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International audience; Understanding how the young infant brain starts to categorize the flurry of ambiguous sensory inputs coming in from its complex environment is of primary scientific interest. Here, we test the hypothesis that senses other than vision play a key role in initiating complex visual categorizations in 20 4-mo-old infants exposed either to a baseline odor or to their mother’s odor while their electroencephalogram (EEG) is recorded. Various natural images of objects are presented at a 6-Hz rate (six images/second), with face-like object configurations of the same object categories (i.e., eliciting face pareidolia in adults) interleaved every sixth stimulus (i.e., 1 Hz). In the baseline odor context, a weak neural categorization response to face-like stimuli appears at 1 Hz in the EEG frequency spectrum over bilateral occipitotemporal regions. Critically, this face-like–selective response is magnified and becomes right lateralized in the presence of maternal body odor. This reveals that nonvisual cues systematically associated with human faces in the infant’s experience shape the interpretation of face-like configurations as faces in the right hemisphere, dominant for face categorization. At the individual level, this intersensory influence is particularly effective when there is no trace of face-like categorization in the baseline odor context. These observations provide evidence for the early tuning of face-(like)–selective activity from multisensory inputs in the developing brain, suggesting that perceptual development integrates information across the senses for efficient category acquisition, with early maturing systems such as olfaction driving the acquisition of categories in later-developing systems such as vision.

International audience; 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.

International audience; According to recent evidence, rapid categorization of natural face images in the infant brain is enhanced by concomitant maternal odor (Leleu et al., 2029). To test whether this effect is selective to faces, we recorded scalp electroencephalogram (EEG) in 4-month-old infants presented with variable exemplars of a nonface visual category - cars - appearing every 6 stimuli in 6-Hz streams of natural object images. At the same time, infants were exposed to the maternal or to a control odor context. A relatively weak neural categorization response to cars (i.e., a differential response to cars that generalizes across exemplars) was observed at 1 Hz over the right occipital cortex in both odor conditions, revealing rapid categorization of an unfamiliar object in the infant brain. However, the car categorization response was not modulated by maternal odor, suggesting that odors selectively prime neural activity in the infant visual cortex to categorize congruent incoming inputs.

International audience; Visual categorization is the rapid and automatic ability to provide a similar response to different exemplars of a single category despite widely variable sensory inputs. Whether visual categorization is solely visually-driven or can be influenced by other sensory modalities remains unclear. Here we test whether odor contexts can modulate visual categorization in humans, expecting that congruent odors facilitate the categorization of visual objects as a function of their ambiguity. Scalp electroencephalogram (EEG) was recorded in N = 26 participants while naturalistic object stimuli were displayed in rapid 12-Hz streams (i.e., 12 images / second, leading to a 12-Hz general response in the EEG frequency spectrum). Variable exemplars of a target category (human faces, cars, or facelike objects in different sequences) were interspersed every 9 stimuli to tag category-selective EEG responses at 12/9 = 1.33 Hz. During visual stimulation, odor contexts (body, gasoline or baseline odors) were implicitly diffused. Category-selective responses to every category are clearly isolated over the occipito-temporal cortex, with the largest response for human faces and the lowest for facelike objects. Importantly, body odors enhance the right-hemispheric response to the ambiguous category, i.e., facelike objects, which are either perceived as nonface objects or faces. This odor effect is especially found in participants who noticed the illusory faces during visual stimulation. In contrast, odor contexts do not modulate other category-selective responses, nor the 12-Hz general response, revealing a selective facilitation of the visual categorization of congruent ambiguous stimuli. Altogether, these observations indicate that the visual system can rely on non-visual cues for efficient categorization, and that olfaction, which is often considered as poor in humans, is ideally suited to assist visual categorization.

