Abstract
BackgroundThe recording of olfactory and trigeminal chemosensory event-related potentials (ERPs) has been proposed as an objective and non-invasive technique to study the cortical processing of odors in humans. Until now, the responses have been characterized mainly using across-trial averaging in the time domain. Unfortunately, chemosensory ERPs, in particular, olfactory ERPs, exhibit a relatively low signal-to-noise ratio. Hence, although the technique is increasingly used in basic research as well as in clinical practice to evaluate people suffering from olfactory disorders, its current clinical relevance remains very limited. Here, we used a time-frequency analysis based on the wavelet transform to reveal EEG responses that are not strictly phase-locked to onset of the chemosensory stimulus. We hypothesized that this approach would significantly enhance the signal-to-noise ratio of the EEG responses to chemosensory stimulation because, as compared to conventional time-domain averaging, (1) it is less sensitive to temporal jitter and (2) it can reveal non phase-locked EEG responses such as event-related synchronization and desynchronization.Methodology/Principal FindingsEEG responses to selective trigeminal and olfactory stimulation were recorded in 11 normosmic subjects. A Morlet wavelet was used to characterize the elicited responses in the time-frequency domain. We found that this approach markedly improved the signal-to-noise ratio of the obtained EEG responses, in particular, following olfactory stimulation. Furthermore, the approach allowed characterizing non phase-locked components that could not be identified using conventional time-domain averaging.Conclusion/SignificanceBy providing a more robust and complete view of how odors are represented in the human brain, our approach could constitute the basis for a robust tool to study olfaction, both for basic research and clinicians.
Highlights
In 1978, Kobal and Plattig [1] introduced a device capable of delivering transient chemosensory stimuli over the olfactory neuroepithelium of the nasal mucosa, without concurrent mechanical and/or thermal stimulation
As shown in the group-level average waveforms displayed in Figure 1, trigeminal stimulation elicited a negative deflection (TRI-N1: 391655 ms) followed by a positive deflection (TRI-P2: 554657 ms)
Contrasting with the relatively high signal-to-noise ratio of the event-related potentials (ERPs) elicited by trigeminal stimulation, olfactory stimulation elicited a clearly identifiable negative (OLF-N1) and positive (OLF-P2) peak in only a few subjects, and neither the measure of OLF-N1 amplitude nor the measure of OLF-P2 amplitude were able to discriminate between the presence vs. absence of an olfactory response
Summary
In 1978, Kobal and Plattig [1] introduced a device capable of delivering transient chemosensory stimuli over the olfactory neuroepithelium of the nasal mucosa, without concurrent mechanical and/or thermal stimulation. The device reported by Kobal and Plattig [1] delivers pulses of odorant embedded within a constant airflow, avoiding any concomitant mechanical stimulation of the nasal mucosa, making it possible to study the brain responses related to the activation of chemosensitive afferents. We used a time-frequency analysis based on the wavelet transform to reveal EEG responses that are not strictly phase-locked to onset of the chemosensory stimulus We hypothesized that this approach would significantly enhance the signal-to-noise ratio of the EEG responses to chemosensory stimulation because, as compared to conventional time-domain averaging, (1) it is less sensitive to temporal jitter and (2) it can reveal non phase-locked EEG responses such as event-related synchronization and desynchronization
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