Shared retronasal identifications of vapor-phase 18-carbon fatty acids.

The long-chain 18-carbon fatty acids linoleic, oleic, and stearic acids, retronasally in vapor phase, are discriminated from blanks and each other. However, ability to linguistically identify them was unknown. To explore this, a Focus Group and then Check-All-That-Apply measures gave 9 identifiers for the 3 fatty acids plus phenylethyl alcohol (PEA) and geraniol. Next, participants selected 1 of the 9 identifiers from a computer-based display. It was found that the modal identification for linoleic acid was 23% "Rubbery" (next 18% "Oily" and "New Plastic"), oleic acid was 21% Oily (next 19% Rubbery), and stearic acid was 43% Rubbery (next 22% New Plastic), but linoleic acid received ~40% food-related identifiers. Geraniol was 96% "Lemon," and PEA was 67% "Flowers." Identifications for fatty acids differed significantly (P ≤ 0.05) from those for geraniol for most participants (86%) and from those for PEA for 59% of participants. Stearic acid identifications differed significantly from those for linoleic and oleic acids for 32% of participants. However, identification for linoleic acid differed significantly from those for oleic acid for only 14% of participants. Overall, retronasal vapor-phase stearic acid was identified differently from other 18-carbon fatty acids by a substantial minority of participants, but linoleic and oleic acids were not, suggesting that these 2 vapor-phase 18-carbon fatty acids can be identified retronasally as a group but not separately.

Oral cavity discrimination of vapor-phase long-chain 18-carbon fatty acids.

Linoleic, oleic, and stearic fatty acids, presented vapor-phase retronasally, were discriminable from blanks and each other, but the same concentrations, oral-cavity-only (OCO), were not discriminable from blanks. It remained possible that higher concentrations might be discriminable OCO. To evaluate this, participants attempted to discriminate undiluted linoleic, oleic, or stearic acids, vapor-phase OCO, from blanks. For each fatty acid, participants received 5 stimulus delivery containers (SDCs) in 2 trials; 4 SDC held blanks, the fifth, a fatty acid. As a "positive control" in 2 trials, participants received vapor-phase OCO peppermint extract and blanks. For all trials, the task was to select the 1 different SDC. It was found that the 1 different SDC was selected in 24% of stearic, 32% of linoleic, 47% of oleic acid, and in 92% of peppermint trials; discriminations (the 1 different SDC selected in both trials) occurred in 0%, 16%, 26%, and 84% of pairs, respectively. Correct selections for oleic acid differed from chance, P = 0.0004, but not for linoleic acid, P = 0.125, or stearic acid, P = 0.345, Bonferroni corrected. Vapor-phase oleic acid can be an oral cavity trigeminal stimulus, linoleic acid might be (uncorrected P = 0.0384), but vapor-phase stearic acid cannot be.

Orthonasal and retronasal but not oral-cavity-only discrimination of vapor-phase fatty acids.

Discrimination of vapor-phase linoleic, oleic, and stearic fatty acids was studied using triangle tests. For each trial, 2 of the 3 modified odorant delivery containers (MODCs) had the same content and 1 was different. Contents were either mineral oil-diluted linoleic or oleic acids, with mineral oil in the other MODC (blanks) or undiluted stearic acid with NaCl in the other MODC (blanks). The task was to indicate which of the 3 MODC had the most different odor. Vapor-phase fatty acids and blanks were presented orthonasally, retronasally, or oral-cavity-only. It was found that all 3 fatty acids were discriminated from the blanks both orthonasally and retronasally, P 0.05 (30% of 30 participants discriminated linoleic acid from blanks, P = 0.71; 47%, oleic and stearic acids, P = 0.09). These results demonstrate that human participants can discriminate linoleic, oleic, and stearic fatty acids both orthonasally and retronasally, confirming that humans can smell fatty acids.

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.

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.

An oral-cavity component in retronasal smelling of natural extracts.

Retronasal and oral-cavity-only identifications of six natural extract odorants, presented in air-phase, were compared in an initial experiment. Prior to identification testing, the 21 participants were given experience with air-phase presentations, and with the odorants and their correct identifications. Retronasal correct identifications for anise, cinnamon, coffee, orange, peppermint, and strawberry were 88%, 81%, 98%, 95%, 91%, and 83%; oral-cavity-only, 19%, 21%, 19%, 21%, 33%, and 24%. All participants correctly identified retronasal odorants above chance. Across participants only peppermint received correct oral-cavity-only identifications, but two participants gave correct oral-cavity-only identifications for all odorants. In a second experiment, different participants attempted to discriminate oral-cavity-only odorants from their solvents. Fifteen participants discriminated orange, peppermint, and strawberry odorants from their solvents, and five discriminated all odorants from their solvents. It had been hypothesized that peppermint would provide unique trigeminal stimulation; this was supported by correct oral-cavity-only identification of only peppermint. A second hypothesis posited oral-cavity-only discrimination of orange and peppermint from their solvents because of trigeminal stimuli, but strawberry extract discrimination was unexpected. Furthermore, oral-cavity-only discrimination of all odorants by one-quarter of the participants was not anticipated. Overall, these outcomes suggest that peppermint-like odorants can initiate sufficiently differential responses in the oral cavity to permit identification, indicate that not only odorants with known trigeminal stimulus components but also others may elicit oral-cavity-only air-phase responses, and imply that for a substantial minority of individuals, trigeminal input may enhance oral-cavity effectiveness of many odorants during retronasal smelling.

Experience-induced changes in taste identification of monosodium glutamate (MSG) are reversible.

A few studies have reported experience-inducible changes in human taste and olfactory sensitivities. However, no study thus far has systematically characterized the stability of the enhanced sensitivities. In our previous study, we found increases in taste identification ability for monosodium glutamate (MSG) in subjects who had been briefly exposed to MSG in food for 10 days. Here, we tested the temporal stability of the enhanced taste identification ability. First, we exposed a group of 20 subjects to MSG in food and then compared their sensitivities to MSG with those of a control group. When tested on day 11 or 12, the mean MSG taste identification ability of the MSG-exposed group was significantly higher than the control group. Next, 11 of the subjects who were exposed to MSG in food initially, and then stopped being exposed performed significantly poorer in identifying MSG after 10 days of the nonexposure than they did 10 days before. In contrast, nine subjects who were exposed to MSG initially and continued being exposed maintained their high identification levels. These results support earlier finding of the plasticity in the taste identification of MSG and show that the enhanced identification ability can be reversed rapidly when MSG exposure is not sustained.

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.