Maximum Comfortable Loudness Levels Research Articles

Electrically evoked auditory cortical responses elicited from individually fitted stimulation parameters in cochlear implant users

Objective To investigate electrically evoked auditory cortical responses (eACR) elicited from the stimulation of intracochlear electrodes based on individually fitted stimulation parameters in cochlear implant (CI) users. Design An eACR setup based on individual fitting parameters is proposed. A 50-ms alternating biphasic pulse train was used to stimulate apical, medial, and basal electrodes and to evoke auditory cortical potentials (N1-P2 complex). Study sample The eACR setup proposed was validated with 14 adult CI users. Results Individual and grand-average eACR waveforms were obtained. The eACR amplitudes were lower in the basal than in the apical and medial regions. Earlier N1 latencies were found in CI users with lower maximum comfortable loudness levels and shorter phase duration in response to apical stimulation, while medial and basal stimulation resulted in earlier N1 latencies and larger N1-P2 amplitudes in users with longer CI experience. Conclusions eACR could be elicited by direct intracochlear stimulation using individual fitting parameters with a success rate of 71%. The highest cortical peak-to-peak amplitudes were obtained in response to apical stimulation. Unlike the P2, the N1 component appeared to be a consistent cortical potential to determine eACR and gain knowledge of the auditory processing beyond the cochlea in CI users. Highlights eACR can be elicited through direct stimulation of intracochlear electrodes. Stimulation of apical and medial regions yielded the highest N1-P2 amplitudes. CI users with lower maximum comfortable loudness levels had shorter N1 latencies during apical stimulation. The present dataset of mainly well-performing CI users suggests better cortical processing, that is, higher amplitudes and shorter latencies of N1. The N1 potential appears a more consistent and reliable potential than the P2 to determine eACR responses in CI users.

ARTFit-A Quick and Reliable Tool for Performing Initial Fittings in Users of MED-EL Cochlear Implants.

This study assessed the safety and performance of ARTFit, a new tool embedded in MAESTRO, the cochlear implant (CI) system software by MED-EL GmbH (Innsbruck, Austria). ARTFit automatically measures thresholds of the electrically evoked compound action potential (ECAP) to produce initial ‘maps’ (ECAPMAPs), i.e., configuration settings of the audio processor that the audiologist switches to live mode and adjusts for comfortable loudness (LiveECAPMAPs). Twenty-three adult and ten pediatric users of MED-EL CIs participated. The LiveECAPMAPs were compared to behavioral maps (LiveBurstMAPs) and to the participants’ everyday clinical maps (ClinMAPs). Four evaluation measures were considered: average deviations of the maximum comfortable loudness (MCL) levels of the LiveECAPMAPs and the LiveBurstMAPs from the MCLs of the ClinMAPs; correlations between the MCLs of the LiveECAPMAPs (MCLecap) and the LiveBurstMAPs (MCLburst) with the MCLs of the ClinMAPs (MCLclin); fitting durations; and speech reception thresholds (SRTs). All evaluation measures were analyzed separately in the adult and pediatric subgroups. For all evaluation measures, the deviations of the LiveECAPMAPs from the ClinMAPs were not larger than those of the LiveBurstMAPs from the ClinMAPs. The Pearson correlation between the MCLecap and the MCLclin across all channels was r2 = 0.732 (p < 0.001) in the adult and r2 = 0.616 (p < 0.001) in the pediatric subgroups. The mean fitting duration in minutes for the LiveECAPMAPs was significantly shorter than for that of the LiveBurstMAPs in both subgroups: adults took 5.70 (range 1.90–11.98) vs. 9.27 (6.83–14.72) min; children took 3.03 (1.97–4.22) vs. 7.35 (3.95–12.77). SRTs measured with the LiveECAPMAPs were non-inferior to those measured with the ClinMAPs and not statistically different to the SRTs measured with the LiveBurstMAPs. ARTFit is a safe, quick, and reliable tool for audiologists to produce ECAP-based initial fitting maps in adults and young children who are not able to provide subjective feedback.

Accessing the Stapedius Muscle Via Novel Surgical Retrofacial Approach: A Cadaveric Feasibility Study.

