Intra- and interindividual correlations between auditory evoked potentials and speech perception in cochlear implant users

Evaluation of auditory pathway excitability using a pre-operative trans-tympanic electrically evoked auditory brainstem response under local anesthesia in cochlear implant candidates

OBJECTIVE

Subjective promontory stimulation is used to evaluate cochlear implant (CI) candidacy, but the test reliability is low. Electrically evoked auditory brainstem response (EABR) can verify the function of the auditory system objectively. This study's procedure uses a trans-tympanic rounded bent-tip electrode to perform pre-operative EABR under local anaesthesia (LA-TT-EABR) using MED-EL Software and Hardware. This study aimed to determine usability and effectiveness for CI candidates.

DESIGN

We hypothesised that LA-TT-EABR waveforms of good quality would be related to successful hearing outcomes. We assumed that the duration of hearing loss/deafness was a confounding factor to study outcomes.

STUDY SAMPLE

19 borderline CI candidates.

RESULTS

Positive LA-TT-EABR results were confirmed in 14 patients. LA-TT-EABR's mean latency was 2.05 ± 0.31 ms (eII/eIII) and 4.24 ± 0.39 ms (eIV/eV). Latencies weren't statistically different from intra-operative EABR elicited by basal CI contacts. All positive LA-TT-EABR patients benefitted from CI and speech performance improved one year after implantation. One patient with negative LA-TT-EABR was cochlear-implanted and had no hearing sensation.

CONCLUSIONS

LA-TT-EABR is a tool in the frame of pre-operative objective testing the auditory pathway. It seems useful for clinical testing CI candidacy. Based on this study's outcomes, LA-TT-EABR should be recommended for uncertain CI candidates.

The Role of the P1 Latency in Auditory and Speech Performance Evaluation in Cochlear Implanted Children

Auditory deprivation affects normal age-related changes in the central auditory maturation. Cochlear implants (CIs) have already become the best treatment strategy for severe to profound hearing impairment. However, it is still hard to evaluate the speech-language outcomes of the pediatric CI recipients because of hearing-impaired children with limited speech-language abilities. The cortical auditory evoked potential (CAEP) provides a window into the development of the auditory cortical pathways. This preliminary study is aimed at assessing electrophysical characteristics of P1-N1 of electrically CAEP in children with CIs and at exploring whether these changes could be accounted for in auditory and speech outcomes of these patients. CAEP responses were recorded in 48 children with CIs in response to electrical stimulus to determine the presence of the P1-N1 response. Speech perception and speech intelligibility of the implanted children were further evaluated with the categories of auditory performance (CAP) test and speech intelligibility rating (SIR) test, respectively, to explore the relationship between the latency of P1-N1 and auditory and speech performance. This study found that P1 and N1 of the intracochlear CAEP were reliably evoked in children fitted with CIs and that the latency of the P1 as opposed to that of N1 was negative in relation to the wearing time of the cochlear implant. Moreover, the latency of the P1 produced significantly negative scores in both CAP and SIR tests, which indicates that P1 latency may be reflective of the auditory performance and speech intelligibility of pediatric CI recipients. These results suggest that the latency of P1 could be used for the objective assessment of auditory and speech function evaluation in cochlear-implanted children, which would be helpful in clinical decision-making regarding intervention for young hearing-impaired children.

Do the minimum and maximum comfortable stimulation levels influence the cortical potential latencies or the speech recognition in adult cochlear implant users?

INTRODUCTION

Cochlear implants (CI) programming is based on both the measurement of the minimum levels required to stimulate the auditory nerve and the maximum levels to generate loud, yet comfortable loudness. Seeking for guidance in the adequacy of this programming, the cortical auditory evoked potentials (CAEP) have been gaining space as an important tool in the evaluation of CI users, providing information on the central auditory system.

OBJECTIVE

To evaluate the influence of mishandling of electrical stimulation levels on speech processor programming on hearing thresholds, speech recognition and cortical auditory evoked potential in adult CI users.

