Delayed maturation and sensitive periods in the auditory cortex

Reorganization of the connectivity of cortical field DZ in congenitally deaf cat

Psychophysics and brain imaging studies in deaf patients have revealed a functional crossmodal reorganization that affects the remaining sensory modalities. Similarly, the congenital deaf cat (CDC) shows supra-normal visual skills that are supported by specific auditory fields (DZ-dorsal zone and P-posterior auditory cortex) but not the primary auditory cortex (A1). To assess the functional reorganization observed in deafness we analyzed the connectivity pattern of the auditory cortex by means of injections of anatomical tracers in DZ and A1 in both congenital deaf and normally hearing cats. A quantitative analysis of the distribution of the projecting neurons revealed the presence of non-auditory inputs to both A1 and DZ of the CDC which were not observed in the hearing cats. Firstly, some visual (areas 19/20) and somatosensory (SIV) areas were projecting toward DZ of the CDC but not in the control. Secondly, A1 of the deaf cat received a weak projection from the visual lateral posterior nuclei (LP). Most of these abnormal projections to A1 and DZ represent only a small fraction of the normal inputs to these areas. In addition, most of the afferents to DZ and A1 appeared normal in terms of areal specificity and strength of projection, with preserved but smeared nucleotopic gradient of A1 in CDCs. In conclusion, while the abnormal projections revealed in the CDC can participate in the crossmodal compensatory mechanisms, the observation of a limited reorganization of the connectivity pattern of the CDC implies that functional reorganization in congenital deafness is further supported also by normal cortico-cortical connectivity.

Direct recordings from the auditory cortex in a cochlear implant user

Electrical stimulation of the auditory nerve with a cochlear implant (CI) is the method of choice for treatment of severe-to-profound hearing loss. Understanding how the human auditory cortex responds to CI stimulation is important for advances in stimulation paradigms and rehabilitation strategies. In this study, auditory cortical responses to CI stimulation were recorded intracranially in a neurosurgical patient to examine directly the functional organization of the auditory cortex and compare the findings with those obtained in normal-hearing subjects. The subject was a bilateral CI user with a 20-year history of deafness and refractory epilepsy. As part of the epilepsy treatment, a subdural grid electrode was implanted over the left temporal lobe. Pure tones, click trains, sinusoidal amplitude-modulated noise, and speech were presented via the auxiliary input of the right CI speech processor. Additional experiments were conducted with bilateral CI stimulation. Auditory event-related changes in cortical activity, characterized by the averaged evoked potential and event-related band power, were localized to posterolateral superior temporal gyrus. Responses were stable across recording sessions and were abolished under general anesthesia. Response latency decreased and magnitude increased with increasing stimulus level. More apical intracochlear stimulation yielded the largest responses. Cortical evoked potentials were phase-locked to the temporal modulations of periodic stimuli and speech utterances. Bilateral electrical stimulation resulted in minimal artifact contamination. This study demonstrates the feasibility of intracranial electrophysiological recordings of responses to CI stimulation in a human subject, shows that cortical response properties may be similar to those obtained in normal-hearing individuals, and provides a basis for future comparisons with extracranial recordings.

Feline deafness

Cats have among the best hearing of all mammals in that they are extremely sensitive to a broad range of frequencies. The ear is a highly complex structure that is delicately balanced in terms of its biochemistry, types of receptors, ion channels, mechanical properties, and cellular organization. Sensorineural deafness is caused by "flawed" genes that are inherited from one or both parents. Hearing loss can also be acquired as a result of noise trauma from industrialized environment, viral infection, or blunt trauma. To date, it is not practical to intervene and attempt to correct these forms of deafness in cats.

Critical period for acoustic preference in mice

Preference behaviors are often established during early life, but the underlying neural circuit mechanisms remain unknown. Adapting a unique nesting behavior assay, we confirmed a "critical period" for developing music preference in C57BL/6 mice. Early music exposure between postnatal days 15 and 24 reversed their innate bias for silent shelter, which typically could not be altered in adulthood. Instead, exposing adult mice treated acutely with valproic acid or carrying a targeted deletion of the Nogo receptor (NgR(-/-)) unmasked a strong plasticity of preference consistent with a reopening of the critical period as seen in other systems. Imaging of cFos expression revealed a prominent neuronal activation in response to the exposed music in the prelimbic and infralimbic medial prefrontal cortex only under conditions of open plasticity. Neither behavioral changes nor selective medial prefrontal cortex activation was observed in response to pure tone exposure, indicating a music-specific effect. Open-field center crossings were increased concomitant with shifts in music preference, suggesting a potential anxiolytic effect. Thus, music may offer both a unique window into the emotional state of mice and a potentially efficient assay for molecular "brakes" on critical period plasticity common to sensory and higher order brain areas.

Plasticity in the developing auditory cortex: evidence from children with sensorineural hearing loss and auditory neuropathy spectrum disorder

The developing auditory cortex is highly plastic. As such, the cortex is both primed to mature normally and at risk for reorganizing abnormally, depending upon numerous factors that determine central maturation. From a clinical perspective, at least two major components of development can be manipulated: (1) input to the cortex and (2) the timing of cortical input. Children with sensorineural hearing loss (SNHL) and auditory neuropathy spectrum disorder (ANSD) have provided a model of early deprivation of sensory input to the cortex and demonstrated the resulting plasticity and development that can occur upon introduction of stimulation. In this article, we review several fundamental principles of cortical development and plasticity and discuss the clinical applications in children with SNHL and ANSD who receive intervention with hearing aids and/or cochlear implants.

