Effects of Stimulus Polarity and Artifact Reduction Method on the Electrically Evoked Compound Action Potential.

OBJECTIVE: Previous research from our laboratory comparing electrically evoked compound action potential (ECAP) artifact reduction methods has shown larger amplitudes and lower thresholds with cathodic-leading forward masking (CathFM) than with alternating polarity (AltPol). One interpretation of this result is that the anodic-leading phase used with AltPol elicits a less excitatory response (in contrast to results from recent studies with humans), which when averaged with responses to cathodic-leading stimuli, results in smaller amplitudes. Another interpretation is that the latencies of the responses to anodic- and cathodic-leading pulses differ, which when averaged together, result in smaller amplitudes than for either polarity alone due to temporal smearing. The purpose of this study was to separate the effects of stimulus polarity and artifact reduction method to determine the relative effects of each. DESIGN: This study used a within-subjects design. ECAP growth functions were obtained using CathFM, anodic-leading forward masking (AnodFM), and AltPol for 23 CI recipients (N = 13 Cochlear and N = 10 Advanced Bionics). N1 latency, amplitude, slope of the amplitude growth function, and threshold were compared across methods. Data were analyzed separately for each manufacturer due to inherent differences between devices. RESULTS: N1 latencies were significantly shorter for AnodFM than for CathFM and AltPol for both Cochlear and Advanced Bionics participants. Amplitudes were larger for AnodFM than for either CathFM or AltPol for Cochlear recipients; amplitude was not significantly different across methods for Advanced Bionics recipients. Slopes were shallowest for CathFM for Cochlear subjects, but were not significantly different among methods for Advanced Bionics subjects. Thresholds with AltPol were significantly higher than both FM methods for Cochlear recipients; there was no difference in threshold across methods for the Advanced Bionics recipients. CONCLUSIONS: For Cochlear devices, the smaller amplitudes and higher thresholds observed for AltPol seem to be the result of latency differences between polarities. These results suggest that AltPol is not ideal for managing stimulus artifact for ECAP recordings. For the Advanced Bionics group, there were no significant differences among methods for amplitude, slope, or threshold, which suggests that polarity and artifact reduction method have little influence in these devices. We postulate that polarity effects are minimized for symmetrical biphasic pulses that lack an interphase gap, such as those used with Advanced Bionics devices; however, this requires further investigation.

Effects of stimulus level and rate on psychophysical thresholds for interleaved pulse trains in cochlear implants.

This study examined channel interactions using interleaved pulse trains to assess masking and potential facilitative effects in cochlear-implant recipients using clinically relevant stimuli. Psychophysical thresholds were measured for two adjacent mid-array electrodes; one served as the masker and the other as the probe. Two rates representative of those found in present-day strategies were tested: 1700 and 3400 pulses per second per channel. Four masker levels ranging from sub-threshold to loud-but-comfortable were tested. It was hypothesized that low-level maskers would produce facilitative effects, shifting to masking effects at high levels, and that faster rates would yield smaller masking effects due to greater stochastic neural firing patterns. Twenty-nine ears with Cochlear or Advanced Bionics devices were tested. High-level maskers produced more masking than low-level maskers, as expected. Facilitation was not observed for sub-threshold or threshold-level maskers in most cases. High masker levels yielded reduced probe thresholds for two Advanced Bionics subjects. This was partly eliminated with a longer temporal offset between each masker-probe pulse pair, as was used with Cochlear subjects. These findings support the use of temporal gaps between stimulation of subsequent electrodes to reduce channel interactions.

A Comparison of Alternating Polarity and Forward Masking Artifact-Reduction Methods to Resolve the Electrically Evoked Compound Action Potential.

OBJECTIVE: Cochlear implant manufacturers utilize different artifact-reduction methods to measure electrically evoked compound action potentials (ECAPs) in the clinical software. Two commercially available artifact-reduction techniques include forward masking (FwdMsk) and alternating polarity (AltPol). AltPol assumes that responses to the opposing polarities are equal, which is likely problematic. On the other hand, FwdMsk can yield inaccurate waveforms if the masker does not effectively render all neurons into a refractory state. The goal of this study was to compare ECAP thresholds, amplitudes, and slopes of the amplitude growth functions (AGFs) using FwdMsk and AltPol to determine whether the two methods yield similar results. DESIGN: ECAP AGFs were obtained from three electrode regions (basal, middle, and apical) across 24 ears in 20 Cochlear Ltd. recipients using both FwdMsk and AltPol methods. AltPol waveforms could not be resolved for recipients of devices with the older-generation chip (CI24R(CS); N = 6). RESULTS: Results comparing FwdMsk and AltPol in the CI24RE- and CI512-generation devices showed significant differences in threshold, AGF slope, and amplitude between methods. FwdMsk resulted in lower visual-detection thresholds (p < 0.001), shallower slopes (p = 0.004), and larger amplitudes (p = 0.03) compared with AltPol. CONCLUSIONS: Results from this study are consistent with recent findings showing differences in ECAP amplitude and latency between polarities for human CI recipients. When averaged, these differences likely result in a reduced ECAP response with AltPol. The next step will be to separate the effects of artifact-reduction method and stimulus polarity to determine the relative effects of each.

