Paul M. Wise

Synergistic Mixture Interactions in Detection of Perithreshold Odors by Humans

Laboratory demonstrations of synergistic mixture interactions in human odor perception have been rare. The current study examined perithreshold mixture interactions between maple lactone (ML) and selected carboxylic acids. An air-dilution olfactometer allowed precise stimulus control. Experimenters measured stimulus concentrations in vapor phase using a combination of solid-phase microextraction and gas chromatography/mass spectrometry. A probability of detection versus concentration, or a psychometric, functions was measured for pure ML. Psychometric functions were also measured for ML with the addition of fixed, subthreshold concentrations of carboxylic acids. Relative to statistical independence in detection, clear synergy occurred over a range of ML concentrations. To the best of our knowledge, the current results constitute the first clear demonstration of synergy in odor detection by humans from an experiment that combined precise stimulus control, vapor-phase calibration of stimuli, and a clear statistical definition of synergy.

Sweet taste and menthol increase cough reflex thresholds.

Cough is a vital protective reflex that is triggered by both mechanical and chemical stimuli. The current experiments explored how chemosensory stimuli modulate this important reflex. Cough thresholds were measured using a single-inhalation capsaicin challenge. Experiment 1 examined the impact of sweet taste: Cough thresholds were measured after rinsing the mouth with a sucrose solution (sweet) or with water (control). Experiment 2 examined the impact of menthol: Cough thresholds were measured after inhaling headspace above a menthol solution (menthol vapor) or headspace above the mineral oil solvent (control). Experiment 3 examined the impact of rinsing the mouth with a (bitter) sucrose octaacetate solution. Rinsing with sucrose and inhaling menthol vapor significantly increased measured cough thresholds. Rinsing with sucrose octaacete caused a non-significant decrease in cough thresholds, an important demonstration of specificity. Decreases in cough reflex sensitivity from sucrose or menthol could help explain why cough syrups without pharmacologically active ingredients are often almost as effective as formulations with an added drug. Further, the results support the idea that adding menthol to cigarettes might make tobacco smoke more tolerable for beginning smokers, at least in part, by reducing the sensitivity of an important airway defense mechanism.

Reduced dietary intake of simple sugars alters perceived sweet taste intensity but not perceived pleasantness

BACKGROUND Individuals who adhere to reduced-sodium diets come to prefer less salt over time, but it is unclear whether sweet taste perception is modulated by reduced sugar intake. OBJECTIVE The objective was to determine how a substantial reduction in dietary intake of simple sugars affects sweetness intensity and pleasantness of sweet foods and beverages. DESIGN Healthy men and women aged 21-54 y participated for 5 mo. After the baseline month, 2 subject groups were matched for demographic characteristics, body mass index, and intake of simple sugars. One group (n = 16; 13 of whom completed key experimental manipulations) was randomly assigned to receive a low-sugar diet during the subsequent 3 mo, with instructions to replace 40% of calories from simple sugars with fats, proteins, and complex carbohydrates. The other (control) group (n = 17; 16 of whom completed the study) did not change their sugar intake. During the final month, both groups chose any diet they wished. Each month subjects rated the sweetness intensity and pleasantness of vanilla puddings and raspberry beverages that varied in sucrose concentration. RESULTS ANOVA showed no systematic differences between groups in rated sweetness during the baseline or first diet month. During the second diet month, the low-sugar group rated low-sucrose pudding samples as more intense than did the control group (significant group-by-concentration interaction, P = 0.002). During the third diet month, the low-sugar subjects rated both low and high concentrations in puddings as ∼40% sweeter than did the control group (significant effect of group, P = 0.01). A weaker effect on rated sweetness was obtained for the beverages. Rated pleasantness was not affected for either of the stimuli. CONCLUSIONS This experiment provides empirical evidence that changes in consumption of simple sugars influence perceived sweet taste intensity. More work is needed to determine whether sugar intake ultimately shifts preferences for sweet foods and beverages. This trial was registered at clinicaltrials.gov as NCT02090478.

