Neural response telemetry in patients with the double-array cochlear implant

This study aimed to evaluate the neural response in double-array cochlear implant as well as to describe the refractory recovery and the spread of excitation functions. In a prospective study 11 patients were implanted with the double-array cochlear implant. Neural response telemetry (NRT) was performed intra-operatively. NRT threshold could be registered in 6 of the 11 patients, at least in one electrode. The remaining five patients did not show measurable neural response intra-operatively. It was noted that although recovery and spread of excitation functions could be recorded in all the tested electrodes with measurable neural responses, the responses were shown to be different from the usual register in patients with other etiologies.     

神经反应遥测技术应用于婴幼儿人工耳蜗植入的作用和体会

目的探讨应用神经反应遥测技术（NRT）在人工耳蜗植入术中监测,术后调机中的作用和经验体会。方法回顾性分析38例经历人工耳蜗植入的婴幼儿的临床资料,收集每例患者在术中、术后开机、调机应用NRT测试电诱发听神经复合动作电位（ECAP）的阈值数据,应用听觉整合量表（IT MAIS）评估行为听觉言语功能。结果38例婴幼儿在术中植电极进入耳蜗后均实施NRT测试,每例测试5个电极,分别为1、6、11、16、22号电极,共测试了190个电极,其中163个电极（85.8%）引出了ECAP,其平均阈值为（163.8±21.2）CL。近端1号电极ECAP阈值显著高于远端22号电极（P<0.05）。从开机到开机后12个月,各电极ECAP阈值无显著变化。3岁以下患儿中,25例患儿在各次NRT测试时所有电极都能引出ECAP,而8例患儿存在1~5个电极不能引出ECAP,在开机12个月后,后者的IT MAIS积分与前者比较差异无统计学意义（P>0.05）。结论人工耳蜗植入后NRT测试的ECAP阈值在近端电极显著高于远端电极,耳蜗内数个电极引不出ECAP不影响术后的听觉言语康复。     

Elimination of facial nerve stimulation by reimplantation in cochlear implant subjects.

HYPOTHESIS: Perimodiolar intracochlear electrodes with contacts facing towards the modiolus have limited current flow towards the outer wall of the cochlea and therefore, may reduce the occurrence of facial nerve stimulation (FN) in cochlear implant subjects. BACKGROUND: Facial nerve stimulation is a well-known complication in cochlear implant treatment especially in the group of subjects with otosclerosis. The possible explanation of this side effect is a change of the electrical properties of the otosclerotic bone leading to leakage current and resulting in facial nerve stimulation. METHODS: Four CI subjects who had been implanted with a Nucleus Mini22 device with a Nucleus Straight electrode between 9 to 12 years ago suffered from severe FN stimulation. Electrode contacts had to be switched off so that they could only use 4, 11, 13, and 15 electrodes of their usual set of 22. The switch off resulted in deteriorating speech understanding over time. Therefore, all subjects were reimplanted with a Nucleus 24R device with a Contour electrode. Preoperatively, the threshold of FN stimulation was obtained on all electrodes subjectively. Intraoperatively, FN stimulation thresholds were measured objectively with both, the old and the new device and were compared. NRT and SRT thresholds were also obtained with the reimplanted device to assure effective electrical stimulation of the auditory nerve. RESULTS: In all four cases the postoperative fitting demonstrated no FN stimulation on all electrodes up to maximum comfortable level. The insertion of the Contour electrode array was complete in three cases, in one case the array could only be inserted partially similarly to the situation before the reimplantation. Speech perception tests showed a significant improvement in all subjects with the new device. CONCLUSION: Electrodes with modiolar facing contacts and perimodiolar position like the Nucleus Contour electrode reduce the possibility of facial nerve stimulation significantly due to more focused electrical stimulation.     

Effect of stylet removal on neural response telemetry and stapedial reflex thresholds during cochlear implantation

Objective

The purpose of this study was to analyze changes in neural response telemetry (NRT) and electrically evoked stapedial reflex thresholds (ESRT) before and after stylet withdrawal during cochlear implant surgery.

Methods

Thirty children (21–92 months old) who were candidates for cochlear implantation took part in this study. In all of them Nucleus Contour Advance was implanted. NRT and ESRT responses were recorded initially with the stylet in and then when the stylet was taken out during the implant procedure. The recordings were performed in the basal, middle, and apical areas of the electrode array.

