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.

Past, Present, and Future of Human Chemical Communication Research

Although chemical signaling is an essential mode of communication in most vertebrates, it has long been viewed as having negligible effects in humans. However, a growing body of evidence shows that the sense of smell affects human behavior in social contexts ranging from affiliation and parenting to disease avoidance and social threat. This article aims to (a) introduce research on human chemical communication in the historical context of the behavioral sciences; (b) provide a balanced overview of recent advances that describe individual differences in the emission of semiochemicals and the neural mechanisms underpinning their perception, that together demonstrate communicative function; and (c) propose directions for future research toward unraveling the molecular principles involved and understanding the variability in the generation, transmission, and reception of chemical signals in increasingly ecologically valid conditions. Achieving these goals will enable us to address some important societal challenges but are within reach only with the aid of genuinely interdisciplinary approaches.

The role of olfaction and sex-hormone status in empathy-related measures

Reports of a female advantage in empathy-related measures suggest a role for sex hormones, although the data are inconsistent. Studies also report similar sex differences in human olfactory perception. In rodents, olfaction is involved in detecting and integrating socially relevant information and is modulated by the brain actions of estrogens. We hypothesized that olfaction may untangle the mixed evidence on the relationship between sex hormones and empathy-related measures (cognitive and affective) in humans. To test this, we examined 60 healthy participants in three sex-hormone-status groups: free-cycling women tested in cycle phases with higher 17-β estradiol and progesterone, oral-contraceptive users (low estradiol and progesterone), and men. We assessed empathy-related measures, facial mimicry (from zygomaticus and corrugator muscle activity), and odor discrimination ability. In the empathy-related measures and facial mimicry, we did not find overall group effects or meaningful associations with salivary levels of estradiol, progesterone, or testosterone. Free-cycling women only outperformed men in the recognition of emotions from pictures of the eye region, but sex hormones were unrelated to emotion recognition performance. Oral contraceptive users showed higher scores in the affective empathy-related measure when viewing negative emotions, with no relation to hormone levels. Free-cycling women exhibited the strongest facial mimicry (viewing female, but not male protagonists), positively associated with progesterone. Finally, the groups differed in odor discrimination, with free-cycling women outperforming men. However, odor discrimination ability and empathy-related performance were not correlated. Our results support a role of sex hormones in odor perception and in empathy-related measures, to a certain extent. However, no common underlying mechanism was found.

Chemosensory communication of aggression: women's fine-tuned neural processing of male aggression signals

The current study is the first to examine the central nervous processing of aggression chemosignals within men and women by means of chemosensory event-related potential (CSERP) analysis. Axillary sweat was collected from 17 men and 17 women participating in a competitive computer game (aggression condition) and playing a construction game (control condition). Sweat samples were pooled with reference to donor gender and condition, and presented to 23 men and 25 women via a constant flow olfactometer. Ongoing electroencephalogram was recorded from 61 scalp locations, CSERPs (P2, P3-1, P3-2) were analysed and neuronal sources calculated (low-resolution electromagnetic tomography, LORETA). Women, especially, showed larger P3-1 and P3-2 amplitudes in response to male as compared with female aggression signals (all p values < 0.01). The peak activation of this effect was related to activity within the dorsomedial prefrontal cortex (Brodmann area 8). As male aggression commonly targets physical harm, the competence of the human brain to sensitively detect male aggression signals is considered to be highly adaptive. The detection of male aggression signals seems to be of higher importance for women than for men. It is suggested that the processing of male aggression signals in women induces an immediate response selection. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

[Olfactory Dysfunctions: References to Neuropsychiatric Disorders and Diagnostics]

Past research has revealed a variety of olfactory deficits associated with psychiatric and neurodegenerative disorders. These deficits are evident in psychophysical olfactory testing as well as neurophysiological and neuroanatomical examinations. The specific type of olfactory dysfunction appears disorder specific. For example, with regard to affective disorders, the functional and anatomical overlap between olfactory and emotion-specific brain areas has been suggested as a major underlying factor for olfactory dysfunction. Based on converging evidence of changes in olfactory perception related to Major Depression, Schizophrenia, Alzheimer's and Parkinson's Disease, olfactory testing has been discussed as an important additional diagnostic marker. Hence, valid methods for objective and reliable olfactory testing as well as guidelines for the interpretation of the respective diagnostic findings are required. The aim of this review is to provide an overview of reported olfactory deficits in psychiatric and neurodegenerative disorders. In addition, a selection of olfactory tests, available in German-speaking countries, with regard to the respective disorder-related olfactory deficit in question are presented and classified. Original data regarding an empirical validation of the Düsseldorf Odour Discrimination Test in a clinical population are presented.

