Advantage of bimodal fitting in prosody perception for children using a cochlear implant and a hearing aid.

Cochlear implants are largely unable to encode voice pitch information, which hampers the perception of some prosodic cues, such as intonation. This study investigated whether children with a cochlear implant in one ear were better able to detect differences in intonation when a hearing aid was added in the other ear ("bimodal fitting"). Fourteen children with normal hearing and 19 children with bimodal fitting participated in two experiments. The first experiment assessed the just noticeable difference in F0, by presenting listeners with a naturally produced bisyllabic utterance with an artificially manipulated pitch accent. The second experiment assessed the ability to distinguish between questions and affirmations in Dutch words, again by using artificial manipulation of F0. For the implanted group, performance significantly improved in each experiment when the hearing aid was added. However, even with a hearing aid, the implanted group required exaggerated F0 excursions to perceive a pitch accent and to identify a question. These exaggerated excursions are close to the maximum excursions typically used by Dutch speakers. Nevertheless, the results of this study showed that compared to the implant only condition, bimodal fitting improved the perception of intonation.

Sound localization and binaural hearing in children with a hearing aid and a cochlear implant.

The aims of the study were to investigate whether sound localization acuity improved when children with 1 cochlear implant use a hearing aid in the contralateral ear (bimodal fitting), and whether this enabled them to benefit from a binaural masking level difference. Four different noise bursts were used as stimuli for a minimal audible angle localization test. On average, localization acuity remained poor with the cochlear implant alone, but also with bimodal fitting. A significant benefit of bimodal fitting was only shown when the most complicated stimulus with roved amplitude and spectrum was presented (minimal audible angle of 151 degrees with bimodal fitting vs. 175 degrees with cochlear implant alone). No significant binaural masking level difference was found between the cochlear implant alone and the bimodal condition.

Comparing signal to noise ratios of amplitude modulation following responses from four EEG derivations in awake normally hearing adults.

Amplitude modulation following responses (AMFR) or steady-state evoked potentials (SSEPs) can be used for the objective and frequency-specific estimation of hearing thresholds in awake and sleeping subjects. To be useful as a clinical tool, a high signal to noise ratio (SNR) is required for a minimal testing time. Four EEG derivations were compared in six relaxed, cooperative awake normally hearing adults to compare the SNRs. Stimuli comprised a 1 kHz carrier wave, amplitude modulated at 39 Hz and 90 Hz. They were presented for about 4.4 min in one ear at a level slightly above the behavioural threshold (10 dB SL). In the 39 Hz condition, the SNRs from the different derivations did not differ significantly. However, in the 90 Hz condition one novel derivation (Cz-inion with ground at Pz) yielded a significantly lower noise level, about 60 per cent of the noise level of the other derivations. The SNR of this derivation was about 50 per cent larger than from other derivations. Such an increase of the SNR implies that testing time is reduced by about 56 per cent as compared to conventional derivations in these subjects.

Modelling surface potentials from intracochlear electrical stimulation.

Volume conduction models were used qualitatively to model surface potentials from cochlear implant patients recorded earlier by the authors. These recorded potentials reflected the equivalent dipole orientation in the head in patients who are deaf due to otosclerosis, but increased uniformly with the distance between the stimulating electrodes along the basilar membrane in other patients, which suggested a low and high resistivity of the cochlear bone, respectively. Several models of the head were constructed, with compartments representing the skin, skull, brain, cochlea, internal and external ear canal. In the "petrous bone" model, the cochlea was modelled as a cavity in a bony layer surrounded by the brain compartment. Of all models, the petrous bone model using a high resistivity ratio (1:100) between the bony and the other compartments was the only one that produced outcomes similar to the potentials observed in non-otosclerosis patients. In conclusion, the results suggested that the surface potentials observed in non-otosclerosis patients are sufficiently explained by a high impedance between cochlear turns and a non-specific return of current via the wall of the petrous bone into the larger brain compartment.

Predictors of cochlear implant performance.