The impact of early identification of permanent childhood hearing impairment on speech and language outcomes

It is well established that permanent childhood hearing impairment (PCHI) has a detrimental impact on speech and language development. The past two decades have seen the gradual introduction of universal newborn hearing screening (UNHS) programmes coupled with early intervention programmes. We review studies that have capitalised on the advent of newborn hearing screening to assess the impact of early identification of PCHI on language outcomes in deaf children. The research supports the conclusion that, in children with PCHI, newborn hearing screening and early identification lead to beneficial effects on language development, with the most consistent evidence provided for links between early identification of PCHI and positive language outcomes. Future research needs to encompass a wider range of outcomes and to assess the impact of UNHS in adolescents and young adults.

A sensitive period for musical training: contributions of age of onset and cognitive abilities

The experiences we engage in during childhood can stay with us well into our adult years. The idea of a sensitive period--a window during maturation when our brains are most influenced by behavior--has been proposed. Work from our laboratory has shown that early-trained musicians (ET) performed better on visual-motor and auditory-motor synchronization tasks than late-trained musicians (LT), even when matched for total musical experience. Although the groups of musicians showed no cognitive differences, working memory scores correlated with task performance. In this study, we have replicated these findings in a larger sample of musicians and included a group of highly educated nonmusicians (NM). Participants performed six woodblock rhythms of varying levels of metrical complexity and completed cognitive subtests measuring verbal abilities, working memory, and pattern recognition. Working memory scores correlated with task performance across all three groups. Interestingly, verbal abilities were stronger among the NM, while nonverbal abilities were stronger among musicians. These findings are discussed in context of the sensitive period hypothesis as well as the debate surrounding cognitive differences between musicians and NM.

A sensitive period for cochlear implantation in deaf children

The absence of hearing, as in congenital deafness, affects normal development of the auditory brain resulting in deficits in spoken language. Cochlear implants provide direct stimulation to the central auditory nervous system of hearing impaired children allowing cortical development to progress. However, implantation needs to take place within a brief window in early childhood for it to be maximally effective to allow children to acquire speech and oral language. In this review, we describe age cut-offs for a sensitive period for central auditory development in children who receive cochlear implants. We review consequences for cortical re-organization and cortical de-coupling when children receive cochlear implants after the end of the sensitive period.

Developmental neuroplasticity after cochlear implantation

Cortical development is dependent on stimulus-driven learning. The absence of sensory input from birth, as occurs in congenital deafness, affects normal growth and connectivity needed to form a functional sensory system, resulting in deficits in oral language learning. Cochlear implants bypass cochlear damage by directly stimulating the auditory nerve and brain, making it possible to avoid many of the deleterious effects of sensory deprivation. Congenitally deaf animals and children who receive implants provide a platform to examine the characteristics of cortical plasticity in the auditory system. In this review, we discuss the existence of time limits for, and mechanistic constraints on, sensitive periods for cochlear implantation and describe the effects of multimodal and cognitive reorganization that result from long-term auditory deprivation.

Synaptic morphology and the influence of auditory experience

The auditory experience is crucial for the normal development and maturation of brain structure and the maintenance of the auditory pathways. The specific aims of this review are (i) to provide a brief background of the synaptic morphology of the endbulb of Held in hearing and deaf animals; (ii) to argue the importance of this large synaptic ending in linking neural activity along ascending pathways to environmental acoustic events; (iii) to describe how the re-introduction of electrical activity changes this synapse; and (iv) to examine how changes at the endbulb synapse initiate trans-synaptic changes in ascending auditory projections to the superior olivary complex, the inferior complex, and the auditory cortex.

Bilateral simultaneous cochlear implantation in children: report of a case and review of literature

The benefits of bilateral cochlear implant in adults are well established. Auditory ability in children is also substantially improved by binaural hearing. We report the first case of bilateral simultaneous paediatric cochlear implant in India and discuss the merits of bilateral implant.

The impact of perinatal stress on the functional maturation of prefronto-cortical synaptic circuits: implications for the pathophysiology of ADHD?

Enriched as well as impoverished or adverse perinatal environment plays an essential role in the development and refinement of neuronal pathways, which are the neural substrate of intellectual capacity and socioemotional competence. Perinatal experience and learning events continuously interact with the adaptive shaping of excitatory, inhibitory, and neuromodulatory synaptic as well as the endocrine stress systems, including the neuronal corticotropin-releasing factor (CRF) pathways. Adverse environments, such as stress and emotional deprivation can not only delay experience-dependent maturation of these pathways, but also induce permanent changes in prefronto-cortical wiring patterns. We assume that such dysfunctional connections are the neuronal basis for the development of psychosocially induced mental disorders during later life. The aim of this review is to focus on the impact of perinatal stress on the neuronal and synaptic reorganization during brain development and possible implications for the etiology and therapy of mental disorders such as ADHD.

Interdisciplinary approach to design, performance, and quality management in a multicenter newborn hearing screening project: introduction, methods, and results of the newborn hearing screening in Hamburg (Part I)

From the actual point of view, the "sensitive period" for the effects of hearing impairment on speech and language development is within the first year of life. Early exposure to acoustic or electric stimulation can compensate for the acoustic deficit. A regional-based, specifically designed concept of a universal newborn hearing screening (UNHS) was started in Hamburg in the year 2002. For the first time in Germany, a comprehensive protocol including screening measurement, follow-up procedures, tracking, and early intervention was implemented. An interdisciplinary approach from the very beginning could be realized. Sixty-three thousand, four hundred fifty-nine out of 65,466 births were registered during the period August 2002 to July 2006, 93% were primarily screened. 3.3% failed the test and 31.3% were lost to follow-up. A total of 118 children were diagnosed with hearing loss in the follow-up. The median age at time of diagnosis was 3.5 months. Seventy-four children received hearing aids. Out of these 74 children, 6 were subsequently supplied with cochlear implants. The high lost-to-follow-up rate is the biggest challenge for the tracking. Our results will be discussed in part II.