Results

The threshold levels required to obtain NRT and ESRT responses after stylet removal were lower. This decrease was observed in all parts of the cochlea. It was statistically significant in all areas with the exception of the basal ESRT measurements.

Conclusion

Withdrawing the stylet results in better NRT and ESRT responses, most probably due to a favorable position change of the electrode array within the scala tympani.     

Spatial spread of neural excitation in cochlear implant recipients: comparison of improved ECAP method and psychophysical forward masking

This study introduces and evaluates a method for measurement of the longitudinal spread of electrically evoked neural excitation in the cochlea, using the Neural Response Telemetry system (NRT) available with the Nucleus((R)) 24 cochlear implant system. The recently released version of the NRT software (version 3.0) enables presentation of the 'masker' and 'probe' on different electrodes. In the present method the probe position was fixed, while the masker position was varied across the electrode array. The amplitude of the response to the partially masked probe provides a measure of the amount of masking, which is dependent on the extent of overlap of the excitation regions of the masker and probe. These measurements were performed in seven subjects implanted with the Nucleus 24 cochlear implant system (four with straight and three with Contour electrode arrays), for basal, middle and apical probe electrodes. Similar excitation profiles were obtained using either the standard NRT subtraction paradigm or an alternative 'Miller' method. The excitation profiles were compared with those obtained from psychophysical forward masking and good agreement was found. The widths of electrically evoked compound action potential (ECAP) and forward masking profiles did not differ significantly. Whereas the width of the ECAP measure was significantly correlated with both the maximum comfortable level and the distance of the electrode band from the modiolus, the width of the forward masking profile was not.     

神经反应遥测技术在人工耳蜗植入术中的监测应用

目的探讨在人工耳蜗植入术中能快速、准确地判断人工耳蜗装置的完好性和患者客观听觉反应的监测方法.方法在40例患儿人工耳蜗植入术中先测定电极阻抗,然后使用神经反应遥测技术(neuralresponsetelemetry,NRT)监测6个电极的电诱发听神经复合动作电位(electricallyevokedauditorynervecompoundactionpotentials,ECAP).结果患儿所有电极阻抗正常,ECAP的检出率分别为97.5%(39例/40例)和92.1%(221个电极/240个电极).其中33例内耳无畸形的患儿所有198个测试电极中有195个电极测出清晰的ECAP波形(98.5%).7例内耳Mondini畸形患者共42个测试电极中有26个电极测得ECAP波形(61.9%),两组之间差异有极显著性.靠近耳蜗底回(高频区)的电极比靠近蜗尖(低频区)的电极具有较高的ECAP反应阈值和较高的ECAP饱和阈值.结论NRT技术可以简便、快速和准确地判断患者的听神经反应,可望成为术中常规监测方法,内耳Mondini畸形是影响ECAP检出的重要因素.     

神经反应遥测技术在人工耳蜗植入术中的监测应用

目的：探讨在人工耳蜗植入术中能快速,准确地判断人工蜗装置的完好性和患者客观听觉反应的监测方法。方法：在40例患儿人工耳蜗植入术中先测定电极阻抗,然后使用神经反应遥测技术（neural response telemetry,NRT)监测6个电极的电诱发听神经复合动作电位（electrically evoked auditory nerve compound action potentials,ECAP)。结果：患儿所有电极阻抗正常,ECAP的检出率分别为97．5％（39例／40例）和92．1％（221个电极／240个电极）,其中33例内耳无畸形的患儿所有198个测试电极中有195个电极测出清晰的ECAP波形（98．5％）,7例内耳Mondini畸形患者共42个测试电极中有26个电极测得ECAP波形（61．9％）,两组之间差异有极显著性,靠近耳蜗底回（高频区）的电极比靠近蜗尖（低频区）的电极具有较高的ECAP反应阈值的ECAP饱和阈值。结论：NRT技术可以简便,快速和准确地判断患者的听神经反应,可望成为术中常规监测方法。内耳Mondini畸形是影响ECAP检出的重要因素。     

Three-dimensional spiraling finite element model of the electrically stimulated cochlea.

