Acceptance and Benefits of Electro-Acoustic Stimulation for Conventional-Length Electrode Arrays.

BACKGROUND: Prior studies have shown an advantage for electro-acoustic stimulation (EAS) in cochlear implant (CI) patients with residual hearing, but the degree of benefit can vary. The objective was to explore which factors relate to performance with and acceptance of EAS for CI users with conventional-length electrodes. METHODS: A retrospective chart review was conducted for adults with an average threshold of 75 dB hearing loss or better across 250 and 500 Hz preoperatively (n = 83). All patients underwent cochlear implantation with a conventional-length electrode. Low-frequency audiometric thresholds were measured at initial activation as well as 3 and 12 months postoperatively to determine who met the criteria for EAS. Speech perception for CNC words and AzBio sentences in quiet and +10 dB SNR noise was evaluated 3 and 12 months after activation. RESULTS: Speech perception in quiet and noise was similar regardless of whether or not the patient was eligible for EAS. Less than half of the patients who met the EAS criteria chose to use it, citing reasons such as physical discomfort or lack of perceived benefit. EAS users performed better on CNC words but not sentence recognition than EAS nonusers. CONCLUSIONS: EAS use is dependent on audiologic and nonaudiologic issues. Hearing preservation is possible with conventional electrodes, but hearing preservation alone does not guarantee superior speech perception.

The benefits of preserving residual hearing following cochlear implantation: a systematic review.

OBJECTIVE: Systematically review the current literature for evidence on the "real-life" benefits of hearing preservation cochlear implantation (HPCI) for children and adults. DESIGN: Systematic search of Pubmed, MEDLINE, EMBASE, CINHAL and Cochrane Library for MesH terms hearing¸ preservation and cochlear implantation. Inclusion criteria were the "real-life" benefit of HPCI i.e. other than pre- and post-operative pure tone thresholds. Exclusion criteria were non-English language, conference abstracts, reviews and animal and cadaveric studies. Risk of bias was assessed using the Evidence Project Tool. STUDY SAMPLE: 37 studies that matched criteria for review with 8/37 including children and 29/37 including adults. RESULTS: HPCI was associated with better speech perception in noise in 18/26 papers and better music perception in 4/5 papers. There was no significant benefit reported in speech perception in quiet (14/20 papers) or binaural cues (3/4 papers), nor was there convincing evidence of HPCI outperforming bimodal users (5/7 papers). QoL scores were high amongst HPCI patients (2/2 papers). Interpretation of findings was hindered by small study groups and significant heterogeneity in various parameters. CONCLUSION: Current literature on the "real-life" benefit of HPCI, although limited, supports the existence of meaningful benefit, especially in speech perception in noise and music perception.

Preservation of residual hearing after cochlear implant surgery with slim modiolar electrode.

PURPOSE: To evaluate the insertion results and hearing preservation of a novel slim modiolar electrode (SME) in patients with residual hearing. METHODS: We retrospectively collected the data from the medical files of 17 patients (18 ears) implanted with a SME. All patients had functional low frequency hearing (PTA (0.125-0.5 kHz) ≤ 80 dB HL). The insertion results were re-examined from the postoperative cone-beam computed tomography scans. Postoperative thresholds were obtained at the time of switch-on of the sound processors (mean 43 days) and at latest follow-up (mean 582 days). The speech recognition in noise was measured with the Finnish matrix sentence test preoperatively and at follow-up. RESULTS: The mean insertion depth angle (IDA) was 395°. Neither scala dislocations nor tip fold over were detected. There were no total hearing losses. Functional low-frequency hearing was preserved in 15/18 (83%) ears at switch-on and in 14/17 (82%) ears at follow-up. According to HEARRING classification, 55% (10/18) had complete HP at switch-on and 41% (7/17) still at follow-up. Thirteen patients (14 ears) were initially fitted with electric-acoustic stimulation and seven patients (8 ears) continued to use it after follow-up. CONCLUSIONS: The preliminary hearing preservation results with the SME were more favorable than reported for other perimodiolar electrodes. The results show that the array may also be feasible for electro-acoustic stimulation; it is beneficial in that it provides adequate cochlear coverage for pure electrical stimulation in the event of postoperative or progressive hearing loss.