Despite the complete embodiment of the stapedius muscle (SM) into the pyramidal eminence, it is possible to safely gain access to the SM belly via a retrofacial approach. This presents a novel approach to directly measure the electrically evoked stapedius reflex threshold (eSRT). Objective fitting of maximum comfortable loudness levels for cochlear implant users can improve the benefit introduced by the device. Sensing SM activity via direct surgical access represents a potential tool for objective eSRT fitting. Eighteen human temporal bones (TBs) were used. Micro-computed tomography was performed for six TBs. Standard computed tomography for six TBs. Manual 3D-segmentation of the relevant middle and inner ear anatomy was performed on 12 TBs. Mastoidectomy and posterior tympanotomy allowed the access to middle ear of all 18 the TBs. Once identified the mastoidal segment of the facial nerve (FN), the retrofacial access to the SM was drilled. The total access rate was 72.2%. Only in the first three cases the posterior semi-circular canal was hit. The SM access was identified posterior to the FN at a 4 ± 0.78 mm distance from the stapes' head, almost halfway to the chorda tympani's branching point along the FN direction. The drilling depth to access the SM posterior to the external surface of FN on average was 2 ± 0.30 mm. The exposure took on average of 5 to 8 minutes. The retrofacial approach seems to offer a feasible and reproducible access to the SM belly opening an avenue to electromyographic sensing of eSRT.

Optimizing stimulation parameters to record electrically evoked cortical auditory potentials in cochlear implant users

Objectives: To measure the effect of burst duration, stimulated electrode position, and stimulation level on the P1–N1–P2 electrically evoked cortical auditory potentials (eCAEPs) elicited via the direct stimulation of selected electrode contacts on a cochlear implant (CI) electrode array. Methods: Prospective observational study of 20 adult cochlear implant users with a MED-EL CI system. eCAEPs were recorded simultaneously with the Eclipse (Interacoustics) and the Neuropack S1 MEB-9400 (Nihon Kohden) recording systems. Tone bursts with durations of 50, 100, and 150 ms were used for stimulation at the maximum comfortable loudness level (MCL) and MCL minus 50% dynamic range (DR) at selected apical, medial, and basal intracochlear electrodes. Results: Individual P1–N1 and N1–P2 amplitudes were significantly higher at the MCL level of stimulation than at the MCL minus 50% DR. Burst length and stimulated electrode position did not affect the eCAEP responses. Residual noise was lower in the Neuropack S1 MEB-9400 system. Conclusions: This study shows the feasibility of eCAEP recording using the MAESTRO software. The eCAEP morphology was independent of the burst duration and the stimulated electrode position. This allows a large flexibility in using direct cochlear stimulation to elicit eCAEPs.

Feasibility of Day Surgery for Cochlear Implantation under Conscious Sedation with Same-Day Fitting.

To evaluate the feasibility of performing cochlear implantation under conscious sedation (CS) as day surgery with same-day fitting. All patients underwent cochlear implantation under CS between November 2017 and April 2018. The data collected included demographic information, preoperative clinical characteristics, surgical details, postoperative fitting information, and side effects, if any. Nine patients had 11 cochlear implants (CIs) placed under CS (2 patients received bilateral CIs). One patient's data were excluded from the audiological results because conversion to general anesthesia (GA) was necessary. One patient (11%) vomited just before the end of the procedure. Seven patients had uneventful procedures. Eight (88%) patients were discharged home the same day. There was a statistically significant difference in recovery time between the CS group and the GA group (t=-2.26, df=12, p<0.05). In the CS group, there was no statistically significant change in the maximum comfortable loudness level for all electrodes from the day of the surgery to the following day. However, there was a statistically significant difference in the threshold levels of all electrodes from the day of the surgery to the following day (Z=-2.04, N=120, p<0.05). Further analysis revealed a statistically significant difference in the four most apical electrodes (Z=-3.496, N=40, p<0.0001), but not in the middle or basal electrodes. Cochlear implantation can be performed under CS with careful patient selection. This approach facilitates same-day fitting and day surgery by minimizing comorbidity.