Effect of electrode impedance on spread of excitation and pitch perception using electrically coupled "dual-electrode" stimulation.

OBJECTIVES: In newer-generation Cochlear Ltd. cochlear implants, two adjacent electrodes can be electrically coupled to produce a single contact or "dual electrode" (DE). The goal of the present study was to evaluate whether relatively large impedance differences (>3.0 kOhms) between coupled electrodes affect the excitation pattern and pitch percepts produced by the DE. DESIGN: Fifteen electrode pairs in six recipients were tested. Neural spread-of-excitation patterns and pitch perception were measured for adjacent physical electrodes (PEs) and the resulting DE to determine if the lower-impedance PE in the pair dominates the DE response pattern. The results were compared with a "normative sample" (impedance differences <3.0 kOhms) from two earlier studies. RESULTS: In general, spread-of-excitation patterns for DEs more closely approximated those of the lower-impedance PE in each pair. The DE was more easily distinguished in pitch from the higher-impedance PE than the lower-impedance PE. The electrically evoked compound action potential and perceptual results generally differed from those of the normative group. CONCLUSIONS: Impedance differences between adjacent PEs should be considered if DE stimulation is implemented in future research studies or clinical coding strategies.

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

The first objective of this study was to determine whether adaptive pitch-ranking and electrode-discrimination tasks with cochlear-implant (CI) recipients produce similar results for perceiving intermediate "virtual-channel" pitch percepts using current steering. Previous studies have not examined both behavioral tasks in the same subjects with current steering. A second objective was to determine whether a physiological metric of spatial separation using the electrically evoked compound action potential spread-of-excitation (ECAP SOE) function could predict performance in the behavioral tasks. The metric was the separation index (Σ), defined as the difference in normalized amplitudes between two adjacent ECAP SOE functions, summed across all masker electrodes. Eleven CII or 90 K Advanced Bionics (Valencia, CA) recipients were tested using pairs of electrodes from the basal, middle, and apical portions of the electrode array. The behavioral results, expressed as d', showed no significant differences across tasks. There was also no significant effect of electrode region for either task. ECAP Σ was not significantly correlated with pitch ranking or electrode discrimination for any of the electrode regions. Therefore, the ECAP separation index is not sensitive enough to predict perceptual resolution of virtual channels.

Pitch ranking, electrode discrimination, and physiological spread-of-excitation using Cochlear's dual-electrode mode.

This study compared pitch ranking, electrode discrimination, and electrically evoked compound action potential (ECAP) spatial excitation patterns for adjacent physical electrodes (PEs) and the corresponding dual electrodes (DEs) for newer-generation Cochlear devices (Cochlear Ltd., Macquarie, New South Wales, Australia). The first goal was to determine whether pitch ranking and electrode discrimination yield similar outcomes for PEs and DEs. The second goal was to determine if the amount of spatial separation among ECAP excitation patterns (separation index, Σ) between adjacent PEs and the PE-DE pairs can predict performance on the psychophysical tasks. Using non-adaptive procedures, 13 subjects completed pitch ranking and electrode discrimination for adjacent PEs and the corresponding PE-DE pairs (DE versus each flanking PE) from the basal, middle, and apical electrode regions. Analysis of d' scores indicated that pitch-ranking and electrode-discrimination scores were not significantly different, but rather produced similar levels of performance. As expected, accuracy was significantly better for the PE-PE comparison than either PE-DE comparison. Correlations of the psychophysical versus ECAP Σ measures were positive; however, not all test/region correlations were significant across the array. Thus, the ECAP separation index is not sensitive enough to predict performance on behavioral tasks of pitch ranking or electrode discrimination for adjacent PEs or corresponding DEs.

