Histopathology of the Clarion cochlear implant electrode positioner in a human subject.

A Silastic electrode positioner was introduced by the Advanced Bionics Corporation in 1999 and it was designed to achieve a perimodiolar position of the stimulating electrode. The positioner was voluntarily recalled in the United States in July 2002 due to an apparent higher risk of bacterial meningitis in patients in whom the electrode positioner had been placed. A detailed histopathologic study of the positioner in the human has not previously been published. The histopathologic findings in a 74-year-old woman who underwent bilateral cochlear implantation using the positioner are presented. Findings include a large track caused by the combined electrode and its positioner with considerable disruption of the basilar membrane and osseous spiral lamina. Although there was a fibrous sheath around the electrode and positioner at the cochleostomy in both ears, this fibrous sheath did not extend deeply into the cochlea except at the apical end of the electrode beyond the positioner. This resulted in a large fluid space around and between the positioner and electrode within the cochlea and presumably in fluid continuity with the cerebrospinal fluid space. Possible clinical implications are discussed.

Histopathology of cochlear implants in humans.

The insertion of an intrascalar electrode array during cochlear implantation causes immediate damage to the inner ear and may result in delayed onset of additional damage that may interfere with neuronal stimulation. To date, there have been reports on fewer than 50 temporal bone specimens from patients who had undergone implantation during life. The majority of these were single-channel implants, whereas the majority of implants inserted today are multichannel systems. This report presents the histopathologic findings in temporal bones from 8 individuals who in life had undergone multichannel cochlear implantation, with particular attention to the type and location of trauma and to long-term changes within the cochlea. The effect of these changes on spiral ganglion cell counts and the correlation between speech comprehension and spiral ganglion cell counts were calculated. In 4 of the 8 cases, the opposite, unimplanted ear was available for comparison. In 3 of the 4 cases, there was no significant difference between the spiral ganglion cell counts on the implanted and unimplanted sides. In addition, in this series of 8 cases, there was an apparent negative correlation between residual spiral ganglion cell count and hearing performance during life as measured by single-syllable word recognition. This finding suggests that abnormalities in the central auditory pathways are at least as important as spiral ganglion cell loss in limiting the performance of implant users.

Effects of channel-to-electrode mappings on speech reception with the ineraid cochlear implant.

OBJECTIVE: Research was conducted to assess multichannel saliency with the Ineraid cochlear prosthesis. The goal was to determine whether tonotopically ordered stimulation benefits speech reception. DESIGN: In a single high-performing subject, changes in speech reception were studied during two dramatic alterations to the normal (tonotopic) mapping between the sound-processors's four filter channels and the intracochlear electrode array: In one alteration, the four filter outputs were summed and delivered to one electrode; this single-channel mapping was worn for 7 days. In another alteration, the four filter outputs were connected in reverse tonotopic order to four intracochlear electrodes; this reversed mapping was worn for 3 days. RESULTS: When using the implant in conjunction with speechreading, all three mappings provided large gains over speechreading alone on the recognition of words in sentences. When using the implant alone (without speechreading), tests of consonant and vowel recognition, and the recognition of words in isolation and in sentences all showed substantial decreases in performance across the three mapping conditions: normal > single-channel > reversed. The patterns of segmental confusions and the relations among scores on different tests were highly consistent. Finally, the performance with the altered mappings was, in some ways, remarkably good. With single-channel mapping, 15% word recognition was obtained, much less than the 54% obtained with the normal mapping, but demonstrative of some open-set speech reception. With the reversed mapping, high levels of consonantal voicing and manner information were received, indicating good reception of time-intensity cues, but open-set word recognition was near zero. CONCLUSIONS: Despite the extensive spread of current associated with monopolar intracochlear stimulation, the Ineraid electrode array affords a degree of perceptual selectivity that substantially aids speech reception.

Relations among different measures of speech reception in subjects using a cochlear implant.

A comprehensive set of speech reception measures were obtained in a group of about 20 postlingually deafened adult users of the Ineraid multichannel cochlear implant. The measures included audio, visual, and audiovisual recognition of words embedded in two types of sentences (with differing degrees of difficulty) and audio-only recognition of isolated monosyllabic words, consonant identification (12 alternatives, /Ca/), and vowel identification (8 alternatives, /bVt/). For most implantees, the audiovisual gains in the sentence tests were very high. Quantitative relations among audio-only scores were assessed using power-law transformations suggested by Boothroyd and Nittrouer [J. Acoust. Soc. Am. 84, 101-114 (1988)] that can account for the benefit of sentence context (via a factor k) and the relation between word and phoneme recognition (via a factor j). Across the broad range of performance that existed among the subjects, substantial order was observed among measures of speech reception along the continuum from recognition of words in sentences, words in isolation, speech segments, and the retrieval of underlying phonetic features. Correlations exceeded 0.85 among direct and sentence-derived measures of isolated word recognition as well as among direct and word-derived measures of segmental recognition. Results from a variety of other studies involving presentation of limited auditory signals, single-channel and multichannel implants, and tactual systems revealed a similar pattern among word recognition, overall consonant identification performance, and consonantal feature recruitment. Finally, improving the reception of consonantal place cues was identified as key to producing the greatest potential gains in speech reception.

Better speech recognition with cochlear implants.

