The auditory nerve overlapped waveform (ANOW) originates in the cochlear apex

The Origin Along the Cochlea of Otoacoustic Emissions Evoked by Mid-Frequency Tone Pips

PURPOSE

Tone-pip-evoked otoacoustic emissions (PEOAEs) are transient-evoked otoacoustic emissions (OAEs) that are hypothesized to originate from reflection of energy near the best-frequency (BF) cochlear place of the stimulus frequency. However, individual PEOAEs have energy with a wide range of delays. We sought to determine whether some PEOAE energy is consistent with having been generated far from BF.

METHODS

PEOAEs from 35 and 47 dB SPL tone pips were obtained by removing pip-stimulus energy by subtracting the ear-canal sound pressure from scaled-down 59 dB SPL tone pips (which evoke relatively small OAEs). PEOAE delays were measured at each peak in the PEOAE absolute-value waveforms. While measuring PEOAEs and auditory-nerve compound action potentials (CAPs), amplification was blocked sequentially from apex to base by cochlear salicylate perfusion. The perfusion time when a CAP was reduced identified when the perfusion reached the tone-pip BF place. The perfusion times when each PEOAE peak was reduced identified where along the cochlea it received cochlear amplification. PEOAEs and CAPs were measured simultaneously using one pip frequency in each ear (1.4 to 4 kHz across 16 ears).

RESULTS

Most PEOAE peaks received amplification primarily between the BF place and 1-2 octaves basal of the BF place. PEOAE peaks with short delays received amplification basal of BF place. PEOAE peaks with longer delays sometimes received amplification apical of BF place, consistent with previous stimulus-frequency-OAE results.

CONCLUSION

PEOAEs provide information about cochlear amplification primarily within ~ 1.5 octave of the tone-pip BF place, not about regions > 3 octaves basal of BF.

A Guinea Pig Model Suggests That Objective Assessment of Acoustic Hearing Preservation in Human Ears With Cochlear Implants Is Confounded by Shifts in the Spatial Origin of Acoustically Evoked Potential Measurements Along the Cochlear Length

OBJECTIVES

Our recent empirical findings have shown that the auditory nerve compound action potential (CAP) evoked by a low-level tone burst originates from a narrow cochlear region tuned to the tone burst frequency. At moderate to high sound levels, the origins shift to the most sensitive audiometric regions rather than the extended high-frequency regions of the cochlear base. This means that measurements evoked from extended high-frequency sound stimuli can shift toward the apex with increasing level. Here we translate this study to understand the spatial origin of acoustically evoked responses from ears that receive cochlear implants, an emerging area of research and clinical practice that is not completely understood. An essential step is to first understand the influence of the cochlear implant in otherwise naive ears. Our objective was to understand how function of the high-frequency cochlear base, which can be excited by the intense low-frequency sounds that are frequently used for objective intra- and postoperative monitoring, can be influenced by the presence of the cochlear implant.

DESIGN

We acoustically evoked responses and made measurements with an electrode placed near the guinea pig round window. The cochlear implant was not utilized for either electrical stimulation or recording purposes. With the cochlear implant in situ, CAPs were acoustically evoked from 2 to 16 kHz tone bursts of various levels while utilizing the slow perfusion of a kainic acid solution from the cochlear apex to the cochlear aqueduct in the base, which sequentially reduced neural responses from finely spaced cochlear frequency regions. This cochlear perfusion technique reveals the spatial origin of evoked potential measurements and provides insight on what influence the presence of an implant has on acoustical hearing.

RESULTS

Threshold measurements at 3 to 11 kHz were elevated by implantation. In an individual ear, thresholds were elevated and lowered as cochlear implant was respectively inserted and removed, indicative of "conductive hearing loss" induced by the implant. The maximum threshold elevation occurred at most sensitive region of the naive guinea pig ear (33.66 dB at 8 kHz), making 11 kHz the most sensitive region to acoustic sounds for guinea pig ears with cochlear implants. Conversely, the acute implantation did not affect the low-frequency, 500 Hz thresholds and suprathreshold function, as shown by the auditory nerve overlapped waveform. As the sound pressure level of the tone bursts increased, mean data show that the spatial origin of CAPs along the cochlear length shifted toward the most sensitive cochlear region of implanted ears, not the extended high-frequency cochlear regions. However, data from individual ears showed that after implantation, measurements from moderate to high sound pressure levels originate in places that are unique to each ear.

CONCLUSIONS

Alterations to function of the cochlear base from the in situ cochlear implant may influence objective measurements of implanted ears that are frequently made with intense low-frequency sound stimuli. Our results from guinea pigs advance the interpretation of measurements used to understand how and when residual acoustic hearing is lost in human ears receiving a cochlear implant.

Assessment of cochlear synaptopathy by electrocochleography to low frequencies in a preclinical model and human subjects

Cochlear synaptopathy is the loss of synapses between the inner hair cells and the auditory nerve despite survival of sensory hair cells. The findings of extensive cochlear synaptopathy in animals after moderate noise exposures challenged the long-held view that hair cells are the cochlear elements most sensitive to insults that lead to hearing loss. However, cochlear synaptopathy has been difficult to identify in humans. We applied novel algorithms to determine hair cell and neural contributions to electrocochleographic (ECochG) recordings from the round window of animal and human subjects. Gerbils with normal hearing provided training and test sets for a deep learning algorithm to detect the presence of neural responses to low frequency sounds, and an analytic model was used to quantify the proportion of neural and hair cell contributions to the ECochG response. The capacity to detect cochlear synaptopathy was validated in normal hearing and noise-exposed animals by using neurotoxins to reduce or eliminate the neural contributions. When the analytical methods were applied to human surgical subjects with access to the round window, the neural contribution resembled the partial cochlear synaptopathy present after neurotoxin application in animals. This result demonstrates the presence of viable hair cells not connected to auditory nerve fibers in human subjects with substantial hearing loss and indicates that efforts to regenerate nerve fibers may find a ready cochlear substrate for innervation and resumption of function.

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.

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.

Auditory nerve phase-locked response recorded from normal hearing adults using electrocochleography

OBJECTIVE

The auditory nerve overlapped waveform response (ANOW), a new measure that can be recorded non-invasively from humans, holds promise for providing more accurate assessment of low frequency hearing thresholds than currently used objective measures. This research aims to investigate the robustness and the nature of the ANOW response in humans.

DESIGN

Repeated within-session recordings of the ANOW response using low-frequency Tone Bursts (TBs) were obtained at multiple stimulus levels. ANOW's absolute amplitude and phase locking value (PLV) measures were analysed to obtain normative data and to test the reliability of the ANOW response.

STUDY SAMPLE

Thirteen normal hearing adults within the age range of 25 to 40 years.

