Autonomic nervous system responses to odours: the role of pleasantness and arousal

Perception of odours can provoke explicit reactions such as judgements of intensity or pleasantness, and implicit output such as skin conductance or heart rate variations. The main purpose of the present experiment was to ascertain: (i) the correlation between odour ratings (intensity, arousal, pleasantness and familiarity) and activation of the autonomic nervous system, and (ii) the inter-correlation between self-report ratings on intensity, arousal, pleasantness and familiarity dimensions in odour perception. Twelve healthy volunteers were tested in two separate sessions. Firstly, subjects were instructed to smell six odorants (isovaleric acid, thiophenol, pyridine, L-menthol, isoamyl acetate, and 1-8 cineole), while skin conductance and heart rate variations were being measured. During this phase, participants were not asked to give any judgement about the odorants. Secondly, subjects were instructed to rate the odorants on dimensions of intensity, pleasantness, arousal and familiarity (self-report ratings), by giving a mark between 1 (not at all intense, arousing, pleasant or familiar) and 9 (extremely intense, arousing, pleasant or familiar). Results indicated: (i) a pleasantness factor correlated with heart rate variations, (ii) an arousal factor correlated with skin conductance variations, and (iii) a strong correlation between the arousal and intensity dimensions. In conclusion, given that these correlations are also found in other studies using visual and auditory stimuli, these findings provide preliminary information suggesting that autonomic variations in response to olfactory stimuli are probably not modality specific, and may be organized along two main dimensions of pleasantness and arousal, at least for the parameters considered (i.e. heart rate and skin conductance).     

Auditory-olfactory integration: congruent or pleasant sounds amplify odor pleasantness

Even though we often perceive odors while hearing auditory stimuli, surprisingly little is known about auditory-olfactory integration. This study aimed to investigate the influence of auditory cues on ratings of odor intensity and/or pleasantness, with a focus on 2 factors: "congruency" (Experiment 1) and the "halo/horns effect" of auditory pleasantness (Experiment 2). First, in Experiment 1, participants were presented with congruent, incongruent, or neutral sounds before and during the presentation of odor. Participants rated the odors as being more pleasant while listening to a congruent sound than while listening to an incongruent sound. In Experiment 2, participants received pleasant or unpleasant sounds before and during the presentation of either a pleasant or unpleasant odor. The hedonic valence of the sounds transferred to the odors, irrespective of the hedonic tone of the odor itself. The more the participants liked the preceding sound, the more pleasant the subsequent odor became. In contrast, the ratings of odor intensity appeared to be little or not at all influenced by the congruency or hedonic valence of the auditory cue. In conclusion, the present study for the first time provides an empirical demonstration that auditory cues can modulate odor pleasantness.     

Predicting Odor Pleasantness with an Electronic Nose

A primary goal for artificial nose (eNose) technology is to report perceptual qualities of novel odors. Currently, however, eNoses primarily detect and discriminate between odorants they previously “learned”. We tuned an eNose to human odor pleasantness estimates. We then used the eNose to predict the pleasantness of novel odorants, and tested these predictions in naïve subjects who had not participated in the tuning procedure. We found that our apparatus generated odorant pleasantness ratings with above 80% similarity to average human ratings, and with above 90% accuracy at discriminating between categorically pleasant or unpleasant odorants. Similar results were obtained in two cultures, native Israeli and native Ethiopian, without retuning of the apparatus. These findings suggest that unlike in vision and audition, in olfaction there is a systematic predictable link between stimulus structure and stimulus pleasantness. This goes in contrast to the popular notion that odorant pleasantness is completely subjective, and may provide a new method for odor screening and environmental monitoring, as well as a critical building block for digital transmission of smell.     