The relationship between the electrical stapedial muscle reflex threshold and electrical and behavioral measures in cochlear implant patients

Programming of multichannel cochlear implants (CIs) requires subjective responses to a series of sophisticated psychophysical percepts. It is often difficult for young prelinguistically deaf children to provide adequate responses for device fitting. This is especially true in setting levels of maximum comfortable loudness, whereby failure to indicate growth of loudness may result in elevation of stimulus levels to the threshold of pain. The acoustic or stapedial muscle reflex has been used previously to provide objective confirmation of acoustic stimulation, and there have been attempts to use the reflex in hearing-aid fitting. It has also been suggested that electrically elicited middle-ear muscle reflexes [electrically evoked stapedial reflex threshold (ESRT)] may have applicability in confirming and quantifying electrical stimulation through a CI. To assess the relationship between ESRT characteristics and levels of loudness perception with CIs, determine the reliability of the response, and investigate the potential use of ESRT in CI programming, 26 prelinguistically deafened CI users were evaluated. Reflexes have also been attempted on 312 electrodes, with responses present in 213 (68.3%). Comfort levels predicted by subjective judgments were highly correlated with the ESRT in individuals with CI. ESRT provides an objective, accurate, and rapid method of estimating maximum comfortable loudness levels, which may be useful in the initial programming of young implant recipients.

Radiologic and functional evaluation of electrode dislocation from the scala tympani to the scala vestibuli in patients with cochlear implants.

Localization of the electrode after cochlear implantation seems to have an impact on auditory outcome, and conebeam CT has emerged as a reliable method for visualizing the electrode array position within the cochlea. The aim of this retrospective study was to evaluate the frequency and clinical impact of scalar dislocation of various electrodes and surgical approaches and to evaluate its influence on auditory outcome. This retrospective single-center study analyzed a consecutive series of 63 cochlear implantations with various straight electrodes. The placement of the electrode array was evaluated by using multiplanar reconstructed conebeam CT images. For the auditory outcome, we compared the aided hearing thresholds and the charge units of maximum comfortable loudness level at weeks 6, 12, and 24 after implantation. In 7.9% of the cases, the electrode array showed scalar dislocation. In all cases, the electrode array penetrated the basal membrane within 45° of the electrode insertion. All 3 cases of cochleostomy were dislocated in the first 45° segment. No hearing differences were noted, but the charge units of maximum comfortable loudness level seemed to increase with time in patients with dislocations. The intracochlear dislocation rate of various straight electrodes detected by conebeam CT images is relatively low. Scalar dislocation may not negatively influence the hearing threshold but may require an increase of the necessary stimulus charge and should be reported by the radiologist.

Bimodal Cochlear Implants: The Role of Acoustic Signal Level in Determining Speech Perception Benefit

The aim of this project was to determine for bimodal cochlear implant (CI) patients, i.e. patients with low-frequency hearing in the ear contralateral to the implant, how speech understanding varies as a function of the difference in level between the CI signal and the acoustic signal. The data suggest that (1) acoustic signals perceived as significantly softer than a CI signal can contribute to speech understanding in the bimodal condition, (2) acoustic signals that are slightly softer than, or balanced with, a CI signal provide the largest benefit to speech understanding, and (3) acoustic signals presented at maximum comfortable loudness levels provide nearly as much benefit as signals that have been balanced with a CI signal.

Characteristics of detection thresholds and maximum comfortable loudness levels as a function of pulse rate in human cochlear implant users

