Compound Action Potential Measurements Research Articles

Association Between Intracochlear Electrode Design and Electrically-Evoked Compound Action Potential Measures in Cochlear Implant Users.

Cochlear implant (CI) electrode design has changed over time. Changes in intracochlear electrode design might influence the spread of neural activation along the auditory nerve and the number of independent channels. This study aimed to investigate the impact of intracochlear electrode design on the electrode-neuron interface using electrophysiological measures. Prospective cohort study. A single tertiary hospital. Fifty-two ears who were implanted with CI divided into 3 groups based on the design of intracochlear electrode arrays. Twenty-three ears were implanted with lateral wall straight electrodes. Eighteen ears were implanted with the slim perimodiolar electrode, and 11 ears were implanted with the old perimodiolar electrode. Various electrically-evoked compound action potential (ECAP) metrics were measured to quantify spread of excitation and channel interaction. ECAP threshold and slope were not significantly different among groups. ECAP spread of excitation (SOE) half-width and channel interaction index (CII) were significantly larger in subjects implanted with the lateral wall straight electrodes, indicating a wider spread of excitation compared to those with perimodiolar electrodes. Electrode impedance was significantly lower in subjects implanted with perimodiolar electrodes than those with lateral wall electrodes. Perimodiolar electrode groups yielded significantly narrower SOE half-widths and smaller CII than the lateral wall straight electrode group. This may indicate that the electrode array that hugged the modiolus had less overlap in neural excitation between adjacent electrodes, resulting in reduced channel interaction and potentially better spectral resolution than the electrode array positioned more laterally.

Cochlear Implant Outcomes in Patients With TMTC2 -Associated Sensorineural Hearing Loss and Auditory Neuropathy/Auditory Dys-Synchrony.

To investigate the outcomes of cochlear implantation in patients with TMTC2 -associated sensorineural hearing loss and auditory neuropathy/auditory dys-synchrony. Adult and pediatric cochlear implant (CI) patients followed in an academic center who tested positive for TMTC2 genetic variant rs35725509. Cochlear implantation. Speech perception scores in quiet. Ten CI patients were identified with TMTC2 variant rs35725509 out of 157 patients who underwent genetic testing (i.e., 6.3% of patients tested). All demonstrated progressive, bilateral hearing loss with severe-to-profound audiometric thresholds preoperatively. Pre-CI and 1-year post-CI speech recognition percent correct scores were compared. Post-CI speech perception (mean 61.0%, standard deviation 31.4%) was significantly higher than pre-CI speech perception (mean 21.0%, standard deviation 27.0%) ( p = 0.002). Individually, 9 of the 10 subjects experienced significant improvements in speech perception pre- to post-CI ( p < 0.05). Electrically evoked compound action potential measures were available for five patients, and all showed normal electrically evoked compound action potential thresholds. Patients with TMTC2 -associated sensorineural hearing loss and auditory neuropathy/auditory dys-synchrony have significantly improved speech perception outcomes with cochlear implantation and should be considered candidates for this intervention if there are no other contraindications.

Relation between electrical compound action potential measures and speech perception in cochlear implanted children: audiological and phonological outcomes

PurposeTo study ECAP measures (threshold and amplitude growth function 'AGF') in children CI users and find the relation between these ECAP measures and speech outcomes using audiological and phonological assessment.Subjects and methodTwenty-one children were unilaterally implanted with Medel CI, and all subjects were submitted to phonological assessment, basic audiological assessment, speech recognition tests (WRS and BKB-SIN) and Medel maestro software measures (IFT, AutoART and AGF measures "thresholds and slopes" across apical, middle and basal electrodes).ResultsThis study demonstrated no statistically significant difference between AGF thresholds at apical, middle and basal electrodes and a statistically significant difference between AGF slopes at apical and both middle and basal electrodes. There was no statistically significant correlation between the ECAP threshold and speech perception tests. In contrast, a positive statistically significant correlation was found between the AGF slope of the apical electrode and word recognition score, and a negative statistically significant correlation between AGF slopes at apical, middle and basal electrodes and SNR loss of BKB-SIN. High sensitivity and specificity of AGF slope at apical electrode were found to differentiate between good and poor performers as regards SNR loss of BKB-SIN and language test.ConclusionsThe AGF slope reflects neural survival better than the ECAP threshold. AGF slope at apical electrodes correlated with better CI performance in both phoniatric and audiological measures of speech perception and can be used as an objective tool to predict CI outcome.

