Mechanisms of flavor perception : how odor and taste interact when we eat

Abstract: Every time we eat, our brains are bombarded with sensory information from the olfactory and gustatory modalities. Through a binding process that is not fully understood, the odor and taste are then merged into flavor, a unitary sensation that appears to arise from inside the mouth. Flavors are used to guide our behavioral responses to potential sources of nutrition: flavorsome foods can easily be consumed in excess, while distasteful foods are generally rejected. Recent studies have shown that familiar flavors with congruent odor and taste components are perceived and evaluated differently than less familiar flavors with incongruent components. In this context, congruency denotes the extent to which an odor and a taste are associated with the same food object. A citrus odor mixed with a sweet/sour taste is an example of a congruent combination, because both sensations are associated with lemons. By contrast, chicken odor and sweet/sour taste would by most people be perceived as an incongruent combination. That congruency affects flavor perception suggests that associative learning within the olfactory-gustatory network regulates the binding process that gives rise to flavor. Investigating how odors and tastes that are frequently encountered together become mentally linked to one another has the potential to advance our understanding of how food preferences are formed and evolve over time. In this dissertation, I will present three lab experiments that in different ways have explored the role of associative learning in flavor perception. Study I (n=30) and Study II (n=23) investigated perceptual and hedonic effects of learning that has already taken place. In these studies, participants rated several flavors with varying degrees of congruency. Congruency was manipulated in a linear fashion: some flavors were highly congruent (e.g. citrus odor+sweet/sour taste), some were highly incongruent (e.g. chicken odor + sweet/sour taste), and others were moderately congruent (e.g. chicken/lemon odor mixture + sweet/sour taste). Study I first showed that congruency has a positive, linearlike effect on flavor pleasantness. The more congruent the particular odor-taste mixture, the more pleasant the flavor sensation. This study also provided weak evidence that congruency increases the probability that a flavor’s odor will be referred (or mislocalized) to the mouth, a perceptual illusion that has been suggested to reflect that the unisensory components have been bound together as a unified and meaningful whole. Previous research has shown that hunger makes food more appetizing. By adhering to a preregistered analysis plan, the primary aim of Study II was to test whether the amplifying effect of congruency on pleasantness interacts with the hunger state of the perceiver. To promote consumption of familiar (and safe) foods, we expected the amplifying effect of hunger to be stronger on congruent than on incongruent flavors. Participants attended two experimental sessions spaced approximately one week apart. One session was completed during hunger and the other session was completed during satiety. This study first replicated the positive effect of congruency on pleasantness from Study I. However, contrary to expectations, the congruency by hunger state interaction was not significant. Although Study II provided no evidence that the effect of hunger on flavor pleasantness is stronger for congruent than for incongruent flavors, this fining should be considered preliminary due to the small sample size. Taken together, Study I and Study II provide strong evidence that congruent flavors are more appetizing than incongruent flavors (at least for some odor-taste combinations). This suggests that frequently encountered foods that have been determined through experience to be safe are preferred to novel foods that are associated with greater risks of negative metabolic consequences. An exploratory analysis of the combined datasets from Study I and Study II showed that the amplifying effect of congruency was linear. This suggests that while congruent flavors indeed are most pleasant, minor differences between what is perceived (a specific odor-taste mixture) and what is expected (a perfect prototype of the encountered food item) will likely be tolerable. Such graded response pattern may underlie our ability to accommodate fluctuations in the chemical composition of different foods. Study III (n=60) was designed to create new odor-taste associations. In this pre-registered study, two relatively unfamiliar odors were repeatedly presented during a five-day exposure phase. One odor was presented with a sweet taste and the other odor was presented alone. Four outcomes that are thought to be affected by associative learning were rated before and after the exposure phase: odor sweetness, odor pleasantness, odor intensity enhancement by taste, and odor referral to the mouth. Contrary to expectations, repeatedly presenting odor and taste together had no effect on any of the outcomes. Moreover, exploratory equivalence tests suggested that the effects were either absent, or substantially smaller than in previous studies. High-powered, transparently conducted, direct replications of studies with significant results are needed to confirm that associative learning effects can reliably be observed in experimental settings. If this turns out to be the case, follow-up studies should focus on identifying contextual factors that modulate these effects. In this dissertation, Study I, II, and III will be discussed in light of the so-called replication crisis (or credibility revolution) in experimental psychology. Methodological advancements that can be adopted to increase the trustworthiness of flavor research are highlighted, together with some recommendations on how the field should proceed to determine what associative learning actually contributes to the eating experience.