The ability of an implanted ear to integrate multiple pulses, as measured by the slopes of detection threshold level (T level) versus pulse rate functions, may reflect cochlear health in the cochlea, as suggested by previous animal studies (Kang et al., 2010; Pfingst et al., 2011). In the current study, we examined the slopes of T level versus pulse rate functions in human subjects with cochlear implants. Typically, T levels decrease as a function of pulse rate, consistent with a multipulse integration mechanism. The magnitudes of the slopes of the T level versus pulse rate functions obtained from the human subjects were comparable to those reported in the animal studies. The slopes varied across stimulation sites, but did not change systematically along the tonotopic axis. This suggests that the slopes are dependent on local conditions near the individual stimulation sites. The characteristics of these functions were also similar to those found in animals in that the slopes for higher pulse rates were steeper than those for the lower pulse rates, consistent with a combined effect of multipulse integration and cumulative partial depolarization mechanisms at rates above 1000 pps. The maximum comfortable loudness level (C level) versus pulse rate functions were also examined to determine the effect of level on the slopes. Slopes of C-level functions were shallower than those for the T-level functions and were not correlated with those of the T-level functions, so the mechanisms underlying these two functions are probably not identical. The slopes of the T- or C-level functions were not dependent on stimulus-current level. Based on these results, we suggest that slopes of T level versus pulse rate functions might be a useful measure for estimating nerve survival in the cochlea in regions close to the stimulation sites.

Effects of electrode configuration on cochlear implant modulation detection thresholds

Cochlear implant function, as assessed by psychophysical measures, varies from one stimulation site to another within a patient's cochlea. This suggests that patient performance might be improved by selection of the best-functioning sites for the processor map. In evaluating stimulation sites for such a strategy, electrode configuration is an important variable. Variation across stimulation sites in loudness-related measures (detection thresholds and maximum comfortable loudness levels), is much larger for stimulation with bipolar electrode configurations than with monopolar configurations. The current study found that, in contrast to the loudness-related measures, magnitudes of across-site means and the across-site variances of modulation detection thresholds were not dependent on electrode configuration, suggesting that the mechanisms underlying variation in these various psychophysical measures are not all the same. The data presented here suggest that bipolar and monopolar electrode configurations are equally effective in identifying good and poor stimulation sites for modulation detection but that the across-site patterns of modulation detection thresholds are not the same for the two configurations. Therefore, it is recommended to test all stimulation sites using the patient's clinically assigned electrode configuration when performing psychophysical evaluation of a patient's modulation detection acuity to select sites for the processor map.

Relationship between gap detection thresholds and loudness in cochlear-implant users

Gap detection threshold (GDT) is a commonly used measure of temporal acuity in cochlear-implant (CI) recipients. This measure, like other measures of temporal acuity, shows considerable variation across subjects and also varies across stimulation sites within subjects. The aims of this study were (1) to determine whether across-site variation in GDTs would be reduced or maintained with increased stimulation levels; (2) to determine whether across-site variation in GDTs at low stimulation levels was related to differences in loudness percepts at those same levels; and (3) to determine whether matching loudness levels could reduce across-site differences in GDTs. Thresholds and maximum comfortable loudness levels were measured in postlingually deaf adults using all available sites in their electrode arrays. All sites were then surveyed at 30% of the dynamic range (DR) to examine across-site variation. Two sites with the largest difference in GDTs were then selected and for those two sites GDTs were measured at multiple levels of the DR (10%, 30%, 50%, 70%, and 90%). Stimuli consisted of 500 ms trains of symmetric-biphasic pulses, 40 μs/phase, presented at a rate of 1000 pps using a monopolar (MP1+2) electrode configuration. To examine perceptual differences in loudness, the selected sites were loudness-matched at the same levels of the DR. Variations in GDTs and loudness patterns were observed across stimulation sites and across subjects. Variations in GDTs across sites tended to decrease with increasing stimulation levels. For the majority of the subjects, stimuli at a given level in %DR were perceived louder at sites with better GDTs than those presented at the same level in %DR at sites with poorer GDTs. These results suggest that loudness is a contributing factor to across-site variation in GDTs and that CI fittings based on more detailed loudness matching could reduce across-site variation and improve perceptual acuity.