Effect on vestibular function of cochlear implantation by partial deafness treatment-electro acoustic stimulation (PDT-EAS).

PURPOSE: Although the cochlear implantation procedure does not interfere with vestibular structures directly, both the vestibulum and the cochlea share the same inner ear fluid space, and this fluid may be responsible for transferring possibly damaging forces from one to the other. The purpose of the study is to assess postoperative vestibular function after partial deafness treatment-electro-acoustic stimulation (PDT-EAS) cochlear implantation. METHODS: Fifty-five patients were included in the study (30 females, 25 males, age 11-80, mean 41.8 ± 19.35). cVEMP and oVEMP were performed preoperatively and 1-3 months after cochlear implantation. Caloric and vHIT tests were conducted preoperatively and 4-6 months after cochlear implantation. RESULTS: Our study shows that, based on a wide range of electrodes, use of PDT-EAS is protective in terms of preserving vestibular function. It gives a rate of saccular damage of 15.79%, utricular damage of 19.04%, and a horizontal semicircular canal response reduction of 15.79%. CONCLUSIONS: PDT-EAS is protective in terms of preserving vestibular function. Nevertheless, it should be emphasized that the risk of vestibular damage cannot be totally eliminated even when hearing preservation techniques are adopted.

Auditory and cognitive development in a partially deaf child with bilateral electro-acoustic stimulation: a case study.

OBJECTIVE: To study the long-term evolution of speech and intelligence in a child with partial deafness and normal hearing in the low frequencies after sequentially receiving cochlear implants in both ears. DESIGN: Retrospective chart review. STUDY SAMPLE: Male child aged 6 years was followed over a time period of four years. RESULTS: The paediatric patient had normal hearing up to 1 kHz and profound hearing loss at all higher frequencies symmetrical in both ears. Deprivation of high-frequency sounds resulted in retarded development of speech, language and cognitive skills. The choice for rehabilitation was cochlear implantation with the aim of preserving a considerable amount of low-frequency hearing. With natural hearing at low frequencies and electrical stimulation at high frequencies, the child was able to compensate most of his developmental deficits. Moreover, spatial hearing was almost normal. CONCLUSIONS: Electro-natural stimulation without amplification of the low frequencies (electro-natural hearing) provides access to the whole audible frequency range for children, who suffer from partial deafness in the high frequencies and are normal hearing at low frequencies. Such provision allows for regular speech development and favours the development of spatial hearing. The case report also demonstrates a strong impact on intellectual performance.

Clinical Applications of Intracochlear Electrocochleography in Cochlear Implant Users With Residual Acoustic Hearing.

We herein review the use of electrocochleography (ECoG) to assess peripheral auditory system responsiveness in a growing population of cochlear implant (CI) users with preserved hearing in ears with implants. Twenty-eight recently published intracochlear ECoG articles were thoroughly reviewed to investigate the prognostic utility of intraoperative ECoG monitoring to assess hearing preservation, and the clinical applicability of postoperative ECoG for estimating audiometric thresholds and monitoring longitudinal changes in residual acoustic hearing in patients with EAS. Intraoperative ECoG studies have focused on monitoring the changes in the cochlear microphonics (CM) amplitudes during and after electrode insertion. Mixed results have been reported regarding the relationship between changes in CM amplitude in the operating room and changes in hearing thresholds after surgery. Postoperative ECoG studies have shown that CM and auditory nerve neurophonics thresholds correlate significantly with behavioral thresholds. ECoG thresholds sensitively detect changes as residual acoustic hearing decreases over time in some CI users. This indicates its potential clinical value for monitoring the post-implantation status of the peripheral auditory system. Intracochlear ECoG can provide real-time intraoperative feedback and monitor postoperative hearing preservation in a growing population of CI users.

Acoustic stimulation of the human round window by laser-induced nonlinear optoacoustics.