The relation between auditory-nerve temporal responses and perceptual rate integration in cochlear implants.

The purpose of this study was to examine auditory-nerve temporal response properties and their relation to psychophysical threshold for electrical pulse trains of varying rates ("rate integration"). The primary hypothesis was that better rate integration (steeper slope) would be correlated with smaller decrements in ECAP amplitude as a function of stimulation rate (shallower slope of the amplitude-rate function), reflecting a larger percentage of the neural population contributing more synchronously to each pulse in the train. Data were obtained for 26 ears in 23 cochlear-implant recipients. Electrically evoked compound action potential (ECAP) amplitudes were measured in response to each of 21 pulses in a pulse train for the following rates: 900, 1200, 1800, 2400, and 3500 pps. Psychophysical thresholds were obtained using a 3-interval, forced-choice adaptive procedure for 300-ms pulse trains of the same rates as used for the ECAP measures, which formed the rate-integration function. For each electrode, the slope of the psychophysical rate-integration function was compared to the following ECAP measures: (1) slope of the function comparing average normalized ECAP amplitude across pulses versus stimulation rate ("adaptation"), (2) the rate that produced the maximum alternation depth across the pulse train, and (3) rate at which the alternating pattern ceased (stochastic rate). Results showed no significant relations between the slope of the rate-integration function and any of the ECAP measures when data were collapsed across subjects. However, group data showed that both threshold and average ECAP amplitude decreased with increased stimulus rate, and within-subject analyses showed significant positive correlations between psychophysical thresholds and mean ECAP response amplitudes across the pulse train. These data suggest that ECAP temporal response patterns are complex and further study is required to better understand the relative contributions of adaptation, desynchronization, and firing probabilities of individual neurons that contribute to the aggregate ECAP response.

ECAP spread of excitation with virtual channels and physical electrodes.

The primary goal of this study was to evaluate physiological spatial excitation patterns for stimulation of adjacent physical electrodes and intermediate virtual channels. Two experiments were conducted that utilized electrically evoked compound action potential (ECAP) spread-of-excitation (SOE) functions obtained with the traditional forward-masking subtraction method. These two experiments examined spatial excitation patterns for virtual-channel maskers and probes, respectively. In Experiment 1, ECAP SOE patterns were obtained for maskers applied to physical electrodes and virtual channels to determine whether virtual-channel maskers yield SOE patterns similar to those predicted from physical electrodes. In Experiment 2, spatial separation of SOE functions was compared for two adjacent physical probe electrodes and the intermediate virtual channel to determine the extent to which ECAP SOE patterns for virtual-channel probes are spatially separate from those obtained with physical electrodes. Data were obtained for three electrode regions (basal, middle, apical) for 35 ears implanted with Cochlear (N = 16) or Advanced Bionics (N = 19) devices. Results from Experiment 1 showed no significant difference between predicted and measured ECAP amplitudes for Advanced Bionics subjects. Measured ECAP amplitudes for virtual-channel maskers were significantly larger than the predicted amplitudes for Cochlear subjects; however, the difference was <2 µV and thus is likely not clinically significant. Results from Experiment 2 showed that the probe set in the apical region demonstrated the least amount of spatial separation amongst SOE functions, which may be attributed to more uniform nerve survival patterns, closer electrode spacing, and/or the tapered geometry of the cochlea. As expected, adjacent physical probes demonstrated greater spatial separation than for comparisons between each physical probe and the intermediate virtual channel. Finally, the virtual-channel SOE functions were generally weighted toward the basal electrode in the pair.

Effect of ECAP-based choice of stimulation rate on speech-perception performance.

