Abstract
Propylene glycol, also denoted as 1.2 propanediol (C3H8O2), often serves as a solvent for dilution of olfactory stimuli. It is supposed to serve as a neutral substance and has been used in many behavioral and electrophysiological studies to dilute pure olfactory stimuli. However, the effect of propylene glycol on perception and on neuronal responses has hitherto never been studied. In this study we tested by means of a threshold test, whether a nasal propylene glycol stimulation is recognizable by humans. Participants were able to recognize propylene glycol at a threshold of 42% concentration and reported a slight cooling effect. In addition to the threshold test, we recorded electroencephalography (EEG) during nasal propylene glycol stimulation to study the neuronal processing of the stimulus. We used a flow olfactometer and stimulated 15 volunteers with three different concentrations of propylene glycol (40 trials each) and water as a control condition (40 trials). To evaluate the neuronal response, we analyzed the event-related potentials (ERPs) and power modulations. The task of the volunteers was to identify a change (olfactory, thermal, or tactile) in the continuous air flow generated by the flow olfactometer. The analysis of the ERPs showed that propylene glycol generates a clear P2 component, which was also visible in the frequency domain as an evoked power response in the theta-band. The source analysis of the P2 revealed a widespread involvement of brain regions, including the postcentral gyrus, the insula and adjacent operculum, the thalamus, and the cerebellum. Thus, it is possible that trigeminal stimulation can at least partly account for sensations and brain responses elicited by propylene glycol. Based on these results, we conclude that the use of high propylene glycol concentrations to dilute fragrances complicates the interpretation of presumed purely olfactory effects.
Highlights
The human nasal mucosa contains a variety of sensory receptors and nerve endings enabling it to respond to odors, and to tactile, thermal, and noxious stimuli
The development of tangible flow-olfactometers enabled the controlled stimulation of the nasal cavity time-locked to electroencephalography (EEG) recordings and opened a way to study the neural processing of chemical stimulation of the olfactory and trigeminal senses using the methodology of (ERPs; Kobal and Plattig, 1978)
Pure propylene glycol was detected with a high probability
Summary
The human nasal mucosa contains a variety of sensory receptors and nerve endings enabling it to respond to odors, and to tactile, thermal, and noxious stimuli. These additional sensations arise from activation of the trigeminal nerve, which divides into three main branches two of which innervate the nasal cavity. The development of tangible flow-olfactometers enabled the controlled stimulation of the nasal cavity time-locked to electroencephalography (EEG) recordings and opened a way to study the neural processing of chemical stimulation of the olfactory and trigeminal senses using the methodology of (ERPs; Kobal and Plattig, 1978). Source analyses of ERPs showed that the neuronal responses in both modalities clearly differ at early stages of processing (
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