Competitive (but not cooperative) body odors bias the discrimination of action intentions towards cooperation

Odors help us to interpret the environment, including the nature of social interactions. But, whether and how they influence the ability to discriminate the intentional states embedded in actions is unclear. In two experiments, we asked two independent groups of participants to discriminate motor intentions from videos showing one agent performing a reach-to-grasp movement with another agent with a cooperative or a competitive intent, and the same movement performed alone at either natural- or fast-speed, as controls. Task-irrelevant odor primes preceded each video presentation. Experiment 1 (N = 19) included masked cooperative and competitive body odors (human sweat collected while the donors were engaged in cooperative and competitive activities), whereas Experiment 2 (N = 20) included a common odor (cedarwood oil) and no odor (clean air) as primes. In an odor-primed, two-alternative forced choice task, participants discriminated the intention underlying the observed action. The results indicated that the odor exposure modulated the discrimination speed across different intentions, but only when the action intentions were hard to discriminate (cooperative vs. individual natural-speed, and competitive vs. individual fast-speed). Contrary to our hypothesis, a direct odor-action intention compatibility effect was not found. Instead, we propose a negative arousal compatibility-like effect to explain our results. Discrimination of high arousing action intentions (i.e., competitive) took longer when primed by high arousing odors (common odor and competitive body odor) than by low arousing odors (cooperative body odor and no odor). Discrimination of low arousing action intentions (i.e., cooperative) took longer when primed by low arousing odors than by high arousing odors. All in all, competitive (but not cooperative) body odors bias the discrimination of action intentions towards cooperation.

Investigating the human chemical communication of positive emotions using a virtual reality-based mood induction

Humans can communicate their emotions to others via volatile emissions from their bodies. Although there is now solid evidence for human chemical communication of fear, stress and anxiety, investigations of positive emotions remain scarce. In a recent study, we found that women's heart rate and performance in creativity tasks were modulated by body odors of men sampled while they were in a positive vs. neutral mood. However, inducing positive emotions in laboratory settings remains challenging. Therefore, an important step to further investigate the human chemical communication of positive emotions is to develop new methods to induce positive moods. Here, we present a new mood induction procedure (MIP) based on virtual reality (VR), that we assumed to be more powerful than videos (used in our previous study) to induce positive emotions. We hypothesized that, consequently, given the more intense emotions created, this VR-based MIP would induce larger differences between the receivers' responses to the positive body odor versus a neutral control body odor, than the Video-based MIP. The results confirmed the higher efficacy of VR to induce positive emotions compared with videos. More specifically, VR had more repeatable effects between individuals. Although positive body odors had similar effects to those found in the previous video study, especially faster problem solving, these effects did not reach statistical significance. These outcomes are discussed as a function of the specificities of VR and of other methodological parameters, that may have prevented the observation of such subtle effects and that should be understood more in-depth for future studies on human chemical communication.

Social Odour Perception and Stress Responses in Women’s Quality of Partner Relationship and Attachment Style

The perception of body and social odours (SOP) is crucial for interpersonal chemosensory signalling and mate choice, yet little is known about the role of the SOP on the quality of partnerships and the attachment style. The aim of this study was to investigate the role of the SOP in women’s stress responses by considering the role of biopsychosocial variables in the quality of interpersonal relationships (also considering intimate partner violence). In total, 253 women filled out an online survey that included a series of questionnaires to investigate self-perceived stress (PSS), emotional regulation (ERQ), olfactory social assessment (SOS), quality of partnership (RRQ), attachment style (RQ), and the Conflict Tactile Scale 2 (CTS-2). The main results highlight that a high awareness of social odours correlates with a good quality of relationship and with an emotional regulation capacity; the PSS correlates negatively with the ERQ (i.e., as the PSS increases, the ERQ decreases). The level of IPV predicts an interpersonal style characterized by a low desire to develop meaningful relationships but with a tendency to depend on and trust another. The idea of being hurt by the other is not central in women who experience this type of relationship. The study’s main conclusion is that social odour perception is important for emotional regulation and in partner relationships.

Sex-specific effects of human chemosignal on perception of angry but not fearful faces

Androsta-4,16,-dien-3-one (androstadienone), a steroids implicated as a human social chemosignal, has been reported to impact one's emotional perception along the valence axis. The current study takes a step further to examine whether it modulates the perception of angry and fearful faces, two negative emotions that are similar with respect to valence and arousal, but signal different social values. Systematic comparisons of psychophysical data collected from 40 heterosexual men and 45 heterosexual women revealed that androstadienone subconsciously biased heterosexual men toward perceiving the male faces as less angry, while it biased the heterosexual women toward perceiving the female faces as angrier. Meanwhile, androstadienone did not affect the perception of fearful faces in either men or women. These findings indicate that the modulation of androstadienone on negative emotional perceptions is not uniform, suggesting that it alters the perception of specific rather than general negative emotions. In particular, it impacts one's perception of anger, which signals impending aggression, and hence could further impact an individual's social interaction in a sex-specific manner.

How priming with body odors affects decision speeds in consumer behavior

To date, odor research has primarily focused on the behavioral effects of common odors on consumer perception and choices. We report a study that examines, for the first time, the effects of human body odor cues on consumer purchase behaviors. The influence of human chemosignals produced in three conditions, namely happiness, fear, a relaxed condition (rest), and a control condition (no odor), were examined on willingness to pay (WTP) judgments across various products. We focused on the speed with which participants reached such decisions. The central finding revealed that participants exposed to human odors reached decisions significantly faster than the no odor control group. The main driving force is that human body odors activate the presence of others during decision-making. This, in turn, affects response speed. The broader implications of this finding for consumer behavior are discussed.

Different armpits under my new nose: Olfactory sex but not gender affects implicit measures of embodiment

Conflicting multisensory signals may alter embodiment to produce self-identification with a foreign body, but the role of olfaction in this process has been overlooked. We studied in healthy participants how sex (male and female sweat odors) and gender (male and female cosmetic scents) olfactory stimuli contribute to embodiment. Participants saw, on a head mounted display, the first-person perspective of a sex mismatching person. Synchronous visuotactile stimulation was applied to enhance illusory embodiment. Simultaneously, they smelled either sex- or gender- congruent or incongruent stimuli. We assessed implicit (skin conductance responses to visual threats) and explicit (questionnaire) measures of embodiment. Stronger responses to threat were found when participants smelled the sex-congruent compared to the sex-incongruent odor, while no such differences were found for the cosmetic scents. According to the questionnaire, embodiment did not differ between conditions. Post-experimental assessment of the presented cues, suggest that while both sweat odors were considered generally male, cosmetic scents were not. The presented scents were generally not associated with the embodied body. Our results suggest that sex-related body odors influence implicit but not explicit aspects of embodiment and are in line with unique characteristics of olfaction in other aspects of cognition.

