Speech Comprehension in Children and Adolescents After Sequential Bilateral Cochlear Implantation With Long Interimplant Interval

Dissociated Representation of Binaural Cues in Single-Sided Deafness: Implications for Cochlear Implantation

Congenital single-sided deafness (SSD) leads to an aural preference syndrome that is characterized by overrepresentation of the hearing ear in the auditory system. Cochlear implantation (CI) of the deaf ear is an effective treatment for SSD. However, the newly introduced auditory input in congenital SSD often does not reach expectations in late-implanted CI recipients with respect to binaural hearing and speech perception. In a previous study, a reduction of the interaural time difference (ITD) sensitivity has been shown in unilaterally congenitally deaf cats (uCDCs). In the present study, we focused on the interaural level difference (ILD) processing in the primary auditory cortex. The uCDC group was compared with hearing cats (HCs) and bilaterally congenitally deaf cats (CDCs). The ILD representation was reorganized, replacing the preference for the contralateral ear with a preference for the hearing ear, regardless of the cortical hemisphere. In accordance with the previous study, uCDCs were less sensitive to interaural time differences than HCs, resulting in unmodulated ITD responses, thus lacking directional information. Such incongruent ITDs and ILDs cannot be integrated for binaural sound source localization. In normal hearing, the predominant effect of each ear is excitation of the auditory cortex in the contralateral cortical hemisphere and inhibition in the ipsilateral hemisphere. In SSD, however, auditory pathways reorganized such that the hearing ear produced greater excitation in both cortical hemispheres and the deaf ear produced weaker excitation and preserved inhibition in both cortical hemispheres.

Importance of age at 2nd implantation and interimplant interval to the outcome of bilateral prelingually deafened pediatric cochlear implantation

BACKGROUND

In Taiwan, the number of cases of sequential bilateral pediatric cochlear implantation (CI) is increasing but data regarding its effectiveness and impact of the reimbursement policy are lacking. We examined the speech perception and quality of life (QOL) of bilateral prelingually deaf children who underwent sequential CI, considering the effects of age at the time of second implantation and interimplant interval.

METHODS

We enrolled 124 Mandarin-speaking participants who underwent initial cochlear implant (CI1) in 2001-2019 and a second CI (CI2) in 2015-2020. Patients were followed up for ≥2 years and were categorized into groups based on age at the time of CI2 implantation (<3.5, 3.6-7, 7.1-10, 10.1-13, and 13.1-18 years) and interimplant interval (0.5-3, 3.1-5, 5.1-7, 7.1-10, and >10 years). We evaluated speech perception, device usage rates, and QOL using subjective questionnaires (Speech, Spatial, and Qualities of Hearing and Comprehension Cochlear Implant Questionnaire).

RESULTS

Speech perception scores of CI2 were negatively correlated with ages at the time of CI1 and CI2 implantation and interimplant interval. Older age and a longer interimplant interval were associated with higher nonuse rates for CI2 and worse auditory performance and QOL. Among individuals aged >13 years with interimplant intervals >10 years, up to 44% did not use their second ear. Patients aged 7.1 to 10 years had better speech perception and higher questionnaire scores than those aged 10.1 to 13 and 13.1 to 18 years. Furthermore, patients aged 10.1 to 13 years had a lower rate of continuous CI2 usage compared to those aged 7.1 to 10 years.

CONCLUSION

Timely implantation of CI2 is essential to achieve optimal outcomes, particularly among sequentially implanted patients with long-term deafness in the second ear and no improvement with hearing aids following CI1 implantation. For CI2 implantation, an upper limit of age of 10 years and interimplant interval of 7 years are essential to prevent suboptimal outcomes. These data can provide useful information to implant recipients, their families, and medical and audiological professionals, enabling a comprehensive understanding of the benefits and potential impacts of the timing of CI2 implantation.

Relationships between bilateral auditory brainstem activity and inter-implant interval in children with cochlear implants

PURPOSE

To investigate the effect of the interval between bilateral cochlear implantation on the development of bilateral peripheral auditory pathways as revealed by the electrically evoked auditory brainstem response (EABR).

METHODS

Fifty-eight children with profound bilateral sensorineural hearing loss were recruited. Among them, 33 children received sequential bilateral cochlear implants (CIs), and 25 children received simultaneous bilateral CIs. The bilateral EABRs evoked by electrical stimulation from the CI electrode were recorded on the day of second-side CI activation.

RESULTS

The latencies of wave III (eIII) and wave V (eV) were significantly shorter on the first CI side than on the second CI side in children with sequential bilateral CIs but were similar between the two sides in children with simultaneous bilateral CIs. Furthermore, the latencies were prolonged from apical to basal channels along the cochlea in the two groups. In children with sequential CIs, the inter-implant interval was negatively correlated with the eV latency on the first CI side and was positively correlated with bilateral differences in the eIII and eV latencies.

CONCLUSIONS

Unilateral CI use promotes the maturation of ipsilateral auditory conduction function. However, a longer inter-implant interval results in more unbalanced development of bilateral auditory brainstem pathways. Bilateral cochlear implantation with no or a short interval is recommended.

Variability in Receptive Language Development Following Bilateral Cochlear Implantation

OBJECTIVES

The primary aim was to investigate the variability in language development in children aged 5-7.5 years after bilateral cochlear implantation (CI) up to the age of 2 years, and any impact of the age at implantation and additional noncognitive or anatomical disorders at implantation.

DESIGN

Data of 84 congenitally deaf children that had received simultaneous bilateral CI at the age of ≤ 24 months were included in this retrospective study. The results of language comprehension acquisition were evaluated using a standardized German language acquisition test for normal hearing preschoolers and first graders. Data on speech perception of monosyllables and sentences in quiet and noise were added.

RESULTS

In a monosyllabic test, the children achieved a median performance of 75.0 ± 12.88%. In the sentence test in quiet, the median performance was 89 ± 12.69%, but dropped to 54 ± 18.92% in noise. A simple analysis showed a significant main effect of age at implantation on monosyllabic word comprehension (p < .001), but no significant effect of comorbidities that lacked cognitive effects (p = .24). Language acquisition values correspond to the normal range of children with normal hearing. Approximately 25% of the variability in the language acquisition tests is due to the outcome of the monosyllabic speech perception test.

CONCLUSIONS

Congenitally deaf children who were fitted bilaterally in the 1st year of life can develop age-appropriate language skills by the time they start school. The high variability in the data is partly due to the age of implantation, but additional factors such as cognitive factors (e.g., working memory) are likely to influence the variability.

