Plasticity of binaural interaction. II. Critical period and changes in midline response

Auditory processing remains sensitive to environmental experience during adolescence in a rodent model

Elevated neural plasticity during development contributes to dramatic improvements in perceptual, motor, and cognitive skills. However, malleable neural circuits are vulnerable to environmental influences that may disrupt behavioral maturation. While these risks are well-established prior to sexual maturity (i.e., critical periods), the degree of neural vulnerability during adolescence remains uncertain. Here, we induce transient hearing loss (HL) spanning adolescence in gerbils, and ask whether behavioral and neural maturation are disrupted. We find that adolescent HL causes a significant perceptual deficit that can be attributed to degraded auditory cortex processing, as assessed with wireless single neuron recordings and within-session population-level analyses. Finally, auditory cortex brain slices from adolescent HL animals reveal synaptic deficits that are distinct from those typically observed after critical period deprivation. Taken together, these results show that diminished adolescent sensory experience can cause long-lasting behavioral deficits that originate, in part, from a dysfunctional cortical circuit.

Temporary Unilateral Hearing Loss Impairs Spatial Auditory Information Processing in Neurons in the Central Auditory System

Temporary conductive hearing loss (CHL) can lead to hearing impairments that persist beyond resolution of the CHL. In particular, unilateral CHL leads to deficits in auditory skills that rely on binaural input (e.g., spatial hearing). Here, we asked whether single neurons in the auditory midbrain, which integrate acoustic inputs from the two ears, are altered by a temporary CHL. We introduced 6 weeks of unilateral CHL to young adult chinchillas via foam earplug. Following CHL removal and restoration of peripheral input, single-unit recordings from inferior colliculus (ICC) neurons revealed the CHL decreased the efficacy of inhibitory input to the ICC contralateral to the earplug and increased inhibitory input ipsilateral to the earplug, effectively creating a higher proportion of monaural responsive neurons than binaural. Moreover, this resulted in a ∼10 dB shift in the coding of a binaural sound location cue (interaural-level difference, ILD) in ICC neurons relative to controls. The direction of the shift was consistent with a compensation of the altered ILDs due to the CHL. ICC neuron responses carried ∼37% less information about ILDs after CHL than control neurons. Cochlear peripheral-evoked responses confirmed that the CHL did not induce damage to the auditory periphery. We find that a temporary CHL altered auditory midbrain neurons by shifting binaural responses to ILD acoustic cues, suggesting a compensatory form of plasticity occurring by at least the level of the auditory midbrain, the ICC.

Children With Congenital Unilateral Sensorineural Hearing Loss: Effects of Late Hearing Aid Amplification-A Pilot Study

OBJECTIVES

Although children with unilateral hearing loss (uHL) have high risk of experiencing academic difficulties, speech-language delays, poor sound localization, and speech recognition in noise, studies on hearing aid (HA) outcomes are few. Consequently, it is unknown when and how amplification is optimally provided. The aim was to study whether children with mild-to-moderate congenital unilateral sensorineural hearing loss (uSNHL) benefit from HAs.

DESIGN

All 6- to 11-year-old children with nonsyndromic congenital uSNHL and at least 6 months of HA use were invited (born in Stockholm county council, n = 7). Participants were 6 children (9.7- to 10.8-years-old) with late HA fittings (>4.8 years of age). Unaided and aided hearing was studied with a comprehensive test battery in a within-subject design. Questionnaires were used to study overall hearing performance and disability. Sound localization accuracy (SLA) and speech recognition thresholds (SRTs) in competing speech were measured in sound field to study hearing under demanding listening conditions. SLA was measured by recording eye-gaze in response to auditory-visual stimuli presented from 12 loudspeaker-video display pairs arranged equidistantly within ±55° in the frontal horizontal plane. The SRTs were measured for target sentences at 0° in spatially separated (±30° and ±150°) continuous speech. Auditory brainstem responses (ABRs) were obtained in both ears separately to study auditory nerve function at the brainstem level.

RESULTS

The mean ± SD pure-tone average (0.5, 1, 2, and 4 kHz) was 45 ± 8 dB HL and 6 ± 4 dB HL in the impaired and normal hearing ear, respectively (n = 6). Horizontal SLA was significantly poorer in the aided compared with unaided condition. A significant relationship was found between aided SLA (quantified by an error index) and the impaired ear's ABR I to V interval, suggesting a relationship between the two. Results from questionnaires revealed aided benefit in one-to-one communication, whereas no significant benefit was found for communication in background noise or reverberation. No aided benefit was found for the SRTs in competing speech.