An Overview of Telepractice Applications for Comprehensive Cochlear Implant Service Delivery

Purpose:Numerous follow-up appointments are required to program a cochlear implant (CI) sound processor and monitor the patient's progress over time, particularly within the first year of implantation. This, coupled with the specialty nature of CI service delivery and sparsely located clinics, can pose burdens on recipients' time and access to necessary resources for consistent follow-up. Advances in the application of telepractice for CI service delivery have evolved and expanded over recent years, with further utilization spurred by the COVID-19 pandemic that emerged in late 2019. This review article provides a summary of telepractice applications for adult and pediatric CI processor programming, impedance, and electrically evoked compound action potential testing, speech-perception testing, evaluations of patient and clinician experiences, troubleshooting, and aural rehabilitation for CI recipients. Future directions and opportunities for further implementation of telepractice for CI service delivery are discussed.Conclusions:Studies to date suggest that telepractice can be reliably used for remote sound processor programming for both children and adults, impedance and evoked compound action potential measures, speech-perception testing in adult CI users, and self-monitoring. However, further technological advancements are needed to make remote speech-perception testing more easily implemented, and well-controlled studies are needed to better evaluate the efficacies of aural rehabilitation through teletherapy, particularly for children.

Conductive hearing loss during development does not appreciably alter the sharpness of cochlear tuning

An increasing number of studies show that listeners often have difficulty hearing in situations with background noise, despite normal tuning curves in quiet. One potential source of this difficulty could be sensorineural changes in the auditory periphery (the ear). Signal in noise detection deficits also arise in animals raised with developmental conductive hearing loss (CHL), a manipulation that induces acoustic attenuation to model how sound deprivation changes the central auditory system. This model attributes perceptual deficits to central changes by assuming that CHL does not affect sensorineural elements in the periphery that could raise masked thresholds. However, because of efferent feedback, altering the auditory system could affect cochlear elements. Indeed, recent studies show that adult-onset CHL can cause cochlear synapse loss, potentially calling into question the assumption of an intact periphery in early-onset CHL. To resolve this issue, we tested the long-term peripheral effects of CHL via developmental bilateral malleus displacement. Using forward masking tuning curves, we compared peripheral tuning in animals raised with CHL vs age-matched controls. Using compound action potential measurements from the round window, we assessed inner hair cell synapse integrity. Results indicate that developmental CHL can cause minor synaptopathy. However, developmental CHL does not appreciably alter peripheral frequency tuning.

Effects of Static Magnetic Field on Compound Action Potential of Isolated Frog Sciatic Nerve

The use of Static Magnetic Field (SMF) in medicine is currently under consideration. In this experimental study, the aim is to investigate the possible effects of SMF on nerve excitation and conduction characteristics. Our objectives are to take Compound Action Potential (CAP) measurements from an isolated frog sciatic nerve under SMF, and to calculate the amplitude and latency parameters of the CAP in order to assess possible effects. The experimental study was carried on twelve Rana Ridibundas. The sciatic nerve of the frogs were isolated from their locations and transferred to the nerve chamber. The nerve was stimulated with an electrical impulse of 0.2 ms duration and 1.4 V amplitude at the proximal end and its response was recorded at the distal end. The possible effects of SMF on the frog sciatic nerve were examined. A sequence of CAP measurements were taken with and without SMF exposure. Changes in four variables were observed. Two of the measurement variables were peak-to-peak amplitudes. The other two variables were the durations of stimulus artifact from the onset to the appearance of the first negative and first positive peaks respectively. After the first, second and third SMF exposure periods, there was a significant increase in the height of PP-1 and PP-2 which are peak-to-peak variables of CAP in both during and after exposure. Similarly, after the first, second and third SMF exposure periods, there was a significant increase in the length of Latency-1 and Latency-2 which are linked with the duration of CAP. In this study, it was observed that SMF exposure increases both the amplitude and duration of nerve CAP. Our study gave a different perspective on the effects of SMF on neuronal excitation mechanism of sciatic nerves. Besides, it provided a better understanding of the pain perception phenomenon based on transmembrane Na+ channel dynamics and nerve conduction velocity.

