Psychophysical evaluation of cochlear prostheses in a monkey model

Cochlear Health and Cochlear-implant Function

The cochlear implant (CI) is widely considered to be one of the most innovative and successful neuroprosthetic treatments developed to date. Although outcomes vary, CIs are able to effectively improve hearing in nearly all recipients and can substantially improve speech understanding and quality of life for patients with significant hearing loss. A wealth of research has focused on underlying factors that contribute to success with a CI, and recent evidence suggests that the overall health of the cochlea could potentially play a larger role than previously recognized. This article defines and reviews attributes of cochlear health and describes procedures to evaluate cochlear health in humans and animal models in order to examine the effects of cochlear health on performance with a CI. Lastly, we describe how future biologic approaches can be used to preserve and/or enhance cochlear health in order to maximize performance for individual CI recipients.

Primate optogenetics: Progress and prognosis

Monkeys are a premier model organism for neuroscience research. Activity in the central nervous systems of monkeys can be recorded and manipulated while they perform complex perceptual, motor, or cognitive tasks. Conventional techniques for manipulating neural activity in monkeys are too coarse to address many of the outstanding questions in primate neuroscience, but optogenetics holds the promise to overcome this hurdle. In this article, we review the progress that has been made in primate optogenetics over the past 5 years. We emphasize the use of gene regulatory sequences in viral vectors to target specific neuronal types, and we present data on vectors that we engineered to target parvalbumin-expressing neurons. We conclude with a discussion of the utility of optogenetics for treating sensorimotor hearing loss and Parkinson's disease, areas of translational neuroscience in which monkeys provide unique leverage for basic science and medicine.

Functional and Histological Effects of Chronic Neural Electrode Implantation

Objectives

Permanent injury to the cranial nerves can often result in a substantial reduction in quality of life. Novel and innovative interventions can help restore form and function in nerve paralysis, with bioelectric interfaces among the more promising of these approaches. The foreign body response is an important consideration for any bioelectric device as it influences the function and effectiveness of the implant. The purpose of this review is to describe tissue and functional effects of chronic neural implantation among the different categories of neural implants and highlight advances in peripheral and cranial nerve stimulation.

Data Sources: PubMed, IEEE, and Web of Science literature search.

Review Methods: A review of the current literature was conducted to examine functional and histologic effects of bioelectric interfaces for neural implants.

Results

Bioelectric devices can be characterized as intraneural, epineural, perineural, intranuclear, or cortical depending on their placement relative to nerves and neuronal cell bodies. Such devices include nerve‐specific stimulators, neuroprosthetics, brainstem implants, and deep brain stimulators. Regardless of electrode location and interface type, acute and chronic histological, macroscopic and functional changes can occur as a result of both passive and active tissue responses to the bioelectric implant.

Conclusion

A variety of chronically implantable electrodes have been developed to treat disorders of the peripheral and cranial nerves, to varying degrees of efficacy. Consideration and mitigation of detrimental effects at the neural interface with further optimization of functional nerve stimulation will facilitate the development of these technologies and translation to the clinic.

Level of Evidence

3.

Discrimination of Simultaneous Frequency and Level Changes in Electrical Stimuli

Frequency difference limens for sinusoidal electrical stimuli were measured at operationally defined equal-loudness points in a behaviorally trained monkey that was deafened and implanted in one ear. The equal-loudness points were defined as the levels at which the discrimination of a frequency change was minimal when frequency and level were varied simultaneously. To determine accurately these points, we varied the level in very fine steps (as small as 0.05 dB) above and below the estimated equal-loudness point. With this method we also determined precise equal-loudness contours and level difference limens. Frequency difference limens ranged from 7% at 100 Hz, 17 dB sensation level (SL) to about 30% at 100, 300, and 600 Hz, 7 to 9 dB SL. Level difference limens ranged from 0.4 to 1.9 dB. Slopes of the equal-loudness contours were 0 at 100 Hz, about 6 dB/octave at 300 Hz, and leveled off to about 2 dB/octave above 600 Hz.

Three-dimensional models of cochlear implants: A review of their development and how they could support management and maintenance of cochlear implant performance

Three-dimensional (3D) computational modeling of the auditory periphery forms an integral part of modern-day research in cochlear implants (CIs). These models consist of a volume conduction description of implanted stimulation electrodes and the current distribution around these, coupled with auditory nerve fiber models. Cochlear neural activation patterns can then be predicted for a given input stimulus. The objective of this article is to present the context of 3D modeling within the field of CIs, the different models, and approaches to models that have been developed over the years, as well as the applications and potential applications of these models. The process of development of 3D models is discussed, and the article places specific emphasis on the complementary roles of generic models and user-specific models, as the latter is important for translation of these models into clinical application.