OBJECTIVES: The objective determination of an optimal stimulation rate for CI users could save time and take the uncertainty out of choosing a rate based on patient preference. Electrically evoked compound action potential (ECAP) temporal response patterns vary across stimulation rates and cochlear regions, and could be useful in objectively predicting an optimal rate. Given that only one rate of stimulation can be used for current CI devices, we propose two potential ways to investigate whether a rate that produces stochastic ECAP responses (termed stochastic rate) can be used to predict an optimal stimulation rate. The first approach follows that of , which compared performance across three cochlear regions using limited electrode sets. This approach, which has inherent limitations, may provide insight into the effects of region-specific stochastic rates on performance. The second, more direct, approach is to compare speech perception for full-array maps that each uses a stochastic rate from a different region of the cochlea. Using both of these methods in a set of two acute experiments, the goal of the present study was to assess the effects of stochastic rate on speech perception. DESIGN: Speech-perception stimuli included the Hearing in Noise Test (HINT sentences), Consonant-Nucleus-Consonant (CNC) phonemes, and Iowa Medial Consonants. For Experiment 1, 22 ears in 20 CI recipients were tested in three map conditions (basal-only, middle-only, and apical-only electrode sets) using the subject's daily-use stimulation rate to first explore the level of performance possible with region-specific maps. A one-way repeated-measures analysis of variance (RM ANOVA) was used to examine the effect of electrode region on performance. A subset of nine subjects was tested with three additional maps (basal-only, middle-only, and apical-only electrode sets) using the region-specific stochastic rate, as measured in a previous study. A two-way RM ANOVA was used to assess the effects of electrode region and per-channel stimulation rate on performance for this subgroup. For Experiment 2, the same subset of nine subjects was tested with four full-array maps that each used either the daily-use stimulation rate or one of the stochastic rates. A one-way RM ANOVA was used to examine the effect of stimulation rate on performance. RESULTS: For Experiment 1, average performance with the daily-use rate and the stochastic rate was significantly better using the middle electrode set for HINT sentences and CNC phonemes. Perception of medial consonants was similar using the basal and middle electrode sets, and both of these were better than consonant perception with the apical region. For the subgroup of subjects tested with both the daily and stochastic rates, results revealed that stimulation rate did not have a significant effect on performance. For Experiment 2, results revealed no significant differences in performance using full-array maps with different stochastic rates or with the daily-use rate. CONCLUSIONS: Speech-perception scores were higher using the middle electrode set than with the basal or apical sets; however, this may have resulted from less spectral compression for the middle-region map. The effect of using stochastic rate as an optimal stimulation rate requires further investigation. A longer acclimatization period may be more likely to show differences in performance using stochastic rates versus daily-use rates.

How well do cochlear implant intraoperative impedance measures predict postoperative electrode function?

OBJECTIVE: Objectives were as follows: 1) to evaluate the incidence of abnormal cochlear implant electrode impedance intraoperatively and at the initial activation, 2) to identify the percentage of abnormalities that resolve by the initial activation, and 3) to determine the incidence of normal intraoperative impedances that present as abnormal at the initial activation. STUDY DESIGN: Retrospective records review of intraoperative and postoperative cochlear implant electrode impedances. SETTING: Tertiary referral center. PATIENTS: Records were examined for 194 devices implanted in 165 pediatric and adult patients. RESULTS: Results indicate at least 1 open (OC) or short circuit (SC) in 12.4% (24/194) of devices intraoperatively, decreasing to 8.2% (16/194) postoperatively. OCs were more prevalent than SCs for intraoperative (92% versus 8%) and postoperative (94% versus 6%) intervals. Of the 3,430 total electrodes, 78 had abnormal impedance intraoperatively. Sixty-four of those (82%) resolved by the postoperative interval (62 OC, 2 SC), whereas 18% (14/78) remained abnormal postoperatively (12 OC, 2 SC). Six (0.17%) of 3,430 electrodes had normal impedance intraoperatively but were abnormal postoperatively. CONCLUSION: The incidence of SCs in the present study is likely underestimated because of poor sensitivity of monopolar coupling for detecting SCs. Intraoperative OCs have a high probability of resolving by the initial activation, particularly when contiguous electrodes are affected and suggests limited need for the use of a backup device in these cases. Surgical technique and/or complications, such as explant/reimplant or perilymphatic gushers, may result in increased incidence of bubbles in the cochlea and may play a role in abnormal intraoperative impedance results.

The effect of technology and testing environment on speech perception using telehealth with cochlear implant recipients.

PURPOSE: In this study, the authors evaluated the effect of remote system and acoustic environment on speech perception via telehealth with cochlear implant recipients. METHOD: Speech perception was measured in quiet and in noise. Systems evaluated were Polycom visual concert (PVC) and a hybrid presentation system (HPS). Each system was evaluated in a sound-treated booth and in a quiet office. RESULTS: For speech in quiet, there was a significant effect of environment, with better performance in the sound-treated booth than in the office; there was no effect of system (PVC or HPS). Speech in noise revealed a significant interaction between environment and system. Subjects' performance was poorer for PVC in the office, whereas performance in the sound-treated booth was not significantly different for the two systems. Results from the current study were compared to results for the same group of subjects from an earlier study; these results suggested that poorer performance at remote sites in the previous study was primarily due to environment, not system. CONCLUSIONS: Speech perception was best when evaluated in a sound-treated booth. HPS was superior for speech in noise in a reverberant environment. Future research should focus on modifications to non-sound-treated environments for telehealth service delivery in rural areas.

