Sexual Orientation Affects Neural Responses to Subtle Social Aggression Signals

The current series of studies are the first to examine brain responses to social aggression signals as a function of male and female sexual orientation. For the first set of studies (1a, 1b), axillary sweat had been collected from 17 heterosexual men and 17 heterosexual women aggressively responding to frustrating opponents (aggression condition) and while playing a construction game (control condition). Sweat samples were pooled according to sex and condition, and presented via a constant flow olfactometer to 17 gay and 23 heterosexual men (Study 1a), and 19 lesbian and 25 heterosexual women (Study 1b). Ongoing EEG was recorded from 61 scalp locations, chemosensory event-related potentials (CSERPs; P2, P3-1, P3-2) were analyzed, and neuronal sources calculated (low resolution electromagnetic tomography). Within the second set of studies (2a, 2b), pictures of males' and females' weak angry and neutral facial expressions were presented to 21 gay and 23 heterosexual men (Study 2a), and 19 lesbian and 26 heterosexual women (Study 2b), and ERPs (N170, P3) were analyzed. Gay men showed larger P3-1 amplitudes than heterosexual men upon presentation of male aggression sweat, accompanied by activation of the right inferior frontal gyrus (IFG, BA 10). Gay men also displayed longer N170 latencies in response to men's compared to women's angry facial expressions, while heterosexual men did not. In women, sexual orientation did not affect the processing of aggression sweat or anger expressions. Gay men showed preferential processing of chemosensory aggression signals (P3-1 amplitudes), indicating fine-tuned socioemotional sensitivity, related to activation of brain areas involved in emotion regulation (IFG). They further process the relative relevance of visual aggression signals (N170 latency). These results were in line with theories proposing a common evolutionary pathway for same-sex attraction and traits easing social integration.

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.

Emotional expression in human odour

Recent work has demonstrated that human body odour alters with changing emotional state and that emotionally laden odours can affect the physiology and behaviour of people exposed to them. Here we review these discoveries, which we believe add to a growing recognition that the human sense of smell and its potential role in social interactions have been underappreciated. However, we also critically evaluate the current evidence, with a particular focus on methodology and the interpretation of emotional odour studies. We argue that while the evidence convincingly indicates that humans retain a capacity for olfactory communication of emotion, the extent to which this occurs in ordinary social interaction remains an open question. Future studies should place fewer restrictions on participant selection and lifestyle and adopt more realistic experimental designs. We also need to devote more consideration to underlying mechanisms and to recognise the constraints that these may place on effective communication. Finally, we outline some promising approaches to address these issues, and raise some broader theoretical questions that such approaches may help us to answer.

Harvesting Context and Mining Emotions Related to Olfactory Cultural Heritage

This paper presents an Artificial Intelligence approach to mining context and emotions related to olfactory cultural heritage narratives, particularly to fairy tales. We provide an overview of the role of smell and emotions in literature, as well as highlight the importance of olfactory experience and emotions from psychology and linguistic perspectives. We introduce a methodology for extracting smells and emotions from text, as well as demonstrate the context-based visualizations related to smells and emotions implemented in a novel smell tracker tool. The evaluation is performed using a collection of fairy tales from Grimm and Andersen. We find out that fairy tales often connect smell with the emotional charge of situations. The experimental results show that we can detect smells and emotions in fairy tales with an F1 score of 91.62 and 79.2, respectively.