Bilateral effects of unilateral cochlear implantation in congenitally deaf cats

Congenital deafness results in synaptic abnormalities in auditory nerve endings. These abnormalities are most prominent in terminals called endbulbs of Held, which are large, axosomatic synaptic endings whose size and evolutionary conservation emphasize their importance. Transmission jitter, delay, or failures, which would corrupt the processing of timing information, are possible consequences of the perturbations at this synaptic junction. We sought to determine whether electrical stimulation of the congenitally deaf auditory system via cochlear implants would restore the endbulb synapses to their normal morphology. Three and 6-month-old congenitally deaf cats received unilateral cochlear implants and were stimulated for a period of 10-19 weeks by using human speech processors. Implanted cats exhibited acoustic startle responses and were trained to approach their food dish in response to a specific acoustic stimulus. Endbulb synapses were examined by using serial section electron microscopy from cohorts of cats with normal hearing, congenital deafness, or congenital deafness with a cochlear implant. Synapse restoration was evident in endbulb synapses on the stimulated side of cats implanted at 3 months of age but not at 6 months. In the young implanted cats, postsynaptic densities exhibited normal size, shape, and distribution, and synaptic vesicles had density values typical of hearing cats. Synapses of the contralateral auditory nerve in early implanted cats also exhibited synapses with more normal structural features. These results demonstrate that electrical stimulation with a cochlear implant can help preserve central auditory synapses through direct and indirect pathways in an age-dependent fashion.

The influence of a sensitive period for auditory-visual integration in children with cochlear implants

PURPOSE

Children who experience long periods of auditory deprivation are susceptible to large-scale reorganization of auditory cortical areas responsible for the perception of speech and language. One consequence of this reorganization is that integration of combined auditory and visual information may be altered after hearing is restored with a cochlear implant. Our goal was to investigate the effects of reorganization in a task that examines performance during multisensory integration.

METHODS

Reaction times to the detection of basic auditory (A), visual (V), and combined auditory-visual (AV) stimuli were examined in a group of normally hearing children, and in two groups of cochlear implanted children: (1) early implanted children in whom cortical auditory evoked potentials (CAEPs) fell within normal developmental limits, and (2) late implanted children in whom CAEPs were outside of normal developmental limits. Miller's test of the race model inequality was performed for each group in order to examine the effects of auditory deprivation on multisensory integration abilities after implantation.

RESULTS

Results revealed a significant violation of the race model inequality in the normally hearing and early implanted children, but not in the group of late implanted children.

CONCLUSION

These results suggest that coactivation to multi-modal sensory input cannot explain the decreased reaction times to multi-modal input in late implanted children. These results are discussed in regards to current models for coactivation to redundant sensory information.

Postnatal Maturation of Primary Auditory Cortex in the Mustached Bat, Pteronotus parnellii

The primary auditory cortex (AI) of adult Pteronotus parnellii features a foveal representation of the second harmonic constant frequency (CF2) echolocation call component. In the corresponding Doppler-shifted constant frequency (DSCF) area, the 61 kHz range is over-represented for extraction of frequency-shift information in CF2 echoes. To assess to which degree AI postnatal maturation depends on active echolocation or/and reflects ongoing cochlear maturation, cortical neurons were recorded in juveniles up to postnatal day P29, before the bats are capable of active foraging. At P1-2, neurons in posterior AI are tuned sensitively to low frequencies (22–45 dB SPL, 28–35 kHz). Within the prospective DSCF area, neurons had insensitive responses (>60 dB SPL) to frequencies <40 kHz and lacked sensitive tuning curve tips. Up to P10, when bats do not yet actively echolocate, tonotopy is further developed and DSCF neurons respond to frequencies of 51–57 kHz with maximum tuning sharpness ( Q10dB) of 57. Between P11 and 20, the frequency representation in AI includes higher frequencies anterior and dorsal to the DSCF area. More multipeaked neurons (33%) are found than at older age. In the oldest group, DSCF neurons are tuned to frequencies close to 61 kHz with Q10dB values ≤212, and threshold sensitivity, tuning sharpness and cortical latencies are adult-like. The data show that basic aspects of cortical tonotopy are established before the bats actively echolocate. Maturation of tonotopy, increase of tuning sharpness, and upward shift in the characteristic frequency of DSCF neurons appear to strongly reflect cochlear maturation.

Early continuous white noise exposure alters l-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit glutamate receptor 2 and gamma-aminobutyric acid type a receptor subunit beta3 protein expression in rat auditory cortex

Auditory experience during the postnatal critical period is essential for the normal maturation of auditory function. Previous studies have shown that rearing infant rat pups under conditions of continuous moderate-level noise delayed the emergence of adult-like topographic representational order and the refinement of response selectivity in the primary auditory cortex (A1) beyond normal developmental benchmarks and indefinitely blocked the closure of a brief, critical-period window. To gain insight into the molecular mechanisms of these physiological changes after noise rearing, we studied expression of the AMPA receptor subunit GluR2 and GABA(A) receptor subunit beta3 in the auditory cortex after noise rearing. Our results show that continuous moderate-level noise rearing during the early stages of development decreases the expression levels of GluR2 and GABA(A)beta3. Furthermore, noise rearing also induced a significant decrease in the level of GABA(A) receptors relative to AMPA receptors. However, in adult rats, noise rearing did not have significant effects on GluR2 and GABA(A)beta3 expression or the ratio between the two units. These changes could have a role in the cellular mechanisms involved in the delayed maturation of auditory receptive field structure and topographic organization of A1 after noise rearing.

Characteristics of malfunctioning channels in pediatric cochlear implants

OBJECTIVES/HYPOTHESIS

To examine the characteristics of pediatric cochlear implant channel malfunction preceding device failure.

STUDY DESIGN

: Retrospective review.