OBJECTIVE: The objective of the article is to provide an accurate model of the human cochlea with which potential distributions and thus neural excitation patterns around cochlear implant electrodes can be determined. Improvements on previous models of the implanted cochlea are that this model 1) includes the spiral nature of the cochlea as well as many other anatomical details (and it is a model of the human cochlear rather than the guinea pig cochlea), and 2) facilitates modeling of different electrode geometries, array locations and electrode separations without changing the structure of the model. DESIGN: A three-dimensional spiraling finite element model of the human cochlea was created. The model incorporates the effect of neighboring canals and conduction along the fluid-filled canals of the cochlea. Potential distributions are used as inputs to a nerve fiber model to investigate auditory nerve excitation patterns around intracochlear electrode arrays. RESULTS: Potential distributions around intracochlear electrodes generated with the finite element model are presented. The effects of electrode separation, electrode geometry and array location on excitation threshold, excitation spread and ectopic excitation (i.e., excitation of nerve fibers at an undesirable location) are demonstrated. CONCLUSIONS: The following conclusions should be considered preliminary, as their accuracy depends on the exactness of the underlying model. The spiraling geometry of the cochlea causes asymmetry in potential distributions. The location of electrodes along the length of the basilar membrane has a stronger influence on the site of excitation than the polarity of the leading phase of the stimulus. Array location is the primary parameter that controls excitation spread. Threshold currents and the effect of ongoing loss of peripheral dendrites on threshold currents can be limited by placing arrays close to the modiolus. Point electrode geometries are recommended above banded electrode geometries only when the array can be placed close to the modiolus. There is a tradeoff between array location and the degree of ectopic stimulation caused by a specific array location. Bimodal excitation patterns exist at comfortable stimulus intensities for longitudinal bipolar electrode configurations. It is shown that an electrode configuration with an electrode separation of approximately half that of the bipolar electrode separation of the Nucleus electrode can be used instead of radial and offset radial electrode configurations to create unimodal excitation patterns. The stimulation resolution of cochlear implant electrode arrays can potentially be improved by increasing the number of electrode contacts in an array.     

Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients

The feasibility of using the electrically evoked compound action potential (ECAP), measured with the NRT system (Neural Response Telemetry) and the NRT software (version 2.04), to quantify the longitudinal spread of neural excitation was examined in four subjects fitted with the Nucleus CI24M cochlear implant. The ECAP and psychophysical forward-masking profiles were measured using stimulation on each of three electrodes, in basal, middle and apical positions. Spatial spread profiles derived from the ECAP measure produced broader functions than those derived from the psychophysical measure. These results, together with investigation of the change of ECAP spread pattern with stimulation current, suggest that functions derived from this ‘simple’ method were more influenced by the spread of electric field from excited neuron to electrode array than by breadth of the neural excitation pattern. The recently released NRT version 3.0 permits the masker and probe pulses to be delivered to separate electrodes, thus removing a fundamental limitation of version 2.04. Results from a pilot study, in which this capability was exploited, suggest that the spatial functions from this ‘advanced’ method may provide a better correlation with results from psychophysical forward masking.     

Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients

The feasibility of using the electrically evoked compound action potential (ECAP), measured with the NRT system (Neural Response Telemetry) and the NRT software (version 2.04), to quantify the longitudinal spread of neural excitation was examined in four subjects fitted with the Nucleus C124M cochlear implant. The ECAP and psychophysical forward-masking profiles were measured using stimulation on each of three electrodes, in basal, middle and apical positions. Spatial spread profiles derived from the ECAP measure produced broader functions than those derived from the psychophysical measure. These results, together with investigation of the change of ECAP spread pattern with stimulation current, suggest that functions derived from this 'simple' method were more influenced by the spread of electric field from excited neuron to electrode array than by breadth of the neural excitation pattern. The recently released NRT version 3.0 permits the masker and probe pulses to be delivered to separate electrodes, thus removing a fundamental limitation of version 2.04. Results from a pilot study, in which this capability was exploited, suggest that the spatial functions from this 'advanced' method may provide a better correlation with results from psychophysical forward masking.     

Efficacy of using NRT thresholds in cochlear implants fitting,in prelingual pediatric patients

ObjectiveTo evaluate the efficacy of using neural response telemetry (NRT) thresholds in predicting behavioural thresholds during programming of cochlear implant in prelingual children.MethodProspective study of 28 cochlear implants implanted with Nucleus 24 cochlear implant. We recorded NRT-thresholds on electrode numbers 1, 6, 11, 16 and 22 of the electrode array in each patient, the neural response thresholds were correlated with the behavioural map after six months of programming the device.ResultsThe mean neural response telemetry level was significantly higher than the mean threshold level (T-level) but lower than the comfortable level (C-level) in all the electrodes tested. NRT levels could statistically significantly predict T behavioural levels and comfortable behavioural levels, p < 0.01. There was a strong positive correlation between comfortable thresholds and neural response telemetry level measurements and behavioural threshold level and neural response telemetry threshold measurements.ConclusionThere is a useful role for neural response telemetry values in predicting the behavioural threshold and comfortable values in prelingual children. Combining the NRT values with behavioural observations can improve the programming of cochlear implants.     

