Differential context effects in taste perception

A series of three experiments explored differential contexteffects in judgements of taste intensity. For example, a givenconcentration of NaCl was judged stronger than a given concentrationof sucrose in one contextual condition (where the stimulus setcomprised low concentrations of NaCl and high ones of sucrose),but was judged weaker in another condition (where the concentrationsreversed). Overall, differential context effects were largerwhen subjects judged NaCl and sucrose than when they judgedNaCl and NaCl sucrose mixtures, an outcome predicted by thehypothesis that the magnitude of differential context effectsdepends on the degree of qualitative similarity between thesubsets of stimuli. This pattern of results mimics those previouslyobtained for judgements of loudness of tones at different soundfrequencies, and thereby suggests that a common mechanism mayunderlie contextual interactions in different modalities; moreover,this mechanism may bear implications for current theories ofthe coding of taste quality and intensify.     

Enhancement of retronasal odors by taste

Psychophysical studies of interactions between retronasal olfaction and taste have focused most often on the enhancement of tastes by odors, which has been attributed primarily to a response bias (i.e., halo dumping). Based upon preliminary evidence that retronasal odors could also be enhanced by taste, the present study measured both forms of enhancement using appropriate response categories. In the first experiment, subjects rated taste ("sweet," "sour," "salty," and "bitter") and odor ("other") intensity for aqueous samples of 3 tastants (sucrose, NaCl, and citric acid) and 3 odorants (vanillin, citral, and furaneol), both alone and in taste-odor mixtures. The results showed that sucrose, but not the other taste stimuli, significantly increased the perceived intensity of all 3 odors. Enhancement of tastes by odors was inconsistent and generally weaker than enhancement of odors by sucrose. A second experiment used a flavored beverage and a custard dessert to test whether the findings from the first experiment would hold for the perception of actual foods. Adding sucrose significantly enhanced the intensity of "cherry" and "vanilla" flavors, whereas adding vanillin did not significantly enhance the intensity of sweetness. It is proposed that enhancement of retronasal odors by a sweet stimulus results from an adaptive sensory mechanism that serves to increase the salience of the flavor of nutritive foods.     

Concentration-dependent changes of perceived odor quality

In order to assess the dependence of perceived odor qualityon odorant concentration, we studied 21 subjects. For eightsubjects all possible pairs from a pool of six odorants at threedecimal dilutions were presented, and subjects were requestedto state whether members of the pair were qualitatively ‘similar’or ‘different’ It was found that while pairs withthe same odorant at identical concentrations were judged ‘similar’in >90% of the cases by all subjects, scores went down to10% ‘similar’ judgements in some cases when thesame odorant was presented at a 100-fold concentration difference.Large time-invariable differences were found among subjectsand among odorants. For the additional 13 subjects, all possiblepairs from a pool of four odorants at three decimal dilutionswere presented. Subjects were instructed to state whether membersof the pair were qualitatively ‘same’ or ‘different’,and were also requested to rank the degree of difference ona visual analogue scale. Results for this group were, in general,similar to the results of the former group of subjects and goodagreement between the two tasks was found. The results suggestthat variations in olfactory stimulus magnitude may be perceivedas quality differences, as previously shown for vision and audition.     

Olfactory discrimination ability for aliphatic odorants as a function of oxygen moiety

We tested the ability of human subjects to distinguish between aliphatic odorants sharing the same number of carbon atoms but differing in their functional groups. 1-Alcohols, n-aldehydes, 2-ketones and n-carboxylic acids of four, six and eight carbon atoms, respectively, were employed. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with 18 odor pairs and asked to identify the bottle containing the odd stimulus. We found (i) that as a group, the subjects performed significantly above chance level in all tasks and thus were clearly able to discriminate between all odor pairs presented; (ii) marked interindividual differences in discrimination performance, ranging from subjects who were able to significantly distinguish between all 18 odor pairs to subjects who failed to do so with 1/3 of the tasks; (iii) a lack of significant differences in performance between male and female, and between Japanese and German subjects; (iv) that odor pairs that involved 2-ketones and/or n-carboxylic acids were significantly easier to discriminate compared to odor pairs that involved 1-alcohols and/or n-aldehydes, and thus a clear dependence of discriminability on type of functional group; and (v) that aliphatic odorants with eight carbon atoms (irrespective of their oxygen moiety) were significantly more difficult to discriminate from each other compared to substances with four or six carbon atoms. The results suggest that functional groups may be an important determinant of the interaction between stimulus molecule and olfactory receptor in aliphatic substances, and thus may be a molecular property affecting odor quality in a substance class-specific manner.     