Pleasant and unpleasant odor identification ability is associated with distinct dimensions of negative symptoms transdiagnostically in psychotic disorders

Negative symptoms are among the greatest sources of functional impairment for individuals with schizophrenia, yet their mechanisms remain poorly understood. Olfactory impairment is associated with negative symptoms. The processing of pleasant olfactory stimuli is subserved by reward-related neural circuitry while unpleasant olfactory processing is subserved by emotion-related neural circuitry, suggesting that these two odor dimensions may offer a window into differential mechanisms of negative symptoms. We examined whether pleasant and unpleasant odor identification bears differential relationships with avolition and inexpressivity dimensions of negative symptoms, whether these relationships are transdiagnostic, and whether pleasant and unpleasant odor processing also relate differently to other domains of functioning in a sample of individuals diagnosed with schizophrenia (N = 54), other psychotic disorders (N = 65), and never-psychotic adults (N = 160). Hierarchical regressions showed that pleasant odor identification was uniquely associated with avolition, while unpleasant odor identification was uniquely associated with inexpressivity. These relationships were largely transdiagnostic across groups. Additionally, pleasant and unpleasant odor identification displayed signs of specificity with other functional and cognitive measures. These results align with past work suggesting dissociable pathomechanisms of negative symptoms and provide a potential avenue for future work using valence-specific olfactory dysfunction as a semi-objective and low-cost marker for understanding and predicting the severity of specific negative symptom profiles.

There is chemistry in social chemistry

Nonhuman terrestrial mammals sniff themselves and each other to decide who is friend or foe. Humans also sniff themselves and each other, but the function of this is unknown. Because humans seek friends who are similar to themselves, we hypothesized that humans may smell themselves and others to subconsciously estimate body odor similarity, which, in turn, may promote friendship. To test this, we recruited nonromantic same-sex friend dyads and harvested their body odor. We found that objective ratings obtained with an electronic nose, and subjective ratings obtained from independent human smellers converged to suggest that friends smell more similar to each other than random dyads. Last, we recruited complete strangers, smelled them with an electronic nose, and engaged them in nonverbal same-sex dyadic interactions. We observed that dyads who smelled more similar had more positive dyadic interactions. In other words, we could predict social bonding with an electronic nose. We conclude that there is indeed chemistry in social chemistry.

Localizing the human brain response to olfactory stimulation: A meta-analytic approach

The human sense of smell and the ability to detect and distinguish odors allows for the extraction of valuable information from the environment, thereby driving human behavior. Not only can the sense of smell help to monitor the safety of inhaled air, but it can also help to evaluate the edibility of food. Therefore, in an effort to further our understanding of the human sense of smell, the aim of this meta-analysis was to provide the scientific community with activation probability maps of the functional anatomy of the olfactory system, in addition to separate activation maps for specific odor categories (pleasant, food, and aversive odors). The activation likelihood estimation (ALE) method was utilized to quantify all relevant and available data to perform a formal statistical analysis on the inter-study concordance of various odor categories. A total of 81 studies (108 contrasts, 1053 foci) fulfilled our inclusion criteria. Significant ALE peaks were observed in all odor categories in brain areas typically associated with the functional neuroanatomy of olfaction including the piriform cortex, amygdala, insula, and orbitofrontal cortex, amongst others. Additional contrast analyses indicate clear differences in neural activation patterns between odor categories.

The social odor scale: Development and initial validation of a new scale for the assessment of social odor awareness

The degree of attention individuals pay to olfactory cues (called odor awareness) influences the role of odors in everyday life. Particularly, odors produced by the human body (i.e., social odors) are able to carry a wide variety of information and to elicit a broad spectrum of emotional reactions, making them essential in interpersonal relationships. Hence, despite the assessment of awareness toward social odors is crucial, a proper tool is still lacking. Here, we designed and initially validated the Social Odor Scale (SOS), a 12-item scale designed to measure the individual differences in awareness towards different social odors. In Study 1, an exploratory factor analysis (EFA; KMO test: MSA = 0.78; Bartlett's test: χ2(78) = 631.34, p < 0.001; Chi-squared test: χ2(42) = 71.84, p = 0.003) suggests that the three factors structure was the model that best fit with the Italian version of the scale. The confirmatory factor analysis (CFA) supports a second-order model with one higher-order factor representing social odor awareness in general and three lower-order factors representing familiar, romantic partner, and stranger social odors. The final version of the scale presented a good fit (RMSEA = 0.012, SRMR = 0.069, CFI = 0.998, TLI = 0.997). In Study 2, CFA was performed in the German version of the scale confirming the validity of scale structure. Study 3 and 4 revealed that SOS total score and its subscales were positively correlated with other validated olfactory scales, but not with olfactory abilities. Moreover, SOS was found to be related to the gender of the participants: women reported to be more aware to social odors and, specifically, to familiar social odors than men. Overall, the results indicated that SOS is a valid and reliable instrument to assess awareness toward social odors in everyday life.

Maternal chemosignals enhance infant-adult brain-to-brain synchrony

Maternal body odors serve as important safety-promoting and social recognition signals, but their role in human brain maturation is largely unknown. Utilizing ecological paradigms and dual- electroencephalography recording, we examined the effects of maternal chemosignals on brain-to-brain synchrony during infant-mother and infant-stranger interactions with and without the presence of maternal body odors. Neural connectivity of right-to-right brain theta synchrony emerged across conditions, sensitizing key nodes of the infant’s social brain during its maturational period. Infant-mother interaction elicited greater brain-to-brain synchrony; however, maternal chemosignals attenuated this difference. Infants exhibited more social attention, positive arousal, and safety/approach behaviors in the maternal chemosignals condition, which augmented infant-stranger neural synchrony. Human mothers use interbrain mechanisms to tune the infant’s social brain, and chemosignals may sustain the transfer of infant sociality from the mother-infant bond to life within social groups.