Bilateral cochlear implantation in the ferret: a novel animal model for behavioral studies

Bilateral cochlear implantation has recently been introduced with the aim of improving both speech perception in background noise and sound localization. Although evidence suggests that binaural perception is possible with two cochlear implants, results in humans are variable. To explore potential contributing factors to these variable outcomes, we have developed a behavioral animal model of bilateral cochlear implantation in a novel species, the ferret. Although ferrets are ideally suited to psychophysical and physiological assessments of binaural hearing, cochlear implantation has not been previously described in this species. This paper describes the techniques of deafening with aminoglycoside administration, surgical implantation of an intracochlear array and chronic intracochlear electrical stimulation with monitoring for electrode integrity and efficacy of stimulation. Experiments have been presented elsewhere to show that the model can be used to study behavioral and electrophysiological measures of binaural hearing in chronically implanted animals. This paper demonstrates that cochlear implantation and chronic intracochlear electrical stimulation are both safe and effective in ferrets, opening up the possibility of using this model to study potential protective effects of bilateral cochlear implantation on the developing central auditory pathway. Since ferrets can be used to assess psychophysical and physiological aspects of hearing along with the structure of the auditory pathway in the same animals, we anticipate that this model will help develop novel neuroprosthetic therapies for use in humans.

Effects of deafening and cochlear implantation procedures on postimplantation psychophysical electrical detection thresholds

Previous studies have shown large decreases in cochlear implant psychophysical detection thresholds during the weeks following the onset of electrical testing. The current study sought to determine the variables underlying these threshold decreases by examining the effects of four deafening and implantation procedures on detection thresholds and implant impedances. Thirty-two guinea pigs were divided into four matched groups. Group I was deafened and implanted Day 0 and began electrical testing Day 1. Group II was deafened and implanted Day 0 and began electrical testing Day 45. Group III was deafened Day 0, implanted Day 45 and began electrical testing Day 46. Group IV was not predeafened but was implanted Day 0 and began electrical testing Day 1. All groups showed threshold decreases over time but the magnitude of change, time course and final stable threshold levels depended on the type and time course of treatment. Impedances increased over the first two weeks following the onset of electrical testing except in Group II. Results suggest that multiple mechanisms underlie the observed threshold shifts including (1) recovery of the cochlea from a temporary pathology caused by the deafening and/or implantation procedures, (2) effects of electrical stimulation on the auditory pathway, and (3) tissue growth in the implanted cochlea. They also suggest that surviving hair cells influence electrical threshold levels.

Cochlear implantation in rats: a new surgical approach

The laboratory rat has been used extensively in auditory research but has had limited use in cochlear implant related research due mainly to the surgically restricted access to the scala tympani. We have developed a new surgical method for cochlear implantation in rats. The key to this protocol was cauterizing the stapedial artery (SA) and making a small cochleostomy near the round window in order to enlarge the surgical access to the scala tympani. Five normal hearing Hooded Wistar rats were used to investigate the effect of cauterizing the SA on hearing and auditory nerve survival. Results showed that cauterizing the SA was surgically feasible, afforded excellent exposure of the round window niche for cochleostomy, and did not adversely affect acoustic thresholds measured electrophysiologically. Moreover, there was no difference in spiral ganglion cell densities for any cochlear turn when compared with the contralateral control ears. Three deafened rats were subsequently implanted with a scala tympani electrode array using this new surgical approach. Electrically evoked auditory brainstem responses using bipolar stimulation, and subsequent cochlear histopathology demonstrated that cochlear implantation using a custom-made rat electrode array was safe and effective. The surgical approach presented in this paper presents a safe and effective procedure for acute or chronic cochlear implantation in the rat model.

Electrophysiologic channel interaction, electrode pitch ranking, and behavioral threshold in straight versus perimodiolar cochlear implant electrode arrays

The primary goal of this study was to examine electrophysiologic measures of channel interaction, electrode pitch discrimination ability using a pitch-ranking task, and behavioral threshold levels in individuals implanted with a straight electrode array versus a perimodiolar array. It was hypothesized that perimodiolar arrays should yield lower thresholds, less channel interaction as measured with the electrically evoked compound action potential (ECAP), and better electrode pitch-ranking ability. Results from ten adult Nucleus 24 recipients (N=5 straight array, N=5 perimodiolar Contour array) showed no significant difference in threshold between the two electrode designs; however, there was significantly better electrode pitch-ranking ability and less channel interaction as measured with the ECAP for perimodiolar electrodes. Additionally, there was a significant positive correlation between behavioral threshold and width of the ECAP interaction function for Contour group data. There was no significant correlation between behavioral threshold and electrode pitch-ranking ability. These data suggest that electrode design and/or perimodiolar position may reduce physiologic channel interaction in the cochlea and improve electrode pitch discrimination ability; however, this positive finding did not translate into significantly better speech perception ability for Contour subjects.

