Short inter-stimulus intervals can be used for olfactory electroencephalography in patients of varying olfactory function

Measurement and Analyses of Olfactory Event-Related Potentials

Olfactory event-related potentials (OERPs) are brain electrical activities time-locked to olfactory stimuli, detectable via scalp electrodes. They offer a noninvasive means to study cortical olfactory processing in humans. Previous research suggests that olfactory cortical processing occurring within several seconds after the onset of an odor can be tracked using OERPs. This enables the investigation of the temporal dynamics of neural activity, spanning from those associated with odor properties to subjects' states. Moreover, OERPs are influenced by diseases, including olfactory, neurological, or psychiatric conditions, and have the potential to serve as biomarkers. This chapter describes the measurement and analysis methods required to obtain OERPs, with a particular focus on odor delivery, aiming to provide a primer for those unfamiliar with OERP measurement.

Effect of physical activity on olfaction acuity: A systematic review

Olfaction acuity, which includes detection thresholds, discrimination and identification, appears to decline with age, obesity, and various neurological disorders. Knowing that smell influences energy intake, there is a growing interest in protecting this sense. Physical activity could be a key intervention to counteract the loss of olfaction. This systematic review aims to explore the literature on the effect of physical activity on olfaction acuity. The search strategy consisted of using index terms and keywords in MEDLINE, EMBASE, EBM Reviews - Cochrane Central Register of Controlled Trials, CINAHL, SPORTDiscus, and Web of Science search engine. Data from 17 trials involving 10,861 participants showed that physical activity improved olfaction thresholds, discrimination, identification and perceived intensity. Regular practice of physical activity seemed to have better effects on olfaction components than acute exercise. Although this review has clarified the evidence on the effects of physical activity on olfaction, better methodological consistency is needed.

[Breath changes induced by short chemosensory stimuli cannot be intentionally suppressed]

INTRODUCTION

Breathing changes induced by repeated short olfactory stimuli are used as an objective indicator of the integrity of the olfactory system. Until now, it has not been investigated whether chemosensorically induced changes in inspiratory and expiratory time parameters can be suppressed intentionally. The same applies to breathing changes due to weak CO₂ stimuli.

METHODOLOGY

34 healthy adult normosmics were stimulated during relaxed regular nasal breathing using a flow olfactometer with nine differently concentrated H₂S and three weak CO₂ stimulation pulses. They were instructed to intentionally maintain regular nasal breathing during the stimulation. A significant breathing change was present if the duration of the inspiration (DIN) or the expiration (DEX) of the first stimulatory breath was outside the double standard deviation of the mean of five prestimulatory regular breaths. These could be shortened or extended the DIN or DEX.

RESULTS

Despite high motivation, the chemosensorically induced breathing changes could not be suppressed intentionally. Rest breathing reacted to both stimulants by changes in both the inspiratory and the expiratory time parameters. However, it outweighed the expiratory reactions. CO₂ evoked more breathing changes than H₂S. The frequency of reaction rate of H₂S stimuli was not concentration-dependent. Strong H₂S stimuli induced more frequent shortening than prolongation of DEX.

CONCLUSIONS

Chemosensorically triggered breathing changes cannot be suppressed intentionally. They therefore provide an additional objective tool to check the functionality of nasal chemosensory afferents.

Erkennungsraten von chemosensorisch evozierten Potenzialen bei gesunden und anosmischen Erwachsenen

Hintergrund Die Detektionsrate von olfaktorischen chemosensorischen ereignisbezogenen EEG-Potenzialen (oCSERP) und trigeminalen chemosensorischen ereignisbezogenen EEG-Potenzialen (tCSERP) ist auch abhängig vom Reizstoff und vom Beurteiler der gemittelten EEG-Kurven.

Methodik Es wurden bei 45 Probanden mit altersentsprechendem Riechvermögen (NP) und 20 Anosmikern (AN) oCSERP (Reizung mit Schwefelwasserstoff, H2S 6 ppm, und Phenylethylalkohol, PEA 30 % v/v) sowie tCSERP (Reizung mit CO2 20–60 % v/v, intensitätsnormiert) standardmäßig abgeleitet. Die gemittelten EEG-Kurven wurden von 2 Untersuchern kategorisiert in: CSERP erkennbar, kein CSERP erkennbar, Artefakt. Untersucher 2 war in Bezug auf die Gruppenzuordnung der Probanden verblindet, Untersucher 1 hingegen nicht.

Ergebnisse AN benötigten im Vergleich zu NP höhere trigeminale Reizkonzentrationen bei gleichem Intensitätsrating. Die Aufmerksamkeit (Trackingspiel) verschlechterte sich im Testverlauf nicht. Die Detektionsrate von H2S-CSERP war höher als die von CO2-CSERP und PEA-CSERP. Die Übereinstimmung der Detektionsraten zwischen beiden Untersuchern bei H2S-CSERP war höher als bei CO2-CSERP und bei PEA-CSERP.

Schlussfolgerung Für die standardmäßige Ableitung von oCSERP reicht gewöhnlich eine Reizung mit H2S. Auf eine Stimulation mit PEA und CO2 wie auch auf einen verblindeten zweiten Untersucher kann verzichtet werden.