METHODS

All pediatric patients who underwent cochlear implantation at a tertiary academic medical center were reviewed regarding device type, reason for replacement, time to replacement, and timing and pattern of channel faults in failed versus nonfailed devices.

RESULTS

Between 1993 and 2008, 264 pediatric cochlear implantations were performed. With an average 894-day follow-up, the replacement rate was 9.5% (25/264). Reasons for replacement were device failure (6.4%), medical/surgical failure (2.3%), and obsolescence (0.8%). Replacement rates were comparable among Advanced Bionics (13.3%), Cochlear Corporation (6.3%), and MED-EL (10.3%) devices. Fifty-two cochlear implants developed at least one channel fault, and 13 eventually progressed to failure requiring replacement. MED-EL devices comprised 12 of these 13 failures. At the 12-month follow-up interval, one, three, and five channel faults predicted 40%, 75%, and 100% probabilities of eventual electrode failure, respectively. Channels destined to fail demonstrated small, yet statistically significant, impedance elevations 12 months before failure and large elevations 3 months before failure.

CONCLUSIONS

Replacement of cochlear implants in pediatric patients is common and is due to device malfunction about one half of the time. Earlier initial channel fault, earlier subsequent channel faults, adjacent channel faults, and a greater total number of channel faults were associated with the need for replacement surgery. Elevations in a channel's impedance should raise the concern for an impending failure. These predictors can help the cochlear implant team when considering surgery to replace the device.

The prelexical development in children implanted by 16 months compared with normal hearing children

OBJECTIVE

Few studies exist which document the early speech development of German-speaking children or German-speaking children who are deaf and using cochlear implants. The current study aims to: (1) document the pre-canonical and canonical speech development of German-speaking children who are deaf and receive cochlear implants by the age of 16 months and (2) compare these children's results with those of children with normal hearing.

DESIGN

This longitudinal study included 5 German-speaking children with normal hearing and 5 with sensorineural deafness. All children from the deaf group received hearing amplification before cochlear implantation, received their first implant by 16 months of age, and became bilateral implant users by 31 months of age. The pre-canonical and canonical vocalisations of each child were recorded on video- and audiotapes in a semi-standardised playing situation every 4 weeks over a span of 1 year. In the cochlear implant group, the recording started 4-5 days postoperatively (first implant); in the normal hearing group it began between the ages of 4 and 5 months. The video and audio recordings were analysed using EUDICO Linguistic Annotator version 2.4 (Nijmegen, The Netherlands) and International Phonetic Alphabet transcription.

RESULTS

Both groups showed individual patterns of babbling acquisition, though the groups' patterns of acquisition were similar when analysed for consonant manner and place. Some children started with plosives and others, with nasals, but all acquired fricatives and laterals next. Onset of canonical babbling for children in the cochlear implant group began 0-4 months after first fitting of the first device, while children from the normal hearing group demonstrated an onset of canonical babbling between 4 and 9 months of age.

CONCLUSION

Our results show that deaf children who receive cochlear implants at an early age are capable of reaching the canonical babbling milestone in a shorter time than children with normal hearing typically do and that their consonant phoneme acquisition follows a similar sequence to normal hearing peers'. These results are consistent with the literature indicating that early identification and intervention are important for allowing children with cochlear implants the opportunity to catch up to hearing peers.

Neural prostheses and brain plasticity

The success of modern neural prostheses is dependent on a complex interplay between the devices' hardware and software and the dynamic environment in which the devices operate: the patient's body or 'wetware'. Over 120 000 severe/profoundly deaf individuals presently receive information enabling auditory awareness and speech perception from cochlear implants. The cochlear implant therefore provides a useful case study for a review of the complex interactions between hardware, software and wetware, and of the important role of the dynamic nature of wetware. In the case of neural prostheses, the most critical component of that wetware is the central nervous system. This paper will examine the evidence of changes in the central auditory system that contribute to changes in performance with a cochlear implant, and discuss how these changes relate to electrophysiological and functional imaging studies in humans. The relationship between the human data and evidence from animals of the remarkable capacity for plastic change of the central auditory system, even into adulthood, will then be examined. Finally, we will discuss the role of brain plasticity in neural prostheses in general.

Unimodal and cross-modal plasticity in the ‘deaf’ auditory cortex

Congenital auditory deprivation leads to deficits in the auditory cortex. The present review focuses on central aspects of auditory deprivation: development, plasticity, corticocortical interactions, and cross-modal reorganization. We compile imaging data from human subjects, electroencephalographic data from cochlear implanted children, and animal research on congenital deafness. Behavioral, electroencephalographic, and imaging data in humans correspond well to data behavioral and neurophysiological data obtained from congenitally deaf cats. The available data indicate that auditory deprivation leads to 'decoupling' of the primary auditory cortex from cognitive modulation of higher-order auditory areas. Higher-order auditory areas undergo a strong cross-modal reorganization and take-over new functions. Due to these and other deficits of intrinsic microcircuitry, the cortical column can not integrate bottom-up and top-down influences in deaf auditory cortex. In the ultimate consequence perceptual learning is compromised, resulting in sensitive periods.

Deprivation-induced cortical reorganization in children with cochlear implants

A basic finding in developmental neurophysiology is that some areas of the cortex cortical areas will reorganize following a period of stimulus deprivation. In this review, we discuss mainly electroencephalography (EEG) studies of normal and deprivation-induced abnormal development of the central auditory pathways in children and in animal models. We describe age cut-off for sensitive periods for central auditory development in congenitally deaf children who are fitted with a cochlear implant. We speculate on mechanisms of decoupling and reorganization which may underlie the end of the sensitive period. Finally, we describe new magentoencephalography (MEG) evidence of somatosensory cross-modal plasticity following long-term auditory deprivation.

