Signal-to-noise ratio of chemosensory event-related potentials

doi:10.1016/j.clinph.2006.11.004

Clinical Neurophysiology

Volume 118, Issue 3, March 2007, Pages 690-695

https://doi.org/10.1016/j.clinph.2006.11.004 Get rights and content

Abstract

Objective

We investigated the influence of the number of stimuli on signal-to-noise (S/N) ratio of CSERP.

Methods

CSERP from 20 normosmic subjects were obtained in response to stimulation with two olfactory (H₂S and PEA) and a trigeminal (CO₂) stimulant. For each of these odors, 160 stimuli were delivered into the right nostril (duration 200 ms, mean ISI 30 s) using a constant-flow, air-dilution olfactometer. For each EEG recording site (Fz, Cz, Pz, C3, C4), peak-to-peak amplitude N1P2 and noise amplitude levels were determined. Subsequently, S/N ratios were calculated.

Results

The S/N ratios for olfactory ERP generally improved for H₂S and PEA. For responses to PEA, S/N ratios increased significantly up to 80 averages (S/N ratio = 5.6). The number of stimuli for an optimal S/N ratio for trigeminal ERP was slightly lower, i.e. 60 averages (S/N ratio = 7.9).

Conclusions

S/N N1P2 ratios in olfactory and trigeminal ERP significantly improve with an increasing number of responses averaged under these experimental conditions. This is mainly due to a reduction of noise level. Applying more stimuli has little additional effect on S/N ratio due to a concomitant decrease in signal amplitude.

Significance

An optimal S/N ratio is essential when recording CSERP in neurodegenerative disorders, where responses may be of low amplitude, and for medico-legal purposes.

Introduction

In 1966, Finkenzeller, and in 1967, Allison and Goff first described cerebral potentials, which they assumed to be of olfactory origin. Measurement of chemosensory event-related potentials (CSERP) has since become a useful method to quantify olfactory function in a manner relatively independent of subjective biases (for review see (Hummel and Kobal, 2001). Despite this long-standing use, methods for appropriate stimulation are still under debate, as olfactory ERP (OERP) components are affected by the same factors that influence ERP in other modalities, such as variations in interstimulus interval (ISI), stimulus duration, stimulus concentration, and type of stimulus (Covington et al., 1996, Hummel and Kobal, 1999, Murphy et al., 2000, Geisler and Murphy, 2000, Frasnelli et al., 2003).

Since ERP reflect cognitive processing, attention has a major influence on their appearance (Krauel et al., 1998). Most subjects have difficulty maintaining vigilance and attention during long test sessions. Experiments should therefore not be excessively lengthy. Other than by choosing short ISIs, this can be achieved by minimizing the number of stimuli. However, little is known about the influence of the number of stimuli on CSERP latency, amplitude, and signal-to-noise ratio. According to previous research (Covington et al., 1996, Hummel et al., 2000), the absolute minimum number of averages per ERP is 8 records. Although this number of stimuli may produce meaningful results, there tends to be a high noise level. So far, this issue has not been investigated systematically.

The aim of the present study was to determine the number of stimuli that is required to obtain an optimal signal-to-noise ratio in recording of the general amplitude of the CSERP.

Section snippets

Subjects

Twenty subjects (11 male, 9 female, aged between 15 and 35 years, mean age 23.9 years), recruited from the University of Dresden Medical School, were included in this study. Only subjects with normal olfactory function, as determined by administration of the odor identification part of the “Sniffin’ Sticks” test battery (Hummel et al., 1997, Kobal et al., 2000), were included. Subjects provided written informed consent. The study was approved by the Ethics Committee of the University of Dresden

Chemosensory event-related potentials

Descriptive statistics for results obtained at position Cz are shown in Table 1 (see also Fig. 1).

Trigeminal stimulation with CO₂

For A-N1P2 a significant effect of the factor “sequence” was found (F[7, 98] = 16.3, p < 0.001; η² = 0.54) suggesting that the amplitude decreased with the number of averages. Similar observations were made for the general noise level (F[7, 98] = 33.6, p < 0.001; η² = 0.71). Specifically, noise levels for averages over trials 1–20 and 1–40 were significantly higher compared to all other averages, except for the

Discussion

The present study revealed that the S/N ratio of olfactory ERP significantly improves with an increasing number of stimuli. For PEA the optimal number of stimuli was found to be approximately 80. For H₂S, a significant effect of averaging was found, indicating an increase of S/N ratio for repeated averaging. However, no significant differences were found after post hoc testing, probably because Bonferroni correction is too conservative an adjustment for this analysis. The optimal number of

Conclusions

The results of the present study highlight the importance of using a sufficient number of stimuli when recording CSERP, which can be of particular value in patients with (neurological) disorders associated with olfactory impairments, such as Parkinson’s disease, where responses might be reduced in amplitude. Moreover, these results will also have an effect on the practical conduct of medico-legal investigations in patients with olfactory loss where measures of utmost reliability are required.

Acknowledgements

This research was funded partly by Philip Morris USA Inc., and the Dutch Parkinson’s Disease Association. We thank Johannes Frasnelli, Jens Reden, and Benno Schuster for their valuable technical assistance.

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Signal-to-noise ratio of chemosensory event-related potentials

Abstract

Objective

Methods

Results

Conclusions

Significance

Introduction

Section snippets

Subjects

Chemosensory event-related potentials

Trigeminal stimulation with CO2

Discussion

Conclusions

Acknowledgements

Int J Psychophysiol

Int J Psychophysiol

Electroencephalogr Clin Neurophysiol

Brain Res

Int J Psychophysiol

Int J Psychophysiol

Biol Psychol

Electroencephalogr Clin Neurophysiol

Visuell evozierte Potentiale (VEP) und Elektroretinogramm (ERG)

Optimal number of trials to obtain a reliable olfactory event-related potential

Chem Senses

Trigeminal stimulation with CO₂