Techniques for Obtaining High-quality Recordings in Electrocochleography

Human Olivocochlear Effects: A Statistical Detection Approach Applied to the Cochlear Microphonic Evoked by Swept Tones

The human medial olivocochlear (MOC) reflex was assessed by observing the effects of contralateral acoustic stimulation (CAS) on the cochlear microphonic (CM) across a range of probe frequencies. A frequency-swept probe tone (125-4757 Hz, 90 dB SPL) was presented in two directions (up sweep and down sweep) to normal-hearing young adults. This study assessed MOC effects on the CM in individual participants using a statistical approach that calculated minimum detectable changes in magnitude and phase based on CM signal-to-noise ratio (SNR). Significant increases in CM magnitude, typically 1-2 dB in size, were observed for most participants from 354 to 1414 Hz, where the size and consistency of these effects depended on participant, probe frequency, sweep direction, and SNR. CAS-related phase lags were also observed, consistent with CM-based MOC studies in laboratory animals. Observed effects on CM magnitude and phase were in the opposite directions of reported effects on otoacoustic emissions (OAEs). OAEs are sensitive to changes in the motility of outer hair cells located near the peak region of the traveling wave, while the effects of CAS on the CM likely originate from MOC-related changes in the conductance of outer hair cells located in the basal tail of the traveling wave. Thus, MOC effects on the CM are complementary to those observed for OAEs.

Effects of Tympanic Membrane Electrodes on Sound Transmission From the Ear Canal to the Middle and Inner Ears

OBJECTIVES

The first objective of the study was to compare approaches to eardrum electrode insertion as they relate to the likelihood of introducing an acoustic leak between the ear canal and eartip. A common method for placing a tympanic membrane electrode involves securing the electrode in the canal by routing it underneath a foam eartip. This method is hypothesized to result in a slit leak between the canal and foam tip due to the added bulk of the electrode wire. An alternative approach involves creating a bore in the wall of the foam tip that the electrode can be threaded through. This method is hypothesized to reduce the likelihood of a slit leak before the electrode wire is integrated into the foam tip. The second objective of the study was to investigate how sound transmission in the ear is affected by placing an electrode on the eardrum. It was hypothesized that an electrode in contact with the eardrum increases the eardrum's mass, with the potential to reduce sound transmission at high frequencies.

DESIGN

Wideband acoustic immittance and distortion product otoacoustic emissions (DPOAEs) were measured in eight human ears. Measurements were completed for five different conditions: (1) baseline with no electrode in the canal, (2) dry electrode in the canal but not touching the eardrum, secured underneath the eartip, (3) dry electrode in the canal not touching the eardrum, secured through a bore in the eartip (subsequent conditions were completed using this method), (4) hydrated electrode in the canal but not touching the eardrum, and (5) hydrated electrode touching the eardrum. To create the bore, a technique was developed in which a needle is heated and pushed through the foam eartip. The electrode is then thread through the bore and advanced slowly by hand until contacting the eardrum. Analysis included comparing absorbance, admittance phase angle, and DPOAE levels between measurement conditions.

RESULTS

Comparison of the absorbance and admittance phase angle measurements between the electrode placement methods revealed significantly higher absorbance and lower admittance phase angle from 0.125 to 1 kHz when the electrode is routed under the eartip. Absorbance and admittance phase angle were minimally affected when the electrode was inserted through a bore in the eartip. DPOAE levels across the different conditions showed changes approximating test-retest variability. Upon contacting the eardrum, the absorbance tended to decrease below 1 kHz and increase above 1 kHz. However, changes were within the range of test-retest variability. There was evidence of reduced levels below 1 kHz and increased levels above 1 kHz upon the electrode contacting the eardrum. However, differences between conditions approximated test-retest variability.

CONCLUSIONS

Routing the eardrum electrode through the foam tip reduces the likelihood of incurring an acoustic leak between the canal walls and eartip, compared with routing the electrode under the eartip. Changes in absorbance and DPOAE levels resulting from electrode contact with the eardrum implicate potential stiffening of eardrum; however, the magnitude of changes suggests minimal effect of the electrode on sound transmission in the ear.

