Pitch ranking, electrode discrimination, and physiological spread of excitation using current steering in cochlear implants

Assessing Array-Type Differences in Cochlear Implant Users Using the Panoramic ECAP Method

OBJECTIVES

Cochlear implant companies manufacture devices with different electrode array types. Some arrays have a straight geometry designed for minimal neuronal trauma, while others are precurved and designed to position the electrodes closer to the cochlear neurons. Due to their differing geometries, it is possible that the arrays are not only positioned differently inside the cochlea but also produce different patterns of the spread of current and of neural excitation. The panoramic electrically evoked compound action potential method (PECAP) provides detailed estimates of peripheral neural responsiveness and current spread for individual patients along the length of the cochlea. These estimates were assessed as a function of electrode position and array type, providing a normative dataset useful for identifying unusual patterns in individual patients.

DESIGN

ECAPs were collected from cochlear implant users using the forward-masking artifact-reduction technique for every combination of masker and probe electrode at the most comfortable level. Data were available for 91 ears using Cochlear devices, and 53 ears using Advanced Bionics devices. The Cochlear users had straight arrays (Slim Straight, CI-22 series, n = 35), or 1 of 2 precurved arrays (Contour Advance, CI-12 series, n = 43, or Slim Modiolar, CI-32 series, n = 13). Computed tomography scans were also available for 41 of them, and electrode-modiolus distances were calculated. The Advanced Bionics users had 1 of 2 straight arrays (1J, n = 9 or SlimJ, n = 20), or precurved arrays (Helix, n = 4 or Mid-Scala, n = 20). The ECAPs were submitted to the PECAP algorithm to estimate current spread and neural responsiveness along the length of the electrode array for each user. A linear mixed-effects model was used to determine whether there were statistically significant differences between different array types and/or for different electrodes, both for the PECAP estimates of current spread and neural responsiveness, as well as for the available electrode-modiolus distances. Correlations were also conducted between PECAP's estimate of current spread and the electrode-modiolus distances.

RESULTS

For Cochlear users, significant effects of array type (p = 0.001) and of electrode (p < 0.001) were found on the PECAP's current-spread estimate, as well as a significant interaction (p = 0.006). Slim Straight arrays had a wider overall current spread than both the precurved arrays (Contour Advance and Slim Modiolar). The interaction revealed the strongest effect at the apex. A significant correlation between PECAP's current-spread estimate and the electrode-modiolus distances was also found across subjects (r = 0.516, p < 0.001). No effect of array type was found on PECAP's estimate of current spread for the Advanced Bionics users (p = 0.979).

CONCLUSIONS

These results suggest that for users of the Cochlear device, precurved electrode arrays show narrower current spread within the cochlea than those with lateral-wall electrode arrays, with the strongest effect present at the apex. No corresponding effects of array type were found in the Advanced Bionics device. This could have implications for device selection in clinical settings, although the authors underscore that this is a post-hoc analysis and does not demonstrate a causal link wherein device selection can be expected to give rise to specific neural excitation patterns.

Full-array channel discrimination in cochlear implants: validation and clinical application

OBJECTIVE

We sought to validate our proposed tool for estimating channel discrimination of cochlear implant (CI) users along the full electrode array and to assess associations between place-pitch discrimination and speech perception.

DESIGN

In two tests, participants identified one stimulus (probe) as the odd-one-out compared with two reference stimuli. Probe stimuli were evoked using dual electrode stimulation characterised by the current steering coefficient α. The first test measured psychometric functions (PFs) on pre-defined contacts, with just a noticeable difference (JNDα) as the outcome variable. The second test estimated channel discrimination on the full electrode array, yielding a discrimination score of Dα. We measured speech perception as free-field consonant-vowel-consonant phoneme recognition scores.

STUDY SAMPLE

We included 25 adults with at least 6 months of CI experience.

RESULTS

JNDα and Dα scores measured on the same contact correlated significantly (rs = 0.64, p < 0.001). Mean JNDα and speech perception scores showed significant relationships in quiet and in noise.

CONCLUSIONS

Dα correlated strongly with JNDα scores obtained with the PFs. For poor performers, the full-array test may underestimate JNDα. The full-array pitch discrimination test could be a helpful clinical tool, such as for fitting regions of lesser pitch discrimination ability.