Intranasal localizability of odorants: influence of stimulus volume

When an odorant is presented to one side of the nose and air to the other, the ability to localize which side received the odorant depends upon trigeminal nerve stimulation. It has been shown that performance on this lateralization task increases as stimulus concentration increases. In this study, we determined the influences of stimulus volume and sex on the ability to localize each of 8 odorants presented at neat concentrations: anethole, geraniol, limonene, linalool, menthol, methyl salicylate, phenyl ethanol, and vanillin. At a low stimulus volume (11 mL), only menthol was localized at an above-chance level. At a high stimulus volume (21 mL), above-chance localization occurred for all odorants except vanillin. Women were significantly better than men in localizing menthol. Stimuli rated as most intense were those that were most readily localized. The detection performance measures, as well as rated intensity values, significantly correlated with earlier findings of the trigeminal detectability of odorants presented to anosmic and normosmic subjects. This study suggests that differences in stimulus volume may explain some discrepant findings within the trigeminal chemosensory literature and supports the concept that vanillin may be a "relatively pure" olfactory stimulus.     

No oral-cavity-only discrimination of purely olfactory odorants

The purely olfactory odorants coumarin, octanoic acid, phenylethyl alcohol, and vanillin had been found to be consistently identified when presented retronasally but could not be identified when presented oral-cavity only (OCO). However, OCO discrimination of these odorants was not tested. Consequently, it remained possible that the oral cavity trigeminal system might provide sufficient information to differentiate these purely olfactory odorants. To evaluate this, 20 participants attempted to discriminate vapor-phase coumarin, octanoic acid, phenylethyl alcohol, and vanillin and, as a control, the trigeminal stimulus peppermint extract, from their glycerin solvent, all presented OCO. None of the purely olfactory odorants could be discriminated OCO, but, as expected, peppermint extract was consistently discriminated. This inability to discriminate clarifies and expands the previous report of lack of OCO identification of purely olfactory odorants. Taken together with prior data, these results suggest that the oral cavity trigeminal system is fully unresponsive to these odorants in vapor phase and that coumarin, octanoic acid, phenylethyl alcohol, and vanillin are indeed purely olfactory stimuli. The OCO discrimination of peppermint extract demonstrated that the absence of discrimination for the purely olfactory odorants was odorant dependent and confirmed that the oral cavity trigeminal system will provide differential response information to some vapor-phase stimuli.     

Olfactory discrimination of aliphatic odorants at 1 ppm: too easy for CD-1 mice to show odor structure–activity relationships?

Using an operant conditioning paradigm we tested the ability of CD-1 mice to discriminate between 25 odorants comprising members of five homologous series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found (a) that all mice significantly discriminated between all 50 stimulus pairs that involved odorants sharing the same functional group, but differing in carbon chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional group, (b) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes, and n-carboxylic acids tested, and (c) that odorant pairs differing in functional group were significantly better discriminated than odorant pairs differing in carbon chain length. These findings demonstrate that CD-1 mice have excellent discrimination ability for structurally related aliphatic odorants, that correlations between discrimination performance and structural similarity of odorants are odorant class-specific rather than a general phenomenon, and that both carbon chain length and type of functional group play an important role for odor quality coding in mice.     

Olfactory discrimination ability for aromatic odorants as a function of oxygen moiety.