Communication of gender from human breath odors: Relationship to perceived intensity and pleasantness

During five consecutive daily test sessions, 10 men and women rated the relative intensity and pleasantness of breath odors from 14 males and 19 females on a no-oral-hygiene regimen. In addition, the likely gender of the donor of each odor was also estimated. The breath odors of males were rated, on the average, as more intense and less pleasant than the breath odors of females. Women consistently gave lower pleasantness ratings to the odors than did men. Both the male and female judges assigned the breath odors to the correct gender classes at a frequency unlikely due to chance, although the females were more accurate in this regard. An inverse relation between breath odor intensity and pleasantness was noted. Systematic changes in the rated intensity and pleasantness of the odors were present across the 5 days of the study period. These data suggest that differences exist between the breath odors of men and women, and that humans, like many other mammals, may be capable of assessing gender from oral odors. However, such assignments conceivably reflect the strategy of assigning stronger and less pleasant odors to the male category, and weaker and less unpleasant odors to the female category, regardless of the true sex of the odor donor.     

Effect of Aging on Hedonic Appreciation of Pleasant and Unpleasant Odors

Does hedonic appreciation evolve differently for pleasant odors and unpleasant odors during normal aging? To answer this question we combined psychophysics and electro-encephalographic recordings in young and old adults. A first study showed that pleasant odorants (but not unpleasant ones) were rated as less pleasant by old adults. A second study validated this decrease in hedonic appreciation for agreeable odors and further showed that smelling these odorants decreased beta event-related synchronization in aged participants. In conclusion, the study offers new insights into the evolution of odor hedonic perception during normal aging, highlighting for the first time a change in processing pleasant odors.     

The impact of natural odors on affective states in humans

Laboratory studies have shown a significant influence of certain fragrances on affective as well as cognitive states in humans. The aim of the current study was to measure the relationship between complex, natural odors and affective states, that is, calmness, alertness, and mood, in the field. In 4 experiments, the emotional impact, intensity, and hedonics of complex, natural plant odors were assessed in 32 healthy human subjects and compared with control conditions involving a similar outdoor environment without the tested fragrant plants. In all experiments, the selected fragrances were evaluated as more intense than the odors in the control conditions but pleasantness ratings differed only in 2 of the 4 experiments. The fragrances improved subjective ratings of calmness, alertness, and mood depending on the sequence of the conditions but independent of visual features of the environment. In contrast, a fifth experiment which tested the influence of natural and artificial pleasant odors and an artificial unpleasant odor on calmness, alertness, and mood in 22 subjects showed that the unpleasant odor impaired these affective states in humans independent of the order of presentation. On the other hand, no effects of the pleasant odors on mood and calmness were observed in this experiment.     

Odor Hedonics: Connection With Emotional Response Estimated by Autonomic Parameters

The aim of this paper is to analyse the relationship betweenself-report hedonic evaluations and the physiological expressionof emotion in response to odorants. We try to solve the followingquestions: (1) Is it possible to find any experimental evidencethat the sense of smell is linked with emotion? (2) What kindof odorants can be distinguished by autonomic analysis? (3)Is there a link between hedonics and autonomic information?The effects of odorants on the emotional process were estimated,in terms of autonomic nervous system (ANS) activity. Fifteensubjects inhaled five odorants as olfactory stimuli: lavender(LAV), ethyl acetoacetate (EAA), camphor (CAM), acetic acid(AA) and butyric acid (BA). After inhaling the odorant, subjectswere requested to fill out an 11-point hedonic scale to rateits pleasantness versus unpleasantness. ANS parameters wereas follows: two electrodermal responses, skin potential (SP)and resistance (SR); two thermovascular parameters, skin bloodflow (SBF) and skin temperature (ST); and two cardiorespiratoryparameters: instantaneous respiratory frequency (IRF) and instantaneousheart rate (IHR). Simultaneous recording of six parameters showedthat specific autonomic patterns were associated with each odorant.An analysis of variance made it possible to differentiate amongthe five odorants. Two-by-two odorant comparisons for autonomicresponses using Tukey's HSD multiple comparison test only permitteddifferentiation between pleasant odorants (LAV and EAA) andunpleasant (AA and BA) ones, but camphor was differentiatedfrom both pleasant and unpleasant odorants. Each odorant elicitedresponses in the different parameters, yet subjects respondedthrough their preferential channels; an average of two channelswas used by each subject. These results when compared with thoseobtained with other senses (visual and auditory). did not evidencethe postulated preferential link between olfaction and emotion.A'strong link between hedonics and ANS response could be demonstratedwhen considering each subject and mainly through his/her preferentialchannel(s); conversely a weak correlation (SR duration excepted)was obtained between inter-subjects' hedonic evaluation. Itseems that for a given population the autonomic response reflectthe odor valence only through some parameters related to themain preferential channel(s) and thus the global autonomic patternhas to be considered. Chem. Senses 22: 237–248, 1997.     

