Kevin H. Franck

Estimation of psychophysical levels using the electrically evoked compound action potential measured with the neural response telemetry capabilities of Cochlear Corporation's CI24M device.

Objective The goal of this study was to estimate psychophysical levels using the electrically evoked compound action potential (EAP), measured with the Neural Response Telemetry capabilities of Cochlear Corporation’s Nucleus CI24M cochlear implant system. Design Twelve postlingually deafened adults with at least 3 mo of implant experience with the CI24M were subjects in this study. EAP growth functions were successfully quantified on each active electrode of every subject. Correlation and regression analyses were performed between EAP measures and cochlear implant fitting psychophysics. Other information including performance, etiology and duration of hearing loss, and individual electrode impedance was considered. Results EAP thresholds were found to be highly correlated with psychophysical thresholds. The rate of EAP growth with increasing stimulation levels was also found to be correlated with the dynamic range of loudness limits and psychophysical thresholds in some subjects. No relationship was evident between EAP measures and speech perception tests. Conclusions Information from EAP growth function measurements may be used to estimate psychophysical information used in cochlear implant fitting but not to predict performance with the device.

Electrically evoked compound action potential amplitude growth functions and HiResolution programming levels in pediatric CII implant subjects.

Objective: To characterize the amplitude growth functions of the electrically evoked compound action potential (ECAP) in pediatric subjects implanted with the Clarion HiFocus electrode array with respect to electrode position and the presence or absence of a Silastic positioner. Electrophysiologic growth function data are compared with HiResolution (HiRes) psychophysical programming levels. Design: ECAP growth functions were measured for all electrodes along the implant’s array in 16 pediatric subjects. Nine of the patients were implanted with a Silastic positioner, whereas seven had no positioner. ECAP thresholds and growth function slopes were calculated. Fifteen of the 16 patients had psychophysical threshold and maximum comfort levels available. Programming levels and ECAP thresholds were compared within and among the subjects. Results: ECAP thresholds showed variability among patients, ranging from 178 to 920 nA at 32 μsec pulse width. ECAP thresholds did not depend on electrode position along the cochlea but were lower in the presence of the Silastic positioner (p < 0.001). Thresholds determined with the masker-probe versus the alternating polarity paradigms revealed moderate (r = 0.76) correlation. Growth function slopes also showed considerable variation among patients. Unlike thresholds, slopes decreased from apical to basal cochlear locations (p < 0.001) but showed no difference between the absence and presence of the positioner. Programming levels in HiRes were correlated with ECAP threshold levels. When ECAP thresholds were adjusted for each patient by the difference between M level and ECAP threshold at electrode 9, however, overall correlation between the two measurements was excellent (r = 0.98, N = 224). Conclusions: In pediatric subjects with the Clarion HiFocus electrode, ECAP growth function thresholds appear to decrease with the presence of the Silastic positioner but are unaffected by electrode position along the array. Growth function slope, however, depends on electrode position along the array but not on the presence of the positioner. ECAP programming levels can reliably predict stimulus intensities within the patients’ dynamic ranges, but considerable variability is seen between ECAP thresholds and HiRes programming levels.

A model of a nucleus 24 cochlear implant fitting protocol based on the electrically evoked whole nerve action potential.

Objective The goal of this study was to model a cochlear implant fitting protocol based on the electrically evoked whole nerve action potential (EAP) measured using the Neural Response Telemetry capabilities of the Nucleus CI24M cochlear implant. The model and data were based on the significant correlations found between the EAP threshold and growth function slope and psychophysical threshold and dynamic range, respectively, in 12 subjects (Franck, Reference Note 3; Franck & Norton, 2001). Design Using a retrospective split-half study design, these correlations found between psychophysical mapping levels and EAP data from six of the subjects were used in a model to predict psychophysical mapping levels from EAP data of the other six subjects. Results Predicted psychophysical mapping levels from the model of the EAP-based fitting protocol closely approximated measured cochlear implant fitting psychophysics. Conclusions The close approximation of the measured data to the model data indicates the feasibility of the clinical use of an EAP-based cochlear implant fitting protocol. The realization of this model would only require two loudness judgments from the patient, whereas traditional fitting requires 44, and would be fit in a live-voice mode, accounting for across-electrode loudness summation.

Electrode interaction in pediatric cochlear implant subjects.