International audience; Humans exhibit a marked specialization to process the most experienced facial morphologies. In particular, nonhuman primate faces are poorly discriminated compared to human faces in behavioral tasks. So far however, a clear and consistent marker that quantifies our expertise in human over monkey face discrimination directly from brain activity is lacking. Here, using scalp electroencephalography (EEG), we isolate a direct signature of individuation abilities for human and nonhuman (i.e., macaque faces) primate faces. Human or monkey faces were rapidly presented at a base rate of 12Hz in upright or inverted orientations while participants performed an orthogonal behavioral task. In each stimulation sequence, eight face images of one individual were used as base stimuli, while images of other individuals were briefly introduced every 9th stimulus to quantify an identity-change response at 1.33Hz and harmonics (i.e., integer multiples) in the EEG frequency spectrum. The brain response to upright human faces was twice as large as to monkey faces, and reduced following picture-plane inversion for human faces only. This reflects the disruption of high-level face identity discrimination developed for the canonical upright human face. No difference was observed between upright monkey faces and inverted human faces, suggesting non-expert visual processes for those two face formats associated with little experience. In addition, the size of the inversion effect for human, but not monkey faces, was predictive of the expertise effect (i.e., difference between upright human and monkey faces) at the individual level. This result suggests a selective ability to discriminate human faces that does not contribute to the individuation of other unexperienced face morphologies such as monkey faces. Overall, these findings indicate that human expertise for conspecific face discrimination can be isolated and quantified in individual human brains.

Although various psychiatric disorders present with social-cognitive impairment, a measure assessing social-cognitive processes implicitly and reliably, with high selectivity and with enough signal-to-noise ratio (SNR) for individual evaluation of any population at any age, is lacking. Here we isolate a neural marker quantifying impaired visual coding of facial expression in individuals with 22q11.2 deletion syndrome (22q11DS) using frequency-tagging with electroencephalography (EEG). Twenty-two 22q11DS participants and 22 healthy controls were presented with changes of facial expression displayed at low, moderate, and high intensities every five cycles in a stream of one neutral face repeating 6 times per second (i.e., at a 6 Hz base rate). The brain response to expression changes tagged at the 1.2 Hz (i.e., 6 Hz/5) predefined frequency was isolated over occipito-temporal regions in both groups of participants for moderate- and high-intensity facial expressions. Neural sensitivity to facial expression was reduced by about 36% in 22q11DS, revealing impaired visual coding of emotional facial signals. The significance of the expression-change response was estimated for each single participant thanks to the high SNR of the approach. Further analyses revealed the high reliability of the response and its immunity from other neurocognitive skills. Interestingly, response magnitude was associated with the severity of positive symptoms, pointing to a potential endophenotype for psychosis risk. Overall, the present study reveals an objective, selective, reliable, and behavior-free signature of impaired visual coding of facial expression implicitly quantified from brain activity with high SNR. This novel tool opens avenues for clinical practice, providing a potential early biomarker for later psychosis onset and offering an alternative for individual assessment of social-cognitive functioning in even difficult-to-test participants.

Automatic responses to brief expression changes from a neutral face have been recently isolated in the human brain using fast periodic visual stimulation (FPVS) coupled with scalp electroencephalography (EEG). Based on these observations, here we isolate specific neural signatures for the rapid categorization of each of 5 basic expressions, i.e., when they are directly discriminated from all other facial expressions. Scalp EEG was recorded in 15 participants presented with pictures alternating at a rapid 6 Hz rate (i.e., one fixation/face, backward- and forward-masked). In different stimulation sequences, an expressive (angry, disgusted, happy, fearful, or sad) or a neutral face arose every 5 pictures (i.e., at 6/5 = 1.2 Hz), among pictures of the same individual expressing the other emotions randomly. Frequency-domain analysis indicated a robust (i.e., recorded in every individual participant) and objective (i.e., at the predefined 1.2 Hz frequency and its harmonics) expression-specific brain response over occipito-temporal sites for each emotion and neutrality. In this context of variable expressions, while neural responses to the different expressions (Anger, Disgust, Happiness, Sadness) were dissimilar qualitatively, a much larger specific signature for neutral faces as compared to facial expressions was found. Interestingly, Fear also elicited a strong contrasted response with other facial expressions, associated with a specific neural signature over ventral occipito-temporal sites. Collectively, these findings reveal that specific EEG signatures for different facial expressions can be isolated in the human brain, pointing to partially different neural substrates. In addition, they provide support for a strong and highly selective neural response to fear at the system-level, in line with the importance of this emotional expression for biological survival.