The feasibility of low frequency pure tone generation in the inner ear by laser-induced nonlinear optoacoustic effect at the round window was demonstrated in three human cadaveric temporal bones (TB) using an integral pulse density modulation (IPDM). Nanosecond laser pulses with a wavelength in the near-infrared (NIR) region were delivered to the round window niche by an optical fiber with two spherical lenses glued to the end and a viscous gel at the site of the laser focus. Using IPDM, acoustic tones with frequencies between 20 Hz and 1 kHz were generated in the inner ear. The sound pressures in scala tympani and vestibuli were recorded and the intracochlear pressure difference (ICPD) was used to calculate the equivalent sound pressure level (eq. dB SPL) as an equivalent for perceived loudness. The results demonstrate that the optoacoustic effect produced sound pressure levels ranging from 140 eq. dB SPL at low frequencies ≤ 200 Hz to 90 eq. dB SPL at 1 kHz. Therefore, the produced sound pressure level is potentially sufficient for patients requiring acoustic low frequency stimulation. Hence, the presented method offers a potentially viable solution in the future to provide the acoustic stimulus component in combined electro-acoustic stimulation with a cochlear implant.

Cochlear implantation and partial deafness - A retrospective review on processor programming.

BACKGROUND: To decide what programming parameters to use for cochlear implants (CIs) in partial deaf patients can be challenging. OBJECTIVE: The processor programming form, categorised as electrical complement (EC), electro-acoustic-stimulation (EAS) or electric stimulation (ES), and difficulties switching programming form were investigated. METHODS: A retrospective investigation of medical records and audiograms was conducted in adult patients intended for EC and EAS. RESULTS: Eighty-four ears (80 patients) were included. Twenty ears were initially fitted with EC, 32 with EAS, 30 with ES and 2 with both EC and EAS. Sixty-four ears met the criteria to use EC or EAS at initial fitting, however only 54 ears were fitted with EC or EAS initially. Twenty-eight patients altered between at least two programming forms and six of those experienced difficulties to adapt to a new form when their low-frequency hearing deteriorated. Twenty-five percent of patients initially fitted with EC or EAS switched programming form within two years. DISCUSSION: Further studies on how to choose the most beneficial sound processor programming parameters for EC and EAS, and when to change between programming forms, are warranted as well as clear guidance on choosing the right candidates for EC and EAS.

Effects of Cochlear Implantation and Steroids on the Aging Guinea Pig Cochlea.

OBJECTIVE: The objective was to determine the effects of older age on hearing preservation after cochlear implantation (CI), and whether steroids improve hearing preservation in older animals. We hypothesized greater hearing preservation would be observed in (1) young animals compared to older animals and (2) older animals receiving steroids compared to no steroids. The secondary objective was to assess levels of fibrosis utilizing optical coherence tomography (OCT). STUDY DESIGN: Experimental Animal Study. SETTING: Laboratory. METHODS: Three groups of guinea pigs: young (YCI; 8.5 ± 0.5 weeks; n = 10), old (OCI; 19.1 ± 1.0 months; n = 9) and old + steroids (OCI+S; 19.1 ± 1.0 months; n = 9) underwent CI. The OCI+S group received a steroid taper over 7 days starting 2 days before surgery to 4 days after. Auditory brainstem response (ABR) measurements were performed preoperatively and postoperatively. OCT imaging was performed to assess cochleae for extent of fibrotic tissue growth in the scala tympani. RESULTS: The YCI group had significantly better hearing preservation as measured by smaller increases in ABR thresholds [mean shift: 2.79 ± 0.66] compared to the OCI group [mean shift = 12.44 ± 5.6]. The OCI+S group had significantly better hearing preservation [2.66 ± 1.50] compared to the OCI group. No significant differences was seen in fibrosis across groups. CONCLUSIONS: Young animals and older animals that received steroids had better hearing after CI than older animals not given steroids, but hearing preservation was not correlated with the level of fibrosis assessed using OCT. This work is the first to investigate differences in hearing preservation by age in an animal model, and supports the protective effects of steroids on hearing preservation in older individuals.

Chronic cochlear implantation with and without electric stimulation in a mouse model induces robust cochlear influx of CX3CR1+/GFP macrophages.

