Habituation and adaptation to odors in humans.

Habituation, or decreased behavioral response, to odors is created by repeated exposure and several detailed characteristics, whereas adaptation relates to the neural processes that constitute this decrease in a behavioral response. As with all senses, the olfactory system continually encounters an enormous variety of odorants which is why mechanisms must exist to segment them and respond to changes. Although most olfactory habitation studies have focused on animal models, this non-systematic review provides an overview of olfactory habituation and adaptation in humans, and techniques that have been used to measure them. Thus far, psychophysics in combination with modern techniques of neural measurement indicate that habituation to odors, or decrease of intensity, is relatively fast with adaptation occurring more quickly at higher cerebral processes than peripheral adaptation. Similarly, it has been demonstrated that many of the characteristics of habitation apply to human olfaction; yet, evidence for some characteristics such as potentiation of habituation or habituation of dishabituation need more support. Additionally, standard experimental designs should be used to minimize variance across studies, and more research is needed to define peripheral-cerebral feedback loops involved in decreased responsiveness to environmental stimuli.

Learning to (dis)like: The effect of evaluative conditioning with tastes and faces on odor valence assessed by implicit and explicit measurements.

Evaluative conditioning may be an important mechanism for learning food preferences and aversions; however, in both real life and experimental settings it has not been consistently successful. The current study aimed to gain more insight into which underlying factors may contribute to a successful outcome of olfactory evaluative conditioning. Two groups of 18 participants came in on three consecutive days, and were repeatedly exposed to four novel, neutral odors (CS) coupled to varying disliked, neutral, liked, or no stimuli (taste and/or pictures, US), following a 50% reinforcement schedule, leading to 40 odor presentations per session. Liking ratings, as well as changes in the autonomic nervous system were assessed before, during and after conditioning. We were able to induce negative, but not positive, affective changes by pairing neutral odors with tastes and pictures differing in valence. Negative as well as multimodal stimuli appear to be more potent US, since they may be considered more salient. Lastly, results of the current study imply that heart rate is responsive to changes in valence of olfactory stimuli, and perhaps even more sensitive than explicit ratings of liking.

The effects of food viscosity on bite size, bite effort and food intake.

Two studies investigated the effect of a food's viscosity on bite size, bite effort and food intake using a standardized protocol in which subjects sipped through a straw every 20 s for a period of 15 min from one of two products, a chocolate-flavored dairy drink and a chocolate-flavored dairy semi-solid, matched for energy density. In the first study, subjects consumed 47% more from the liquid than from the semi-solid to reach the same degree of satiation, with larger bite sizes for the liquid throughout the 15 minute period (8.7+/-0.45 g) compared to the semi-solid (5.8+/-0.3 g, p<0.01). In the second study bite effort was eliminated by using a peristaltic pump to present the products every 20 s. Oral processing time before swallowing was set at 5 s (both products) or 8 s (semi-solid). With the elimination of bite effort and a standardized oral processing time, subjects consumed as much from the semi-solid as from the liquid to reach the same degree of satiation. Bite size for liquids started relatively small and grew gradually over successive bites, whereas the bite size for the semi-solid food started relatively large and became gradually smaller. The latter effect was even more pronounced when the oral processing time was increased from 5 to 8 s. In conclusion, semi-solids resulted in smaller bite sizes and lower intake than liquids, but these differences disappeared when differences in bite effort were eliminated.

The effect of viscosity on ad libitum food intake.

BACKGROUND: Energy-yielding liquids elicit weak suppressive appetite responses and weak compensatory responses, suggesting that liquid calories might lead to a positive energy balance. However, data is often derived from foods differing in many characteristics other than viscosity. OBJECTIVE: To investigate the effect of viscosity on ad libitum food intake in real-life setting and to investigate whether a difference in ad libitum intake is related to eating rate and/or eating effort. DESIGN: In real-life setting 108 nonrestrained subjects (26+/-7 years, BMI 22.7+/-2.4 kg m(-2)) received a chocolate flavored liquid, semi-liquid and semi-solid milk-based product, similar in palatability, macronutrient composition and energy density. In laboratory setting 49 nonrestrained subjects (24+/-6 years, BMI 22.2+/-2.3 kg m(-2)) received the liquid or semi-solid product. Effort and eating rate were controlled by means of a peristaltic pump. RESULTS: In real-life setting the intake of the liquid (809+/-396 g) was respectively 14 and 30% higher compared to the semi-liquid (699+/-391 g) and semi-solid product (566+/-311 g; P<0.0001). In laboratory setting, removing eating effort, resulted in a 29% (P<0.0001) intake difference between liquid (319+/-176 g) and semi-solid (226+/-122 g). Standardizing eating rate resulted in 12% difference between liquid (200+/-106 g) and semi-solid (176+/-88 g; P=0.24). If not controlled, the difference in intake between liquid (419+/-216 g) and semi-solid (277+/-130 g) was comparable to the real-life setting (34%; P<0.0001). CONCLUSIONS: Products different in viscosity but similar in palatability, macronutrient composition and energy density lead to significant differences in intake. This difference is partially explained by the higher eating rate of liquids.