You See What You Smell: Preferential Processing of Chemosensory Satiety Cues and Its Impact on Body Shape Perception

The current study examines neural responses to satiety- and fasting-related volatiles and their effect on the processing of body shapes. Axillary sweat was sampled with cotton pads from 10 individuals after 12 h of fasting, and after having consumed a standard breakfast. Pure cotton pads served as the control. The chemosensory stimuli were presented to 20 participants (via a constant-flow olfactometer) exclusively, and additionally as context to images of overweight and underweight avatars. EEG was recorded (61 electrodes), and chemosensory (CSERPs; P1, N1, P2, P3) and visual event-related potentials (VERPs; N1, P2, P3a, P3b) were analyzed. The amplitudes of all positive CSERP components differed more strongly from cotton in response to chemosensory satiety cues as compared to fasting cues (P1: p = 0.023, P2: p = 0.083, P3: p = 0.031), paralleled by activity within the middle frontal and temporal gyrus. Overweight compared to underweight body shapes tended to elicit larger VERP P2 amplitudes (p = 0.068), and chemosensory satiety cues amplified the VERP amplitudes in response to any body shape (P2, P3a, P3b; all p_s ≤ 0.017) as compared to the cotton control. The results indicate that chemosensory satiety cues transmit complex social information, overriding the processing of analogous visual input.

Acquired olfactory loss alters functional connectivity and morphology

Removing function from a developed and functional sensory system is known to alter both cerebral morphology and functional connections. To date, a majority of studies assessing sensory-dependent plasticity have focused on effects from either early onset or long-term sensory loss and little is known how the recent sensory loss affects the human brain. With the aim of determining how recent sensory loss affects cerebral morphology and functional connectivity, we assessed differences between individuals with acquired olfactory loss (duration 7-36 months) and matched healthy controls in their grey matter volume, using multivariate pattern analyses, and functional connectivity, using dynamic connectivity analyses, within and from the olfactory cortex. Our results demonstrate that acquired olfactory loss is associated with altered grey matter volume in, among others, posterior piriform cortex, a core olfactory processing area, as well as the inferior frontal gyrus and angular gyrus. In addition, compared to controls, individuals with acquired anosmia displayed significantly stronger dynamic functional connectivity from the posterior piriform cortex to, among others, the angular gyrus, a known multisensory integration area. When assessing differences in dynamic functional connectivity from the angular gyrus, individuals with acquired anosmia had stronger connectivity from the angular gyrus to areas primary responsible for basic visual processing. These results demonstrate that recently acquired sensory loss is associated with both changed cerebral morphology within core olfactory areas and increase dynamic functional connectivity from olfactory cortex to cerebral areas processing multisensory integration.

Olfactory impairment and close social relationships. A narrative review

Olfactory impairment is one of the more unique symptoms of COVID-19 infection and has therefore enjoyed increased public attention in recent months. Olfactory impairment has various implications and consequences ranging from difficulty detecting dangerous pathogens to hindering social functioning and social behaviors. We provide an overview of how olfactory impairment can impact 3 types of close social relationships: family relationships, friendships, and romantic relationships. Evidence is divided into several categories representing potential mechanisms by which olfactory impairment can impact close social relationships: bonding disruptions, decreased social support, missed group-eating experiences, hygiene concerns, and altered sexual behaviors. We conclude with a discussion of emerging future research questions.

The scent of attraction and the smell of success: crossmodal influences on person perception

In recent decades, there has been an explosion of research into the crossmodal influence of olfactory cues on multisensory person perception. Numerous peer-reviewed studies have documented that a variety of olfactory stimuli, from ambient malodours through to fine fragrances, and even a range of chemosensory body odours can influence everything from a perceiver's judgments of another person's attractiveness, age, affect, health/disease status, and even elements of their personality. The crossmodal and multisensory contributions to such effects are reviewed and the limitations/peculiarities of the research that have been published to date are highlighted. At the same time, however, it is important to note that the presence of scent (and/or the absence of malodour) can also influence people's (i.e., a perceiver's) self-confidence which may, in turn, affect how attractive they appear to others. Several potential cognitive mechanisms have been put forward to try and explain such crossmodal/multisensory influences, and some of the neural substrates underpinning these effects have now been characterized. At the end of this narrative review, a number of the potential (and actual) applications for, and implications of, such crossmodal/multisensory phenomena involving olfaction are outlined briefly.

The lasting smell of emotions: The effects of reutilizing fear sweat samples

A growing body of research has shown that human apocrine sweat carries information about the emotional state of its donor. Exposure to sweat produced in a fear-inducing context triggers in its receivers a simulacrum of this emotional state, as evidenced by increased medial frontalis and corrugator supercilii (facial electromyography; fEMG) activity – two facial muscles involved in the display of fear facial expressions. However, despite the increased interest in the effects of emotional sweat, little is known about the properties of these chemical sweat samples. The goal of this study was to examine whether a second application of the same sweat sample would yield reliable results. Specifically, we assessed whether sweat samples collected from Portuguese males (N = 8) in fear (vs. neutral)-inducing contexts would produce similar fEMG activations (i.e., in the medial frontalis and corrugator supercilii) in female receivers (N = 60) across two independent applications (the first with Dutch and the second with Portuguese receivers). Our findings showed that exposure to fear (vs. neutral) sweat resulted in higher activation of both muscles compared with neutral odors, revealing a similar data pattern across the two applications and underlining the feasibility of reusing emotional sweat samples. The implications of these findings for properties of these sweat volatiles are discussed.

Olfactory Perception and Different Decongestive Response of the Nasal Mucosa During Menstrual Cycle

BACKGROUND

Sex hormones are known to have some influence on nasal functions, but their effect on the decongestive response of the nasal mucosa during menstrual cycle is still undetermined.

OBJECTIVES

The aim of this study was to examine the nasal physiology, the interconnectedness of olfactory and respiratory nasal function and the decongestive response of the nasal mucosa during menstrual cycle.

METHODS

This study included 101 healthy women aged 23.26 ± 4.81 years with a regular menstrual cycle. The nasal respiratory function and the decongestive response of the nasal mucosa were examined by rhinomanometry. Subjective sense of nasal obstruction and the subjective odor intensity were assessed by standardized questionnaires. The odor identification ability was assessed by Sniffin' Sticks test.