RESULTS

ANOW response was obtained to both 250 Hz and 500 Hz TBs and was traced down to 30-40 dB nHL. ANOW response showed significantly higher amplitude and stronger phase locking using 250 Hz TB compared to 500 Hz TB. High degree of test retest reliability of the ANOW response was found using 250 Hz TB at presentation levels higher than 40 dB nHL.

CONCLUSIONS

ANOW response is recordable noninvasively using low-frequency TBs and shows higher robustness as the stimulus frequency decreases.

Development and Characteristics of Hearing Loss With the Progression of Endolymphatic Hydrops

OBJECTIVES

This study aims to explore how patients' hearing loss developed with the progression of endolymphatic hydrops and the characteristics of hearing loss at different stages.

MATERIALS AND METHODS

We collected 73 patients with definite or possible unilateral Meniere's disease or sudden hearing loss who underwent magnetic resonance imaging after intravenous contrast agent injection. There were 25 cases of isolated cochlear hydrops, 24 cases of isolated vestibular hydrops, and 24 cases of cochlear and vestibular hydrops. Primary outcome analyses included their evaluation of endolymphatic hydrops and hearing thresholds at low and high frequencies.

RESULTS

The overall hearing threshold of patients with vestibular and cochlear hydrops was significantly higher than that of patients with isolated cochlear hydrops and patients with isolated vestibular hydrops. There was a significant correlation between low-frequency hearing loss and cochlear hydrops, and the low-frequency hearing threshold was proportional to the grade of cochlear hydrops. At low frequency, the hearing threshold of patients with isolated vestibular hydrops was lower than that of patients with isolated cochlear hydrops and patients with both cochlear and vestibular hydrops. The audiogram configurations of patients with isolated cochlear hydrops consist largely of flat type and up-sloping type. The audiogram configurations of patients with isolated vestibular hydrops and patients with both cochlear and vestibular hydrops are mainly flat type and down-sloping type.

CONCLUSIONS

Patients present with low-frequency hearing loss in the early stage of endolymphatic hydrops. When the hydrops involves the whole cochlea and vestibule, the patients' hearing is impaired at both low and high frequencies.

Medial Olivocochlear Reflex Effect on Cochlear Response in Humans: Elicitor Side and Level

BACKGROUND

Understanding the functional differences between crossed and uncrossed medial olivocochlear (MOC) neurons has been of interest to researchers for decades. Previous reports revealed conflicting results about which MOC pathway, crossed or uncrossed, is stronger in humans. Both crossed and uncrossed MOC neurons synapse at the base of the outer hair cells (OHCs) in each ear. OHCs generate the cochlear microphonic, which is a major contributor to the cochlear response (CR) PURPOSE:  The current study investigated the effects of eliciting the crossed and uncrossed MOC reflex (MOCR) on CR in humans with three levels of noise.

RESEARCH DESIGN

Normal-hearing, young adults (n = 16) participated in this study. The CR was recorded using 500 Hz tone-burst stimuli presented at 80 dB nHL. To examine the crossed and uncrossed MOCR, CR was recorded without and with continuous ipsilateral or contralateral broadband noise (BBN) at three levels (40, 50, and 60 dB SPL).

DATA ANALYSIS

Analysis of the CR was completed using the amplitude of the response extracted using fast Fourier transform. Statistical analysis was completed using repeated measures analysis of variance and post-hoc analysis.

RESULTS

Compared with baseline, the presentation of BBN, specifically contralaterally, resulted in CR enhancement with no significant difference as a function of the three BBN levels. Greater enhancement of the CR amplitude was observed with contralateral than ipsilateral BBN elicitor.

CONCLUSIONS

The current findings suggest that a contralateral elicitor of the uncrossed MOC pathway results in a larger CR amplitude enhancement compared with an ipsilateral elicitor of the crossed MOC pathway, regardless of the elicitor level. Eliciting the MOCR appears to modulate the OHCs function. Furthermore, assessing the MOCR with the 500 Hz CR with BBN elicitors at moderate levels should separate its effects (i.e., increase response amplitude) from those associated with the middle ear muscle reflex (i.e., reduce response amplitude).

Measurements From Ears With Endolymphatic Hydrops and 2-Hydroxypropyl-Beta-Cyclodextrin Provide Evidence That Loudness Recruitment Can Have a Cochlear Origin

Background: Loudness recruitment is commonly experienced by patients with putative endolymphatic hydrops. Loudness recruitment is abnormal loudness growth with high-level sounds being perceived as having normal loudness even though hearing thresholds are elevated. The traditional interpretation of recruitment is that cochlear amplification has been reduced. Since the cochlear amplifier acts primarily at low sound levels, an ear with elevated thresholds from reduced cochlear amplification can have normal processing at high sound levels. In humans, recruitment can be studied using perceptual loudness but in animals physiological measurements are used. Recruitment in animal auditory-nerve responses has never been unequivocally demonstrated because the animals used had damage to sensory and neural cells, not solely a reduction of cochlear amplification. Investigators have thus looked for, and found, evidence of recruitment in the auditory central nervous system (CNS). While studies on CNS recruitment are informative, they cannot rule out the traditional interpretation of recruitment originating in the cochlea.

Design: We used techniques that could assess hearing function throughout entire frequency- and dynamic-range of hearing. Measurements were made from two animal models: guinea-pig ears with endolymphatic-sac-ablation surgery to produce endolymphatic hydrops, and naïve guinea-pig ears with cochlear perfusions of 13 mM 2-Hydroxypropyl-Beta-Cyclodextrin (HPBCD) in artificial perilymph. Endolymphatic sac ablation caused low-frequency loss. Animals treated with HPBCD had hearing loss at all frequencies. None of these animals had loss of hair cells or synapses on auditory nerve fibers.

Results: In ears with endolymphatic hydrops and those perfused with HPBCD, auditory-nerve based measurements at low frequencies showed recruitment compared to controls. Recruitment was not found at high frequencies (> 4 kHz) where hearing thresholds were normal in ears with endolymphatic hydrops and elevated in ears treated with HPBCD.

Conclusions: We found compelling evidence of recruitment in auditory-nerve data. Such clear evidence has never been shown before. Our findings suggest that, in patients suspected of having endolymphatic hydrops, loudness recruitment may be a good indication that the associated low-frequency hearing loss originates from a reduction of cochlear amplification, and that measurements of recruitment could be used in differential diagnosis and treatment monitoring of Ménière's disease.

