Psychophysics of a prototype peri-modiolar cochlear implant electrode array

Psychophysical measurements were performed in three hearing-impaired adult subjects implanted with a CI22 cochlear prosthesis (Cochlear Ltd.) fitted with a developmental peri-modiolar electrode array. The array was manufactured with a curvature approximating that of the inner wall of the scala tympani but, after straightening and insertion, lay on average about half way between the inner and outer walls of the scala. All subjects were tested with bipolar stimulation; two were also tested with monopolar, employing the most basal electrode as the return. Maximum comfortable level and threshold reduced with decreasing distance of electrode from the modiolus, whereas dynamic range increased. The linearity of the loudness growth function did not vary significantly with electrode position but the function was more non-linear for lower maximum comfortable levels. Current level discrimination, normalized with respect to dynamic range, improved with decreasing distance of electrode from the modiolus in two subjects. Pitch varied regularly with insertion depth of the stimulated electrode for bipolar stimulation in two subjects and also for monopolar stimulation in one subject. Electrode discrimination was enhanced by closeness to the modiolus. Whereas the forward masking patterns for bipolar stimulation of electrodes close to the modiolus had a sharp double-peaked structure, those for monopolar stimulation were flatter and had a single peak.

The contour electrode array: safety study and initial patient trials of a new perimodiolar design

OBJECTIVE

The aim of these studies was to investigate the insertion properties and safety of a new intracochlear perimodiolar electrode array design (Contour).

BACKGROUND

An electrode array positioned close to the neural elements could be expected to reduce stimulation thresholds and might potentially reduce channel interaction.

METHODS

Two sequential studies were conducted. In study 1, the Contour electrode array was inserted in 12 human temporal bones. After cochlear surface preparation, the position of the array was noted and the basilar membrane was examined for insertion damage. On the basis of the outcome of this temporal bone study, study 2 investigated the Contour array, mounted on a Nucleus CI-24 M device and implanted in three adult patients.

RESULTS

Study I showed that in 10 temporal bones, the Contour array was positioned close to the modiolus, and the basilar membrane was intact. In the two remaining bones, the arrays had pierced the basilar membrane and were positioned in the scala vestibuli apical to the penetration. Statistical analysis showed an equivalent probability of insertion-induced damage of the two array designs. In study 2, image analysis indicated that the Contour electrodes were positioned closer to the modiolus than the standard Nucleus straight array. Lower T and C levels, but higher impedance values, were recorded from electrodes close to the modiolus. Initial speech perception data showed that all patients gained useful open-set speech perception, two patients achieving scores of 100% on sentence material 3 months postoperatively.

CONCLUSIONS

The temporal bone studies showed the Contour electrode array to be generally positioned closer to the modiolus than the standard Nucleus straight array, and to have an equivalent probability of causing insertion-induced damage.

Comparison of electrode position in the human cochlea using various perimodiolar electrode arrays

OBJECTIVE

This study was conducted to evaluate the insertion properties and intracochlear trajectories of three perimodiolar electrode array designs and to compare these designs with the standard Cochlear/Melbourne array.

BACKGROUND

Advantages to be expected of a perimodiolar electrode array include both a reduction in stimulus thresholds and an increase in dynamic range, resulting in a more localized stimulation pattern of the spiral ganglion cells, reduced power consumption, and, therefore, longer speech processor battery life.

METHODS

The test arrays were implanted into human temporal bones. Image analysis was performed on a radiograph taken after the insertion. The cochleas were then histologically processed with the electrode array in situ, and the resulting sections were subsequently assessed for position of the electrode array as well as insertion-related intracochlear damage.

RESULTS

All perimodiolar electrode arrays were inserted deeper and showed trajectories that were generally closer to the modiolus compared with the standard electrode array. However, although the precurved array designs did not show significant insertion trauma, the method of insertion needed improvement. After insertion of the straight electrode array with positioner, signs of severe insertion trauma in the majority of implanted cochleas were found.

