Thierry Thomas-Danguin

Just Noticeable Differences in Component Concentrations Modify the Odor Quality of a Blending Mixture

The odors we perceive are mainly the result of mixtures of odorants that, however, are commonly perceived as single undivided entities; nevertheless, the processes involved remain poorly explored. It has been recently reported that perceptual blending based on configural olfactory processing can cause odorant mixtures to give rise to an emergent odor not present in the components. The present study examined whether specific component proportions are required to elicit an emergent odor. Starting from the composition of a ternary target mixture in which an emergent pineapple odor was perceived, 4 concentration levels of each component were chosen to elicit just noticeable differences (JNDs). Each combination of levels was used to design sample mixtures. Fifteen subjects evaluated the intensity, typicality, and pleasantness of each sample mixture against the target mixture in a paired-comparison protocol. Statistical modeling showed that a variation of less than 1 JND in one of the components was sufficient to induce a significant decrease in pineapple odor typicality in the ternary mixture. This finding confirms previous findings on perceptual blending in simple odorant mixtures and underscores the human ability to discriminate between odor percepts induced by mixtures including very similar odorant proportions.

The perception of odor objects in everyday life: a review on the processing of odor mixtures.

Smelling monomolecular odors hardly ever occurs in everyday life, and the daily functioning of the sense of smell relies primarily on the processing of complex mixtures of volatiles that are present in the environment (e.g., emanating from food or conspecifics). Such processing allows for the instantaneous recognition and categorization of smells and also for the discrimination of odors among others to extract relevant information and to adapt efficiently in different contexts. The neurophysiological mechanisms underpinning this highly efficient analysis of complex mixtures of odorants is beginning to be unraveled and support the idea that olfaction, as vision and audition, relies on odor-objects encoding. This configural processing of odor mixtures, which is empirically subject to important applications in our societies (e.g., the art of perfumers, flavorists, and wine makers), has been scientifically studied only during the last decades. This processing depends on many individual factors, among which are the developmental stage, lifestyle, physiological and mood state, and cognitive skills; this processing also presents striking similarities between species. The present review gathers the recent findings, as observed in animals, healthy subjects, and/or individuals with affective disorders, supporting the perception of complex odor stimuli as odor objects. It also discusses peripheral to central processing, and cognitive and behavioral significance. Finally, this review highlights that the study of odor mixtures is an original window allowing for the investigation of daily olfaction and emphasizes the need for knowledge about the underlying biological processes, which appear to be crucial for our representation and adaptation to the chemical environment.

Relationships Between Molecular Structure and Perceived Odor Quality of Ligands for a Human Olfactory Receptor

Perception of thousands of odors by a few hundreds of olfactory receptors (ORs) results from a combinatorial coding, in which one OR recognizes multiple odorants and an odorant is recognized by a specific group of ORs. Moreover, odorants could act both as agonists or antagonists depending on the OR. This dual agonist-antagonist combinatorial coding is in good agreement with behavioral and psychophysical observations of mixture perception. We previously described the odorant repertoire of a human OR, OR1G1, identifying both agonists and antagonists. In this paper, we performed a 3D-quantitative structure-activity relationship (3D-QSAR) study of these ligands. We obtained a double-alignment model explaining previously reported experimental activities and permitting to predict novel agonists and antagonists for OR1G1. These model predictions were experimentally validated. Thereafter, we evaluated the statistical link between OR1G1 response to odorants, 3D-QSAR categorization of OR1G1 ligands, and their olfactory description. We demonstrated that OR1G1 recognizes a group of odorants that share both 3D structural and perceptual qualities. We hypothesized that OR1G1 contributes to the coding of waxy, fatty, and rose odors in humans.