Smell what you hardly see: Odors assist visual categorization in the human brain

Visual categorization is the brain ability to rapidly and automatically respond to a certain category of inputs. Whether category-selective neural responses are purely visual or can be influenced by other sensory modalities remains unclear. Here, we test whether odors modulate visual categorization, expecting that odors facilitate the neural categorization of congruent visual objects, especially when the visual category is ambiguous. Scalp electroencephalogram (EEG) was recorded while natural images depicting various objects were displayed in rapid 12-Hz streams (i.e., 12 images / second) and variable exemplars of a target category (either human faces, cars, or facelike objects in dedicated sequences) were interleaved every 9^th stimulus to tag category-selective responses at 12/9 = 1.33 Hz in the EEG frequency spectrum. During visual stimulation, participants (N = 26) were implicitly exposed to odor contexts (either body, gasoline or baseline odors) and performed an orthogonal cross-detection task. We identify clear category-selective responses to every category over the occipito-temporal cortex, with the largest response for human faces and the lowest for facelike objects. Critically, body odor boosts the response to the ambiguous facelike objects (i.e., either perceived as nonface objects or faces) over the right hemisphere, especially for participants reporting their presence post-stimulation. By contrast, odors do not significantly modulate other category-selective responses, nor the general visual response recorded at 12 Hz, revealing a specific influence on the categorization of congruent ambiguous stimuli. Overall, these findings support the view that the brain actively uses cues from the different senses to readily categorize visual inputs, and that olfaction, which has long been considered as poorly functional in humans, is well placed to disambiguate visual information.

The Importance of Intact Senses in Mating and Social Assessments Made by Deaf Individuals

Social perception is a multimodal process involving vision and audition as central input sources for human social cognitive processes. However, it remains unclear how profoundly deaf people assess others in the context of mating and social interaction. The current study explored the relative importance of different sensory modalities (vision, smell, and touch) in assessments of opposite- and same-sex strangers. We focused on potential sensory compensation processes in mate selection (i.e., increased importance of the intact senses in forming impressions of an opposite-sex stranger as a potential partner). A total of 74 deaf individuals and 100 normally hearing controls were included in the study sample. We found diminished importance of vision and smell in deaf participants compared with controls for opposite- and same-sex strangers, and increased importance of touch for the assessment of same-sex strangers. The results suggested that deaf people rely less on visual and olfactory cues in mating and social assessments, highlighting a possible role of sign language in shaping interpersonal tactile experience in non-romantic relationships.

Olfaction in the Multisensory Processing of Faces: A Narrative Review of the Influence of Human Body Odors

A recent body of research has emerged regarding the interactions between olfaction and other sensory channels to process social information. The current review examines the influence of body odors on face perception, a core component of human social cognition. First, we review studies reporting how body odors interact with the perception of invariant facial information (i.e., identity, sex, attractiveness, trustworthiness, and dominance). Although we mainly focus on the influence of body odors based on axillary odor, we also review findings about specific steroids present in axillary sweat (i.e., androstenone, androstenol, androstadienone, and estratetraenol). We next survey the literature showing body odor influences on the perception of transient face properties, notably in discussing the role of body odors in facilitating or hindering the perception of emotional facial expression, in relation to competing frameworks of emotions. Finally, we discuss the developmental origins of these olfaction-to-vision influences, as an emerging literature indicates that odor cues strongly influence face perception in infants. Body odors with a high social relevance such as the odor emanating from the mother have a widespread influence on various aspects of face perception in infancy, including categorization of faces among other objects, face scanning behavior, or facial expression perception. We conclude by suggesting that the weight of olfaction might be especially strong in infancy, shaping social perception, especially in slow-maturing senses such as vision, and that this early tutoring function of olfaction spans all developmental stages to disambiguate a complex social environment by conveying key information for social interactions until adulthood.

Odor-evoked hedonic contexts influence the discrimination of facial expressions in the human brain

The influence of odor valence on expressive-face perception remains unclear. Here, three "valenced" odor contexts (pleasant, unpleasant, control) were diffused while scalp electroencephalogram (EEG) was recorded in 18 participants presented with expressive faces alternating at a 6-Hz rate. One facial expression (happiness, disgust or neutrality) repeatedly arose every 6 face pictures to isolate its discrimination from other expressions at 1 Hz and harmonics in the EEG spectrum. The amplitude of the brain response to neutrality was larger in the pleasant vs. control odor context, and fewer electrodes responded in the unpleasant odor context. The number of responding electrodes was reduced for disgust in both odor contexts. The response to happiness was unchanged between odor conditions. Overall, these observations suggest that valenced odors influence the neural discrimination of facial expressions depending on both face and odor hedonic valence, especially for the emotionally ambiguous neutral expression.