Midbrain responses to micro-stimulation of the cochlea using high density thin-film arrays

A broader activation of auditory nerve fibres than normal using a cochlear implant contributes to poor frequency discrimination. As cochlear implants also deliver a restricted dynamic range, this hinders the ability to segregate sound sources. Better frequency coding and control over amplitude may be achieved by limiting current spread during electrical stimulation of the cochlea and positioning electrodes closer to the modiolus. Thin-film high density microelectrode arrays and conventional platinum ring electrode arrays were used to stimulate the cochlea of urethane-anaesthetized rats and responses compared. Neurophysiological recordings were taken at 197 multi-unit clusters in the central nucleus of the inferior colliculus (CIC), a site that receives direct monaural innervation from the cochlear nucleus. CIC responses to both the platinum ring and high density electrodes were recorded and differences in activity to changes in stimulation intensity, thresholds and frequency coding of neural activation were examined. The high density electrode array elicited less CIC activity at nonspecific frequency regions than the platinum ring electrode array. The high density electrode array produced significantly lower thresholds and larger dynamic ranges than the platinum ring electrode array when positioned close to the modiolus. These results suggest that a higher density of stimulation sites on electrodes that effectively 'aim' current, combined with placement closer to the modiolus would permit finer control over charge delivery. This may equate to improved frequency specific perception and control over amplitude when using future cochlear implant devices.     

Intraneural stimulation for auditory prosthesis: modiolar trunk and intracranial stimulation sites

We have demonstrated recently in an animal model that stimulation with a penetrating auditory nerve electrode array is a feasible means of activating the ascending auditory pathway for auditory prosthesis. Compared to a conventional intrascalar cochlear implant, intraneural stimulation provides access to fibers serving a broader frequency range, activation of more tonotopically restricted fiber populations, lower thresholds, and reduced interference between simultaneously stimulated channels. The spread of excitation by a single intraneural electrode is broader than that by an acoustic tone but narrower than that by a cochlear-implant electrode. In the present study, we compare in an animal model two sites of intraneural stimulation: the modiolar trunk of the nerve accessed using a transcochlear approach and the intracranial portion of the nerve accessed using a posterior fossa approach. The two stimulation sites offer very similar thresholds, spread of activation, and dynamic ranges. The intracranial site differed in that there was greater between-animal variation in tonotopic patterns. We discuss the implications of these results for possible improvements in hearing prosthesis for human subjects.     

儿童人工耳蜗植入术后神经反应遥测阈值的变化

目的:通过对儿童患者术后神经反应遥测 (NRT) 阈值的分析,总结其变化规律,为儿童人工耳蜗植入术后调试提供帮助.方法:测试40例接受Nucleus CI24R 型多导人工耳蜗单侧植入术的患儿.测试使用澳大利亚Cochlear公司提供的便携式调试系统及NRT3.1版本软件.采用削减算法提取NRT反应波形,分别测试每位患者的第1、5、10、15、20号电极,NRT阈值使用NRT软件计算.分别在开机时、开机3、6和12个月时进行测试.结果:在术后同一测试时间,位于耳蜗不同部位电极的NRT阈值存在差异.耳蜗底部和顶部电极的NRT阈值较低,而位于耳蜗中部的电极NRT阈值较高.经单因素方差分析,不同部位NRT阈值均差异有统计学意义.同一测试电极,随术后开机时间的增长,NRT阈值呈逐渐增加的趋势.经单因素方差分析,术后不同测试时间NRT阈值均差异有统计学意义.结论:术后开机1年内,NRT阈值呈逐渐增加的趋势.若人工耳蜗植入术后调试需要NRT进行辅助,每次均应测试NRT阈值,以便更准确估算患者的行为反应T、C级,尤其对植入术后1年内的儿童患者更为重要.     