Effects of contralateral noise on envelope-following responses, auditory-nerve compound action potentials, and otoacoustic emissions measured simultaneously

This study assessed whether the effects of contralateral acoustic stimulation (CAS) are consistent with eliciting the medial olivocochlear (MOC) reflex for measurements sensitive to outer hair cell (otoacoustic emissions, OAEs), auditory-nerve (AN; compound action potential, CAP), and brainstem/cortical (envelope-following response, EFR) function. The effects of CAS were evaluated for simultaneous measurement of OAEs, CAPs, and EFRs in participants with normal hearing. Clicks were presented at 40 or 98 Hz in three ipsilateral noise conditions (no noise, 45 dB SPL, and 55 dB SPL). For the no noise condition, CAS suppressed or enhanced EFR amplitudes for 40- and 98-Hz clicks, respectively, while CAS had no significant effect on CAP amplitudes. A follow-up experiment using slower rates (4.4-22.2 Hz) assessed whether this insignificant CAS effect on CAPs was from ipsilateral MOC stimulation or AN adaptation; however, CAS effects remained insignificant despite favorable signal-to-noise ratios. CAS-related enhancements of EFR and CAP amplitudes in ipsilateral noise were not observed, contrary to the anti-masking effect of the MOC reflex. EFR and OAE suppression from CAS were not significantly correlated. Thus, the effects of CAS on EFRs may not be solely mediated by the MOC reflex and may be partially mediated by higher auditory centers.

A Time-Saving Alternative to "Peak-Picking" Algorithms: A Gaussian Mixture Model Feature Extraction Technique for the Neurodiagnostic Auditory Brainstem Response

OBJECTIVES

The accurate and efficient analysis of neurodiagnostic auditory brainstem responses (ABR) plays a critical role in assessing auditory pathway function in human and animal research and in clinical diagnosis. Traditional analysis of the neurodiagnostic ABR analysis involves visual inspection of the waveform and manually marking peaks and troughs. Visual inspection is a tedious and time-consuming task, especially in research where there may be hundreds or thousands of waveforms to analyze. "Peak-picking" algorithms have made this task faster; however, they are prone to the same errors as visual inspection. A Gaussian mixture model-based feature extraction technique (GMM-FET) is a descriptive model of ABR morphology and an alternative to peak-picking algorithms. The GMM-FET is capable of modeling multiple waves and accounting for wave interactions, compared with other template-matching approaches that fit single waves.

DESIGN

The present study is a secondary analysis applying the GMM-FET to 321 ABRs from adult humans from 2 datasets using different stimuli and recording parameters. Goodness-of-fit of the GMM-FET to waves I and V and surrounding waves, that is, the summating potential and waves IV and VI, was assessed, and latency and amplitude estimations by the GMM-FET were compared with estimations from visual inspection.

RESULTS

The GMM-FET had a similar success rate to visual inspection in extracting peak latency and amplitude, and there was low RMS error and high intraclass correlation between the model and response waveform. Mean peak latency differences between the GMM-FET and visual inspection were small, suggesting the two methods chose the same peak in the majority of waveforms. The GMM-FET estimated wave I amplitudes within 0.12 µV of visual inspection, but estimated larger wave V amplitudes than visual inspection.

CONCLUSIONS

The results suggest the GMM-FET is an appropriate method for extracting peak latencies and amplitudes for neurodiagnostic analysis of ABR waves I and V.

Estimation of Cochlear Frequency Selectivity Using a Convolution Model of Forward-Masked Compound Action Potentials

PURPOSE

Frequency selectivity is a fundamental property of the peripheral auditory system; however, the invasiveness of auditory nerve (AN) experiments limits its study in the human ear. Compound action potentials (CAPs) associated with forward masking have been suggested as an alternative to assess cochlear frequency selectivity. Previous methods relied on an empirical comparison of AN and CAP tuning curves in animal models, arguably not taking full advantage of the information contained in forward-masked CAP waveforms.