Electrically Evoked Compound Action Potential Recording in Peripheral Nerves

Applications for bioelectric medicine can be found in all parts of the nervous system. The CNS – brain and spinal cord – contain targets for commercial neuromodulation therapies. Peripheral nerves are also modulated with commercially available systems during treatment for chronic pain and epilepsy, and developments are in progress for treating many other diseases. The electrically evoked compound action potential is a measure of the electrical response from the tissue to stimulation. It provides a direct insight into the electrophysiology of the stimulation, and despite its incorporation into cochlear implants it is a technology that is yet to find its way into commercial peripheral nerve stimulation applications. This review outlines the status of evoked compound action potential measurements on peripheral nerves and highlights the challenges which need to be overcome.

Loudness optimized registration of compound action potential in cochlear implant recipients

Background Postoperative measurements of compound action potentials are not always possible due to the insufficient acceptance of the CI-recipients. This study investigated the impact of different parameters on the acceptance of the measurements. Methods Compound action potentials of 16 CI recipients were measured with different pulse-widths. Recipients performed a loudness rating at the potential thresholds with the different sequences. Results Compound action potentials obtained with higher pulse-widths were rated softer than those obtained with smaller pulse-widths. Conclusions Compound action potentials measured with higher pulse-widths generate a gap between loudest acceptable presentation level and potential threshold. This gap contributes to a higher acceptance of postoperative measurements.

Understanding bone-conduction hearing: Measurements and model development

Bone conduction (BC), the transmission of sound to the inner ear by way of skull vibration, is thought to stimulate the inner ear by several mechanisms: external ear (vibration of the ear-canal walls), middle ear (ossicular inertia), and inner ear (inertia of the cochlear fluids, compression of the cochlear walls, and sound flow through “third windows”). In chinchilla, we explore the inner-ear mechanisms with measurements of compound action potentials and intra-cochlear sound pressures after manipulations of the middle and inner ear. These measurements suggest that while contribution of cochlear fluid inertia to BC is reduced by occlusion of both cochlear windows, effects of cochlear compression and/or sound pressure transmission via “third window” pathways increase. We also explore external-ear mechanisms by applying an analytically-derived reverse input impedance of the external-ear canal, pre-existing measurements of middle-ear input impedance, and new measurements of ear canal sound pressure. We define an equivalent volume velocity source representative of the combined BC external-ear mechanisms. Estimates of the source magnitude are in agreement with experimental data (Chhan et al., 2016), showing that during ear-canal occlusion, the ear-canal source is a significant contributor to BC hearing at mid-frequencies (1-3 kHz).

Use of spatiotemporal templates for pathway discrimination in peripheral nerve recordings: a simulation study

Objective. Extraction of information from the peripheral nervous system can provide control signals in neuroprosthetic applications. However, the ability to selectively record from different pathways within peripheral nerves is limited. We investigated the integration of spatial and temporal information for pathway discrimination in peripheral nerves using measurements from a multi-contact nerve cuff electrode. Approach. Spatiotemporal templates were established for different neural pathways of interest, and used to obtain tailored matched filters for each of these pathways. Simulated measurements of compound action potentials propagating through the nerve in different test cases were used to evaluate classification accuracy, percentage of missed spikes, and ability to reconstruct the original firing rates of the neural pathways. Main results. The mean Pearson correlation coefficients between the original firing rates and estimated firing rates over all tests cases was found to be 0.832 ± 0.161, 0.421 ± 0.145, 0.481 ± 0.340 for our algorithm, Bayesian spatial filters, and velocity selective recordings respectively. Significance. The proposed method shows that the spatiotemporal templates were able to provide more robust spike detection and reliable pathway discrimination than these existing algorithms.

Evaluating the Noise in Electrically Evoked Compound Action Potential Measurements in Cochlear Implants

Electrically evoked compound action potentials (ECAPs) are widely used to study the excitability of the auditory nerve and stimulation properties in cochlear implant (CI) users. However, ECAP detection can be difficult and very subjective at near-threshold stimulation levels or in spread of excitation measurements. In this study, we evaluated the statistical properties of the background noise (BN) and the postaverage residual noise (RN) in ECAP measurements in order to determine an objective detection criterion. For the estimation of the BN and the RN, a method currently used in auditory brainstem response measurements was applied. The potential benefit of using weighted (Bayesian) averages was also examined. All estimations were performed with a set of approximately 360 ECAP measurements recorded from five human CI users of the CII or HiRes90K device (advanced bionics). Results demonstrated that the BN was normally distributed and the RN decreased according to the square root of the number of averages. No additional benefit was observed by using weighted averaging. The noise was not significantly different either at different stimulation intensities or across recording electrodes along the cochlea. The analysis of the statistical properties of the noise indicated that a signal-to-noise ratio of 1.7 dB as a detection criterion corresponds to a false positive detection rate of 1% with the used measurement setup.