Predicting Dynamic Range and Intensity Discrimination for Electrical Pulse-Train Stimuli Using a Stochastic Auditory Nerve Model: The Effects of Stimulus Noise

This work investigates dynamic range and intensity discrimination for electrical pulse-train stimuli that are modulated by noise using a stochastic auditory nerve model. Based on a hypothesized monotonic relationship between loudness and the number of spikes elicited by a stimulus, theoretical prediction of the uncomfortable level has previously been determined by comparing spike counts to a fixed threshold, Nucl. However, no specific rule for determining Nucl has been suggested. Our work determines the uncomfortable level based on the excitation pattern of the neural response in a normal ear. The number of fibers corresponding to the portion of the basilar membrane driven by a stimulus at an uncomfortable level in a normal ear is related to Nucl at an uncomfortable level of the electrical stimulus. Intensity discrimination limens are predicted using signal detection theory via the probability mass function of the neural response and via experimental simulations. The results show that the uncomfortable level for pulse-train stimuli increases slightly as noise level increases. Combining this with our previous threshold predictions, we hypothesize that the dynamic range for noise-modulated pulse-train stimuli should increase with additive noise. However, since our predictions indicate that intensity discrimination under noise degrades, overall intensity coding performance may not improve significantly.

Across-species comparisons of psychophysical detection thresholds for electrical stimulation of the cochlea: I. Sinusoidal stimuli

Several species have been, and continue to be, used as subjects in studies of electrical stimulation of the cochlea. Few attempts, however, have been made to determine if data obtained from different species are quantitatively or qualitatively similar. The present work compares psychophysical absolute detection threshold vs. frequency functions for sinusoidal stimuli obtained from humans, nonhuman primates, cats, and guinea pigs. Threshold data for monopolar and bipolar electrode configurations from both previously published and unpublished studies are compared. In general, within all four species, significant intersubject variation in detection threshold level was found, but slopes of threshold vs. frequency functions were relatively well conserved within a species, under the conditions studied. With one exception (cat bipolar stimulation), threshold functions reached a minimum at or near 100 Hz across species and electrode configurations. In all cases, thresholds were significantly lower for monopolar, as compared with bipolar, configurations. Statistically, there were no significant differences in absolute threshold level across species. Threshold levels increased with frequency above 100 Hz at a rate of 3.0-7.9 dB/octave, depending on both electrode configuration and species. Slopes were steeper for monopolar than for bipolar configurations. When slopes were averaged between 200 and 2000 Hz, no statistically significant differences in overall slopes were found, nor was there a significant interaction between electrode configuration and species. There were, however, consistent species differences within more restricted regions of the function. Human functions for both monopolar and bipolar stimulation were steeper than all animal functions in the range of 100-300 Hz. Within this range, the differences between slopes for human and nonhuman subjects were statistically significant. In addition, differences were noted in the frequency at which slope decreased, with slopes for nonhuman subjects showing the decrease at higher frequencies than did those for human subjects. These differences may be true species differences, or may reflect the influence of confounding variables associated with each experimental-subject model.

Interactions between pulse separation and pulse polarity order in cochlear implants

Interactions between pulse separation and pulse polarity order were examined using psychophysical studies of electrical detection thresholds in nonhuman primates. Subjects were trained using acoustic stimuli, then deafened in one ear and implanted with an electrode array for electrical stimulation of the cochlea. Threshold vs pulse separation functions for trains of biphasic electrical pulses were compared for constant and alternating leading phase polarity. When leading phase polarity was held constant, threshold vs pulse separation functions were nonmonotonic (U-shaped). Small polarity-dependent (cathodic vs anodic leading phase) differences in absolute thresholds were observed at long pulse separations, but function shape was independent of leading phase. When leading phase polarity alternated, there was a pronounced reduction in thresholds at short pulse separations (below about 1 ms), resulting in monotonically increasing threshold vs pulse separation functions. At long pulse separations, functions for alternating and constant polarity stimuli were similar. Polarity effects were most apparent for longer duration trains (20 pulses) at long pulse durations (1-2 ms/phase). For stimuli consisting of only two biphasic pulses, alternating polarity effects depended on whether cathodic or anodic phases were adjacent. The neural mechanisms underlying these effects probably include refractory properties and/or residual potentials.