BACKGROUND: Cochlear implantation is an effective auditory rehabilitation strategy for those with profound hearing loss, including those with residual low frequency hearing through use of hybrid cochlear implantation techniques. Post-mortem studies demonstrate the nearly ubiquitous presence of intracochlear fibrosis and neo-ossification following cochlear implantation. Current evidence suggests post-implantation intracochlear fibrosis is associated with delayed loss of residual acoustic hearing in hybrid cochlear implant (CI) recipients and may also negatively influence outcomes in traditional CI recipients. This study examined the contributions of surgical trauma, foreign body response and electric stimulation to intracochlear fibrosis and the innate immune response to cochlear implantation and the hierarchy of these contributions. METHODS: Normal hearing CX3CR1+/GFP mice underwent either round window opening (sham), acute CI insertion or chronic CI insertion with no, low- or high-level electric stimulation. Electric stimulation levels were based on neural response telemetry (NRT), beginning post-operative day 7 for 5 h per day. Subjects (n=3 per timepoint) were sacrificed at 4 h, 1,4,7,8,11,14 and 21 days. An unoperated group (n=3) served as controls. Cochleae were harvested at each time-point and prepared for immunohistochemistry with confocal imaging. The images were analyzed to obtain CX3CR1+ macrophage cell number and density in the lateral wall (LW), scala tympani (ST) and Rosenthal's canal (RC). RESULTS: A ST peri-implant cellular infiltrate and fibrosis occurred exclusively in the chronically implanted groups starting on day 7 with a concurrent infiltration of CX3CR1+ macrophages not seen in the other groups. CX3CR1+ macrophage infiltration was seen in the LW and RC in all experimental groups within the first week, being most prominent in the 3 chronically implanted groups during the second and third week. CONCLUSIONS: The cochlear immune response was most prominent in the presence of chronic cochlear implantation, regardless of electric stimulation level. Further, the development of intracochlear ST fibrosis was dependent on the presence of the indwelling CI foreign body. An innate immune response was evoked by surgical trauma alone (sham and acute CI groups) to a lesser degree. These data suggest that cochlear inflammation and intrascalar fibrosis after cochlear implantation are largely dependent on the presence of a chronic indwelling foreign body and are not critically dependent on electrical stimulation. Also, these data support a role for surgical trauma in inciting the initial innate immune response.

Molecular mechanisms and roles of inflammatory responses on low-frequency residual hearing after cochlear implantation.

Preservation of low-frequency residual hearing is very important for combined electro-acoustic stimulation after cochlear implantation. However, in clinical practice, loss of low-frequency residual hearing often occurs after cochlear implantation and its mechanisms remain unclear. Factors affecting low-frequency residual hearing after cochlear implantation are one of the hot spots in current research. Inflammation induced by injury associated with cochlear implantation is deemed to be significant, as it may give rise to low-frequency residual hearing loss by interfering with the blood labyrinth barrier and neural synapses. Pathological changes along the pathway for low-frequency auditory signals transmission may include latent factors such as damage to neuroepithelial structures, synapses, stria vascularis and other ultrastructures. In this review, current research on mechanisms of low-frequency residual hearing loss after cochlear implantation and possible roles of inflammatory responses are summarized.

Comparison of speech recognition in cochlear implant patients with and without residual hearing: A review of indications.

OBJECTIVE: This study evaluated existing guidelines for cochlear implantation (CI) according to the residual hearing of the patient. Speech recognition performance of adults implanted according to their degree of residual hearing was retrospectively evaluated. METHODS: Speech recognition results of 174 adult cochlear implant recipients were analyzed. All participants had pure tone thresholds of >75 dB at 2 and 4 kHz and were divided into three groups based on differing levels of low-frequency residual hearing. RESULTS: Findings indicate that CI was of benefit for all patients with bilateral severe hearing loss and patients with nonfunctional residual hearing both in quiet and noisy conditions. Patients with residual low-frequency hearing could benefit from combined electro-acoustic stimulation compared to CI alone or to hearing aids alone preoperatively. The speech recognition performance of all CI recipients with different levels of residual hearing in the low frequencies was similar except that superior performance was observed in the group using electro-acoustic stimulation compared to electrical stimulation alone. DISCUSSION AND CONCLUSIONS: Study results demonstrate the success of CI in accordance with current guidelines and also support expanding implantation guidelines to include patients who have severe-to-profound high-frequency sensorineural hearing loss and usable residual low frequency hearing.