Exponential fitting of spread of excitation response measurements in cochlear implants

BACKGROUND

Hearing performance in cochlear implant (CI) users is variable. An objective measure which can allow a prediction of this performance is desirable. Spread of neural excitation (SoE) curves are an objective measure that can be obtained using the fitting software of cochlear implants and might be able to be used as a predictor. A novel method to interpret SoE curves is presented. New Method Spread of excitation measurements for three recording sites were fitted using two exponential functions. An asymmetric width measure was developed, defined as the distance in mm to the point, where 25% or 50% of peak normalized amplitude was reached, for each half of the SoE separately. Also, a novel population of subjects with MED-EL CIs is used. Furthermore, speech perception (speech reception threshold, SRT) was evaluated using a matrix sentence test in a multi-source noise field.

RESULTS

SoE width was narrowest for the basal recording site and widest for the apical recording site. Fitted SoE exponential functions were most asymmetric for the apical recording site. A significant positive correlation between sentence test SRT and SoE width at the apical recording site was found. Comparison with Existing Methods The use of an asymmetric width measure correlated strongly and positively with speech perception for apical recording sites, unlike the symmetric width measure used in previous studies. Presumably, longer electrodes allow stimulation of a more apical part of the cochlear. At the apical part of the cochlea, dendrites from a large region of the basilar membrane map to a narrow portion on the spiral ganglion, which might explain the observed asymmetry.

CONCLUSIONS

For subjects implanted with long electrode arrays, an asymmetric width measure correlates positively with apical SoE distance. However, due to lack of a sufficient amount of data, the results are currently less conclusive and need to be consolidated in a larger cohort of subjects.

Piloting the recording of electrode voltages (REVS) using surface electrodes as a test to identify cochlear implant electrode migration, extra-cochlear electrodes and basal electrodes causing discomfort

OBJECTIVES

To determine if Electrode Voltage (EV) measurements are potentially suitable as a test for detecting extra-cochlear electrodes in cochlear implants (CIs).

METHODS

EV measurements were made using surface electrodes in live mode in 17 adult cochlear implant (CI) users. Repeatability, the effects of stimulation level, CI active electrode position, (active) recording electrode position and stimulation mode (for Nucleus devices) were investigated.

RESULTS/DISCUSSION

Recordings made in monopolar mode showed good repeatability when the active recording electrode was placed on the ipsilateral earlobe; voltages increased linearly with stimulation level as expected. EVs for basal electrodes differed greatly between partially inserted/migrated devices, fully inserted devices with all electrodes activated, and those with deactivated basal electrodes [χ²(2) = 10.2, p < 0.05 for the most basal electrode]. EVs for Nucleus devices were small for electrodes on the array when compared to those for monopolar return electrodes, except for the participant with extra-cochlear electrodes. We argue that fibrosis around the electrode array facilitated current flow across the round window in this case.

CONCLUSION

The test appears to be a viable approach to detect electrode migration and extra-cochlear electrodes in adult CI users and may also be sensitive to discomfort caused by current leakage from the basal end of the cochlea.

Audiologists' preferences in programming cochlear implants: A preliminary report

Background: There is little data available addressing how clinical audiologists handle cochlear implant (CI) programming between device manufacturers and make decisions on related services, particularly in the United States. Objectives: This study sought to understand the techniques and settings professionals use with their patients, how they approach bimodal fitting, which tests they use to evaluate patient and device performance, and their overall preferences of (re)habilitative options. Methods: A questionnaire was developed and distributed to CI audiologists throughout the United States electronically. Results: All respondents reported either always or almost always using Cochlear's default signal processing strategy in contrast to the 64% reported for Med-El and 40% for Advanced Bionics. A trend of less use of electrically evoked stapedial reflex threshold (eSRT) than electrically evoked compound action potentials (eCAP) for objective measures was revealed. Higher likelihood of performing speech recognition tests in quiet than in noise was revealed. Preferences for bimodal fitting trend toward using a partner company's hearing aid, although preferences were comparable in adopting four types of hearing aid formulas surveyed in the questionnaire. Conclusions: These data confirm high variability among audiologists' CI programming practices, and documenting these differences is an important step to understanding how to best treat patients.

Channel discrimination along all contacts of the cochlear implant electrode array and its relation to speech perception

OBJECTIVE

To test the channel discrimination of cochlear implant (CI) users along all contacts of the electrode array and assess whether this is related to speech perception.