Across-site patterns of modulation detection in listeners with cochlear implants

In modern cochlear implants, much of the information required for recognition of important sounds is conveyed by temporal modulation of the charge per phase in interleaved trains of electrical pulses. In this study, modulation detection thresholds (MDTs) were used to assess listeners' abilities to detect sinusoidal modulation of charge per phase at each available stimulation site in their 22-electrode implants. Fourteen subjects were tested. MDTs were found to be highly variable across stimulation sites in most listeners. The across-site patterns of MDTs differed considerably from subject to subject. The subject-specific patterns of across-site variability of MDTs suggest that peripheral site-specific characteristics, such as electrode placement and the number and condition of surviving neurons, play a primary role in determining modulation sensitivity. Across-site patterns of detection thresholds (T levels), maximum comfortable loudness levels (C levels) and dynamic ranges (DRs) were not consistently correlated with across-site patterns of MDTs within subjects, indicating that the mechanisms underlying across-site variation in these measures differed from those underlying across-site variation in MDTs. MDTs sampled from multiple sites in a listener's electrode array might be useful for diagnosing across-subject differences in speech recognition with cochlear implants and for guiding strategies to improve the individual's perception.

Objective methods in postlingually and prelingually deafened adults for programming cochlear implants: ESR and NRT

This study compared responses of prelingually and postlingually deafened adult Nucleus 24 cochlear implant users on two objective measures employed to predict programming levels: neural response telemetry (NRT) and electrically evoked stapedial reflexes (eSR). Thirty experienced postlingually and prelingually deafened adult implant users underwent standard behavioural judgements of maximum comfortable loudness levels (C levels) and thresholds (Ts) followed by eSR and NRT measurements. Two different programs were created based on both the subjective judgement and the objective estimates of C levels (eSR thresholds) and these were compared. Relationships between the subjective and the objective measures were statistically analysed. Maximum stimulation levels estimated by both eSR and NRT were highly correlated with C levels. Variability of NRT results was higher than for eSR results. Mean NRT thresholds for postlingually deafened patients were higher than for prelingually deafened patients. A number of prelingually deafened users could distinguish no difference between programs; however, the majority of postlingually deafened users were sensitive to the difference and many reported preference for the program with eSR-estimated C levels. Neural response telemetry thresholds and eSRTs obtained in Nucleus 24 patients are highly correlated with C levels and Ts. Results suggest that estimation of C levels and Ts using NRT or eSR requires different correction factors for prelingually versus postlingually deafened adult subjects. Copyright © 2006 John Wiley & Sons, Ltd.

Objective methods in postlingually and prelingually deafened adults for programming cochlear implants: ESR and NRT

This study compared responses of prelingually and postlingually deafened adult Nucleus 24 cochlear implant users on two objective measures employed to predict programming levels: neural response telemetry (NRT) and electrically evoked stapedial reflexes (eSR). Thirty experienced postlingually and prelingually deafened adult implant users underwent standard behavioural judgements of maximum comfortable loudness levels (C levels) and thresholds (Ts) followed by eSR and NRT measurements. Two different programs were created based on both the subjective judgement and the objective estimates of C levels (eSR thresholds) and these were compared. Relationships between the subjective and the objective measures were statistically analysed. Maximum stimulation levels estimated by both eSR and NRT were highly correlated with C levels. Variability of NRT results was higher than for eSR results. Mean NRT thresholds for postlingually deafened patients were higher than for prelingually deafened patients. A number of prelingually deafened users could distinguish no difference between programs; however, the majority of postlingually deafened users were sensitive to the difference and many reported preference for the program with eSR-estimated C levels. Neural response telemetry thresholds and eSRTs obtained in Nucleus 24 patients are highly correlated with C levels and Ts. Results suggest that estimation of C levels and Ts using NRT or eSR requires different correction factors for prelingually versus postlingually deafened adult subjects.

Psychophysical Metrics and Speech Recognition in Cochlear Implant Users

Intersubject variability in perception is a prominent characteristic of people with cochlear implants. This study characterized intersubject differences using simple metrics based on psychophysical measures: maximum comfortable loudness levels (C levels) and dynamic ranges (DRs). In a group of 17 subjects, we assessed across-site variation (ASV) and across-site mean (ASM) values of C levels and DRs for bipolar (BP) and monopolar (MP) stimulation, and examined the relation of these metrics to speech recognition across subjects. Significant negative correlations with speech recognition were found for ASVs of C levels for BP stimulation; i.e., subjects with high ASVs of BP C levels had poor speech recognition. Positive correlations with speech recognition were found for ASMs of C levels and ASMs of DRs for both BP and MP stimulation; i.e., subjects with high mean C levels and large mean DRs had better speech recognition. Thus, these psychophysical metrics are effective for diagnosis of individual differences in performance of subjects with cochlear implants. Furthermore, they point to some potentially useful treatment procedures.