Protection of Inner Ear Function after Cochlear Implantation: Compound Action Potential Measurements after Local Application of Glucocorticoids in the Guinea Pig Cochlea

Background: Cochlear implant users with residual hearing often benefit greatly from simultaneous electric and acoustic stimulation. However, implantation can cause trauma to the inner ear, resulting in poorer hearing postoperatively. We investigated whether a single local injection of glucocorticoids can reduce hearing loss in long-term implanted guinea pigs. Methods: Three groups of animals underwent bilateral surgery. One ear was implanted with an electrode, and the contralateral ear received a cochleostomy only. A single dose of the glucocorticoids triamcinolone or dexamethasone, or of artificial perilymph was infused into cochleae via cochleostomy. Compound action potentials were measured before and after application and for 3 months postoperatively. Tissue growth was measured as the percentage of the total area of the scala tympani that was obliterated. Results: Ears subjected to cochleostomy only and treated with glucocorticoids demonstrated a mild hearing loss. In the implanted ears, both glucocorticoids preserved hearing at least temporarily. The volume of tissue growth within the scala tympani was not reduced, and there was no relation between the amount of tissue and hearing loss. Conclusions: Both glucocorticoids show a potential benefit for hearing preservation in implanted ears. Glucocorticoid therapy may be useful to protect residual hearing during cochlear implantation.

Intracochlear Recordings of Electrophysiological Parameters Indicating Cochlear Damage

: The pathophysiologic mechanisms resulting in hearing loss during electrode implantation are largely unknown. To better understand the functional implications of electrode implantation, we recorded the effects of cochlear damage on acoustically evoked intracochlear measurements using normal-hearing gerbils. : A metal electrode was placed on the surface of the round window, and recordings of the cochlear microphonic (CM) and compound action potential (CAP) were made in response to stimulation with tone-bursts at various frequencies in 1-octave intervals and at intensities of 15 to 72 dB sound pressure level. The electrode was then advanced incrementally, with CM and CAP measurements obtained at each step. These data were compared with data obtained at the round window, and the electrode was withdrawn when a significant change was observed. After electrophysiological analysis, the cochlea was examined histologically. : Results show that on electrode insertion, loss of amplitude in the CM and CAP occurs after damage to cochlear structures. Loss of activity was typically first apparent in the CAP rather than the CM. : These results suggest that a reduction of the CAP can be an early marker of interaction of the electrode with cochlear structures. Such measurements are potentially available with slight modifications to current cochlear implant technology.

Time‐dependent effects of pre‐conditioning activation on muscle fiber conduction velocity and twitch torque

In this study we investigate the range of interstimulus intervals influencing the recovery functions for conduction velocity (velocity recovery function, or VRF) and twitch torque (twitch torque recovery function, or TRF) of muscle fibers. We studied the tibialis anterior muscle of 8 healthy men by varying the time interval between a pair of stimuli in the range of 4-1000 ms, with (triplet) and without (doublet) a preconditioning stimulus. For triplet stimulations, the interval between the first and second stimulus or between the second and third stimulus was fixed, and the other interval was varied. VRF and TRF were influenced by a preconditioning stimulus only when it was delivered within 125 ms and 30 ms, respectively, before the doublet stimulation. The results provide evidence of the range of preconditioning intervals influencing the VRF and TRF; this is relevant for understanding the mechanisms of force generation and for standardizing compound action potential measurements.