Delayed access to bilateral input alters cortical organization in children with asymmetric hearing.

Bilateral hearing in early development protects auditory cortices from reorganizing to prefer the better ear. Yet, such protection could be disrupted by mismatched bilateral input in children with asymmetric hearing who require electric stimulation of the auditory nerve from a cochlear implant in their deaf ear and amplified acoustic sound from a hearing aid in their better ear (bimodal hearing). Cortical responses to bimodal stimulation were measured by electroencephalography in 34 bimodal users and 16 age-matched peers with normal hearing, and compared with the same measures previously reported for 28 age-matched bilateral implant users. Both auditory cortices increasingly favoured the better ear with delay to implanting the deaf ear; the time course mirrored that occurring with delay to bilateral implantation in unilateral implant users. Preference for the implanted ear tended to occur with ongoing implant use when hearing was poor in the non-implanted ear. Speech perception deteriorated with longer deprivation and poorer access to high-frequencies. Thus, cortical preference develops in children with asymmetric hearing but can be avoided by early provision of balanced bimodal stimulation. Although electric and acoustic stimulation differ, these inputs can work sympathetically when used bilaterally given sufficient hearing in the non-implanted ear.

A physiologically-inspired model reproducing the speech intelligibility benefit in cochlear implant listeners with residual acoustic hearing.

This study introduces a speech intelligibility model for cochlear implant users with ipsilateral preserved acoustic hearing that aims at simulating the observed speech-in-noise intelligibility benefit when receiving simultaneous electric and acoustic stimulation (EA-benefit). The model simulates the auditory nerve spiking in response to electric and/or acoustic stimulation. The temporally and spatially integrated spiking patterns were used as the final internal representation of noisy speech. Speech reception thresholds (SRTs) in stationary noise were predicted for a sentence test using an automatic speech recognition framework. The model was employed to systematically investigate the effect of three physiologically relevant model factors on simulated SRTs: (1) the spatial spread of the electric field which co-varies with the number of electrically stimulated auditory nerves, (2) the "internal" noise simulating the deprivation of auditory system, and (3) the upper bound frequency limit of acoustic hearing. The model results show that the simulated SRTs increase monotonically with increasing spatial spread for fixed internal noise, and also increase with increasing the internal noise strength for a fixed spatial spread. The predicted EA-benefit does not follow such a systematic trend and depends on the specific combination of the model parameters. Beyond 300 Hz, the upper bound limit for preserved acoustic hearing is less influential on speech intelligibility of EA-listeners in stationary noise. The proposed model-predicted EA-benefits are within the range of EA-benefits shown by 18 out of 21 actual cochlear implant listeners with preserved acoustic hearing.

Electro-tactile stimulation (ETS) enhances cochlear-implant Mandarin tone recognition.

OBJECTIVE: Electro-acoustic stimulation (EAS) is an effective method to enhance cochlear-implant performance in individuals who have residual low-frequency acoustic hearing. To help the majority of cochlear implant users who do not have any functional residual acoustic hearing, electro-tactile stimulation (ETS) may be used because tactile sensation has a frequency range and perceptual capabilities similar to that produced by acoustic stimulation in the EAS users. METHODS: Following up the first ETS study showing enhanced English sentence recognition in noise,1 the present study evaluated the effect of ETS on Mandarin tone recognition in noise in two groups of adult Mandarin-speaking individuals. The first group included 11 normal-hearing individuals who listened to a 4-channel, noise-vocoded, cochlear-implant simulation. The second group included 1 unilateral cochlear-implant user and 2 bilateral users with each of their devices being tested independently. Both groups participated in a 4-alternative, forced-choice task, in which they had to identify a tone that was presented in noise at a 0-dB signal-to-noise ratio via electric stimulation (actual or simulated cochlear implants), tactile stimulation or the combined ETS. RESULTS: While electric or tactile stimulation alone produced similar tone recognition (∼40% correct), the ETS enhanced the cochlear-implant tone recognition by 17-18 percentage points. The size of the present ETS enhancement effect was similar to that of the previously reported EAS effect on Mandarin tone recognition. Psychophysical analysis on tactile sensation showed an important role of frequency discrimination in the ETS enhancement. CONCLUSION: Tactile stimulation can potentially enhance Mandarin tone recognition in cochlear-implant users who do not have usable residual acoustic hearing. To optimize this potential, high fundamental frequencies need to be transposed to a 100-200 Hz range.