To assess the significance of the type of oxygen moiety on odor quality of aromatic compounds, I tested the ability of human subjects to distinguish between odorants sharing a benzene ring and the same total number of carbon atoms but differing in their functional groups. Phenyl ethanol, phenyl acetaldehyde, phenyl methyl ketone, methyl benzoate and phenyl acetic acid, were employed. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with all possible binary combinations of the five odorants, and asked to identify the bottle containing the odd stimulus. I found (i) that as a group, the subjects performed significantly above chance level in six of the tasks whereas they failed to do so with the four other tasks; (ii) marked interindividual differences in discrimination performance, ranging from subjects who were able to significantly distinguish between all 10 odor pairs to subjects who failed to do so with the majority of the tasks; and (iii) that odor pairs that involved methyl benzoate or phenyl methyl ketone were significantly easier to discriminate than those that involved phenyl acetaldehyde or phenyl ethanol, and thus there was a clear dependence of discriminability on type of functional group. Additional tests of the degree of trigeminality of the five aromatic substances indicated that the discriminability of the odor pairs is indeed due to differences in odor quality. A comparison of the present results with those of an earlier study that employed aliphatic odorants suggests that functional oxygen-containing groups may generally be an important determinant of the interaction between the stimulus molecule and the olfactory receptor, and thus may generally be a molecular property affecting odor quality in a substance class-specific manner. The poorer discriminatory performance of the subjects with aromatic odorants compared with corresponding aliphatic substances suggests that the structure of the alkyl rest attached to a functional group may also play a crucial role for recognition of ligands at the olfactory receptor and thus for odor quality.     

Contrast and range effects for category, magnitude and labeled magnitude scales in judgements of sweetness intensity

The labeled magnitude scale (LMS) is a verbally anchored quasi-logarithmically spaced response scale with properties similar to magnitude estimation. Three experiments examined whether the LMS showed context effects similar to those found with magnitude estimation and category scales. Two versions of the LMS were used, one anchored at the high end to the strongest imaginable sweetness and the other to the strongest imaginable oral sensation. In a simple contrast experiment, subjects judged the sweetness of a 10% sucrose fruit beverage in the context of a less sweet (5%) beverage or a more sweet (20%) beverage. Consistent with previous literature, the sweetness was judged more intense in the low context and less intense in the high context, for all scaling methods. In a second experiment, this effect persisted (although was smaller) when the contextual item preceded the to-be-rated item, a so-called 'reversed-pair' design. Once again, the effect was highly significant for all scaling methods. In a third experiment, a range effect was examined using wide and narrow ranges of concentration. Psychophysical functions were flatter in a wide context and steeper in a narrow context, consistent with previous observations on range-mapping bias. This result was obtained for all scales. In three common contextual effects, the labeled magnitude scale behaved similarly to other scaling procedures. Its application to comparisons across individuals may be limited if those individuals have different experiential contexts within which they make their judgements.     

Differences in human evoked potentials related to olfactory or trigeminal chemosensory activation

The aim of the present study was to determine, whether there are differences in the topographical distribution of chemosensory evoked potentials (CSEPs) due to stimulation with different odous substances. The odorants used in the study which mainly excited the olfactory nerve were vanillin and acetaldehyde; those which additionally excited the trigeminal nerve were sulphur dioxide and ammonia. Twelve subjects participated in the study. The subjects separately estimated the intensity of the odorous and of the painful/pricking sensation caused by the stimuli, and described the odorous qualities in their own words. CSEPs were recorded from 7 positions.After stimulation with “olfactory” subsances maximum CSEP amplitudes were recorded at parietol-central sites, and after stimulation with “trigeminal” substances maximum amplitudes were obtained at the vertex. Following stimulation with ammonia and sulphur dioxide amplitudes were largest contralateral to the stimulated nostril. In contrast, little difference in CSEP amplitudes was observed between hemispheres after stimulation with vanillin or acetyldehyde.Thus, the topographical distribution of CSEP amplitudes may provide information with regard to the sensory system (olfactory or trigeminal) activated by the presentation of an odorous stimulus.     

Taste confusions following chlorhexidine treatment.