Resume La microdeletion 22q11.2 est le syndrome microdeletionnel le plus frequent de la population generale. Le phenotype associe des anomalies de l’appareil pharynge embryonnaire a un phenotype neurocomportemental. La presentation clinique du syndrome est extremement variable d’un individu a l’autre, quelle que soit la taille de la deletion, et plus de 180 manifestations ont ete decrites, aucune n’etant pathognomonique. Les symptomes psychiatriques, particulierement de nature psychotique, sont frequents dans la microdeletion 22q11.2 et de nombreux psychiatres sont amenes a rencontrer ces patients. La prise en charge doit tenir compte des particularites du syndrome. L’evaluation de la neurocognition, de la cognition sociale et la recherche de symptomes psychiatriques dans le contexte medical general sont une etape fondamentale. Concernant le phenotype neurocomportemental, les soins de remediation cognitive doivent etre combines a une prescription mesuree de psychotropes pour aboutir a une meilleure qualite de vie de ces patients.

We studied the time course of the cerebral integration of olfaction in the visual processing of emotional faces during an orthogonal task asking for detection of red-colored faces among expressive faces. Happy, angry, disgust, fearful, sad, and neutral faces were displayed in pleasant, aversive or no odor control olfactory contexts while EEG was recorded to extract event-related potentials (ERPs). Results indicated that the expressive faces modulated the cerebral responses at occipito-parietal, central and central-parietal electrodes from around 100 ms and until 480 ms after face onset. The response was divided in different successive stages corresponding to different ERP components (P100, N170, P200 and N250 (EPN), and LPP). The olfactory contexts influenced the ERPs in response to facial expressions in two phases. First, regardless of their emotional content, the response to faces was enhanced by both odors compared with no odor approximately 160 ms after face-onset at several central, centro-parietal and left lateral electrodes. The topography of this effect clearly depended on the valence of odors. Then, a second phase occurred, but only in the aversive olfactory context, which modulated differentially the P200 at occipital sites (starting approximately 200 ms post-stimulus) by amplifying the differential response to expressions, especially between emotional neutrality and both happiness and disgust. Overall, the present study suggests that the olfactory context first elicits an undifferentiated effect around 160 ms after face onset, followed by a specific modulation at 200 ms induced by the aversive odor on neutral and affectively congruent/incongruent expressions.

Resume L’alteration de la cognition sociale associee a la schizophrenie, dont fait partie la perception des emotions faciales, a fait l’objet d’une recente meta-analyse qui a revele son impact majeur sur l’insertion sociale et professionnelle. Or l’utilisation de programmes de remediation cognitive portant sur le traitement des emotions faciales est freinee par un manque d’outil permettant d’identifier et de quantifier les troubles pour un sujet donne. Le test de reconnaissance des emotions faciales (TREF) permet d’evaluer la capacite a reconnaitre 6 emotions universelles (joie, colere, tristesse, peur, degout et mepris), presentees a neuf intensites d’expression s’etalant de 20 % a 100 %. Les resultats de l’etude comparative realisee aupres d’un echantillon de 64 sujets temoins sains et de 45 personnes souffrant de schizophrenie montrent, de maniere coherente avec les donnees de la litterature, une diminution significative des performances de reconnaissance des emotions faciales dans la schizophrenie. Des scores seuils sont proposes afin de permettre l’utilisation du TREF pour l’identification et la specification de la presence d’un deficit de reconnaissance des emotions faciales chez un sujet donne, et permettre son orientation vers une prise en charge specifique.

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.