MATERIAL AND METHODS

This is a prospective cross-sectional study, with a sample of adult unilateral CI users of both sexes, aged at least 18 years, post-lingual deafness, with minimum experience of 12 months of device use. Selected subjects should have average free field hearing thresholds with cochlear implant equal to or better than 34 dBHL and monosyllable recognition different from 0%. Individuals who could not collaborate with the procedures or who had no CAEP recordings were excluded. Participants were routinely programmed, and the map was named MO (optimized original map). Then three experimentally wrong maps were made: optimized original map with 10 current units below the maximum comfort level (C), named MC- (map minus C); optimized original map with minus 10 current units at minimum threshold level (T), named MT- (map minus T) and optimized original map with 10 current units above minimum level (T), named MT + (map plus T). In all programs, participants underwent free-field auditory thresholds from 250Hz to 6000Hz, recorded sentences and monosyllabic recognition tests presented at 65dB SPL in quiet and in noise, and free field CAEP evaluation. All tests were performed in an acoustically treated booth, in a randomized order of map presentation. Data were compared by Wilcoxon test.

RESULTS

Thirty individuals were selected and signed an informed consent form. The MC- map provided worsening of all free field thresholds, quiet and noise speech recognition, and P1 wave latency delay with significant difference from the results with the MO map. The MT- map worsened the hearing thresholds and statistically significantly reduced the P2 wave latency; MT+ map improved free field thresholds except 6000Hz, worsening speech recognition, without statistical significance.

CONCLUSIONS

The results suggest that maximum levels below the optimal thresholds lead to worse cochlear implant performance in both hearing thresholds and speech recognition tests in quiet and noise, increasing CAEP component P1 latency. On the other hand, the manipulation of minimum threshold levels showed alteration in audibility without significant impact on speech recognition.

Speech-ABRs in cochlear implant recipients: feasibility study

Objective: The aim of this study was to assess the feasibility of recording speech-ABRs from cochlear implant (CI) recipients, and to remove the artefact using a clinically applicable single-channel approach. Design: Speech-ABRs were recorded to a 40 ms [da] presented via loudspeaker using a two-channel electrode montage. Additionally, artefacts were recorded using an artificial-head incorporating a MED-EL CI with stimulation parameters as similar as possible to those of three MED-EL participants. A single-channel artefact removal technique was applied to all responses. Study sample: A total of 12 adult CI recipients (6 Cochlear Nucleus and 6 MED-EL CIs). Results: Responses differed according to the CI type, artefact removal resulted in responses containing speech-ARB characteristics in two MED-EL CI participants; however, it was not possible to verify whether these were true responses or were modulated by artefacts, and artefact removal was successful from the artificial-head recordings. Conclusions: This is the first study that attempted to record speech-ABRs from CI recipients. Results suggest that there is a potential for application of a single-channel approach to artefact removal. However, a more robust and adaptive approach to artefact removal that includes a method to verify true responses is needed.

Change in Speech Perception and Auditory Evoked Potentials over Time after Unilateral Cochlear Implantation in Postlingually Deaf Adults

Speech perception varies widely across cochlear implant (CI) users and typically improves over time after implantation. There is also some evidence for improved auditory evoked potentials (shorter latencies, larger amplitudes) after implantation but few longitudinal studies have examined the relationship between behavioral and evoked potential measures after implantation in postlingually deaf adults. The relationship between speech perception and auditory evoked potentials was investigated in newly implanted cochlear implant users from the day of implant activation to 9 months postimplantation, on five occasions, in 10 adults age 27 to 57 years who had been bilaterally profoundly deaf for 1 to 30 years prior to receiving a unilateral CI24 cochlear implant. Changes over time in middle latency response (MLR), mismatch negativity, and obligatory cortical auditory evoked potentials and word and sentence speech perception scores were examined. Speech perception improved significantly over the 9-month period. MLRs varied and showed no consistent change over time. Three participants aged in their 50s had absent MLRs. The pattern of change in N1 amplitudes over the five visits varied across participants. P2 area increased significantly for 1,000- and 4,000-Hz tones but not for 250 Hz. The greatest change in P2 area occurred after 6 months of implant experience. Although there was a trend for mismatch negativity peak latency to reduce and width to increase after 3 months of implant experience, there was considerable variability and these changes were not significant. Only 60% of participants had a detectable mismatch initially; this increased to 100% at 9 months. The continued change in P2 area over the period evaluated, with a trend for greater change for right hemisphere recordings, is consistent with the pattern of incremental change in speech perception scores over time. MLR, N1, and mismatch negativity changes were inconsistent and hence P2 may be a more robust measure of auditory plasticity in adult implant recipients. P2 was still improving at 9 months postimplantation. Future studies should explore longitudinal changes over a longer period.