Perceptual Blending in Odor Mixtures Depends on the Nature of Odorants and Human Olfactory Expertise

Our olfactory system is confronted with complex mixtures of odorants, often recognized as single entities due to odor blending (e.g., coffee). In contrast, we are also able to discriminate odors from complex mixtures (e.g., off-odors). Therefore, the olfactory system is able to engage either configural or elemental processes when confronted with mixtures. However, the rules that govern the involvement of these processes during odor perception remain poorly understood. In our first experiment, we examined whether simple odorant mixtures (binary/ternary) could elicit configural perception. Twenty untrained subjects were asked to evaluate the odor typicality of mixtures and their constituents. The results revealed a significant increase in odor typicality in some but not all mixtures as compared with the single components, which suggest that perceptual odor blending can occur only in specific mixtures (configural processing). In our second experiment, we tested the hypothesis that general olfactory expertise can improve elemental perception of mixtures. Thirty-two trained subjects evaluated the odor typicality of the stimuli presented during the first experiment, and their responses were compared with those obtained from the untrained panelists. The results support the idea that general training with odors increases the elemental perception of binary and ternary blending mixtures.

Perception of odor blending mixtures in the newborn rabbit

In adult mammals, the processing of complex odor mixtures is elemental or configural. Here, we challenged these processes in newborn rabbits, in evaluating their perception of a binary odor mixture for which perceptual blending occurs in humans. This model of newborn animal was interesting since general questions remain on how odor cues are processed in immature organisms, and since rabbit pups present abilities of rapid odor learning. In the present study, we first demonstrated (Exp. 1) that rabbit pups rapidly acquired the odor of the binary mixture through associative conditioning (when the mammary pheromone was used as unconditioned stimulus). Then, we compared how they responded to the mixture, its components and the mammary pheromone, after they had learned the mixture or one of its constituents. After they had learned the odor of the mixture, they responded to its odor and the odor of its constituents (Exp. 2). However, after they had learned one constituents odor, they responded to this odor but not to the mixtures odor (Exp. 3). The response to the mixture appeared nevertheless when pups successively acquired the odor of the two components (Exp. 4). Therefore, both elemental and configural processing of the mixture seem to be displayed by rabbit pups, suggesting that neonatal perception of a simple odor mixture may involve more than the perception of its constituents.

A pheromone to behave, a pheromone to learn: the rabbit mammary pheromone

Birth is part of a continuum and is a major developmental change. Newborns need to adapt rapidly to the environment in terms of physiology and behaviour, and ability to locate the maternal source of milk is vital. Mechanisms have evolved resulting in the emission of olfactory cues by the mother and the processing of these cues by the young. Here, we focus on some sensory, cognitive and behavioural strategies developed by the European rabbit (Oryctolagus cuniculus) that optimize the early development of offspring. In this species, chemosensory communication between the mother and young plays a critical role in eliciting adaptive neonatal responses. In particular, lactating females release a molecule, the mammary pheromone, which has several functional impacts. It triggers orocephalic responses involved in the quick localization of nipples and sucking. Moreover, this unconditioned signal promotes rapid appetitive learning of novel odorants, acting as a potent organizer of neonatal cognition. The mammary-pheromone-induced odour memory requires consolidation/reconsolidation processes to be maintained in the long term. Finally, as this mode of conditioning also promotes learning of mixtures of odorants, it supports investigations related to the capacity of neonatal olfaction to extract biological value from the complex environment.

Interactions of odorants with olfactory receptors and receptor neurons match the perceptual dynamics observed for woody and fruity odorant mixtures

The present study aimed to create a direct bridge between observations on peripheral and central responses to odorant mixtures and their components. Three experiments were performed using mixtures of fruity (isoamyl acetate; ISO) and woody (whiskey lactone; WL) odorants known to contribute to some of the major notes in Burgundy red wine. These experiments consisted of (i) calcium imaging of human embryonic kidney cells (HEK293T) transfected with olfactory receptors (ORs); (ii) single‐unit electrophysiological recordings from olfactory receptor neurons (ORNs) and analyses of electro‐olfactogram (EOG) responses in the rat nose in vivo; and (iii) psychophysical measurements of the perceived intensity of the mixtures as rated by human subjects. The calcium imaging and electrophysiological results revealed that ISO and WL can act simultaneously on single ORs or ORNs and confirm that receptor responses to mixtures are not the result of a simple sum of the effects of the individual mixture compounds. The addition of WL to ISO principally suppressed the ORN activation induced by ISO alone and was found to enhance this activation in a subset of cases. In the human studies, the addition of high concentrations of WL to ISO decreased the perceived intensity of the ISO. In contrast, the addition of low concentrations of WL enhanced the perceived intensity of the fruity note (ISO) in this mixture, as it enhanced EOG responses in ORNs. Thus, both OR and ORN responses to ISO + WL mixtures faithfully reflected perceptual response changes, so the odour mixture information is set up after the peripheral stage of the olfactory system.