Is fat perception a thermal effect?

It has been generally assumed that fat is detected by its flavour and by its lubrication of the oral mucosa. A recent study reported a correlation of -.99 between perceived temperature of a product and its fat content. This was significantly higher than correlations of sensory scores for fat flavour, mouthfeel, and afterfeel. This suggested a third detection mechanism; fat may be detected via its effect on the thermal conductivity of the food. In 3 studies, thermal sensitivity in humans was investigated to verify whether oral thermal receptors are sufficiently rapid and accurate to play a role in the perception of fats. The thermal sensitivity of the lips and oral mucosa of the anterior and middle one-third of the tongue were assessed using a Peltier device. Subjects detected 0.5 Hz fluctuations in temperature of 0.08'C on the lower lip, 0.26 degrees C and 1.36 degrees C at the tip and dorsum of the tongue, demonstrating that the lips are sufficiently sensitive to detect small differences in temperature. In two further experiments subjects ingested custards and mayonnaises and then spat out samples after 5, 10, or 20 sec. The temperature of the food and oral mucosa was measured before and after spitting and the rates of heating were calculated. Results suggest assessment of thermal conductivity of food may be used to assess fat content.

Sensory correlation of glucose levels in a starch-based semi-solid model system before and after alpha-amylase breakdown.

In this study a diabetic glucose monitor was used to measure the glucose content of 40 model vanilla custard desserts, these were compared with sensory ratings from a trained quantitative descriptive analysis panel. Stimuli varied in starch type, concentration and homogenisation but had identical sugar, flavour and fat contents. Despite this, mean sensory ratings of sweetness varied from 21 to 43 on a 100-point scale. Glucose levels in the stimuli were measured, in vitro, before and after the addition of 0.1% whole human saliva to the product. In all cases glucose concentration increased after the addition of saliva as starch was converted to glucose. There were highly significant correlations of some odour, flavour, mouth feel and after-feel attributes with glucose concentration. The correlation of sweet flavour with glucose concentration was higher in unmodified stimuli than in stimuli with added saliva, suggesting that sweetness is perceived soon after ingestion. Additional glucose produced through starch breakdown appeared to have a relatively small impact on the sensory scores of the stimuli.

Instrumental and sensory quantification of oral coatings retained after swallowing semi-solid foods.

After a mouthful of food has been swallowed, some food material is always retained in the mouth. With semi-solid foods this is in the form of a coating that adheres to the oral mucosa. The amount and location of this material may play an important role in food sensations. In this study two quantitative methods of describing the coatings, generated by a set of 16 model custards varying in degree of lubrication (fat content) and degree of viscosity (starch concentration) are investigated. In the first method, a trained quantitative descriptive analysis panel (N=8) was instructed to take single mouthfuls of semi-solid foods, swallow and then rinse twice for 5s with water and spit out. The turbidity of the rinse water was then measured. During the same session, sensory assessments of the products were obtained. In the second method the thickness of the coating on the anterior and middle one-thirds of the tongue was quantified using a pair of opto-electronic reflectance sensors mounted on a probe which was placed on the tongue, one sensor measuring the anterior part of the tongue the other the posterior (middle third) of the tongue. Turbidity of the first rinse related strongly to the food's viscosity, as well as to sensory attributes associated with the food's fat content and viscosity, such as perceived thickness, creaminess and fattiness. Turbidity of subsequent rinses related primarily to fat content. These results indicate that turbidity of rinse water is a useful tool in fundamental (e.g., food texture research) as well as applied research (e.g., product development and quality assurance in food industry). Reflectance varied primarily with fat content and did not relate well to sensory attributes.

Odor and chemesthesis from brief exposures to TXIB.