Multielectrode cochlear implants rely on differential stimulation of the cochlear nerve for presenting the brain with the spectral and timing information required to understand speech. In implant patients, the degree of overlap among cochlear nerve fibers stimulated by the different electrodes constitutes the electrode interaction. Electrode interaction degrades the spectral resolution of the implant’s stimulus. We sought to define electrode interaction in a cohort of pediatric cochlear implant subjects as a function of both stimulus intensity and electrode location along the array. The 27 pediatric subjects that completed the study were implanted with either the Clarion Hi-Focus array with or without positioner, the Nucleus 24 Contour array, or the Nucleus 24 Straight array. All but two of the patients had congenital hearing loss, and none of the patients had meningitis prior to the onset of deafness. The cochlear nerve response was measured with the electrically evoked compound action potential (ECAP). A forward masking protocol was used such that a probe stimulus electrode remained fixed while a preceding masker was moved across the array. Electrode interaction was estimated by measuring the unmasked probe response minus the masked probe response. Three probe locations and three probe intensities were examined for each subject. At all probe locations, electrode interaction increased as probe intensity increased (p < 0.05). Interaction at the basal probe was less than that at either the middle or apical probe locations (p < 0.05), and significant correlation found between probe distance from the basal end of the array and electrode interaction (p < 0.001). These results demonstrate that in this cohort of pediatric subjects, electrode interaction depended on both stimulus intensity and probe location. Implications of these findings on future implant array design and current implant fitting strategies are discussed. The impact of electrode interaction on implant performance is yet to be elucidated.

Effects of stimulus level on speech perception with cochlear prostheses.

This study is one of a series that examines stimulus features important for cochlear implant function. Here, we examine effects of stimulus level. In subjects with cochlear implants, a number of psychophysical tests of temporal discrimination (pulse rate discrimination, gap detection, etc.) show marked improvement as a function of stimulus level through most or all of the dynamic range, while electrode-place discrimination can improve or degrade as a function of level. In this study, effects of these combined potential influences were studied by examining the effects of stimulus level on syllable identification. We tested two hypotheses: that syllable identification varies as a function of stimulus level and that level and electrode configuration interact in affecting syllable identification. We examined vowel and consonant identification as a function of stimulus level for bipolar and monopolar electrode configurations. We used experimental processor maps where upper and lower stimulation limits of each electrode pair were equated to eliminate confounding effects of dynamic range, which varies across subjects and electrodes. For each channel, stimulation amplitude was set to a fixed percentage of its dynamic range. Eight adult subjects with Nucleus CI24M implants were tested using the SPEAK processing strategy. With each electrode configuration, stimulus levels were tested from 0% to 90% of the dynamic range in nine steps. The effects on consonant and vowel identification were similar. Phoneme identification was usually better for monopolar than for bipolar stimulation. In the lower half of the dynamic range, syllable identification usually increased as a function of stimulus level. In the upper half of the dynamic range, syllable identification continued to increase as a function of level to 90% of the dynamic range for some subjects, while for others there was no appreciable change or a decrease as a function of level. Decreases in performance at high levels were more common with monopolar than bipolar stimulation. These results suggest that if speech processors are programmed to optimize level for each individual, speech perception performance could be improved.

Effects of Clarion electrode design on mapping levels in children.

The design of the placement of the Clarion cochlear implants intracochlear electrode array has undergone 2 revisions since its introduction, each to improve modiolar proximity. Stimulation with modiolar proximity may reduce current requirements for threshold levels and most comfortable levels of stimulation. This study analyzed the effects of electrode design on programming levels for deaf children implanted with the 3 cochlear implant designs and followed at The Childrens Hospital of Philadelphia. Psychophysical data were reported if measurements were taken approximately 3 months after initial activation, and programming parameters included nonsequential monopolar stimulation of 75-μs-per-phase biphasic pulses presented at 813 Hz per electrode. The threshold level and most comfortable programming level were measured by standard clinical techniques appropriate for children. The results indicate that the 2 electrode placement revisions have each significantly reduced threshold levels and most comfortable stimulation levels. These results are discussed in the context of device aesthetics, safety, and function.

Frequency and electrode discrimination in children with cochlear implants.