ECAP, ESR and Subjective Levels for Two Different Nucleus 24 Electrode Arrays

We compared behavioral judgment of maximum comfortable loudness levels (C levels) and behavioral thresholds (Ts) for straight and Contour electrode arrays with two objective thresholds, electrically elicited stapedial reflex thresholds (eSRTs) and electrically elicited compound action potential thresholds (eCAP thresholds), on experienced adult cochlear implant users. Next, we evaluated the predictive value of objective measures for the straight and Contour electrode arrays, respectively. This is a prospective, two-group comparison study of two objective and subjective levels for two different Nucleus 24 electrode arrays. Thirty experienced adults with Nucleus 24 cochlear implant were subjects in this study. Half the subjects used the straight electrode array, and the other half used the Contour electrode array. Subjective C levels, Ts, and eSRTs were successfully identified for each active electrode. eCAP thresholds were measured on 5 representative basal, medial, and apical electrodes. Correlation and regression analyses between subjective levels and objective thresholds were performed. For our study subjects, there were no significant differences between the straight and Contour electrode array in regard to stimulation requirements between C levels, Ts, and thresholds of eCAP thresholds and eSRTs. Both eSRTs and eCAP thresholds may be used equally for estimation of subjective levels for either straight electrode array or Contour electrode array.

Correlation of speech recognition by cochlear implant users with metrics based on thresholds and loudness levels

Intersubject variability in speech recognition is a persistent characteristic of cochlear implant users, despite significant improvements in average performance. This study examines simple psychophysical measures, maximum comfortable loudness levels (C-levels) and dynamic ranges (DRs) that may serve for diagnosis of conditions that affect speech recognition. Eighteen subjects who had used Nucleus CI24 cochlear implants for a minimum of 1 year participated in the study. T and C levels were measured using the method of adjustment. Consonant, vowel, and sentence recognition were tested using the subjects everyday speech processors (SPEAK or ACE). Across-site variation (ASV) was quantified for each subject using the variance of the T and C levels across all tested stimulation sites. Mean DRs (C levels minus T levels) for all tested sites were calculated for each subject. The results showed a moderate but significant negative correlation between speech-recognition performance and ASV of the T and C levels measured with both monopolar and bipolar electrode configurations. Mean DRs also correlated with speech performance. These results support the hypothesis that the large across-site variation in detection thresholds and small dynamic range reflect conditions that are detrimental to speech-recognition with cochlear implants. [Work supported by NIDCD Grant DC03808.]

Modeling the relationship between psychophysical perception and electrically evoked compound action potential threshold in young cochlear implant recipients: clinical implications for implant fitting

Objective: In cochlear implant recipients, the threshold of the electrically evoked compound action potential (ECAP) has been shown to correlate with the perceptual detection threshold and maximum comfortable loudness levels (respectively, T- and C-levels) used for implant programming. Our general objective was to model the relationship between ECAP threshold and T/ C-levels by taking into account their relative changes within each subject. In particular, we were interested in investigating further the validity of ECAP threshold as a predictor of psychophysical levels, depending on intra-cochlear electrode location and time of testing (from 1 to 18 months post-implantation). Methods: A total of 370 ECAP thresholds, measured in 49 children, using a Nucleus ® 24 cochlear implant, were compared with the corresponding T- and C-levels obtained at the same visit, for the same electrode. Response profiles for the whole group of patients were modeled across four test electrodes spaced equally along the electrode array from base towards apex. A linear regression model was constructed and the quality of the ECAP threshold-based predictions was assessed by testing for correlation between measured and predicted psychophysics. Comparison was made with a more simplistic model (described here as the ‘parallel profiles method’) stipulating, within each subject, a 1 μA increase in psychophysical levels for every 1 μA increase in ECAP threshold. Results: Offset between ECAP threshold and psychophysics profiles was found to vary significantly along the electrode array for the T-, but not for the C-level. In contrast with the parallel profiles method, our regression model predicted, within each subject, an average increase of 0.23 μA (95% confidence interval: 0.18–0.28) in T-level for every 1 μA increase in ECAP threshold. This correction improved the quality of T-level prediction when our model was run using measured T-level and ECAP threshold from a reference electrode ( r=0.77 vs. r=0.62). The shorter the distance between the electrode for which T-level was predicted and the one used as reference, the stronger the correlation between measured and predicted T-levels. In addition, poorer T-level predictions were obtained at the basal end of the array during the first 3 months post-implantation. In contrast to T-level, individual changes in C-level with ECAP threshold exhibited heterogeneous patterns across subjects so that no common coefficient could account for these changes. However, applying the parallel profiles method led to high-quality C-level prediction. Conclusions and significance: The results suggest that covariation between ECAP thresholds and psychophysics plays a decisive role in the relationship of ECAP threshold with T-, but not with C-level. Therefore, our regression model and the parallel profiles method should both be used for predicting, respectively, the T- and the C-levels. Although the predictability of our regression model seems to be better for middle and apical electrodes, its utilization should be extended to basal electrodes after 6 months' implant use.