Speech Perception of Second Cochlear Implant after 6 Years of Age in Prelingually Deaf Children

Sequential bilateral cochlear implantation in children becomes less effective as the inter-implant interval increases. However, the cause of this and the age at which speech perception becomes impossible are unclear. We examined the cases of 11 prelingually deaf children who underwent unilateral cochlear implantation at our hospitals before the age of 5 years old, followed by a second implantation procedure on the contralateral side when they were aged ≥ 6 years old (6-12 years old). The subjects' hearing thresholds and speech discrimination scores for the second cochlear implant were evaluated at 3 postoperative months and 1-7 years. All of the subjects demonstrated improvements in their hearing thresholds to a mean of 30 dB HL at 1 year. Regarding speech perception, one patient (a 12-year-old), who had developed bilateral hearing loss at 30 months of age after contracting mumps, demonstrated a 90% improvement in his speech discrimination score at 1 year. However, among the other congenitally deaf children, there were two patients whose speech discrimination scores had improved by ≥ 80% at > 4 postoperative years. The congenitally deaf children exhibited poor speech perception despite showing improved hearing thresholds in the ears that received second cochlear implants. Assuming that the auditory pathway beyond the superior olivary complex remained functional, the reduced speech perception abilities associated with the second cochlear implants may have been attributable to the loss of the spiral ganglion and cochlear nucleus cells due to a lack of auditory input since birth.

Longitudinal Effects of Simultaneous and Sequential Bilateral Cochlear Implantation on Cortical Auditory-Evoked Potentials Recorded at Cz in a Large Cohort of Children

OBJECTIVES

Auditory development after bilateral cochlear implantation in children has been measured using source localization of multi-channel late latency responses. It is not clear, however, whether this development can be tracked using a more clinically feasible method of recording from one active recording electrode placed at mid-line center of the head (Cz).

DESIGN

In this prospective cohort study, cortical auditory-evoked potential responses (CAEPs) were recorded from Cz referenced to each earlobe (Cz-CAEP) from 222 children with bilateral cochlear implant (CI); 128 (mean ± SD age: 2.78 ± 3.30 years) received both CIs in the same surgery (simultaneous group) and 94 (aged 7.72 ± 4.45 years) received a second CI after 4.21 ± 2.98 years of unilateral CI use. We sought to (1) identify cortical development over the first couple of years of bilateral CI use; (2) measure known asymmetries in auditory development between the CIs; and (3) detect the effects of bilateral rather than unilateral CI use. 4556 Cz-CAEPs were recorded across the cohort over 33.50 ± 7.67 months duration of bilateral CI use. Given concerns related to peak picking, amplitude areas were measured across two response time windows (50 to 199 ms and 200 to 400 ms).

RESULTS

Results indicated that small response amplitudes occur at initial CI use and amplitudes increase in the negative or positive direction rapidly over the first months of CI use in both time windows. Asymmetries between Cz-CAEPs evoked by each CI were found in the sequential group and reduced with bilateral CI use, particularly in the first time window; these differences increased with longer inter-implant delay. Bilaterally evoked Cz-CAEPs were larger in amplitude than unilateral responses from either CI in the simultaneous group. In the sequential group, bilateral responses were similar to responses from the first implanted side but increased in relative amplitude with bilateral CI use. The Cz-CAEP measures were not able to predict asymmetries or bilateral benefits in speech perception measures.

CONCLUSIONS

The Cz-CAEP was able to indicate cortical detection of CI input and showed gross morphological changes with bilateral CI use. Findings indicate Cz-CAEPs can be used to identify gross changes in auditory development in children with bilateral CIs, but they are less sensitive to tracking the remaining abnormalities that are measured by multi-channel CAEPs and speech perception testing.

Effects of Sequential Bilateral Cochlear Implantation in Children: Evidence from Speech-Evoked Cortical Potentials and Tests of Speech Perception

INTRODUCTION

Benefits of bilateral cochlear implants (CI) may be compromised by delays to implantation of either ear. This study aimed to evaluate the effects of sequential bilateral CI use in children who received their first CI at young ages, using a clinical set-up.

METHODS

One-channel cortical auditory evoked potentials and speech perception in quiet and noise were evoked at repeated times (0, 3, 6, 12 months of bilateral CI use) by unilateral and bilateral stimulation in 28 children with early-onset deafness. These children were unilaterally implanted before 3.69 years of age (mean ± SD of 1.98 ± 0.73 years) and received a second CI after 5.13 ± 2.37 years of unilateral CI use. Comparisons between unilaterally evoked responses were used to measure asymmetric function between the ears and comparisons between bilateral responses and each unilateral response were used to measure the bilateral benefit.

RESULTS

Chronic bilateral CI promoted changes in cortical auditory responses and speech perception performance; however, large asymmetries were present between the two unilateral responses despite ongoing bilateral CI use. Persistent cortical differences between the two sides at 1 year of bilateral stimulation were predicted by increasing age at the first surgery and inter-implant delay. Larger asymmetries in speech perception occurred with longer inter-implant delays. Bilateral responses were more similar to the unilateral responses from the first rather than the second CI.

CONCLUSION

These findings are consistent with the development of the aural preference syndrome and reinforce the importance of providing bilateral CIs simultaneously or sequentially with very short delays.

Effects of long-term unilateral cochlear implant use on large-scale network synchronization in adolescents

Unilateral cochlear implantation (CI) limits deafness-related changes in the auditory pathways but promotes abnormal cortical preference for the stimulated ear and leaves the opposite ear with little protection from auditory deprivation. In the present study, time-frequency analyses of event-related potentials elicited from stimuli presented to each ear were used to determine effects of unilateral CI use on cortical synchrony. CI-elicited activity in 34 adolescents (15.4±1.9 years of age) who had listened with unilateral CIs for most of their lives prior to bilateral implantation were compared to responses elicited by a 500Hz tone-burst in normal hearing peers. Phase-locking values between 4 and 60Hz were calculated for 171 pairs of 19-cephalic recording electrodes. Ear specific results were found in the normal hearing group: higher synchronization in low frequency bands (theta and alpha) from left ear stimulation in the right hemisphere and more high frequency activity (gamma band) from right ear stimulation in the left hemisphere. In the CI group, increased phase synchronization in the theta and beta frequencies with bursts of gamma activity were elicited by the experienced-right CI between frontal, temporal and parietal cortical regions in both hemispheres, consistent with increased recruitment of cortical areas involved in attention and higher-order processes, potentially to support unilateral listening. By contrast, activity was globally desynchronized in response to initial stimulation of the naïve-left ear, suggesting decoupling of these pathways from the cortical hearing network. These data reveal asymmetric auditory development promoted by unilateral CI use, resulting in an abnormally mature neural network.

Bilateral Cochlear Implants or Bimodal Hearing for Children with Bilateral Sensorineural Hearing Loss

Purpose of review

This review describes speech perception and language outcomes for children using bimodal hearing (cochlear implant (CI) plus contralateral hearing aid) as compared to children with bilateral CIs and contrasts said findings with the adult literature. There is a lack of clinical evidence driving recommendations for bimodal versus bilateral CI candidacy and as such, clinicians are often unsure about when to recommend a second CI for children with residual acoustic hearing. Thus the goal of this review is to identify scientific information that may influence clinical decision making for pediatric CI candidates with residual acoustic hearing.