Cortical Auditory Evoked Potentials to Evaluate Cochlear Implant Candidacy in an Ear With Long-standing Hearing Loss: A Case Report

OBJECTIVES

We describe a novel use of cortical auditory evoked potentials in the preoperative workup to determine ear candidacy for cochlear implantation.

METHODS

A 71-year-old male was evaluated who had a long-deafened right ear, had never worn a hearing aid in that ear, and relied heavily on use of a left-sided hearing aid. Electroencephalographic testing was performed using free field auditory stimulation of each ear independently with pure tones at 1000 and 2000 Hz at approximately 10 dB above pure-tone thresholds for each frequency and for each ear.

RESULTS

Mature cortical potentials were identified through auditory stimulation of the long-deafened ear. The patient underwent successful implantation of that ear. He experienced progressively improving aided pure-tone thresholds and binaural speech recognition benefit (AzBio score of 74%).

CONCLUSIONS

Findings suggest that use of cortical auditory evoked potentials may serve a preoperative role in ear selection prior to cochlear implantation.

Auditory cortical activity to different voice onset times in cochlear implant users

OBJECTIVE

Voice onset time (VOT) is a critical temporal cue for perception of speech in cochlear implant (CI) users. We assessed the cortical auditory evoked potentials (CAEPs) to consonant vowels (CVs) with varying VOTs and related these potentials to various speech perception measures.

METHODS

CAEPs were recorded from 64 scalp electrodes during passive listening in CI and normal-hearing (NH) groups. Speech stimuli were synthesized CVs from a 6-step VOT /ba/-/pa/ continuum ranging from 0 to 50 ms VOT in 10-ms steps. Behavioral measures included the 50% boundary point for categorical perception ("ba" to "pa") from an active condition task.

RESULTS

Behavioral measures: CI users with poor speech perception performance had prolonged 50% VOT boundary points compared to NH subjects. The 50% boundary point was also significantly correlated to the ability to discriminate consonants in quiet and noise masking. Electrophysiology: The most striking difference between the NH and CI subjects was that the P2 response was significantly reduced in amplitude in the CI group compared to NH. N1 amplitude did not differ between NH and CI groups. P2 latency increased with increases in VOT for both NH and CI groups. P2 was delayed more in CI users with poor speech perception compared to NH subjects. N1 amplitude was significantly related to consonant perception in noise while P2 latency was significantly related to vowel perception in noise. When dipole source modelling in auditory cortex was used to characterize N1/P2, more significant relationships were observed with speech perception measures compared to the same N1/P2 activity when measured at the scalp. N1 dipole amplitude measures were significantly correlated with consonants in noise discrimination. Like N1, the P2 dipole amplitude was correlated with consonant discrimination, but additional significant relationships were observed such as sentence and word identification.

CONCLUSIONS

P2 responses to a VOT continuum stimulus were different between NH subjects and CI users. P2 responses show more significant relationships with speech perception than N1 responses.

SIGNIFICANCE

The current findings indicate that N1/P2 measures during a passive listening task relate to speech perception outcomes after cochlear implantation.