Middle latency responses to acoustical and electrical stimulation of the cochlea in cats

The middle latency responses (MLR) to acoustical stimulation (A-MLR) as well as to electrical stimulation (E-MLR) of the inner ear were recorded in pentobarbital-anaesthetised cats. Monopolar and bipolar MLR recordings were performed with electrodes located at different places on the primary auditory cortex (AI). The cochlea was electrically stimulated (ES) through a single round-window electrode or through a multichannel intracochlear implant. The slope of amplitude-intensity functions of the A-MLR was steeper when the stimulus frequency of the acoustical stimuli corresponded to the tonotopical recording place on the auditory cortex. Other response properties (waveshape, thresholds and latencies) were related to the recording site and stimulus frequency in only two-thirds of animals. Parameters of E-MLRs evoked by high-frequency ( > 4 kHz) and low-intensity ES in hearing cats, which produced an electrophonic effect, were similar to parameters of acoustically evoked MLRs. In deafened cats, the properties of responses to extracochlear ES were different from those recorded to acoustical stimulation and they were almost uniform in all cortical places. Variations in thresholds, in latencies and in the slope of the amplitude-intensity functions of the E-MLRs recorded in individual tonotopical cortical places were observed when the auditory nerve was stimulated with different configurations of electrodes through a multichannel intracochlear implant.

Partial hearing loss in the macaque following the co-administration of kanamycin and ethacrynic acid

Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) rapidly produces a profound hearing loss in the cat while maintaining normal renal function [Xu et al., Hear. Res. 70, 205-215 (1993)]. In the present paper we have applied this deafening procedure to the old world monkey Macaca fascicularis (macaque). Following the co-administration of KA and EA, the hearing loss in the macaque developed far slower than we observed in the cat. Moreover, unlike the cat, there was evidence of a partial recovery in the animal's hearing, resulting in a bilaterally symmetrical high frequency hearing loss. The extent of this hearing loss was dependent on the dose of the EA administered. Finally, the most unexpected result of the present study was the degree of acute nephrotoxicity experienced by these animals following the drug administration. The sensitivity of this species to renal failure restricted the dose of EA that could be safely administered. In conclusion, the co-administration of KA and EA cannot reliably produce a profound hearing loss in the macaque. While it can produce a dose dependent high frequency hearing loss the animal will also experience acute renal failure that requires careful management.

Cochlear wall titanium implants for auditory nerve stimulation

Genetically deaf dalmatian dogs and ototoxically deafened macaque monkeys were implanted with electrodes housed in cochlear wall titanium implants to assess long-term stability, tolerance, and performance. Short-term human implantation, followed by trials of stimulation, was performed in 4 unilaterally deaf patients. In the dog experiments, cochlear wall electrode stimulation produced consistent electrophysiologic thresholds that were higher, by approximately 6 dB, than those obtained with bipolar scala tympani stimulation. Clinical testing revealed electrically evoked middle latency response, auditory brain stem response, and/or behavioral detection responses in 3 of 4 patients, at levels below those for facial nerve activation and pain sensation. Electrode place discrimination studies, with controls for loudness cues, revealed near-perfect discrimination in a monkey subject. Eleven of the 12 animal implants were found to be rigidly fixed in the cochlear bone, with direct contract between bone and implant over 8% to 23% of the implant surface for the 6 implants examined in detail. These results suggest that long-term fixation of titanium cochlear wall implants occurs by virtue of intimate implant-bone contact in restricted areas. This approach to prosthetic stimulation demonstrates encouraging performance characteristics in achieving auditory activation.

Middle latency responses to electrical stimulation of the auditory nerve in unanaesthetized guinea pigs

Middle latency responses (MLR) to sinusoidal and pulsatile electrical stimulation (ES) of the cochlea and to acoustical stimulation (AS) were evaluated in awake guinea pigs with chronically implanted electrodes. The ear, which was later electrically stimulated, was deafened by local intracochlear application of gentamicin, the opposite ear was left intact. Waveforms and P1-P2 interpeak intervals of the electrically evoked MLR (ES-MLR) were similar to those evoked by acoustical stimulation of the intact ear (AS-MLR) and the latencies of the ES-MLR were shorter by about 1-3 ms. Thresholds of ES-MLR in the frequency range 0.5-32 kHz increased with increasing ES frequency (slope 3.2 dB/octave), thresholds were 3.5-9.5 dB lower for intracochlear than for extracochlear ES. Dynamic ranges for ES-MLR varied between 6-20 dB. MLR amplitude-intensity functions for ES were steeper (slope 2-12 microV/dB) than those for AS (slope 0.2-2 microV/dB). Maximal ES-MLR amplitudes exceeded usually 1.5-3 times the amplitudes of the acoustically evoked MLR. Both types of stimulations evoked larger MLR amplitudes to contralateral stimulation than to ipsilateral stimulation (average ratio = 4.1 +/- 2.2 for AS and 3.3 +/- 2.2 for ES). Because of the relatively long latency and therefore insensitivity to electrical artifact, the ES-MLR can be used for the evaluation of different strategies of the electrical stimulation of the cochlea in awake guinea pig.

