Progress in Development and Application of the University of California at San Francisco/Storz Multichannel Cochlear Implant

A four-channel bipolar cochlear implant developed at the University of California at San Francisco through a long series of animal experiments, engineering development studies, and speech processor design-optimization studies with experimental prosthetic devices in patients, is now being applied in a clinical investigation, as a joint project of the UCSF group and the manufacturer, Storz Medical Instruments.

The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation

This report examines the effects of intracochlear electrode configuration and mode of stimulation (bipolar or monopolar) on neural threshold and spatial selectivity in the inferior colliculus (IC) of the cat. Single and multiunit IC recordings were made in three groups of animals; acutely deafened adults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thresholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured at 6 dB above threshold. Spiral ganglion cell (SG) survival was examined histologically in all neonatally deafened animals and correlated with physiological results. Animals studied at less than 1.5 years had SG densities of 23.5%-64.4% of normal (mean=42.7%) while animals studied at greater than 2.5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiological results include the following. (1) Monopolar thresholds were 7-8 dB lower than bipolar thresholds in the same animals. (2) Varying the configuration of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or short-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6 dB)) of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulation. (4) The spatial selectivity of neonatally deafened animals studied at ages up to 1.5 years was equal to that of control animals with normal auditory experience. However, selectivity was degraded in the older animals. (5) Selectivity was decreased in some animals with the longitudinal bipolar configuration and multiple response peaks were seen in several cases using this stimulus configuration.

Plasticity in central representations in the inferior colliculus induced by chronic single- vs. two-channel electrical stimulation by a cochlear implant after neonatal deafness

The goal of this research is to examine the functional consequences of patterned electrical stimulation delivered by a cochlear implant in the deafened developing auditory system. In previous electrophysiological experiments conducted in the inferior colliculus (IC), we have demonstrated that the precise cochleotopic organization of the central nucleus (ICC) develops normally in neonatally deafened unstimulated cats and is unaltered despite the lack of normal auditory input during development. However, these studies also showed that chronic electrical stimulation delivered at a single intracochlear location by one bipolar channel of a cochlear implant induces significant expansion of the central representation of the stimulated cochlear sector and degrades the cochleotopic organization of the IC. This report presents additional data from a new experimental series of neonatally deafened cats that received chronic stimulation on two adjacent bipolar intracochlear channels of a cochlear implant. Results suggest that competing inputs elicited by electrical stimulation delivered by two adjacent channels can maintain the selective representations of each activated cochlear sector within the central auditory system and prevent the expansion seen after single-channel stimulation. Alternating stimulation of two channels and use of highly controlled electrical signals may be particularly effective in maintaining or even sharpening selectivity of central representations of stimulated cochlear sectors. In contrast, simultaneous stimulation using two channels of a model analog cochlear implant processor in one experimental animal failed to maintain channel selectivity and resulted in marked expansion and fusion of the central representations of the stimulated channels. This potentially important preliminary result suggests that under some conditions the central auditory system may be unable to discriminate simultaneous, overlapping inputs from adjacent cochlear implant channels as distinct.

Responses of inferior colliculus neurons to amplitude-modulated intracochlear electrical pulses in deaf cats

Current cochlear prostheses use amplitude-modulated pulse trains to encode acoustic signals. In this study we examined the responses of inferior colliculus (IC) neurons to sinusoidal amplitude-modulated pulses and compared the maximum unmodulated pulse rate (Fmax) to which they responded with the maximum modulation frequency (maxFm) that they followed. Consistent with previous results, responses to unmodulated pulses were all low-pass functions of pulse rate. Mean Fmax to unmodulated pulses was 104 pulses per second (pps) and modal Fmax was 60 pps. Above Fmax IC neurons ceased responding except for an onset burst at the beginning of the stimulus. However, IC neurons responded to much higher pulse rates when these pulses were amplitude modulated; 74% were relatively insensitive to carrier rate and responded to all modulated carriers including those exceeding 600 pps. In contrast, the responses of these neurons (70%) were low-pass functions of modulation frequency, and the remaining (30%) had band-pass functions with a maxFm of 42 and 34 Hz, respectively. Thus temporal resolution of IC neurons for modulated frequencies is significantly lower than that for unmodulated pulses. These two measures of temporal resolution (Fmax and maxFm) were uncorrelated (r(2) = 0.101). Several parameters influenced the amplitude and temporal structure of modulation responses including modulation depth, overall intensity and modulation-to-carrier rate ratio. We observed distortions in unit responses to amplitude-modulated signals when this ratio was 1/4 to 1/6. Since most current cochlear implant speech processors permit ratios that are significantly greater than this, severe distortion and signal degradation may occur frequently in these devices.