Open set speech understanding with cochlear implants, without speechreading, is nowadays a common finding. However, there is a large variation in speech understanding between cochlear implant users. We tried to find pre-operative parameters which predicted the post-operative results. Thirty-seven adult post-lingually deafened Nucleus cochlear implant users with a mean age of 46 years (range 16 68) and a mean duration of deafness of 15 years (range 1.5-47) were studied. Pre-operatively, we performed pure-tone audiometry, round window and ear canal electrical stimulation, psychological tests and imaging. Additionally, we measured pre-operatively speech understanding in the auditory, the visual and the audiovisual conditions with several tests which were also administered after 6 and 12 months' implant experience. Correlation analysis between the pre-operative variables and the post-operative factors showed that duration of deafness and residual hearing are the most important predictors. The temporal difference limen in pre-operative round window electrical stimulation is a secondary predictor.

Electrical impedance of the cochlear implant lubricants hyaluronic acid, oxycellulose, and glycerin.

Hyaluronic acid (Healon), oxycellulose (hydroxypropyl methylcellulose), and glycerin are lubricants used in cochlear implant surgery for atraumatic deep insertion of the electrode array into the scala tympani. The electrical impedances of these three lubricants were measured to assess possible effects on intraoperative evoked response measurements, such as the electrically evoked stapedius reflex and auditory brain stem response. The impedances of hyaluronic acid, oxycellulose, and saline were very similar and independent of frequency (20 Hz to 1 MHz). Glycerin had an excessively high impedance at low frequencies. A film of hyaluronic acid or oxycellulose around the electrode array immersed in saline did not have any measurable effect on the impedance; a film of glycerin resulted in a strongly reactive polarized layer. However, neither the far-field current spread nor the impedance between stimulated electrodes was affected by any of the lubricants applied as a thin film. This suggests that none of these lubricants affect intraoperative responses, when applied as a thin film.

Stapedius reflex measurements during surgery for cochlear implantation in children.

Electrically evoked stapedius reflex measurements were obtained in 19 children during surgery for cochlear implantation. They all received the Nucleus device. Stapedius reflexes could be elicited in all the children with congenital deafness but not in all the children with an etiology of meningitis. The intraoperative stapedius reflex thresholds were compared with postoperative values obtained after fitting of the speech processor and with the children's long-term behavioral most comfortable levels (C-levels). The intraoperative reflex thresholds were considerably higher than the postoperative reflex thresholds (44 "stimulus level steps" on the average), which could in part be ascribed to the influence of anesthetics used during surgery. It was concluded that, especially in children with an etiology of meningitis, the intraoperative stapedius reflex threshold (even after corrections for the concentration of the volatile anesthetics used) was a weak predictor of the C-level.

Averaged electrode voltages: management of electrode failures in children, fluctuating threshold and comfort levels, and otosclerosis.

Implant-generated surface potentials, or averaged electrode voltages (AEVs), were collected by means of the electrode-by-electrode (E-E) mapping variable mode strategy. Three topics were investigated. 1) Eighteen children under the age of 7 were tested and the E-E map of 4 of them was found deviant; all 4 children were deaf owing to meningitis. Some electrodes marked as failing by E-E mapping did not cause problems during device fitting, and electrodes not usable in device fitting showed normal AEVs in 1 child. Overall, the AEVs agreed well with abnormalities in the behavioral threshold (T) and comfort (C) levels. The E-E maps provided useful clues for the audiologist in most cases. 2) Repeated E-E mapping in 2 children who displayed large fluctuations over time of T levels suggested a fluctuation in (neural) responsiveness in 1 child and new bone formation in the other. 3) Although massive phase reversals of AEVs in 2 patients deafened by otosclerosis seemed to indicate a very permeable cochlear bone, stimulation in the pseudomonopolar mode across the basal turn did not affect T levels, and affected pitch perception in only 1 patient. Deviant AEVs from abnormal cochleas should, therefore, not be interpreted too easily as an indication of an electrode failure, faulty electrode placement, or inadequate tonotopy.

Performance of prelingually and postlingually deaf patients using single-channel or multichannel cochlear implants.