Body position-dependent shift in odor percept present only for perithreshold odors

We recently demonstrated that a supine position causes a decrease in olfactory sensitivity compared with an upright position. We pursued that initial finding in 3 separate experiments in which we explored the extent of, and mechanism underlying, this phenomenon. In Experiment 1, we replicated the decrease in olfactory sensitivity when in a supine compared with an upright position. In Experiment 2, we measured body position-dependent shifts in physiological variables and sniff measures while smelling suprathreshold odorants and performing a perithreshold odor intensity discrimination task. Olfactory performances were reduced while supine. However, no relationships between the shift in olfactory performances and either the physiological variables or sniff measures were found. In Experiment 3, we determined that there were no position-dependent shifts in ability to discriminate or identify suprathreshold odors or rate them for pleasantness, intensity, or familiarity. However, a drop in scores was observed, and performance was slowed, on a cognitive skill while supine. These results demonstrate a body position-dependent shift in olfactory sensitivity only for perithreshold odors that appears to be mediated by cognitive rather than physiological factors. Implications for olfactory imaging studies are discussed.     

Odor recognition memory: two encoding trials are better than one

The primary purpose of this study was to determine the effect of one versus two encoding trials in the classical yes/no recognition memory paradigm using olfactory stimuli. A group of 24 young adults rated 18 standard microencapsulated odorant targets for familiarity (first encoding block) or pleasantness (second encoding block). Once-encoded targets were in only one block and twice-encoded targets were in both, with items counterbalanced across participants. Participants performed a 20-min nonverbal distractor task followed by a yes/no recognition test incorporating 18 additional odors as foils. Memory performance for twice-encoded targets was superior to that for once-encoded targets. For once-encoded targets, performance did not differ between those rated for familiarity versus those rated for pleasantness. Less pleasant odors produced overall better recognition, with a tendency for less familiar odors to produce overall better recognition. There was a tendency for the second encoding trial to have a larger effect for less pleasant or familiar odors than for more pleasant or familiar odors. The main conclusion is that recognition memory for odors is better for items encoded two times than for items encoded only once. Implications of these findings and suggestions for future research are discussed.     

Characteristics of electroencephalographic responses induced by a pleasant and an unpleasant odor

