subtle hedonic variations (e.g., slightly from strongly pleasant)? Thirdly, how sensitive is sniffing to other perceptual dimensions of odors such as familiarity or edibility? Fourthly, is the sniffing/hedonic valence relationship valid in other populations than young adults (e.g., in the elderly)? With regard to the possible use of sniffing measurement in the specific populations mentioned above, these four questions are essential and were addressed through four distinct experiments involving, for the first time, a very wide range of odorants. These aims were achieved through the use of an experimental sniffing measurement system developed in our laboratory.
One important characteristic of the human sense of smell is that it is a highly emotional sense. Affective responses to odors, and especially the most obvious ones such as attraction and disgust, serve important adaptive functions (Stevenson, 2010). They are involved in the regulation of behavioral response to events in the surrounding environment. Some particular smells can warn against toxic or dangerous substances (e.g., spoiled food, fire), enabling us to avoid serious environmental hazards. Other types of odor play a major role in sensory pleasure, modulating the ingestion of food, or contributing to social communication through attraction toward mates or attachment to kin. Such emotional responses to odors are expressed at different levels, from conscious and possibly verbalized subjective feelings to physiological changes and motor expression (e.g., Scherer, 2000). Measuring them thus requires differing methodological approaches, at the verbal (Churchill and Behan, 2010; Ferdenzi et al., 2013a), autonomic (e.g., Alaoui-Ismaïli et al., 1997; Bensafi et al., 2002a) and motor levels (such as sniffing behavior: Bensafi et al., 2003, 2007). Research in animals and in humans has shown that sniffing, which is the active sampling of olfactory information through the nasal cavity, is of considerable importance in odor perception. The mere act of sniffing (whether or not an odorant is present) induces activation in the piriform cortex (Sobel et al., 1998), thus probably preparing the primary olfactory cortex for the arrival of olfactory information and detection of odors by the olfactory system. Laing, who was one of the first to investigate sniffing in humans, wrote (Laing, 1983, p. 99–102): “Perception of an [odor] in the environment usually initiates a sniffing episode [. . . ]. Each sniff appears to be of shorter duration and to have a greater inhalation velocity than a normal breath” and “this [behavior] may enhance [odor] perception by increasing the amount and rate at which [odor] molecules reach the olfactory receptor epithelium.” He also reported that sniff volume, duration and number during a sniffing episode decreased with increasing odor concentration, thus reducing the amount of inhaled odor when strong. Sniff volume and duration were also found to be inversely related to odor concentration in later studies (Warren et al., 1994; Walker et al., 2001; Johnson et al., 2003) and topdown accommodation to stimulus properties seems to occur very rapidly (160–260 ms) after onset of the first sniff (Johnson et al., 2003). This “concentration-dependent” characteristic of sniffing behavior was later exploited to set up a simple test of olfactory sensitivity based on the reduction in sniff volume and duration in presence of an odor compared to non-odorized air (Frank et al., 2003). Although some authors have argued that other perceptual dimensions of odors such as hedonics occur too late in the neural cascade to have an influence on the nearly reflexive sniffing behavior (Johnson et al., 2003), there is now psychophysiological evidence that sniffing is modulated not only by odor intensity but also by subjective pleasantness. For example, breathed volume was visibly lower for the unpleasant odor of acetic acid than for the pleasant rose-like odor of phenylethanol (Warren et al., 1994). Similar findings were obtained comparing sniff volume in
Frontiers in Psychology | www.frontiersin.org
MATERIALS AND METHODS Participants A total of 102 volunteers participated in 4 experiments (Experiment 1: 14 females, 6 males, mean age ± standard deviation = 24.45 ± 1.63 years; Experiment 2: 16 females, 6 males, 23 ± 2.71 years; Experiment 3: 14 females, 16 males, 29.40 ± 1.05 years; Experiment 4: 16 females, 14 males, 67.37 ± 0.77 years). Participants were tested individually and
2
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
paid e16 for their participation. Exclusion criteria included self-reported olfactory impairment and/or neurological disease. All participants claimed normal sense of smell. The study was conducted in accordance with the Declaration of Helsinki and experimental procedures were approved by the local Lyon SudEst II review board.
TABLE 1 | List of the odorants used in Experiments 1, 2, 3, and 4. Odorant
CAS number
Concentration
Experiments
(volume/volume)
Odorants Forty-eight odorants were used in Experiment 1, and 20 in Experiment 3 and 4 (19 of which were also used in Experiment 1; see Table 1). These stimuli were chosen to represent a wide range of perceived pleasantness. All odorants (molecules provided by Sigma-Aldrich) were diluted in mineral oil and presented in 15 ml flasks (opening diameter: 1.7 cm; height: 5.8 cm; filled with 5 ml solution). Stimuli were absorbed on a scentless polypropylene fabric (3 × 7 cm; 3 M, Valley, NE, USA) to optimize evaporation and air/oil partitioning. In Experiment 2, 16 complex aromas were used (see Table 1). These stimuli were chosen because they represent subtle variations within the positive pole of the pleasantness scale. They were used to further investigate (after Experiment 1) the link between sniffing and pleasantness with a different, more evocative, set of odorants. All odorants (provided by Firmenich SA) were diluted in odorless dipropyleneglycol to obtain similar subjective intensities (see Delplanque et al., 2008). Solutions (4 ml) were injected into the absorbent core of cylindrical felt-tip pens (14 cm long, inner diameter 1.3 cm, Burghart, Germany).