RESULTS

Statistically significant higher values of nasal resistance (0.311 ± 0.107 Pa/cm³/s) and NOSE score (11.893 ± 13.83) were observed in the ovulatory phase compared to the luteal (0.281 ± 0.084 Pa/cm³/s and 9.029 ± 11.12). An odor identification test score was significantly higher in luteal phase (12.476 ± 1.48) compared to the ovulatory phase (11.971 ± 1.51), opposite of odor intensity. The difference of nasal resistance before and after decongestion was significantly higher in ovulatory phase (0.105 ± 0.097 Pa/cm³/s) compared to the luteal (0.084 ± 0.079 Pa/cm³/s). Correlation between subjective and objective parameters of the examinated nasal functions was not statistically significant in any menstrual phase.

CONCLUSION

In the population of women studied, total nasal resistance and NOSE score were significantly lower in the luteal phase of the menstrual cycle. Odor identification was significantly higher in the luteal phase but odor intensity significantly higher in the ovulatory phase. The decongestive response of nasal mucosa was better in the ovulatory phase of the menstrual cycle.

Male brain processing of the body odor of ovulating women compared to that of pregnant women

Female chemical signals underlie the advertising of sexual receptivity and fertility. Whether the body odor of a pregnant woman also has a signaling function with respect to male behavior is yet to be conclusively established. This study examines how the body odors of ovulating and pregnant women differentially affect the behavior of heterosexual men. Body odor samples were collected from 5 pregnant women and 5 matched controls during ovulation. In a double-blind functional magnetic resonance imaging design, 18 heterosexual men were exposed to female body odors during ovulation (OV) and pregnancy (PRG) while being required to indicate the attractiveness of concurrently presented female portrait images. The participants were also required to indicate whether they assumed a depicted woman was pregnant. While neither OV nor PRG altered the perceived attractiveness of a presented face, the men tended to identify the women as pregnant while exposed to a PRG body odor. On the neural level, OV activated a network of the frontotemporal and limbic regions, while PRG activated the superior medial frontal gyrus. The results suggest that the detection of sexual availability activates the male brain regions associated with face processing and reward/motivation, whereas sensing pregnancy activates a region responsible for empathy and prosocial behavior. Thus, the female body odor during pregnancy likely helps foster circumstances conducive to the future care of offspring while the body odor advertising sexual availability promotes mating behavior. The brains of heterosexual men may be capable of unconsciously discriminating between these two types of olfactory stimuli.

Pheromone effects on the human hypothalamus in relation to sexual orientation and gender

Pheromones are chemicals that serve communicational purposes within a species. In most terrestrial mammals, pheromones are detected by either the olfactory epithelium or the vomeronasal organ and processed by various downstream structures including the medial amygdala and the hypothalamus to regulate motivated behaviors and endocrine responses. The search for human pheromones began in the 1970s. Whereas bioactive ligands are yet to be identified, there has been accumulating evidence that human body odors exert a range of pheromone-like effects on the recipients, including triggering innate behavioral responses, modulating endocrine levels, signaling social information, and affecting mood and cognition. In parallel, results from recent brain imaging studies suggest that body odors evoke distinct neural responses from those observed with common nonsocial odors. Two endogenous steroids androsta-4,16,- dien-3-one and estra-1,3,5(10),16-tetraen-3-ol are considered by some as candidates for human sex pheromones. The two substances produce sexually dimorphic effects on human perception, mood, and physiological arousal. Moreover, they reportedly elicit different hypothalamic response patterns in manners contingent on the recipients' sex and sexual orientation. Neuroendocrine mechanisms underlying the effects of human chemosignals are not yet clear and await future detailed analyses.

Human Chemosignals and Brain Activity: A Preliminary Meta-analysis of the Processing of Human Body Odors

Across phyla, chemosignals are a widely used form of social communication and increasing evidence suggests that chemosensory communication is present also in humans. Chemosignals can transfer, via body odors, socially relevant information, such as specific information about identity or emotional states. However, findings on neural correlates of processing of body odors are divergent. The aims of this meta-analysis were to assess the brain areas involved in the perception of body odors (both neutral and emotional) and the specific activation patterns for the perception of neutral body odor (NBO) and emotional body odor (EBO). We conducted an activation likelihood estimation (ALE) meta-analysis on 16 experiments (13 studies) examining brain activity during body odors processing. We found that the contrast EBO versus NBO resulted in significant convergence in the right middle frontal gyrus and the left cerebellum, whereas the pooled meta-analysis combining all the studies of human odors showed significant convergence in the right inferior frontal gyrus. No significant cluster was found for NBOs. However, our findings also highlight methodological heterogeneity across the existing literature. Further neuroimaging studies are needed to clarify and support the existing findings on neural correlates of processing of body odors.

Brain response to intranasal trimethylamine stimulation: A preliminary human fMRI study

The trace amine-associated receptors (TAARs) are a second class of olfactory receptors in humans. They are activated by volatile amines, including pheromone-like odors. However, in humans the neural processing of TAAR-associated signals is not known. Using functional magnetic resonance imaging, the current study investigated brain activation following intra-nasal stimulation with trimethylamine (TMA), an agonist of human TAAR5, and three "canonical" odors with varied valence (an unpleasant odor [n-butanol], a neutral to unpleasant odor [civet musk], and a pleasant odor [phenyl ethyl alcohol]) in 12 healthy young participants. Our hypothesis driven analysis showed that TMA induced a trend for stronger left amygdala activation as compared to the other odors (Family-Wise Error corrected p = 0.08). Whole-brain exploratory analyses revealed superior activation of the cerebellum and caudate to TMA compared to canonical odors, and stronger activation of the anterior cingulate and somatosensory regions (postcentral gyrus and mid cingulate) in response to canonical odors compared to TMA. The current results provide initial evidence on differential central processes of a TAAR mediated stimulus compared to odors targeting canonical olfactory receptors. Future research are needed to elucidate the physiological and psychological relevance of TAARs in humans.

Giving meaning to the social world in autism spectrum disorders: Olfaction as a missing piece of the puzzle?