METHODS

To improve the estimation of cochlear frequency selectivity based on the CAP, we introduce a convolution model to fit forward-masked CAP waveforms. The model generates masking patterns that, when convolved with a unitary response, can predict the masking of the CAP waveform induced by Gaussian noise maskers. Model parameters, including those characterizing frequency selectivity, are fine-tuned by minimizing waveform prediction errors across numerous masking conditions, yielding robust estimates.

RESULTS

The method was applied to click-evoked CAPs at the round window of anesthetized chinchillas using notched-noise maskers with various notch widths and attenuations. The estimated quality factor Q10 as a function of center frequency is shown to closely match the average quality factor obtained from AN fiber tuning curves, without the need for an empirical correction factor.

CONCLUSION

This study establishes a moderately invasive method for estimating cochlear frequency selectivity with potential applicability to other animal species or humans. Beyond the estimation of frequency selectivity, the proposed model proved to be remarkably accurate in fitting forward-masked CAP responses and could be extended to study more complex aspects of cochlear signal processing (e.g., compressive nonlinearities).

Exploring Electrode Placements to Optimize the Identification and Measurement of Early Auditory Evoked Potentials

Cochlear synaptic loss (termed cochlear synaptopathy) has been suggested to contribute to suprathreshold hearing difficulties. However, its existence and putative effects in humans remain inconclusive, largely due to the heterogeneous methods used across studies to indirectly evaluate the health of cochlear synapses. There is a need to standardize proxies of cochlear synaptopathy to appropriately compare and interpret findings across studies. Early auditory evoked potentials (AEPs), including the compound action potential (AP)/Wave I of the auditory brainstem response are a popular proxy, yet remain variable based on technical considerations. This study evaluated one such consideration-electrode array (i.e., montage)-to optimize the use of early AEP waveforms. In 35 young adults, electrocochleography (ECochG) responses were collected using vertical and horizontal montages. Standard ECochG measures and AP/Wave I and Wave II peak-to-trough amplitudes and latencies were compared between montages. Vertical montage recordings consistently produced significantly larger AP/Wave I peak-to-trough amplitudes compared to horizontal recordings. These findings support the use of a vertical electrode montage for optimal recordings of peripheral cochlear nerve activity. As cochlear synaptopathy continues to be explored in humans, the methods highlighted here should be considered in the development of a standardized assessment.

Minimum Detectable Differences in Electrocochleography Measurements: Bayesian-Based Predictions

Physiology of the cochlea and auditory nerve can be assessed with electrocochleography (ECochG), a technique that involves measuring auditory evoked potentials from an electrode placed near or within the cochlea. Research, clinical, and operating room applications of ECochG have in part centered on measuring the auditory nerve compound action potential (AP) amplitude, the summating potential (SP) amplitude, and the ratio of the two (SP/AP). Despite the common use of ECochG, the variability of repeated amplitude measurements for individuals and groups is not well understood. We analyzed ECochG measurements made with a tympanic membrane electrode in a group of younger normal-hearing participants to characterize the within-participant and group-level variability for the AP amplitude, SP amplitude, and SP/AP amplitude ratio. Results show that the measurements have substantial variability and that, especially with smaller sample sizes, significant reduction in variability can be obtained by averaging measurements across repeated electrode placements within subjects. Using a Bayesian-based model of the data, we generated simulated data to predict minimum detectable differences in AP and SP amplitudes for experiments with a given number of participants and repeated measurements. Our findings provide evidence-based recommendations for the design and sample size determination of future experiments using ECochG amplitude measurements, and the evaluation of previous publications in terms of sensitivity to detecting experimental effects on ECochG amplitude measurements. Accounting for the variability of ECochG measurements should result in more consistent results in the clinical and basic assessments of hearing and hearing loss, either hidden or overt.