The objective of this study was to develop reliable pediatric psychophysical methodologies in order to address the limits of frequency and electrode discrimination in children with cochlear implants. Discrimination was measured with a two-alternative, adaptive, forced choice design using a video game graphical user interface. Implanted children were compared to normal-hearing children in the same age ranges. Twenty-nine implanted children and 68 children with normal-hearing performed frequency discrimination studies at varying frequencies. Electrode discrimination was assessed in thirty-four implanted children at varying electrode locations and stimulation intensities. Older children had better frequency discrimination than younger children, both for implanted and hearing subjects. Implanted children had worse frequency discrimination overall and exhibited learning effects at older ages than hearing children. Frequency discrimination Weber fractions were smallest in low frequencies. Electrode discrimination improved with stimulus intensity level for older but not younger children at all electrode locations. These results support the premise that developmental changes in signal processing contribute to discrimination of simple acoustic stimuli. For implanted children, auditory discrimination improved at lower frequencies and with electrodes at higher intensity. These findings imply that spatial separation may not be the key determinant in creating discriminable electrical stimuli for this population.

Pediatric cochlear implantation-I : Candidacy

The cochlear implant is a successful means of intervention to facilitate the perception of sound and oral communication for children with severe to profound sensorineural hearing loss. Often, the cochlear implantation process is viewed as being limited to a surgical procedure, rather than an entire process of medical, audiological, psychosocial, educational and therapeutic, management. In India, cochlear implantation is available in only a few large cities. Here, the otolaryngologist will direct the cochlear implant programme. Besides determination of medical candidacy, device implantation and medical management, the otolaryngologist is responsible to ensure that other aspects of cochlear implant management are implemented. This paper, the first of two that describe the multidisciplinary, team approach of the Pediatric Cochlear Implant Program of The Children’s Hospital of Philadelphia (CHOP), in Pennsylvania, USA, discusses the non-medical aspects of cochlear implant candidacy. A second article will follow that covers post-implantation follow- up. The various speech tests used at CHOP for assessment are based on the English language. They may be translated into the regional Indian languages where the assessment and training can be carried out accordingly.

Pediatric cochlear implantation — II: postoperative follow-up

A child with a cochlear implant is expected to achieve the successful outcome of facilitated perception of sound and more oral communication. To achieve this goal, ongoing intervention from a variety of professionals is required. These professionals may represent the disciplines of medicine, audiology, social work, education, and speech / language pathology. In India, cochlear implantation is available in only a few large cities. Here, the otolaryngologist will direct the cochlear implant program. Besides determination of medical candidacy, device implantation and medical management, the otolaryngologist is responsible to ensure that other aspects of cochlear implant management are implemented. This paper, the second of two that describe the multidisciplinary, team approach of the Pediatric Cochlear Implant Program of The Children’s Hospital of Philadelphia (CHOP), in Pennsylvania, USA, discusses the non-medical aspects of cochlear implant post-implantation follow-up. The first article, previously published, discussed cochlear implant candidacy. The various speech tests used at CHOP for assessment are based on the English language. They may be translated into the regional Indian languages where the assessment and training can be carried out accordingly.

The positioner effects electrode interaction in pediatric cochlear implants

Abstract Problem: Address the effect of an implant electrode’s cochlear position on the degree of electrode interaction in pediatric patients with Clarion HiFocus arrays using objective measurements well-suited for very young implant recipients. Methods: PATIENTS: A volunteer sample of 16 patients (range 2.2 to 12.8; mean age, 6.3 years) implanted with the HiFocus electrode were tested; 9 patients with the positioner and 7 without. MAIN OUTCOME MEASURES: Electrically evoked whole nerve action potentials (EAPs) were measured using the CII’s neural response imaging. An electrode interaction function was measured by varying the masker electrode position while maintaining probe and recording electrode positions. Interaction functions were obtained at 3 probe locations (electrodes 5, 9, and 13) and 3 supra-threshold intensities. Results: While EAP thresholds did not vary along the electrode array, growth function slope increased from basal to apical location ( P P P > 0.01). Conclusion: Both electrode interaction and growth function slope were greater toward the cochlear apex than at the cochlear base. We attribute this difference to a combination of systematic variations in electrode/neural element distances and surviving cochlear ganglion cells. Behavioral testing will determine the implication of different degrees of electrode interaction on electrical hearing. Significance: No systematic study of EAPs in children has been reported. These findings will help us understand the relationship between the auditory nerve response to electrical stimuli and the perception of hearing. Support: None reported.