Altered social cognition is a core feature of Autism Spectrum Disorders (ASD). These impairments have been explained as the consequence of compromised social motivational mechanisms that limit social interest and activate a cascade of social deficits. Following this rational, we argue that approaches capable of surpassing ASD usual restraints (e.g., deficits in verbal abilities), and able to assign social meaning, could be more effective at responding to these difficulties. In this framework, we propose that olfaction, as well as cross-modal integration strategies involving both visual and olfactory domains, may have such potential. In fact, most of socioemotional processing deficits in ASD have been shown in an uni-modal perspective, mainly with visual stimuli. However, the social environment involves other modalities and is typically multisensorial. Given the potential of olfaction as a gateway for socioemotional information in ASD, we argue in favor of studying olfactory perception, as well as visuo-olfactory integration, given the potential of these approaches to drive effective interventions and give the access to a meaningful social world in ASD.

Are humans constantly but subconsciously smelling themselves?

All primates, including humans, engage in self-face-touching at very high frequency. The functional purpose or antecedents of this behaviour remain unclear. In this hybrid review, we put forth the hypothesis that self-face-touching subserves self-smelling. We first review data implying that humans touch their faces at very high frequency. We then detail evidence from the one study that implicated an olfactory origin for this behaviour: This evidence consists of significantly increased nasal inhalation concurrent with self-face-touching, and predictable increases or decreases in self-face-touching as a function of subliminal odourant tainting. Although we speculate that self-smelling through self-face-touching is largely an unconscious act, we note that in addition, humans also consciously smell themselves at high frequency. To verify this added statement, we administered an online self-report questionnaire. Upon being asked, approximately 94% of approximately 400 respondents acknowledged engaging in smelling themselves. Paradoxically, we observe that although this very prevalent behaviour of self-smelling is of concern to individuals, especially to parents of children overtly exhibiting self-smelling, the behaviour has nearly no traction in the medical or psychological literature. We suggest psychological and cultural explanations for this paradox, and end in suggesting that human self-smelling become a formal topic of investigation in the study of human social olfaction. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Interdisciplinary challenges for elucidating human olfactory attractiveness

Many species use chemicals to communicate. In humans, there is increasing evidence that chemicals conveyed by the body are extremely important in interpersonal relationships. However, many aspects of chemical communication remain to be explored to fully understand this function in humans. The aim of this article is to identify relevant challenges in this field, with a focus on human attractiveness in the context of reproduction, and to put forward roadmaps for future studies that will hopefully extend to a wider range of social interactions. The first challenge consists in not being limited to body (mal)odours from the axilla. Preliminary data on how the odour of the face and head is perceived are presented. Second, there is a crucial need to increase our knowledge of the chemical bases of human chemical communication. Third, cross-cultural approaches must not be overlooked, because they have a major input in understanding the universal and culture-specific aspects of chemical communication. Fourth, the influence of specific cultural practices such as contraceptive and fragrance use is likely to be prominent and, therefore, needs to be well described. The fifth and last challenge for research projects in this field is the integration of different disciplines such as behavioural sciences, social sciences, neurosciences and microbiology. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Human body odor increases familiarity for faces during encoding-retrieval task

Odors can increase memory performance when presented as context during both encoding and retrieval phases. Since information from different sensory modalities is integrated into a unified conceptual knowledge, we hypothesize that the social information from body odors and faces would be integrated during encoding. The integration of such social information would enhance retrieval more so than when the encoding occurs in the context of common odors. To examine this hypothesis and to further explore the underlying neural correlates of this behavior, we have conducted a functional magnetic resonance imaging study in which participants performed an encoding‐retrieval memory task for faces during the presentation of common odor, body odor or clean air. At the behavioral level, results show that participants were less biased and faster in recognizing faces when presented in concomitance with the body odor compared to the common odor. At the neural level, the encoding of faces in the body odor condition, compared to common odor and clean air conditions, showed greater activation in areas related to associative memory (dorsolateral prefrontal cortex), odor perception and multisensory integration (orbitofrontal cortex). These results suggest that face and body odor information were integrated and as a result, participants were faster in recognizing previously presented material.

Individual Differences as a Key Factor to Uncover the Neural Underpinnings of Hedonic and Social Functions of Human Olfaction: Current Findings from PET and fMRI Studies and Future Considerations

The hedonic and social dimensions of olfactory perception are characterized by a great diversity across people. Whereas the cerebral processing underlying these aspects of odor perception have been widely explored in the last decades, very few brain imaging studies considered individual differences. This lack of consideration weakens the current models in the field, where the paradigm of universality is the norm. The present review is aimed at examining this issue. Through a synthetic summary, we will first present past studies suggesting that (1) hedonics are represented consistently throughout the olfactory system from primary to secondary areas, with a progressive cognitive modulation and integration with other senses, (2) social dimension of odors may be represented in a distinct pathway involving social and attentional networks. In a second, and more critical part, we will highlight the importance of individual differences for the cerebral study of human olfaction.

Smelling Anxiety Chemosignals Impairs Clinical Performance of Dental Students

Despite the fact that human body odors can transfer anxiety-related signals, the impact of such signals in real-life situations is scant. In this study, the effects of anxiety chemosignals on the performance of dental students operating on simulation units, wearing T-shirts imbued with human sweat and masked with eugenol were tested. A total of 24 fourth-year dental students (17 F) donated their body odors in two sessions (Anxiety and Rest). Twenty-four normosmic, sex- and age-matched test subjects who were third-year dental students performed 3 dental procedures while smelling masked anxiety body odors, masked rest body odors, or masker alone. The intensity and pleasantness ratings showed that the test subjects could not report perceptual differences between the odor conditions. When exposed to masked anxiety body odors, the test subjects' dental performance was significantly worse than when they were exposed to masked rest body odors and masker alone, indicating that their performance was modulated by exposure to the emotional tone of the odor. These findings call for a careful evaluation of the anxiety-inducing effects of body odors in performance-related tasks and provide the first ecological evaluation of human anxiety chemosignal communication.

