Structure–activity relationships on the odor detectability of homologous carboxylic acids by humans

We measured concentration detection functions for the odor detectability of the homologs: formic, acetic, butyric, hexanoic, and octanoic acids. Subjects (14 ≤ n ≤ 18) comprised young (19–37 years), healthy, nonsmoker, and normosmic participants from both genders. Vapors were delivered by air dilution olfactometry, using a three-alternative forced-choice procedure against carbon-filtered air, and an ascending concentration approach. Delivered concentrations were established by gas chromatography (flame ionization detector) in parallel with testing. Group and individual olfactory functions were modeled by a sigmoid (logistic) equation from which two parameters are calculated: C, the odor detection threshold (ODT) and D, the steepness of the function. Thresholds declined with carbon chain length along formic, acetic, and butyric acid where they reached a minimum (ODTs = 514, 5.2, and 0.26 ppb by volume, respectively). Then, they increased for hexanoic (1.0 ppb) and octanoic (0.86 ppb) acid. Odor thresholds and interindividual differences in olfactory acuity among these young, normosmic participants were lower than traditionally thought and reported. No significant effects of gender on odor detectability were observed. The finding of an optimum molecular size for odor potency along homologs confirms a prediction made by a model of ODTs based on a solvation equation. We discuss the mechanistic implications of this model for the process of olfactory detection

Dissociated Representations of Pleasant and Unpleasant Olfacto-Trigeminal Mixtures: An fMRI Study

How the pleasantness of chemosensory stimuli such as odorants or intranasal trigeminal compounds is processed in the human brain has been the focus of considerable recent interest. Yet, so far, only the unimodal form of this hedonic processing has been explored, and not its bimodal form during crossmodal integration of olfactory and trigeminal stimuli. The main purpose of the present study was to investigate this question. To this end, functional magnetic resonance imaging (fMRI) was used in an experiment comparing brain activation related to a pleasant and a relatively unpleasant olfacto-trigeminal mixture, and to their individual components (CO2 alone, Orange alone, Rose alone). Results revealed first common neural activity patterns in response to both mixtures in a number of regions: notably the superior temporal gyrus and the caudate nucleus. Common activations were also observed in the insula, although the pleasant mixture activated the right insula whereas the unpleasant mixture activated the left insula. However, specific activations were observed in anterior cingulate gyrus and the ventral tegmental area only during the perception of the pleasant mixture. These findings emphasized for the firs time the involvement of the latter structures in processing of pleasantness during crossmodal integration of chemosensory stimuli

Olfactory and trigeminal interaction of menthol and nicotine in humans

The purpose of the study was to investigate the interactions between two stimuli—menthol and nicotine—both of which activate the olfactory and the trigeminal system. More specifically, we wanted to know whether menthol at different concentrations modulates the perception of burning and stinging pain induced by nicotine stimuli in the human nose. The study followed an eightfold randomized, double-blind, cross-over design including 20 participants. Thirty phasic nicotine stimuli at one of the two concentrations (99 and 134 ng/mL) were applied during the entire experiment every 1.5 min for 1 s; tonic menthol stimulation at one of the three concentrations (0.8, 1.5 and 3.4 μg/mL) or no-menthol (placebo control conditions) was introduced after the 15th nicotine stimulus. The perceived intensities of nicotine’s burning and stinging pain sensations, as well as perceived intensities of menthol’s odor, cooling and pain sensations, were estimated using visual analog scales. Recorded estimates of stinging and burning sensations induced by nicotine initially decreased (first half of the experiment) probably due to adaptation/habituation. Tonic menthol stimulation did not change steady-state nicotine pain intensity estimates, neither for burning nor for stinging pain. Menthol-induced odor and cooling sensations were concentration dependent when combined with low-intensity nicotine stimuli. Surprisingly, this dose dependency was eliminated when combining menthol stimuli with high-intensity nicotine stimuli. There was no such nicotine effect on menthol’s pain sensation. In summary, we detected interactions caused by nicotine on menthol perception for odor and cooling but no effect was elicited by menthol on nicotine pain sensation