Abstract
Exploring the potential of eucalyptol as a masking agent for aversive odors, we found that eucalyptol masks the olfactory but not the trigeminal sensation of ammonia in a previous study. Here, we further investigate the processing of a mixture consisting of eucalyptol and ammonia, two olfactory–trigeminal stimuli. We presented the two pure odors and a mixture thereof to 33 healthy participants. The nostrils were stimulated alternately (monorhinal application). We analyzed the behavioral ratings (intensity and pleasantness) and functional brain images. First, we replicated our previous finding that, within the mixture, the eucalyptol component suppressed the olfactory intensity of the ammonia component. Second, mixture pleasantness was rated differently by participants depending on which component dominated their mixture perception. Approximately half of the volunteers rated the eucalyptol component as more intense and evaluated the mixture as pleasant (pleasant group). The other half rated the ammonia component as more intense and evaluated the mixture as unpleasant (unpleasant group). Third, these individual differences were also found in functional imaging data. Contrasting the mixture either to eucalyptol or to both single odors, neural activation was found in the unpleasant group only. Activation in the anterior insula and SII was interpreted as evidence for an attentional shift towards the potentially threatening mixture component ammonia and for trigeminal enhancement. In addition to insula and SII, further regions of the pain matrix were involved when assessing all participant responses to the mixture. Both a painful sensation and an attentional shift towards the unpleasant mixture component complicates the development of an efficient mask because a pleasant perception is an important requirement for malodor coverage.
Highlights
We are rarely surrounded by pure olfactory or trigeminal stimuli but rather by bimodal olfactory–trigeminal mixtures
We focused on the interaction effect “stimulus” × “target” to assess whether intensity differences in the eucalyptol component between pure and mixed conditions reached significance compared to differences in the ammonia component between pure and mixed conditions
Pairwise comparisons showed that perceived intensity depended on both what was presented and what was rated: Eucalyptol component intensity did not differ between pure eucalyptol and mixture while ammonia component intensity was perceived as significantly less intense in the mixture compared to pure ammonia
Summary
We are rarely surrounded by pure olfactory or trigeminal stimuli but rather by bimodal olfactory–trigeminal mixtures. In our investigation of an efficient tool for malodor coverage, we, focus on masking behavior within a mixture consisting of two bimodal components. We proved that eucalyptol is an effective mask to cover an aversive odor: the fresh smell of eucalyptol masked the urine-like smell of ammonia, indicating potential industry applications such as improving the smell of hair dye or used animal litter. The trigeminal sensation of ammonia could not be reduced by eucalyptol, instead, the trigeminal sensation of the mixture was enhanced (Müschenich et al 2019). We repeated the experiment and focused on the underlying neural processing using fMRI.
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