Electrical stimulation of the auditory nerve: effects of pulse width on frequency discrimination

Effects of pulse width on discrimination of simultaneous changes in frequency and level of electrical pulse trains were studied in a monkey subject with a cochlear implant. At test-stimulus levels where performance was minimum, frequency difference limens were larger for longer-duration pulses than that for shorter-duration pulses. Several factors may have contributed to these differences.

Discrimination of complex electrical stimulation through a multichannel intracochlear implant

A model has been developed to describe the electric fields generated in the inner ear when electrical stimuli are presented through a multichannel implant in the scala tympani of the cochlea. The model relies on the hypothesis that stimuli which excite the largest number of neural elements provide the greatest probability of successful discrimination by the implanted subject. It suggests that the effective stimulus is determined by the linear combination of electrical fields produced by the individual channels, and that excitation takes place in a spatially restricted area of the auditory nerve in the vicinity of the stimulating electrodes. The model was tested by biophysical measurements of the potential developed in the stimulated cochlea, and by a psychophysical study of the ability of a monkey to discriminate complex electrical signals using dual channel stimulation. The experimental findings are in agreement with the computer simulations.

Changes over time in thresholds for electrical stimulation of the cochlea

The purpose of this paper is to better characterize changes over time that occurred in psychophysical detection thresholds for electrical stimulation of the cochlea. Threshold changes observed in nonhuman primates implanted with cochlear electrode arrays can be divided into at least three types based on the patterns of change over time. Short-term increases and subsequent decreases in threshold were commonly observed during the first months after implantation and were often followed by periods of long-term threshold stability. Long-term slow increases in thresholds and more rapid increases after a period of threshold stability have also been observed. The threshold changes may be divided into at least two classes based on their dependence on the waveforms used for the threshold measurements. Some changes occurred primarily in thresholds for long phase-duration signals while other changes were equal in magnitude (in decibels) for all tested stimuli. This suggests that at least two mechanisms underlay these threshold changes. The observed changes in thresholds have implications for experimental studies of electrical stimulation and for clinical application of auditory prostheses.

Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli

The purpose of this experiment was to study the effects of stimulus level on discrimination of frequency as represented in the temporal waveforms of acoustic and electrical signals. The subjects were four nonhuman primates in which one ear had been deafened and implanted with an electrode array and the other ear was untreated. Frequency difference limens for 100 Hz electrical sinusoidal stimulation via a cochlear implant in the deafened ear were compared to those for 100 Hz sinusoidally amplitude-modulated white noise (SAM noise) acoustic stimuli to the normal-hearing contralateral ear. To correct for loudness cues, levels of the test stimuli were varied relative to the reference-stimulus level. The test-stimulus levels at which the percent responses were minimum were determined. These levels were used to measure the frequency difference limens. Frequency difference limens for the electrical stimuli decreased as a function of reference-stimulus level through most of the dynamic range, while those for the acoustic stimuli reached a minimum at 20 dB to 40 dB above threshold. For the electrical stimuli the slopes and relative positions of the frequency difference limen vs. level functions varied from subject to subject, and with changes in electrode configuration within a subject. These differences were related to threshold level and dynamic range. At higher levels of stimulation, frequency difference limens for acoustic and electrical stimuli fell in the same range. The slopes and relative positions of the frequency difference limen vs. level functions for electrical stimuli did not parallel those of level difference limen vs. level functions collected simultaneously from the same ears. The data suggest that nonspectral frequency discrimination may depend on the number of nerve fibers stimulated. With prostheses in cochleas with less than a full complement of auditory nerve fibers, the data suggest that stimulation level is an important variable influencing discriminability.

Chronic skull-anchored percutaneous implants in non-human primates

Three groups of chronic, skull-anchored, percutaneous implants differing in materials, design and surgical procedures used for implantation, were tested in macaque monkeys in conjunction with studies of an inner ear stimulation device. Implants from the first two groups in which high-speed drilling methods and stainless steel materials were used, showed a high percentage of failures during the first 3 months after implantation of the percutaneous connector. Implants in the third group, in which measures were taken to preserve living bone tissue, all survived for greater than 7 months. Probable factors relating to implant survival are care of the bone during surgery, postsurgical mechanical trauma, materials and other details of the surgical procedure.