Recent findings

Bilateral CIs are considered standard of care for children with bilateral severe-to-profound sensorineural hearing loss. For children with aidable acoustic hearing-even in just the low frequencies-an early period of bimodal stimulation has been associated with significantly better speech perception, vocabulary, and language development. HA audibility, however, is generally poorer than that offered by a CI resulting in interaural asymmetry in speech perception, head shadow, as well as brainstem and cortical activity and development. Thus there is a need to optimize "two-eared" hearing while maximizing a child's potential with respect to hearing, speech, and language while ensuring that we limit asymmetrically driven auditory neuroplasticity. A recent large study of bimodal and bilateral CI users suggested that a period of bimodal stimulation was only beneficial for children with a better-ear pure tone average (PTA) ≤ 73 dB HL. This 73-dB-HL cutoff applied even to children who ultimately received bilateral CIs.

Summary

Though we do not yet have definitive guidelines for determining bimodal versus bilateral CI candidacy, there is increasing evidence that 1) bilateral CIs yield superior outcomes for children with bilateral severe-to-profound hearing loss and, 2) an early period of bimodal stimulation is beneficial for speech perception and language development, but only for children with better-ear PTA ≤ 73 dB HL. For children with residual acoustic hearing, even in just the low-frequency range, rapid sequential bilateral cochlear implantation following a trial period with bimodal stimulation will yield best outcomes for auditory, language, and academic development. Of course, there is also an increasing prevalence of cochlear implantation with acoustic hearing preservation allowing for combined electric and acoustic stimulation even following bilateral implantation.

Measurements of the local evoked potential from the cochlear nucleus in patients with an auditory brainstem implant and its implication to auditory perception and audio processor programming

The measurement of the electrically evoked compound action potential (ECAP) in cochlear implant (CI) patients is widely used to provide evidence of a functioning electrode-nerve interface, to confirm proper location of the electrode array and to program the sound processor. In patients with an auditory brainstem implant (ABI), a likewise versatile measurement would be desirable. The ECAP measurement paradigm “Alternating Polarity” was utilized to record responses via the implanted ABI electrode array placed on the cochlear nucleus. Emphasizing on the different location of stimulation and recording, these responses are called local evoked potentials (LEP). LEP measurements were conducted during the clinical routine in 16 ABI patients (12 children and 4 adults), corresponding to 191 electrode contacts. A retrospective analysis of these data revealed, that LEP responses were observed in 64.9% of all measured electrode contacts. LEP responses predicted auditory perception with a sensitivity of 90.5%. False-positive rate was 33.7%. Objective LEP thresholds were highly significantly (p < 0.001) correlated both to behavioral thresholds (Pearson’s r = 0.697) and behavioral most comfortable levels (r = 0.840). Therefore, LEP measurements have the potential to support fitting in ABI patients.

The impact of unilateral, simultaneous, or sequential cochlear implantation on pediatric language outcomes

OBJECTIVES

The purpose of this study was to determine the impact of unilateral versus bilateral cochlear implantation on receptive and expressive spoken language outcomes. Secondary aims were to investigate factors timing of first and second implant placement and reliance on government funded health plans on language outcome.

METHODS

This was a retrospective chart review of spoken language users with bilateral severe-to-profound hearing loss. A total of 204 children were included, 105 in the bilateral group and 99 in the unilateral group. Multiple regression analyses were completed to investigate factors that influence language outcomes at age 5.

RESULTS

Recipients who received bilateral CIs performed significantly higher on measures of receptive and expressive language than those with unilateral implants.

DISCUSSION

This study demonstrates that bilateral implantation has a significant impact on receptive and expressive aspects of language development, and should be strongly considered as standard practice for children with bilateral severe to profound hearing loss.

CONCLUSION

These results indicate that families should be counseled that language outcomes are better with bilateral cochlear implantation than unilateral implantation. Cochlear implant teams should continue to consider the impact of socioeconomic status on outcomes and explore new methods to reduce the impacts and barriers of poverty to language development.

Sequential Bilateral Cochlear Implantation With Prolonged Time Intervals

Purpose The aim of the study was to assess whether sequential cochlear implantation (CI) with a prolonged interimplant interval (M = 15.2 years) between the first and second CIs benefited speech recognition and health-related quality of life. Method This prospective study included 14 prelingually deafened participants who received their second CI after a prolonged interimplant interval (M = 15.2 years). Additionally, speech recognition ability over a 12-month period of bilateral implant use was investigated. The results of the speech recognition test in both quiet and noisy conditions were statistically analyzed for each CI alone and both CIs together. Nijmegen Cochlear Implant Questionnaire scores were also collected at activation and at 12 months after activation. Results Improvements in speech recognition ability were observed following the use of the first implant alone and with the use of both implants together; however, progress was much slower with the use of the second implant alone, following its introduction. Furthermore, a significant difference in the trajectory of speech recognition ability was observed between the first and the second implanted ear. According to Nijmegen Cochlear Implant Questionnaire scores, all participants benefitted from bilateral CI after 12 months. Conclusions Prolonged interimplant intervals resulted in asymmetrical speech recognition abilities. A significant improvement in the speech recognition scores was observed with the first implanted ear, and much slower progress was observed with the second implanted ear. However, the "poorer" second implanted ear could provide a considerable beneficial effect on the improved speech recognition and health-related quality of life with the bilateral CI. Supplemental Material https://doi.org/10.23641/asha.12861152.

The Effect of Cochlear Implant Interval on Spoken Language Skills of Pediatric Bilateral Cochlear Implant Users

OBJECTIVE

To examine the effects of cochlear implant (CI) interval (time between CI surgeries) on receptive vocabulary and receptive language skills for children with bilateral CIs.

STUDY DESIGN

A prospective cross-sectional study design.

SETTING

Participants were recruited from, and tested at, oral schools for the deaf and pediatric audiology clinics across the United States.

PATIENTS

Eighty-eight children, 4 to 9 years of age, with bilateral CIs and known hearing histories. Twenty-three participants received CIs simultaneously and 65 received CIs sequentially. Of those implanted sequentially, 86% wore a hearing aid (HA) on the non-implanted ear during the CI interval.

INTERVENTION

Bilateral cochlear implantation.

MAIN OUTCOME MEASURES

Receptive vocabulary was measured via the Peabody Picture Vocabulary Test (PPVT). Receptive language skills were measured via the Clinical Evaluation of Language Fundamentals (CELF).

RESULTS

Multiple linear regression models indicate better receptive vocabulary and receptive language skills are associated with earlier ages at first CI (CI 1), but not with shorter CI intervals.

