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

Abstract

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.

Keywords: animal behavior; configural; elemental; human applications; interactions; odor mixture; perception.

FIGURE 1

Theoretical outcomes on odor quality (A) and odor intensity (B) when two odorants are perceived in the mixture. One odorant has an odor noted A and the other B, while odor U is specific to the mixture (Unique-cue, see section on configural processing of odorants in mixtures) (this figure was partially adapted from Thomas-Danguin (1997).

FIGURE 2

Proportions (%) of 2- or 3-day-old newborn rabbits responding behaviorally (sucking response) to the odorant A (ethyl isobutyrate), the odorant B (ethyl maltol), their AB or A’B’ mixtures (respectively, at a ratio of 30/70 and 68/32 of the two components), and the AC mixture (C: guaïacol; ratio 50/50) after a single conditioning to the AB mixture or to one of its components. The results show that after conditioning to AB, the pups respond both to the odorants and the different mixtures. Therefore they perceive the elements A and B during the learning episode. However, after conditioning to odorant A or B, they respond to the conditioned odorant but not to the AB mixture; nevertheless, they respond to the A’B’ and AC mixtures. Thus, newborn rabbits perceive the odor of a configuration in the AB mixture in addition to the odors of each odorant, while they perceive only (or mainly) the elements in the A’B’ and AC mixtures (adapted from Coureaud et al., 2008, 2009, 2011).

FIGURE 3

Mean typicality ratings (gray bars) of the term pineapple obtained with a group of 20 untrained subjects for a binary mixture of ethyl isobutyrate and ethyl maltol, each single odorant and a control odorant (allyl caproate carrying a typical pineapple odor). The error bars represent the standard error of the mean. The same letters indicate that the means were not different at a significance level of 5%. The results indicated that the binary mixture carried a pineapple odor that was significantly less present in the single odorants. This finding supports the idea that the odor quality of the mixture is different from those of its components (adapted from Le Berre et al., 2008b).