Delayed Auditory Brainstem Responses in Prelingually Deaf and Late-Implanted Cochlear Implant Users

Neurophysiological studies in animals and humans suggest that severe hearing loss during early development impairs the maturation of the auditory brainstem. To date, studies in humans have mainly focused on the neural activation of the auditory brainstem in children treated with a cochlear implant (CI), but little is known about the pattern of activation in adult CI users with early onset of deafness (prelingual, before the age of 2 years). In this study, we compare auditory brainstem activation in prelingually deaf and late-implanted adult CI users to that in postlingually deaf CI users. Electrically evoked auditory brainstem responses (eABRs) were recorded by monopolar stimulation, separately using a middle and an apical electrode of the CI. Comparison of the eABR latencies revealed that wave V was significantly delayed in the prelingually deaf CI users on both electrode locations. Accordingly, when the apical electrode was stimulated, the III–V interwave interval was significantly longer in the prelingually deaf group. These findings suggest a slower neural conduction in the auditory brainstem, probably caused by impairment of maturation during the long duration of severe hearing loss in infancy. Shorter wave V latencies, reflecting a more mature brainstem, appeared to be a predictor for better speech perception.

Peripheral and Central Contributions to Cortical Responses in Cochlear Implant Users

OBJECTIVES

The primary goal of this study was to describe relationships between peripheral and central electrophysiologic measures of auditory processing within individual cochlear implant (CI) users. The distinctiveness of neural excitation patterns resulting from the stimulation of different electrodes, referred to as 'spatial selectivity,' was evaluated. The hypothesis was that if central representations of spatial interactions differed across participants semi-independently of peripheral input, then the within-subject relationships between peripheral and central electrophysiologic measures of spatial selectivity would reflect those differences. Cross-subject differences attributable to processing central to the auditory nerve may help explain why peripheral electrophysiologic measures of spatial selectivity have not been found to correlate with speech perception.

DESIGN

Eleven adults participated in this and a companion study. All were peri- or post-lingually deafened with more than 1 year of CI experience. Peripheral spatial selectivity was evaluated at 13 cochlear locations using 13 electrodes as probes to elicit electrically evoked compound action potentials (ECAPs). Masker electrodes were varied across the array for each probe electrode to derive channel-interaction functions. The same 13 electrodes were used to evaluate spatial selectivity represented at a cortical level. Electrode pairs were stimulated sequentially to elicit the auditory change complex (ACC), an obligatory cortical potential suggestive of discrimination. For each participant, the relationship between ECAP channel-interaction functions (quantified as channel-separation indices) and ACC N1-P2 amplitudes was modeled using the saturating exponential function y = a * (1-e). Both a and b coefficients were varied using a least-squares approach to optimize the fits.

RESULTS

Electrophysiologic measures of spatial selectivity assessed at peripheral (ECAP) and central (ACC) levels varied across participants. The results indicate that differences in ACC amplitudes observed across participants for the same stimulus conditions were not solely the result of differences in peripheral excitation patterns. This finding supports the view that processing at multiple points along the auditory neural pathway from the periphery to the cortex may vary across individuals with different etiologies and auditory experiences.

CONCLUSIONS

The distinctiveness of neural excitation resulting from electrical stimulation varies across CI recipients, and this variability was observed in both peripheral and cortical electrophysiologic measures. The ACC amplitude differences observed across participants were partially independent from differences in peripheral neural spatial selectivity. These findings are clinically relevant because they imply that there may be limits (1) to the predictive ability of peripheral measures and (2) in the extent to which improving the selectivity of electrical stimulation via programming options (e.g., current focusing/steering) will result in more specific central neural excitation patterns or will improve speech perception.

The Relationship Between EABR and Auditory Performance and Speech Intelligibility Outcomes in Pediatric Cochlear Implant Recipients

PURPOSE

The primary purpose of the present study was to investigate the relationship between postimplant electrically evoked auditory brainstem response (EABR) measures (wave V threshold, wave V latency, input-output functions) and auditory performance and speech intelligibility outcomes measured using parental rating scales, such as Categories of Auditory Performance (CAP; Archbold, Lutman, & Marshall, 1995) and the Speech Intelligibility Rating scale (SIR; Allen, Nikolopoulos, Dyar, & O'Donoghue, 2001), respectively. The secondary purpose was to evaluate the relationship between age at implantation and ratings on the CAP and SIR.