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.

Sexual Chemosignals: Evidence that Men Process Olfactory Signals of Women's Sexual Arousal

Research suggests that humans can communicate emotional states (e.g., fear, sadness) via chemosignals. However, thus far little is known about whether sexual arousal can also be conveyed through chemosignals and how these signals might influence the receiver. In three experiments, and a subsequent mini meta-analysis, support was found for the hypothesis that men can process the scent of sexually aroused women and that exposure to these sexual chemosignals affect the subsequent perceptions and sexual motivation of men. Specifically, Experiment 1 revealed that men evaluate the axillary sweat of sexually aroused women as more attractive, compared to the scent of the same women when not sexually aroused. In addition, Experiment 2 showed that exposure to sexual chemosignals increased the men's sexual arousal. Experiment 3 found support for the thesis that exposure to sexual chemosignals would increase sexual motivation. As predicted, men devoted greater attention to and showed greater interest in mating with women who displayed sexual cues (e.g., scantily dressed, in seductive poses). By contrast, exposure to the sexual chemosignals did not alter males' attention and mating interest toward women who displayed no sexual cues. It is discussed how sexual chemosignals may function as an additional channel in the communication of sexual interest and how contextual factors can influence the dynamics of human sexual communication.

Not All Emotions Are Equal: Fear Chemosignals Lower Awareness Thresholds Only for Fearful Faces

Exposure to body odors (chemosignals) collected under different emotional states (i.e., emotional chemosignals) can modulate our visual system, biasing visual perception. Recent research has suggested that exposure to fear body odors, results in a generalized faster access to visual awareness of different emotional facial expressions (i.e., fear, happy, and neutral). In the present study, we aimed at replicating and extending these findings by exploring if these effects are limited to fear odor, by introducing a second negative body odor—that is, disgust. We compared the time that 3 different emotional facial expressions (i.e., fear, disgust, and neutral) took to reach visual awareness, during a breaking continuous flash suppression paradigm, across 3 body odor conditions (i.e., fear, disgust, and neutral). We found that fear body odors do not trigger an overall faster access to visual awareness, but instead sped-up access to awareness specifically for facial expressions of fear. Disgust odor, on the other hand, had no effects on awareness thresholds of facial expressions. These findings contrast with prior results, suggesting that the potential of fear body odors to induce visual processing adjustments is specific to fear cues. Furthermore, our results support a unique ability of fear body odors in inducing such visual processing changes, compared with other negative emotional chemosignals (i.e., disgust). These conclusions raise interesting questions as to how fear odor might interact with the visual processing stream, whilst simultaneously giving rise to future avenues of research.

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.

The scent of emotions: A systematic review of human intra- and interspecific chemical communication of emotions

OBJECTIVE

The sense of olfaction has been considered of minor importance in human communication. In recent years, evidence has emerged that humans might be influenced by unconscious messages sent through chemosignals in body odors. Data concerning the ability of humans to recognize fear, maybe related to the evolutionary role of these emotions in the fight-or-flight reactions, are well known.

METHODS

To further understand the role of emotional chemosignals in mediating communication in humans and its influence on animal behaviors, we conducted a systematic literature review.

RESULTS

Chemosignals derived from axillary odors collected under a variety of emotional stimuli and sad tears in humans affect receivers' social interactions, danger detection and risk-taking behavior, social aspects of eating, and performance under stressing conditions. In addition, beyond the fight-or-flight response, even the body odors of happiness can be perceived by others. Furthermore, human chemosignals can influence behaviors and stressful responses in animals, particularly dogs and horses, which may partially explain their special relationship with humans.

CONCLUSION

Our review highlights the importance of chemosignaling in human intra- and interspecific interactions and suggests the need for further investigations, both in physiological conditions and in patients with psychiatric or neurodegenerative disorders.