The properties of the face-sensitive N170 component of the event-related brain potential (ERP) were explored through an orientation discrimination task using natural faces, objects, and Arcimboldo paintings presented upright or inverted. Because Arcimboldo paintings are composed of non-face objects but have a global face configuration, they provide great control to disentangle high-level face-like or object-like visual processes at the level of the N170, and may help to examine the implication of each hemisphere in the global/holistic processing of face formats. For upright position, N170 amplitudes in the right occipito-temporal region did not differ between natural faces and Arcimboldo paintings but were larger for both of these categories than for objects, supporting the view that as early as the N170 time-window, the right hemisphere is involved in holistic perceptual processing of face-like configurations irrespective of their features. Conversely, in the left hemisphere, N170 amplitudes differed between Arcimboldo portraits and natural faces, suggesting that this hemisphere processes local facial features. For upside-down orientation in both hemispheres, N170 amplitudes did not differ between Arcimboldo paintings and objects, but were reduced for both categories compared to natural faces, indicating that the disruption of holistic processing with inversion leads to an object-like processing of Arcimboldo paintings due to the lack of local facial features. Overall, these results provide evidence that global/holistic perceptual processing of faces and face-like formats involves the right hemisphere as early as the N170 time-window, and that the local processing of face features is rather implemented in the left hemisphere.

Sex-related hemispheric lateralization and interhemispheric transmission times (IHTTs) were examined in twenty-four participants at the level of the first visual ERP components (P1 and N170) during face identity encoding in a divided visual-field paradigm. While no lateralization-related and sex-related differences were reflected in the P1 characteristics, these two factors modulated the N170. Indeed, N170 amplitudes indicated a right hemisphere (RH) dominance in men (and a more bilateral functioning in women). N170 latencies and the derived IHTTs confirmed the RH advantage in men but showed the reverse asymmetry in women. Altogether, the results of this study suggest a clear asymmetry in men and a more divided work between the hemispheres in women, with a tendency toward a left hemisphere (LH) advantage. Thus, by extending the pattern to the right-sided face processing, our results generalize previous findings from studies using other materials and indicating longer transfers from the specialized to the non-specialized hemisphere, especially in the male brain. Because asymmetries started from the N170 component, the first electrophysiological index of high-level perceptual processing on face representations, they also suggest a functional account for hemispheric lateralization and sex-related differences rather than a structural one. (c) 2013 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.

