The multi-channel cochlear implant and the relief of severe-to-profound deafness

Abstract

This personal reflection outlines the discoveries at the University of Melbourne leading to the multi-channel cochlear implant, and its development industrially by Cochlear Limited. My earlier experimental electrophysiological research demonstrated temporal coding occurred for only low frequencies, i.e. below 200–500 pulses/second. I was able to confirm these findings perceptually in behaviourally conditioned animals. In addition, these studies showed that temporal discrimination occurred across spatial coding channels. These experimental results correlated with the later conscious experience for electrical stimulation in my implant patients. In addition, the mid-to-high frequencies were coded in part by place of stimulation using bipolar and monopolar stimulation to restrict current spread. Furthermore, place of stimulation had the qualities of sharpness and dullness, and was also experienced as vowels. Owing to the limitation in coding speech with a physiological model due to the overlap of electrical current leading to unpredictable variations in loudness, a speech coding strategy that extracted the most important speech features for transmission through an electro-neural ‘bottle-neck’ to the brain was explored. Our inaugural strategy, discovered in 1978, extracted the second formant for place of stimulation, voicing for rate of stimulation, and sound pressure for current level. This was the first coding strategy to provide open-set speech understanding, as shown by standard audiological tests, and it became the first clinically successful interface between the world and human consciousness. This strategy was improved with place coding for the third formant or high-frequency spectrum, and then the spectral maxima. In 1989, I operated on our first patient to receive a bilateral implant, and in 1990, the first with a bimodal processor. The psychophysics and speech perception for these showed that the stimuli from each side could be fused into a single image, and localized according to differences in intensity and time of arrival of the stimuli. There were significant improvements for speech perception in noise. In 1985, I implanted our first children with the multi-channel prosthesis and found that speech understanding and spoken language were greatly improved the younger the child at surgery, and especially when younger than 12 months. Speech understanding was strongly related to the development of place coding. In 1990, the US Food and Drug Administration approved the implant for deaf children, the first by any world health regulatory body making it the first major advance in helping deaf children to communicate.     

Iowa cochlear implant clinical project: results with two single-channel cochlear implants and one multi-channel cochlear implant

Cochlear implants have become a realistic alternative for the management of profoundly deaf patients. A variety of implants with differing electrode designs and coding strategies have been developed by nine major implant centers around the world. Each center has their "star" patient, but objective comparisons between these different implant designs are unavailable. In order to determine the performance characteristics of the present generation of cochlear implants, comparison data are vital. We have developed an independent center where uniform objective comparisons of different cochlear implants can be performed longitudinally. This report will present results of nine patients implanted with three different cochlear implant prostheses. Four patients have been implanted with the Los Angeles (House) single-channel implant, three patients have received the Vienna (Hochmair) single-channel intracochlear device, and two patients have been implanted with the Melbourne (Clark) 21-channel unit. All patients have had 11 months or more of experience with their cochlear prostheses. The results of a comprehensive audiologic battery which includes audiovisual and environmental tasks are presented. All implants provide significant improvement in speechreading and sound awareness. The findings to date suggest that there is strong correlation between top-down cognitive processing (as reflected by lip reading skills) and performance with cochlear implants.     

The surgical rehabilitation of total deafness with the multichannel cochlear implant. Indications and results

In case of total deafness the multichannel cochlear implant supplies a substantial improvement. It allows recognition of known words without lip reading, with a high percentage of good responses. Clinical and phoniatric results are reported in 21 cases of implanted patients.     

Real-life performance considerations of four pediatric multi-channel cochlear implant recipients.

Most pediatric cochlear implant programs support a team approach for post-implant services. However, individuals directly involved in the care of these children often have differing opinions on the child's performance. We describe our experience with four children, aged 3 to 10 at the time of implantation, who have used the Nucleus 22-channel cochlear implant device for at least nine months. A questionnaire, focusing on the individual's observations of the child's speech and hearing performance in their particular setting (i.e., clinic, home, school), was completed by the child's parent(s), therapist and classroom teacher. Overall, performance in structured settings (i.e., testing and therapy sessions) was not in agreement with performance in unstructured settings (i.e., classroom and home environments). The results suggest that different individuals interacting with the same child in different environments often have differing perceptions of the child's performance. Based on the information obtained on the four cases, we discuss factors to consider in assessing the real-life performance of pediatric cochlear implant recipients.     