Excitation Patterns of Simultaneous and Sequential Dual-Electrode Stimulation in Cochlear Implant Recipients

Both simultaneous (SI) and sequential stimulation of intracochlear electrodes can be used to generate pitches that are intermediate to the physical electrodes (PEs). The goal of this study was to compare the spread of neural excitation for SI and sequential dual-electrode stimulation with the spread of neural excitation for the intermediate electrode using electrically evoked compound action potentials. Seven Advanced Bionics cochlear implant users with either CII or HiRes 90k implant and HiFocus 1 or HiFocus 1j electrode array participated in this study. A masker-probe subtraction method was used to derive neural excitation patterns for SI nonadjacent dual-electrode stimulation, apical and basal-first sequential nonadjacent dual-electrode stimulation, and the intermediate PE. For apical-first sequential (SEa) stimulation, the masker pulse on the apical electrode immediately preceded the masker pulse on the basal electrode, and vice versa for basal-first sequential stimulation (SEb). The electrodes used for dual-electrode stimulation were separated by an intermediate PE, which represents a spatial distance of approximately 2 mm. Current levels necessary to achieve comfortable loudness were determined for each masker and probe stimulus. During the evoked compound action potential measurements, the masker was fixed in location, whereas the probe was varied across a subset of electrodes in the array. Neural responses were calculated by subtracting the response to the probe from the masked response. Neural excitation patterns were normalized to their peak and analyzed in terms of their area and center of gravity. The area and center of gravity for SI nonadjacent dual-electrode stimulation were similar to those of the intermediate PE. In contrast, the area for the two modes of sequential nonadjacent dual-electrode (SEa and SEb) stimulation differed significantly from the intermediate PE. The center of gravity for SEa stimulation also differed significantly from the intermediate PE, whereas there was no significant difference in the center of gravity between SEb stimulation and the intermediate PE. Peripheral neural activation patterns suggest a similar spread of excitation for SI dual-electrode stimulation and the intermediate PE. The spread of excitation associated with sequential dual-electrode stimulation is generally different from the intermediate PE, and it varies depending on the order of the sequential pulses.

Glutamate Excitotoxicity Inflicts Paranodal Myelin Splitting and Retraction

Paranodal myelin damage is observed in white matter injury. However the culprit for such damage remains unknown. By coherent anti-Stokes Raman scattering imaging of myelin sheath in fresh tissues with sub-micron resolution, we observed significant paranodal myelin splitting and retraction following glutamate application both ex vivo and in vivo. Multimodal multiphoton imaging further showed that glutamate application broke axo-glial junctions and exposed juxtaparanodal K+ channels, resulting in axonal conduction deficit that was demonstrated by compound action potential measurements. The use of 4-aminopyridine, a broad-spectrum K+ channel blocker, effectively recovered both the amplitude and width of compound action potentials. Using CARS imaging as a quantitative readout of nodal length to diameter ratio, the same kind of paranodal myelin retraction was observed with applications of Ca2+ ionophore A23187. Moreover, exclusion of Ca2+ from the medium or application of calpain inhibitor abolished paranodal myelin retraction during glutamate exposure. Examinations of glutamate receptor agonists and antagonists further showed that the paranodal myelin damage was mediated by NMDA and kainate receptors. These results suggest that an increased level of glutamate in diseased white matter could impair paranodal myelin through receptor-mediated Ca2+ overloading and subsequent calpain activation.

Collateral sprouting of sensory axons after end-to-side nerve coaptation—A longitudinal study in the rat

The end-to-side nerve coaptation is able to induce collateral sprouting of axons from the donor nerve and to provide functional reinnervation of the target tissue. Sensory axon sprouting and its effects on the donor nerve up to 9 months after the end-to-side nerve coaptation were studied in the rat. Peroneal, tibial and saphenous nerves were transected and ligated, and the distal stump of the transected peroneal nerve was sutured to the side of the uninjured sural nerve. The average skin area of the residual sensitivity to pinch due to the axons sprouting through the recipient peroneal nerve did not change statistically significantly between 4 and 9 months after surgery. Axon counting, measurements of compound action potentials and retrograde neuron labeling indicate that the sprouting of the myelinated sensory axons and unmyelinated axons through the recipient nerve was largely completed by 2 months and 4 months after the end-to-side nerve coaptation, respectively, and remained stable thereafter for at least 9 months. A decrease in the amplitude and area of the CAP of myelinated fibers, observed in the donor nerve up to 4 months after surgery, was probably due to mild degeneration of nerve fibers and a tendency of the diameter of myelinated axons to decline. However, no significant changes in functional, electrophysiological or morphological properties of the donor nerve could be observed at the end of the observational period, indicating that end-to-side nerve coaptation has no detrimental effect on the donor nerve on a long-term scale.