Threshold functions for electrical stimulation of the human cochlear nucleus

Thresholds to sinusoidal and biphasic pulsatile electrical stimuli were measured in two patients with electrodes positioned on the cochlear nucleus. The threshold functions differ from those observed in patients with scala tympani electrodes, primarily at low sinusoidal frequencies and long pulse widths. This difference is probably due to differences in the biophysical properties of the stimulated neural tissues in the two regions.

Inner ear implants for experimental electrical stimulation of auditory nerve arrays

Electrode arrays chronically implanted in the inner ear are gaining increased use for experimental studies of the auditory nervous system, as well as for studies related to development of improved auditory prostheses. Commercially available electrode arrays are designed for human use and thus may be unsuitable for experimental studies, particularly in small animals. This paper describes a simple, inexpensive method for making custom electrode arrays in a variety of configurations, suitable for animals ranging from small rodents to non-human primates.

Effect of electrical pulse shape on AVCN unit responses to cochlear stimulation

Electrical stimulation of the cochlea with a multiple-electrode array is best accomplished using pulsatile instead of continuous stimulation. The optimum shapes of electrical pulses for this purpose are still uncertain due to a lack of knowledge about their stimulation efficiency and requirements of the encoding strategy. We presented an extensive set of charge-balanced, rectangular pulse shapes to the guinea pig cochlea. Durations per phase for these constant-current pulses ranged from 20 microseconds to 900 microseconds with initially positive and initially negative polarities. Spike counts from single units in the anteroventral cochlear nucleus differed significantly for different pulse shapes, as did their initial latencies. Implications for stimulation efficiency and encoding strategies are discussed.

Comparisons of psychophysical and neurophysiological studies of cochlear implants

This paper compares psychophysical and neural studies of electrical stimulation of the auditory nerve with the goal of evaluating the relevance of single-unit animal models for the development of cochlear prostheses for profoundly deaf humans. Comparative psychophysical studies with implanted deaf subjects indicate that animal models, at least nonhuman primates, provide a close match to humans, though this is not always true for acoustic stimulation of normal-hearing subjects. However, the human-animal comparisons, especially those involving electrical stimuli, need further study using more carefully matched conditions. Comparisons of psychophysical and neurophysiological thresholds for electrical stimulation in animals reveal consistently higher thresholds in the neural studies. A number of factors which may account for these differences are discussed. A partial resolution of the problem could result from conducting neurophysiological and behavioral studies in the same animal. Finally, comparison of psychophysical and neurophysiological studies of temporal encoding suggest that there may be more information encoded in the auditory nerve than is used by the system, at least for nonspectral frequency discrimination. This points to a need for further analysis of the processing of this information at higher levels in the auditory pathway.

Brainstem auditory pathway degeneration associated with chronic cochlear implants in the monkey

The form and pattern of first-order and transsynaptic degeneration in the central auditory pathway was studied in monkeys following inner ear stimulation by a cochlear implant. Multielectrode, scala tympani, and modiolar systems were implanted; in some cases, neomycin was perfused into the cochlea to destroy the organ of Corti at the time of implantation. The monkeys were maintained chronically for 5 to 120 weeks, then the cochleas and brainstems were examined histologically. The extent of spiral ganglion cell loss across animals showed variability, reflecting the different procedures and devices used. The degree and distribution of spiral ganglion cell loss was related to the degree and distribution of neural degeneration seen in the cochlear nucleus in all cases. Peripheral damage progressed toward the cochlear apex as survival time increased, and this progression was reflected in the cochlear nucleus by a ventrolateral shift in the locus of degeneration over time. In addition, evidence for transneuronal degeneration was seen at the superior olive, the lateral lemniscus and the inferior colliculus. Our findings indicate that several factors inherent in the use of a cochlear prosthesis, i.e., insertion trauma, host reaction, and/or electrical stimulation, may be associated with a long-term, continuing process of central degeneration visible at several levels of the auditory system.

Responses of cochlear nucleus units to electrical stimulation through a cochlear prosthesis: channel interaction

The responses of 39 single units in the ventral cochlear nucleus of acute anesthetized guinea-pigs were studied with continuous electrical stimuli presented through a dual-channel implant in the scala tympani. Implants had four electrodes placed along the axis of the cochlea with 1 mm separations. With a specific pair (either apical or basal) of stimulating electrodes, about half of the units responded when current was flowing apically, while the rest responded to current in the opposite direction. No obvious relation existed between the effective polarity of the basal and apical pairs of electrodes. Two response types were observed while stimulating through both pairs simultaneously. Seventy-nine percent of the units responded to the sum of the current waveforms presented through the two pairs. Twenty-one percent responded to the difference between the waveforms. Both types of responses were observed for suprathreshold as well as for some intensities of stimulation that alone were subthreshold. The type of response was not dependent on the absolute threshold or threshold difference between the two pairs. For equal peak-intensities of stimuli, two-channel stimulation evoked larger responses than single-channel stimulation, provided the two channels were in their effective polarities. Responses to dual-channel stimulation were consistently larger than the summed responses to the two individual single-channel stimuli. The observed responses to the dual-channel stimulation indicate that the adequate stimulus was determined by the linear combination of fields produced by the individual channels in the vicinity of the stimulating electrodes before the auditory nerve is stimulated, and that excitation takes place in a spatially restricted area of the auditory nerve.