Speech perception with multi-channel cochlear implant of short duration pulse strategy

A multi-channel cochlear implant (Nucleus type) was implanted in a 40-year-old female Japanese patient who became totally deaf after meningitis. The formant-based speech processing strategy was used, but a narrow pulse width of 22-42 microseconds was required because of intermittent difficulty in controlling the pulse amplitude. The patient was tested with a speech tracking test and could recognize 24 bunsetsues (the minimum meaningful unit of the Japanese sentence) per minute using the cochlear implant plus lipreading and 14.3 bunsetsues for the lipreading alone after 3.5 months' training. The patient was also able to understand usual conversational sentences spoken a little slowly. Scores of vowel and consonant tests reached 70% and 54% respectively for the cochlear implant alone, and 100% and 73% for the cochlear implant plus lipreading. This study has also shown that cochlear stimulation with very narrow pulse widths can be used, and restore speech comprehension ability for the Japanese.     

Maturation of the mismatch negativity: effects of profound deafness and cochlear implant use

The use of cochlear implants to restore auditory sensation in deaf children is increasing, with a trend toward earlier implantation. However, little is known about how auditory deprivation and subsequent cochlear implant use affect the maturing human central auditory system. Our previous studies have demonstrated that the obligatory auditory evoked potentials (AEPs) of implanted children are very different from those of normal-hearing children. Unlike the obligatory potentials, which primarily reflect neural responses to stimulus onset, the mismatch negativity (MMN) provides a neurophysiological measure of auditory short-term memory and discrimination processes. The purpose of this investigation is to review our studies of the effects of auditory deprivation due to profound deafness and cochlear implant use on the maturation of the MMN in children, placed in the context of overall age-related changes in the AEPs. The development and application of a statistical technique to assess the MMN in individuals is also reviewed. Results show that although the morphology of the obligatory AEPs is substantially altered by the absence of a normal N(1) peak, the MMN is robustly present in a group of implanted children who have good spoken language perception through their device. Differences exist in the scalp distribution of the MMN between implanted and normal-hearing children. Specifically, the MMN appears to be more symmetrical in amplitude over both hemispheres, whereas it is initially much larger over the contralateral hemisphere in normal-hearing children. These findings suggest that, compared to N(1), the MMN is a better measure of basic auditory processes necessary for the development of spoken language perception skills in profoundly deaf children and adults who use a cochlear implant.     

语后聋患者多通道人工耳蜗植入后听觉效果分析

目的:探讨使用心理物理测试及听觉行为分级、言语可懂度分级对语后聋多通道人工耳蜗植入患者听觉言语康复效果进行评估的意义,比较语前聋和语后聋患者术后康复效果.方法:1997-03-2007-06共植入人工耳蜗353例,选择语后聋植入者31例,语前聋植入者59例参加研究.对语后聋和语前聋受试者的纯音听阈、听觉行为分级标准(CAP)和言语可懂度分级标准(SIR)进行评估分析,并比较相同植入电极的语后聋和语前聋患者开机0.5年后的电听觉阈值、最大舒适强度和动态范围有无差异,用统计学方法加以分析.结果:所有受试者声场测试语言频率平均听阈(啭音)为25～45 dB(nHL).植入同种电极(澳大利亚直电极、澳大利亚弯电极、奥地利Medel C40+)的语前聋和语后聋患者之间听觉阈值最大舒适强度和动态范围差异无统计学意义(P>0.05).语后聋患者的CAP和SIR得分高于语前聋患者.结论:CAP和SIR是一种对人工耳蜗植入者日常生活中听说能力的方便有效的评估方法.植入相同电极的语前聋和语后聋患者之间的纯音听阈、电听觉阈值、最大舒适强度、动态范围无明显差异,但语后聋患者听觉水平和言语可懂度效果好与语前聋患者.     