The scent of the other women: Body odor-induced behavioral and physiological effects on face categorization

In body odor research, the interaction of female donors and receivers is scarcely investigated. With the aim to investigate effects of female body odor in a competitive context, we tested 51 women divided into two groups (i.e., a competitive and a non-competitive group, based on verbal instructions). Between groups, we explored whether female body odor exposure (vs. masker odor) modulates emotion categorization (via RT variance and distribution) and physiological reactions (via instantaneous heart rate) in a task with dynamic male and female faces as either angry or happy. Women in the competitive group reported to feel more competitive and performed more accurately. They gathered more emotional information to categorize dynamic faces and when additionally exposed to female body odor, they showed a resistance to cardiac deceleration. Lapses of attention (via RT distribution) occurred irrespective of body odor exposure. Our results support the idea that female body odors, presented in a competitive context, contrast cardiac deceleration and, by tendency, modulate emotion categorization. Data are discussed in the context of chemosignaling and social interactions among women.

Background odors affect behavior in a dot-probe task with emotionally expressive faces

Odors affect perception of social cues in visual environments. Although often underestimated, people use their sense of smell to guide approach or avoidance behavior in social contexts. However, underlying psychological mechanisms are not well known. Prior work suggested olfactory effects are due to increased attention or arousal, or depend on the congruency between olfactory and visual cues. Our aim was to assess how odors influence attentional processes using a dot-probe task with odor-congruent and odor-incongruent facial expressions (happy, disgusted and neutral expressions paired with pleasant odor, unpleasant odor and no-odor). In a preregistered analysis plan, we hypothesized either faster reaction times attributed to arousal from odors in general, or to faces that were emotionally congruent with the odors. We also hypothesized time-on-task effects specific to the odor compared to the control condition. Using Bayesian linear models, we found strong evidence that the faces were rated as more arousing and emotional in odor contexts. However, the dot-probe task did in fact not provide an effective cue to selective visual attention, and odors did not modulate overall attention to the faces. However, we found a time-on-task effect such that in the unpleasant odor condition, response times decreased over time, whereas in the no-odor and pleasant condition there was a slight increase in response times. We conclude that time-on-task effects is an interesting venue for odor-visual interaction research, and such effects might explain inconsistent findings in the prior research literature.

Body odors (even when masked) make you more emotional: behavioral and neural insights

Morality evolved within specific social contexts that are argued to shape moral choices. In turn, moral choices are hypothesized to be affected by body odors as they powerfully convey socially-relevant information. We thus investigated the neural underpinnings of the possible body odors effect on the participants’ decisions. In an fMRI study we presented to healthy individuals 64 moral dilemmas divided in incongruent (real) and congruent (fake) moral dilemmas, using different types of harm (intentional: instrumental dilemmas, or inadvertent: accidental dilemmas). Participants were required to choose deontological or utilitarian actions under the exposure to a neutral fragrance (masker) or body odors concealed by the same masker (masked body odor). Smelling the masked body odor while processing incongruent (not congruent) dilemmas activates the supramarginal gyrus, consistent with an increase in prosocial attitude. When processing accidental (not instrumental) dilemmas, smelling the masked body odor activates the angular gyrus, an area associated with the processing of people’s presence, supporting the hypothesis that body odors enhance the saliency of the social context in moral scenarios. These results suggest that masked body odors can influence moral choices by increasing the emotional experience during the decision process, and further explain how sensory unconscious biases affect human behavior.

Women smelling men's masked body odors show enhanced harm aversion in moral dilemmas

Among the most unnoticeable stimuli providing social information, body odors are powerful social tools that can modulate behavioral and neural processing. It has recently been shown that body odors can affect moral decision-making, by increasing the activations in neural areas processing social and emotional information during the decision process. The aim of the present study was twofold: 1) to test whether body odors selectively affect decisions to real dilemmatic moral scenario (incongruent) vs. fake (congruent) dilemmas, and 2) to characterize whether the impact of masked body odors is modulated by four conceptual factors: personal force, intentionality, benefit recipient and evitability. Women chose between utilitarian (sacrificing a person's life in order to save other lives) or deontological actions (deciding against the harmful action) in 64 moral dilemmas under the exposure of a neutral fragrance (masker) or a masked male body odor. Our results showed that the masked male body odor did not specifically affect the answers to real and fake dilemmas but instead, its effect is modulating whether the agent harms the victim in a direct or indirect manner (personal force) to save herself or only other people (benefit recipient). In particular, when exposed to the masked body odor participants gave more deontological answers when the harm was indirect and only other people were saved. These data support the hypothesis that body odors induce participants to perceive the individuals described in moral dilemmas as more real, triggering harm avoidance.

The Design Matters: How to Detect Neural Correlates of Baby Body Odors

Functional magnetic resonance imaging of body odors is challenging due to methodological obstacles of odor presentation in the scanner and low intensity of body odors. Hence, few imaging studies investigated neural responses to body odors. Those differ in design characteristics and have shown varying results. Evidence on central processing of baby body odors has been scarce but might be important in order to detect neural correlates of bonding in mothers. A suitable paradigm for investigating perception of baby body odors has still to be established. We compared neural responses to baby body odors in a new to a conventional block design in a sample of ten normosmic mothers. For the new short design, 6 s of continuous odor presentation were followed by 19 s baseline and 13 repetitions were performed. For the conventional long design, 15 s of pulsed odor presentation were followed by 30 s of baseline and eight repetitions were performed. Neural responses were observed in brain structures related to basal and higher-order olfactory processing, such as insula, orbitofrontal cortex, and amygdala. Neural responses following the short design were significantly higher in comparison to the long design. This effect was based on higher number of repetitions but affected olfactory areas differently. The BOLD signal in the primary olfactory structures was enhanced by short and continuous stimulation, secondary structures did profit from longer stimulations with many repetitions. The short design is recommended as a suitable paradigm in order to detect neuronal correlates of baby body odors.

Medical aromatherapy revisited-Basic mechanisms, critique, and a new development

OBJECTIVE

According to a series of recent meta-analyses and systematic reviews, aromatherapy has shown to be effective in treating patients with different medical conditions. However, many of the clinical studies are of rather low methodological quality. Moreover, there is much conceptual ambiguity with regard to what aromatherapy actually constitutes.

METHOD

In this paper, we discuss the conditions under which aromatherapy is most likely to be of medical value by outlining the workings of the olfactory system and the necessary requirements of odors to be therapeutic. We then introduce an aromatherapeutic inhaler that was tested in a series of studies involving 465 participants.

RESULTS

This inhaler (AromaStick®) produced large to very large effects across a variety of physiological target systems (e.g., cardiovascular, endocrine, blood oxygenation, and pain), both short term and long term.