Chlorhexidine, a bitter bis-biguanide antiseptic, is the only known blocker of the human salty taste. In order to characterize the effects of chlorhexidine on stimulus identification, taste confusion matrix (TCM) performance was measured for subjects treated with 1.34 mM chlorhexidine gluconate (n = 9) and water controls (n = 9). Ten stimuli [water, 0.1 M NaCl, 0.1 M KCl, 0.1 mM quinine-HCl (QHCl), 0.1 M monosodium glutamate (MSG), 3 mM citric acid, 0.3 M sucrose and mixtures of NaCl, QHCl and citric acid with sucrose] were presented in 10 replicates for identification from a list of 10 stimulus names. T(10), a measure of performance consistency from information theory, was lower for chlorhexidine-treated subjects (2.02 +/- 0.11 bits) than controls (2.73 +/- 0.11 bits) (P < 0.0001). T(2), an indirect measure of pairwise stimulus discrimination, approached chance levels (0.40 bit) in chlorhexidine-treated subjects for all possible pairs of NaCl, KCl, QHCl and water, as well as pairs composed of sucrose and the NaCl-sucrose and quinine-sucrose mixtures. In controls T(2) values approached perfect scores (1.00 bit) for all stimulus pairs except NaCl-KCl and NaCl-MSG. The results demonstrate a decreased ability to identify taste stimuli that is consistent with alterations in the ability of stimuli to elicit salty and bitter taste perceptions. As a selective, effective, persistent and reversible blocker of taste perceptions, chlorhexidine should prove useful in defining taste mechanisms in humans.     

Influence of experimental context on the development of anhedonia in male mice exposed to chronic social stress

Consumption of 1% sucrose solution supplemented with 0.2% vanillin was studied in two experimental contexts in male mice living under chronic social stress induced by daily experience of defeats in agonistic interactions and leading to development of depression. In the first experiment, vanillin sucrose solution was made available as an option along with water during 10 days for mice living in group home cages. Then the mice were subjected to repeated social defeat stress and during exposure to stress they were provided with both vanillin sucrose solution and water using a free two-bottle choice paradigm. In the other experiment, vanillin sucrose solution was first offered to mice after 8 days of exposure to social defeat stress. Males familiar with vanillin sucrose solution showed vanillin sucrose preference while experiencing defeat stress: consumption of vanillin sucrose solution was about 70% of total liquid consumption. However, the consumption of vanillin sucrose solution per gram of body weight in mice exposed to social stress during 20 days was significantly lower than in control males. In the second experiment, males after 8 days of social defeat stress were found to consume significantly less vanillin sucrose solution as compared to control males. On average, during two weeks of measurements, vanillin sucrose solution intake was less than 20% of total liquid consumption in males. Consumption per gram of body weight also appeared to be significantly lower than in control group. The influence of experimental context on the development of anhedonia measured as a reduction of sucrose solution intake by chronically stressed male mice is discussed.     

Retronasal but not oral-cavity-only identification of "purely olfactory" odorants

Identifications of 5 odorants selected to be nontrigeminal stimuli were compared using retronasal and oral-cavity-only (OCO) air-phase presentations, with OCO produced by both exhalation through the mouth and a nose clip that closed the nostrils. Nine identifiers were available on each trial; 1 or 2 were correct for each odorant. Correct retronasal identifications were more common than OCO identifications and exceeded chance across subjects and for each subject; OCO correct identifications did not exceed chance. Retronasal reaction times were briefer than OCO reaction times. Correct retronasal identifications for vanillin, octanoic acid, phenylethyl alcohol, coumarin, and octane were 88%, 73%, 87%, 70%, and 85%, respectively; correct OCO identifications were, respectively, 10%, 12%, 18%, 35%, and 33%. Identifiers selected for retronasally presented odorants differed from those for other retronasally presented odorants, but identifiers for OCO-presented odorants did not differ between odorants. Overall, the retronasal identifications of nontrigeminal odorants both depended upon the odorant that was presented and corresponded to previous reported orthonasal identifications. In contrast, the OCO identifications, characterized by low percentages of correct identifications and an absence of differences between odorants in selected identifiers, suggested that OCO responses to nontrigeminal, purely olfactory odorants lack sufficient sensory information for either correct or differential identification.     