CONCLUSIONS

Early cochlear implantation (i.e., age at CI 1) is critical for better receptive vocabulary and receptive language skills. Shorter CI intervals are not associated with better receptive vocabulary and receptive language skills for these 88 children, who nearly all used bimodal hearing during the interval. Use of a HA at the non-implanted ear, before receipt of a second CI (CI 2), may mitigate the effects of early bilateral auditory deprivation.

Critical Factors for Binaural Hearing in Children with Bilateral Sequential Cochlear Implantation: First Implant Performance and Inter-Implant Interval

Objectives: This study evaluated the influence of the performance of the first cochlear implant (CI-1) on the second implant (CI-2) and the significance of inter-implant intervals in children with sequential cochlear implantation. Method: Thirty-four patients were included for speech perception outcome and 38 patients were included for subjective hearing satisfaction in daily life in this study. The patients were classified into 3 groups according to inter-implant interval: group I, <4 years; group II, 4–7 years; group III, >7 years. Open-set speech perception scores before and at 3, 6, 12, and 24 months after the second implantation was compared among the CI-1, CI-2, and bilateral CI conditions. Subjective hearing benefits in daily life were measured using a questionnaire of Speech, Spatial, and Qualities of Hearing Scale (SSQ) for parents of children with impaired hearing, and device use and preference were evaluated (by phone interview.) Results: The speech perception of children using the CI-2 only improved quickly within 3 months postoperatively and were similar to those using the CI-1 only. The results of monosyllabic and disyllabic word tests using CI-2 at 3 months after second implantation were not significantly different from those in patients using the CI-1 only. In the sentence test, the scores using the CI-2 only were not significantly different from those using the first implant only at 6 months after second implantation. SSQ scores were similar among groups and the worst score in each section was shown from the questions about performance under noisy conditions in each section. Device usage habits showed that 17 (44.7%) and 16 (42.1%) patients preferred bilateral and the CI-1, respectively. Only 5 (13.2%) patients wanted the CI-2 rather than the CI-1. While group I and II showed a prominent positive attitude to bilateral use, group III, for which the inter-implant interval was 7 years or more, showed a definite preference for the CI-1 (p < 0.01). Conclusions: Functional binaural benefits were achieved in patients who were good performers with the CI-1 after the second implantation irrespective of the inter-implant interval. Sequential CI should be strongly recommended for patients with unilateral CI showing a good performance.

Binaural advantages in using a cochlear implant for adults with profound unilateral hearing loss

BACKGROUND

Recent studies of cochlear implants (CIs) in profound unilateral hearing loss (UHL) patients have demonstrated a restoration of some binaural hearing.

AIMS/OBJECTIVES

The objective was to evaluate three possible advantages of binaural hearing in CIs adult users with UHL including single-side deafness (SSD) and asymmetric hearing loss (AHL) subgroups.

MATERIAL AND METHODS

A prospective study was conducted that included 70 sequentially implanted patients. Subgroups of these subjects included 64 with a postlingual onset of a profound hearing loss on the implanted side and 6 with a prelingual onset of that loss. Three binaural effects - redundancy, head shadow, and squelch - were evaluated.

RESULTS

Significant differences between the 'CI on' and 'CI off' conditions were found for all three binaural effects for the study group as a whole and for the postlingual subgroup. However, results for the subjects in the prelingual subgroup did not demonstrate any of the binaural advantages.

CONCLUSION AND SIGNIFICANCE

Patients with a postlingual onset of a profound hearing loss in one ear and normal hearing or only a moderate loss in the other ear are able to make the effective use of a CI in the profound-loss ear in conjunction with acoustic stimulation of the other ear.

Limiting asymmetric hearing improves benefits of bilateral hearing in children using cochlear implants

Neurodevelopmental changes occur with asymmetric hearing loss, limiting binaural/spatial hearing and putting children at risk for social and educational challenges. These deficits may be mitigated by providing bilateral hearing in children through auditory prostheses. Effects on speech perception and spatial hearing were measured in a large cohort of >450 children who were deaf and used bilateral cochlear implants or bimodal devices (one cochlear implant and a contralateral hearing aid). Results revealed an advantage of bilateral over unilateral device use but this advantage decreased as hearing in the two ears became increasingly asymmetric. Delayed implantation of an ear with severe to profound deafness allowed asymmetric hearing, creating aural preference for the better hearing ear. These findings indicate that bilateral input with the most appropriate device for each ear should be provided early and without delay during development.

Sequential bilateral cochlear implant: results in children and adolescents

Introduction

The use of the bilateral cochlear implants can promote the symmetrical development of the central auditory pathways, thus benefiting the development of auditory abilities and improving sound localization and the ability of auditory speech perception in situations of competitive noise.

Objective

To evaluate the ability of speech perception in children and adolescents using sequential bilateral cochlear implants, considering the association of these variables: age at surgery, time of device use and interval between surgeries.

Methods

A total of 14 individuals between 10 and 16 years of age, who demonstrated surgical indication for the use of sequential bilateral cochlear implants as intervention in the auditory habilitation process, were assessed. The speech perception ability was assessed through sentence lists constructed in the Portuguese language, presented in two situations: in silence, with fixed intensity of 60 dB SPL, and in competitive noise, with a signal-to-noise ratio of +15 dB. The evaluation was performed under the following conditions: unilateral with the first activated cochlear implant, unilateral with the second activated cochlear implant and bilateral with both devices activated.

Results

The results of the speech perception tests showed better performance in both silence and in noise for the bilateral cochlear implant condition when compared to the 1st cochlear implant and the 2nd cochlear implant alone. A worse result of speech perception was found using the 2nd cochlear implant alone. No statistically significant correlation was found between age at the surgical procedure, interval between surgeries and the time of use of the 2nd cochlear implant, and the auditory speech perception performance for all assessed conditions. The use of a hearing aid prior to the 2nd cochlear implant resulted in benefits for auditory speech perception with the 2nd cochlear implant, both in silence and in noise.

Conclusion

The bilateral cochlear implant provided better speech perception in silence and in noise situations when compared to the unilateral cochlear implant, regardless of the interval between surgeries, age at the surgical procedure and the time of use of the 2nd cochlear implant. Speech perception with the 1st cochlear implant was significantly better than with the 2nd cochlear implant, both in silence and in noise. The use of the hearing aid prior to the 2nd cochlear implant influenced speech perception performance with the 2nd cochlear implant, both in silence and in noise.