DESIGN

CI recipients were tested with a custom-made channel discrimination test. They were asked to distinguish a target stimulus from two reference stimuli in a three-alternative forced choice (3AFC) task. The target stimulus was evoked using current steering, with current steering coefficients (α) of 1, 0.5 and 0.25. The test provided a discrimination score (Dα) for each electrode contact along the array.

STUDY SAMPLE

Thirty adults implanted with a CI from Advanced Bionics.

RESULTS

Large variations in Dα scores were observed, both across the electrode array and between subjects. Statistical analysis revealed a significant channel-to-channel variability in Dα score (p < 0.01). Further, there was a significant relationship between subjects' Dα scores and their speech perception in quiet (p < 0.001).

CONCLUSIONS

The large variations in Dα score emphasise the importance of testing pitch discrimination across the complete electrode array. The relationship between Dα score and speech perception indicates that pitch discrimination might be a contributing factor to the performance of individual implant users.

The Effect of Stimulus Polarity on the Relation Between Pitch Ranking and ECAP Spread of Excitation in Cochlear Implant Users

Although modern cochlear implants (CIs) use cathodic-leading symmetrical biphasic pulses to stimulate the auditory nerve, a growing body of evidence suggests that anodic-leading pulses may be more effective. The positive polarity has been shown to produce larger electrically evoked compound action potential (ECAP) amplitudes, steeper slope of the amplitude growth function, and broader spread of excitation (SOE) patterns. Polarity has also been shown to influence pitch perception. It remains unclear how polarity affects the relation between physiological SOE and psychophysical pitch perception. Using a within-subject design, we examined the correlation between performance on a pitch-ranking task and spatial separation between SOE patterns for anodic and cathodic-leading symmetric biphasic pulses for 14 CI ears. Overall, there was no effect of polarity on either ECAP SOE patterns, pitch ranking performance, or the relation between the two. This result is likely due the use of symmetric biphasic pulses, which may have reduced the size of the effect previously observed for pseudomonophasic pulses. Further research is needed to determine if a pseudomonophasic stimulus might further improve the relation between physiology and pitch perception.

Effect of Stimulus Polarity on Physiological Spread of Excitation in Cochlear Implants

BACKGROUND

Contemporary cochlear implants (CIs) use cathodic-leading, symmetrical, biphasic current pulses, despite a growing body of evidence that suggests anodic-leading pulses may be more effective at stimulating the auditory system. However, since much of this research on humans has used pseudomonophasic pulses or biphasic pulses with unusually long interphase gaps, the effects of stimulus polarity are unclear for clinically relevant (i.e., symmetric biphasic) stimuli.

PURPOSE

The purpose of this study was to examine the effects of stimulus polarity on basic characteristics of physiological spread-of-excitation (SOE) measures obtained with the electrically evoked compound action potential (ECAP) in CI recipients using clinically relevant stimuli.

RESEARCH DESIGN

Using a within-subjects (repeated measures) design, we examined the differences in mean amplitude, peak electrode location, area under the curve, and spatial separation between SOE curves obtained with anodic- and cathodic-leading symmetrical, biphasic pulses.

STUDY SAMPLE

Fifteen CI recipients (ages 13-77) participated in this study. All were users of Cochlear Ltd. devices.

DATA COLLECTION AND ANALYSIS

SOE functions were obtained using the standard forward-masking artifact reduction method. Probe electrodes were 5-18, and they were stimulated at an 8 (of 10) loudness rating ("loud"). Outcome measures (mean amplitude, peak electrode location, curve area, and spatial separation) for each polarity were compared within subjects.

RESULTS

Anodic-leading current pulses produced ECAPs with larger average amplitudes, greater curve area, and less spatial separation between SOE patterns compared with that for cathodic-leading pulses. There was no effect of polarity on peak electrode location.

CONCLUSIONS

These results indicate that for equal current levels, the anodic-leading polarity produces broader excitation patterns compared with cathodic-leading pulses, which reduces the spatial separation between functions. This result is likely due to preferential stimulation of the central axon. Further research is needed to determine whether SOE patterns obtained with anodic-leading pulses better predict pitch discrimination.