Approaches for ear-targeted delivery systems in neurosensory disorders to avoid chronic hearing loss mediated neurological diseases

BACKGROUND & OBJECTIVE

Hearing loss is a common audio-vestibular-related neurosensory disability of inner ears, in which patients exhibit clinical symptoms of dizziness, gait unsteadiness, and oscillopsia, at an initial stage. While, if such disorders are untreated for a prolonged duration then the progression of disease into a chronic state significantly decreases GABA level as well as an alteration in the neurotransmission of CNS systems. Hence, to control the progression of disease into a chronic state, timely and targeted delivery of the drug into the site of action in the ear is now attracting the interest of neurologists for effective and safe treatment of such disorders. Among delivery systems, owing to small dimension, better penetration, rate-controlled release, higher bioavailability; nanocarriers are preferred to overcome delivery barriers, improvement in residence time, and enhanced the performance of loaded drugs. Subsequently, these carriers also stabilize encapsulated drugs while the opportunity to modify the surface of carriers favors guided direction for site-specific targeting. Conventional routes of drug delivery such as oral. intravenous, and intramuscular are poorer in performance because of inadequate blood supply to the inner ear and limited penetration of blood-inner ear barrier.

CONCLUSION

This review summarized novel aspects of non-invasive and biocompatible nanoparticles-based approaches for targeted delivery of drugs into the cochlea of the ear to reduce the rate, and extent of the emergence of any hearing loss mediated neurological disorders.

Altered mapping of sound frequency to cochlear place in ears with endolymphatic hydrops provide insight into the pitch anomaly of diplacusis

A fundamental property of mammalian hearing is the conversion of sound pressure into a frequency-specific place of maximum vibration along the cochlear length, thereby creating a tonotopic map. The tonotopic map makes possible systematic frequency tuning across auditory-nerve fibers, which enables the brain to use pitch to separate sounds from different environmental sources and process the speech and music that connects us to people and the world. Sometimes a tone has a different pitch in the left and right ears, a perceptual anomaly known as diplacusis. Diplacusis has been attributed to a change in the cochlear frequency-place map, but the hypothesized abnormal cochlear map has never been demonstrated. Here we assess cochlear frequency-place maps in guinea-pig ears with experimentally-induced endolymphatic hydrops, a hallmark of Ménière’s disease. Our findings are consistent with the hypothesis that diplacusis is due to an altered cochlear map. Map changes can lead to altered pitch, but the size of the pitch change is also affected by neural synchrony. Our data show that the cochlear frequency-place map is not fixed but can be altered by endolymphatic hydrops. Map changes should be considered in assessing hearing pathologies and treatments.

Is cochlear synapse loss an origin of low-frequency hearing loss associated with endolymphatic hydrops?

There is a strong association between endolymphatic hydrops and low-frequency hearing loss, but the origin of the hearing loss remains unknown. A reduction in the number of cochlear afferent synapses between inner hair cells and auditory nerve fibres may be the origin of the low-frequency hearing loss, but this hypothesis has not been directly tested in humans or animals. In humans, measurements of hearing loss and postmortem temporal-bone based measurements of endolymphatic hydrops are generally separated by large amounts of time. In animals, there has not been a good objective, physiologic, and minimally invasive measurement of low-frequency hearing. We overcame this obstacle with the combined use of a reliable surgical approach to ablate the endolymphatic sac in guinea pigs and create endolymphatic hydrops, the Auditory Nerve Overlapped Waveform to measure low-frequency hearing loss (≤ 1 kHz), and immunohistofluorescence-based confocal microscopy to count cochlear synapses. Results showed low- and mid-(1-4 kHz) frequency hearing loss at all postoperative days, 1, 4, and 30. There was no statistically significant loss of cochlear synapses, and there was no correlation between synapse loss and hearing function. We conclude that cochlear afferent synaptic loss is not the origin of the low-frequency hearing loss in the early days following endolymphatic sac ablation. Understanding what is, and is not, the origin of a hearing loss can help guide preventative and therapeutic development.

Bottom-up and top-down neural signatures of disordered multi-talker speech perception in adults with normal hearing

In social settings, speech waveforms from nearby speakers mix together in our ear canals. Normally, the brain unmixes the attended speech stream from the chorus of background speakers using a combination of fast temporal processing and cognitive active listening mechanisms. Of >100,000 patient records,~10% of adults visited our clinic because of reduced hearing, only to learn that their hearing was clinically normal and should not cause communication difficulties. We found that multi-talker speech intelligibility thresholds varied widely in normal hearing adults, but could be predicted from neural phase-locking to frequency modulation (FM) cues measured with ear canal EEG recordings. Combining neural temporal fine structure processing, pupil-indexed listening effort, and behavioral FM thresholds accounted for 78% of the variability in multi-talker speech intelligibility. The disordered bottom-up and top-down markers of poor multi-talker speech perception identified here could inform the design of next-generation clinical tests for hidden hearing disorders.

Early Detection of Endolymphatic Hydrops using the Auditory Nerve Overlapped Waveform (ANOW)

Endolymphatic hydrops is associated with low-frequency sensorineural hearing loss, with a large body of research dedicated to examining its putative causal role in low-frequency hearing loss. Investigations have been thwarted by the fact that hearing loss is measured in intact ears, but gold standard assessments of endolymphatic hydrops are made postmortem only; and that no objective low-frequency hearing measure has existed. Yet the association of endolymphatic hydrops with low-frequency hearing loss is so strong that it has been established as one of the important defining features for Ménière's disease, rendering it critical to detect endolymphatic hydrops early, regardless of whether it serves a causal role or is the result of other disease mechanisms. We surgically induced endolymphatic hydrops in guinea pigs and employed our recently developed objective neural measure of low-frequency hearing, the Auditory Nerve Overlapped Waveform (ANOW). Hearing loss and endolymphatic hydrops were assessed at various time points after surgery. The ANOW detected low-frequency hearing loss as early as the first day after surgery, well before endolymphatic hydrops was found histologically. The ANOW detected low-frequency hearing loss with perfect sensitivity and specificity in all ears after endolymphatic hydrops developed, where there was a strong linear relationship between degree of endolymphatic hydrops and severity of low-frequency hearing loss. Further, histological data demonstrated that endolymphatic hydrops is seen first in the high-frequency cochlear base, though the ANOW demonstrated that dysfunction begins in the low-frequency apical cochlear half. The results lay the groundwork for future investigations of the causal role of endolymphatic hydrops in low-frequency hearing loss.