METHOD

Forty children with congenital sensorineural hearing loss participated. Preimplant parental ratings on the CAP and SIR were obtained, and all the children underwent cochlear implantation. Intracochlear EABRs were recorded postimplantation. Postimplant parental ratings on the CAP and SIR were obtained. The relationships between EABR parameters and auditory performance and speech intelligibility outcomes were studied. The effect of age at implantation on auditory performance and speech intelligibility outcomes was also investigated.

RESULTS

A significant negative correlation was found between EABR wave V thresholds and SIR growth (r = -.415, p = .016). Children with better CAP growths tended to have lower wave V thresholds than those with poorer CAP growths. Age at implantation had an effect on the auditory performance as measured using the CAP.

CONCLUSIONS

The present study provides evidence for the relationship between wave V thresholds of the intracochlear EABR and auditory performance and speech intelligibility outcomes measured using parental rating scales in pediatric cochlear implant recipients. Data also indicate that early intervention has a positive impact on auditory performance outcomes.

Towards a closed-loop cochlear implant system: application of embedded monitoring of peripheral and central neural activity

Although the cochlear implant (CI) is widely considered the most successful neural prosthesis, it is essentially an open-loop system that requires extensive initial fitting and frequent tuning to maintain a high, but not necessarily optimal, level of performance. Two developments in neuroscience and neuroengineering now make it feasible to design a closed-loop CI. One development is the recording and interpretation of evoked potentials (EPs) from the peripheral to the central nervous system. The other is the embedded hardware and software of a modern CI that allows recording of EPs. We review EPs that are pertinent to behavioral functions from simple signal detection and loudness growth to speech discrimination and recognition. We also describe signal processing algorithms used for electric artifact reduction and cancellation, critical to the recording of electric EPs. We then present a conceptual design for a closed-loop CI that utilizes in an innovative way the embedded implant receiver and stimulators to record short latency compound action potentials ( ~1 ms), auditory brainstem responses (1-10 ms) and mid-to-late cortical potentials (20-300 ms). We compare EPs recorded using the CI to EPs obtained using standard scalp electrodes recording techniques. Future applications and capabilities are discussed in terms of the development of a new generation of closed-loop CIs and other neural prostheses.

The effect of changes in stimulus level on electrically evoked cortical auditory potentials

OBJECTIVES

The purpose of this study was to determine whether the electrically evoked acoustic change complex (EACC) could be used to assess sensitivity to changes in stimulus level in cochlear implant (CI) recipients and to investigate the relationship between EACC amplitude and rate of growth of the N1-P2 onset response with increases in stimulus level.

DESIGN

Twelve postlingually deafened adults using Nucleus CI24 CIs participated in this study. Nucleus Implant Communicator (NIC) routines were used to bypass the speech processor and to control the stimulation of the implant directly. The stimulus consisted of an 800 msec burst of a 1000 pps biphasic pulse train. A change in the stimulus level was introduced 400 msec after stimulus onset. Band-pass filtering (1 to 100 Hz) was used to minimize stimulus artifact. Four to six recordings of 50 sweeps were obtained for each condition, and averaged responses were analyzed in the time domain using standard peak picking procedures.

RESULTS

Cortical auditory change potentials were recorded from CI users in response to both increases and decreases in stimulation level. The amplitude of the EACC was found to be dependent on the magnitude of the stimulus change. Increases in stimulus level elicited more robust EACC responses than decreases in stimulus level. Also, EACC amplitudes were significantly correlated with the slope of the growth of the onset response.