Feasibility of Using Electrocochleography for Objective Estimation of Electro-Acoustic Interactions in Cochlear Implant Recipients with Residual Hearing.

Although cochlear implants (CI) traditionally have been used to treat individuals with bilateral profound sensorineural hearing loss, a recent trend is to implant individuals with residual low-frequency hearing. Patients who retain some residual acoustic hearing after surgery often can benefit from electro-acoustic stimulation (EAS) technologies, which combine conventional acoustic amplification with electrical stimulation. However, interactions between acoustic and electrical stimulation may affect outcomes adversely and are time-consuming and difficult to assess behaviorally. This study demonstrated the feasibility of using the Advanced Bionics HiRes90K Advantage implant electronics and HiFocus Mid Scala/1j electrode to measure electrocochleography (ECochG) responses in the presence of electrical stimulation to provide an objective estimate of peripheral physiologic EAS interactions. In general, electrical stimulation reduced ECochG response amplitudes to acoustic stimulation. The degree of peripheral EAS interaction varied as a function of acoustic pure tone frequency and the intra-cochlear location of the electrically stimulated electrode. Further development of this technique may serve to guide and optimize clinical EAS system fittings in the future.

Perception of Sung Speech in Bimodal Cochlear Implant Users.

Combined use of a hearing aid (HA) and cochlear implant (CI) has been shown to improve CI users' speech and music performance. However, different hearing devices, test stimuli, and listening tasks may interact and obscure bimodal benefits. In this study, speech and music perception were measured in bimodal listeners for CI-only, HA-only, and CI + HA conditions, using the Sung Speech Corpus, a database of monosyllabic words produced at different fundamental frequencies. Sentence recognition was measured using sung speech in which pitch was held constant or varied across words, as well as for spoken speech. Melodic contour identification (MCI) was measured using sung speech in which the words were held constant or varied across notes. Results showed that sentence recognition was poorer with sung speech relative to spoken, with little difference between sung speech with a constant or variable pitch; mean performance was better with CI-only relative to HA-only, and best with CI + HA. MCI performance was better with constant words versus variable words; mean performance was better with HA-only than with CI-only and was best with CI + HA. Relative to CI-only, a strong bimodal benefit was observed for speech and music perception. Relative to the better ear, bimodal benefits remained strong for sentence recognition but were marginal for MCI. While variations in pitch and timbre may negatively affect CI users' speech and music perception, bimodal listening may partially compensate for these deficits.

Morphological correlates of hearing loss after cochlear implantation and electro-acoustic stimulation in a hearing-impaired Guinea pig model.