Temporal properties of chronic cochlear electrical stimulation determine temporal resolution of neurons in cat inferior colliculus

As cochlear implants have become increasingly successful in the rehabilitation of adults with profound hearing impairment, the number of pediatric implant subjects has increased. We have developed an animal model of congenital deafness and investigated the effect of electrical stimulus frequency on the temporal resolution of central neurons in the developing auditory system of deaf cats. Maximum following frequencies (Fmax) and response latencies of isolated single neurons to intracochlear electrical pulse trains (charge balanced, constant current biphasic pulses) were recorded in the contralateral inferior colliculus (IC) of two groups of neonatally deafened, barbiturate-anesthetized cats: animals chronically stimulated with low-frequency signals (< or = 80 Hz) and animals receiving chronic high-frequency stimulation (> or = 300 pps). The results were compared with data from unstimulated, acutely deafened and implanted adult cats with previously normal hearing (controls). Characteristic differences were seen between the temporal response properties of neurons in the external nucleus (ICX; approximately 16% of the recordings) and neurons in the central nucleus (ICC; approximately 81% of all recordings) of the IC: 1) in all three experimental groups, neurons in the ICX had significantly lower Fmax and longer response latencies than those in the ICC. 2) Chronic electrical stimulation in neonatally deafened cats altered the temporal resolution of neurons exclusively in the ICC but not in the ICX. The magnitude of this effect was dependent on the frequency of the chronic stimulation. Specifically, low-frequency signals (30 pps, 80 pps) maintained the temporal resolution of ICC neurons, whereas higher-frequency stimuli significantly improved temporal resolution of ICC neurons (i.e., higher Fmax and shorter response latencies) compared with neurons in control cats. Furthermore, Fmax and latencies to electrical stimuli were not correlated with the tonotopic gradient of the ICC, and changes in temporal resolution following chronic electrical stimulation occurred uniformly throughout the entire ICC. In all three experimental groups, increasing Fmax was correlated with shorter response latencies. The results indicate that the temporal features of the chronically applied electrical signals critically influence temporal processing of neurons in the cochleotopically organized ICC. We suggest that such plastic changes in temporal processing of central auditory neurons may contribute to the intersubject variability and gradual improvements in speech recognition performance observed in clinical studies of deaf children using cochlear implants.

Strategies to improve electrode positioning and safety in cochlear implants

An injection-molded internal supporting rib has been produced to control the flexibility of silicone rubber encapsulated electrodes designed to electrically stimulate the auditory nerve in human subjects with severe to profound hearing loss. The rib molding dies, and molds for silicone rubber encapsulation of the electrode, were designed and machined using AutoCad and MasterCam software packages in a PC environment. After molding, the prototype plastic ribs were iteratively modified based on observations of the performance of the rib/silicone composite insert in a clear plastic model of the human scala tympani cavity. The rib-based electrodes were reliably inserted farther into these models, required less insertion force and were positioned closer to the target auditory neural elements than currently available cochlear implant electrodes. With further design improvements the injection-molded rib may also function to accurately support metal stimulating contacts and wire leads during assembly to significantly increase the manufacturing efficiency of these devices. This method to reliably control the mechanical properties of miniature implantable devices with multiple electrical leads may be valuable in other areas of biomedical device design.

A transparent model of the human scala tympani cavity

A dimensionally accurate clear model of the human scala tympani has been produced to evaluate the insertion and position of clinically applied intracochlear electrodes for electrical stimulation. Replicates of the human scala tympani were made from low melting point metal alloy (LMA) and from polymethylmeth-acrylate (PMMA) resin. The LMA metal casts were embedded in blocks of epoxy and in clear silicone rubber. After removal of the metal alloy, a cavity was produced that accurately models the human scala tympani. Investment casting molds were made from the PMMA scala tympani casts to enable production of multiple LMA casts from which identical models were fabricated. Total dimensional distortion of the LMA casting process was less than 1% in length and 2% in diameter. The models have been successfully integrated into the design process for the iterative development of advanced intracochlear electrode arrays at UCSF. These fabrication techniques are applicable to a wide range of biomedical design problems that require modelling of visually obscured cavities.