Cortical development, plasticity and re-organization in children with cochlear implants

A basic tenet of developmental neurobiology is that certain areas of the cortex will re-organize, if appropriate stimulation is withheld for long periods. Stimulation must be delivered to a sensory system within a narrow window of time (a sensitive period) if that system is to develop normally. In this article, we will describe age cut-offs for a sensitive period for central auditory development in children who receive cochlear implants. We will review de-coupling and re-organization of cortical areas, which are presumed to underlie the end of the sensitive period in congenitally deaf humans and cats. Finally, we present two clinical cases which demonstrate the use of the P1 cortical auditory evoked potential as a biomarker for central auditory system development and re-organization in congenitally deaf children fitted with cochlear implants.

LEARNING OUTCOMES

Readers of this article should be able to (i) describe the importance of the sensitive period as it relates to development of central auditory pathways in children with cochlear implants; (ii) discuss the hypothesis of de-coupling of primary from higher-order auditory cortex as it relates to the end of the sensitive period; (iii) discuss cross-modal re-organization which may occur after long periods of auditory deprivation; and (iv) understand the use of the P1 response as a biomarker for development of central auditory pathways.

Tinnitus is prevalent in children with cochlear implants

OBJECTIVES

To explore the prevalence and the perceived impact of tinnitus in children using cochlear implants.

METHOD

Cross-sectional study of implanted children attending a cochlear implant family event organized annually by our academic tertiary pediatric care center. Children were interviewed together with their parents, using open-questioning and structured interview qualitative methodologies. The main outcome measures were the prevalence of tinnitus and any impact of these symptoms.

RESULTS

40 children (age range: 3-15, mean: 7 years) and their families were interviewed. These included unilateral implantees (n=21), and bilateral implantees (n=19) whose two procedures were simultaneous (n=6), within 6-12 months (n=3), or >2 years apart (n=10). Tinnitus was reported by 38% (n=15). Tinnitus occurred most commonly in the implanted ear, when the implants were not in use (e.g. in bed at night). The children were generally untroubled by the tinnitus, although two reported difficulty sleeping. Tinnitus was most frequent in children aged 6-8 years (8/17, 47%), and in bilateral implantees with an inter-procedure delay of at least 2 years (6/10, 60%). Tinnitus was least reported in those implanted bilaterally simultaneously (1/6, 17%), and in those 5 years old or younger (3/11, 27%). No obvious relationship was identified between the prevalence of tinnitus and the etiology of deafness, age of implantation, or time elapsed since implantation.

CONCLUSIONS

To our knowledge this is the first study to report the widespread prevalence of tinnitus in implanted children. Further work, particularly examining the effect of inter-implant delay on tinnitus in bilateral implantees, may contribute to our understanding of the neuronal plasticity after implantation.

[Early hearing experience and sensitive developmental periods]

This article reviews the studies on functional deficits in the auditory cortex of congenitally deaf animals. It compares their results with psychophysical and imaging data obtained from prelingually deaf humans. The studies demonstrate that the development of the auditory cortex is affected by the absence of hearing experience. In humans, the restoration of hearing after congenital deafness shows a sensitive period of 4 years, whereas even within this sensitive period cortical plasticity is already decreasing with increasing age. The reasons for the sensitive period are developmental changes of synaptic plasticity, developmentally modified synaptogenesis and synaptic pruning as well as changes in connectivity of the auditory cortex. Absence of top-down interactions from higher order auditory areas is another cardinal reason for the sensitive period. All these mechanisms contribute to the decreasing capacity for cortical plasticity during postnatal development. From the developmental and neurophysiological point of view, an early identification of hearing loss is an important prerequisite for effective therapy.

Visual skills and cross-modal plasticity in deaf readers: possible implications for acquiring meaning from print

Most research on reading skill acquisition in deaf individuals has been conducted from the perspective of a hearing child learning to read. This approach may limit our understanding of how a deaf child approaches the task of learning to read and successfully acquires reading skills. An alternative approach is to consider how the cognitive skills that a deaf child brings to the reading task may influence the route by which he or she achieves reading fluency. A review of the literature on visual spatial attention suggests that deaf individuals are more distracted by visual information in the parafovea and periphery. We discuss how this may have an influence upon the perceptual processing of written text in deaf students.

Cochlear implant use following neonatal deafness influences the cochleotopic organization of the primary auditory cortex in cats

Electrical stimulation of spiral ganglion neurons in a deafened cochlea, via a cochlear implant, provides a means of investigating the effects of the removal and subsequent restoration of afferent input on the functional organization of the primary auditory cortex (AI). We neonatally deafened 17 cats before the onset of hearing, thereby abolishing virtually all afferent input from the auditory periphery. In seven animals the auditory pathway was chronically reactivated with environmentally derived electrical stimuli presented via a multichannel intracochlear electrode array implanted at 8 weeks of age. Electrical stimulation was provided by a clinical cochlear implant that was used continuously for periods of up to 7 months. In 10 long-term deafened cats and three age-matched normal-hearing controls, an intracochlear electrode array was implanted immediately prior to cortical recording. We recorded from a total of 812 single unit and multiunit clusters in AI of all cats as adults using a combination of single tungsten and multichannel silicon electrode arrays. The absence of afferent activity in the long-term deafened animals had little effect on the basic response properties of AI neurons but resulted in complete loss of the normal cochleotopic organization of AI. This effect was almost completely reversed by chronic reactivation of the auditory pathway via the cochlear implant. We hypothesize that maintenance or reestablishment of a cochleotopically organized AI by activation of a restricted sector of the cochlea, as demonstrated in the present study, contributes to the remarkable clinical performance observed among human patients implanted at a young age.

Language skills of profoundly deaf children who received cochlear implants under 12 months of age: a preliminary study

CONCLUSION

This study demonstrated that children who receive a cochlear implant below the age of 2 years obtain higher mean receptive and expressive language scores than children implanted over the age of 2 years.