More than sensory stimuli, odorous stimuli were employed to facilitate the evocation of emotional responses in the present study. The odor-stimulated emotion was evaluated by investigating specific features of encephalographic (EEG) responses produced thereof. In this study, the concentrations of the same odor were altered; viz., the changes in odor-induced emotional level were compared with the concurrently monitored EEG response features. In addition, we performed the mental task to evoke the arousal state of the brain and investigated the resemblance of response characteristics of the resting state to the post-mental task resting state. Subjects having no abnormalities in the sense of smell included 12 male undergraduate and graduate students (age range: 22-26 years). Experiment I involved 2 types of odors that induced favorable odorous stimuli (pleasant induction); test-solutions were either diluted 150 (easily perceptible odorous sensation) or 500 (slightly perceptible odorous stimuli) times. Experiment II had 2 types of odors that evoked unfavorable odorous stimuli (unpleasant induction), and test-solutions with dilution rates similar to those of pleasant induction were prepared. Odorless distilled water was used as the control in both experiments. From results of rating the odorous stimuli of our compounds used, the candidates were respectively found to be appropriate in inducing the pleasant and unpleasant smell sensations. The analyses of EEG responses on inducing pleasant and unpleasant smell sensations revealed that the EEG activities of the left frontal region were enhanced. This finding may establish the hypothesis of a relationship prevailing between the positive approach-related emotion evoked by the visual sensation and the left hemisphere (Davidson, 1992; Tomarken et al., 1989). In other words, it can be interpreted that the negative withdrawal-related emotion may be associated with activities of the right hemisphere. However, this hypothesis may not be applicable to the unpleasant odors, as the unpleasant emotions are activated by the unpleasant odors not only in the bilateral frontal regions but also over an extensive area of the brain. As such, the pleasant emotions are evoked in the left frontal brain region while the unpleasant emotions are incited in the bilateral frontal and extensive regions in the brain with the odorous stimuli. Moreover, intrinsic EEG activities in response to the pleasant and unpleasant inputs were not observed after performing the mental tasks. In other words, EEG responses reflecting central nervous system activities elevated by loading of the mental tasks as a result of exposure to the pleasant and unpleasant odors may not apparently be observed.     

Olfactory cues modulate facial attractiveness

We report an experiment designed to investigate whether olfactory cues can influence people's judgments of facial attractiveness. Sixteen female participants judged the attractiveness of a series of male faces presented briefly on a computer monitor using a 9-point visual rating scale. While viewing each face, the participants were simultaneously presented with either clean air or else with 1 of 4 odorants (the odor was varied on a trial-by-trial basis) from a custom-built olfactometer. We included 2 pleasant odors (geranium and a male fragrance) and 2 unpleasant odors (rubber and body odor) as confirmed by pilot testing. The results showed that the participants rated the male faces as being significantly less attractive in the presence of an unpleasant odor than when the faces were presented together with a pleasant odor or with clean air (these conditions did not differ significantly). These results demonstrate the cross-modal influence that unpleasant odors can have on people's judgments of facial attractiveness. Interestingly, this pattern of results was unaffected by whether the odors were body relevant (the body odor and the male fragrance) or not (the rubber and geranium odors).     

Visual stimuli in distraction strategies for increasing pain tolerance. The confounding of affect with other stimulus characteristics

Recent experimental studies in pain control have questioned the value of pleasant affect in strategies employing distraction. It appears that pleasant affect may have been systematically confounded with task complexity or novelty in past research that found pleasant imagery or slides effective in increasing pain tolerance with the cold pressor test. The present study was a follow-up to a study conducted by this author (Greenstein, 1984) in which unpleasant slides had significantly increased pain tolerance above pleasant slide level. In the present study, 69 college students (35 females, 34 males) rated either the pleasant or unpleasant slides used in the original study on their perceived pleasantness, complexity, and uniqueness (novelty). Results indicated that the unpleasant slides were rated significantly more complex (P less than 0.001) and unique (P less than 0.001) than the pleasant slides. Additionally, as in the earlier study, ratings of the unpleasant slides on pleasantness deviated significantly farther from neutrality than did ratings of the pleasant slides (t = 5.04, P less than 0.001). Thus the unpleasant slides were also perceived as being more significant (i.e., pertinent) than were the pleasant slides. The results indicate that affect was confounded with other stimulus characteristics in the Greenstein (1984) pain control study and probably in a significant number of other studies as well. Researchers are cautioned to control for the stimulus characteristics of visual distraction strategies used in pain control studies. The assumption that pleasantness, per se, contributes to strategy effectiveness is no longer tenable; future research must demonstrate an independent effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dissociated representations of pleasant and unpleasant olfacto-trigeminal mixtures: an FMRI study