Consequences of chronic extracochlear electrical stimulation in neonatally deafened cats

This investigation examined the consequences of neonatal deafness and chronic electrical stimulation of the cochlea in the developing auditory system. Cats were bilaterally deafened by daily ototoxic drug administration for two weeks after birth. Electrical stimulation was initiated at 6-9 weeks of age and continued for up to 6 months, using monopolar round window electrodes that synchronously excited auditory neurons throughout the cochlea. Morphometric evaluation of the density of spiral ganglion cell somata within Rosenthal's canal demonstrated that chronic stimulation induced an increase of about 6% in neuronal survival. Although this difference was statistically significant, extracochlear stimulation in these cats was less effective in preventing neural degeneration than lower intensity, more restricted intracochlear stimulation that was shown in a previous study to induce an average increase of about 13% in neuronal survival. Electrophysiological recording experiments conducted in the inferior colliculus in these animals indicated that monopolar extracochlear stimulation can induce profound alterations in the spatial (frequency) selectivity of the auditory midbrain. On average, results were similar to those previously reported for bipolar intracochlear stimulation, showing about a two-fold expansion of the central representation of chronically stimulated electrodes. However, results with extracochlear stimulation showed much greater variability among individual animals. The results presented suggest that it is problematic to effect consistent 'whole' nerve stimulation using monopolar round window electrodes. Moreover, this mode of stimulation can induce profound functional alterations in the central nervous system and is substantially less effective in forestalling the degeneration of auditory neurons than intracochlear stimulation. Both these results contraindicate the implantation of such electrodes in young children for the purpose of maintaining the integrity of the auditory system for later application of a multichannel cochlear implant.

Changes in the cat cochlear nucleus following neonatal deafening and chronic intracochlear electrical stimulation

The effects of chronic intracochlear electrical stimulation on the cochlear nucleus (CN) were studied in eight cats that were neonatally deafened by daily intramuscular injections of neomycin. Profound hearing loss was confirmed in each animal by auditory brainstem response (ABR) and frequency following response (500 Hz) testing. Five of the kittens were implanted unilaterally with a scala tympani electrode array at ages 8-16 weeks. These kittens were stimulated daily for four hours at 2 dB above the evoked ABR threshold, over a period of three months, and subsequently euthanized for histological analysis at 26-32 weeks of age. The three remaining deaf kittens were maintained without stimulation over prolonged periods in order to study the long-term consequences of neonatal deafening, and were euthanized at 66-133 weeks of age. This study compares the CN of these deafened experimental animals and the CN of normal adult cats. Three experimental parameters were examined: CN volume, cross-sectional area of spherical cells in the rostral anteroventral cochlear nucleus (AVCN), and spherical cell density in this same region. The CN in animals that received electrical stimulation showed significant bilateral degenerative changes in all three measured parameters. Total nuclear volume was reduced by 35-36%, spherical cell size was reduced by 20-26%, and spherical cell density decreased by 36-42%, as compared to the normal cat CN. Comparisons were also made in the stimulated animals between CN ipsilateral to the stimulated cochlea and the contralateral, unstimulated CN.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic intracochlear electrical stimulation in neonatally deafened cats: effects of intensity and stimulating electrode location

An earlier study conducted in this laboratory suggested that chronic intracochelear electrical stimulation at moderate current levels can at least partially delay or prevent the retrograde degeneration of primary auditory (spiral ganglion) neurons that otherwise is progressive after neonatal deafness induced by ototoxic drug administration. Increased survival of spiral ganglion neurons was observed within the basal cochlear region near the stimulating biopolar electrode pairs, while in more apical regions there was no significant difference between the stimulated and control cochleas. The mechanisms underlying this maintenance of spiral ganglion neurons induced by chronic electrical stimulation are uncertain, especially since increased neuronal survival was observed over broader sectors of the ganglion than would be expected to be directly activated by the bipolar electrodes and moderate stimulation intensity (6 dB above electrically evoked auditory brainstem response threshold) used. In this report, data are presented from a second series of neonatally deafened and chronically stimulated cats. The parameters for chronic electrical stimulation were manipulated in two simple ways. First, the intensity of the electrical stimulus was reduced from the earlier study, while the duration of chronic stimualtion periods was increased; and secondly, two different intracochlear positions of stimulating electrodes were employed in different experimental groups. Results indicate that elecrical stimulation of the cochlea at an extremely low intensity (2 dB above electrically evoked auditory brainstem response threshold) is sufficient to at least partially prevent or delay ganglion cell degeneration in the deafened cochlea. In addition, data suggest a differential distribution of the maintained or conserved ganglion cells, such that when the stimulating electrode pair was positioned near the base of the cochlea increased ganglion survival in a more basal cochlear sector, while stimulation at a more apical site resulted in increased neuronal survival extending to more apical regions.