Auditory responses in cochlear implant users with and without GJB2 deafness

OBJECTIVE/HYPOTHESIS: It is reasonable to suppose that the pattern of sensorineural damage along the length of the cochlea depends on the etiology of a hearing loss (HL). In GJB2-related deafness, we hypothesize that gap junction deficits are uniformly distributed and will result in similar damage along the length of the cochlea as compared with non-GJB2 subjects. We assessed this by measuring patterns of neural activity and hearing from apical versus basal cochlear implant electrode regions. STUDY DESIGN: This was a prospective, blind, controlled study. METHODS: Blood from 301 pediatric cochlear implant users was analyzed for mutations in GJB2 by direct sequencing. After exclusion of patients with monoallelic GJB2 mutations, associated syndromes, or risk factors for HL that were not congenital, 39 children with biallelic GJB2 mutations and 58 without GJB2 mutations were evaluated. Hearing was measured before implantation at frequencies ranging from 250 Hz to 8 kHz. After implantation, neural activity at the apical and basal ends of the implanted array was measured using electrically evoked compound action potentials of the auditory nerve (ECAPs) and evoked stapedius reflexes (ESRs). RESULTS: GJB2 and non-GJB2 groups were not significantly different with respect to sex, age at implantation, duration of auditory deprivation, hearing aid use, duration of aided hearing, ear implanted, implant model, or depth of insertion (P>.05). Children with GJB2-related HL had greater similarities between low- and high-frequency residual hearing and between neural activity electrically evoked at apical and basal regions of the cochlea as compared with children with non-GJB2-related HL who demonstrated larger deficits in basal regions. CONCLUSION: Results suggest more consistent spiral ganglion survival along the length of the cochlea in GJB2-related HL as compared with non-GJB2-related HL, which appears to involve a decreasing gradient of spiral ganglion survival from the apex to the base of the cochlea. Our findings support our premise that in GJB2-related HL, dysfunction of gap junctions likely occurs to a similar degree in the apical and basal regions of the cochlea. This knowledge might be used to customize implantable devices for patients with HL in the future.     

An X-ray fluorescence microscopic analysis of the tissue surrounding the multi-channel cochlear implant electrode array

Objectives: The aim of this study was to analyse the tissue surrounding the University of Melbourne's (UOMs) multi-channel cochlear implant electrode array and cochlear limited replacements, after long-term implantations. In particular, it aimed to identify the particulate material in the fibrous tissue capsule of the arrays implanted in 1978, 1983, and 1998, by using the Australian Synchrotron for X-ray fluorescence microscopy (XFM) to reveal the characteristic spectrum of metal, in particular platinum. This also helped to determine its format and chemical state. Tissue was retrieved following the recipient's death in 2007.

Methods: Tissue was fixed and sections taken across the UOM and Cochlear Corporation (CI-22 and CI-24) electrode tracks. These were stained with Masson's trichrome. The Australian Synchrotron enabled XFM to accurately identify platinum from its characteristic fluorescence spectrum.

Results: There was a fibrous tissue capsule (about 100-µm thick) and small regions of calcification around the UOM and CI-22 arrays, but a thinner capsule (40–60-µm thick) around CI-24, and a greater degree of calcification. Dark particulate matter was observed within macrophages and especially in fibrous tissue in proximity to the UOM and CI-22 arrays. This was identified as platinum using X-ray fluorescence. There was also diffusion of platinum into the tissue surrounding the UOM and CI-22 electrodes and fine particles had penetrated the spiral ligament.

Discussion: The larger particulate matter in the tissue around the UOM and CI-22 arrays suggested that it had flaked off in the manufacturing of the UOM electrodes. The more diffuse spread of platinum in the tissue around the UOM and CI-22 electrodes was likely due to electrolysis, probably from charge imbalance with the bipolar pulses from the UOM implant. This did not occur with the Cochlear CI-24 device. Furthermore, the widespread fine particles of platinum could have also been due to corrosion, especially from the UOM electrodes.     