Mu-Suppression as an Indicator of Empathic Processes in Lesbian, Gay, and Heterosexual Adults

Self-reported empathy differs with gender and sexual orientation. The current study investigated whether mu-suppression, reflecting brain activity especially related to state empathy, also is modulated by gender and sexual orientation. Pictures of painful and non-painful actions were presented to 20 lesbians, 20 gay men, 20 heterosexual men and 20 heterosexual women, while EEG was recorded. Individual peak frequencies of mu-activity (electrodes C3, C4) were detected within the 6-11 Hz band for each participant, and mu-suppression indices were calculated. Further, verbal indicators of state empathy (pain ratings) and compassion were assessed. Only heterosexual individuals showed the typical pattern of enhanced mu-suppression in response to painful relative to non-painful pictures. Lesbian women and gay men did not show a differential mu-response. Moreover, they felt less compassion compared to heterosexual individuals. In line with this finding, the more compassion the participants reported, the stronger the mu-suppression in response to painful relative to non-painful pictures was. Pain ratings did not vary with sexual orientation. The lesser compassion reported by lesbian women and gay men is discussed as a mediator of their non-differential mu-suppression response. It is hypothesized that this pattern might relate to gay men and lesbian women tending to perceive the anonymous depicted actors as outgroup members, hence showing less compassion and reduced mu-suppression. As empathy is often related to negative feelings (empathic stress), a clear distinction between individuals to empathize with versus individuals not to emphasize with may well be an adaptive feature in same-sex oriented individuals.

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.

Sensitivity of the human mirror neuron system for abstract traces of actions: An EEG-study

Theories of neuroaesthetics assume, that looking at traces of actions used in creating artwork (e.g. brush marks) is associated with a simulation of these actions in the observer's sensorimotor-cortex. The aim of the current study is to dissociate the activation of the sensorimotor-cortex by the observation of action traces from associated visual processes. Twenty-eight participants observed handmade graphics (acrylic paint on paper) of different complexity (line, triangle, shape of a house) and computer-generated counterparts. Central mu-activity, as an index of sensorimotor-cortex activity, and occipital alpha-activity, as an index of visual cortex activity were recorded in the 8-13Hz EEG-band. In line with the hypothesis, mu-activity at electrode C4 is sensitive for the complexity of handmade (p=0.001), but not computer-generated graphics (p>0.500). In contrast, occipital alpha-activity is sensitive for the complexity of both handmade and computer-generated graphics (p<0.001). Furthermore, the more empathic the participants rated themselves, the stronger mu-suppression was induced by handmade graphics compared to computer-generated graphics (electrode C4; r=-0.612, p=0.001). These results support the involvement of the sensorimotor-cortex in the recognition of action traces and strengthen evidence that individuals scoring high in emotional empathy feature a particularly responsive mirror neuron system.

Chemosensory anxiety signals prime defensive behavior in prepubertal girls

Chemosensory anxiety signals effectively prime motor responses related to withdrawal behavior, such as the startle reflex, in adult humans. As the reproductive status strongly affects the response to social chemosignals, the current study examined whether chemosensory anxiety signals would augment the startle response in prepubertal children as it does in adults. Using cotton pads, axillary sweat was collected from 28 men while waiting for an important oral examination (anxiety condition), and during ergometer training (sport control condition). Using a constant-flow olfactometer, sweat samples and pure cotton samples (cotton control) were presented to 10 prepubertal girls aged 9-13years (M=11.25, SD=1.25) for 3000ms during inhalation. White noise bursts of 102dB(A) served as startle probes, and startle responses were recorded via electromyography of the orbicularis oculi muscle. The girls showed larger startle amplitudes to probes presented in the context of chemosensory anxiety signals as compared to a context of sport control sweat (p<0.01) as well as cotton control (p<0.05). This effect was not attributable to differences in stimulus detection rates or stimulus hedonics. The results show that in prepubertal girls, similar to adults, chemosensory anxiety signals prime defensive motor behavior. This effect appears unrelated to the odorous quality of anxiety sweat, but seems to reflect a specific preparedness to respond to the underlying social alarm signal. Thus, chemosensory communication supporting individual harm protection is independent of the reproductive status in humans.