Chronic intracochlear electrical stimulation in the neonatally deafened cat. II. Temporal properties of neurons in the inferior colliculus

The major focus of this study was to define the effects of chronic intracochlear electrical stimulation (ICES) on single unit responses in the inferior colliculus from three experimental groups: 1) normal adults 2) neonatally-deafened/unstimulated adults; and 3) neonatally-deafened/chronically stimulated adults. The major findings include: 1) IC neurons in normal adults showed a diversity of perstimulus responses to ICES which were qualitatively similar to those evoked by acoustic stimuli. They responded with: an onset burst, a sustained discharge, a decrease in their spontaneous activity, or a strong post-stimulus response. The excitatory responses showed either a monotonic or a nonmonotonic increase in activity with increasing stimulus intensity. Response latencies ranged from 5 to over 40 ms. 2) Responses to ICES in normal and deafened/unstimulated animals were virtually indistinguishable from one another. 3) In contrast, responses to ICES in neonatally deafened stimulated animals were different from normal and from deafened, unstimulated animals. Their perstimulus response latencies were significantly shorter, their late response latencies were significantly longer, and the frequency of occurrence of inhibitory and late responses were significantly higher. From these results we conclude that the responses to intracochlear electrical stimulation are directly comparable to those observed following normal acoustic stimulation; that development of cochleotopic organization of the inferior colliculus is not affected by the almost complete lack of normal acoustic input experienced by neonatally deafened animals; and that the basic response properties of IC units are likewise unaffected by neonatal deafening. Moreover, the results suggest that, although the limited regime of electrical stimulation employed in these studies produced no major qualitative distortions in the perstimulus response patterns of IC neurons, it did result in some quantitative changes in those responses.

Chronic intracochlear electrical stimulation induces selective survival of spiral ganglion neurons in neonatally deafened cats

Ten newborn kittens were deafened by systemic administration of neomycin sulfate. Profound hearing losses were documented by ABR and FFR (500 Hz) testing. At 9-17 weeks of age, the young deafened cats were unilaterally implanted with a multichannel scala tympani electrode. Six of the animals were chronically stimulated at 6 dB above electrically evoked ABR thresholds for 1 h/day for periods of 1 month or 3 months. Stimuli were charge-balanced biphasic pulses (200 microseconds/phase, 30 pps.) The remaining 4 cats underwent identical deafening and implantation schedules but were not stimulated. Results indicate that administration of neomycin in neonatal cats induced degeneration of hair cells and spiral ganglion cell loss that was bilaterally symmetrical between the two cochleas of each individual animal, although there was variation between animals in the severity of the ototoxic drug effect. In animals receiving passive (unstimulated) implants, morphometric analysis of spiral ganglion cell density showed no significant difference in ganglion cell survival between the implanted cochleas and the contralateral control ears. In contrast, animals that were chronically stimulated for 3 months showed significantly better neuronal survival in implanted and stimulated cochleas as compared to contralateral deafened control ears. The induced conservation of spiral ganglion neurons was observed consistently within the basal cochlear region near the stimulating electrodes. In more apical regions there was no significant difference between the stimulated and control cochleas. The mechanisms underlying this selective conservation of spiral ganglion neurons induced by chronic intracochlear electrical stimulation are uncertain. Since no comparable chronic stimulation studies have been conducted in adults, it is not known whether similar conservation effects could be induced in mature animals.