This thesis examines whether and how odors contribute to the development of visual categorization in the human brain using fast periodic visual stimulation coupled with scalp electroencephalography (FPVS-EEG). Specifically, we sought to characterize if a neural visual response selective to the face category is modulated by the presence of a body odor in both infants (Study 1, 2, 3) and adults (Study 4).In infants, the selectivity of the odor effect on visual categorization was addressed by testing separately three categories in three groups of 4-month-old infants presented with a control odor or their mother’s odor. We observed that a face-selective response is largely enhanced by maternal odor over the right occipito-temporal cortex (Study 1). By contrast, a car-selective response, observed over the right occipital region, is insensitive to the presence of maternal odor (Study 2). Finally, when using nonface objects configured as faces (i.e., facelike objects), a facelike-selective response is amplified over the right hemisphere when infants are exposed to maternal odor, and even initiated in some infants who do not selectivly respond to facelike objects in the absence of maternal odor (Study 3). In adults, the selective responses to human faces, cars and facelike objects were recorded in a body, gasoline, or control odor context (Study 4). While the categorization of human faces or cars is immune to the presence of odors, the body odor enhances the facelike-selective response, suggesting a facilitating effect of a congruent odor when the visual categroy is ambiguous. For these four studies, the general visual response elicited by all stimuli in the rapid visual sequence remains stable across odor contexts, excluding any general effect of odors in terms of arousal/attention.Altogether, these findings demonstrate a strong facilitative effect of body odors on congruent visual categorizations (i.e., human or illusory faces). We thus illustrate through olfactory-visual associations that information from different senses are integrated to facilitate visual categorization, especially early in life when the visual system is still immature, and that these intersensory congruent associations are maintained until adulthood in the case of ambiguous visual stimuli., Cette thèse examine dans quelle mesure les odeurs contribuent au développement de la catégorisation visuelle dans le cerveau humain à l’aide d’une stimulation visuelle périodique rapide couplée à l’électroencéphalographie de surface (FPVS-EEG). Nous avons en particulier cherché à caractériser si une réponse cérébrale sélective à la catégorie visuelle des visages est modulées par la présence d’une odeur corporelle chez le nourrisson (Etudes 1 à 3) et l'adulte (Etude 4).Chez le nourrisson, la sélectivité de l'effet de l’odeur sur la categorisation visuelle a été testée à l’aide de trois catégories étudiées séparément chez trois groupes de nourrissons de 4 mois exposés à une odeur contrôle ou à l’odeur de leur mère. Nous avons observé qu’une réponse sélective aux visages est largement amplifiée par l'odeur maternelle en regard du cortex occipito-temporal droit (Etude 1). En revanche, une réponse sélective aux voitures, mesurée au niveau de la région occipitale droite, ne présente aucun effet de l'odeur (Etude 2). Enfin, en utilisant des objets non faciaux configurés comme des visages (visages illusoires), une réponse sélective à ces visages illusoires est amplifiée au niveau de l’hémisphère droit en présence de l'odeur maternelle, et même intiée chez certains nourrissons qui ne répondent pas à ces visages illusoire en l’absence de l’odeur maternelle (Etude 3). Chez l’adulte, les réponses sélectives aux visages humains, voitures et visages illusoires ont été mesurées en présence d’odeurs corporelles, de gasoil ou d’une odeur contrôle (Etude 4). Tandis que la categorisation des visages humains ou des voitures n’est pas affectée par la présence des odeurs, l’odeur corporelle amplifie la réponse sélective aux visages illusoires, suggérant un effet facilitateur d’une odeur congruente quand la catégorie visuelle est ambigüe. Dans ces quatre études, la réponse visuelle générale élicitée par l’ensemble des stimuli de la séquence visuelle rapide reste stable entre les contextes olfactifs testés, excluant un effet général des odeurs en termes d’éveil/d’attention.Dans l’ensemble, ces résultats démontrent un effet facilitateur important des odeurs corporelles sur la catégorisation d’informations visuelles congruentes (visages humains ou illusoires). Nous illustrons ainsi par ces associations olfacto-visuelles que les informations provenant de différents sens sont intégrées pour favoriser la catégorisation visuelle, particulièrement en début de vie lorsque le système visuel est encore immature, et que ces associations intersensorielles congruentes se maintiennent jusqu’à l’âge adulte dans le cas de stimuli visuels ambigus.