Computational modeling of the human compound action potential

The auditory nerve (AN) compound action potential (CAP) is an important tool for assessing auditory disorders and monitoring the health of the auditory periphery during surgical procedures. The CAP has been mathematically conceptualized as the convolution of a unit response (UR) waveform with the firing rate of a population of AN fibers. Here, an approach for predicting experimentally recorded CAPs in humans is proposed, which involves the use of human-based computational models to simulate AN activity. CAPs elicited by clicks, chirps, and amplitude-modulated carriers were simulated and compared with empirically recorded CAPs from human subjects. In addition, narrowband CAPs derived from noise-masked clicks and tone bursts were simulated. Many morphological, temporal, and spectral aspects of human CAPs were captured by the simulations for all stimuli tested. These findings support the use of model simulations of the human CAP to refine existing human-based models of the auditory periphery, aid in the design and analysis of auditory experiments, and predict the effects of hearing loss, synaptopathy, and other auditory disorders on the human CAP.

Middle ear muscle and medial olivocochlear activity inferred from individual human ears via cochlear potentials

The peripheral auditory system is influenced by the medial olivocochlear (MOC) and middle ear muscle (MEM) reflexes. When elicited by contralateral acoustic stimulation (CAS), these reflexes reduce cochlear amplification (MOC reflex) and limit low-frequency transmission through the middle ear (MEM reflex). The independent roles of these reflexes on auditory physiology and perception are difficult to distinguish. The amplitude of the cochlear microphonic (CM) is expected to increase or decrease when the MOC and MEM reflexes are elicited by CAS, respectively, which could lead to a straightforward interpretation of what reflex is dominant for a given CAS level. CM and ear canal sound pressure level (SPL) were measured for a 500 Hz, 90 dB SPL probe in the presence of contralateral broadband noise (CBBN) for levels ranging from 45-75 dB SPL. In most subjects, CM amplitude increased for CBBN levels of 45 and 55 dB SPL, while no change in ear canal SPL was observed, consistent with eliciting the MOC reflex. Conversely, CM amplitude decreased, and ear canal SPL increased in the presence of 65 and 75 dB SPL CBBN, consistent with eliciting the MEM reflex. A CM-based test of the MOC reflex may facilitate detection of MEM effects and the assessment of adults with cochlear hearing loss.

Temporal Envelope Coding of the Human Auditory Nerve Inferred from Electrocochleography: Comparison with Envelope Following Responses

Neural coding of the slow amplitude fluctuations of sound (i.e., temporal envelope) is thought to be essential for speech understanding; however, such coding by the human auditory nerve is poorly understood. Here, neural coding of the temporal envelope by the human auditory nerve is inferred from measurements of the compound action potential in response to an amplitude modulated carrier (CAPENV) for modulation frequencies ranging from 20 to 1000 Hz. The envelope following response (EFR) was measured simultaneously with CAPENV from active electrodes placed on the high forehead and tympanic membrane, respectively. Results support the hypothesis that phase locking to higher modulation frequencies (> 80 Hz) will be stronger for CAPENV, compared to EFR, consistent with the upper-frequency limits of phase locking for auditory nerve fibers compared to auditory brainstem/cortex neurons. Future work is needed to determine the extent to which (1) CAPENV is a useful tool for studying how temporal processing of the auditory nerve is affected by aging, hearing loss, and noise-induced cochlear synaptopathy and (2) CAPENV reveals the relationship between auditory nerve temporal processing and perception of the temporal envelope.

Comparison of Glycerol Test, ECochG and VEMP Findings in Patients with Meniere’s Disease