Hybrid or electro-acoustic stimulation (EAS) cochlear implants (CIs) are designed to provide high-frequency electric hearing together with residual low-frequency acoustic hearing. However, 30-50% of EAS CI recipients lose residual hearing after implantation. The objective of this study was to determine the mechanisms of EAS-induced hearing loss in an animal model with high-frequency hearing loss. Guinea pigs were exposed to 24 h of noise (12-24 kHz at 116 dB) to induce a high-frequency hearing loss. After recovery, two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem responses were recorded biweekly to monitor changes in hearing. The organ of Corti was immunolabeled with phalloidin, anti-CtBP2, and anti-GluR2 to quantify hair cells, ribbons and post-synaptic receptors. The lateral wall was immunolabeled with phalloidin and lectin to quantify stria vascularis capillary diameters. Bimodal or trimodal diameter distributions were observed; the number and location of peaks were objectively determined using the Aikake Information Criterion and Expectation Maximization algorithm. Noise exposure led to immediate hearing loss at 16-32 kHz for all groups. Cochlear implantation led to additional hearing loss at 4-8 kHz; this hearing loss was negatively and positively correlated with minimum and maximum peaks of the bimodal or trimodal distributions of stria vascularis capillary diameters, respectively. After chronic stimulation, no significant group changes in thresholds were seen; however, elevated thresholds at 1 kHz in implanted, stimulated animals were significantly correlated with decreased presynaptic ribbon and postsynaptic receptor counts. Inner and outer hair cell counts did not differ between groups and were not correlated with threshold shifts at any frequency. As in the previous study in a normal-hearing model, stria vascularis capillary changes were associated with immediate hearing loss after implantation, while little to no hair cell loss was observed even in cochlear regions with threshold shifts as large as 40-50 dB. These findings again support a role of lateral wall blood flow changes, rather than hair cell loss, in hearing loss after surgical trauma, and implicate the endocochlear potential as a factor in implantation-induced hearing loss. Further, the analysis of the hair cell ribbons and post-synaptic receptors suggest that delayed hearing loss may be linked to synapse or peripheral nerve loss due to stimulation excitotoxicity or inflammation. Further research is needed to separate these potential mechanisms of delayed hearing loss.

Plasticity in human pitch perception induced by tonotopically mismatched electro-acoustic stimulation.

Under normal conditions, the acoustic pitch percept of a pure tone is determined mainly by the tonotopic place of the stimulation along the cochlea. Unlike acoustic stimulation, electric stimulation of a cochlear implant (CI) allows for the direct manipulation of the place of stimulation in human subjects. CI sound processors analyze the range of frequencies needed for speech perception and allocate portions of this range to the small number of electrodes distributed in the cochlea. Because the allocation is assigned independently of the original resonant frequency of the basilar membrane associated with the location of each electrode, CI users who have access to residual hearing in either or both ears often have tonotopic mismatches between the acoustic and electric stimulation. Here we demonstrate plasticity of place pitch representations of up to three octaves in Hybrid CI users after experience with combined electro-acoustic stimulation. The pitch percept evoked by single CI electrodes, measured relative to acoustic tones presented to the non-implanted ear, changed over time in directions that reduced the electro-acoustic pitch mismatch introduced by the CI programming. This trend was particularly apparent when the allocations of stimulus frequencies to electrodes were changed over time, with pitch changes even reversing direction in some subjects. These findings show that pitch plasticity can occur more rapidly and on a greater scale in the mature auditory system than previously thought possible. Overall, the results suggest that the adult auditory system can impose perceptual order on disordered arrays of inputs.

Combined electric and acoustic hearing performance with Zebra® speech processor: speech reception, place, and temporal coding evaluation.

OBJECTIVE: To assess the auditory performance of Digisonic(®) cochlear implant users with electric stimulation (ES) and electro-acoustic stimulation (EAS) with special attention to the processing of low-frequency temporal fine structure. METHOD: Six patients implanted with a Digisonic(®) SP implant and showing low-frequency residual hearing were fitted with the Zebra(®) speech processor providing both electric and acoustic stimulation. Assessment consisted of monosyllabic speech identification tests in quiet and in noise at different presentation levels, and a pitch discrimination task using harmonic and disharmonic intonating complex sounds ( Vaerenberg et al., 2011 ). These tests investigate place and time coding through pitch discrimination. All tasks were performed with ES only and with EAS. RESULTS: Speech results in noise showed significant improvement with EAS when compared to ES. Whereas EAS did not yield better results in the harmonic intonation test, the improvements in the disharmonic intonation test were remarkable, suggesting better coding of pitch cues requiring phase locking. DISCUSSION: These results suggest that patients with residual hearing in the low-frequency range still have good phase-locking capacities, allowing them to process fine temporal information. ES relies mainly on place coding but provides poor low-frequency temporal coding, whereas EAS also provides temporal coding in the low-frequency range. Patients with residual phase-locking capacities can make use of these cues.