The Spatial Origins of Cochlear Amplification Assessed by Stimulus-Frequency Otoacoustic Emissions

Cochlear amplification of basilar membrane traveling waves is thought to occur between a tone's characteristic frequency (CF) place and within one octave basal of the CF. Evidence for this view comes only from the cochlear base. Stimulus-frequency otoacoustic emissions (SFOAEs) provide a noninvasive alternative to direct measurements of cochlear motion that can be measured across a wide range of CF regions. Coherent reflection theory indicates that SFOAEs arise mostly from the peak region of the traveling wave, but several studies using far-basal suppressor tones claimed that SFOAE components originate many octaves basal of CF. We measured SFOAEs while perfusing guinea pig cochleas from apex to base with salicylate or KCl solutions that reduced outer-hair-cell function and SFOAE amplification. Solution effects on inner hair cells reduced auditory nerve compound action potentials (CAPs) and provided reference times for when solutions reached the SFOAE-frequency CF region. As solution flowed from apex to base, SFOAE reductions generally occurred later than CAP reductions and showed that the effects of cochlear amplification usually peaked ∼1/2 octave basal of the CF region. For tones ≥2 kHz, cochlear amplification typically extended ∼1.5 octaves basal of CF, and the data are consistent with coherent reflection theory. SFOAE amplification did not extend to the basal end of the cochlea, even though reticular lamina motion is amplified in this region, which indicates that reticular lamina motion is not directly coupled to basilar membrane traveling waves. Previous reports of SFOAE-frequency residuals produced by suppressor frequencies far above the SFOAE frequency are most likely due to additional sources created by the suppressor. For some tones <2 kHz, SFOAE amplification extended two octaves apical of CF, which highlights that different vibratory motions produce SFOAEs and CAPs, and that the amplification region depends on the cochlear mode of motion considered. The concept that there is a single "cochlear amplification region" needs to be revised.

Residual Cochlear Function in Adults and Children Receiving Cochlear Implants: Correlations With Speech Perception Outcomes

OBJECTIVES

Variability in speech perception outcomes with cochlear implants remains largely unexplained. Recently, electrocochleography, or measurements of cochlear potentials in response to sound, has been used to assess residual cochlear function at the time of implantation. Our objective was to characterize the potentials recorded preimplantation in subjects of all ages, and evaluate the relationship between the responses, including a subjective estimate of neural activity, and speech perception outcomes.

DESIGN

Electrocochleography was recorded in a prospective cohort of 284 candidates for cochlear implant at University of North Carolina (10 months to 88 years of ages). Measurement of residual cochlear function called the "total response" (TR), which is the sum of magnitudes of spectral components in response to tones of different stimulus frequencies, was obtained for each subject. The TR was then related to results on age-appropriate monosyllabic word score tests presented in quiet. In addition to the TR, the electrocochleography results were also assessed for neural activity in the forms of the compound action potential and auditory nerve neurophonic.

RESULTS

The TR magnitude ranged from a barely detectable response of about 0.02 µV to more than 100 µV. In adults (18 to 79 years old), the TR accounted for 46% of variability in speech perception outcome by linear regression (r = 0.46; p < 0.001). In children between 6 and 17 years old, the variability accounted for was 36% (p < 0.001). In younger children, the TR accounted for less of the variability, 15% (p = 0.012). Subjects over 80 years old tended to perform worse for a given TR than younger adults at the 6-month testing interval. The subjectively assessed neural activity did not increase the information compared with the TR alone, which is primarily composed of the cochlear microphonic produced by hair cells.

CONCLUSIONS

The status of the auditory periphery, particularly of hair cells rather than neural activity, accounts for a large fraction of variability in speech perception outcomes in adults and older children. In younger children, the relationship is weaker, and the elderly differ from other adults. This simple measurement can be applied with high throughput so that peripheral status can be assessed to help manage patient expectations, create individually-tailored treatment plans, and identify subjects performing below expectations based on residual cochlear function.

An analytic approach to identifying the sources of the low-frequency round window cochlear response

The cochlear microphonic, traditionally thought of as an indication of electrical current flow through hair cells, in conjunction with suppressing high-pass noise or tones, is a promising method of assessing the health of outer hair cells at specific locations along the cochlear partition. We propose that the electrical potential recorded from the round window in gerbils in response to low-frequency tones, which we call cochlear response (CR), contains significant responses from multiple cellular sources, which may expand its diagnostic purview. In this study, CR is measured in the gerbil and modeled to identify its contributing sources. CR was recorded via an electrode placed in the round window niche of sixteen Mongolian gerbils and elicited with a 45 Hz tone burst embedded in 18 high-pass filtered noise conditions to target responses from increasing regions along the cochlear partition. Possible sources were modeled using previously-published hair cell and auditory nerve response data, and then weighted and combined using linear regression to produce a model response that fits closely to the mean CR waveform. The significant contributing sources identified by the model are outer hair cells, inner hair cells, and the auditory nerve. We conclude that the low-frequency CR contains contributions from several cellular sources.

Cochlear compound action potentials from high-level tone bursts originate from wide cochlear regions that are offset toward the most sensitive cochlear region

Little is known about the spatial origins of auditory nerve (AN) compound action potentials (CAPs) evoked by moderate to intense sounds. We studied the spatial origins of AN CAPs evoked by 2- to 16-kHz tone bursts at several sound levels by slowly injecting kainic acid solution into the cochlear apex of anesthetized guinea pigs. As the solution flowed from apex to base, it sequentially reduced CAP responses from low- to high-frequency cochlear regions. The times at which CAPs were reduced, combined with the cochlear location traversed by the solution at that time, showed the cochlear origin of the removed CAP component. For low-level tone bursts, the CAP origin along the cochlea was centered at the characteristic frequency (CF). As sound level increased, the CAP center shifted basally for low-frequency tone bursts but apically for high-frequency tone bursts. The apical shift was surprising because it is opposite the shift expected from AN tuning curve and basilar membrane motion asymmetries. For almost all high-level tone bursts, CAP spatial origins extended over 2 octaves along the cochlea. Surprisingly, CAPs evoked by high-level low-frequency (including 2 kHz) tone bursts showed little CAP contribution from CF regions ≤ 2 kHz. Our results can be mostly explained by spectral splatter from the tone-burst rise times, excitation in AN tuning-curve "tails," and asynchronous AN responses to high-level energy ≤ 2 kHz. This is the first time CAP origins have been identified by a spatially specific technique. Our results show the need for revising the interpretation of the cochlear origins of high-level CAPs-ABR wave 1. NEW & NOTEWORTHY Cochlear compound action potentials (CAPs) and auditory brain stem responses (ABRs) are routinely used in laboratories and clinics. They are typically interpreted as arising from the cochlear region tuned to the stimulus frequency. However, as sound level is increased, the cochlear origins of CAPs from tone bursts of all frequencies become very wide and their centers shift toward the most sensitive cochlear region. The standard interpretation of CAPs and ABRs from moderate to intense stimuli needs revision.