Cochlear implant - state of the art

Cochlear implants are the treatment of choice for auditory rehabilitation of patients with sensory deafness. They restore the missing function of inner hair cells by transforming the acoustic signal into electrical stimuli for activation of auditory nerve fibers. Due to the very fast technology development, cochlear implants provide open-set speech understanding in the majority of patients including the use of the telephone. Children can achieve a near to normal speech and language development provided their deafness is detected early after onset and implantation is performed quickly thereafter. The diagnostic procedure as well as the surgical technique have been standardized and can be adapted to the individual anatomical and physiological needs both in children and adults. Special cases such as cochlear obliteration might require special measures and re-implantation, which can be done in most cases in a straight forward way. Technology upgrades count for better performance. Future developments will focus on better electrode-nerve interfaces by improving electrode technology. An increased number of electrical contacts as well as the biological treatment with regeneration of the dendrites growing onto the electrode will increase the number of electrical channels. This will give room for improved speech coding strategies in order to create the bionic ear, i.e. to restore the process of natural hearing by means of technology. The robot-assisted surgery will allow for high precision surgery and reliable hearing preservation. Biological therapies will support the bionic ear. Methods are bio-hybrid electrodes, which are coded by stem cells transplanted into the inner ear to enhance auto-production of neurotrophins. Local drug delivery will focus on suppression of trauma reaction and local regeneration. Gene therapy by nanoparticles will hopefully lead to the preservation of residual hearing in patients being affected by genetic hearing loss. Overall the cochlear implant is a very powerful tool to rehabilitate patients with sensory deafness. More than 1 million of candidates in Germany today could benefit from this high technology auditory implant. Only 50,000 are implanted so far. In the future, the procedure can be done under local anesthesia, will be minimally invasive and straight forward. Hearing preservation will be routine.

Perceived improvements and challenges following sequential bilateral cochlear implantation in children and adults

OBJECTIVE

Semi-structured interviews were conducted with sequentially implanted bilateral cochlear-implant (biCI) recipients to examine functional aspects of communication that are affected by listening with one versus two CIs.

DESIGN

Participants were 15 adult biCI recipients and parents of 30 children (categorized into three groups by age) with biCIs. All CI users had sequential placement of biCIs with at least six months' experience with the first CI before activation of the second device, and at least three months' experience with both CIs prior to the interview. The parent/paediatric and adult interviews were all conducted by the same examiner. Electronic transcripts of the interview responses were coded for perceived changes or lack thereof in 23 behaviours following biCI. Extent of reported benefit was quantified for each subject within and across these behaviours and at the group level as a function of age.

RESULTS

Most adults and parents of children reported multiple functional changes following biCI use, and changes often translated to enhanced social communication. Nearly all participants were consistent users of biCIs, and were satisfied with their perceived gains in communicating in everyday settings. Most reported ongoing challenges listening in noisy settings. Although many reports on children paralleled those of adults, developmental differences were apparent. Thirteen percent of adults and twenty percent of parents of children in each of the respective groups reported low levels of change.

CONCLUSIONS

Results suggest that many biCI users experience meaningful functional benefits that may be underestimated by traditional outcome measures. We suggest the need to expand measurement approaches to better quantify the nature of these benefits.

Simultaneous bilateral cochlear implants: Developmental advances do not yet achieve normal cortical processing

BACKGROUND

Simultaneous bilateral cochlear implantation promotes symmetric development of bilateral auditory pathways but binaural hearing remains abnormal. To evaluate whether bilateral cortical processing remains impaired in such children, cortical activity to unilateral and bilateral stimuli was assessed in a unique cohort of 16 children who received bilateral cochlear implants (CIs) simultaneously at 1.97 ± 0.86 years of age and had ~4 years of CI experience, providing the first opportunity to assess electrically driven cortical development in the absence of reorganized asymmetries from sequential implantation.

METHODS

Cortical activity to unilateral and bilateral stimuli was measured using multichannel electro-encephalography. Cortical processing in children with bilateral CIs was compared with click-elicited activity in 13 normal hearing children matched for time-in-sound. Source activity was localized using the Time Restricted, Artefact and Coherence source Suppression (TRACS) beamformer method.

RESULTS

Consistent with dominant crossed auditory pathways, normal P1 activity (~100 ms) was weaker to ipsilateral stimuli relative to contralateral and bilateral stimuli and both auditory cortices preferentially responded to the contralateral ear. Right hemisphere dominance was evident overall. Children with bilateral CIs maintained the expected right dominance but differences from normal included: (i) minimal changes between ipsilateral, contralateral and bilateral stimuli, (ii) weaker than normal contralateral stimulus preference, (iii) symmetric activity to bilateral stimuli, and (iv) increased occipital lobe recruitment during bilateral relative to unilateral stimulation. Between-group contrasts demonstrated lower than normal activity in the inferior parieto-occipital lobe (suggesting deficits in sensory integration) and greater than normal left frontal lobe activity (suggesting increased attention), even during passive listening.

CONCLUSIONS

Together, findings suggest that early simultaneous bilateral cochlear implantation promotes normal-like auditory symmetry but that abnormalities in cortical processing consequent to deafness and/or electrical stimulation through two independent speech processors persist.

United Kingdom national paediatric bilateral project: Demographics and results of localization and speech perception testing

OBJECTIVES

To assess longitudinal outcomes in a large and varied population of children receiving bilateral cochlear implants both simultaneously and sequentially.

METHODS

This observational non-randomized service evaluation collected localization and speech recognition in noise data from simultaneously and sequentially implanted children at four time points: before bilateral cochlear implants or before the sequential implant, 1 year, 2 years, and 3 years after bilateral implants. No inclusion criteria were applied, so children with additional difficulties, cochleovestibular anomalies, varying educational placements, 23 different home languages, a full range of outcomes and varying device use were included.

RESULTS

1001 children were included: 465 implanted simultaneously and 536 sequentially, representing just over 50% of children receiving bilateral implants in the UK in this period. In simultaneously implanted children the median age at implant was 2.1 years; 7% were implanted at less than 1 year of age. In sequentially implanted children the interval between implants ranged from 0.1 to 14.5 years. Children with simultaneous bilateral implants localized better than those with one implant. On average children receiving a second (sequential) cochlear implant showed improvement in localization and listening in background noise after 1 year of bilateral listening. The interval between sequential implants had no effect on localization improvement although a smaller interval gave more improvement in speech recognition in noise. Children with sequential implants on average were able to use their second device to obtain spatial release from masking after 2 years of bilateral listening. Although ranges were large, bilateral cochlear implants on average offered an improvement in localization and speech perception in noise over unilateral implants.

CONCLUSION

These data represent the diverse population of children with bilateral cochlear implants in the UK from 2010 to 2012. Predictions of outcomes for individual patients are not possible from these data. However, there are no indications to preclude children with long inter-implant interval having the chance of a second cochlear implant.