Auditory cortical activation and speech perception in cochlear implant users: effects of implant experience and duration of deafness

This study aimed to investigate the relationship between outcome following cochlear implantation and auditory cortical activation. It also studied the effects of length of implant use and duration of deafness on the auditory cortical activations. Cortical activity resulting from auditory stimulation was measured using [(18)F]FDG positron emission tomography. In a group of 18 experienced adult cochlear implant users, we found a positive correlation between speech perception and activations in both the primary and association auditory cortices. This correlation was present in a subgroup of experienced implant users but absent in a group of new implant users with similar speech perception abilities. There was a significant negative correlation between duration of deafness and auditory cortical activation. This study gives insights into the relationship between implant speech perception and auditory cortical activation and the influence of duration of preceding deafness and implant experience.     

Better speech performance in cochlear implant patients with GJB2-related deafness.

OBJECTIVE: We applied mutation screening in seven cochlear implant users to identify those persons with GJB2-related deafness to determine whether etiology of deafness was predictive of speech performance after implantation. METHODS: Direct sequence of GJB2 was conducted over seven cochlear implant users with prelingual hearing impairment and their speech, language and cognitive performance was examined. RESULTS: The three persons with GJB2-related deafness had a mean vocabulary of 1243 words compared to a mean vocabulary of 195 words in the four children with GJB2-unrelated deafness, although the number of patients examined here was limited. The developmental quotient (DQ) of cognitive ability also was higher in those children with GJB2-related deafness. CONCLUSIONS: These preliminary results suggest that better speech performance after cochlear implantation may be observed in persons with GJB2-related deafness. In the future, detailed phenotypic studies and mutation screening for non-syndromic hearing loss may play an important role in the preoperative assessment of prelingually-deafened children.     

Of kittens and kids: altered cortical maturation following profound deafness and cochlear implant use

Profoundly deaf children who use a cochlear implant (CI) provide a unique opportunity to investigate the effects of auditory sensory deprivation on the maturing human central nervous system. Previous results suggest that children fitted with a CI show evidence of altered auditory cortical maturation, based on evoked potentials. This altered maturation was characterized by both latency delays and morphological changes in the cortical auditory evoked potentials (AEPs). Based on prolonged P(1) latencies compared to age-matched normal-hearing (NH) peers, these data suggested a delayed maturation nearly equivalent to the period of deafness. However, rates of maturation for this AEP peak were essentially the same in NH and CI children. This suggests that, given enough time, the AEPs of CI children would assume the characteristic morphology found in older NH teens and NH adults. However, the data also indicated a substantial alteration of the typical set of obligatory P(1)-N(1b)-P(2) peaks, specifically related to the absence of the N(1) potential. Recent analyses of more extensive sets of longitudinal and cross-sectional data indicate that even after many years of implant use, the AEPs of CI users in their late teens remain very different from those of their NH peers. The P(1) peak latency remains prolonged and P(1) amplitude remains much larger in CI users than in age-matched NH teens. These findings suggested that age-related changes in the P(1) peak are completed by 12 years of age. In addition, the normal N(1b) peak fails to emerge in virtually all of the CI children tested in our laboratory. A major new interpretation of the abnormal maturation of AEP waveforms in CI children is presented. It is based on direct evidence showing that a persistent immaturity of the superficial layer axons has persistent negative effects on the generation of the N(1b) and, consequently, on the morphology of the AEPs. A comparison of scalp-recorded AEPs from implanted children with local field potentials measured from the cortical surface in deaf white kittens suggests the effects of deafness and CI use are similar across these mammalian species. For both species, a period of profound deafness followed by CI stimulation reveals a substantial immaturity in cortical activation even after a period of electrical stimulation by the CI.     