DISCUSSION

Inhalation of volatile compounds from essential oils yields almost immediate, large, and clinically relevant effects as long as the scents are delivered highly concentrated from an appropriate device. The changes caused in the body seem side effect-free and can be sustained when inhalation is repeated.

Human chemosignals of disgust facilitate food judgment

Choosing food is not a trivial decision that people need to make daily, which is often subject to social influences. Here, we studied a human homolog of social transmission of food preference (STFP) as observed in rodents and other animals via chemosignals of body secretions. Human social chemosignals (sweat) produced during a disgust or neutral state among a group of donors were presented to participants undergoing a 2-alternative-forced-choice food healthiness judgment task during functional magnetic resonance imaging (fMRI). Response speed and two key signal detection indices—d’ (discrimination sensitivity) and β (response bias)—converged to indicate that social chemosignals of disgust facilitated food healthiness decisions, in contrast to primary disgust elicitors (disgust odors) that impaired the judgment. fMRI analyses (disgust vs. neutral sweat) revealed that the fusiform face area (FFA), amygdala, and orbitofrontal cortex (OFC) were engaged in processing social chemosignals of disgust during food judgment. Importantly, a double contrast of social signaling across modalities (olfactory vs. visual—facial expressions) indicated that the FFA and OFC exhibited preferential response to social chemosignals of disgust. Together, our findings provide initial evidence for human STFP, where social chemosignals are incorporated into food decisions by engaging social and emotional areas of the brain.

Sex differences in own and other body perception

Own body perception, and differentiating and comparing one's body to another person's body, are common cognitive functions that have relevance for self‐identity and social interactions. In several psychiatric conditions, including anorexia nervosa, body dysmorphic disorder, gender dysphoria, and autism spectrum disorder, self and own body perception, as well as aspects of social communication are disturbed. Despite most of these conditions having skewed prevalence sex ratios, little is known about whether the neural basis of own body perception differs between the sexes. We addressed this question by investigating brain activation using functional magnetic resonance imaging during a Body Perception task in 15 male and 15 female healthy participants. Participants viewed their own body, bodies of same‐sex, or opposite‐sex other people, and rated the degree that they appeared like themselves. We found that men and women did not differ in the pattern of brain activation during own body perception compared to a scrambled control image. However, when viewing images of other bodies of same‐sex or opposite‐sex, men showed significantly stronger activations in attention‐related and reward‐related brain regions, whereas women engaged stronger activations in striatal, medial‐prefrontal, and insular cortices, when viewing the own body compared to other images of the opposite sex. It is possible that other body images, particularly of the opposite sex, may be of greater salience for men, whereas images of own bodies may be more salient for women. These observations provide tentative neurobiological correlates to why women may be more vulnerable than men to conditions involving own body perception.

The scent of security: Odor of romantic partner alters subjective discomfort and autonomic stress responses in an adult attachment-dependent manner

When in a stressful situation, access to adult attachment figures (e.g., romantic partners) is an important means by which adults regulate stress responses. The practice of smelling a partner's worn garment is reported as a self-treatment against stress. Here, we experimentally determined whether exposure to a partner's body odor attenuates adults' subjective discomfort and psychophysiological responses, and whether such effects are qualified by adult attachment security. In a blocked design, participants (N = 34) were presented with their partner's body odor, their own body odor, the odor of a clean t-shirt and rose odor, while exposed to weak electric shocks to induce discomfort and stress responses. Results showed that partner body odor reduces subjective discomfort during a stressful event, as compared with the odor of oneself. Also, highly secure participants had attenuated skin conductance when exposed to partner odor. We conclude that partner odor is a scent of security, especially for attachment-secure adults.

Prefrontal Control Over Occipital Responses to Crossmodal Overlap Varies Across the Congruency Spectrum

While matched crossmodal information is known to facilitate object recognition, it is unclear how our perceptual systems encode the more gradual congruency variations that occur in our natural environment. Combining visual objects with odor mixtures to create a gradual increase in semantic object overlap, we demonstrate high behavioral acuity to linear variations of olfactory–visual overlap in a healthy adult population. This effect was paralleled by a linear increase in cortical activation at the intersection of occipital fusiform and lingual gyri, indicating linear encoding of crossmodal semantic overlap in visual object recognition networks. Effective connectivity analyses revealed that this integration of olfactory and visual information was achieved by direct information exchange between olfactory and visual areas. In addition, a parallel pathway through the superior frontal gyrus was increasingly recruited towards the most ambiguous stimuli. These findings demonstrate that cortical structures involved in object formation are inherently crossmodal and encode sensory overlap in a linear manner. The results further demonstrate that prefrontal control of these processes is likely required for ambiguous stimulus combinations, a fact of high ecological relevance that may be inappropriately captured by common task designs juxtaposing congruency and incongruency.

On the Communicative Function of Body Odors

Humans use multiple senses to navigate the social world, and the sense of smell is arguably the most underestimated one. An intriguing aspect of the sense of smell is its social communicative function. Research has shown that human odors convey information about a range of states (e.g., emotions, sickness) and traits (e.g., individuality, gender). Yet, what underlies the communicability of these states and traits via smell? We fill this explanatory gap with a framework that highlights the dynamic and flexible aspects of human olfactory communication. In particular, we explain how chemical profiles, associative learning (i.e., the systematic co-occurrence of chemical profiles with state- or trait-related information), and top-down contextual influences could interact to shape human odor perception. Our model not only helps to integrate past research on human olfactory communication but it also opens new avenues for future research on this fascinating, yet to date poorly understood, field.

Smelling is Telling: Human Olfactory Cues Influence Social Judgments in Semi-Realistic Interactions

How does a person's smell affect others' impressions of them? Most body odor research asks perceivers to make social judgments based on armpit sweat without perfume or deodorant, presented on t-shirts. Yet, in real life, perceivers encounter fragranced body odor, on whole bodies. Our "raters" wore blindfolds and earplugs and repeatedly smelled same-sex "donors" in live interactions. In one condition, donors wore their normal deodorant and perfume ("diplomatic" odor) while in the other condition, donors were asked to avoid all outside fragrance influences ("natural" odor). We assessed the reliability of social judgments based on such live interactions, and the relationships between live judgments and traditional t-shirt based judgments, and between natural- and diplomatic odor-based judgments. Raters' repeated live social judgments (e.g., friendliness, likeability) were highly consistent for both diplomatic and natural odor, and converged with judgments based on t-shirts. However, social judgments based on natural odor did not consistently predict social judgments based on diplomatic odor, suggesting that natural and diplomatic body odor may convey different types of social information. Our results provide evidence that individuals can perceive reliable, meaningful social olfactory signals from whole bodies, at social distances, regardless of the presence or absence of perfume. Importantly, however, the social value of these signals is modified by the addition of exogenous fragrances. Further, our focus on judgments in same-sex dyads suggests that these olfactory cues hold social value in non-mating contexts. We suggest that future research employ more ecologically relevant methods.