Bilateral cochlear implants in children: Effects of auditory experience and deprivation on auditory perception

Spatial hearing skills are essential for children as they grow, learn and play. These skills provide critical cues for determining the locations of sources in the environment, and enable segregation of important sounds, such as speech, from background maskers or interferers. Spatial hearing depends on availability of monaural cues and binaural cues. The latter result from integration of inputs arriving at the two ears from sounds that vary in location. The binaural system has exquisite mechanisms for capturing differences between the ears in both time of arrival and intensity. The major cues that are thus referred to as being vital for binaural hearing are: interaural differences in time (ITDs) and interaural differences in levels (ILDs). In children with normal hearing (NH), spatial hearing abilities are fairly well developed by age 4-5 years. In contrast, most children who are deaf and hear through cochlear implants (CIs) do not have an opportunity to experience normal, binaural acoustic hearing early in life. These children may function by having to utilize auditory cues that are degraded with regard to numerous stimulus features. In recent years there has been a notable increase in the number of children receiving bilateral CIs, and evidence suggests that while having two CIs helps them function better than when listening through a single CI, these children generally perform worse than their NH peers. This paper reviews some of the recent work on bilaterally implanted children. The focus is on measures of spatial hearing, including sound localization, release from masking for speech understanding in noise and binaural sensitivity using research processors. Data from behavioral and electrophysiological studies are included, with a focus on the recent work of the authors and their collaborators. The effects of auditory plasticity and deprivation on the emergence of binaural and spatial hearing are discussed along with evidence for reorganized processing from both behavioral and electrophysiological studies. The consequences of both unilateral and bilateral auditory deprivation during development suggest that the relevant set of issues is highly complex with regard to successes and the limitations experienced by children receiving bilateral cochlear implants. This article is part of a Special Issue entitled .

The effect of device use after sequential bilateral cochlear implantation in children: An electrophysiological approach

OBJECTIVES

In many studies evaluating the effect of sequential bilateral cochlear implantation in congenitally deaf children, device use is not taken into account. In this study, however, device use was analyzed in relation to auditory brainstem maturation and speech recognition, which were measured in children with early-onset deafness, 5-6 years after bilateral cochlear implantation. We hypothesized that auditory brainstem maturation is mostly functionally driven by auditory stimulation and is therefore influenced by device use and not mainly by inter-implant delay.

METHODS

Twenty-one children participated and had inter-implant delays between 1.2 and 7.2 years. The electrically-evoked auditory brainstem response was measured for both implants separately. The difference in interaural wave V latency and speech recognition between both implants were used in the analyses. Device use was measured with a Likert scale.

RESULTS

Results showed that the less the second device is used, the larger the difference in interaural wave V latencies is, which consequently leads to larger differences in interaural speech recognition.

CONCLUSIONS

In children with early-onset deafness, after various periods of unilateral deprivation, full-time device use can lead to similar auditory brainstem responses and speech recognition between both ears. Therefore, device use should be considered as a relevant factor contributing to outcomes after sequential bilateral cochlear implantation. These results are indicative for a longer window between implantations in children with early-onset deafness to obtain symmetrical auditory pathway maturation than is mentioned in the literature. Results, however, must be interpreted as preliminary findings as actual device use with data logging was not yet available at the time of the study.

Balancing current levels in children with bilateral cochlear implants using electrophysiological and behavioral measures

Children have benefited from bilateral cochlear implants (CIs) over unilateral CIs despite often missing important periods in bilateral auditory development. This suggests a remarkable perceptual ability by children to "work around" abnormal changes in the auditory pathways. Nonetheless, these children rely primarily on interaural level differences as interaural timing cues are more difficult to access or detect. Mismatched levels provided to the two implants could distort interaural level cues thus compromising the benefits of bilateral CI use. We asked whether "balanced" or "centered" perception of bilateral input can be predicted by physiological or behavioral measures. Twenty-four children who had used unilateral CIs for 9.21 ± 2.66 years prior to bilateral implantation participated. "Balanced bilateral levels" were identified by responses occurring with a probability of 50% on the right side of the head and 50% on the left in a two choice lateralization task. Loudness judgments of current presented unilaterally by each implant were measured on a continuous visual scale. Maximum wave eV amplitudes were evoked unilaterally by each implant and matched amplitudes were identified. Balanced bilateral levels were predicted within 10 Clinical Units (CU) in 9 of 13 (69%) children using matched wave eV amplitudes. Bilaterally balanced levels were reasonably predicted by similar loudness judgments (<10% difference between CIs) in only 6 of 13 (46%) children. Results indicate that matching amplitudes of physiological responses can produce a balanced perception of bilateral input despite unilateral strengthening of the auditory pathways and can potentially be used clinically to provide a first approximation of balance/centered levels.

Monaural Congenital Deafness Affects Aural Dominance and Degrades Binaural Processing

Cortical development extensively depends on sensory experience. Effects of congenital monaural and binaural deafness on cortical aural dominance and representation of binaural cues were investigated in the present study. We used an animal model that precisely mimics the clinical scenario of unilateral cochlear implantation in an individual with single-sided congenital deafness. Multiunit responses in cortical field A1 to cochlear implant stimulation were studied in normal-hearing cats, bilaterally congenitally deaf cats (CDCs), and unilaterally deaf cats (uCDCs). Binaural deafness reduced cortical responsiveness and decreased response thresholds and dynamic range. In contrast to CDCs, in uCDCs, cortical responsiveness was not reduced, but hemispheric-specific reorganization of aural dominance and binaural interactions were observed. Deafness led to a substantial drop in binaural facilitation in CDCs and uCDCs, demonstrating the inevitable role of experience for a binaural benefit. Sensitivity to interaural time differences was more reduced in uCDCs than in CDCs, particularly at the hemisphere ipsilateral to the hearing ear. Compared with binaural deafness, unilateral hearing prevented nonspecific reduction in cortical responsiveness, but extensively reorganized aural dominance and binaural responses. The deaf ear remained coupled with the cortex in uCDCs, demonstrating a significant difference to deprivation amblyopia in the visual system.

Prevalence and audiological profiles of GJB2 mutations in a large collective of hearing impaired patients

Mutations in the GJB2 gene are known to represent the commonest cause of hereditary and congenital hearing loss. In this study, a complete sequencing of the GJB2 gene in a cohort of 506 patients from a single, large cochlear implant program in Europe was performed. Audiological testing for those patients who could actively participate was performed using pure tone audiometry (PTA). Those unable to undergo PTA were measured using click-auditory brainstem response (ABR). Data analysis was performed to determine genotype-phenotype correlations of the mutational status vs. audiological profiles and vs. age at the time of presentation. An overall prevalence of biallelic mutations of 13.4% was found for the total collective. When subsets of younger patients were examined, the prevalence increased to 27% of those up to age 18 and 35% of those up to age 5 at the time of testing, respectively. This increase was found to be highly significant (p < 0.001). Analysis of the mean PTA thresholds revealed a strong correlation between allele combination status and mean PTA (p = 0.021). The prevalence of simple heterozygotes was found to be approximately 10.1%, which is around 3.3 times the value expected in the general population. As GJB2 follows a recessive pattern of inheritance, the question arises as to why such a large fraction of simple heterozygotes was observed among the hearing impaired patients included in this study.