How the pleasantness of chemosensory stimuli such as odorants or intranasal trigeminal compounds is processed in the human brain has been the focus of considerable recent interest. Yet, so far, only the unimodal form of this hedonic processing has been explored, and not its bimodal form during crossmodal integration of olfactory and trigeminal stimuli. The main purpose of the present study was to investigate this question. To this end, functional magnetic resonance imaging (fMRI) was used in an experiment comparing brain activation related to a pleasant and a relatively unpleasant olfacto-trigeminal mixture, and to their individual components (CO(2) alone, Orange alone, Rose alone). Results revealed first common neural activity patterns in response to both mixtures in a number of regions: notably the superior temporal gyrus and the caudate nucleus. Common activations were also observed in the insula, although the pleasant mixture activated the right insula whereas the unpleasant mixture activated the left insula. However, specific activations were observed in anterior cingulate gyrus and the ventral tegmental area only during the perception of the pleasant mixture. These findings emphasized for the firs time the involvement of the latter structures in processing of pleasantness during crossmodal integration of chemosensory stimuli.     

Judgement of odor intensity is influenced by subjects' knowledge of the odor source

Odor perception, including intensity, is affected by knowledge of odor source. For 76 subjects tested with 24 everyday odorants, ratings of intensity, pleasantness and familiarity were enhanced when subjects either could identify the odor source themselves or were provided with the name by the experimenter. Ratings were highest when subjects judged that the names provided matched their own perception, suggesting an interaction between individuals' cognitive representation of odors and their immediate perceptual experience.     

Basic emotions evoked by eugenol odor differ according to the dental experience. A neurovegetative analysis.

Subjective individual experiences seem to indicate that odors may form strong connections with memories, especially those charged with emotional significance. In the dental field, this could be the case with the odorant eugenol, responsible for the typical clinging odor impregnating the dental office. The odor of eugenol could evoke memories of unpleasant dental experiences and, therefore, negative feelings such as anxiety and fear, since eugenates (cements containing eugenol) are used in potentially painful restorative dentistry. This hypothesis was tested by evaluating the emotional impact of the odor of eugenol through autonomic nervous system (ANS) analysis. The simultaneous variations of six ANS parameters (two electrodermal, two thermovascular and two cardiorespiratory), induced by the inhalation of this odorant, were recorded on volunteer subjects. Vanillin (a pleasant odorant) and propionic acid (an unpleasant one) served as controls. After the experiment, subjects were asked to rate the pleasantness versus unpleasantness of each odorant on an 11-point hedonic scale. The patterns of autonomic responses, obtained for each odorant and each subject, were transcribed into one of the six basic emotions defined by Ekman et al. (happiness, surprise, sadness, fear, anger and disgust). Results were compared between two groups of subjects divided according to their dental experience (fearful and non-fearful dental care subjects) and showed significant differences only for eugenol. This odorant was rated as pleasant by non-fearful dental subjects but unpleasant by fearful dental subjects. The evoked autonomic responses were mainly associated with positive basic emotions (happiness and surprise) in non-fearful dental subjects and with negative basic emotions (fear, anger, disgust) in fearful dental subjects. These results suggest that eugenol can be responsible for different emotional states depending on the subjects' dental experience, which seems to confirm the potential role of odors as elicitors of emotional memories. This study also supports the possible influence of the ambient odor impregnating the dental office, strengthening a negative conditioning toward dental care in some anxious patients.     

CONTEXTUAL SHIFTS IN HEDONIC JUDGMENTS

Taste intensity ratings are higher when many low-intensity stimuli are presented than when many high-intensity stimuli are presented. It is investigated whether similar contextual shifts occur for hedonic judgments. Experiment 1 assessed the perceived pleasantness of a heterogeneous stimulus set containing either a large number of quinine (unpleasant context) or sucrose (pleasant context) solutions. In Experiment 2 subjects made hedonic judgments for a set of sucrose solutions. They were classified pre-experimentally as ‘sweet-likers,’‘neutrals,’ or ‘sweet-dislikers.’ Stimulus frequency distributions were positively or negatively skewed. In Experiment 1, hedonic ratings in the pleasant context were lower than those obtained in the unpleasant context. In Experiment 2, no effect of stimulus context was found. It is argued that contextual shifts in hedonic judgments are found only if the stimuli differ substantially along the hedonic dimension. In addition, the subject groups should be homogeneous with regard to preference rank orders, and should be sufficiently large to allow testing with adequate statistical power.     