The cochlear implant electrode-pitch function

The cochlear frequency-place function in normal hearing ears has been found to be an exponential relationship in a wide variety of species [D.D. Greenwood, J. Acoust. Soc. Am. 87 (1990) 2592-2605]. Although it seems reasonable to assume a similar function for electrical stimulation by means of an intra-cochlear electrode array, the exact frequency-place function for this special type of stimulation needs to be investigated. Six users of the MED-EL COMBI 40+ cochlear implant device with moderate to profound hearing loss between 125 and 1000 Hz in the non-implanted ear took part in a binaural pitch adjustment experiment. The COMBI 40+ electrode array provides a deep insertion into the scala tympani and a wide spatial separation between the stimulating electrodes. Insertion depth was controlled by Stenver's view plain radiographs and the insertion angle was estimated. The task of the subjects was to adjust the frequency of a sinusoid presented in the non-implanted ear by means of an adjusting knob until they perceived the same pitch as was elicited by a reference stimulus in the implanted ear. The results show adjustments corresponding to electrode positions along the cochlea, with the exception of the two most apical electrodes for most of the subjects. Pitch increased in an orderly fashion with an average of 98 Hz per electrode separation (40 Hz/mm). In contrast to the exponential predictions according to [D.D. Greenwood, J. Acoust. Soc. Am. 87 (1990) 2592-2605] for normal hearing, the average electrode-pitch function shows a linear relationship.     

The Vienna cochlear implant program

The cochlear implant program in Vienna has now gathered 160 patient years of experience with 25 patients who have received an extracochlear implant prosthesis and 31 patients who have received an intracochlear implant prosthesis. The sound processor avoids the transformation of speech into pulses and provides a processed analog broad-band stimulation signal. Approximately one half of the patients are prelingually deaf. The selection criteria include electrical promontory stimulation and acoustic and electrical brain-stem audiometry. The rehabilitation program consisting of counseling, communication training, and auditory training is considered an important part of the program. The results are assessed using the Vienna Auditory Test battery. Sixty per cent of the postlingually deafened patients achieve some open-set speech understanding without lip-reading. Sixty-five per cent of the users show a substantial improvement in aided speech understanding versus lip-reading alone.     

Re-training the deaf ear: Auditory training for adult cochlear implant users with singlesided deafness

Objective: While cochlear implant (CI) provision for adults with single-sided deafness (SSD) is now an accepted treatment option, auditory training programs specific to this group of CI users have not been described. This paper details the auditory training protocol and critical factors required to rehabilitate CI users with post-lingual SSD.

Outcomes and Results: Several key factors are integral to the success of the rehabilitation program; these include 1) CI users with SSD require a map that is balanced as closely as possible to their normal hearing ear and has optimal mapping levels; 2) the auditory training program needs to be stimulating, rewarding, and directly stimulate the implanted ear via Direct Auditory Input (DAI); 3) CI users need to achieve some success in the early post-implantation stages to maintain or increase their motivation; 3) CI users need to be fully committed to the auditory training; and 5) a well-defined structured auditory training program with immediate feedback and markers of success helps ensure optimal communication outcomes. As an indication of success, from the foundation of the program in 2008 until the present all adults with SSD who have received a CI at our clinic (N?=?114) only 5 have elected to stop using their device.

Conclusion: The auditory training program described herein has been developed to optimize hearing and quality of life outcomes for adult CI users with SSD.     

The spiral ganglion and cochlear nuclei of deafness mice

Deafness mice (dn/dn) never develop hearing, but, except for the associated physical defects, have no other known abnormalities. For this study, cochleas and brains of five adult homozygotes (dn/dn) and five adult heterozygotes (+/dn), matched by weight and sex, were prepared for serial section light microscopy. In the homozygotes, the organ of Corti was totally degenerated basally, gradually improving toward the apex where supporting cells, border cells and pillar cells were present; however, the stria vascularis was dystrophic in the apex. The saccular macula was atrophied or dystrophic in seven of the ten homozygote ears. The homozygotes had only 23% of the number of spiral ganglion cells found in the heterozygotes, but they appeared robust. In six of the ears of each group there was clumping of apical spiral ganglion neurons. In the homozygotes, the volume of Rosenthal's canal was 121%, that of the dorsal nuclei was 90%, and that of the ventral cochlear nuclei was 63% of the comparable volumes in heterozygotes. The globular cells of the homozygote ventral cochlear nucleus were 72% of the size of those of heterozygotes. These quantitative morphological abnormalities of the homozygote spiral ganglion and cochlear nuclei may result from organ of Corti atrophy.