Early unilateral cochlear implantation promotes mature cortical asymmetries in adolescents who are deaf

Unilateral cochlear implant (CI) stimulation establishes hearing to children who are deaf but compromises bilateral auditory development if a second implant is not provided within ∼ 1.5 years. In this study we asked: 1) What are the cortical consequences of missing this early sensitive period once children reach adolescence? 2) What are the effects of unilateral deprivation on the pathways from the opposite ear? Cortical responses were recorded from 64-cephalic electrodes within the first week of bilateral CI activation in 34 adolescents who had over 10 years of unilateral right CI experience and in 16 normal hearing peers. Cortical activation underlying the evoked peaks was localized to areas of the brain using beamformer imaging. The first CI evoked activity which was more strongly lateralized to the contralateral left hemisphere than normal, with abnormal recruitment of the left prefrontal cortex (involved in cognition/attention), left temporo-parietal-occipital junction (multi-modal integration), and right precuneus (visual processing) region. CI stimulation in the opposite deprived ear evoked atypical cortical responses with abnormally large and widespread dipole activity across the cortex. Thus, using a unilateral CI to hear beyond the period of cortical maturation causes lasting asymmetries in the auditory system, requires recruitment of additional cortical areas to support hearing, and does little to protect the unstimulated pathways from effects of auditory deprivation. The persistence of this reorganization into maturity could signal a closing of a sensitive period for promoting auditory development on the deprived side.

Mental health problems in adolescents with cochlear implants: peer problems persist after controlling for additional handicaps

The aims of the present multi-center study were to investigate the extent of mental health problems in adolescents with a hearing loss and cochlear implants (CIs) in comparison to normal hearing (NH) peers and to investigate possible relations between the extent of mental health problems of young CI users and hearing variables, such as age at implantation, or functional gain of CI. The survey included 140 adolescents with CI (mean age = 14.7, SD = 1.5 years) and 140 NH adolescents (mean age = 14.8, SD = 1.4 years), their parents and teachers. Participants were matched by age, gender and social background. Within the CI group, 35 adolescents were identified as “risk cases” due to possible and manifest additional handicaps, and 11 adolescents were non-classifiable. Mental health problems were assessed with the Strengths and Difficulties Questionnaire (SDQ) in the versions “Self,” “Parent,” and “Teacher.” The CI group showed significantly more “Peer Problems” than the NH group. When the CI group was split into a “risk-group” (35 “risk cases” and 11 non-classifiable persons) and a “non-risk group” (n = 94), increased peer problems were perceived in both CI subgroups by adolescents themselves. However, no further differences between the CI non-risk group and the NH group were observed in any rater. The CI risk-group showed significantly more hyperactivity compared to the NH group and more hyperactivity and conduct problems compared to the CI non-risk group. Cluster analyses confirmed that there were significantly more adolescents with high problems in the CI risk-group compared to the CI non-risk group and the NH group. Adolescents with CI, who were able to understand speech in noise had significantly less difficulties compared to constricted CI users. Parents, teachers, and clinicians should be aware that CI users with additionally special needs may have mental health problems. However, peer problems were also experienced by CI adolescents without additional handicaps.

Asymmetric Hearing During Development: The Aural Preference Syndrome and Treatment Options

Deafness affects ∼2 in 1000 children and is one of the most common congenital impairments. Permanent hearing loss can be treated by fitting hearing aids. More severe to profound deafness is an indication for cochlear implantation. Although newborn hearing screening programs have increased the identification of asymmetric hearing loss, parents and caregivers of children with single-sided deafness are often hesitant to pursue therapy for the deaf ear. Delayed intervention has consequences for recovery of hearing. It has long been reported that asymmetric hearing loss/single-sided deafness compromises speech and language development and educational outcomes in children. Recent studies in animal models of deafness and in children consistently show evidence of an "aural preference syndrome" in which single-sided deafness in early childhood reorganizes the developing auditory pathways toward the hearing ear, with weaker central representation of the deaf ear. Delayed therapy consequently compromises benefit for the deaf ear, with slow rates of improvement measured over time. Therefore, asymmetric hearing needs early identification and intervention. Providing early effective stimulation in both ears through appropriate fitting of auditory prostheses, including hearing aids and cochlear implants, within a sensitive period in development has a cardinal role for securing the function of the impaired ear and for restoring binaural/spatial hearing. The impacts of asymmetric hearing loss on the developing auditory system and on spoken language development have often been underestimated. Thus, the traditional minimalist approach to clinical management aimed at 1 functional ear should be modified on the basis of current evidence.

Strengthening of hearing ear representation reduces binaural sensitivity in early single-sided deafness

Single-sided deafness initiates extensive adaptations in the central auditory system, with the consequence that a stronger and a weaker ear representation develops in the auditory brain. Animal studies demonstrated that the effects are substantially stronger if the condition starts early in development. Sequential binaural cochlear implantations with longer interimplant delays demonstrate that the speech comprehension at the weaker ear is substantially compromised. A pronounced loss of the ability to extract and represent binaural localisation cues accompanies this condition, as shown in animal models.

Long-term outcomes on spatial hearing, speech recognition and receptive vocabulary after sequential bilateral cochlear implantation in children

Sequential bilateral cochlear implantation in profoundly deaf children often leads to primary advantages in spatial hearing and speech recognition. It is not yet known how these children develop in the long-term and if these primary advantages will also lead to secondary advantages, e.g. in better language skills. The aim of the present longitudinal cohort study was to assess the long-term effects of sequential bilateral cochlear implantation in children on spatial hearing, speech recognition in quiet and in noise and receptive vocabulary. Twenty-four children with bilateral cochlear implants (BiCIs) were tested 5-6 years after sequential bilateral cochlear implantation. These children received their second implant between 2.4 and 8.5 years of age. Speech and language data were also gathered in a matched reference group of 26 children with a unilateral cochlear implant (UCI). Spatial hearing was assessed with a minimum audible angle (MAA) task with different stimulus types to gain global insight into the effective use of interaural level difference (ILD) and interaural timing difference (ITD) cues. In the long-term, children still showed improvements in spatial acuity. Spatial acuity was highest for ILD cues compared to ITD cues. For speech recognition in quiet and noise, and receptive vocabulary, children with BiCIs had significant higher scores than children with a UCI. Results also indicate that attending a mainstream school has a significant positive effect on speech recognition and receptive vocabulary compared to attending a school for the deaf. Despite of a period of unilateral deafness, children with BiCIs, participating in mainstream education obtained age-appropriate language scores.