In Vivo Electrocochleography in Hybrid Cochlear Implant Users Implicates TMPRSS3 in Spiral Ganglion Function

Cochlear implantation, a surgical method to bypass cochlear hair cells and directly stimulate the spiral ganglion, is the standard treatment for severe-to-profound hearing loss. Changes in cochlear implant electrode array design and surgical approach now allow for preservation of acoustic hearing in the implanted ear. Electrocochleography (ECochG) was performed in eight hearing preservation subjects to assess hair cell and neural function and elucidate underlying genetic hearing loss. Three subjects had pathogenic variants in TMPRSS3 and five had pathogenic variants in genes known to affect the cochlear sensory partition. The mechanism by which variants in TMPRSS3 cause genetic hearing loss is unknown. We used a 500-Hz tone burst to record ECochG responses from an intracochlear electrode. Responses consist of a cochlear microphonic (hair cell) and an auditory nerve neurophonic. Cochlear microphonics did not differ between groups. Auditory nerve neurophonics were smaller, on average, in subjects with TMPRSS3 deafness. Results of this proof-of-concept study provide evidence that pathogenic variants in TMPRSS3 may impact function of the spiral ganglion. While ECochG as a clinical and research tool has been around for decades, this study illustrates a new application of ECochG in the study of genetic hearing and deafness in vivo.

Behavioral Pure-Tone Threshold Shifts Caused by Tympanic Membrane Electrodes

OBJECTIVE

To determine whether tympanic membrane (TM) electrodes induce behavioral pure-tone threshold shifts.

DESIGN

Pure-tone thresholds (250 to 8000 Hz) were measured twice in test (n = 18) and control (n = 10) groups. TM electrodes were placed between first and second threshold measurements in the test group, whereas the control group did not receive electrodes. Pure-tone threshold shifts were compared between groups. The effect of TM electrode contact location on threshold shifts was evaluated in the test group.

RESULTS

TM electrodes significantly increased average low-frequency thresholds, 7.5 dB at 250 Hz and 4.2 dB at 500 Hz, and shifts were as large as 25 dB in individual ears. Also, threshold shifts did not appear to vary at any frequency with TM electrode contact location.

CONCLUSIONS

Low-frequency threshold shifts occur when using TM electrodes and insert earphones. These findings are relevant to interpreting electrocochleographic responses to low-frequency stimuli.

A Model-Based Approach for Separating the Cochlear Microphonic from the Auditory Nerve Neurophonic in the Ongoing Response Using Electrocochleography

Electrocochleography (ECochG) is a potential clinically valuable technique for predicting speech perception outcomes in cochlear implant (CI) recipients, among other uses. Current analysis is limited by an inability to quantify hair cell and neural contributions which are mixed in the ongoing part of the response to low frequency tones. Here, we used a model based on source properties to account for recorded waveform shapes and to separate the combined signal into its components. The model for the cochlear microphonic (CM) was a sinusoid with parameters for independent saturation of the peaks and the troughs of the responses. The model for the auditory nerve neurophonic (ANN) was the convolution of a unit potential and population cycle histogram with a parameter for spread of excitation. Phases of the ANN and CM were additional parameters. The average cycle from the ongoing response was the input, and adaptive fitting identified CM and ANN parameters that best reproduced the waveform shape. Test datasets were responses recorded from the round windows of CI recipients, from the round window of gerbils before and after application of neurotoxins, and with simulated signals where each parameter could be manipulated in isolation. Waveforms recorded from 284 CI recipients had a variety of morphologies that the model fit with an average r² of 0.97 ± 0.058 (standard deviation). With simulated signals, small systematic differences between outputs and inputs were seen with some variable combinations, but in general there were limited interactions among the parameters. In gerbils, the CM reported was relatively unaffected by the neurotoxins. In contrast, the ANN was strongly reduced and the reduction was limited to frequencies of 1,000 Hz and lower, consistent with the range of strong neural phase-locking. Across human CI subjects, the ANN contribution was variable, ranging from nearly none to larger than the CM. Development of this model could provide a means to isolate hair cell and neural activity that are mixed in the ongoing response to low-frequency tones. This tool can help characterize the residual physiology across CI subjects, and can be useful in other clinical settings where a description of the cochlear physiology is desirable.

Putative Auditory-Evoked Neurophonic Measurements Using a Novel Signal Processing Technique: A Pilot Case Study

With changes to cochlear implant candidacy and improvements in surgical technique, there is a need for accurate intraoperative assessment of low-frequency hearing thresholds during cochlear implantation. In electrocochleography, onset compound action potentials (CAPs) typically allow estimation of auditory threshold for frequencies above 1 kHz, but they are less accurate at lower frequencies. Auditory nerve neurophonic (ANN) waveforms, on the other hand, may overcome this limitation by allowing phase-locked neural activity to be tracked during a prolonged low-frequency stimulus rather than just at its onset (Henry, 1995). Lichtenhan et al. (2013) have used their auditory nerve overlapped waveform (ANOW) technique to measure these potentials from the round windows of cats and guinea pigs, and reported that in guinea pigs these potentials originate in the cochlear apex for stimuli below 70 dB SPL (Lichtenhan et al., 2014). Human intraoperative round window neurophonic measurements have been reported by Choudhury et al. (2012). We have done the same in hearing impaired awake participants, and present here the results of a pilot study in which we recorded responses evoked by 360, 525, and 725 Hz tone bursts from the cochlear promontory of one participant. We also present a modification to the existing measurement technique which halves recording time, extracting the auditory neurophonic by recording a single averaged waveform, and then subtracting from it a 180° group-delayed version of itself, rather than using alternating condensation and rarefaction sound stimuli. We cannot conclude that the waveforms we measured were purely neural responses originating from the apex of the cochlea: as with all neurophonic measurement procedures, the neural responses of interest cannot be separated from higher harmonics of the cochlear microphonic without forward masking, regardless of electrode location, stimuli or post-processing algorithm. In conclusion, the extraction of putative neurophonic waveforms can easily be incorporated into existing electrocochleographic measurement paradigms, but at this stage such measurements should be interpreted with caution.