CONCLUSIONS

Children with congenital uSNHL benefit from late HA intervention in one-to-one communication but not in demanding listening situations, and there is a risk of degraded SLA. The results indicate that neural transmission time from the impaired cochlea to the upper brainstem may have an important role in unilaterally aided spatial hearing, warranting further study in children with uHL receiving early HA intervention.

Chronic Conductive Hearing Loss Is Associated With Speech Intelligibility Deficits in Patients With Normal Bone Conduction Thresholds

OBJECTIVES

The main objective of this study is to determine whether chronic sound deprivation leads to poorer speech discrimination in humans.

DESIGN

We reviewed the audiologic profile of 240 patients presenting normal and symmetrical bone conduction thresholds bilaterally, associated with either an acute or chronic unilateral conductive hearing loss of different etiologies.

RESULTS

Patients with chronic conductive impairment and a moderate, to moderately severe, hearing loss had lower speech recognition scores on the side of the pathology when compared with the healthy side. The degree of impairment was significantly correlated with the speech recognition performance, particularly in patients with a congenital malformation. Speech recognition scores were not significantly altered when the conductive impairment was acute or mild.

CONCLUSIONS

This retrospective study shows that chronic conductive hearing loss was associated with speech intelligibility deficits in patients with normal bone conduction thresholds. These results are as predicted by a recent animal study showing that prolonged, adult-onset conductive hearing loss causes cochlear synaptopathy.

Dichotic listening deficits in amblyaudia are characterized by aberrant neural oscillations in auditory cortex

OBJECTIVE

Children diagnosed with auditory processing disorder (APD) show deficits in processing complex sounds that are associated with difficulties in higher-order language, learning, cognitive, and communicative functions. Amblyaudia (AMB) is a subcategory of APD characterized by abnormally large ear asymmetries in dichotic listening tasks.

METHODS

Here, we examined frequency-specific neural oscillations and functional connectivity via high-density electroencephalography (EEG) in children with and without AMB during passive listening of nonspeech stimuli.

RESULTS

Time-frequency maps of these "brain rhythms" revealed stronger phase-locked beta-gamma (~35 Hz) oscillations in AMB participants within bilateral auditory cortex for sounds presented to the right ear, suggesting a hypersynchronization and imbalance of auditory neural activity. Brain-behavior correlations revealed neural asymmetries in cortical responses predicted the larger than normal right-ear advantage seen in participants with AMB. Additionally, we found weaker functional connectivity in the AMB group from right to left auditory cortex, despite their stronger neural responses overall.

CONCLUSION

Our results reveal abnormally large auditory sensory encoding and an imbalance in communication between cerebral hemispheres (ipsi- to -contralateral signaling) in AMB.

SIGNIFICANCE

These neurophysiological changes might lead to the functionally poorer behavioral capacity to integrate information between the two ears in children with AMB.

Consistent and chronic cochlear implant use partially reverses cortical effects of single sided deafness in children

Potentially neuroprotective effects of CI use were studied in 22 children with single sided deafness (SSD). Auditory-evoked EEG confirmed strengthened representation of the intact ear in the ipsilateral auditory cortex at initial CI activation in children with early-onset SSD (n = 15) and late-onset SSD occurring suddenly in later childhood/adolescence (n = 7). In early-onset SSD, representation of the hearing ear decreased with chronic CI experience and expected lateralization to the contralateral auditory cortex from the CI increased with longer daily CI use. In late-onset SSD, abnormally high activity from the intact ear in the ipsilateral cortex reduced, but responses from the deaf ear weakened despite CI use. Results suggest that: (1) cortical reorganization driven by unilateral hearing can occur throughout childhood; (2) chronic and consistent CI use can partially reverse these effects; and (3) CI use may not protect children with late-onset SSD from ongoing deterioration of pathways from the deaf ear.

Sound Localization in Preweanling Mice Was More Severely Affected by Deleting the Kcna1 Gene Compared to Deleting Kcna2, and a Curious Inverted-U Course of Development That Appeared to Exceed Adult Performance Was Observed in All Groups