This thesis examines whether and how odors contribute to the development of visual categorization in the human brain using fast periodic visual stimulation coupled with scalp electroencephalography (FPVS-EEG). Specifically, we sought to characterize if a neural visual response selective to the face category is modulated by the presence of a body odor in both infants (Study 1, 2, 3) and adults (Study 4).In infants, the selectivity of the odor effect on visual categorization was addressed by testing separately three categories in three groups of 4-month-old infants presented with a control odor or their mother’s odor. We observed that a face-selective response is largely enhanced by maternal odor over the right occipito-temporal cortex (Study 1). By contrast, a car-selective response, observed over the right occipital region, is insensitive to the presence of maternal odor (Study 2). Finally, when using nonface objects configured as faces (i.e., facelike objects), a facelike-selective response is amplified over the right hemisphere when infants are exposed to maternal odor, and even initiated in some infants who do not selectivly respond to facelike objects in the absence of maternal odor (Study 3). In adults, the selective responses to human faces, cars and facelike objects were recorded in a body, gasoline, or control odor context (Study 4). While the categorization of human faces or cars is immune to the presence of odors, the body odor enhances the facelike-selective response, suggesting a facilitating effect of a congruent odor when the visual categroy is ambiguous. For these four studies, the general visual response elicited by all stimuli in the rapid visual sequence remains stable across odor contexts, excluding any general effect of odors in terms of arousal/attention.Altogether, these findings demonstrate a strong facilitative effect of body odors on congruent visual categorizations (i.e., human or illusory faces). We thus illustrate through olfactory-visual associations that information from different senses are integrated to facilitate visual categorization, especially early in life when the visual system is still immature, and that these intersensory congruent associations are maintained until adulthood in the case of ambiguous visual stimuli., Cette thèse examine dans quelle mesure les odeurs contribuent au développement de la catégorisation visuelle dans le cerveau humain à l’aide d’une stimulation visuelle périodique rapide couplée à l’électroencéphalographie de surface (FPVS-EEG). Nous avons en particulier cherché à caractériser si une réponse cérébrale sélective à la catégorie visuelle des visages est modulées par la présence d’une odeur corporelle chez le nourrisson (Etudes 1 à 3) et l'adulte (Etude 4).Chez le nourrisson, la sélectivité de l'effet de l’odeur sur la categorisation visuelle a été testée à l’aide de trois catégories étudiées séparément chez trois groupes de nourrissons de 4 mois exposés à une odeur contrôle ou à l’odeur de leur mère. Nous avons observé qu’une réponse sélective aux visages est largement amplifiée par l'odeur maternelle en regard du cortex occipito-temporal droit (Etude 1). En revanche, une réponse sélective aux voitures, mesurée au niveau de la région occipitale droite, ne présente aucun effet de l'odeur (Etude 2). Enfin, en utilisant des objets non faciaux configurés comme des visages (visages illusoires), une réponse sélective à ces visages illusoires est amplifiée au niveau de l’hémisphère droit en présence de l'odeur maternelle, et même intiée chez certains nourrissons qui ne répondent pas à ces visages illusoire en l’absence de l’odeur maternelle (Etude 3). Chez l’adulte, les réponses sélectives aux visages humains, voitures et visages illusoires ont été mesurées en présence d’odeurs corporelles, de gasoil ou d’une odeur contrôle (Etude 4). Tandis que la categorisation des visages humains ou des voitures n’est pas affectée par la présence des odeurs, l’odeur corporelle amplifie la réponse sélective aux visages illusoires, suggérant un effet facilitateur d’une odeur congruente quand la catégorie visuelle est ambigüe. Dans ces quatre études, la réponse visuelle générale élicitée par l’ensemble des stimuli de la séquence visuelle rapide reste stable entre les contextes olfactifs testés, excluant un effet général des odeurs en termes d’éveil/d’attention.Dans l’ensemble, ces résultats démontrent un effet facilitateur important des odeurs corporelles sur la catégorisation d’informations visuelles congruentes (visages humains ou illusoires). Nous illustrons ainsi par ces associations olfacto-visuelles que les informations provenant de différents sens sont intégrées pour favoriser la catégorisation visuelle, particulièrement en début de vie lorsque le système visuel est encore immature, et que ces associations intersensorielles congruentes se maintiennent jusqu’à l’âge adulte dans le cas de stimuli visuels ambigus.