Neural response telemetry in cochlear implant users

Programming of multichannel cochlear implants requires subjective responses to a series of sophisticated psychophysical percepts. It is often difficult for cochlear implant patients (especially young prelinguistically deaf children) to provide adequate responses for device fitting. However, the neural response telemetry (NRT) system renders possible the measurement of the compound action potential threshold. We performed NRT examinations in 27 cochlear implant users with Nucleus 24-channel cochlear implants. Measurements were obtained from five electrodes (3, 5, 10, 15, and 20) in each patient. Our goal was to look for correlation between behavioral subjective thresholds and compound action potentials. The action potentials could be elicited in 23 patients in all measured electrodes. The NRT threshold values were highly correlated with electrical threshold levels obtained through subjective responses. Our results suggest that the electrically elicited neural responses may yield very important information for device fitting in patients with cochlear implants.     

Degree of modiolar coiling, electrical thresholds, and speech perception after cochlear implantation.

OBJECTIVE: Perimodiolar electrode arrays were developed to improve stimulation of specific neural populations and to decrease power consumption. Postoperative radiographs suggest that some arrays are more tightly coiled than others. The purpose of this study was to evaluate whether the degree of modiolar coil correlates with electrical threshold and/or performance measures postimplantation with the Nucleus CI24RCS (Contour) device. STUDY DESIGN: Retrospective review. SETTING: Cochlear implant center and a tertiary care hospital. PATIENTS: Twenty-eight adult cochlear implant recipients with normal cochlear anatomy who underwent implantation with the Nucleus C124RCS perimodiolar electrode and are at least 1 year postimplantation. INTERVENTIONS: Therapeutic. MAIN OUTCOME MEASURES: Radiologic degree of perimodiolar electrode placement (determined by a computer algorithmic analysis) compared with electrical thresholds and standard speech perception outcome measures at 1 year postcochlear implantation. RESULTS: No significant correlations were found between the degree of modiolar coiling of the electrode array and electrical thresholds and speech perception outcome measures at 1 year postcochlear implantation. CONCLUSIONS: The degree of coiling of a modiolar hugging electrode array was not directly correlated with the level of electrical thresholds or postoperative speech perception outcome measures. Appearance of coil tightness on postoperative radiographs could reflect either differences in array placement or intrinsic variations in cochlear anatomy, and variations in speech perception performance can be influenced by other factors, including length of deafness.     

Correlation between NRT measurement level and behavioral levels in pediatrics cochlear implant patients

Objective

To determine the relationship between the electrically evoked nerve action potential (Neural Response Telemetry [NRT]) and behavioral levels (T- and C-level) for pediatric patients using the Nucleus 24 cochlear implant system.

Method

A hospital based study of pediatric cochlear implant patients in the period between June 2000 and December 2008. At King Abdul-Aziz University Hospital (KAUH) Saudi Arabia the Neural Response Telemetry was administered to 47 children (mean age at implantation: 4 years) with the Nucleus 24 cochlear implants. Four intra cochlear electrodes (numbers 5, 10, 15, and 20) were tested one-month post-implantation, the neural response threshold compared with the behavioral threshold and the maximum comfort level estimated at the same time.

Results

At all the electrode numbers, the mean for NRT level measurements was significantly higher than that for the T-level measurements and the mean for the C-level measurements was significantly higher than that for NRT level measurements The correlation analyses showed positive correlation between C-level and NRT level measurements and T-level and NRT level measurements.

Conclusion

There was a positive correlation between NRT value measurements and both T and C value measurements. Therefore, it is useful to use the NRT values to predict the behavioral T and C values in prelingual children.     

A silastic positioner for a modiolus-hugging position of intracochlear electrodes: electrophysiologic effects