Intra-aural reflexes elicited by a cochlear prosthesis in monkeys

Objective audiological tests are needed for pre- and postsurgical evaluation of cochlear prosthesis patients who are unable to give reliable subjective responses. In this study we demonstrated that contralateral intra-aural reflexes were elicited by a cochlear prosthesis in the monkey. Reflex variables measured include threshold, latency and amplitude. These findings indicate that the electrically elicited intra-aural reflex response may be useful to evaluate the peripheral auditory system in subjects with sensory deafness.

Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.

Tuning characteristics of cochlear nucleus units in response to electrical stimulation of the cochlea

The responses of single units in the ventral cochlear nucleus of acute anesthetized guinea pigs were studied with continuous sinusoidal electrical stimuli presented through a multi-electrode implant in the scala tympani. Implants had two or four electrodes along the axis of the scala with 1 mm separations. Best frequencies were consistently in the 100 Hz range (50-250 Hz) with thresholds of about 0.063 mA peak-to-peak. Tuning curves were usually symmetrical with slopes of 3-4 dB/octave, both below and above the best frequency. The relative sharpness of the tuning curves, as measured by Q10dB, averaged 0.2. Dynamic ranges as determined by the intensity-rate functions for the various frequencies were 2-15 dB. No significant difference was found between tuning characteristics of units in response to stimulation via the apical or basal pair of implant electrodes. The findings suggest some limitations on the applicability of independent stimulating channels in multi-electrode implants.

Cochlear prostheses: stimulation-induced damage

The effects of 4 weekly, three-hour exposures to continuous sinusoidal (l kHz) electrical stimulation of the inner ear at various current levels were assessed in the chronically implanted guinea pig. With scala tympani stimulation, histopathological damage, including new bone growth, was observed for currents at and above 100 microA rms. No changes were observed in similarly implanted, but not stimulated cochleas. At equal current levels, less damage was found in subjects stimulated via electrodes placed on the round window and promontory, as compared to the scala tympani. Consistent reversible changes in threshold and suprathreshold features of the electrically evoked auditory brainstem response (EABR) were found. The magnitude of EABR change was directly related to exposure stimulus current level and to cochlear stimulation site. Suprathreshold features of the EABR were more sensitive to continuous stimulation exposures than threshold measures. Reversible EABR changes were found in the presence and absence of stimulation-induced histopathology.

Multichannel electrical stimulation of the auditory nerve in man. II. Channel interaction

A multichannel cochlear implant can be an effective prosthesis only if its channels are independent of each other. Presumably independence is achieved by stimulating different populations of surviving neurons. Two types of interaction might occur between channels: electrical current field summation peripheral to stimulation of the nerves and neural-perceptual interaction following stimulation. Two psychophysical techniques to assess channel independence are discussed. In one technique a masker is presented on one channel in order to adapt the nerves responding to that channel. The forward masked threshold of a signal is then measured on all other channels and elevation of threshold is assumed to indicate overlapping neural populations. In the second procedure channel interaction is evaluated by measuring the loudness summation of stimuli presented simultaneously to two channels. The magnitude, distribution, and phasic components of the loudness summation are measures of interaction between channels. Data from two subjects suggests that monopolar stimulation produces broader interaction patterns than bipolar stimulation as a function of electrode separation. Considerable differences in the extent of channel interaction were observed between the two subjects, possibly because of the difference in the absolute current levels needed for equivalent sensation levels.

Multichannel electrical stimulation of the auditory nerve in man. I. Basic psychophysics

Basic psychophysical measurements were obtained from three patients implanted with multichannel cochlear implants. This paper presents measurements from stimulation of a single channel at a time (either monopolar or bipolar). The shape of the threshold vs. frequency curve can be partially related to the membrane biophysics of the remaining spiral ganglion and/or dendrites. Nerve survival in the region of the electrode may produce some increase in the dynamic range on that electrode. Loudness was related to the stimulus amplitude by a power law with exponents between 1.6 and 3.4, depending on frequency. Intensity discrimination was better than for normal auditory stimulation, but not enough to offset the small dynamic range for electrical stimulation. Measures of temporal integration were comparable to normals, indicating a central mechanism that is still intact in implant patients. No frequency analysis of the electrical signal was observed. Each electrode produced a unique pitch sensation, but they were not simply related to the tonotopic position of the stimulated electrode. Pitch increased over more than 4 octaves (for one patient) as the frequency was increased from 100 to 300 Hz, but above 300 Hz no pitch change was observed. Possibly the major limitation of single channel cochlear implants is the 1-2 ms integration time (probably due to the capacitative properties of the nerve membrane which acts as a low-pass filter at 100 Hz). Another limitation of electrical stimulation is that there is no spectral analysis of the electrical waveform so that temporal waveform alone determines the effective stimulus.