This thesis examines the mechanisms subtending the perception of emotional facial expressions and their early development using a Fast Periodic Visual Presentation (FPVS) approach coupled with electroencephalography (EEG). More specifically, we tried to characterize brain responses reflecting facial expression discrimination and to determine whether hedonic odor contexts influence these responses in adults (studies 1 and 2), and in infants at different developmental stages (studies 3 and 4).We showed specific responses to the discrimination of every facial expression in the adult brain, indicating rapid and automatic categorization of basic facial expressions (study 1). In addition, we revealed that hedonic odor contexts influence these expression-specific brain responses. While the response to happiness is unchanged, the response to disgust is weaker in both pleasant and unpleasant odor contexts. The odor effect is strong for the response to neutrality, this response being nearly absent in the unpleasant odor context vs. larger in the pleasant context. Hedonic odors could thus orient facial expression perception toward their own emotional valence, or toward the valence of the odor for the emotionally ambiguous neutral expression, hampering or facilitating their discrimination from the other mainly negative expressions (study 2). In infants, a first study isolated brain responses to the discrimination of facial expressions in 3.5- and 7-month-olds (study 3). In particular, we observed that the response to happiness evolves between 3.5- and 7-month-old (additional fronto-central activity), suggesting the progressive integration of the emotional meaning of this positive expression. The second study has been initiated in 7-month-old infants to test the influence of a pleasant hedonic odor context on the specific brain response to happiness (study 4, ongoing study). This study seems to indicate that the response is larger in a pleasant rather than neutral odor context.These results reveal that facial expressions are readily discriminated in adults, hedonic odor contexts orienting perception toward emotional valence rather than related basic emotions. Emotional valence may be acquired early in development, before 7 months, since the discrimination of happiness (positive) from other expressions (mainly negative) evolves at this age. Moreover, at 7 months, the perception of happiness seems facilitated by the presence of a pleasant odor context. Hence, due to the early functional maturity of the olfactory system, odors may actively participate in acquiring the emotional meaning of facial expressions during early development and facilitate the perception of expressions whose emotional meaning is ambiguous in adults. Overall, our work sheds new light on the role of multisensory integration during early perceptual development and opens interesting perspectives to investigate typical and atypical socio-cognitive and affective development., Cette thèse se propose d’examiner les mécanismes de la perception des expressions faciales émotionnelles et leur développement précoce, grâce à une approche de présentation visuelle périodique rapide (Fast Periodic Visual Presentation : FPVS) couplée à l’électroencéphalographie (EEG). Plus spécifiquement, nous avons cherché à caractériser des réponses cérébrales de discrimination des expressions faciales et à déterminer l’influence d’un contexte olfactif hédonique sur ces réponses, chez l’adulte (Etudes 1 et 2) ainsi que chez le nourrisson à différents stades de développement (Etudes 3 et 4).Nous avons montré que l’adulte présente des réponses cérébrales distinctes selon l’expression faciale cible à discriminer, indiquant une catégorisation spontanée des expressions faciales de base (Étude 1). Par ailleurs, nous avons mis en évidence que la présence d’un contexte olfactif hédonique influence les réponses cérébrales de catégorisation des expressions faciales. Tandis que la réponse à la joie reste inchangée, la réponse au dégoût est moindre en présence d’un contexte olfactif hédonique agréable ou désagréable. L’effet de l’odeur est particulièrement retrouvé pour la réponse à la neutralité, la réponse étant presque absente en contexte désagréable vs. amplifiée en contexte agréable. Les odeurs hédoniques orienteraient ainsi la perception des expressions faciales vers leur propre valence émotionnelle, ou vers la valence de l’odeur pour l’expression neutre émotionnellement ambiguë, entravant ou facilitant leur discrimination des autres expressions, majoritairement négatives (Etude 2). Chez le nourrisson, une première étude a permis d’isoler des réponses cérébrales de discrimination des expressions faciales à 3,5 et 7 mois (Étude 3). Une évolution de la réponse cérébrale à la joie a été observée entre 3,5 et 7 mois (activité fronto-centrale additionnelle), suggérant l’intégration progressive de la signification émotionnelle de cette expression positive. La seconde étude a été initiée chez les nourrissons de 7 mois pour tester l’influence d’un contexte olfactif agréable sur la réponse cérébrale à l’expression de joie (Etude 4, en cours). Elle semble indiquer que la réponse à la joie est amplifiée par un contexte olfactif agréable plutôt que neutre.Ces résultats révèlent que les expressions faciales sont facilement discriminées chez l’adulte, la présence d’un contexte olfactif hédonique orientant la perception vers la valence émotionnelle de l’expression faciale plutôt que vers l’émotion de base associée en tant que telle. La valence émotionnelle des expressions pourrait être acquise précocement au cours du développement, avant 7 mois, âge auquel évolue la discrimination de la joie (positive) par rapport aux autres expressions (majoritairement négatives). En outre, à cet âge, la perception de la joie semble facilitée en présence d’un contexte olfactif agréable. Ainsi, du fait de la maturité fonctionnelle précoce du système olfactif, les odeurs pourraient activement participer à l’acquisition de la signification émotionnelle des expressions faciales au cours du développement, et faciliter la perception des expressions dont la signification émotionnelle est ambiguë chez l’adulte. Dans l’ensemble, nos travaux éclairent le rôle de l’intégration multisensorielle au cours du développement perceptif précoce et ouvrent d’intéressantes perspectives pour investiguer le développement sociocognitif et affectif typique et atypique.