Intraoperative monitoring of olfactory function: a feasibility study

OBJECTIVE

Intraoperative neuromonitoring of the chemical senses (smell and taste) has never been performed. The objective of this study was to determine if olfactory-evoked potentials could be obtained intraoperatively under general anesthesia.

METHODS

A standard olfactometer was used in the surgical theater with hydrogen sulfide (4 ppm, 200 msec). Olfactory-evoked potentials were recorded in 8 patients who underwent neurosurgery for resection of cerebral lesions. These patients underwent routine target-controlled propofol and sufentanil general anesthesia. Frontal, temporal, and parietal scalp subdermal electrodes were recorded ipsilaterally and contralaterally at the site of the surgery. Evoked potentials were computed if at least 70 epochs (0.5-100 Hz) satisfying the artifact rejection criterion (threshold 45 μV) could be extracted from signals of electrodes.

RESULTS

Contributive recordings were obtained for 5 of 8 patients (3 patients had fewer than 70 epochs with an amplitude < 45 μV). Olfactory-evoked potentials showed N1 responses (mean 442.8 ± 40.0 msec), most readily observed in the patient who underwent midline anterior fossa neurosurgery. No component of later latencies could be recorded consistently.

CONCLUSIONS

The study confirms that olfactory-evoked potentials can be measured in response to olfactory stimuli under general anesthesia. This demonstrates the feasibility of recording olfactory function intraoperatively and opens the potential for neuromonitoring of olfactory function during neurosurgery.

Electrophysiologic assessment of olfactory and gustatory function

This chapter reviews approaches for assessing human and gustatory function using electrophysiologic methods. Its focus is on changes in electrical signals, including summated generator potentials that occur after nasal or oral stimulation. In the first part of the review, we describe tools available to the clinician for assessing olfactory and nasotrigeminal function, including modern electroencephalography (EEG) analysis of brain responses both in the time domain and in the time-frequency (TF) domain. Particular attention is paid to chemosensory event-related potentials (CSERPs) and their potential use in medical-legal cases. Additionally, we focus on the changes of summated generator potentials from the olfactory and respiratory nasal epithelium that could provide new diagnostic insights. In the second part, we describe gustatory event-related potentials (gCSERPs) obtained using a relatively new computer controlled gustometer. A device for presenting different pulses of electrical current to the tongue is also described, with weaker pulses likely reflecting gCSERPs and stronger ones trigeminal CSERPs. Finally, summated generator potentials from the surface of the tongue during gustatory stimulation are described that may prove useful for examining peripheral taste function.

Separating normosmic and anosmic patients based on entropy evaluation of olfactory event-related potentials

OBJECTIVE

Methods based on electroencephalography (EEG) are used to evaluate brain responses to odors which is challenging due to the relatively low signal-to-noise ratio. This is especially difficult in patients with olfactory loss. In the present study, we aim to establish a method to separate functionally anosmic and normosmic individuals by means of recordings of olfactory event-related potentials (OERP) using an automated tool. Therefore, Shannon entropy was adopted to examine the complexity of the averaged electrophysiological responses.

METHODS

A total of 102 participants received 60 rose-like odorous stimuli at an inter-stimulus interval of 10 s. Olfactory-related brain activity was investigated within three time-windows of equal length; pre-, during-, and post-stimulus.

RESULTS

Based on entropy analysis, patients were correctly diagnosed for anosmia with a 75% success rate.

CONCLUSION

This novel approach can be expected to help clinicians to identify patients with anosmia or patients with early symptoms of neurodegenerative disorders.

SIGNIFICANCE

There is no automated diagnostic tool for anosmic and normosmic patients using OERP. However, detectability of OERP in patients with functional anosmia has been reported to be in the range of 50%.

Structural Plasticity of the Primary and Secondary Olfactory cortices: Increased Gray Matter Volume Following Surgical Treatment for Chronic Rhinosinusitis

Functional plasticity of the adult brain is well established. Recently, the structural counterpart to such plasticity has been suggested by neuroimaging studies showing experience-dependent differences in gray matter (GM) volumes. Within the primary and secondary olfactory cortices, reduced GM volumes have been demonstrated in patients with olfactory loss. However, these cross-sectional studies do not provide causal evidence for GM volume change, and thereby structural plasticity. Disorders of the peripheral olfactory system, such as chronic rhinosinusitis (CRS), provide an ideal model to study GM structural plasticity, given that patients may experience long periods of olfactory impairment, followed by near complete recovery with treatment. We therefore performed a prospective longitudinal study in patients undergoing surgical treatment for CRS. We used voxel-based morphometry (VBM) to investigate GM volume change in 12 patients (M:F = 7:5; 47.2 ± 14.9 years), 3 months post-op. There was a significant improvement in olfactory function according to birhinal psychophysical testing. We performed a voxel-wise region of interest analysis, with significance corrected for number of regions (p < 0.0036_corr). We found significantly increased post-operative GM volumes within the primary (left piriform cortex, right amygdala) and secondary (right orbitofrontal cortex, caudate nucleus, hippocampal-parahippocampal complex and bilateral temporal poles) olfactory networks, and decreased GM volumes within the secondary network only (left caudate nucleus and temporal pole, bilateral hippocampal-parahippocampal complex). As a control measure, we assessed GM change within V1, S1 and A1, where there were no suprathreshold voxels. To our knowledge, this is the first study to demonstrate GM structural plasticity within the primary and secondary olfactory cortices, following restoration of olfaction.