Effects of selective adaptation on coding sugar and salt tastes in mixtures

Little is known about coding of taste mixtures in complex dynamic stimulus environments. A protocol developed for odor stimuli was used to test whether rapid selective adaptation extracted sugar and salt component tastes from mixtures as it did component odors. Seventeen human subjects identified taste components of "salt + sugar" mixtures. In 4 sessions, 16 adapt-test stimulus pairs were presented as atomized, 150-μL "taste puffs" to the tongue tip to simulate odor sniffs. Stimuli were NaCl, sucrose, "NaCl + sucrose," and water. The sugar was 98% identified but the suppressed salt 65% identified in unadapted mixtures of 2 concentrations of NaCl, 0.1 or 0.05 M, and sucrose at 3 times those concentrations, 0.3 or 0.15 M. Rapid selective adaptation decreased identification of sugar and salt preadapted ambient components to 35%, well below the 74% self-adapted level, despite variation in stimulus concentration and adapting time (<5 or >10 s). The 96% identification of sugar and salt extra mixture components was as certain as identification of single compounds. The results revealed that salt-sugar mixture suppression, dependent on relative mixture-component concentration, was mutual. Furthermore, like odors, stronger and recent tastes are emphasized in dynamic experimental conditions replicating natural situations.     

Mood induction with olfactory stimuli reveals differential affective responses in males and females

Olfactory perception is characterized by interpersonal variability.Although gender has been identified as a potential influencingfactor, currently little is known about its effect on perceivedhedonicity of individual odorants. This study assessed genderdifferences in emotional appraisal of 3 odorants (eugenol, vanillin,and hydrogen sulfide [H₂S]), presented to 25 healthy subjects(13 males, 12 females) in a blocked design. Standardized scalesrating valence and judgments of emotional experience were usedfor stimulus evaluation. Results indicate ambiguous pleasantnessratings for eugenol as well as stronger responses to vanillinodorant in female subjects; furthermore, in emotional experienceratings, the effect of eugenol was found to be gender dependent,evoking more positive and less negative emotions in female subjectsthan in males. The gender dependence of the mood response toeugenol necessitates reconsideration of this odorant as a reliablegender independent olfactory stimulus for studies on olfactionand emotion.     

Olfactory discrimination ability of human subjects for ten pairs of enantiomers.

We tested the ability of human subjects to distinguish between enantiomers, i.e. odorants which are identical except for chirality. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with 10 enantiomeric odor pairs and asked to identify the bottle containing the odd stimulus. We found (i) that as a group, the subjects were only able to significantly discriminate the optical isomers of alpha-pinene, carvone and limonene, whereas they failed to distinguish between the (+)- and (-)-forms of menthol, fenchone, rose oxide, camphor, alpha-terpineol, beta-citronellol and 2-butanol; (ii) marked individual differences in discrimination performance, ranging from subjects who were able to significantly discriminate between 6 of the 10 odor pairs to subjects who failed to do so with 9 of the 10 tasks; (iii) that with none of the 10 odor pairs were the antipodes reported to differ significantly in subjective intensity when presented at equal concentrations; and (iv) that error rates were quite stable and did not differ significantly between sessions, and thus, we observed a lack of learning or training effects. Additional tests of the degree of trigeminality and threshold measurements of the optical isomers of alpha-pinene, carvone and limonene suggest that the discriminability of these three enantiomeric odor pairs is indeed due to differences in odor quality. These findings support the assumption that enantioselective molecular odor receptors may only exist for some but not all volatile enantiomers and thus that chiral recognition of odorants may not be a general phenomenon but is restricted to some substances.     

Evidence for deficiencies in perceptual and semantic olfactory processes in Parkinson's disease

Olfactory deficits have been reported in Parkinson's disease (PD) and are thought to represent a sensitive marker of the disease. The aim of the present study was to examine the differential contribution in olfactory dysfunction of perceptual and semantic processes of odours in PD patients. Twenty-four PD patients (12 males and 12 females) and 24 control subjects (12 males and 12 females) were tested. The experiment included two sessions. Initially, 12 odorants were delivered, one per minute. For each odour, subjects were asked to rate intensity, pleasantness, familiarity and edibility using linear rating scales. The odorants were again presented and the subjects were asked to identify them. The four olfactory judgements and odour identification were severely disturbed in PD patients when compared to control subjects. These findings demonstrate major deficits for all cognitive tasks of olfactory judgement in PD, and suggest that PD is associated with disruption of olfactory areas situated in the temporal lobes and also in the prefrontal cortex.     