The submillisecond acuity for detecting rapid spatial and temporal fluctuations in acoustic stimuli observed in humans and laboratory animals depends in part on select groups of auditory neurons that preserve synchrony from the ears to the binaural nuclei in the brainstem. These fibers have specialized synapses and axons that use a low-threshold voltage-activated outward current, IKL, conducted through Kv1 potassium ion channels. These are in turn coupled with HCN channels that express a mixed cation inward mixed current, IH, to support precise synchronized firing. The behavioral evidence is that their respective Kcna1 or HCN1 genes are absent in adult mice; the results are weak startle reflexes, slow responding to noise offsets, and poor sound localization. The present behavioral experiments were motivated by an in vitro study reporting increased IKL in an auditory nucleus in Kcna2-/- mice lacking the Kv1.2 subunit, suggesting that Kcna2-/- mice might perform better than Kcna2+/+ mice. Because Kcna2-/- mice have only a 17-18-day lifespan, we compared both preweanling Kcna2-/- vs. Kcna2+/+ mice and Kcna1-/- vs. Kcna1+/+ mice at P12-P17/18; then, the remaining mice were tested at P23/P25. Both null mutant strains had a stunted physique, but the Kcna1-/- mice had severe behavioral deficits while those in Kcna2-/- mice were relatively few and minor. The in vitro increase of IKL could have resulted from Kv1.1 subunits substituting for Kv1.2 units and the loss of the inhibitory "managerial" effect of Kv1.2 on Kv1.1. However, any increased neuronal synchronicity that accompanies increased IKL may not have been enough to affect behavior. All mice performed unusually well on the early spatial tests, but then, they fell towards adult levels. This unexpected effect may reflect a shift from summated independent monaural pathways to integrated binaural processing, as has been suggested for similar observations for human infants.

Reversible external auditory canal ligation (REACL): A novel surgical technique to induce transient and reversible hearing loss in developing rats

BACKGROUND

Cases of sensory loss provide an excellent model to study the plastic nature of cortical sensory systems. Models of reversible sensory loss are particularly useful for establishing the timeline of various critical periods for cortical plasticity. However, there currently is an absence of adequate methods to produce reversible hearing loss in neonatal and developing rodents.

NEW METHOD

We propose a novel and reversible adaptation of an existing surgical technique-external auditory canal ligation (EACL)-that produces a reliable and moderate hearing loss.

RESULTS

Auditory brainstem responses (ABRs) were used to measure both the magnitude of the hearing loss induced by EACL and the auditory thresholds following hearing restoration. The EACL and reopening procedures, as assessed by visual inspection, had success rates of 81% and 78%, respectively. The average hearing thresholds, as assessed with ABRs, increased by nearly 40 decibels across all tested frequencies. Hearing thresholds returned to normal levels following the reopening procedure.

COMPARISON WITH EXISTING METHODS

Our procedure yields similar benefits to other methods, such as producing a reliable and moderate hearing loss that is entirely reversible. Furthermore, to our knowledge, it is the first that can be performed in neonatal rodents, thus allowing researchers the opportunity to assess the effects of sensory loss on behavior and cortical neurophysiology during developmental critical periods.

CONCLUSION

Our modified technique of reversible external auditory canal ligation offers an easy, and reliable method to induce a transient state of hearing loss that mimics naturally occurring congenital conductive hearing loss.

Effects of Cochlear Implantation on Binaural Hearing in Adults With Unilateral Hearing Loss

A FDA clinical trial was carried out to evaluate the potential benefit of cochlear implant (CI) use for adults with unilateral moderate-to-profound sensorineural hearing loss. Subjects were 20 adults with moderate-to-profound unilateral sensorineural hearing loss and normal or near-normal hearing on the other side. A MED-EL standard electrode was implanted in the impaired ear. Outcome measures included: (a) sound localization on the horizontal plane (11 positions, -90° to 90°), (b) word recognition in quiet with the CI alone, and (c) masked sentence recognition with the target at 0° and the masker at -90°, 0°, or 90°. This battery was completed preoperatively and at 1, 3, 6, 9, and 12 months after CI activation. Normative data were also collected for 20 age-matched control subjects with normal or near-normal hearing bilaterally. The CI improved localization accuracy and reduced side bias. Word recognition with the CI alone was similar to performance of traditional CI recipients. The CI improved masked sentence recognition when the masker was presented from the front or from the side of normal or near-normal hearing. The binaural benefits observed with the CI increased between the 1- and 3-month intervals but appeared stable thereafter. In contrast to previous reports on localization and speech perception in patients with unilateral sensorineural hearing loss, CI benefits were consistently observed across individual subjects, and performance was at asymptote by the 3-month test interval. Cochlear implant settings, consistent CI use, and short duration of deafness could play a role in this result.

