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People often confuse smell loss with taste loss, so it is unclear how much gustatory function is reduced in patients self-reporting taste loss. Our pre-registered cross-sectional study design included an online survey in 12 languages with instructions for self-administering chemosensory tests with 10 household items. Between June 2020 and March 2021, 10,953 individuals participated. Of these, 5,225 self-reported a respiratory illness and were grouped based on their reported COVID test results: COVID-positive (COVID+, N = 3,356), COVID-negative (COVID-, N = 602), and COVID unknown for those waiting for a test result (COVID?, N = 1,267). The participants who reported no respiratory illness were grouped by symptoms: sudden smell/taste changes (STC, N = 4,445), other symptoms excluding smell or taste changes (OthS, N = 832), and no symptoms (NoS, N = 416). Taste, smell, and oral irritation intensities and self-assessed abilities were rated on visual analog scales. Compared to the NoS group, COVID+ was associated with a 21% reduction in taste (95% confidence interval (CI): 15-28%), 47% in smell (95% CI: 37-56%), and 17% in oral irritation (95% CI: 10-25%) intensity. There were medium to strong correlations between perceived intensities and self-reported abilities (r = 0.84 for smell, r = 0.68 for taste, and r = 0.37 for oral irritation). Our study demonstrates that COVID-19-positive individuals report taste dysfunction when self-tested with stimuli that have little to none olfactory components. Assessing the smell and taste intensity of household items is a promising, cost-effective screening tool that complements self-reports and may help to disentangle taste loss from smell loss. However, it does not replace standardized validated psychophysical tests., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Recent anecdotal and scientific reports have provided evidence of a link between COVID-19 and chemosensory impairments, such as anosmia. However, these reports have downplayed or failed to distinguish potential effects on taste, ignored chemesthesis, and generally lacked quantitative measurements. Here, we report the development, implementation, and initial results of a multilingual, international questionnaire to assess self-reported quantity and quality of perception in 3 distinct chemosensory modalities (smell, taste, and chemesthesis) before and during COVID-19. In the first 11 days after questionnaire launch, 4039 participants (2913 women, 1118 men, and 8 others, aged 19-79) reported a COVID-19 diagnosis either via laboratory tests or clinical assessment. Importantly, smell, taste, and chemesthetic function were each significantly reduced compared to their status before the disease. Difference scores (maximum possible change ±100) revealed a mean reduction of smell (-79.7 ± 28.7, mean ± standard deviation), taste (-69.0 ± 32.6), and chemesthetic (-37.3 ± 36.2) function during COVID-19. Qualitative changes in olfactory ability (parosmia and phantosmia) were relatively rare and correlated with smell loss. Importantly, perceived nasal obstruction did not account for smell loss. Furthermore, chemosensory impairments were similar between participants in the laboratory test and clinical assessment groups. These results show that COVID-19-associated chemosensory impairment is not limited to smell but also affects taste and chemesthesis. The multimodal impact of COVID-19 and the lack of perceived nasal obstruction suggest that severe acute respiratory syndrome coronavirus strain 2 (SARS-CoV-2) infection may disrupt sensory-neural mechanisms., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Flavorants such as lemon extract that activate olfactory receptors may also evoke or enhance flavor qualities such as sour and sweet that are typically considered gustatory. Similarly, flavorants such as sucrose and citric acid that activate gustatory receptors may enhance flavors such as citrus that are typically considered olfactory. Here, we ask how lemon extract, sucrose, and citric acid, presented separately and together, affect sweet, sour, and citrus flavors. We accomplished this by testing, in the same 12 subjects, lemon extract and sucrose (Experiment 1), lemon extract and citric acid (Experiment 2), and lemon extract, sucrose, and citric acid (Experiment 3). Results showed that both lemon extract and citric acid increased the ratings of citrus and sour intensity. Lemon extract did not affect sweet, but citric acid did, mainly in Experiment 3. Sucrose systematically increased only sweet intensity and modulated the effect of lemon extract on sour. The most robust multiquality effect was the enhancement of sour by lemon extract. These outcomes suggest, first, a role played by experience with the statistical associations of gustatory and olfactory flavorants and, second, that lemon flavor is complex, having citrus and sour qualities that may not be fully separable in perception., (© The Author(s) 2017. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Two experiments presented oral mixtures containing different proportions of the gustatory flavorant sucrose and an olfactory flavorant, either citral (Experiment 1) or lemon (Experiment 2). In 4 different sessions of each experiment, subjects identified each mixture as "mostly sugar" or "mostly citrus/lemon" or rated the perceived intensities of the sweet and citrus components. Different sessions also presented the mixtures in different contexts, with mixtures containing relatively high concentrations of sucrose or citral/lemon presented more often (skew sucrose or skew citral/lemon). As expected, in both experiments, varying stimulus context affected both identification and perceived intensity: Skewing to sucrose versus citral/lemon decreased the probability of identifying the stimuli as "mostly sugar" and reduced the ratings of sweet intensity relative to citrus intensity. Across both contextual conditions of both experiments, flavor identification associated closely with the ratio of the perceived sweet and citrus intensities. The results accord with a model, extrapolated from signal-detection theory, in which sensory events are represented as multisensory-multidimensional distributions in perceptual space. Changing stimulus context can shift the locations of the distributions relative to response criteria, Decision rules guide judgments based on both sensory events and criteria, these rules not necessarily being identical in tasks of identification and intensity rating., (© The Author 2016. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Animal studies have shown that olfactory sensitivity is greater when fasted than when fed. However, human research has generated inconsistent results. One possible explanation for these conflicting findings is metabolic health. Many metabolic peptides, including ghrelin, are moderated by adiposity and influence olfaction and olfactory-guided behaviors. We tested whether the effect of a meal on the perceived intensity of suprathreshold chemosensory stimuli is influenced by body mass index and/or metabolic response to a meal. We found that overweight or obese (n = 13), but not healthy weight (n = 20) subjects perceived odors, but not flavored solutions, as more intense when hungry than when sated. This effect was correlated with reduced postprandial total ghrelin suppression (n = 23) and differential brain response to odors in the cerebellum, as measured with functional magnetic resonance imaging. In contrast, it was unrelated to circulating leptin, glucose, insulin, triglycerides, or free fatty acids; or to odor pleasantness or sniffing (n = 24). These findings demonstrate that the effect of a meal on suprathreshold odor intensity perception is associated with metabolic measures such as body weight and total ghrelin reactivity, supporting endocrine influences on olfactory perception., (© The Author 2016. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