Behavioral and neural correlates to multisensory detection of sick humans

Throughout human evolution, infectious diseases have been a primary cause of death. Detection of subtle cues indicating sickness and avoidance of sick conspecifics would therefore be an adaptive way of coping with an environment fraught with pathogens. This study determines how humans perceive and integrate early cues of sickness in conspecifics sampled just hours after the induction of immune system activation, and the underlying neural mechanisms for this detection. In a double-blind placebo-controlled crossover design, the immune system in 22 sample donors was transiently activated with an endotoxin injection [lipopolysaccharide (LPS)]. Facial photographs and body odor samples were taken from the same donors when "sick" (LPS-injected) and when "healthy" (saline-injected) and subsequently were presented to a separate group of participants (n = 30) who rated their liking of the presented person during fMRI scanning. Faces were less socially desirable when sick, and sick body odors tended to lower liking of the faces. Sickness status presented by odor and facial photograph resulted in increased neural activation of odor- and face-perception networks, respectively. A superadditive effect of olfactory-visual integration of sickness cues was found in the intraparietal sulcus, which was functionally connected to core areas of multisensory integration in the superior temporal sulcus and orbitofrontal cortex. Taken together, the results outline a disease-avoidance model in which neural mechanisms involved in the detection of disease cues and multisensory integration are vital parts.

Mate-choice copying, social information processing, and the roles of oxytocin

Social and sexual behaviors, including that of mate choice, are dependent on social information. Mate choice can be modified by prior and ongoing social factors and experience. The mate choice decisions of one individual can be influenced by either the actual or potential mate choice of another female or male. Such non-independent mate choice, where individuals gain social information and socially learn about and recognizes potential mates by observing the choices of another female or male, has been termed "mate-choice copying". Here we first briefly review how, why, and under what circumstances individuals engage in mate-choice copying. Secondly, we review the neurobiological mechanisms underlying mate-choice copying. In particular, we consider the roles of the nonapeptide, oxytocin, in the processing of social information and the expression of mate-choice copying.

The influence of androstadienone during psychosocial stress is modulated by gender, trait anxiety and subjective stress: An fMRI study

Androstadienone (ANDR), a bodily secreted steroid compound, is a socially relevant chemosignal that modulates subjective and (neuro)physiological responses, predominantly in females. The impact of ANDR on stress responses in males and females has not been explored. Therefore, this fMRI study aimed to examine psychosocial stress reactions induced by mental arithmetic and social evaluation on behavioral and hormonal levels (46 participants: 15 naturally cycling females in their early follicular phase (EF), 15 females on hormonal contraceptives (HC) and 16 males); and on a neural level (40 participants: 13 EF-females, 13 HC-females and 14 males) in an ANDR and placebo treatment repeated-measures design. While no gender differences emerged in subjective ratings and performance during stress, neural activation patterns differed significantly. Besides, ANDR attenuated the post-stress increase of negative mood in all participants. Region of interest analyses showed that irrespective of treatment, males showed stronger activation of the dorsolateral prefrontal cortex (DLPFC) than females. At the whole brain level, gender differences emerged indicating stronger fronto-parietal activation in males compared to HC-females on both treatments. Males showed stronger visual and fusiform activation than EF-females under ANDR. Both female groups did not show stronger activation than males. Further, error ratio in the ANDR-stress condition was positively associated with their post-stress cortisol level and increase in subjective stress in males; and male DLPFC activity in the ANDR-stress condition was negatively associated with trait anxiety. Surprisingly, compared to HC-females, EF-female only showed stronger activation of arousal-related areas under placebo treatment. Taken together, these findings suggest that the male stress reaction under social evaluative threat was stronger than female stress reactions as a function of ANDR. More specifically, this effect on behavioral and neural stress reactions seems to depend on trait anxiety in males only. The study highlights the significance of a chemosignal in enhancing social threat that may facilitate adaptive stress responses.

Testing for Individual Differences in the Identification of Chemosignals for Fear and Happy: Phenotypic Super-Detectors, Detectors and Non-Detectors

Mood odor identification, explicit awareness of mood odor, may be an important emotion skill and part of a complex dual processing system. It has already been shown that mood odors have significant implicit effects, effects that occur without awareness. This study applies methods for examining human individual differences in the identification of chemosignals for fear and happy, important in itself, and a key to understanding the dual processing of emotion in the olfactory system. Axillary mood odors had been collected from 14 male donors during a mood induction task. Pads were collected after 12 and 24 minutes, creating two doses. Sixty -one participants (41 females) identified the mood odor chemosignals. On a single trial, participants identified 2 doses of fear, 2 doses of happy, and a sterile control. There were 15 trials. The first analysis (r_tt) showed that the population was phenotypically heterogeneous, not homogeneous, in identification accuracy. It also showed that a minimum of 10 trials was needed for test reliability. The second analysis, Growth Mixture Modeling, found three distinct groups of detectors: (1) 49.49% were consistently accurate super detectors, (2) 32.52% were accurate above chance level detectors, and (3) 17.98% were non-detectors. Bayesian Posterior Analyses showed reliability of groups at or above 98%. No differences related to mood odor valence (fear or happy), dose (collection at 12 or 24 minutes) or gender were found. Implications for further study of genetic differences, learning and function of identification are noted. It appears that many people can be reliable in explicitly identifying fear and happy mood odors but this skill is not homogeneous.

Conscious olfaction: Content, function, and localization

The target article's emphasis on olfaction is a welcome reminder of the multimodal nature of conscious experience. Here, I explore the distinctive and even unique attributes of our sense of smell from the point of view of their bearing on and fit with a subcortical locus of sensory experience.