Improved directional hearing of children with congenital unilateral conductive hearing loss implanted with an active bone-conduction implant or an active middle ear implant

Different amplification options are available for listeners with congenital unilateral conductive hearing loss (UCHL). For example, bone-conduction devices (BCDs) and middle ear implants. The present study investigated whether intervention with an active BCD, the Bonebridge, or a middle ear implant, the Vibrant Soundbridge (VSB), affected sound-localization performance of listeners with congenital UCHL. Listening with a Bonebridge or VSB might provide access to binaural cues. However, when fitted with the Bonebridge, but not with a VSB, binaural processing might be affected through cross stimulation of the contralateral normal hearing ear, and could interfere with processing of binaural cues. In the present study twenty-three listeners with congenital UCHL were included. To assess processing of binaural cues, we investigated localization abilities of broadband (BB, 0.5-20 kHz) filtered noise presented at varying sound levels. Sound localization abilities were analyzed separately for stimuli presented at the side of the normal-hearing ear, and for stimuli presented at the side of the hearing-impaired ear. Twenty-six normal hearing children and young adults were tested as control listeners. Sound localization abilities were measured under open-loop conditions by recording head-movement responses. We demonstrate improved sound localization abilities of children with congenital UCHL, when listening with a Bonebridge or VSB, predominantly for stimuli presented at the impaired (aided) side. Our results suggest that the improvement is not related to accurate processing of binaural cues. When listening with the Bonebridge, despite cross stimulation of the contralateral cochlea, localization performance was not deteriorated compared to listening with a VSB.

A review of the effects of unilateral hearing loss on spatial hearing

The capacity of the auditory system to extract spatial information relies principally on the detection and interpretation of binaural cues, i.e., differences in the time of arrival or level of the sound between the two ears. In this review, we consider the effects of unilateral or asymmetric hearing loss on spatial hearing, with a focus on the adaptive changes in the brain that may help to compensate for an imbalance in input between the ears. Unilateral hearing loss during development weakens the brain's representation of the deprived ear, and this may outlast the restoration of function in that ear and therefore impair performance on tasks such as sound localization and spatial release from masking that rely on binaural processing. However, loss of hearing in one ear also triggers a reweighting of the cues used for sound localization, resulting in increased dependence on the spectral cues provided by the other ear for localization in azimuth, as well as adjustments in binaural sensitivity that help to offset the imbalance in inputs between the two ears. These adaptive strategies enable the developing auditory system to compensate to a large degree for asymmetric hearing loss, thereby maintaining accurate sound localization. They can also be leveraged by training following hearing loss in adulthood. Although further research is needed to determine whether this plasticity can generalize to more realistic listening conditions and to other tasks, such as spatial unmasking, the capacity of the auditory system to undergo these adaptive changes has important implications for rehabilitation strategies in the hearing impaired.

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Unilateral hearing loss in infancy can disrupt spatial hearing, even after binaural inputs are restored.

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Plasticity in the developing brain enables substantial recovery in sound localization accuracy.

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Adaptation to unilateral hearing loss is based on reweighting of monaural spectral cues and binaural plasticity.

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Training on auditory tasks can partially compensate for unilateral hearing loss, highlighting potential therapies.

The Effect of Age and History of Recurrent Otitis Media on Dichotic Listening and Verbal Memory in Children

OBJECTIVES

To explore the possible effects of recurrent otitis media (ROM) in early childhood on binaural processing and verbal memory in school-aged children.

METHODS

Two hundred eleven children, including 31 children with and 180 children without a history of ROM, were examined. A dichotic digit test (DDT) and a forward and backward digit memory span test (DMST) were administered.

RESULTS

A significant difference was observed between age groups among ROM-free children. The ROM-positive group earned significantly poorer results than the ROM-free group in all measurements, except for the right DDT (rDDT) score. Furthermore, a significant correlation was found between the DDT and EA scores with both DMST scores in the ROM-free group. The correlation between the rDDT and forward DMST scores was not significant in the ROM-positive group, and no significant correlation was observed between the EA score and either DMST score.

CONCLUSIONS

Our findings support that a history of OM in early childhood based on a parental survey of children is associated with differences in DDT and DMST outcomes.

Functional Impairments Due to Unilateral Deafness

The authors investigated the impact of unilateral listening upon two central auditory functions: speech discrimination in a noisy environment, and the ability to process and clearly recognize modestly accelerated speech by 30% compression (shortened exposure). We determined that the negative impact of listening to speech in the milieu of noise was individually variable. The data revealed a wide range of impairment, from mild to relatively severe (ie, 0% to 60% [mean, 34%]; normative data, 4% to 36% [mean, 14%]). The performance scores for recognition of compressed sentences were not markedly influenced by unilateral deafness, but the functional results of the residual ear were correlated with a loss of high-frequency sensitivity (mean of 54% correct in severe unilateral deafness with an associated high-frequency loss versus 93% correct in normal controls). Reorganization of the central nervous system by "plasticity" or biological maturation over time failed to improve the performance of monaural listening. The currently recommended methods of remediation for this sensory deficit are discussed, and newer technologies under investigation are examined.