Temporal response properties of the auditory nerve: data from human cochlear-implant recipients.

The primary goal of this study was to characterize the variability in auditory-nerve temporal response patterns obtained with the electrically evoked compound action potential (ECAP) within and across a relatively large group of cochlear-implant recipients. ECAPs were recorded in response to each of 21 pulses in a pulse train for five rates (900, 1200, 1800, 2400, and 3500 pps) and three cochlear regions (basal, middle, and apical). An alternating amplitude pattern was typically observed across the pulse train for slower rates, reflecting refractory properties of individual nerve fibers. For faster rates, the alternation ceased and overall amplitudes were substantially lower relative to the first pulse in the train, reflecting cross-fiber desynchronization. The following specific parameters were examined: (1) the rate at which the alternating pattern ceased (termed stochastic rate), (2) the alternation depth and the rate at which the maximum alternation occurred, and (3) the average normalized ECAP amplitude across the pulse train (measure of overall adaptation/desynchronization). Data from 29 ears showed that stochastic rates for the group spanned the entire range of rates tested. The majority of subjects (79%) had different stochastic rates across the three cochlear regions. The stochastic rate occurred most frequently at 2400 pps for basal and middle electrodes, and at 3500 pps for apical electrodes. Stimulus level was significantly correlated with stochastic rate, where higher levels yielded faster stochastic rates. The maximum alternation depth averaged 19% of the amplitude for the first pulse. Maximum alternation occurred most often at 1800 pps for basal and apical electrodes, and at 1200 pps for middle electrodes. These differences suggest some independence between alternation depth and stochastic rate. Finally, the overall amount of adaptation or desynchronization ranged from 63% (for 900 pps) to 23% (for 3500 pps) of the amplitude for the first pulse. Differences in temporal response properties across the cochlea within subjects may have implications for developing new speech-processing strategies that employ varied rates across the array.

Use of telehealth for research and clinical measures in cochlear implant recipients: a validation study.

PURPOSE: The goal of this study was to compare clinical and research-based cochlear implant (CI) measures using telehealth versus traditional methods. METHOD: This prospective study used an ABA design (A = laboratory, B = remote site). All measures were made twice per visit for the purpose of assessing within-session variability. Twenty-nine adult and pediatric CI recipients participated. Measures included electrode impedance, electrically evoked compound action potential thresholds, psychophysical thresholds using an adaptive procedure, map thresholds and upper comfort levels, and speech perception. Subjects completed a questionnaire at the end of the study. RESULTS: Results for all electrode-specific measures revealed no statistically significant differences between traditional and remote conditions. Speech perception was significantly poorer in the remote condition, which was likely due to the lack of a sound booth. In general, subjects indicated that they would take advantage of telehealth options at least some of the time, if such options were available. CONCLUSIONS: Results from this study demonstrate that telehealth is a viable option for research and clinical measures. Additional studies are needed to investigate ways to improve speech perception at remote locations that lack sound booths and to validate the use of telehealth for pediatric services (e.g., play audiometry), sound-field threshold testing, and troubleshooting equipment.

Evaluating the Feasibility of Using Remote Technology for Cochlear Implants.

The use of remote technology to provide cochlear implant services has gained popularity in recent years. This article contains a review of research evaluating the feasibility of remote service delivery for recipients of cochlear implants. To date, published studies have determined that speech-processor programming levels and other objective tests (electrode impedance and electrically evoked compound action potentials) are equivalent to those obtained in the face-to-face condition. Despite these promising findings, speech perception using remote technology has proven to be more challenging. Previous investigations have evaluated speech perception with recipients of cochlear implants using videoconference (Polycom) equipment in nonsound-treated rooms (due to lack of access to audiological sound booths in rural areas). Results have revealed poorer speech perception scores using remote technology compared to face-to-face results. Additionally, it has been shown that Polycom transmission of a speech stimulus does not cause significant compression for adequate evaluation; rather, poorer results are due to testing performed in nonsound-treated environments. Based on the literature, telepractice is a feasible option for cochlear implant service delivery. Barriers to the wide-spread use of remote services for recipients of cochlear implants include a uniform system for the evaluation of speech perception, validation of services for pediatric recipients and initial activations, license reciprocity, and reimbursement for services.