CONCLUSIONS

Although it was possible to position the electrode arrays close to the modiolus, none of the three perimodiolar designs investigated fulfilled satisfactorily all three criteria of being easy, safe, and atraumatic to implant.

Cochlear view: postoperative radiography for cochlear implantation

OBJECTIVE

This study aimed to define a spatial position of the cochlea in the skull based on anatomical studies and to design an appropriate method of skull radiography for demonstration of the multichannel intracochlear electrode array and the structures of the inner ear, for use in evaluating the electrode position and its related pitch perception.

BACKGROUND

The conventional skull radiograph (plain radiograph) can offer a complete and direct image of an intracochlear electrode array, if the x-ray is directed to the cochlea and parallel to the axis of the cochlea.

METHODS

Measurement from computed tomography imaging and three-dimensional reconstruction were performed to define the spatial position of the cochlea in the skull.

RESULTS

A radiographic projection, the cochlear view, was designed. A detailed radiographic method and radiologic interpretation of the cochlear view is described. An improved clinical method for measuring the longitudinal and angular position of the electrodes from the cochlear view is recommended.

CONCLUSIONS

The application of the cochlear view has proved that it is beneficial postoperatively in documenting the results of cochlear implantation, and in evaluating the depth of insertion and position of individual electrodes. It serves as a valuable reference for managing frequency mapping, optimizing speech processing strategies, and further research purposes. The method can be widely used in cochlear implant clinics because of its simplicity, low radiation, speed, and minimal cost.

Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array

OBJECTIVE

To develop techniques for measuring the positions of the individual electrodes of a multiple channel cochlear implant and for estimating associated characteristic frequencies.

BACKGROUND

Information concerning the positions of the individual electrodes of a cochlear implant array is important for analyzing speech perception or psychophysical data and for optimizing speech-processing strategies. This study presents two techniques for obtaining such information from postoperative plain film radiographs.

METHODS

A template spiral shape, derived from analysis of the radiographs of 30 cochlear implant patients, is used to obtain measurements of the angular positions of the electrode bands within scala tympani. A research technique measures angular positions and estimates characteristic frequencies for all electrode bands but requires that the positions of two cochlear landmarks and all electrode bands be digitized. A clinical technique provides similar angle and frequency estimates but requires a minimum of information to be extracted visually from the radiograph. The lateral positions of the bands are estimated, in the research technique, using mean outer and inner wall functions obtained from 11 Silastic molds of scala tympani.

RESULTS

The mean position of the implanted array relative to the mean scala tympani outer wall function was consistent with published histologic observations of implanted temporal bones. Measured angles did not vary greatly with experimenter or with rotation of the cochlea relative to the radiographic beam by up to 20 degrees.

CONCLUSIONS

The techniques described allow, principally, measurement of the longitudinal positions of the bands of a cochlear implant in scala tympani and estimation of corresponding characteristic frequencies.

Cochlear implant place psychophysics 1. Pitch estimation with deeply inserted electrodes

Numerical estimation of pitch was performed by 8 adult subjects implanted with cochlear prostheses manufactured by Cochlear Limited. The electrode arrays had been inserted into the scale tympani to between one and one and a half turns of the cochlea. Using bipolar stimulation, the pitch estimates for each subjects showed an overall reduction with insertion depth of the stimulated electrode. However, for several subjects, after decreasing regularly for the more basal electrodes, pitch estimates showed an abrupt decrease, followed in some cases by a region of low pitch. Two of the subjects, implanted with a modified electrode array, the '20 + 2' which allowed monopolar in addition to bipolar stimulation, exhibited an abrupt decrease in pitch estimate with bipolar but not with monopolar stimulation. In these two subjects, for stimulating electrodes inserted more deeply than about three quarters of a turn, bipolar stimuli produced lower pitch sensations, and presumably more apical neural excitation patterns, than monopolar stimuli.