CONCLUSIONS

This work describes the effect of change in stimulus level on electrically evoked auditory change potentials in CI users. The amplitude of the EACC was found to be related both to the magnitude of the stimulus change introduced and to the rate of growth of the N1-P2 onset response. To the extent that the EACC reflects processing of stimulus change, it could potentially be a valuable tool for assessing neural processing of the kinds of stimulation patterns produced by a CI. Further studies are needed, however, to determine the relationships between the EACC and psychophysical measures of intensity discrimination in CI recipients.

The electrically evoked auditory change complex: preliminary results from nucleus cochlear implant users

OBJECTIVES

The purpose of this study was to determine if changes in the position of the stimulating electrode in the cochlea could be used to elicit the electrically evoked auditory change complex (EACC) from Nucleus cochlear implant users.

DESIGN

Nine postlingually deafened adults participated in this study. Each study participant had been using his or her Nucleus CI24 cochlear implant for at least 3 mos before testing. The speech processor was bypassed and the output of the implanted receiver/stimulator was controlled directly. The stimulus was a 600 msec burst of a biphasic pulse train (1000 pps). In control conditions, the stimulating electrode was held constant and stimulation continued throughout the 600 msec recording interval. In experimental conditions, the EACC was elicited by introducing a change in the stimulating electrode 300 msec after the onset of the pulse train. The EACC was recorded using surface electrodes. Three recordings of 100 sweeps each were obtained for each stimulus condition. Bandpass filtering (1-100 Hz) was used to minimize contamination of the recordings by stimulus artifact. Averaged responses were then smoothed using a 40-msec wide boxcar filter and standard peak picking procedures were used to analyze these responses in the time domain.

RESULTS

In each case, a clear onset response (P1-N1-P2) was recorded. In the experimental conditions, a second evoked potential, the EACC, was also recorded after the change in stimulating electrode. This second response had general morphological characteristics that were very similar to those of the onset response. Increasing the separation between the two stimulating electrodes in the experimental conditions resulted in a general trend toward increased EACC amplitudes.

CONCLUSIONS

This report describes results of a set of experiments in which the speech processor of the cochlear implant was bypassed and the EACC was recorded in response to a change in stimulating electrode position. EACC amplitude was shown to increase as the separation between the two stimulating electrodes increased. Although preliminary in nature, these results demonstrate the feasibility of recording the EACC in response to changes in stimulating electrode position from individual cochlear implant users.

Effects of auditory pathway anatomy and deafness characteristics? Part 2: On electrically evoked late auditory responses

The purpose of this study was to distinguish the effects of different parameters on latencies of wave N1, wave P2, and inter-peak interval N1-P2 of electrical late auditory responses (ELARs). ELARs were recorded from four intra-cochlear electrodes in fourteen adult HiRes90K cochlear implant users who had at least three months of experience. The relationship between latencies and stimulation sites in the cochlea was characterized to assess the influence of the auditory pathway anatomy on ELARs, i.e., whether the speed of neural propagation varies according to the place that is activated in the cochlea. Audiograms before implantation, duration of deafness, and psychophysics at first fitting were used to describe the influence of deafness characteristics on latencies. The stimulation sites were found to have no effect on ELAR latency and, while there was no influence of psychophysics on latency, a strong relationship was shown with duration of deafness and the pre-implantation audiogram. Thus, ELAR latency was longer for poorer audiograms and longer durations of deafness and this relationship appeared to be independent of stimulation parameters such as stimulation site. Comparison between these findings and those from the equivalent study on EABR waves IIIe and Ve latency [Guiraud, J., Gallego, S., Arnold, L., Boyle, P., Truy, E., Collet, L., 2007. Effects of auditory pathway anatomy and deafness characteristics? (1): On electrically evoked auditory brainstem responses. Hear. Res. 223 (1-2), 48-60] shows that, while ELAR and EABR latencies are related with parameters that reflect the integrity of the auditory pathway, ELAR latency is less dependent on stimulation parameters than EABR latency.

Electrophysiological and speech perception measures of auditory processing in experienced adult cochlear implant users

OBJECTIVE

This study determined the relationship between auditory evoked potential measures and speech perception in experienced adult cochlear implant (CI) users and compared the CI evoked potential results to those of a group of age- and sex-matched control subjects.