(−)-Fenchone
7787-20-4
0.67
1
(+)-Fenchone
4695-62-9
0.67
1
1,8-Cineol
470-82-6
0.17
1
1-Butanol
71-36-3
0.04
1
1-Propanol
71-23-8
0.07
1
2,3-Butanedione
431-03-8
0.110), whereas they were for the unpleasant odors (R2 s > 0.71, ps < 0.0001 for sniff volume, and for duration with or without outlier).
relationship was significant for prediction of sniff parameters by familiarity and edibility (both of which correlated strongly with pleasantness: R = 0.76 and R = 0.84, respectively, p < 0.001): increasing familiarity was linearly associated with increasing sniff volume (Table 2). The partial correlation between pleasantness and sniff volume revealed a slight decrease in Rvalue and significance level (R = 0.67 instead of 0.73 and p < 0.01 instead of 0.001) when familiarity was a covariate, suggesting that familiarity is involved, although moderately, in the relationship. Again in this experiment pleasantness and intensity were independent (R = −0.11, p = 0.653), but this time intensity predicted sniff duration (significant linear and quadratic relationships, with sniffing duration decreasing with increasing intensity; Table 2).
Experiment 2 (Relationship between Pleasantness and Sniffing for Odors Ranging from Neutral to Pleasant) In the second experiment, odors were rated as rather pleasant on average (5.8 ± 1.4, ranging from 3.8 for Leather to 7.6 for Shampoo). No outliers were found for any of the analyzed variables. In agreement with the results obtained in Experiment 1 on the pleasant sub-group of odorants, Experiment 2 found no significant relationships (linear or quadratic) between pleasantness and any of the sniff parameters (R2 s < 0.03, ps > 0.110; see Figure 3 for all R2 s and ps). Pleasantness was unrelated to perceived intensity (Pearson Correlation: R = 0.27, p = 0.319) and positively correlated with familiarity (R = 0.85, p < 0.0001). No significant linear or quadratic relationships were found between perceived intensity or familiarity and the sniff parameters (Table 2).
Experiment 4 (Relationship between Several Perceptual Dimensions and Sniffing in Older Adults) As in Experiment 3, the 20 odorants received relatively varied pleasantness ratings in a group of elderly participants: mean pleasantness was 5.1 ± 1.0, ranging from 2.5 (for Hexanoic acid) to 6.7 (for L-Carvone). No outliers were found for any of the analyzed variables. The detailed results of the linear and quadratic regressions between pleasantness and sniff parameters are shown in Figure 4 (right column) and are in line with the conclusions of Experiments 1 and 3 on the prediction of sniff volume and sniff duration by odor pleasantness. Although the predictions appeared to be more moderate and had lower levels of significance than in Experiment 3 with younger adults (maximum level of significance: p < 0.05), computation of the difference between the two age-groups’ Rs using the rto-Fisher-z transformation revealed no significant difference (ps > 0.276 for the linear predictions, and ps > 0.104 for the quadratic predictions). When considering the prediction
Experiment 3 (Relationship between Several Perceptual Dimensions and Sniffing in Young Adults) The 20 odorants used in this experiment were relatively varied in pleasantness: mean pleasantness was 4.9 ± 1.4, ranging from 2.1 (for Hexanoic acid) to 7.0 (for Isoamyl acetate). No outliers were found for any of the analyzed variables. The detailed results of the linear and quadratic regressions between pleasantness and sniff parameters are shown in Figure 4 (left column) and are fully in line with the conclusions of Experiment 1 on prediction of sniff volume and sniff duration by odor pleasantness. In contrast with Experiment 1, however, maximum sniff flow rate linearly increased with increasing pleasantness (p < 0.05). Only one
Frontiers in Psychology | www.frontiersin.org
5
November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
FIGURE 2 | Sniff characteristics (A: volume, B: duration, and C: maximum flow rate) as a function of odor pleasantness for 48 odorants in Experiment 1. Linear and quadratic relationships are represented by trend curves, R2 and level of significance (***p < 0.001; ns: non-significant or p > 0.05; linear: dashed line and regular font; quadratic: continuous line and bold font).
Frontiers in Psychology | www.frontiersin.org
FIGURE 3 | Sniff characteristics (A: volume, B: duration, and C: maximum flow rate) as a function of odor pleasantness for 16 odorants in Experiment 2. Linear and quadratic relationships are represented by trend curves, R2 and level of significance (ns: non-significant or p > 0.05; linear: dashed line and regular font; quadratic: continuous line and bold font).
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November 2015 | Volume 6 | Article 1776
Ferdenzi et al.
Olfactory Hedonics and Sniffing Behavior
TABLE 2 | Results of the linear and quadratic regressions illustrating the prediction of sniff parameters (volume, duration, and maximum flow rate) by perceptual variables other than pleasantness (familiarity, intensity, edibility) in Experiments 2, 3, and 4. Sniff volume R2
Experiment 2
Intensity Familiarity
Experiment 3
Intensity Familiarity Edibility
Experiment 4
Intensity Familiarity Edibility
Sniff duration
p
R2
p
Maximum sniff flow rate R2
p
Linear
0.06
0.343
0.00
0.925
0.13
0.174
Quadratic
0.08
0.565
0.00
0.993
0.15
0.338
Linear
0.01
0.670
0.00
0.903
0.00
0.861
Quadratic
0.04
0.751
0.00
0.990
0.00
0.985
Linear
0.18
0.060
0.36