Individual Differences in Sour and Salt Sensitivity: Detection and Quality Recognition Thresholds for Citric Acid and Sodium Chloride

Taste sensitivity is assessed with various techniques, including absolute detection and quality recognition. For any stimulus, one might expect individual differences in sensitivity to be reflected in all measures, but they are often surprisingly independent. Here, we focus on sensitivity to sour and salty taste, in part because processing of these qualities is poorly understood relative to other tastes. In Study 1, we measured retest reliability for detection (modified, forced-choice staircase method) and recognition (modified Harris-Kalmus procedure) for both citric acid (CA) and sodium chloride (NaCl). Despite good retest reliability, individual differences in detection and recognition were weakly correlated, suggesting that detection and recognition of sour and salty stimuli may reflect different physiological processes. In Study 2, a subset of subjects returned to contribute full detection (psychometric) functions for CA and NaCl. Thresholds estimated from full detection functions correlated with both staircase and recognition thresholds, suggesting that both tasks may reflect absolute sensitivity to some extent. However, the ranges of individual differences were systematically compressed for staircase thresholds relative to those from full detection functions. Thus, individual differences in sensitivity appear to interact with different test methodologies in lawful ways. More work will be required to understand how different taste phenotypes relate to one another.

Odor detection of mixtures of homologous carboxylic acids and coffee aroma compounds by humans.

Mixture summation among homologous carboxylic acids, that is, the relationship between detection probabilities for mixtures and detection probabilities for their unmixed components, varies with similarity in carbon-chain length. The current study examined detection of acetic, butyric, hexanoic, and octanoic acids mixed with three other model odorants that differ greatly from the acids in both structure and odor character, namely, 2-hydroxy-3-methylcyclopent-2-en-1-one, furan-2-ylmethanethiol, and (3-methyl-3-sulfanylbutyl) acetate. Psychometric functions were measured for both single compounds and binary mixtures (2 of 5, forced-choice method). An air dilution olfactometer delivered stimuli, with vapor-phase calibration using gas chromatography-mass spectrometry. Across the three odorants that differed from the acids, acetic and butyric acid showed approximately additive (or perhaps even supra-additive) summation at low perithreshold concentrations, but subadditive interactions at high perithreshold concentrations. In contrast, the medium-chain acids showed subadditive interactions across a wide range of concentrations. Thus, carbon-chain length appears to influence not only summation with other carboxylic acids but also summation with at least some unrelated compounds.

Dynamics of nasal chemesthesis.

Dynamics, or how stimulation occurs over time, influences the somatosensory impact of volatile chemicals. Within an experimental session, sensation waxes with steady presentation over seconds to minutes, may reach a plateau, and then may fade. Long‐term occupational exposure can desensitize the trigeminal system. Short‐ and long‐term dynamics might be mediated by different mechanisms. For brief intranasal exposures (i.e., up to about 10 seconds), studies have systematically manipulated both time (duration of exposure) and concentration to maintain a fixed perceived intensity or a fixed level of detection. A simple mass integration model describes the trade‐off between concentration and time quite well: a fixed‐ratio increase in duration compensates for a fixed‐ratio decrease in concentration. However, for most compounds, more than a two‐fold increase in duration are required to compensate for cutting concentration in half. For example, for ethanol, an increase in duration of about six‐fold are required. For such compounds that display highly imperfect integration, a fixed number of molecules might have a much greater sensory impact when presented over 0.2 seconds than over 0.5 seconds. Nasal chemesthesis may be temporally sluggish compared to olfaction, but fine‐grained dynamics still matter. Time–intensity ratings of nasal irritation from dynamic stimuli also support this conclusion. Although integration is generally imperfect, compounds vary widely in how far they fall short of perfect time–concentration trading. Current studies use a structure–activity approach to determine how molecular parameters correlate with how well a compound integrates over time.

4 – TASTE, SMELL AND CHEMESTHESIS IN PRODUCT EXPERIENCE

Publisher Summary The oldest human senses are those that respond to chemicals in the environment. In humans, the senses of taste, smell, and chemesthesis have become secondary to those of vision and audition, but they still play a pre-eminent role in how humans experience a wide range of everyday objects, including all foods and beverages, as well as many important products in the perfume, healthcare, and personal product markets. In spite of the importance of the chemical senses to human experience, they have not received the scientific attention that the higher senses have received. However, exciting discoveries during the past decade have produced greater interest in the chemical senses. This chapter explores taste, smell, and chemesthesis in product experience. It examines the role of three sensory systems that signal the presence of chemicals in the environment and that influence one’s product experiences. Taste(gustation) is the sensory system that detects chemicals dissolved in the liquids one drinks and the foods he or she eats. Smell (olfaction) detects airborne chemicals emitted by natural, living, and synthetic products. Chemesthesis detects chemical irritants via the same skin-sense nerves that give sensitivity to touch, temperature, and pain. These three sensory systems have different specialties but work together to contribute to our overall experience of the products one uses. This chapter outlines the anatomical, physiological, and biochemical mechanisms that underlie chemosensory experience. It discusses the fundamental sensory and perceptual phenomena that operate when products are experienced through these senses. Furthermore, it examines how chemosensory product experiences are quantified for practical applications in industry and the roles of nonsensory information, beliefs, expectations, and product context in experience, concluding with a discussion of how age, gender, social, and cultural variables can influence chemosensory product experience.