Across-site variation in detection thresholds and maximum comfortable loudness levels for cochlear implants.

In cochlear implants, variation across stimulation sites in psychophysical detection thresholds (T levels) and maximum comfortable loudness levels (C levels) can be large when narrow-bipolar (BP) stimulation is used. This across-site variation is typically smaller when monopolar (MP) stimulation is used. At least two models can account for across-site variation and the effects of electrode configuration on the magnitude of the variation. According to one model, across-site variation reflects site-to-site differences in the distances between the stimulating electrodes and the sites of action-potential initiation. Under this model, the lower across-site variation with MP stimulation is due to shallower current versus distance gradients. An alternative model assumes that T and C levels depend on integration of activity across the whole population of neurons and that MP stimulation activates neurons over a larger spatial extent than does BP stimulation. If T and C levels are determined by integration of activity across large overlapping populations of neurons, then their values at adjacent sites should be more similar than if these levels result from integration across smaller, more independent populations. We tested the models by examining the effects on across-site variation of three variables believed to affect the spatial extent of activation: electrode configuration, stimulus level within the dynamic range, and electrode-array design. T levels and C levels were measured in 13 subjects with Nucleus CI24M (straight array) and 9 subjects with Nucleus CI24R(CS) (Contour) cochlear implants using bipolar (BP) and monopolar (MP) electrode configurations. Site-to-site variation in T and C levels for BP stimulation was 2.1-3.3 times larger than that for MP stimulation. Contrary to the across-neuron integration hypothesis, no significant differences were found between across-site variation for T levels and that for C levels for the BP configuration. There was considerable overlap in site-to-site variation values for the two types of implants but mean site-to-site variation in C levels for CI24M implants was significantly lower than that for CI24R(CS) implants. Control studies suggested that these results were not an artifact of the scale, and not due to differences in inherent variability of the psychophysical measures, or to the method of quantifying across-site variation.

Childhood otitis media and electrically elicited stapedius reflexes in adult cochlear implantees.

Electric stapedius reflex thresholds are helpful in programming cochlear implants, but only approximately two-thirds of patients have identifiable reflexes. Childhood otitis media correlates with absent stapedius reflexes or with high electric stimulation needed to elicit a reflex in cochlear implantees. Twenty-five adults with acquired hearing loss who underwent implantation with the MED-EL COMBI 40+ standard electrode array. STUDY PARAMETERS: The extent of temporal bone pneumatization, an indicator of childhood otitis media, was measured from preoperative computed tomographic images. Clinical units (in microamperes) needed to elicit a contralateral stapedius reflex, or maximum stimulation tried, were recorded. No definite association of pneumatization volume with intensity of stimulation was observed. The null hypothesis of no association cannot be rejected. Childhood otitis media does not seem to explain absent electric stapedius reflexes and the wide range of clinical units needed for maximum comfortable loudness level. Remaining potential explanations probably include the wide range of cochlear neurons that can be electrically stimulated, and that the maximum tolerable stimulation is too low to elicit a stapedius reflex.