Combined Electric and Acoustic Stimulation With Hearing Preservation: Effect of Cochlear Implant Low-Frequency Cutoff on Speech Understanding and Perceived Listening Difficulty

OBJECTIVE

The primary objective of this study was to assess the effect of electric and acoustic overlap for speech understanding in typical listening conditions using semidiffuse noise.

DESIGN

This study used a within-subjects, repeated measures design including 11 experienced adult implant recipients (13 ears) with functional residual hearing in the implanted and nonimplanted ear. The aided acoustic bandwidth was fixed and the low-frequency cutoff for the cochlear implant (CI) was varied systematically. Assessments were completed in the R-SPACE sound-simulation system which includes a semidiffuse restaurant noise originating from eight loudspeakers placed circumferentially about the subject's head. AzBio sentences were presented at 67 dBA with signal to noise ratio varying between +10 and 0 dB determined individually to yield approximately 50 to 60% correct for the CI-alone condition with full CI bandwidth. Listening conditions for all subjects included CI alone, bimodal (CI + contralateral hearing aid), and bilateral-aided electric and acoustic stimulation (EAS; CI + bilateral hearing aid). Low-frequency cutoffs both below and above the original "clinical software recommendation" frequency were tested for all patients, in all conditions. Subjects estimated listening difficulty for all conditions using listener ratings based on a visual analog scale.

RESULTS

Three primary findings were that (1) there was statistically significant benefit of preserved acoustic hearing in the implanted ear for most overlap conditions, (2) the default clinical software recommendation rarely yielded the highest level of speech recognition (1 of 13 ears), and (3) greater EAS overlap than that provided by the clinical recommendation yielded significant improvements in speech understanding.

CONCLUSIONS

For standard-electrode CI recipients with preserved hearing, spectral overlap of acoustic and electric stimuli yielded significantly better speech understanding and less listening effort in a laboratory-based, restaurant-noise simulation. In conclusion, EAS patients may derive more benefit from greater acoustic and electric overlap than given in current software fitting recommendations, which are based solely on audiometric threshold. These data have larger scientific implications, as previous studies may not have assessed outcomes with optimized EAS parameters, thereby underestimating the benefit afforded by hearing preservation.

A Cochlear Implant Performance Prognostic Test Based on Electrical Field Interactions Evaluated by eABR (Electrical Auditory Brainstem Responses)

Background

Cochlear implants (CIs) are neural prostheses that have been used routinely in the clinic over the past 25 years. They allow children who were born profoundly deaf, as well as adults affected by hearing loss for whom conventional hearing aids are insufficient, to attain a functional level of hearing. The “modern” CI (i.e., a multi-electrode implant using sequential coding strategies) has yielded good speech comprehension outcomes (recognition level for monosyllabic words about 50% to 60%, and sentence comprehension close to 90%). These good average results however hide a very important interindividual variability as scores in a given patients’ population often vary from 5 to 95% in comparable testing conditions. Our aim was to develop a prognostic model for patients with unilateral CI. A novel method of objectively measuring electrical and neuronal interactions using electrical auditory brainstem responses (eABRs) is proposed.

Methods and Findings

The method consists of two measurements: 1) eABR measurements with stimulation by a single electrode at 70% of the dynamic range (four electrodes distributed within the cochlea were tested), followed by a summation of these four eABRs; 2) Measurement of a single eABR with stimulation from all four electrodes at 70% of the dynamic range. A comparison of the eABRs obtained by these two measurements, defined as the monaural interaction component (MIC), indicated electrical and neural interactions between the stimulation channels.

Speech recognition performance without lip reading was measured for each patient using a logatome test (64 "vowel-consonant-vowel"; VCV; by forced choice of 1 out of 16). eABRs were measured in 16 CI patients (CIs with 20 electrodes, Digisonic SP; Oticon Medical ®, Vallauris, France).

Significant correlations were found between speech recognition performance and the ratio of the amplitude of the V wave of the eABRs obtained with the two measurements (Pearson's linear regression model, parametric correlation: r2 = 0.26, p<0.05).

Conclusions

This prognostic model allowed a substantial amount of the interindividual variance in speech recognition scores to be explained. The present study used measurements of electrical and neuronal interactions by eABR to assess patients' bio-electric capacity to use multiple information channels supplied by the implant. This type of prognostic information may be valuable in several ways. On the patient level, it allows customizing of individual treatments.

ClinicalTrials.gov Identifier: NCT01805167