Changes in Odor Sweetness Resulting from Implicit Learning of a Simultaneous Odor-Sweetness Association: An Example of Learned Synesthesia

doi:10.1006/lmot.1998.0996

Learning and Motivation

Volume 29, Issue 2, May 1998, Pages 113-132

https://doi.org/10.1006/lmot.1998.0996 Get rights and content

Abstract

In two experiments the smelled sweetness of odors was increased by using them as flavorants of sucrose solution. Experiment 1 used blind experimenters to compare a target odor mixed with sucrose with a control odor mixed with water during masked training trials. The increased sweetness of the target odor was unaffected by whether or not subjects revealed some explicit knowledge of the contingencies in a post-conditioning recognition test. Experiment 2 found that such a conditioned increase in odor sweetness occurred whether training solutions were sipped from a cup or sucked through a straw. Using a frequency test designed to provide a sensitive assay of contingency awareness, there was still no indication that this affected conditioning. It was concluded that such modification of the taste-properties of odors results from implicit simultaneous associative learning and provides an example of learned synesthesia.

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Can the mere expectation of drinking a sweeter beverage induce changes in taste perception and blood metabolism? Recent evidence has shown that sweetness perception can be altered by other senses, such as vision, audition, and olfaction. However, it is still unclear whether such crossmodal influences have a physiological basis. The present study examined how subjective mental states, such as the expectation of drinking a sweeter beverage due to colour and odour associations, may influence taste perception and metabolism. Twenty-seven healthy participants consumed two beverages containing identical sucrose and citric acid concentrations but altered in colour and aroma, namely a green, lemon-flavoured beverage and a red, strawberry-flavoured beverage. On average, the strawberry manipulation resulted in significantly higher sweetness ratings and lower sourness ratings compared to the lemon manipulation, although there were clear individual differences in the direction of sweetness enhancement. Blood glucose and insulin levels measured at −15, 0, 10, 20, 30, 45, 60 and 90 min after consumption showed no direct difference between the two beverage conditions. However, we found a significant indirect effect of perceived sweetness on insulin release, whereby consuming the beverage perceived to be sweeter resulted in higher overall levels of insulin release compared to the beverage that was less sweet. These results show a partial role of subjective sweetness perception influencing metabolism in accordance with an anticipatory budging model, suggesting that psychological intervention programs may be important for diabetes and metabolic syndrome management.
The effect of multisensory context and experience on flavor preference decisions in rats
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Flavor is perceived through multiple senses, including gustation and olfaction. Previous studies have shown that different sensory qualities that make up flavor are integrated to inform perceptual judgements. Psychophysical work in humans further suggests a prominent role for congruency (i.e., the learnt correspondence between taste and odor components of flavor through eating experience) in shaping multisensory interactions underlying perceptual judgments of flavor. However, eating experience cannot be controlled in humans, and depending on the type of judgement, these studies yielded mixed findings. Here, we used rats to test how experimentally-controlled experience with specific flavor mixtures (Odor_A+Taste_A and Odor_B +Taste_B) from weaning to adulthood affects subsequent flavor preference judgements in a series of two-bottle preference tests. In unisensory conditions, animals made odor or taste preference decisions (i.e., Odor_A versus Odor_B and Taste_A versus Taste_B, respectively). In multisensory conditions, animals made identical decisions, but the addition of the other modality rendered one solution congruent; the other one incongruent (e.g., Odor_A+Taste_A versus Odor_B+Taste_A). The results show that animals effectively learned congruency associations between the taste and smell components of experienced flavor mixtures. Comparing unisensory and multisensory conditions revealed no systematic effect of congruency on the magnitude of flavor preference, but preferences were less variable in multisensory compared to unisensory conditions. Results from a second group of naïve animals further demonstrate that increased reliability of preference judgements in multisensory conditions was independent of experience.
Orthonasal olfactory influences on consumer food behaviour
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It is often suggested in the popular press that food chains deliberately introduce enticing product aromas into (and in the immediate vicinity of) their premises in order to attract customers. However, despite the widespread use of odours in the field of sensory marketing, laboratory research suggests that their effectiveness in modulating people's food behaviours depends on a range of contextual factors. Given the evidence that has been published to date, only under a subset of conditions is there likely to be a measurable effect of the presence of ambient odours on people's food attitudes and choices. This narrative historical review summarizes the various ways in which food odours appear to bias people's food preferences (appetite) and food choices (food consumption and purchase). Emphasis is placed on those experimental studies that have been designed to investigate how the characteristics of the olfactory stimuli (e.g., the congruency between the olfactory cues and the foods, intensity and duration of exposure to odours, and taste properties of odours) modulate the effects of olfactory cues on food behaviour. The review also explores the moderating roles of individual differences, such as dietary restraint, Body Mass Index (BMI), genetic and cultural differences in odour sensitivity and perception. Ultimately, following a review of empirical studies on food-related olfaction, current approaches in scent marketing are discussed and a research agenda is proposed to help encourage further studies on the effective application of scents in promoting healthy foods.
Influence of obesity on saltiness and sweetness intensity enhancement by odors
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Some odors have a taste dimension likely due to repeated experiences in food with tastes. These odors can significantly enhance the related taste perception; this phenomenon is called Odor-Induced Taste Enhancement (OITE). The study aimed to determine odor-induced taste enhancement (OITE) differences between obese (OB) and normal-weight (NW) people. We hypothesized that OB would perceive a higher sweet taste enhancement than NW. We also tested salty taste enhancement without an oriented hypothesis. 43 NW and 38 OB took part in ranking tasks and evaluated OITE in 13 sweet and 4 salty solutions. The sweet base solutions were apple juice (Aj), cocoa (Coc) and water. The odorants were vanillin, ethyl-vanillin, french vanilla, two lychee odorants, and furaneol. The salty base solutions were a green-pea soup and water, and the odorants were smoky bacon smoked garlic odorants. Each ranking was performed with 4 solutions, 3 with an increasing concentration of salt/sugar, and the fourth contained the lower sugar/salt concentration and one odorant. Participants ranked the four solutions from lowest to highest sweetness/saltiness intensity. As a main result, the OB group perceived OITE more often than NW. The maximum sweetness and saltiness enhancements were observed in OB with vanillin in apple juice and bacon in salty water. Our results also demonstrated that the ranking task efficiently assesses OITE and highlights subtle differences between groups and solutions.
Factors affecting odour-induced taste enhancement
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Citation Excerpt :
Intriguingly, neither chefs nor wine experts appear to be able to consciously discount the influence of olfaction on judgments of the taste of food and drink (Boakes & Hemberger, 2012; Harrar, Smith, Deroy, & Spence, 2013; though see also Bingham, Birch, de Graaf, Behan, & Perring, 1990). While such acquired taste properties have been described by certain researchers as a form of learned ‘olfactory-synaesthesia’ (Stevenson & Boakes, 2004; Stevenson & Tomiczek, 2007; Stevenson et al., 1998), the analogy/description has been criticised as misleading/inappropriate by Auvray and Spence (2008). One potential fly in the ointment as far as the suggestion that odours rapidly acquire taste properties relates to a recent failure to demonstrate the acquisition of sweet taste associations in a study of chewing gum conducted by Fondberg, Lundström, and Seubert (2021).
Reducing the use of calorific sweeteners in sugar-sweetened beverage while, at the same time, maintaining the perceived sweetness is an important issue for the food industry given the growing global obesity crisis, and the need for many consumers around the world to reduce their sugar intake. A number of fruit aromas/volatiles appear to offer a promising route to enhancing the perception of sweetness without adding to the calorific load. However, while a growing body of empirical evidence suggests that this is a promising strategy, as made clear by this narrative review, there are a number of outstanding questions that have yet to be answered in terms of optimising the impact on sweetness offered by the addition of volatile aromas. It is, for example, currently unclear what the maximum sweetness enhancement effect that can be achieved by combining a variety of different volatiles (e.g., those associated with different fruits) before a ceiling or saturation effect is reached. It is also currently unclear how important conscious awareness of the specific volatiles, or aromas, are to odour-induced taste enhancement (OITE) effects. A third outstanding issue concerns whether OITE would occur in the case of olfactory metamers that do not overlap in terms of their volatile composition. One other intriguing issue concerns whether volatiles only come to be associated with taste qualities as a result of associative learning or whether there are some odours that are, for example, naturally (or innately) sweet.