Scent as an element of subjective assessment of young males and females

In 46 male and 19 female students who were both donors and recipients of scent samples, we studied effects of the menstrual cycle phase, reproductive health, sexual motivation, and sexual experience on subjective assessment of intensity and pleasantness of the sweat scent. The sweat samples were collected by the 60-min long exposure of a filter paper in the armpit. Then the male and female samples were reciprocally assessed olfactorily with respect to their intensity, unpleasantness/pleasantness, and association with male or female. It has been established that the cycle phase affects statistically significantly the coincidence of subjective assessments by the unpleasant/pleasant scale, which different recipients were giving to the same scent samples. Their coincidence in the non-receptive state is replaced by the non-coincidence at the receptive cycle phase, which indicates an increase in the contribution of the recipient's individual properties to the variability of scent assessments. In girls in the non-receptive state there is noted the reverse correlation between the strength and pleasantness of the male scent. In the receptive state, they become tolerant to intensive male scents. At non-regular menstrual cycles, the female scent is assessments by young males as less pleasant. At the same time, the girls themselves with the cycle disturbances assess higher the male scent samples as compared with reproducibly healthy girls. The first sexual experience decreases the subjective assessments given by girls to the male scent samples. The young males, on the contrary, increase assessments given by girls to the male scent samples. The young males, on the contrary, increase assessments of the female scent on having acquired the sexual experiment. Coefficients of correlation of the mutual assessments of scent pleasantness of young males and females are progressingly rising with increase of biological and social significance of the search for optimal partner. Thus, effect of psychophysiological and social factors on assessments of pleasantness of scent samples aggress well with requirements, to which the reproductively significant non-verbal signal should fit.     

Emotional stimulation alters olfactory sensitivity and odor judgment

Emotions have a strong influence on the perception of visual and auditory stimuli. Only little is known about the relation between emotional stimulation and olfactory functions. The present study investigated the relationship between the presentation of affective pictures, olfactory functions, and sex. Olfactory performance was assessed in 32 subjects (16 male). Olfactory sensitivity was significantly reduced following unpleasant picture presentation for all subjects and following pleasant picture presentation for male subjects only. Pleasantness and intensity ratings of a neutral suprathreshold odor were related to the valence of the pictures: After unpleasant picture presentation, the odor was rated as less pleasant and more intense, whereas viewing positive pictures induced a significant increase in reported odor pleasantness. We conclude that inducing a negative emotional state reduces olfactory sensitivity. A relation to functional deviations within the primary olfactory cortices is discussed.     

Attitudes toward Olfaction: A Cross-regional Study

This study aimed to determine whether there are regional influences on attitudes toward olfaction. A total of 1082 participants aged 21-50 years from 4 different regions (Mexican, Korean, Czech, and German) were asked to rate general attitudes toward olfaction in everyday life. To examine affective attitudes to odors (i.e., pleasantness), participants were also asked to list 3 odors as being the most pleasant or unpleasant, respectively. Next, the mentioned odor names were attributed to 1 of 4 main categories: "Food & Drink," "Social relationship," "Nature," and "Civilization" and the distribution of these categories was compared across regions. Mexicans were significantly different to the other regions in their general attitudes toward olfaction. In addition, in all regions, in comparison with men, women indicated a higher interest in the sense of smell. Moreover, a significant positive correlation was present between individuals' self-rating of olfactory sensitivity and general attitudes toward olfaction. Finally, there were significant cross-regional differences in affective attitudes toward specific categories of odors. In conclusion, our findings support and extend the notion that regions affect attitudes toward the olfactory world.