Chronic intracochlear electrical stimulation in the neonatally deafened cat. I: Expansion of central representation

Intracochlear electrical stimulation via cochlear prostheses has been employed as a means of providing some hearing to deaf children. Since chronically restricted stimuli are known to have profound effects on central nervous system development, it is important to examine the effects of chronic intracochlear electrical stimulation in a neonatally deafened animal model. In this study neonatally deafened cats were implanted with a scala tympani electrode consisting of two pairs of electrodes. Chronic electrical stimulation was delivered using one electrode pair and consisted of charge-balanced biphasic pulses (200 microseconds/phase, 30 pps) at 2 dB above the electrically evoked auditory brain stem response (EABR) threshold for 4 h/day or at 6 dB 1 h/day, 5 days/week, for up to 3 months. The second electrode pair was unstimulated and served as an internal control. Following chronic stimulation, acute mapping experiments were performed in the central nucleus of the inferior colliculus (ICC) using single unit and multi-unit recording techniques and activating each electrode pair separately. In addition to these chronically stimulated animals, 2 other groups of experimental animals were studied: A normal group consisting of prior normal adult cats that were acutely implanted; and an unstimulated control group consisting of neonatally deafened adult cats that were either acutely implanted or implanted at 8-10 weeks of age but not chronically stimulated. Among the major findings of this study are: Electrical stimulation of the intracochlear bipolar electrode consistently produces activation of a reproducibly limited sector of the ICC. The location of this activated sector was found to be consistent with the known cochleotopic organization of the ICC and the intracochlear location of the stimulating electrodes. No major differences in the spatial representation of activated electrodes were found between prior normal cats and neonatally deafened unstimulated cats. The locations, shapes and widths of these spatial representations were virtually indistinguishable indicating that ICC cochleotopic organizations were equivalent in these two experimental groups. In contrast, the ICC representation of chronically stimulated electrode pairs were found to be significantly different. The average area activated by chronically stimulated electrode pairs at 6 dB above minimum threshold was approximately twice that of unstimulated deafened animals and prior normal animals; and it was larger, but not significantly so, than the average of the unstimulated electrode pair in the same experimental group.(ABSTRACT TRUNCATED AT 400 WORDS)

Surgical considerations and hearing results with the UCSF/Storz cochlear implant

Sixteen patients have been implanted with the UCSF/Storz multichannel implant, 11 of whom have been fitted with their external speech processors and transmitters and administered postoperative audiological evaluations. Both the surgical procedures used and the hearing results for these patients are presented. The potential medical/surgical complications of implant surgery and the future direction of research and development within the UCSF/Storz implant program are discussed.

The UCSF/Storz multichannel cochlear implant: patient results

Using the four channel cochlear implant system with a vocoder-based processor developed at UCSF over an extensive period of research, clinical trials of the UCSF/Storz device were initiated in February 1985, under the sponsorship of Storz Instrument Company. To date, 13 patients have been implanted with this device, nine of whom have been fitted with their external processor and transmitter and have received at least their initial postoperative evaluation. Patient results have been extremely promising, with eight of the nine patients obtaining some open-set auditory only speech understanding. Most patients have demonstrated improvement over time and all patients have attained an enhancement in lipreading ability with the use of the UCSF/Storz device.

The University of California, San Francisco/Storz cochlear implant program

Using a four-channel implant system with a vocoder-based processor developed at UCSF over an extensive period of research, clinical trials of the UCSF/Storz device were initiated in February 1985 under the sponsorship of Storz Instrument Company. To date, 13 patients have been implanted with the UCSF/Storz device, 10 of whom have been fitted with their external processor and transmitter and have received at least their initial postoperative evaluations. Of these 10 patients, nine are able to use all four channels of their implant system. The device fitting/adjusting process for these patients has been remarkably easy, requiring only approximately 30 to 60 minutes. Patient results have been extremely good, with eight of the 10 patients obtaining some open-set auditory only speech understanding. Without extensive rehabilitation and training, most patients have demonstrated an improvement in speech reception over time. Lip-reading and tracking results indicate that all patients have attained an enhancement of lip-reading ability with the use of the device, suggesting that improved general communication skills have been provided for each UCSF/Storz patient.

Cochlear pathology with chronically implanted scala tympani electrodes

In summary, these results with chronic implantation of three types of scala tympani electrode indicate the critical importance of two factors in reducing the risk of additional damage to cochlear structure by implantation surgery: (1) The shape and mechanical characteristics of the array must be precisely controlled such that insertion can be performed with an acceptably low incidence of trauma; and (2) the specific electrode materials and fabrication procedures must be demonstrated to be highly biocompatible in in vivo animal control studies. It appears that the neuronal elements of a prior normal cochlea (in the cat) can withstand chronic implantation of scala tympani electrodes (for at least 1 year) if these two prerequisites are met. The effects of chronic electrical stimulation with these arrays at current levels appropriate for the operation of such devices in patients is not known at present.