A mixture of perceptually congruent gustatory and olfactory flavorants (sucrose and citral) was previously shown to be detected faster than predicted by a model of probability summation that assumes stochastically independent processing of the individual gustatory and olfactory signals. This outcome suggests substantial integration of the signals. Does substantial integration also characterize responses to mixtures of incongruent flavorants? Here, we report simple response times (RTs) to detect brief pulses of 3 possible flavorants: monosodium glutamate, MSG (gustatory: "umami" quality), citral (olfactory: citrus quality), and a mixture of MSG and citral (gustatory-olfactory). Each stimulus (and, on a fraction of trials, water) was presented orally through a computer-operated, automated flow system, and subjects were instructed to press a button as soon as they detected any of the 3 non-water stimuli. Unlike responses previously found to the congruent mixture of sucrose and citral, responses here to the incongruent mixture of MSG and citral took significantly longer (RTs were greater) and showed lower detection rates than the values predicted by probability summation. This outcome suggests that the integration of gustatory and olfactory flavor signals is less extensive when the component flavors are perceptually incongruent rather than congruent, perhaps because incongruent flavors are less familiar., (© The Author 2015. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Two experiments, using different ranges and numbers of stimuli, examined how linguistic labels affect the identification of flavor mixtures containing different proportions of sucrose (gustatory flavorant) and citral (olfactory flavorant). Both experiments asked subjects to identify each stimulus as having either "mostly sugar" or "mostly citrus." In one condition, no labels preceded the flavor stimuli. In another condition, each flavor stimulus followed a label, either SUGAR or CITRUS, which, the subjects were informed, usually though not always named the stronger flavor component; that is, the labels were probabilistically valid. The results of both experiments showed that the labels systematically modified the identification responses: Subjects responded "sugar" or "citrus" more often when the flavor stimulus followed the corresponding label, SUGAR or CITRUS. But the labels hardly affected overall accuracy of identification. Accuracy was possibly limited, however, by both the confusability of the flavor stimuli per se and the way that confusability could limit the opportunity to discern the probabilistic associations between labels and individual flavor stimuli. We describe the results in terms of a decision-theoretic model, in which labels induce shifts in response criteria governing the identification responses, or possibly effect changes in the sensory representations of the flavorants themselves.