HYPOTHESIS: It was postulated that an electrode array that achieved a close modiolar proximity would result in reduced threshold levels and amplitude slopes, as measured with electrically evoked auditory brainstem responses (EABRs). BACKGROUND: Quality and quantity of auditory information transmitted by a cochlear implant to patients with sensorineural hearing loss depend on spatial and temporal resolution achieved by the electrical intracochlear stimulation. METHODS: To improve spatial resolution, a new electrode system was developed by Advanced Bionics Corp., with the intention of obtaining greater modiolar proximity. The implant version specified for animal experiments consists of a straight electrode array of seven embedded platinum discs and a so-called Silastic-positioner. The Silastic positioner is shaped to follow the dimensions of the scala tympani with a concave (triangular) inner side, which fits the form of the electrode array. The aim of the study was to evaluate the influence of a modiolus-hugging electrode position in contrast to a conventional electrode position on EABR in short-term animal experiments. Short-term electrophysiologic studies were performed on six adult cats. After local intracochlear application of neomycin solution (50 mg/mL), electrodes were inserted into the scala tympani. Electrically evoked auditory brainstem response threshold levels and EABR amplitude slopes were systematically investigated with and without the positioner. RESULTS: Electrically evoked auditory brainstem response measurements revealed a distinct apicobasal threshold shift, with increasing thresholds toward the basal end of the electrode. After insertion of the positioner, this shift diminished or was inverted and EABR thresholds and amplitude slopes were reduced significantly. CONCLUSIONS: Threshold and amplitude slope data emphasize the functional benefit of the positioner system, especially for the stimulation of electrodes in the more basal channels.     

Neural response telemetry with the nucleus CI24M cochlear implant

OBJECTIVES: To review our intraoperative and postoperative testing protocol for cochlear implant patients. This study describes the methodologies and applications of a new technique called neural response telemetry (NRT) for the Nucleus CI24M cochlear implant system. NRT uses radiofrequency telemetry technology to measure the action potentials of the auditory nerve. STUDY DESIGN: We have developed a specific protocol for intraoperative testing of the implant device before, during, and after implantation. This includes device integrity tests, visual detection of electrical stapedius muscle reflexes (VESR), and NRT. METHODS: Our methodologies use the commercial software (Windows-based Diagnostic and Programming System [WIN-DPS] and NRT) for the Nucleus CI24M. We describe the details of our protocol used on all of the patients (14 adults and 14 children) who received CI24M implants at Mayo Clinic (Rochester, MN). Our protocol correlates the NRT threshold with the behavioral responses for each patient on at least four electrodes. RESULTS: From August 1, 1998, to December 31,1998, we completed electrode integrity tests, NRT, and VESR testing intraoperatively on 12 patients with the Nucleus CI24M. We have measured normal implant function on all 28 of our CI24M patients with one exception. One of our children had a device failure after approximately 4 months as a result of head trauma. We have also obtained NRT results from an additional 10 patients postoperatively. CONCLUSIONS: The measurement of device and electrode array function is quite simple with the CI24M software. These measurements can be obtained intraoperatively as well as postoperatively. We conclude that VESR and NRT measurements can be very helpful in programming for patients with cochlear implants, especially children, because they provides us with target settings for the speech processor.     

A technique for implantation of a 3-dimensional penetrating electrode array in the modiolar nerve of cats and humans

BACKGROUND: We believe that direct intraneural stimulation of the modiolar nerve using an array of electrodes will have lower thresholds, offer greater frequency selectivity and more stimulation sites, and have a greater frequency representation than conventional cochlear implants. OBJECTIVES: To describe a potential auditory prosthesis based on electrical stimulation of the modiolar cochlear nerve and to report the development of a surgical approach in human and animal models. DESIGN: Cadaveric human and animal studies conducted in temporal bones indicated that an array of penetrating microelectrodes could be implanted in the modiolar nerve. Cat studies using anesthesia were performed to develop the surgical procedure in an animal model. Nerve viability was assessed by measurement of electrically evoked auditory brainstem responses at different stages of the surgery. SUBJECTS: Two fresh cadaveric human temporal bones, 3 cat cadavers, 1 pig cadaver, and 6 anesthetized cats were used in the experiments. RESULTS: We were able to implant arrays containing 20 microelectrodes in the human modiolar nerve after exposure by a modified extended facial recess approach. In animals, the modiolar nerve was accessed by the transbulla and the middle fossa approach. The cat was chosen as the appropriate animal model, and the transbulla approach was selected. The round window was exposed by ventral access to the bulla and after cochleostomy; drilling the modiolar bone exposed the modiolar nerve. The mean +/- SD diameter of the exposed nerve in cats was 1.64 +/- 0.07 mm (n = 9), and the mean +/- SD exposed length was 2.50 +/- 0.11 mm (n = 9); this is adequate to accommodate 20 microelectrodes. The electrically evoked auditory brainstem responses indicated nerve survival during and after the surgery. CONCLUSIONS: The surgical technique allows implantation of up to 20 microelectrodes in the cat and human modiolar nerve. The nerve survives the surgical procedure. This work enables studies in the electrophysiological properties and consequences of long-term implantation.