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Animals are widely believed to sense human emotions through smell. Chemoreception is the most primitive and ubiquitous sense, and brain regions responsible for processing smells are among the oldest structures in mammalian evolution. Thus, chemosignals might be involved in interspecies communication. The communication of emotions is essential for social interactions, but very few studies have clearly shown that animals can sense human emotions through smell. We used a habituation-discrimination protocol to test whether horses can discriminate between human odors produced while feeling fear vs. joy. Horses were presented with sweat odors of humans who reported feeling fear or joy while watching a horror movie or a comedy, respectively. A first odor was presented twice in successive trials (habituation), and then, the same odor and a novel odor were presented simultaneously (discrimination). The two odors were from the same human in the fear or joy condition; the experimenter and the observer were blinded to the condition. Horses sniffed the novel odor longer than the repeated odor, indicating they discriminated between human odors produced in fear and joy contexts. Moreover, differences in habituation speed and asymmetric nostril use according to odor suggest differences in the emotional processing of the two odors.

Faces can be categorized along various dimensions including gender or race, an ability developing in infancy. Infant categorization studies have focused on facial attributes in isolation, but the interaction between these attributes remains poorly understood. Experiment 1 examined gender categorization of other-race faces in 9- and 12-month-old White infants. Nine- and 12-month-olds were familiarized with Asian male or female faces, and tested with a novel exemplar from the familiarized category paired with a novel exemplar from a novel category. Both age groups showed novel category preferences for novel Asian female faces after familiarization with Asian male faces, but showed no novel category preference for novel Asian male faces after familiarization with Asian female faces. This categorization asymmetry was not due to a spontaneous preference hindering novel category reaction (Experiment 2), and both age groups displayed difficulty discriminating among male, but not female, other-race faces (Experiment 3). These results indicate that category formation for male other-race faces is mediated by categorical perception. Overall, the findings suggest that even by 12 months of age, infants are not fully able to form gender category representations of other-race faces, responding categorically to male, but not female, other-race faces.

Infants' ability to discriminate facial expressions has been widely explored, but little is known about the rapid and automatic ability to discriminate a given expression against many others in a single experiment. Here we investigated the development of facial expression discrimination in infancy with fast periodic visual stimulation coupled with scalp electroencephalography (EEG). EEG was recorded in eighteen 3.5- and eighteen 7-month-old infants presented with a female face expressing disgust, happiness, or a neutral emotion (in different stimulation sequences) at a base stimulation frequency of 6 Hz. Pictures of the same individual expressing other emotions (either anger, disgust, fear, happiness, sadness, or neutrality, randomly and excluding the expression presented at the base frequency) were introduced every six stimuli (at 1 Hz). Frequency-domain analysis revealed an objective (i.e., at the predefined 1-Hz frequency and harmonics) expression-change brain response in both 3.5- and 7-month-olds, indicating the visual discrimination of various expressions from disgust, happiness and neutrality from these early ages. At 3.5 months, the responses to the discrimination from disgust and happiness expressions were located mainly on medial occipital sites, whereas a more lateral topography was found for the response to the discrimination from neutrality, suggesting that expression discrimination from an emotionally neutral face relies on distinct visual cues than discrimination from a disgust or happy face. Finally, expression discrimination from happiness was associated with a reduced activity over posterior areas and an additional response over central frontal scalp regions at 7 months as compared to 3.5 months. This result suggests developmental changes in the processing of happiness expressions as compared to negative/neutral ones within this age range.

International audience; 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.

Studies on facial attractiveness in human adults, infants, and newborns have consistently reported a visual preference for faces rated as attractive compared to faces rated as unattractive. Biological accounts of facial attractiveness have typically presented such preferences as arising from adaptations for mate choice or as by-products of general sensory bias. In this cross-species study, we examined whether explicit ratings of attractiveness made by human judges would predict implicit visual preferences in other humans and also in rhesus macaques, and if they do, whether such preferences would extend beyond conspecific faces. Results showed that human ratings of attractiveness can predict implicit preferences in non-human primates (macaque monkeys, Macaca mulatta). However, we also found a species-specific effect of face attractiveness in which humans showed a visual preference for human faces (but not macaque faces) rated as attractive, and macaques displayed a visual preference for macaque faces (but not human faces) rated as attractive. Overall, the findings suggest that attentional bias toward attractive faces is neither the result of an exclusive operation of mate choice adaptation mechanisms, nor does it reflect the sole influence of a general sensory bias, but rather reflects their interaction. The influence of a general sensory bias may be modulated by the categorization of a face as conspecific or heterospecific, leading to species-specific preference for attractive faces.