Perilymph pharmacokinetics of marker applied through a cochlear implant in guinea pigs

Patients undergoing cochlear implantation could benefit from a simultaneous application of drugs into the ear, helping preserve residual low-frequency hearing and afferent nerve fiber populations. One way to apply drugs is to incorporate a cannula into the implant, through which drug solution is driven. For such an approach, perilymph concentrations achieved and the distribution in the ear over time have not previously been documented. We used FITC-labeled dextran as a marker, delivering it into perilymph of guinea pigs at 10 or 100 nL/min though a cannula incorporated into a cochlear implant with the outlet in the mid basal turn. After injections of varying duration (2 hours, 1 day or 7 days) perilymph was collected from the cochlear apex using a sequential sampling technique, allowing dextran levels and gradients along scala tympani to be quantified. Data were interpreted quantitatively using computer simulations of the experiments. For injections of 2 hours duration, dextran levels were critically influenced by the presence or absence of fluid leakage at the cochleostomy site. When the cochleostomy was fluid-tight, substantially higher perilymph levels were achieved at the injection site, with concentration declining along scala tympani towards the apex. Contrary to expectations, large dextran gradients along scala tympani persisted after 24 hours of sustained injection and were still present in some animals after 7 days injection. Functional changes associated with implantation and dextran delivery, and the histological state of the implant and cannula were also documented. The persistent longitudinal gradients of dextan along the ear were not readily explained by computer simulations of the experiments based on prior pharmacokinetic data. One explanation is that inner ear pharmacokinetics are altered in the period after cochlear implantation, possibly by a permeabilization of the blood-labyrinth barrier as part of the immune response to the implant.

Human Summating Potential Using Continuous Loop Averaging Deconvolution: Response Amplitudes Vary with Tone Burst Repetition Rate and Duration

Electrocochleography (ECochG) to high repetition rate tone bursts may have advantages over ECochG to clicks with standard slow rates. Tone burst stimuli presented at a high repetition rate may enhance summating potential (SP) measurements by reducing neural contributions resulting from neural adaptation to high stimulus repetition rates. To allow for the analysis of the complex ECochG responses to high rates, we deconvolved responses using the Continuous Loop Averaging Deconvolution (CLAD) technique. We examined the effect of high stimulus repetition rate and stimulus duration on SP amplitude measurements made with extratympanic ECochG to tone bursts in 20 adult females with normal hearing. We used 500 and 2,000 Hz tone bursts of various stimulus durations (12, 6, 3 ms) and repetition rates (five rates ranging from 7.1 to 234.38/s). A within-subject repeated measures (rate x duration) analysis of variance was conducted. We found that, for both 500 and 2,000 Hz stimuli, the mean deconvolved SP amplitudes were larger at faster repetition rates (58.59 and 97.66/s) compared to slower repetition rates (7.1 and 19.53/s), and larger at shorter stimulus duration compared longer stimulus duration. Our concluding hypothesis is that large SP amplitude to short duration stimuli may originate primarily from neural excitation, and large SP amplitudes to long duration, fast repetition rate stimuli may originate from hair cell responses. While the hair cell or neural origins of the SP to various stimulus parameters remains to be validated, our results nevertheless provide normative data as a step toward applying the CLAD technique to understanding diseased ears.

Intraoperative Electrocochleographic Characteristics of Auditory Neuropathy Spectrum Disorder in Cochlear Implant Subjects

Auditory neuropathy spectrum disorder (ANSD) is characterized by an apparent discrepancy between measures of cochlear and neural function based on auditory brainstem response (ABR) testing. Clinical indicators of ANSD are a present cochlear microphonic (CM) with small or absent wave V. Many identified ANSD patients have speech impairment severe enough that cochlear implantation (CI) is indicated. To better understand the cochleae identified with ANSD that lead to a CI, we performed intraoperative round window electrocochleography (ECochG) to tone bursts in children (n = 167) and adults (n = 163). Magnitudes of the responses to tones of different frequencies were summed to measure the "total response" (ECochG-TR), a metric often dominated by hair cell activity, and auditory nerve activity was estimated visually from the compound action potential (CAP) and auditory nerve neurophonic (ANN) as a ranked "Nerve Score". Subjects identified as ANSD (45 ears in children, 3 in adults) had higher values of ECochG-TR than adult and pediatric subjects also receiving CIs not identified as ANSD. However, nerve scores of the ANSD group were similar to the other cohorts, although dominated by the ANN to low frequencies more than in the non-ANSD groups. To high frequencies, the common morphology of ANSD cases was a large CM and summating potential, and small or absent CAP. Common morphologies in other groups were either only a CM, or a combination of CM and CAP. These results indicate that responses to high frequencies, derived primarily from hair cells, are the main source of the CM used to evaluate ANSD in the clinical setting. However, the clinical tests do not capture the wide range of neural activity seen to low frequency sounds.

The Auditory Nerve Overlapped Waveform (ANOW) Detects Small Endolymphatic Manipulations That May Go Undetected by Conventional Measurements

Electrocochleography (ECochG) has been used to assess Ménière's disease, a pathology associated with endolymphatic hydrops and low-frequency sensorineural hearing loss. However, the current ECochG techniques are limited for use at high-frequencies only (≥1 kHz) and cannot be used to assess and understand the low-frequency sensorineural hearing loss in ears with Ménière's disease. In the current study, we use a relatively new ECochG technique to make measurements that originate from afferent auditory nerve fibers in the apical half of the cochlear spiral to assess effects of endolymphatic hydrops in guinea pig ears. These measurements are made from the Auditory Nerve Overlapped Waveform (ANOW). Hydrops was induced with artificial endolymph injections, iontophoretically applied Ca²⁺ to endolymph, and exposure to 200 Hz tones. The manipulations used in this study were far smaller than those used in previous investigations on hydrops. In response to all hydropic manipulations, ANOW amplitude to moderate level stimuli was markedly reduced but conventional ECochG measurements of compound action potential thresholds were unaffected (i.e., a less than 2 dB threshold shift). Given the origin of the ANOW, changes in ANOW amplitude likely reflect acute volume disturbances accumulate in the distensible cochlear apex. These results suggest that the ANOW could be used to advance our ability to identify initial stages of dysfunction in ears with Ménière's disease before the pathology progresses to an extent that can be detected with conventional measures.

Electrophysiological Measurements of Peripheral Vestibular Function-A Review of Electrovestibulography

Electrocochleography (EcochG), incorporating the Cochlear Microphonic (CM), the Summating Potential (SP), and the cochlear Compound Action Potential (CAP), has been used to study cochlear function in humans and experimental animals since the 1930s, providing a simple objective tool to assess both hair cell (HC) and nerve sensitivity. The vestibular equivalent of ECochG, termed here Electrovestibulography (EVestG), incorporates responses of the vestibular HCs and nerve. Few research groups have utilized EVestG to study vestibular function. Arguably, this is because stimulating the cochlea in isolation with sound is a trivial matter, whereas stimulating the vestibular system in isolation requires significantly more technical effort. That is, the vestibular system is sensitive to both high-level sound and bone-conducted vibrations, but so is the cochlea, and gross electrical responses of the inner ear to such stimuli can be difficult to interpret. Fortunately, several simple techniques can be employed to isolate vestibular electrical responses. Here, we review the literature underpinning gross vestibular nerve and HC responses, and we discuss the nomenclature used in this field. We also discuss techniques for recording EVestG in experimental animals and humans and highlight how EVestG is furthering our understanding of the vestibular system.