The Effect of Exposure to Fear-Related Body Odorants on Anxiety and Interpersonal Trust Toward a Virtual Character

A growing body of literature documents how exposure to another person's fear-related body odorants can increase one's own anxiety and interfere with processing of social information, such as facial expression and impression formation. Building on these results, we aimed to 1) test the hypothesis that exposure to fear-related odorant would affect impression formation through fear contagion and 2) verify whether these effects can be observed in an ecologically valid (i.e., virtual) environment. We proposed that exposure to fear-related odorant would cause receivers to feel more anxious, which in turn would lead them to report less trust toward an unknown virtual character. This study had 2 distinct phases. First, we collected perspiration odorants from the armpits of 12 male senders (i.e., the source of the odorant) during the viewing of either fear or joy inducing film clips. In the second phase, 53 women receivers were exposed to either a fear, joy, or neutral odorant (i.e., between-subjects design) by breathing through a gauze attached to a disposable respirator mask while immersed in a virtual bar. As expected, receivers exposed to fear odorants felt significantly more stressed. Mediation analysis also revealed an indirect effect of exposure on trust through anxiety. More specifically, the more anxious the receiver felt, the less she trusted the virtual character. Our results show for the first time that the impact of exposure to fear-related body odorants on negative interpersonal impression formation is mediated by the anxiety induced in the receiver.

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.

Body-odor based assessments of sex and personality - Non-significant differences between blind and sighted odor raters

People exhibit different sensitivity to the signaling properties of body odors in the social context. Here, we aimed to investigate whether visual status modulates sensitivity to socially-relevant cues carried by body odors and whether it affects psychophysical ratings of such smells. We compared abilities of 19 early-blind, 9 late-blind and 13 sighted people to accurately assess sex, neuroticism and dominance of odor donors based on body odor samples. We showed that early blind, late blind and sighted people do not differ in accuracy of sex and personality assessments based on body odor samples. Additionally, the three participating groups perceived the presented body odor samples as similarly intense, pleasant and attractive. We discuss our findings in the context of interpersonal olfactory communication and olfactory compensation.

Fear Odor Facilitates the Detection of Fear Expressions Over Other Negative Expressions

In a double-blind experiment, participants were exposed to facial images of anger, disgust, fear, and neutral expressions under 2 body odor conditions: fear and neutral sweat. They had to indicate the valence of the gradually emerging facial image. Two alternative hypotheses were tested, namely a "general negative evaluative state" hypothesis and a "discrete emotion" hypothesis. These hypotheses suggest 2 distinctive data patterns for muscle activation and classification speed of facial expressions. The pattern of results that would support a "discrete emotions perspective" would be expected to reveal significantly increased activity in the medial frontalis (eyebrow raiser) and corrugator supercilii (frown) muscles associated with fear, and significantly decreased reaction times (RTs) to "only" fear faces in the fear odor condition. Conversely, a pattern of results characterized by only a significantly increased corrugator supercilii activity together with decreased RTs for fear, disgust, and anger faces in the fear odor condition would support an interpretation in line with a general negative evaluative state perspective. The data support the discrete emotion account for facial affect perception primed with fear odor. This study provides a first demonstration of perception of discrete negative facial expressions using olfactory priming.

Emotional Body Odors as Context: Effects on Cardiac and Subjective Responses

Many studies have indicated that the chemical cues from body odors (BOs) of donors experiencing negative emotions can influence the psychophysiological and behavioral response of the observers. However, these olfactory cues have been used mainly as contextual information for processing visual stimuli. Here, for the first time, we evaluate how emotional BO affects the emotional tone of a subsequent BO message. Axillary sweat samples were taken from 20 donors in 3 separate sessions while they watched fear, disgust, or neutral videos. In a double-blind experiment, we assessed the cardiac and subjective responses from 69 participants who were either exposed to negative emotional or neutral BOs. Our results showed a reduced cardiac parasympathetic activity (HF%)-indicating increased stress-when participants smelled the emotional BOs before the neutral BOs, compared to when they smelled neutral followed by emotional BOs. The intensity of the neutral odor also increased following the exposure to both negative BOs. These findings indicate that BOs contain an emotion-dependent chemical cue that affects the perceiver both at the physiological and subjective levels.