OBJECTIVE

The purpose of this study was to compare the receptive and expressive language skills of children who received a cochlear implant before 1 year of age to the language skills of children who received an implant between 1 and 3 years of age.

SUBJECTS AND METHODS

Standardized language measures, the Reynell Developmental Language Scale (RDLS) and the Preschool Language Scale (PLS), were used to assess the receptive and expressive language skills of 91 children who received an implant before their third birthday.

RESULTS

The mean receptive and expressive language scores for the RDLS and the PLS were slightly higher for the children who were implanted below the age of 2 years compared with the children who were implanted over 2 years old. For the PLS, both the receptive and expressive mean standard scores decreased with increasing age at implantation.

Cochlear implants and brain plasticity

Cochlear implants have been implanted in over 110,000 deaf adults and children worldwide and provide these patients with important auditory cues necessary for auditory awareness and speech perception via electrical stimulation of the auditory nerve (AN). In 1942, Woolsey and Walzl presented the first report of cortical responses to localised electrical stimulation of different sectors of the AN in normal hearing cats, and established the cochleotopic organization of the projections to primary auditory cortex. Subsequently, individual cortical neurons in normal hearing animals have been shown to have well characterized input-output functions for electrical stimulation and decreasing response latencies with increasing stimulus strength. However, the central auditory system is not immutable, and has a remarkable capacity for plastic change, even into adulthood, as a result of changes in afferent input. This capacity for change is likely to contribute to the ongoing clinical improvements observed in speech perception for cochlear implant users. This review examines the evidence for changes of the response properties of neurons in, and consequently the functional organization of, the central auditory system produced by chronic, behaviourally relevant, electrical stimulation of the AN in profoundly deaf humans and animals.

Thyroid hormone deficiency affects postnatal spiking activity and expression of Ca2+ and K+ channels in rodent inner hair cells

Thyroid hormone (TH) is essential for the development of hearing. Lack of TH in a critical developmental period from embryonic day 17 to postnatal day 12 (P12) in rats and mice leads to morphological and functional deficits in the organ of Corti and the auditory pathway. We investigated the effects of TH on inner hair cells (IHCs) using patch-clamp recordings, capacitance measurements, and immunocytochemistry in hypothyroid rats and athyroid Pax8-/- mice. Spontaneous and evoked Ca2+ action potentials (APs) were present in control IHCs from P3-P11 rats and vanished in parallel with the expression of a rapidly activating Ca2+- and voltage-activated K+ (BK) conductance. IHCs of hypothyroid rats and athyroid Pax8-/- mice displayed APs until the end of the third postnatal week because of threefold elevated Ca2+ currents and missing expression of BK currents. After the fourth postnatal week, some IHCs showed BK currents whereas adjacent IHCs did not, demonstrated by electrophysiology and immunocytochemistry. To test whether the prolonged spiking activity during TH deficiency may be transmitted at IHC synapses, capacitance measurements were performed in parallel to analysis of otoferlin expression, a protein thought to play an essential role in exocytosis of IHCs. Strikingly, otoferlin was absent from IHCs of hypothyroid rats but not of Pax8-/- mice, although both cell types showed exocytosis with an efficiency typical for immature IHCs. These results demonstrate for the first time a TH-dependent control of IHC spiking activity before the onset of hearing attributable to effects of TH on Ca2+ and BK channels. Moreover, they question an indispensable role of otoferlin for exocytosis in IHCs.

Auditory cortex stimulation for tinnitus

Functional imaging techniques have demonstrated a relationship between the intensity of tinnitus and the degree of reorganization of the primary auditory cortex. Studies in experimental animals and humans have revealed that tinnitus is associated with a synchronized hyperactivity in the auditory cortex and proposed that the underlying pathophysiological mechanism is thalamocortical dysrhythmia; hence, decreased auditory stimulation results in decreased firing rate, and decreased lateral inhibition. Consequently, the surrounding brain area becomes hyperactive, firing at gamma band rates; this is considered a necessary precondition of auditory consciousness, and also tinnitus. Synchronization of the gamma band activity could possibly induce a topographical reorganization based on Hebbian mechanisms. Therefore, it seems logical to try to suppress tinnitus by modifying the tinnitus-related auditory cortex reorganization and hyperactivity. This can be achieved using neuronavigation-guided transcranial magnetic stimulation (TMS), which is capable of modulating cortical activity. If TMS is capable of suppressing tinnitus, the effect should be maintained by implanting electrodes over the area of electrophysiological signal abnormality on the auditory cortex. The results in the first patients treated by auditory cortex stimulation demonstrate a statistically significant tinnitus suppression in cases of unilateral pure tone tinnitus without suppression of white or narrow band noise. Hence, auditory cortex stimulation could become a physiologically guided treatment for a selected category of patients with severe tinnitus.

Do deaf individuals see better?

The possibility that, following early auditory deprivation, the remaining senses such as vision are enhanced has been met with much excitement. However, deaf individuals exhibit both better and worse visual skills than hearing controls. We show that, when deafness is considered to the exclusion of other confounds, enhancements in visual cognition are noted. The changes are not, however, widespread but are selective, limited, as we propose, to those aspects of vision that are attentionally demanding and would normally benefit from auditory-visual convergence. The behavioral changes are accompanied by a reorganization of multisensory areas, ranging from higher-order cortex to early cortical areas, highlighting cross-modal interactions as a fundamental feature of brain organization and cognitive processing.