Present study aimed to find if multiple audio-vestibular tests can be used to improve in diagnosing Meniere's disease. The study compares the results of four test procedures of glycerol test, Electrocochleography (ECochG), cervical Vestibular Evoked Myogenic Potential (c-VEMP) and ocular Vestibular Evoked Myogenic Potential (o-VEMP) individually and also together to see whether the diagnosis of Meniere's disease can be improved. Experimental group consist of 16 participants (32 ears) diagnosed with Meniere's disease as per American Academy of Otolaryngology (AAO) -Head and Neck Foundation (2020) guidelines. Control group consist of 16 participants with compliant of only vertigo participated in the study. Participants in both group underwent glycerol test, Electrocochleography (ECochG), cervical Vestibular Evoked Myogenic Potential (c-VEMP) and ocular Vestibular Evoked Myogenic Potential (o-VEMP). Glycerol test showed a 38% positive rate for the presence of Meniere's disease. ECochG indicated the presence of endolymphatic hydrops in 62% of the participants. Abnormal finding in c-VEMP and o-VEMP was seen in 31% and 56% of the participants respectively. The combination of two tests such as ECochG and o-VEMP has increased the positive rate upto 87%. Adding one more test of c-VEMP or combining all four audiovestibular tests had a positive rate of 94% only. The present study indicated ECochG is a more sensitive tool in the diagnosis of Meniere disease followed by o-VEMP. Thus, the present study indicated that combination ECochG and o-VEMP could be used to find the presence of Meniere's disease successfully.

Firing Rate Adaptation of the Human Auditory Nerve Optimizes Neural Signal-to-Noise Ratios

Several physiological mechanisms act on the response of the auditory nerve (AN) during acoustic stimulation, resulting in an adjustment in auditory gain. These mechanisms include-but are not limited to-firing rate adaptation, dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex. A potential role of these mechanisms is to improve the neural signal-to-noise ratio (SNR) at the output of the AN in real time. This study tested the hypothesis that neural SNRs, inferred from non-invasive assessment of the human AN, improve over the duration of acoustic stimulation. Cochlear potentials were measured in response to a series of six high-level clicks embedded in a series of six lower-level broadband noise bursts. This paradigm elicited a compound action potential (CAP) in response to each click and to the onset of each noise burst. The ratio of CAP amplitudes elicited by each click and noise burst pair (i.e., neural SNR) was tracked over the six click/noise bursts. The main finding was a rapid (< 24 ms) increase in neural SNR from the first to the second click/noise burst, consistent with a real-time adjustment in the response of the auditory periphery toward improving the SNR of the signal transmitted to the brainstem. Analysis of cochlear microphonic and ear canal sound pressure recordings, as well as the time course for this improvement in neural SNR, supports the conclusion that firing rate adaptation is likely the primary mechanism responsible for improving neural SNR, while dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex played a secondary role on the effects observed in this study. Real-time improvements in neural SNR are significant because they may be essential for robust encoding of speech and other relevant stimuli in the presence of background noise.

Electric hearing and tinnitus suppression by noninvasive ear stimulation

While noninvasive brain stimulation is convenient and cost effective, its utility is limited by the substantial distance between scalp electrodes and their intended neural targets in the head. The tympanic membrane, or eardrum, is a thin flap of skin deep in an orifice of the head that may serve as a port for improved efficiency of noninvasive stimulation. Here we chose the cochlea as a target because it resides in the densest bone of the skull and is adjacent to many deep-brain-stimulation structures. We also tested the hypothesis that noninvasive electric stimulation of the cochlea may restore neural activities that are missing in acoustic stimulation. We placed an electrode in the ear canal or on the tympanic membrane in 25 human adults (10 females) and compared their stimulation efficiency by characterizing the electrically-evoked auditory sensation. Relative to ear canal stimulation, tympanic membrane stimulation was four times more likely to produce an auditory percept, required eight times lower electric current to reach the threshold and produced two-to-four times more linear suprathreshold responses. We further measured tinnitus suppression in 14 of the 25 subjects who had chronic tinnitus. Compared with ear canal stimulation, tympanic membrane stimulation doubled both the probability (22% vs. 55%) and the amount (-15% vs. -34%) of tinnitus suppression. These findings extended previous work comparing evoked perception and tinnitus suppression between electrodes placed in the ear canal and on the scalp. Together, the previous and present results suggest that the efficiency of conventional scalp-based noninvasive electric stimulation can be improved by at least one order of magnitude via tympanic membrane stimulation. This increased efficiency is most likely due to the shortened distance between the electrode placed on the tympanic membrane and the targeted cochlea. The present findings have implications for the management of tinnitus by offering a potential alternative to interventions using invasive electrical stimulation such as cochlear implantation, or other non-invasive transcranial electrical stimulation methods.