The auditory and aided lipreading performance of 8 prelingually and 11 postlingually deaf patients who had received a single-channel or multichannel cochlear implant was evaluated during 2 years of follow-up. Although all the patients improved on both closed-set pattern recognition and speech discrimination tests and on a Continuous Discourse Tracking task, the most significant improvement was observed in the postlingually deaf patients who were using a multichannel implant. These patients were the only ones to achieve open-set speech recognition in the auditory-only condition. Only small differences were found between prelingually deaf patients who were using a single-channel system and those who were using a multichannel system. The users' evaluations, obtained by means of a questionnaire, were generally positive in all patients. Based on the study results, the authors concluded that it is feasible to use cochlear implants in highly motivated prelingually deaf patients who have learned to use oral-aural communication.

The risk of vestibular function loss after intracochlear implantation.

Sixty patients were selected for cochlear implantation and 50 of them received an intracochlear implant (Nucleus). Vestibular function was evaluated before and after surgery using a caloric test and a velocity step test. Sixteen patients had normal or residual vestibular function before surgery, 11 bilateral and 5 unilateral; in 3 of the latter patients, the ear with vestibular areflexia was elected for implantation, which reduced the number of patients at risk for vestibular dysfunction to 13. Vestibular function was preserved in all of these patients except for 4; the risk of vestibular function loss can therefore be rated at about 31%.

Does intracochlear implantation jeopardize vestibular function?

We present the results of the vestibular function tests of 35 patients who were selected for cochlear implantation. Vestibular function was evaluated with a caloric test and a velocity step test. The preimplant data were compared to those in previously reported series. Intracochlear implantation was performed in 25 patients. The vestibular complications encountered in this group are presented and discussed. Six patients had normal or residual (but substantial) vestibular function in the ear eligible for implantation. Vestibular function was preserved in 3 patients and was lost in 3 patients, in 1 case through an iatrogenic cause. We estimate the risk of losing vestibular function as a result of intracochlear implantation as between 50% and 60% on the basis of the present and previously reported data.

Identifying electrode failures with cochlear implant generated surface potentials.

A procedure for measuring surface potentials called electrode-by-electrode (E-E) mapping is described that can detect the nonintermittent malfunctioning of the implanted receiver and electrode array of multichannel cochlear implants, such as the Cochlear (Nucleus) device. E-E mapping is based on the sequential stimulation of all paired combinations of electrodes. The recorded waveforms were averaged and all peak-to-peak amplitudes were combined into one graph for a comprehensive check on open circuit or short-circuited electrodes. Normative data from 21 patients are given. E-E mapping detected electrode failure in three patients whose behavioral thresholds were in agreement with hardware problems, including one case of overstimulation at high stimulus levels. In one patient who was suffering from overstimulation without any deviant thresholds, no failure could be detected. The procedure takes about a quarter of an hour and the stimulus amplitude needed is below threshold for most patients, which makes it especially useful in children. An informative partial E-E map can be made during implantation and requires virtually no extra theater time.

Cochlear implant generated surface potentials: current spread and side effects.

Surface potentials were recorded in 16 users of the Cochlear (Nucleus) Mini System 22 in order to investigate the current flow resulting from the sequential stimulation of all paired combinations of 22 electrodes. In almost all patients the amplitude of the surface potentials increased with the distance between the stimulating electrodes along the scala tympani and decreased with the distance between the basal electrode and the round window. However, in two patients whose cause of deafness was otosclerosis, the largest surface potentials were seen when the stimulation was applied to electrodes which were approximately half a cochlear turn apart. These highly regular patterns suggested two different pathways for the currents that generated surface potentials: (1) through the fluid along the scala and not through the dense cochlear bone, leaving the cochlea only at the basally located openings; (2) through the very permeable cochlear bone in the case of otosclerosis. Stimulation of any electrodes that caused facial twitching and/or unpleasant sensations in the head (four patients) did not give rise to abnormal surface potential amplitudes. Two patients who suffered from frequent threshold and comfort level changes were tested repeatedly. As the recorded amplitudes did not change significantly over time, neurophysiological changes were a more likely cause than fluctuations of the stimulator output.

Results from four cochlear implant patients with Usher's syndrome.