METHODS

CI subjects all used the Nucleus CI-22 implant. Middle latency response (MLR), obligatory cortical potentials (CAEP), mismatch negativity (MMN) and P3a auditory evoked potentials were recorded. Speech perception was evaluated using word and sentence tests.

RESULTS

Duration of deafness correlated with speech scores with poor scores reflecting greater years of deafness. Na amplitude correlated negatively with duration of deafness, with small amplitudes reflecting greater duration of deafness. Overall, N1 amplitude was smaller in CI than control subjects. Earlier P2 latencies were associated with shorter durations of deafness and higher speech scores. In general, MMN was absent or degraded in CI subjects with poor speech scores.

CONCLUSIONS

Auditory evoked potentials are related to speech perception ability and provide objective evidence of central auditory processing differences across experienced CI users.

SIGNIFICANCE

Since auditory evoked potentials relate to CI performance, they may be a useful tool for objectively evaluating the efficacy of speech processing strategies and/or auditory training approaches in both adults and children with cochlear implants.

Activity-dependent developmental plasticity of the auditory brain stem in children who use cochlear implants

OBJECTIVES

1) To determine if a period of early auditory deprivation influences neural activity patterns as revealed by human auditory brain stem potentials evoked by electrical stimulation from a cochlear implant. 2) To examine the potential for plasticity in the human auditory brain stem. Specifically, we asked if electrically evoked auditory potentials from the auditory nerve and brain stem in children show evidence of development as a result of implant use. 3) To assess whether a sensitive or critical period exists in auditory brain stem development. Specifically, is there an age of implantation after which there are no longer developmental changes in auditory brain stem activity as revealed by electrically evoked potentials?

DESIGN

The electrically evoked compound potential of the auditory nerve (ECAP) and the electrically evoked auditory brain stem response (EABR) were recorded repeatedly during the first year of implant use in each of 50 children. The children all had pre- or peri-lingual onset of severe to profound sensorineural hearing loss and received their implants at ages ranging from 12 mo to 17 yr. All children received Nucleus cochlear implant devices. All children were in therapy and in school programs that emphasized listening and required the children to wear their implants consistently.

RESULTS

Initial stimulation from the cochlear implant evoked clear responses from the auditory nerve and auditory brain stem in most children. There was no correlation between minimum latency, maximum amplitude, or slope of amplitude growth of initial responses with age at implantation for ECAP eN1, EABR eIII and eV components (p > 0.05). During the first year of implant use, minimum latency of these waves significantly decreased (p < 0.01, p < 0.0001, p < 0.0001, respectively). Neural conduction time, measured using the interwave latency of ECAP eN1-EABR eIII for lower brain stem and EABR eIII-eV for upper brain stem, decreased during the period of 6 to 12 mo of cochlear implant use (p < 0.01 (lower), p < 0.0001(upper)). The ECAP wave eN1 and the EABR wave eV showed significant increases in amplitude during time of implant use (p < 0.05 and p < 0.01, respectively). There were no correlations between the rate of interwave latency decrease and the rate of amplitude increases and the age at which children underwent implantation (p < 0.05).

CONCLUSIONS

Activity in the auditory pathways to the level of the midbrain can be evoked by acute stimulation from a cochlear implant. EABR measures are not influenced by any period of auditory deprivation. Auditory development proceeds once the implant is activated and involves improvements in neural conduction velocity and neural synchrony. Underlying mechanisms likely include improvements in synaptic efficacy and possibly increased myelination. The developmental plasticity that we have shown in the human auditory brain stem does not appear from EABR data to be limited by a critical period during childhood.

Clinical uses of electrically evoked auditory nerve and brainstem responses

PURPOSE OF REVIEW

The purpose of this review is to summarize current thinking relative to clinical applications for the electrically evoked compound action potential and the electrically evoked auditory brainstem response with the focus on works published between 1998 and 2003.