Electrical resistivity measurements in the mammalian cochlea after neural degeneration

OBJECTIVES/HYPOTHESIS

In the present series of experiments, the effect of neural degeneration on the cochlear structure electrical resistivities was evaluated to test if it alters the current flow in the cochlea and if increased current levels are needed to stimulate the impaired cochlea. In cochlear implants, frequency information is encoded in part by stimulating discrete populations of spiral ganglion cells along the cochlea. However, electrical properties of the cochlear structures result in shunting of the current away from the auditory neurons. This consumes energy, makes cochlear implants less efficient, and drastically reduces battery life. Models of the electrically stimulated cochlea serve to make predictions on current paths using modified and improved cochlear implant electrodes. However, one of the model's shortcomings is that most of the values for tissue impedances are not direct measurements. They are derived from bulk impedance measurements, which are fitted to lumped-element models.

STUDY DESIGN

The four-electrode reflection-coefficient technique was used to measure resistivities in the gerbil cochlea. In vivo and in vitro (the hemicochlea) models were used. Measurements were made in normal and in deafened animals. Cochlear damage was induced by neomycin injection into the animals' middle ears. Neural degeneration was allowed to occur over 2 months before performing the measurements in the deafened animals.

RESULTS

The resistivity values in deafened animals were smaller than in the normal-hearing animals, thus altering the current flow within the cochlea.

CONCLUSIONS

Resistivity changes and subsequent changes in current path should be considered in future designs of cochlear implants.

Abnormalities in central auditory maturation in children with language-based learning problems

OBJECTIVE

To examine maturation of the central auditory pathways in children with language-based learning problems (LP).

METHODS

Cortical auditory evoked potentials (CAEPs) recorded from 26 children with LP were compared to CAEPs recorded from 38 typical children. CAEP responses were recorded in response to a speech sound, /uh/, which was presented in a stimulus train with decreasing inter-stimulus intervals (ISIs) of 2000, 1000, 560, and 360 ms.

RESULTS

We identified three atypical morphological categories of CAEP responses in the LP group. Category 1 responses revealed delayed P1 latencies and absent N1/P2 components. Category 2 responses revealed typical P1 responses, but delayed N1 and P2 responses. Category 3 responses revealed generally low-amplitude CAEP responses. A fourth sub-group of LP children had normal CAEP responses.

CONCLUSIONS

Overall, the majority of children with LP had abnormal CAEP responses. These children fell into distinct categories based on the abnormalities in maturational patterns of their CAEP responses.

SIGNIFICANCE

We describe a rate sensitive stimulation paradigm which may be used to identify and categorize LP children who exhibit abnormal patterns of central auditory maturation.

Morphometric comparison of the human corpus callosum in deaf and hearing subjects: an MRI study

Auditory cortices are interconnected to each other by fibers passing through the corpus callosum (CC). In totally deaf persons no auditory impulses are conveyed to the auditory cortices, hence the auditory pathways become nonfunctional. It was reported that there has been cross-modal plasticity between auditory, visual, and somatosensory cortices. In this study, our aim was to make a comparison in the CC morphometry in hearing subjects and in a selected group in which the auditory system was deprived before the age of 2. 18 deaf and 18 hearing male, handedness matched volunteers, ages varying between 28 and 56 years old were examined. Audiometrical tests were applied to both groups and then T1-weighted midsagittal MR images were obtained. Certain dimensions and areas were measured on these images. There were no statistically significant difference between deaf and hearing subjects, either when dimensions and areas were analyzed by multivariate analysis of variance or when areas were analyzed by univariate analysis of variance. Absence of any significant morphometric difference in the CC of deaf subjects could be thought as an evidence of reflection of functional cortical plasticity.

Effects of cochlear implant use on the electrically evoked middle latency response in children

The electrically evoked middle latency response (eMLR) reflects central auditory activity in cochlear implant users. This response was recorded repeatedly in 50 children over the first year of cochlear implant use and in 31 children with 5.3+/-2.9 years of implant experience. The eMLR was rarely detected at the time of implantation in anaesthetized or sedated children and was detected in only 35% of awake children at initial device stimulation. The detectability of the eMLR increased over the first year of implant use becoming 100% detectable in children after at least one year. Acutely evoked responses were more likely to be present in older children despite longer periods of auditory deprivation. Within six months of implant use, most children had detectable eMLRs. At early stages of device use, eMLR amplitudes were lower in children implanted below the age of 5 years compared to children implanted at older ages; amplitudes increased over time in both groups. Latencies after six months of implant use were prolonged in the younger group and decreased with implant use. EMLR changes with chronic cochlear implant use suggest an activity-dependent plasticity of the central auditory system. Results suggest that the pattern of electrically evoked activity and development in the auditory thalamocortical pathways will be dependent upon the duration of auditory deprivation occurring in early childhood.

Cochlear implants: cortical plasticity in congenital deprivation

Congenital auditory deprivation (deafness) leads to a dysfunctional intrinsic cortical microcircuitry. This chapter reviews these deficits with a particular emphasis on layer-specific activity within the primary auditory cortex. Evidence for a delay in activation of supragranular layers and reduction in activity in infragranular layers is discussed. Such deficits indicate the incompetence of the primary auditory cortex to not only properly process thalamic input and generate output within the infragranular layers, but also incorporate top-down modulations from higher order auditory cortex into the processing within primary auditory cortex. Such deficits are the consequence of a misguided postnatal development. Maturation of primary auditory cortex in deaf animals shows evidence of a developmental delay and further alterations in gross synaptic currents, spread of activation, and morphology of local field potentials recorded at the cortical surface. Additionally, degenerative changes can be observed. When hearing is initiated early in life (e.g., by chronic cochlear-implant stimulation), many of these deficits are counterbalanced. However, plasticity of the auditory cortex decreases with increasing age, so that a sensitive period for plastic adaptation can be demonstrated within the second to sixth months of life in the deaf cat. Potential molecular mechanisms of the existence of sensitive period are discussed. Data from animal research may be compared to electroencephalographic data obtained from cochlear-implanted congenitally deaf children. After cochlear implantation in humans, three phases of plastic adaptation can be observed: a fast one, taking place within the first few weeks after implantation, showing no sensitive period; a slower one, taking place within the first months after implantation (a sensitive period up to 4 years of age); and possibly a third, and the longest one, related to increasing activation of higher order cortical areas.