Empathic Cognitions Affected by Undetectable Social Chemosignals: An EEG Study on Visually Evoked Empathy for Pain in an Auditory and Chemosensory Context

Reduction of mu activity within the EEG is an indicator of cognitive empathy and can be generated in response to visual depictions of others in pain. The current study tested whether this brain response can be modulated by an auditory and a chemosensory context. Participants observed pictures of painful and non-painful actions while pain associated and neutral exclamations were presented (Study 1, N = 30) or while chemosensory stimuli were presented via a constant flow olfactometer (Study 2, N = 22). Chemosensory stimuli were sampled on cotton pads while donors participated in a simulated job interview (stress condition) or cycled on a stationary bike (sport condition). Pure cotton was used as a control. The social chemosignals could not be detected as odors. Activity within the 8–13 Hz band at electrodes C3, C4 (mu activity) and electrodes O1, O2 (alpha-activity) was calculated using Fast-Fourier-Transformation (FFT). As expected, suppression of power in the 8–13 Hz band was stronger when painful as compared to non-painful actions were observed (Study 1, p = 0.020; Study 2, p = 0.005). In addition, as compared to the neutral auditory and chemosensory context, painful exclamations (Study 1, p = 0.039) and chemosensory stress signals (Study 2, p = 0.014) augmented mu-/alpha suppression also in response to non-painful pictures. The studies show that processing of social threat-related information is not dominated by visual information. Rather, cognitive appraisal related to empathy can be affected by painful exclamations and subthreshold chemosensory social information.

Anxiety Body Odors as Context for Dynamic Faces: Categorization and Psychophysiological Biases

Body odors (BOs) can convey social information. In particular, their effects are maximal when their presence is paired with meaningful social contexts. Static faces have been widely used as social stimuli. However, they miss a key feature of our phenomenological experience, characterized by multisensory dynamic stimulations. Here, we investigate how BO sampled from individuals experiencing a transitory anxiety state, (a) induce a stress response and (b) bias the recognition of dynamic facial expressions, compared with BO of relaxed individuals. Participants ( n=46) categorized the emotion of a face, morphing from a neutral expression to either an angry or happy expression, during exposure to either BO condition. In addition, their cardiac activity was measured. Exposure to anxiety BO increased the accuracy of dynamic facial recognition and reduced cardiac parasympathetic activity. These results suggest that in social situations that simulate part of the multisensory and dynamic features of real-life social contexts, anxiety BOs will induce a stress response in recipients, modulating both arousal and cognitive-emotional skills but facilitating emotional facial processing.

Effects of androstadienone on dominance perception in males with low and high social anxiety

Increasing evidence suggests that humans can communicate both trait-dominance and state-dominance via body odor. Androstadienone (androsta-4,16,-dien-3-one), a chemosignal found in human sweat, seems to be a likely candidate for signaling dominance in humans. The aim of the current study was to investigate the effects of androstadienone on the perception of social dominance. Moreover, we examined whether high levels of social anxiety, a psychopathology involving concerns that specifically pertain to social dominance, are associated with increased sensitivity to androstadienone as a chemical cue of dominance. In a double-blind, placebo-controlled, within-subject design, 64 heterosexual male participants (32 with high social anxiety and 32 with low social anxiety) viewed facial images of males depicting dominant, neutral and submissive postures, and were asked to recognize and rate the dominance expressed in those images. Participants completed the task twice, once under exposure to androstadienone and once under exposure to a control solution. The results indicate that androstadienone increased the perceived dominance of men's faces, specifically among participants with high social anxiety. These findings suggest a direct influence of androstadienone on dominance perception and further highlight the preferential processing of dominance and social threat signals evident in social anxiety.