RECENT FINDINGS

During the period of this review, a considerable body of research has been published describing how the electrically evoked compound action potential can be measured and how the electrically evoked compound action potential may be used in the clinical treatment of cochlear implant patients. During this same period, there has been a decline in the number of studies reporting potential clinical applications for the electrically evoked auditory brainstem response. Perhaps the strongest clinical application for the electrically evoked compound action potential today is as a tool to facilitate the process of fitting the speech processor of the cochlear implant. This is particularly important for pediatric patients who may not be able to be programmed using traditional behavioral techniques. The accuracy of these predictions is discussed in addition to the limitations of the studies reviewed.

SUMMARY

The introduction of cochlear implants with the capability of measuring the response of the auditory nerve to electrical stimulation from an intracochlear electrode has tremendous potential to impact clinical practice. Research into how best to use this information is ongoing.

[Registering event-related auditory potentials (P300) in patients with cochlear implants]

Event-related auditory evoked potentials can contribute to the evaluation of discrimination abilities of cochlear implant users. Auditory P300 potentials to a frequency contrast were obtained in six post-lingually deaf adults using a cochlear implant and in a control group of normal hearing subjects. The aim of this study was to investigate how these potentials were determined by the stimulation pattern of the cochlear implant. To visualise these stimulation patterns colour-coded plots (stimulograms) were calculated based on the stimulus and the fitting file of the individual subject as inputs. These stimulograms were used to evaluate the influence of various stimulation parameters on the stimulation contrast used in an oddball paradigm. The influence of discrimination difficulty on the P300 response is demonstrated.

Mismatch negativity: a tool for the assessment of stimuli discrimination in cochlear implant subjects

OBJECTIVES

The performance of cochlear implants varies among users. This variability may be due to the ability to process auditory information. The mismatch negativity should provide an index of discrimination in cochlear implantees (Kraus N, McGee T, Carrell T, Sharma A. Neurophysiologic bases of speech discrimination. Ear Hear. 1995;16:19-37). Our aim was to analyze MMN in cochlear implant (Digisonic) subjects to assess electrode discrimination and to study the relationship between MMN and speech performance.

METHODS

The mismatch was determined by stimulating three pairs of different electrodes. Two sessions were performed with both standard and deviant stimuli reversed. Speech recognition abilities were evaluated using 4 speech tests. The statistics included the results of 6 subjects. They indicated that MMN may be obtained when stimulating two different electrodes. A difference occurred between standard and deviant stimuli within a session but also when the response to the deviant stimulus was compared to the response of the same stimulus in a standard condition, validating the discrimination process. MMN latency was about 140 ms, and amplitude about -2.8 microV. No differences were shown with respect to electrode spacing. No relationship between MMN and speech performance was found. A clinical application of this method might be to assess the auditory processing of electrical stimuli in congenitally deaf subjects at the pre-implantation stage.

Can event-related potentials be evoked by extra-cochlear stimulation and used for selection purposes in cochlear implantation?

To investigate whether electrically evoked event-related responses (P300) could be elicited by extra-cochlear stimulation, measurements were performed on a group of adults fitted with the single-channel extra-cochlear implant. To optimize measurement conditions, and because of the low number of subjects still using an extra-cochlear device in our cochlear implant programme, measurements were also performed on a group of experienced users fitted with the intra-cochlear Nucleus multichannel device. For reference purposes, subjects with normal hearing (control group) were also included in the study. Reproducible late latency responses (N1 and P2 peaks) were found in the five extra-cochlear implant users, while P300s were present in four out of these five subjects. The latencies were longer than those of the control group, but were similar to those obtained in the intra-cochlear implant group. Significant correlations were found for most N1, P2 and P300 measurements evoked by the tonal stimuli and by speech stimuli. The P300 amplitudes, evoked by either tonal or speech stimuli, appeared to be related to speech perception ability. This led to the conclusion that N1, P2 and P300 measurements may have potential as a clinical tool for preoperative prediction and postoperative evaluation of sound processing on a cortical level.