Development of spectral and temporal response selectivity in the auditory cortex

The mechanisms by which hearing selectivity is elaborated and refined in early development are very incompletely determined. In this study, we documented contributions of progressively maturing inhibitory influences on the refinement of spectral and temporal response properties in the primary auditory cortex. Inhibitory receptive fields (IRFs) of infant rat auditory cortical neurons were spectrally far broader and had extended over far longer duration than did those of adults. The selective refinement of IRFs was delayed relative to that of excitatory receptive fields by an approximately 2-week period that corresponded to the critical period for plasticity. Local application of a GABA(A) receptor antagonist revealed that intracortical inhibition contributes to this progressive receptive field maturation for response selectivity in frequency. Conversely, it had no effect on the duration of IRFs or successive-signal cortical response recovery times. The importance of exposure to patterned acoustic inputs was suggested when both spectral and temporal IRF maturation were disrupted in rat pups reared in continuous, moderate-intensity noise. They were subsequently renormalized when animals were returned to standard housing conditions as adults.

[The cochlear implant. Molecular arguments favouring early implantation]

BACKGROUND

An important factor in the clinical outcome of cochlear implantation is the age of the patient. Compared to older patients, children with congenital deafness have a better outcome when the implantation is made before the age of 2 years. The cause may lie in the molecular biology of the brain, which changes during postnatal maturation.

METHODS

Protein probes were obtained from tissue of the rat inferior colliculus at different ages. The probes were analyzed using 2-dimensional SDS electrophoresis.

RESULTS

The expression of GAP-43, a protein expressed by neurons during axonal outgrowth and synaptogenesis, and the total number of the protein species showed a significant reduction during ontogenesis. This shows that while neurons gradually assume their specific function, they downregulate GAP-43 and the molecular complexity decreases.

CONCLUSIONS

Due to a lack of neuronal pluripotency at later developmental stages, the flexibility to adapt to the afferent activation provided by a cochlear implant is increasingly limited.

Is there a critical period for cochlear implantation in congenitally deaf children? Analyses of hearing and speech perception performance after implantation

A range of basic and applied studies have demonstrated that during the development of the auditory system, early experimental manipulations or clinical interventions are generally more effective than those made later. We present a short review of these studies. We investigated this age-related plasticity in relation to the timing of cochlear implantation in deaf-from-birth children. Cochlear implantation is a standard intervention for providing hearing in children with severe to profound deafness. An important practical question is whether there is a critical period or cutoff age of implantation after which hearing outcomes are significantly reduced. In this article, we present data from prelingually deaf children (mostly congenitally deaf) implanted at ages ranging from 1 to 15 years. Each child was tested with auditory and speech understanding tests before implantation, and at regular intervals up to 8 years postimplantation. We measured the improvement in performance of speech understanding tests in younger implanted children and compared it with the results of those implanted at a later age. We also used a binary partitioning algorithm to divide the data systematically at all ages at implant to determine the optimum split, i.e., to determine the age at implant which best separates performance of early implanted versus later implanted children. We observed distinct age-of-implant cutoffs, and will discuss whether these really represent critical periods during development.

Cochlear implantation in adults with prelingual deafness. Part II. Underlying constraints that affect audiological outcomes

OBJECTIVES/HYPOTHESIS

To discuss the underlying physiological and anatomical constraints on audiological performance of late-implanted prelingually deafened adult cochlear implant patients.

STUDY DESIGN

Retrospective review.

METHODS

Published literature on the topic of auditory pathway responses to prolonged congenital deafness was reviewed. In particular, the authors sought to identify the anatomical and physiological changes that take place in both the peripheral and central auditory pathways in response to prolonged deafness, as well as how they are altered by chronic electrical stimulation.

RESULTS

The currently available evidence suggests that the colonization of the auditory cortex by other sensory modalities is the main limiting factor in postimplantation performance, not the pathological degenerative changes of the auditory nerve, cochlear nucleus, or auditory midbrain.

CONCLUSION

The reviewed evidence, although circumstantial, suggests that emphasizing aurally based educational programs before (with hearing aids) and after cochlear implantation could reduce the cortical colonization phenomenon and potentially improve postimplantation audiological performance of patients with long-term prelingual deafness.

Bilateral cochlear implants: a way to optimize auditory perception abilities in deaf children?

OBJECTIVES

The Würzburg bilateral cochlear implant (CI) program was started with the aim to improve the patients' communicative abilities in complex listening situations. In this study, the auditory skills of children using bilateral cochlear implants were evaluated.

STUDY DESIGN AND SETTINGS

Qualitative data based on free observations in the rehabilitation setup were collected in 39 bilaterally implanted children. A speech discrimination in noise test was performed in 18 of these children; lists of bisyllabic words were presented in noise at a signal to noise ratio (SNR) of +15 dB.

RESULTS

Qualitative and quantitative data show clearly that bilateral CI improves the children's communicative behaviour, especially in complex listening situations. Children examined with the speech in noise test scored significantly better under the bilateral condition compared to the unilateral condition. Integration of the second implanted side and use of binaural information was observed to be easier and faster in children with a short time lag between both implants.

CONCLUSIONS

To be able to obtain optimal benefit from bilateral cochlear implants, an intensive rehabilitation program is necessary. The important aspects of such a program are creating realistic expectations in older children before implantation; performing the first processor fitting of the second side with the first side switched on; and separate intensive training with the new system in order to balance out the hearing competence of the second CI with that of the first.