The Influence of Bubbles on the Perception Carbonation Bite

Although many people naively assume that the bite of carbonation is due to tactile stimulation of the oral cavity by bubbles, it has become increasingly clear that carbonation bite comes mainly from formation of carbonic acid in the oral mucosa. In Experiment 1, we asked whether bubbles were in fact required to perceive carbonation bite. Subjects rated oral pungency from several concentrations of carbonated water both at normal atmospheric pressure (at which bubbles could form) and at 2.0 atmospheres pressure (at which bubbles did not form). Ratings of carbonation bite under the two pressure conditions were essentially identical, indicating that bubbles are not required for pungency. In Experiment 2, we created controlled streams of air bubbles around the tongue in mildly pungent CO2 solutions to determine how tactile stimulation from bubbles affects carbonation bite. Since innocuous sensations like light touch and cooling often suppress pain, we predicted that bubbles might reduce rated bite. Contrary to prediction, air bubbles flowing around the tongue significantly enhanced rated bite, without inducing perceived bite in blank (un-carbonated) solutions. Accordingly, though bubbles are clearly not required for carbonation bite, they may well modulate perceived bite. More generally, the results show that innocuous tactile stimulation can enhance chemogenic pain. Possible physiological mechanisms are discussed.

Individual Differences in the Chemical Senses: Is There a Common Sensitivity?

Taste, smell, and chemical irritation (so-called trigeminal sensation) combine in our daily experience to produce the supramodal sensation of flavor, are processed by partly overlapping neural mechanisms, and show functional interconnectivity in experiments. Given their collaboration in flavor formation and the well-established connections between these senses, it is plausible that polymodal detection mechanisms might contribute to individual differences in measured sensitivity. One would expect the existence of a general chemosensory sensitivity factor to result in associations among taste, smell, and trigeminal stimulation thresholds. Measures of 5 detection thresholds from all the chemical senses were assessed in the same group of young healthy subjects (n=57). An unbiased principal components analysis (PCA) yielded a 2-component solution. Component 1, on which taste thresholds loaded strongly, accounted for 29.4% of the total variance. Component 2, on which the odor and trigeminal lateralization thresholds loaded strongly, accounted for 26.9% of the total variance. A subsequent PCA restricted to a 3-component solution cleanly separated the 3 sensory modalities and accounted for 75% of the total variance. Thus, though there may be a common underlying factor that determines some individual differences in odor and trigeminal lateralization thresholds, a general chemical sensitivity that spans chemosensory modalities seems unlikely.

Individuals Reporting Idiopathic Malodor Production: Demographics and Incidence of Trimethylaminuria

BACKGROUND Individuals with the metabolic disorder trimethylaminuria may sporadically produce malodors despite good hygiene. The psychosocial impact of trimethylaminuria can be considerable. However, trimethylaminuria is difficult to diagnose without specialized tests, in part because odor production is diet-dependent, and malodors may not be present during medical examinations. Thus, the prevalence and demographics of trimethylaminuria remain unclear. METHODS We tested 353 patients who had unexplained (idiopathic) malodor production for trimethylaminuria using a standard choline challenge. We also collected basic demographic information. RESULTS Approximately one third of patients (118) tested positive for trimethylaminuria. Consistent with previous reports, women, particularly African American women, were significantly overrepresented among trimethylaminuria-positive patients. Of note, the same pattern was seen among trimethylaminuria-negative patients. Also consistent with previous reports, trimethylaminuria-positive women who were still menstruating tended to produce higher levels of trimethylamine within ± 7 days of menses, although this trend was statistically marginal (P = .07). CONCLUSION If our patient sample is representative of patients with idiopathic malodor, demographic information (race and gender) may not be useful in a differential diagnosis of trimethylaminuria. However, undiagnosed cases of trimethylaminuria may be fairly common among patients with idiopathic malodor. If so, choline challenge testing should be indicated for all such patients because trimethylaminuria is responsive to dietary and other treatments. We speculate that testing also might reveal cases of trimethylaminuria among those diagnosed with certain psychologic disorders, including olfactory reference syndrome.