Cochlear implant place psychophysics. 2. Comparison of forward masking and pitch estimation data

Results for forward masking and numerical estimation of pitch were compared in a group of 6 adult subjects implanted with cochlear prostheses manufactured by Cochlear Limited. Data were collected for bipolar + 1 stimulation in all subjects, and for stimulation in one other mode, either common ground or monopolar, for all subjects but one. The pitch data show various irregularities and in each case can be seen to be broadly consistent with the corresponding forward masking data. It is shown that a 'centre of gravity' of the forward masking distribution varies with masker electrode in a manner that is qualitatively very similar to the variation of pitch estimate. It is suggested that, while pitch estimation results are consistent with those from forward masking, the latter contain more detailed information that may be useful in understanding intersubject variations in speech comprehension.

The automated prediction of hearing thresholds in sleeping subjects using auditory steady-state evoked potentials

OBJECTIVE

To examine the relationship between auditory steady-state evoked potentials (SSEPs) and behavioral thresholds in sleeping subjects.

DESIGN

60 adults and children with hearing thresholds ranging from normal to profound were selected on the basis of appropriate audiograms. Behavioral audiograms were determined at the octave frequencies 250-4000 Hz. These behavioral thresholds were then compared with the SSEP thresholds obtained during natural sleep for adults, or natural or sedated sleep for children.

RESULTS

A strong relationship between behavioral and SSEP thresholds was observed. The strength of the relationship increased with increasing frequency and increasing degree of the loss. On the basis of these data, the prediction of behavioral thresholds from SSEP levels was determined. It was found that the standard deviation of the error in this prediction decreased with increasing frequency and increasing degree of the loss. There was no significant age effect in the results obtained at any of the frequencies.

CONCLUSION

The results suggest the SSEP technique can be used as a predictor of behavioral threshold in adults and children at the frequencies 250-4000 Hz.

Investigations on a curved intracochlear array

A curved electrode array and inserting tool have been developed at the University of Melbourne. This electrode array can be successfully implanted with few modifications to the surgical procedure presently in use. When implanted, the curved electrode array adopts a position closer to the modiolus than does the standard straight electrode array.

Initial investigation of the efficacy and biosafety of sodium hyaluronate (Healon) as an aid to electrode array insertion

Sodium hyaluronate has potential uses as an electrode lubricant in cochlear implant surgery. Deeper electrode insertions were achieved in six temporal bones implanted with sodium hyaluronate, compared to 22 patients implanted without sodium hyaluronate. Preliminary biosafety studies measuring hearing thresholds of six cats 4 months after implantation with dummy electrodes did not show evidence of ototoxicity caused by sodium hyaluronate. Until the biosafety studies are complete, we cannot recommend sodium hyaluronate for routine use, but the results so far are promising.

Auditory steady-state evoked potential in newborns

Steady-state evoked potential responses were recorded from 337 normal full-term sleeping newborns to combined amplitude and frequency modulated tones. Responses were automatically detected by statistical analysis of the response phase. Responses were most easily and consistently recorded at carrier frequencies of 500 Hz, 1500 Hz and 4000 Hz when the modulation frequency was between 60 Hz and 100 Hz. In this modulation frequency range, the response latencies were found to be between 11 ms and 15 ms, depending on carrier frequency, and the mean response thresholds for the three carrier frequencies were found to be 41.36 dB HL, 24.41 dB HL and 34.51 dB HL respectively. The results of this study suggest that steady-state evoked potentials at modulation rates in excess of 60 Hz may be useful for frequency specific, automated hearing screening in newborns.

Steady-state evoked potentials to amplitude modulated tones in the monkey

A frequency-specific, objective assessment of hearing thresholds is required for use in subjects unable to perform behavioural audiometry. One such method using steady-state evoked potentials (SSEPs) in response to amplitude-modulated tones was evaluated in an experimental animal, the macaque monkey. An amplitude-modulation frequency of 165 Hz was found to produce optimum response detection in the anaesthetised animal. Auditory thresholds determined by a computerised automatic response detection system accurately reflected behavioural thresholds previously described in this species.