Human Body Odour Composites Are Not Perceived More Positively than the Individual Samples

It is well established that composite facial images are perceived as more attractive compared with individual images, suggesting a preference for heterozygosity. Similarly, there is evidence that preferences for body odours might be linked to heterozygosity. Here, we tested whether blending individual body odours into composites would follow a similar pattern as observed in the perception of faces. We collected axillary odour samples from 38 individuals, which were subsequently assessed individually and as composites of two (N = 19) or four (N = 9) body odours regarding their pleasantness, attractiveness and intensity. We found no significant differences between mean ratings of individual odour samples or composites of two or four odour samples. Our results indicate that, in contrast to faces, composite body odours are not rated as more attractive. Composite body odours retain similar hedonic perceptual qualities as individual odours, thus highlighting differences in visual and chemosensory perceptual mechanisms.

Pregnancy reduces the perception of anxiety

In humans, stress can be contagiously transmitted via chemosignals on a subconscious level. This study investigates how pregnancy affects neural responses to anxiety chemosignals. Using cotton pads, 28 men donated axillary sweat immediately before an academic examination (anxiety sweat) and during ergometer training (control). Via a constant-flow olfactometer, samples were presented (oddball paradigm) to 12 non-pregnant (NP) women, 14 women in their first (T1), and 18 in their third (T3) trimester of pregnancy. Chemosensory event-related potentials and current source densities (CSD) were analysed (60 electrode setup). Compared to NP-women, pregnant women display diminished evaluative processing of the sweat samples (targets; P3-1/ P3-2 amplitudes) and delayed evaluative processing of the anxiety sweat (targets; P3-2 latency). T3-women show attenuated early processing (targets; N1 amplitude) compared to NP-women, and reduced evaluative processing compared to T1-women (standards; P3-2 amplitude). CSDs (P3-1/ P3-2 latency ranges) reveal that T1- and T3-women show an atypical activation distribution to anxiety sweat. Most participants were unable to detect the sweat samples (anxiety sweat: 79.5%, sport sweat 88.6%). The results demonstrate that the processing of anxiety chemosignals progressively vanishes during pregnancy. This effect is likely to occur without any cognitive control.

Exposure to Hedione Increases Reciprocity in Humans

Cooperation among unrelated humans is frequently regarded as a defining feature in the evolutionary success of our species. Whereas, much research has addressed the strategic and cognitive mechanisms that underlie cooperation, investigations into chemosensory processes have received very limited research attention. To bridge that gap, we build on recent research that has identified the chemically synthesized odorant Hedione (HED) as a ligand for the putative human pheromone receptor (VN1R1) expressed in the olfactory mucosa, and hypothesize that exposure to HED may increase reciprocity. Applying behavioral economics paradigms, the present research shows that exposure to the ligand causes differentiated behavioral effects in reciprocal punishments (Study 1) as well as rewards (Study 2), two types of behaviors that are frequently regarded as essential for the development and maintenance of cooperation.

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.

You Smell Dangerous: Communicating Fight Responses Through Human Chemosignals of Aggression

The ability to detect conspecifics that represent a potential harm for an individual represents a high survival benefit. Humans communicate socially relevant information using all sensory modalities, including the chemosensory systems. In study 1, we investigated whether the body odor of a stranger with the intention to harm serves as a chemosignal of aggression. Sixteen healthy male participants donated their body odor while engaging in a boxing session characterized by aggression-induction methods (chemosignal of aggression) and while performing an ergometer session (exercise chemosignal). Self-reports on aggression-related physical activity, motivation to harm and angry emotions selectively increased after aggression induction. In study 2, we examined whether receivers smelling such chemosignals experience emotional contagion (e.g., anger) or emotional reciprocity (e.g., anxiety). The aggression and exercise chemosignals were therefore presented to 22 healthy normosmic participants in a double-blind, randomized exposure during which affective/cognitive processing was examined (i.e., emotion recognition task, emotional stroop task). Behavioral results indicate that chemosignals of aggression induce an affective/cognitive modulation compatible with an anxiety reaction in the recipients. These findings are discussed in light of mechanisms of emotional reciprocity as a way to convey not only affective but also motivational information via chemosensory signals in humans.