Algorithm for multi-curve-fitting with shared parameters and a possible application in evoked compound action potential measurements

BackgroundExperimental results are commonly fitted by determining parameter values of suitable mathematical expressions. In case a relation exists between different data sets, the accuracy of the parameters obtained can be increased by incorporating this relationship in the fitting process instead of fitting the recordings separately.MethodsAn algorithm to fit multiple measured curves simultaneously was developed. The method accounts for parameters that are shared by some curves. It can be applied to either linear or nonlinear equations. Simulated noisy "measurement results" were created to compare the introduced method to the "straight forward" way of fitting the curves separately.ResultsThe analysis of the simulated measurements confirm, that the introduced method yields more accurate parameters compared to the ones gained by fitting the measurements separately. Therefore it needs more computer time. As an example, the new fitting algorithm is applied to the measurements of the evoked compound action potentials (ECAP) of the auditory nerve: This leads to promising ideas to reduce artefacts generated by the measuring process.ConclusionThe introduced fitting algorithm uses the relationship between multiple measurement results to increase the accuracy of the parameters. Its application in the field of ECAP measurements is promising and should be further investigated.

Normative Findings of Electrically Evoked Compound Action Potential Measurements Using the Neural Response Telemetry of the Nucleus CI24M Cochlear Implant System

One hundred and forty-seven adult recipients of the Nucleus^® 24 cochlear implant system, from 13 different European countries, were tested using neural response telemetry to measure the electrically evoked compound action potential (ECAP), according to a standardised postoperative measurement procedure. Recordings were obtained in 96% of these subjects with this standardised procedure. The group results are presented in terms of peak amplitude and latency, slope of the amplitude growth function and ECAP threshold. The effects of aetiological factors and the duration of deafness on the ECAP were also studied. While large intersubject variability and intrasubject variability (across electrodes) were found, results fell within a consistent pattern and a normative range of peak amplitudes and latencies was established. The aetiological factors had little effect on the ECAP characteristics. However, age affected ECAP amplitude and slope of the amplitude growth function significantly; i.e., the amplitude is higher in the lowest age category (15–30 years). Principal component analysis of the ECAP thresholds shows that the thresholds across 5 electrodes can be described by two factors accounting for 92% of the total variance. The two factors represent the overall level of the threshold profiles (‘shift’) and their slopes across the electrode array (‘tilt’). Correlation between these two factors and the same factors describing the T- and C-levels appeared to be moderate, in the range of 0.5–0.6.

Comparisons between Neural Response Imaging thresholds, electrically evoked auditory reflex thresholds and most comfortable loudness levels in CII Bionic Ear users with HiResolution™ sound processing strategies

Conclusions. The data collected in this study indicated that first Neural Response Imaging (NRI) thresholds had a better correlation with HiResolution™ most comfortable loudness (M) levels than tNRI thresholds. Electrically evoked auditory reflex thresholds (EARTs) had a higher correlation with HiResolution M levels than tNRI thresholds and a lower correlation than first NRI thresholds. NRI is a very useful method for programming the cochlear implants of young children who cannot demonstrate a reliable judgment of loudness. Objective. To investigate how HiResolution sound processing, designed to deliver high-rate stimuli, relates to EARTs and electrically evoked compound action potential measurements produced by low-rate stimuli. Material and methods. Nine profoundly hearing-impaired children and adults aged 6–29 years participated in the study. NRI responses were elicited using pulse trains consisting of biphasic pulses at a pulse width per phase of 32 µs delivered at a frequency of 30 Hz using SoundWave programming software. Stimuli were delivered to the odd electrodes (1, 3, 5, 7, 9, 11, 13 and 15) along the array. tNRI (NRI threshold) and first NRI thresholds were recorded for each stimulating electrode. “Speech bursts” stimuli used in EARTs recording were delivered to four electrodes at a time and stapedial reflexes were recorded from the impedance bridge. The M levels used were those used by each patient in their everyday HiResolution programs. Results. For 8 patients (53 stimulating electrodes) the correlation between tNRI threshold and M level was r = 0.675 (p = 0.000) and that between first NRI thresholds and M level was r=0.741 (p = 0.000). On average the M-level value was 20 CU (Current Unit) lower than the first NRI threshold value and 12 CU higher than the tNRI threshold value. The M-level patterns across the electrode array overall were similar to the tNRI or first NRI threshold patterns. For 7 patients (112 stimulating electrodes) the correlation between EART and M levels was r=0.710 (p = 0.000). On average the EART value was 14 CU higher than the M-level value.