Mechanical tuning and amplification within the apex of the guinea pig cochlea

KEY POINTS

A popular conception of mammalian cochlear physiology is that tuned mechanical vibration of the basilar membrane defines the frequency response of the innervating auditory nerve fibres However, the data supporting these concepts come from vibratory measurements at cochlear locations tuned to high frequencies (>7 kHz). Here, we measured the travelling wave in regions of the guinea pig cochlea that respond to low frequencies (<2 kHz) and found that mechanical tuning was broad and did not match auditory nerve tuning characteristics. Non-linear amplification of the travelling wave functioned over a broad frequency range and did not substantially sharpen frequency tuning. Thus, the neural encoding of low-frequency sounds, which includes most of the information conveyed by human speech, is not principally determined by basilar membrane mechanics.

ABSTRACT

The popular notion of mammalian cochlear function is that auditory nerves are tuned to respond best to different sound frequencies because basilar membrane vibration is mechanically tuned to different frequencies along its length. However, this concept has only been demonstrated in regions of the cochlea tuned to frequencies >7 kHz, not in regions sensitive to lower frequencies where human speech is encoded. Here, we overcame historical technical limitations and non-invasively measured sound-induced vibrations at four locations distributed over the apical two turns of the guinea pig cochlea. In turn 3, the responses demonstrated low-pass filter characteristics. In turn 2, the responses were low-pass-like, in that they occasionally did have a slight peak near the corner frequency. The corner frequencies of the responses were tonotopically tuned and ranged from 384 to 668 Hz. Non-linear gain, or amplification of the vibrations in response to low-intensity stimuli, was found both below and above the corner frequencies. Post mortem, cochlear gain disappeared. The non-linear gain was typically 10-30 dB and was broad-band rather than sharply tuned. However, the gain did reach nearly 50 dB in turn 2 for higher stimulus frequencies, nearly the amount of gain found in basal cochlear regions. Thus, our data prove that mechanical responses do not match neural responses and that cochlear amplification does not appreciably sharpen frequency tuning for cochlear regions that respond to frequencies <2 kHz. These data indicate that the non-linear processing of sound performed by the guinea pig cochlea varies substantially between the cochlear apex and base.

Direct administration of 2-Hydroxypropyl-Beta-Cyclodextrin into guinea pig cochleae: Effects on physiological and histological measurements

2-Hydroxypropyl-Beta-Cyclodextrin (HPβCD) can be used to treat Niemann-Pick type C disease, Alzheimer's disease, and atherosclerosis. But, a consequence is that HPβCD can cause hearing loss. HPβCD was recently found to be toxic to outer hair cells (OHCs) in the organ of Corti. Previous studies on the chronic effects of in vivo HPβCD toxicity did not know the intra-cochlear concentration of HPβCD and attributed variable effects on OHCs to indirect drug delivery to the cochlea. We studied the acute effects of known HPβCD concentrations administered directly into intact guinea pig cochleae. Our novel approach injected solutions through pipette sealed into scala tympani in the cochlear apex. Solutions were driven along the length of the cochlear spiral toward the cochlear aqueduct in the base. This method ensured that therapeutic levels were achieved throughout the cochlea, including those regions tuned to mid to low frequencies and code speech vowels and background noise. A wide variety of measurements were made. Results were compared to measurements from ears treated with the HPβCD analog methyl-β-cyclodextrin (MβCD), salicylate that is well known to attenuate the gain of the cochlear amplifier, and injection of artificial perilymph alone (controls). Histological data showed that OHCs appeared normal after treatment with a low dose of HPβCD, and physiological data was consistent with attenuation of cochlear amplifier gain and disruption of non-linearity associated with transferring acoustic sound into neural excitation, an origin of distortion products that are commonly used to objectively assess hearing and hearing loss. A high dose of HPβCD caused sporadic OHC losses and markedly affected all physiologic measurements. MβCD caused virulent destruction of OHCs and physiologic responses. Toxicity of HPβCD to OHC along the cochlear length is variable even when a known intra-cochlear concentration is administered, at least for the duration of our acute studies.

Using Cochlear Microphonic Potentials to Localize Peripheral Hearing Loss

The cochlear microphonic (CM) is created primarily by the receptor currents of outer hair cells (OHCs) and may therefore be useful for identifying cochlear regions with impaired OHCs. However, the CM measured across the frequency range with round-window or ear-canal electrodes lacks place-specificity as it is dominated by cellular sources located most proximal to the recording site (e.g., at the cochlear base). To overcome this limitation, we extract the "residual" CM (rCM), defined as the complex difference between the CM measured with and without an additional tone (saturating tone, ST). If the ST saturates receptor currents near the peak of its excitation pattern, then the rCM should reflect the activity of OHCs in that region. To test this idea, we measured round-window CMs in chinchillas in response to low-level probe tones presented alone or with an ST ranging from 1 to 2.6 times the probe frequency. CMs were measured both before and after inducing a local impairment in cochlear function (a 4-kHz notch-type acoustic trauma). Following the acoustic trauma, little change was observed in the probe-alone CM. In contrast, rCMs were reduced in a frequency-specific manner. When shifts in rCM levels were plotted vs. the ST frequency, they matched well the frequency range of shifts in neural thresholds. These results suggest that rCMs originate near the cochlear place tuned to the ST frequency and thus can be used to assess OHC function in that region. Our interpretation of the data is supported by predictions of a simple phenomenological model of CM generation and two-tone interactions. The model indicates that the sensitivity of rCM to acoustic trauma is governed by changes in cochlear response at the ST tonotopic place rather than at the probe place. The model also suggests that a combination of CM and rCM measurements could be used to assess both the site and etiology of sensory hearing loss in clinical applications.

Ups and Downs in 75 Years of Electrocochleography

Before 1964, electrocochleography (ECochG) was a surgical procedure carried out in the operating theatre. Currently, the newest application is also an intra-operative one, often carried out in conjunction with cochlear implant surgery. Starting in 1967, the recording methods became either minimal- or not-invasive, i.e., trans-tympanic (TT) or extra tympanic (ET), and included extensive studies of the arguments pro and con. I will review several valuable applications of ECochG, from a historical point of view, but covering all 75 years if applicable. The main topics will be: (1) comparing human and animal cochlear electrophysiology; (2) the use in objective audiometry involving tone pip stimulation-currently mostly pre cochlear implantation but otherwise replaced by auditory brainstem response (ABR) recordings; (3) attempts to diagnose Ménière's disease and the role of the summating potential (SP); (4) early use in diagnosing vestibular schwannomas-now taken over by ABR screening and MRI confirmation; (5) relating human electrophysiology to the effects of genes as in auditory neuropathy; and (6) intracochlear recording using the cochlear implant electrodes. The last two applications are the most recently added ones. The "historical aspects" of this review article will highlight the founding years prior to 1980 when relevant. A survey of articles on Pubmed shows several ups and downs in the clinical interest as reflected in the publication counts over the last 75 years.