The Odor Context Facilitates the Perception of Low-Intensity Facial Expressions of Emotion

It has been established that the recognition of facial expressions integrates contextual information. In this study, we aimed to clarify the influence of contextual odors. The participants were asked to match a target face varying in expression intensity with non-ambiguous expressive faces. Intensity variations in the target faces were designed by morphing expressive faces with neutral faces. In addition, the influence of verbal information was assessed by providing half the participants with the emotion names. Odor cues were manipulated by placing participants in a pleasant (strawberry), aversive (butyric acid), or no-odor control context. The results showed two main effects of the odor context. First, the minimum amount of visual information required to perceive an expression was lowered when the odor context was emotionally congruent: happiness was correctly perceived at lower intensities in the faces displayed in the pleasant odor context, and the same phenomenon occurred for disgust and anger in the aversive odor context. Second, the odor context influenced the false perception of expressions that were not used in target faces, with distinct patterns according to the presence of emotion names. When emotion names were provided, the aversive odor context decreased intrusions for disgust ambiguous faces but increased them for anger. When the emotion names were not provided, this effect did not occur and the pleasant odor context elicited an overall increase in intrusions for negative expressions. We conclude that olfaction plays a role in the way facial expressions are perceived in interaction with other contextual influences such as verbal information.

Rapid stress system drives chemical transfer of fear from sender to receiver

Humans can register another person's fear not only with their eyes and ears, but also with their nose. Previous research has demonstrated that exposure to body odors from fearful individuals elicited implicit fear in others. The odor of fearful individuals appears to have a distinctive signature that can be produced relatively rapidly, driven by a physiological mechanism that has remained unexplored in earlier research. The apocrine sweat glands in the armpit that are responsible for chemosignal production contain receptors for adrenalin. We therefore expected that the release of adrenalin through activation of the rapid stress response system (i.e., the sympathetic-adrenal medullary system) is what drives the release of fear sweat, as opposed to activation of the slower stress response system (i.e., hypothalamus-pituitary-adrenal axis). To test this assumption, sweat was sampled while eight participants prepared for a speech. Participants had higher heart rates and produced more armpit sweat in the fast stress condition, compared to baseline and the slow stress condition. Importantly, exposure to sweat from participants in the fast stress condition induced in receivers (N = 31) a simulacrum of the state of the sender, evidenced by the emergence of a fearful facial expression (facial electromyography) and vigilant behavior (i.e., faster classification of emotional facial expressions).

Always follow your nose: the functional significance of social chemosignals in human reproduction and survival

This article is part of a Special Issue "Chemosignals and Reproduction" Across phyla, chemosensory communication is crucial for mediating a variety of social behaviors, which form the basis for ontogenetic and phylogenetic survival. In the present paper, evidence on chemosensory communication in humans, with special reference to reproduction and survival, will be presented. First, the impact of chemosignals on human reproduction will be reviewed. Work will be presented, showing how chemosensory signals are involved in mate choice and partnership formation by communicating attractiveness and facilitating a partner selection, which is of evolutionary advantage, and furthermore providing information about the level of sexual hormones. In addition to direct effects on phylogenetic survival, chemosignals indirectly aid reproductive success by fostering harm protection. Results will be presented, showing that chemosensory communication aids the emotional bond between mother and child, which in turn motivates parental caretaking and protection, leading to infant survival. Moreover, the likelihood of group survival can be increased through the use of stress-related chemosignals. Stress-related chemosignals induce a stress-related physiology in the perceiver, thereby priming a fight-flight-response, which is necessary for an optimum adaption to environmental harm. Finally, effects of sexual orientation on chemosensory communication will be discussed in terms of their putative role in stabilizing social groups, which might indirectly provide harm protection and foster survival. An integrative model of the presented data will be introduced. In conclusion, an outlook, focusing on the involvement of chemosensory communication in human social behavior and illustrating a novel approach to the significance of chemosensory signals in human survival, will be given.