Acute morphological changes in guinea pig cochlea following electrical stimulation. A preliminary scanning electron microscope study

A major area of concern in the development of the cochlear prosthesis involves the effects of the implant on inner ear anatomy and function. This study examined one aspect of host-implant interaction- the effects of cochlear implantation with and without electrical stimulation on cochlear morphology in the guinea pig. To accomplish this, a series of normal guinea pigs were acutely implanted bilaterally with a scala tympani multiple electrode prosthesis. One ear was stimulated with continuous 1 kHz sinusoidal current of constant intensity for a period of three hours. The contralateral control ear was not stimulated. Current intensities tested ranged from 0.1 mA to 1 mA rms. After stimulation the animals were sacrificed, perfused with fixative and the temporal bones were microdissected for examination using the scanning electron microscope. Morphological changes observed in the stimulated ear ranged from hair cell and supporting cell degeneration to complete destruction of the basilar membrane and organ of Corti overlying the electrode. These changes occurred at current intensities ranging from 0.4 mA to 1 mA and were conspicuously absent in the implanted but unstimulated control ears. The current intensities employed in this experiment were within operating ranges presently used in long-tern behavioral studies in other animal models. To what degree the morphological changes observed in this study may effect the function of the prosthesis or host responses on a long-term basis cannot be determined from this experiment. However, our observations do demonstrate that electrical stimulation of the cochlea is not benign and point out the need for further evaluation to help define safe standards for use of the prosthesis.

Design of the cochlear prosthesis: effects of the flow of current in the implanted ear

When structures within the temporal bone are stimulated electrically it is desirable to maximize the dynamic range of the stimulus. The maximum dynamic range of electrical stimulus seems to be found when the threshold of stimulation is minimum. The minimum threshold of stimulus is likely to be reached when the electrical current that flow through regions containing excitable cells is maximized. By implanting electrodes throughout the temporal bone, it is possible to apply electrical currents to the ear and to measure the distributions of current flowing within the ear. The results of these measurements demonstrate that when current flow is directed outside the scala tympani, lower thresholds can be obtained. Frequency dependence of the paths of current flow cannot be used to explain the frequency dependence of the frequency-threshold functions measured in animals.

Brainstem histopathology following chronic scala tympani implantation in monkeys

Degeneration in brainstem auditory nuclei was studied in monkeys following chronic implantation with scala tympani multielectrode systems. Nauta and Fink/Heimer-stained material from animals with survival times to 16 months were studied. The density and distribution of degeneration in the cochlear nuclei were consistent with observed patterns of spiral ganglion cell degeneration. Transneuronal degeneration was seen up to the level of the inferior colliculus. The density and form of the degeneration material in animals of varying survival times was consistent with the interpretation that a process of continuing degeneration occurred in these implanted ears.

Estimates of essential neural elements for stimulation through a cochlear prosthesis

Electrical stimulation of afferent auditory pathways through electrodes placed within and outside of the cochlea were used to study stimulation and design parameters relevant to a cochlear prosthesis. In the acute guinea pig preparation, the tract response evoked in brachium of the inferior colliculus by electrical stimulation to an ear provided estimates of the effectiveness of various electrode placements. Stimulation between an electrode in the cochlea and a site along the eighth nerve was characterized by the lowest thresholds. Stimulation between intracochlear electrodes was somewhat less effective, and stimulation between external electrodes at the nerve, cochlear nucleus, or distant point was least effective. Thresholds, expressed as current, rose at approximately 6 dB per octave for stimulus frequencies from 1 kHz to 16 kHz. Thresholds below 10 microA rms were seen for optimal placements. These observations suggest that the neural elements being stimulated are the cell bodies of the spiral ganglion cells.

Operating ranges for cochlear implants

The range of simple electrical stimuli that may be used for prosthetic stimulation via scala tympani implants was explored using psychophysical procedures in macaque monkeys. Biophysical considerations placed further limitations on the operating range. The operating range was dependent onstimulus waveform, electrode configuration, and the condition of the implanted cochlea and eighth nerve.