Odorants and flavorants typically contain many components. It is generally easier to detect multicomponent stimuli than to detect a single component, through either neural integration or probability summation (PS) (or both). PS assumes that the sensory effects of 2 (or more) stimulus components (e.g., gustatory and olfactory components of a flavorant) are detected in statistically independent channels, that each channel makes a separate decision whether a component is detected, and that the behavioral response depends solely on the separate decisions. Models of PS traditionally assume high thresholds for detecting each component, noise being irrelevant. The core assumptions may be adapted, however, to signal-detection theory, where noise limits detection. The present article derives predictions of high-threshold and signal-detection models of independent-decision PS in detecting gustatory-olfactory flavorants, comparing predictions in yes/no and 2-alternative forced-choice tasks using blocked and intermixed stimulus designs. The models also extend to measures of response times to suprathreshold flavorants. Predictions derived from high-threshold and signal-detection models differ markedly. Available empirical evidence on gustatory-olfactory flavor detection suggests that neither the high-threshold nor the signal-detection versions of PS can readily account for the results, which likely reflect neural integration in the flavor system.

The insular cortex is implicated in general attention and in taste perception. The effect of selective attention to taste on insular responses may therefore reflect a general effect of attention or it may be (taste) modality specific. To distinguish between these 2 possibilities, we used functional magnetic resonance imaging to evaluate brain response to tastes and odors while subjects passively sampled the stimuli or performed a detection task. We found that trying to detect a taste (attention to taste) resulted in activation of the primary taste cortex (anterior and mid-dorsal insula) but not in the primary olfactory cortex (piriform). In contrast, trying to detect an odor (attention to odor) increased activity in primary olfactory but not primary gustatory cortex. However, we did identify a region of far anterior insular cortex that responded to both taste and odor "searches." These results demonstrate modality-specific activation of primary taste cortex by attention to taste and primary olfactory cortex by attention to odor and rule out the possibility that either response reflects a general effect of attentional deployment. The findings also support the existence of a multimodal region in far anterior insular cortex that is sensitive to directed attention to taste and smell.

It is easier to detect mixtures of gustatory and olfactory flavorants than to detect either component alone. But does the detection of mixtures exceed the level predicted by probability summation, assuming independent detection of each component? To answer this question, we measured simple response times (RTs) to detect brief pulses of one of 3 flavorants (sucrose [gustatory], citral [olfactory], sucrose-citral mixture) or water, presented into the mouth by a computer-operated, automated flow system. Subjects were instructed to press a button as soon as they detected any of the 3 nonwater stimuli. Responses to the mixtures were faster (RTs smaller) than predicted by a model of probability summation of independently detected signals, suggesting positive coactivation (integration) of gustation and retronasal olfaction in flavor perception. Evidence for integration appeared mainly in the fastest 60% of the responses, indicating that integration arises relatively early in flavor processing. Results were similar when the 3 possible flavorants, and water, were interleaved within the same session (experimental condition), and when each flavorant was interleaved with water only (control conditions). This outcome suggests that subjects did not attend selectively to one flavor component or the other in the experimental condition and further supports the conclusion that (late) decisional or attentional strategies do not exert a large influence on the gustatory-olfactory flavor integration.

Selective attention is thought to be associated with enhanced processing in modality-specific cortex. We used functional magnetic resonance imaging to evaluate brain response during a taste detection task. We demonstrate that trying to detect the presence of taste in a tasteless solution results in enhanced activity in insula and overlying operculum. The same task does not recruit orbitofrontal cortex (OFC). Instead, the OFC responds preferentially during receipt of an unpredicted taste stimulus. These findings demonstrate functional specialization of taste cortex in which the insula and the overlying operculum are recruited during taste detection and selective attention to taste, and the OFC is recruited during receipt of an unpredicted taste stimulus.

Independent experimental manipulation of subjective intensity and hedonic tone is required if one wants to study their separate effects on brain activity and behavior. This is problematic because hedonic tone and subjective intensity are related, leading to a pleasantness change each time the stimulus intensity is altered. In the present study, a solution to this problem was explored by combining a pleasant-tasting substance (sucrose) and a bad-tasting substance (quinine sulfate) into a number of different isointense mixtures. Here we show that subjective intensity as well as pleasantness can be accurately predicted, particularly in midrange, only if one corrects for mixture suppression.