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^☆

The authors thank Amber Carver, Scott Gazzard, Dominic Dwyer, Mary-Ellen Harrod, Judi Single and Jason Rolles, for collecting the data in both experiments, Quest International for kindly supplying the odors, and Henry Potts for suggesting the straw condition. This research was supported in part by the University of Sydney/CSIRO Collaborative Grants scheme.

Address correspondence to Robert A. Boakes, Department of Psychology, University of Sydney, NSW 2006, Australia.

^☆☆: T. ArcherL-G. Nilsson

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Regular ArticleChanges in Odor Sweetness Resulting from Implicit Learning of a Simultaneous Odor-Sweetness Association: An Example of Learned Synesthesia☆,☆☆

Abstract

Cognition

Behaviour, Research and Therapy

Learning and Motivation

Contingency awareness in evaluative conditioning: A case for unaware affective-evaluative learning

Cognition and Emotion

Sensory studies with sucrose-maltol mixtures

Chemical Senses

How one might find evidence of conditioning in adult humans

Aversion, avoidance and anxiety: Perspectives on aversively motivated behavior

Limiting response alternatives in time-intensity scaling: an examination of the halo dumping effect

Chemical Senses

Time-intensity evaluation of sweetness and fruitiness and their interaction in a model solution

Journal of Food Science

Synesthesia: a union of the senses

Construct validity of recall and recognition postconditioning measures of awareness

Journal of Experimental Psychology

Human autonomic and skeletal conditioning: The role of conscious cognitive factors

Smell and taste disorders, a study of 750 patients from the University of Pennsylvania smell and taste center

Archives of Otolaryngology, Head and Neck Surgery

Atlas of odor character profiles

The perception of odors

Acquisition and transfer in pattern-vs.-component discrimination learning

Journal of Experimental Psychology

Taste-smell interactions are tastant and odorant dependent

Chemical Senses

Odor qualities: A glossary of usage

British Journal of Psychology

Automatic and effortful processes in memory

Journal of Experimental Psychology: General

Conditioning and learning

Regular Article
Changes in Odor Sweetness Resulting from Implicit Learning of a Simultaneous Odor-Sweetness Association: An Example of Learned Synesthesia☆,☆☆