HIGH levels of speech recognition have been achieved with a new sound processing strategy for multielectrode cochlear implants. A cochlear implant system consists of one or more implanted electrodes for direct electrical activation of the auditory nerve, an external speech processor that transforms a microphone input into stimuli for each electrode, and a transcutaneous (rf-link) or percutaneous (direct) connection between the processor and the electrodes. We report here the comparison of the new strategy and a standard clinical processor. The standard compressed analogue (CA) processor presented analogue waveforms simultaneously to all electrodes, whereas the new continuous interleaved sampling (CIS) strategy presented brief pulses to each electrode in a nonoverlapping sequence. Seven experienced implant users, selected for their excellent performance with the CA processor, participated as subjects. The new strategy produced large improvements in the scores of speech reception tests for all subjects. These results have important implications for the treatment of deafness and for minimal representations of speech at the auditory periphery.

Treatment of sensorineural hearing loss by cochlear implantation.

Cochlear implantation, a form of neural prosthesis, is now being employed for some profoundly deaf individuals. The pathophysiology of profound deafness, the strategies inherent in cochlear implantation, the differences between various cochlear implant strategies and designs, medical and otologic evaluation of implant candidates, and the surgical procedure are each discussed. The results in speech recognition and speech reading that have been obtained with four commonly used cochlear implant systems are also presented.

Speech recognition experience with multichannel cochlear implants.

Four patients received multichannel intracochlear implants before 1978 as part of the University of Utah program. By 1983, sound coding strategies and electronic miniaturization were developed to allow production and use of a portable sound processor/cochlear stimulator unit. Approval for expanded clinical trials has allowed implantation of five additional patients by August 1984. Data on the earlier patients demonstrate stability of electrode thresholds and impedance, low risk of the percutaneous pedestal, and the development of speech recognition scores of greater than 60% with electrical stimulation alone and greater than 90% with electrical stimulation combined with lipreading. Data on recent recipients indicate early electrode threshold stability, lower thresholds for apical electrodes, possibility of replacing single-channel with multi-channel units, and low morbidity of the implantation. Return to function in a verbally communicating environment has been achieved by one earlier patient.

Speech discrimination in deaf subjects with cochlear implants.

Electrical stimulation of the auditory nerve is being investigated as a way to provide information useful for speech communication in the profoundly deaf. Single-channel systems that tend to stimulate all fibers alike have had little success in achieving this goal. Multichannel systems that allow excitation of more complex temporal-spatial patterns of activity are now being introduced. Psychoacoustical experiments providing evidence that electrodes of a multichannel implant are able to separately excite distinct groups of neural elements are reviewed. New results using multiple electrodes and speech-like stimuli are presented. The synthetic stimuli were vowels (/a/, /i/, /u/) and consonant-vowel (CV) syllables (/ba/, /da/, /ga/, /ta/). Vowels and CV syllables were presented in an AXB discrimination task with different signal processing schemes and electrode configurations. A four-channel, frequency-selective system produced faultless discrimination scores for all stimuli and spontaneous recognition of the vowels while the scores for the single-channel system were generally much lower. Although understanding free running speech by the profoundly deaf does not seem imminent, the results presented indicate that the multichannel system tested shows more promise of approaching this goal than the single-channel scheme.

Fundamental considerations in designing auditory implants.

The usual premise underlying developmental work on cochlear prostheses as prospective cures for profound deafness is that the auditory nerve can be electrically stimulated in such a manner that communicative skills can be developed or maintained. Physiologic recordings from single fibers in the cat's auditory nerve and attempts to model these responses have generated a description of how the auditory nerve codes complex sounds such as speech. This work suggests that certain minimal cues might have to be present at the level of the auditory nerve in order that adequate discrimination of specific speech signals can take place. The prospects for achieving a useful prosthesis in the near future will be evaluated in terms of what can be expected from current attempts to code the artificial stimulation properly.

Auditory prostheses research with multiple channel intracochlear stimulation in man.

Although single-channel electrode arrays implanted in the scala tympani of deaf patients are useful as an aid to lip reading and for distinguishing some environmental sounds, they do not transmit intelligible speech. However, multichannel electrode arrays, which take advantage of the cochlea's tonotopic organization, may be capable of generating the complex patterns of neural activity necessary for speech discrimination. In this study, multichannel electrodes were implanted in the cochleas of four volunteers, with access to the connecting wires made through the skin via a percutaneous connector. The major portion of the data presented is from two of these subjects: one has been bilaterally deaf since birth and the other has been unilaterally deaf for 15 years. Preliminary results of experiments with two more recently implanted subjects are described as well as experiments with a fifth volunteer who was implanted with five electrodes by House in 1969. Data on pitch and loudness discrimination as well as the effects of stimulation parameters on threshold, impedance, and electrode interaction are presented. Place pitch and periodicity pitch were observed in all five volunteers. The results of pitch-matching experiments with the unilaterally deaf volunteer were consistent with tonotopic maps of the cochlea, and experiments indicated that a pitch continuum may be achieved by combining place and periodicity pitch modulation. Preliminary experiments in tune recognition with one subject demonstrate his ability to recognize simple melodies based on periodicity pitch cues. These results, coupled with the finding that subjective sensations remain stable over the long-term, support the feasibility of providing artificial hearing with a multichannel cochlear stimulation system.

Artifical hearing for the deaf by cochlear stimulation: pitch modulation and some parametric thresholds.

The periodicity and place-pitch modulation data, along with the data on loudness, offers promise of the development of a multielectrode, multichannel cochlear auditory prosthesis. Also, the currents required for cochlear stimulation are of the magnitude which can pass across the electrode-perilymph interface, by double-layer charging, without production of toxic by-products of electrolysis(6), supporting the feasibility of long-term chronic stimulation.