The potential use of low-frequency tones to locate regions of outer hair cell loss

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Drug delivery into the cochlear apex: Improved control to sequentially affect finely spaced regions along the entire length of the cochlear spiral

BACKGROUND

Administering pharmaceuticals to the scala tympani of the inner ear is a common approach to study cochlear physiology and mechanics. We present here a novel method for in vivo drug delivery in a controlled manner to sealed ears.

NEW METHOD

Injections of ototoxic solutions were applied from a pipette sealed into a fenestra in the cochlear apex, progressively driving solutions along the length of scala tympani toward the cochlear aqueduct at the base. Drugs can be delivered rapidly or slowly. In this report we focus on slow delivery in which the injection rate is automatically adjusted to account for varying cross sectional area of the scala tympani, therefore driving a solution front at uniform rate.

RESULTS

Objective measurements originating from finely spaced, low- to high-characteristic cochlear frequency places were sequentially affected. Comparison with existing methods(s): Controlled administration of pharmaceuticals into the cochlear apex overcomes a number of serious limitations of previously established methods such as cochlear perfusions with an injection pipette in the cochlear base: The drug concentration achieved is more precisely controlled, drug concentrations remain in scala tympani and are not rapidly washed out by cerebrospinal fluid flow, and the entire length of the cochlear spiral can be treated quickly or slowly with time.

CONCLUSIONS

Controlled administration of solutions into the cochlear apex can be a powerful approach to sequentially effect objective measurements originating from finely spaced cochlear regions and allows, for the first time, the spatial origin of CAPs to be objectively defined.

Minimally invasive surgical method to detect sound processing in the cochlear apex by optical coherence tomography

Sound processing in the inner ear involves separation of the constituent frequencies along the length of the cochlea. Frequencies relevant to human speech (100 to 500 Hz) are processed in the apex region. Among mammals, the guinea pig cochlear apex processes similar frequencies and is thus relevant for the study of speech processing in the cochlea. However, the requirement for extensive surgery has challenged the optical accessibility of this area to investigate cochlear processing of signals without significant intrusion. A simple method is developed to provide optical access to the guinea pig cochlear apex in two directions with minimal surgery. Furthermore, all prior vibration measurements in the guinea pig apex involved opening an observation hole in the otic capsule, which has been questioned on the basis of the resulting changes to cochlear hydrodynamics. Here, this limitation is overcome by measuring the vibrations through the unopened otic capsule using phase-sensitive Fourier domain optical coherence tomography. The optically and surgically advanced method described here lays the foundation to perform minimally invasive investigation of speech-related signal processing in the cochlea.

Assessment of low-frequency hearing with narrow-band chirp-evoked 40-Hz sinusoidal auditory steady-state response

Objective To determine the clinical utility of narrow-band chirp-evoked 40-Hz sinusoidal auditory steady state responses (s-ASSR) in the assessment of low-frequency hearing in noisy participants. Design Tone bursts and narrow-band chirps were used to respectively evoke auditory brainstem responses (tb-ABR) and 40-Hz s-ASSR thresholds with the Kalman-weighted filtering technique and were compared to behavioral thresholds at 500, 2000, and 4000 Hz. A repeated measure ANOVA and post-hoc t-tests, and simple regression analyses were performed for each of the three stimulus frequencies. Study sample Thirty young adults aged 18-25 with normal hearing participated in this study. Results When 4000 equivalent response averages were used, the range of mean s-ASSR thresholds from 500, 2000, and 4000 Hz were 17-22 dB lower (better) than when 2000 averages were used. The range of mean tb-ABR thresholds were lower by 11-15 dB for 2000 and 4000 Hz when twice as many equivalent response averages were used, while mean tb-ABR thresholds for 500 Hz were indistinguishable regardless of additional response averaging. Conclusion Narrow-band chirp-evoked 40-Hz s-ASSR requires a ∼15 dB smaller correction factor than tb-ABR for estimating low-frequency auditory threshold in noisy participants when adequate response averaging is used.

An analysis of cochlear response harmonics: Contribution of neural excitation

In this report an analysis of cochlear response harmonics is developed to derive a mathematical function to estimate the gross mechanics involved in the in vivo transfer of acoustic sound into neural excitation (f(Tr)). In a simulation it is shown that the harmonic distortion from a nonlinear system can be used to estimate the nonlinearity, supporting the next phase of the experiment: Applying the harmonic analysis to physiologic measurements to derive estimates of the unknown, in vivo f(Tr). From gerbil ears, estimates of f(Tr) were derived from cochlear response measurements made with an electrode at the round window niche from 85 Hz tone bursts. Estimates of f(Tr) before and after inducing auditory neuropathy-loss of auditory nerve responses with preserved hair cell responses from neurotoxic treatment with ouabain-showed that the neural excitation from low-frequency tones contributes to the magnitude of f(Tr) but not the sigmoidal, saturating, nonlinear morphology.

Peak I of the human auditory brainstem response results from the somatic regions of type I spiral ganglion cells: evidence from computer modeling

Early neural responses to acoustic signals can be electrically recorded as a series of waves, termed the auditory brainstem response (ABR). The latencies of the ABR waves are important for clinical and neurophysiological evaluations. Using a biophysical model of transmembrane currents along spiral ganglion cells, we show that in human (i) the non-myelinated somatic regions of type I cells, which innervate inner hair cells, predominantly contribute to peak I, (ii) the supra-strong postsynaptic stimulating current (400 pA) and transmembrane currents of the myelinated peripheral axons of type I cells are an order smaller; such postsynaptic currents correspond to the short latencies of a small recordable ABR peak I', (iii) the ABR signal involvement of the central axon of bipolar type I cells is more effective than their peripheral counterpart as the doubled diameter causes larger transmembrane currents and a larger spike dipole-length, (iv) non-myelinated fibers of type II cells which innervate the outer hair cells generate essentially larger transmembrane currents but their ABR contribution is small because of the small ratio type II/type I cells, low firing rates and a short dipole length of spikes propagating slowly in non-myelinated fibers. Using a finite element model of a simplified head, peaks In and II (where In is the negative peak after peak I) are found to be stationary potentials when volleys of spikes cross the external electrical conductivity barrier at the bone&dura/CSF and at the CSF/brainstem interface whereas peaks I' and I may be generated by strong local transmembrane currents as postsynaptic events at the distal ending and the soma region of type I cells, respectively. All simulated human inter-peak times (I-I', II-I, In-I) are close to published data.