UNLABELLED: An experiment explored ability of subjects to detect vapors of the plasticizer TXIB (2,2,4-trimethyl-1,3-pentanediol diisobutyrate) and ethanol via olfaction and via ocular and nasal chemesthesis, i.e. chemically stimulated feel. Testing, tailored to the sensitivity of each subject, produced psychometric functions for individuals. Olfactory detection of TXIB began at concentrations below 1 ppb (v/v), with 50% correct detection at 1.2 ppb. (Comparable detection for ethanol occurred almost two orders of magnitude higher.) Chemesthetic detection of TXIB began at about 500 ppb, with 50% correct detection at 2.1 ppm for the eye and 4.6 ppm for the nose, both close to saturated vapor concentration. (Comparable detection for ethanol occurred essentially three orders of magnitude higher.) Suggestions that TXIB plays a role in generation of irritative symptoms at concentrations in the range of parts-per-billion need to reckon with a conservatively estimated 200-fold gap between the levels putatively 'responsible' for the symptoms and those even minimally detectable via chemesthesis. Neither the variable of exposure duration nor that of mixing offers a likely explanation. Inclusion of ethanol in the study allowed comparisons pertinent to issues of variability in human chemoreception. An interpretation of the psychometric functions for individuals across materials and perceptual continua led to the conclusion that use of concentration as the metric of detection in olfaction inflates individual differences. PRACTICAL IMPLICATIONS: This study indicated that the plasticizer TXIB could contribute odor at concentrations in the range of parts-per-billion, but could hardly contribute sensory irritation per se, as alleged in reports of some field studies where TXIB has existed amongst many other organic compounds.

Comparison of models of odor interaction.

Subjects rated the overall perceived intensity of concentrations of the odorants cineole, geraniol, hexyl salicylate, and linalyl acetate smelled alone and in binary mixtures. The subjects also rated intensity of specified constituents (e.g. amount of cineole in cineole, and in mixtures of cineole and linalyl acetate). The intensity of the stronger component alone offered a close description of perceived intensity. In addition to the Stronger Component model, two other psychological models (Vector and U model) and two psychophysical models (UPL2 and Equiratio Mixture model) offered descriptions ranging from fair to very good. Psychological models gave better fits, but lack explanatory power. Some results indicated that weaker odors add more potently than stronger odors, an outcome incompatible with these models. The psychophysical models, based on the additivity of single components, generally overestimated perceived intensity. Judgments of individual qualities gave only slight encouragement to any expectation of differences in masking or maskability among odorants. The results highlight the need to test particular critical hypotheses regarding how people perceive mixtures.

Odor quality: discrimination versus free and cued identification.

Eighty-two adults, ranging in age from young to elderly, performed odor-quality discrimination and both free and cued identification on six odorants presented at two intensity levels. The odorants simulated the real-world substances banana, licorice, cherry/almond, wintergreen, clove, and lemon. Performance on all three tasks declined with age, but improved with stimulus intensity. Performance at discrimination benefited from the mere availability of the six names during testing. Performance in cued identification far exceeded that in free identification and, for young and middle-aged adults, fell close to that for discrimination. For elderly adults, however, performance in cued identification fell substantially below that in discrimination. Although not entirely free of cognitive influences, discrimination seems to offer particularly clear resolution of alterations in olfactory functioning.

Odor identification by name and by edibility: life-span development and safety.

The study explored how subjects from childhood (8-14 years old) to elderly adulthood could identify the odors of 17 everyday products. Performance at naming followed an inverted U-shaped course over age, best in young adults and poorest in children and elderly. A discrimination task given to the adults revealed parallel age-related declines in ability to name and to discriminate odors. Correct identification by edibility exceeded that by name considerably. For dangerous household products, children achieved only 15% correct naming but 79% correct edibility/inedibility. Scenting of dangerous household products, however, may compromise the discriminability of these products from less harmful ones. "Fresh"-scented hypochlorite bleach yielded significantly more errors regarding its possible causticity than did unscented bleach. Such modifications of products would seem to necessitate other compensatory changes to enhance discriminability and thereby maintain safety.

Olfactory-evoked potentials: assessment of young and elderly, and comparison to psychophysical threshold.

Olfactory-evoked potentials (OEPs) were recorded monopolarly at the Fz, Cz and Pz electrode sites in young adults and elderly subjects with an amyl acetate stimulus presented olfactometrically. Psychophysical odor thresholds for amyl acetate were determined using a two-alternative, forced-choice detection procedure. These demonstrated a decreased odor detection sensitivity in the elderly relative to tye young subjects. In accordance with the threshold findings, elderly subjects produced significantly smaller N1 and P2 amplitudes, and a trend towards longer latencies than younger subjects. Moreover, odor threshold was found to correlate with amplitude magnitude. The OEP findings, which are discussed in the context of previous OEP studies, offer promise as an objective, non-invasive measure of sensory function in both clinical and non-clinical settings.