Perception of binaural cues develops in children who are deaf through bilateral cochlear implantation

There are significant challenges to restoring binaural hearing to children who have been deaf from an early age. The uncoordinated and poor temporal information available from cochlear implants distorts perception of interaural timing differences normally important for sound localization and listening in noise. Moreover, binaural development can be compromised by bilateral and unilateral auditory deprivation. Here, we studied perception of both interaural level and timing differences in 79 children/adolescents using bilateral cochlear implants and 16 peers with normal hearing. They were asked on which side of their head they heard unilaterally or bilaterally presented click- or electrical pulse- trains. Interaural level cues were identified by most participants including adolescents with long periods of unilateral cochlear implant use and little bilateral implant experience. Interaural timing cues were not detected by new bilateral adolescent users, consistent with previous evidence. Evidence of binaural timing detection was, for the first time, found in children who had much longer implant experience but it was marked by poorer than normal sensitivity and abnormally strong dependence on current level differences between implants. In addition, children with prior unilateral implant use showed a higher proportion of responses to their first implanted sides than children implanted simultaneously. These data indicate that there are functional repercussions of developing binaural hearing through bilateral cochlear implants, particularly when provided sequentially; nonetheless, children have an opportunity to use these devices to hear better in noise and gain spatial hearing.

Benefit of a contralateral routing of signal device for unilateral cochlear implant users

OBJECTIVE

To investigate objective and subjective effects of an adjunctive contralateral routing of signal (CROS) device at the untreated ear in patients with a unilateral cochlear implant (CI).

DESIGN

Prospective study of 10 adult experienced unilateral CI users with bilateral severe-to-profound hearing loss. Speech in noise reception (SNR) and sound localization were measured with and without the additional CROS device. SNR was measured by applying speech signals at the untreated/CROS side while noise signals came from the front (S90N0). For S0N90, signal sources were switched. Sound localization was measured in a 12-loudspeaker full circle setup. To evaluate the subjective benefit, patients tried the device for 2 weeks at home, then filled out the abbreviated Speech, Spatial and Qualities of Hearing Scale as well as the Bern benefit in single-sided deafness questionnaires.

RESULTS

In the setting S90N0, all patients showed a highly significant SNR improvement when wearing the additional CROS device (mean 6.4 dB, p < 0.001). In the unfavorable setting S0N90, only a minor deterioration of speech understanding was noted (mean -0.66 dB, p = 0.54). Sound localization did not improve substantially with CROS. In the two questionnaires, 12 of 14 items showed an improvement in mean values, but none of them was statistically significant.

CONCLUSION

Patients with unilateral CI benefit from a contralateral CROS device, particularly in a noisy environment, when speech comes from the CROS ear side.

Revisiting the adaptive and maladaptive effects of crossmodal plasticity

One of the most striking demonstrations of experience-dependent plasticity comes from studies of sensory-deprived individuals (e.g., blind or deaf), showing that brain regions deprived of their natural inputs change their sensory tuning to support the processing of inputs coming from the spared senses. These mechanisms of crossmodal plasticity have been traditionally conceptualized as having a double-edged sword effect on behavior. On one side, crossmodal plasticity is conceived as adaptive for the development of enhanced behavioral skills in the remaining senses of early-deaf or blind individuals. On the other side, crossmodal plasticity raises crucial challenges for sensory restoration and is typically conceived as maladaptive since its presence may prevent optimal recovery in sensory-re-afferented individuals. In the present review we stress that this dichotomic vision is oversimplified and we emphasize that the notions of the unavoidable adaptive/maladaptive effects of crossmodal reorganization for sensory compensation/restoration may actually be misleading. For this purpose we critically review the findings from the blind and deaf literatures, highlighting the complementary nature of these two fields of research. The integrated framework we propose here has the potential to impact on the way rehabilitation programs for sensory recovery are carried out, with the promising prospect of eventually improving their final outcomes.

Developmental plasticity of spatial hearing following asymmetric hearing loss: context-dependent cue integration and its clinical implications

Under normal hearing conditions, comparisons of the sounds reaching each ear are critical for accurate sound localization. Asymmetric hearing loss should therefore degrade spatial hearing and has become an important experimental tool for probing the plasticity of the auditory system, both during development and adulthood. In clinical populations, hearing loss affecting one ear more than the other is commonly associated with otitis media with effusion, a disorder experienced by approximately 80% of children before the age of two. Asymmetric hearing may also arise in other clinical situations, such as after unilateral cochlear implantation. Here, we consider the role played by spatial cue integration in sound localization under normal acoustical conditions. We then review evidence for adaptive changes in spatial hearing following a developmental hearing loss in one ear, and show that adaptation may be achieved either by learning a new relationship between the altered cues and directions in space or by changing the way different cues are integrated in the brain. We next consider developmental plasticity as a source of vulnerability, describing maladaptive effects of asymmetric hearing loss that persist even when normal hearing is provided. We also examine the extent to which the consequences of asymmetric hearing loss depend upon its timing and duration. Although much of the experimental literature has focused on the effects of a stable unilateral hearing loss, some of the most common hearing impairments experienced by children tend to fluctuate over time. We therefore propose that there is a need to bridge this gap by investigating the effects of recurring hearing loss during development, and outline recent steps in this direction. We conclude by arguing that this work points toward a more nuanced view of developmental plasticity, in which plasticity may be selectively expressed in response to specific sensory contexts, and consider the clinical implications of this.

Unilateral hearing during development: hemispheric specificity in plastic reorganizations

The present study investigates the hemispheric contributions of neuronal reorganization following early single-sided hearing (unilateral deafness). The experiments were performed on ten cats from our colony of deaf white cats. Two were identified in early hearing screening as unilaterally congenitally deaf. The remaining eight were bilaterally congenitally deaf, unilaterally implanted at different ages with a cochlear implant. Implanted animals were chronically stimulated using a single-channel portable signal processor for two to five months. Microelectrode recordings were performed at the primary auditory cortex under stimulation at the hearing and deaf ear with bilateral cochlear implants. Local field potentials (LFPs) were compared at the cortex ipsilateral and contralateral to the hearing ear. The focus of the study was on the morphology and the onset latency of the LFPs. With respect to morphology of LFPs, pronounced hemisphere-specific effects were observed. Morphology of amplitude-normalized LFPs for stimulation of the deaf and the hearing ear was similar for responses recorded at the same hemisphere. However, when comparisons were performed between the hemispheres, the morphology was more dissimilar even though the same ear was stimulated. This demonstrates hemispheric specificity of some cortical adaptations irrespective of the ear stimulated. The results suggest a specific adaptation process at the hemisphere ipsilateral to the hearing ear, involving specific (down-regulated inhibitory) mechanisms not found in the contralateral hemisphere. Finally, onset latencies revealed that the sensitive period for the cortex ipsilateral to the hearing ear is shorter than that for the contralateral cortex. Unilateral hearing experience leads to a functionally-asymmetric brain with different neuronal reorganizations and different sensitive periods involved.