Fancy Citrus, Feel Good: Positive Judgment of Citrus Odor, but Not the Odor Itself, Is Associated with Elevated Mood during Experienced Helplessness

Aromatherapy claims that citrus essential oils exert mood lifting effects. Controlled studies, however, have yielded inconsistent results. Notably, studies so far did not control for odor pleasantness, although pleasantness is a critical determinant of emotional responses to odors. This study investigates mood lifting effects of d-(+)-limonene, the most prominent substance in citrus essential oils, with respect to odor quality judgments. Negative mood was induced within 78 participants using a helplessness paradigm (unsolvable social discrimination task). During this task, participants were continuously (mean duration: 19.5 min) exposed to d-(+)-limonene (n = 25), vanillin (n = 26), or diethyl phthalate (n = 27). Participants described their mood (Self-Assessment-Manikin, basic emotion ratings) and judged the odors’ quality (intensity, pleasantness, unpleasantness, familiarity) prior to and following the helplessness induction. The participants were in a less positive mood after the helplessness induction (p < 0.001), irrespective of the odor condition. Still, the more pleasant the participants judged the odors, the less effective the helplessness induction was in reducing happiness (p = 0.019). The results show no odor specific mood lifting effect of d-(+)-limonene, but indicate a positive effect of odor pleasantness on mood. The study highlights the necessity to evaluate odor judgments in aromatherapy research.

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.

Does human body odor represent a significant and rewarding social signal to individuals high in social openness?

Across a wide variety of domains, experts differ from novices in their response to stimuli linked to their respective field of expertise. It is currently unknown whether similar patterns can be observed with regard to social expertise. The current study therefore focuses on social openness, a central social skill necessary to initiate social contact. Human body odors were used as social cues, as they inherently signal the presence of another human being. Using functional MRI, hemodynamic brain responses to body odors of women reporting a high (n = 14) or a low (n = 12) level of social openness were compared. Greater activation within the inferior frontal gyrus and the caudate nucleus was observed in high socially open individuals compared to individuals low in social openness. With the inferior frontal gyrus being a crucial part of the human mirror neuron system, and the caudate nucleus being implicated in social reward, it is discussed whether human body odor might constitute more of a significant and rewarding social signal to individuals high in social openness compared to individuals low in social openness process.

Differential processing of social chemosignals obtained from potential partners in regards to gender and sexual orientation

On an individual level, human body odors carry information about whether a person is an eligible mate. The current studies investigate if body odors also transmit information about individuals being potential partners in more general terms, namely in regards to gender and sexual orientation. In study 1, 14 gay and 14 heterosexual men were presented with body odors obtained from potential partners (gay male and heterosexual female body odors, respectively) and heterosexual male body odor as a control. In study 2, 14 lesbian and 14 heterosexual women were presented with lesbian female and heterosexual male body odors representing body odors of potential partners, and heterosexual female body odor as a control. Central nervous processing was analyzed using chemosensory event-related potentials and current source density analysis (64-channel EEG recording). Gay and heterosexual men responded with shorter P2 latencies to the body odors of their preferred sexual partners, and lesbian women responded with shorter P2 latencies to body odors of their preferred gender. In response to heterosexual male body odors, lesbian women displayed the most pronounced P3 amplitude, and distinct neuronal activation in medial frontal and parietal neocortical areas. A similar pattern of neuronal activation was observed in gay men when presented with heterosexual male body odor. Both the early processing advantage (P2) for desirable partners' body odors as well as the enhanced evaluative processing (P3, CSD) of undesirable partners' body odors suggest that human body odors indeed carry information about individuals being potential partners in terms of gender and sexual orientation.