Individual results are presented of 4 patients with Usher's syndrome type 1 who received a cochlear implant. Both single-channel and multichannel implants were used. Because of implant failure, one of the single-channel systems was replaced by a Nucleus multichannel system. Results are compared to the results of 5 other prelingually deaf cochlear implant users. The performance of the patients with Usher's syndrome on suprasegmental and segmental speech perception tests and on a connected discourse tracking task did not differ significantly from the performance of the other prelingually deaf patients. A significant improvement over time was found at the suprasegmental level for the combined group of Usher's and other patients. No obvious differences were found between the scores from the patients with a single-channel and the patients with a multichannel device. The rehabilitation of the Usher's patients required very little extra effort in comparison with that of the other prelingually deaf patients; all patients reported considerable advantages in hearing abilities and social life.

Stapedius reflex and EABR thresholds in experienced users of the Nucleus cochlear implant.

Evoked auditory brainstem responses (EABR) and stapedius reflex thresholds were established in 7 experienced users of the Nucleus cochlear implant. Even using biphasic 400 microseconds/phase clicks for the EABR, responses were observed in only 5 patients; no stapedius reflex (SR) was seen in 3 patients, 2 of whom had a history of middle-ear disorder. The EABR threshold varied widely between subjective threshold and uncomfortable loudness level (ULL) for the same stimulus. The average SR threshold was found somewhat more consistently at 66% of the dynamic range between threshold and ULL, but grossly overestimated the most comfortable level (MCL) in most cases. To obtain equal loudness at the same current level we suggest that broad clicks (300 microseconds/phase) be used for EABR measurements, thus compensating for the lower repetition rate of EABR stimulus compared with the device fitting stimulus.

Performance of prelingually or postlingually deafened adults who were using a single or multichannel cochlear implant.

Long-term (two-year post-implantation) speech recognition was evaluated of prelingually and postlingually deafened adult cochlear implant users who had received either a single or a multichannel device. The patient group comprised 8 pairs of patients matched according to the onset and duration of deafness; in each pair one patient had an extra-cochlear single-channel implant and one had an intra-cochlear multichannel implant. A CDT (Continuous Discourse Tracking) test and a closed-set speech recognition test were used for speech recognition. The results of the postlingually deafened pairs of patients showed that the multichannel implant was superior to the single-channel implant, which is in agreement with other studies. In the pairs of prelingually deafened adult patients, the multichannel system was not found to be superior. This warrants further study and further application of single-channel implants in prelingually deafened patients.

Preoperative electrical stimulation for cochlear implant selection. The use of ear canal electrodes versus transtympanic electrodes.

Preoperative electrostimulation tests were performed on 43 postlingually deaf and 20 prelingually deaf cochlear implant (CI) candidates. The stimulating electrode was placed at three different locations, i.e. the round window, the promontory and the ear canal and the results were compared. The evoked sensations were reported to be of auditory origin by most of the postlingually deaf CI candidates. Prelingually deaf subjects could not always distinguish clearly and reliably between "hearing" and "feeling". The percentage of stimulated ears of postlingually deaf subjects in whom hearing sensations were evoked was almost identical for the three locations of the stimulating electrode. However, in 5 out of the 7 ears without hearing sensations during ear canal stimulation (ECS), hearing sensations could be evoked during either promontory stimulation (PS) or round window stimulation (RWS). The mean threshold level for "hearing" at a stimulation frequency of 62 Hz was lowest during RWS, 7.7 dB higher during PS and 35.8 dB higher during ECS. The mean electrical dynamic range at 62 Hz was most favourable during RWS (23.9 dB), smaller during PS (15.6 dB) and smallest during ECS (10.0 dB). All differences were statistically significant. Placement of the ear canal electrode was easier, less frightening for the patient and required less time than insertion of the needle electrode for PS or RWS. Therefore we recommend the use of ECS to examine whether the CI candidate can be stimulated, and of RWS if more detailed information is required.

Vestibular function in cochlear implant patients.

Thirty-five patients receiving a cochlear implant were evaluated using vestibular function tests. Twenty-five patients received an intracochlear implant (Nucleus). Three out of 6 patients with normo- or hyporeflexia before implantation showed postoperative vestibular damage. In one case this was iatrogenic. Together with available data from the literature the risk of losing preoperative vestibular function is estimated to be around 60%. Improvement of implantation techniques can probably reduce this risk considerably.