International audience; The abilities to identify individuals within the group, and to interpret their expressions and intentions are essential for many social animals. Face recognition in human and nonhuman primates stems from a conjunction of evolutionary inheritance and experience via exposure to faces present in the environment. Individuation is clearly a vital mechanism for any social species. By uncovering similarities across primate face systems, comparative studies allow us to better understand the evolution of face processing capabilities in humans. Some researchers have argued that primates, including humans, may possess an innate face processing system that is predisposed to respond to conspecifics. The argument is supported by a study showing that monkeys raised without experience of own-species faces still prefer to look at faces of conspecifics (Fujita, Int J Primatol 11:553–573, 1990). However, this proposal does not fit well with findings from the human infant literature (Pascalis et al., Science 296:1321–1323, 2002) or with data on monkeys raised without seeing faces (Sugita, Proc Natl Acad Sci, 105, 394–398, 2008) which suggest that face processing is highly shaped by experience at an early age. We argue that human and nonhuman primates possess an evolved system for processing faces that becomes specialized as a consequence of predominant exposure to faces from a single species. According to this interpretation, a limitation of the face processing expertise to own species should be observed.

Despite some interpersonal variability, judgments of facial attractiveness are largely shared by most individuals, both within and between cultures. Infants are also sensitive to attractive faces even before being influenced by cultural standards of beauty. The intercultural agreement on this matter and its emergence during infancy suggest an evolutionary basis for facial attractiveness. Sensitivity to facial attractiveness is typically understood through evolutionary-based frameworks, either reflecting mate selection mechanisms or emerging as by-products of brain processing and perceptual sensory biases. In the current article, we review data on the emergence and the development of attractiveness preferences in infants, focusing on mechanisms that may explain or contribute to these preferences such as familiarity or fluency in processing. We further discuss the possibility that infants’ preference for attractiveness could extend to other stimuli than faces like objects or visual art. Potential directions for future research are proposed for developmental and comparative approaches.

Perceptual narrowing has been observed in human infants for monkey faces: 6-month-olds can discriminate between them, whereas older infants from 9 months of age display difficulty discriminating between them. The difficulty infants from 9 months have processing monkey faces has not been clearly identified. It could be due to the structural characteristics of monkey faces, particularly the key facial features that differ from human faces. The current study aimed to investigate whether the information conveyed by the eyes is of importance. We examined whether the presence of Caucasian human eyes in monkey faces allows recognition to be maintained in 6-month-olds and facilitates recognition in 9- and 12-month-olds. Our results revealed that the presence of human eyes in monkey faces maintains recognition for those faces at 6 months of age and partially facilitates recognition of those faces at 9 months of age, but not at 12 months of age. The findings are interpreted in the context of perceptual narrowing and suggest that the attenuation of processing of other-species faces is not reversed by the presence of human eyes.

In visually complex environments, numerous items compete for attention. Infants may exhibit attentional efficiency—privileged detection, attention capture and holding—for face-like stimuli. However, it remains unknown when these biases develop and what role, if any, experience plays in this emerging skill. Here, nursery-reared infant macaques’ (Macaca mulatta; n = 10) attention to faces in 10-item arrays of non-faces was measured using eye tracking. With limited face experience, 3-week-old monkeys were more likely to detect faces and looked longer at faces compared to non-faces, suggesting a robust face detection system. By 3 months, after peer exposure, infants looked faster to conspecific faces, but not heterospecific faces, suggesting an own-species bias in face attention capture, consistent with perceptual attunement.

Perceptual narrowing has been observed in human infants for monkey faces: 6-month-olds can discriminate between them, whereas older infants from 9 months of age display difficulty discriminating between them. The difficulty infants from 9 months have processing monkey faces has not been clearly identified. It could be due to the structural characteristics of monkey faces, particularly the key facial features that differ from human faces. The current study aimed to investigate whether the information conveyed by the eyes is of importance. We examined whether the presence of Caucasian human eyes in monkey faces allows recognition to be maintained in 6-month-olds and facilitates recognition in 9- and 12-month-olds. Our results revealed that the presence of human eyes in monkey faces maintains recognition for those faces at 6 months of age and partially facilitates recognition of those faces at 9 months of age, but not at 12 months of age. The findings are interpreted in the context of perceptual narrowing and suggest that the attenuation of processing of other-species faces is not reversed by the presence of human eyes.