Elemental and configural processing of odour mixtures in the newborn rabbit.

SUMMARY The processing of odour mixtures by young organisms is poorly understood. Recently, the perception of an AB mixture, known to engage configural perception in adult humans, was suggested also to be partially configural in newborn rabbits. In particular, pups did not respond to AB after they had learned A or B. However, two alternative hypotheses might be suggested to explain this result: the presence in the mixture of a novel odorant that inhibits the response to the learned stimulus, and the unevenness of the sensory and cognitive processes engaged during the conditioning and the behavioural testing. We conducted four experiments to explore these alternative hypotheses. In experiment 1, the learning of A or B ended in responses to mixtures including a novel odorant (AC or BC). Experiment 2 pointed to the absence of overshadowing. Therefore, a novelty effect cannot explain the non-response to AB after the learning of A or B. In experiment 3, pups having learned A or B in AC or BC did not respond to AB. However, they generalized odour information acquired in AB to AC or BC in experiment 4. Thus, the balancing of the perceptual tasks between the conditioning and retention test does not enhance the response to the AB mixture. To sum up, the present experiments give concrete support to the partially configural perception of specific odour mixtures by newborn rabbits.

Experience influences elemental and configural perception of certain binary odour mixtures in newborn rabbits

SUMMARY Elemental and configural olfactory perception allows interaction with the environment from very early in life. To evaluate how newborn rabbits can extract and respond to information from the highly complex chemical surroundings, and how experience acts on this sensory, cognitive and behavioural capability, we ran a study in four steps including a total of eight experiments. We mainly used a binary AB mixture comprising ethyl isobutyrate (component A) and ethyl maltol (component B), previously shown as a bearer of blending properties; in rabbit pups (as in human adults), the mixture elicits a weak configural perception, i.e. the perception of a configural odour different from the odours of the components. First, a repeated exposure to one component of AB led to a more elemental perception of this mixture; conversely, a repeated exposure to AB facilitated its configural processing. Second, similar impact of experience did not appear with a non-blending AC mixture (ethyl isobutyrate-guaïacol). Third, repeated exposure to AB impacted not only the perception of AB, but also and in the same way the perception of the AC mixture sharing one component, and reciprocally. However, facilitation to perceive one mixture in one mode (configural/elemental) was not generalized to a mixture sharing no components with the experienced mixture [AB versus DE (damascenone and vanillin)]. Thus, experience contributes to the neonatal perception of odour mixtures and adds plasticity to the perceptual system. However, this impact remains dependent on the chemical composition of the mixtures.

Synergy and Masking in Odor Mixtures: An Electrophysiological Study of Orthonasal vs. Retronasal Perception

Perceptual interactions in a model of wine woody-fruity binary mixtures were previously reported in a psychophysical study performed through orthonasal stimulation only. However, recent studies suggested that the perception of food-like and nonfood-like odors may depend on the route of stimulation. The aim of the present study was two-fold: first to examine the neural correlates of perceptual interactions using electroencephalogram (EEG)-derived event-related potentials (ERPs) and second to test the influence of the stimulation route on quality perception. Therefore, we designed an experiment with 30 subjects to study perceptual interactions in woody-fruity mixtures and compared ortho- vs. retronasal stimulation sites on perceived odor quality and ERPs. The results revealed synergy or masking of the fruity component, depending on the woody component level. Synergy was supported by larger N1 amplitude of the ERP. Furthermore, mixtures including a medium level of the woody odor elicited a strong increase of P2 amplitude only retronasally. This study evidenced for the first time electrophysiological correlates of both perceptual synergy and masking on the early component of the ERPs and confirmed that retro- vs. orthonasal stimulation route induces different neural processes that are reflected in the late component of the ERP.