Identification of air phase retronasal and orthonasal odorant pairs

Identifications (IDs) of paired retronasal and orthonasal odorants were studied, with stimuli limited to air phase. Odorants were liquid extracts of plant materials, sold as food flavorings, matched by each subject both for retronasal-only and orthonasal-only air phase intensities and then learned to 100% correct veridical name retronasal-only and orthonasal-only IDs. Subjects were tested for ID of (a) retronasal-only and orthonasal-only odorants, (b) homogeneously paired odorant (the same odorant in retronasal and orthonasal locations), and (c) heterogeneously paired odorants (different odorants in retronasal and orthonasal locations). Paired odorants were presented in two different sequences: retronasal location odorant smelled first or orthonasal location odorant smelled first. IDs were reported after odorants were removed. Results were as follows: (a) no significant differences between correct ID of odorants when in retronasal-only versus orthonasal-only locations, although percent correct IDs were lower for half the retronasal-only location odorants; (b) correct ID of a homogeneously paired odorant equaled or exceeded its unpaired ID, with two successive, identical IDs reported on the majority of its trials; (c) with heterogeneous pairs, for all odorants when in the orthonasal location of a pair, correct ID occurred less often than when these odorants were presented orthonasal-only, but for odorants in the retronasal location, correct ID equaled or exceeded retronasal-only correct ID; and (d) perceived order of presentation of heterogeneous pairs was the reverse of the physically presented sequence for both retronasal-first and orthonasal-first conditions. The heterogeneous odorant ID outcome supports the concept that processing of retronasal and orthonasal odorants differ, and the perceived reversal of the presented sequence is in agreement with the importance of recency in odorant memory.     

Differences in human chemosensory evoked potentials to olfactory and somatosensory chemical stimuli presented to left and right nostrils

The aim of the present study was to determine whether the recordingof chemosensory evoked potentials (CSEP) in healthy subjects(n = 11) can be helpful in differentiating the olfactory ortrigeminal component possessed by odorants. By recording fromseveral positions on the surface of the skull it was attemptedto ascertain whether different generators are responsible forCSEP associated with the different sensory components of odorants.Birhinal stimulation was used in order to establish an interactionbetween the stimulated side and the stimulated sensory channel.The four substances carbon dioxide, menthol, hydrogen sulphideand vanillin were tested. EEG was recorded from eight positions. The CSEPs' topographical distribution revealed differences inthe location of maximum amplitudes following stimulation withdifferent types of stimulants. Largest amplitudes always appearedat the vertex when trigeminal stimulants (menthol, carbon dioxide)were presented, whereas olfactory substances (vanillin, hydrogensulphide) elicited maximal amplitudes at parietal and centralsites. This suggests that at least two neuronal populationsare involved in the cortical generation of CSEP. Another interestingfinding was that the evoked potentials differed in relationto the stimulated side. Generally, responses to carbon dioxide,menthol and hydrogen sulphide had shorter latencies and smalleramplitudes after stimulation of the left nostril. In contrast,after stimulation with vanillin latencies were shorter and amplitudestended to be smaller after stimulation of the right side. Sincevanillin was the only substance which always evoked pleasantand positive associations, it was assumed that the differencesin CSEP after stimulation of the two nostrils are related tothe different processing of emotional information within thetwo hemispheres.     

'Thermal taste' predicts higher responsiveness to chemical taste and flavor

Individual differences in taste perception have been explained in part by variations in peripheral innervation associated with the genetic ability to taste the bitter substances PTC and PROP. In the present study we report evidence of another source of individual differences that is independent of taste stimulus, taste quality, or gustatory nerve. Individuals who perceived taste from thermal stimulation alone (thermal taste) gave significantly higher taste ratings to chemical stimuli--often by a factor of >2:1--than did individuals who perceived no taste from thermal stimulation. This was true for all taste stimuli tested (sucrose, saccharin, sodium chloride, citric acid, quinine sulfate, MSG and PROP), for all three gustatory areas of the mouth (anterior tongue, posterior tongue and soft palate) and for whole-mouth stimulation. Moreover, the same individuals reported stronger sensations from the olfactory stimulus vanillin, particularly when it was sensed retronasally. The generality of the thermal-taster advantage and its extension to an olfactory stimulus suggests that it arises from individual differences in CNS processes that are involved in perception of both taste and flavor.