Human Body Scents: Do they Influence our Behavior?

Pheromonal communication in the animal world has been of great research interest for a long time. While extraordinary discoveries in this field have been made, the importance of the human sense of smell was of far lower interest. Humans are seen as poor smellers and therefore research about human olfaction remains quite sparse compared with other animals. Nevertheless amazing achievements have been made during the past 15 years. This is a collection of available data on this topic and a controversial discussion on the role of putative human pheromones in our modern way of living. While the focus was definitely put on behavioral changes evoked by putative human pheromones this article also includes other important aspects such as the possible existence of a human vomeronasal organ. If pheromones do have an influence on human behavior there has to be a receptor organ. How are human body scents secreted and turned into odorous substances? And how can con-specifics detect those very odors and transmit them to the brain? Apart from that the most likely candidates for human pheromones are taken on account and their impact on human behavior is shown in various detail.

Human gender differences in the perception of conspecific alarm chemosensory cues

It has previously been established that, in threatening situations, animals use alarm pheromones to communicate danger. There is emerging evidence of analogous chemosensory "stress" cues in humans. For this study, we collected alarm and exercise sweat from "donors," extracted it, pooled it and presented it to 16 unrelated "detector" subjects undergoing fMRI. The fMRI protocol consisted of four stimulus runs, with each combination of stimulus condition and donor gender represented four times. Because olfactory stimuli do not follow the canonical hemodynamic response, we used a model-free approach. We performed minimal preprocessing and worked directly with block-average time series and step-function estimates. We found that, while male stress sweat produced a comparably strong emotional response in both detector genders, female stress sweat produced a markedly stronger arousal in female than in male detectors. Our statistical tests pinpointed this gender-specificity to the right amygdala (strongest in the superficial nuclei). When comparing the olfactory bulb responses to the corresponding stimuli, we found no significant differences between male and female detectors. These imaging results complement existing behavioral evidence, by identifying whether gender differences in response to alarm chemosignals are initiated at the perceptual versus emotional level. Since we found no significant differences in the olfactory bulb (primary processing site for chemosensory signals in mammals), we infer that the specificity in responding to female fear is likely based on processing meaning, rather than strength, of chemosensory cues from each gender.

Chemosignals communicate human emotions

Can humans communicate emotional states via chemical signals? In the experiment reported here, we addressed this question by examining the function of chemosignals in a framework furnished by embodied social communication theory. Following this theory, we hypothesized that the processes a sender experiences during distinctive emotional states are transmitted to receivers by means of the chemicals that the sender produces, thus establishing a multilevel correspondence between sender and receiver. In a double-blind experiment, we examined facial reactions, sensory-regulation processes, and visual search in response to chemosignals. We demonstrated that fear chemosignals generated a fearful facial expression and sensory acquisition (increased sniff magnitude and eye scanning); in contrast, disgust chemosignals evoked a disgusted facial expression and sensory rejection (decreased sniff magnitude, target-detection sensitivity, and eye scanning). These findings underline the neglected social relevance of chemosignals in regulating communicative correspondence outside of conscious access.

Processing of Body Odor Signals by the Human Brain

Brain development in mammals has been proposed to be promoted by successful adaptations to the social complexity as well as to the social and non-social chemical environment. Therefore, the communication via chemosensory signals might have been and might still be a phylogenetically ancient communication channel transmitting evolutionary significant information. In humans, the neuronal underpinnings of the processing of social chemosignals have been investigated in relation to kin recognition, mate choice, the reproductive state and emotional contagion. These studies reveal that human chemosignals are probably not processed within olfactory brain areas but through neuronal relays responsible for the processing of social information. It is concluded that the processing of human social chemosignals resembles the processing of social signals originating from other modalities, except that human social chemosignals are usually communicated without the allocation of attentional resources, that is below the threshold of consciousness. Deviances in the processing of human social chemosignals might be related to the development and maintenance of mental disorders.