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.

Amblyaudia: Review of Pathophysiology, Clinical Presentation, and Treatment of a New Diagnosis

OBJECTIVE

Similar to amblyopia in the visual system, "amblyaudia" is a term used to describe persistent hearing difficulty experienced by individuals with a history of asymmetric hearing loss (AHL) during a critical window of brain development. Few clinical reports have described this phenomenon and its consequent effects on central auditory processing. We aim to (1) define the concept of amblyaudia and (2) review contemporary research on its pathophysiology and emerging clinical relevance.

DATA SOURCES

PubMed, Embase, and Cochrane databases.

REVIEW METHODS

A systematic literature search was performed with combinations of search terms: "amblyaudia," "conductive hearing loss," "sensorineural hearing loss," "asymmetric," "pediatric," "auditory deprivation," and "auditory development." Relevant articles were considered for inclusion, including basic and clinical studies, case series, and major reviews.

CONCLUSIONS

During critical periods of infant brain development, imbalanced auditory input associated with AHL may lead to abnormalities in binaural processing. Patients with amblyaudia can demonstrate long-term deficits in auditory perception even with correction or resolution of AHL. The greatest impact is in sound localization and hearing in noisy environments, both of which rely on bilateral auditory cues. Diagnosis and quantification of amblyaudia remain controversial and poorly defined. Prevention of amblyaudia may be possible through early identification and timely management of reversible causes of AHL.

IMPLICATIONS FOR PRACTICE

Otolaryngologists, audiologists, and pediatricians should be aware of emerging data supporting amblyaudia as a diagnostic entity and be cognizant of the potential for lasting consequences of AHL. Prevention of long-term auditory deficits may be possible through rapid identification and correction.

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.

Art and science: how musical training shapes the brain

What makes a musician? In this review, we discuss innate and experience-dependent factors that mold the musician brain in addition to presenting new data in children that indicate that some neural enhancements in musicians unfold with continued training over development. We begin by addressing effects of training on musical expertise, presenting neural, perceptual, and cognitive evidence to support the claim that musicians are shaped by their musical training regimes. For example, many musician-advantages in the neural encoding of sound, auditory perception, and auditory-cognitive skills correlate with their extent of musical training, are not observed in young children just initiating musical training, and differ based on the type of training pursued. Even amidst innate characteristics that contribute to the biological building blocks that make up the musician, musicians demonstrate further training-related enhancements through extensive education and practice. We conclude by reviewing evidence from neurobiological and epigenetic approaches to frame biological markers of musicianship in the context of interactions between genetic and experience-related factors.

Context-specific reweighting of auditory spatial cues following altered experience during development

Background

Neural systems must weight and integrate different sensory cues in order to make decisions. However, environmental conditions often change over time, altering the reliability of different cues and therefore the optimal way for combining them. To explore how cue integration develops in dynamic environments, we examined the effects on auditory spatial processing of rearing ferrets with localization cues that were modified via a unilateral earplug, interspersed with brief periods of normal hearing.

Results

In contrast with control animals, which rely primarily on timing and intensity differences between their two ears to localize sound sources, the juvenile-plugged ferrets developed the ability to localize sounds accurately by relying more on the unchanged spectral localization cues provided by the single normal ear. This adaptive process was paralleled by changes in neuronal responses in the primary auditory cortex, which became relatively more sensitive to these monaural spatial cues. Our behavioral and physiological data demonstrated, however, that the reweighting of different spatial cues disappeared as soon as normal hearing was experienced, showing for the first time that this type of plasticity can be context specific.

Conclusions

These results show that developmental changes can be selectively expressed in response to specific acoustic conditions. In this way, the auditory system can develop and simultaneously maintain two distinct models of auditory space and switch between these models depending on the prevailing sensory context. This ability is likely to be critical for maintaining accurate perception in dynamic environments and may point toward novel therapeutic strategies for individuals who experience sensory deficits during development.

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Ferrets reared with a unilateral hearing loss are able to localize sounds accurately

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Adaptation relies on cue reweighting that reverses when normal hearing is available

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Auditory cortical neurons show corresponding context-specific plasticity

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Contextual cue reweighting maintains perceptual stability in dynamic environments

Behavioral consequences of unilateral inferior colliculus lesions in the rat

This study was carried out to determine the behavioral sensitivity to sound of rats with unilateral lesions of inferior colliculus (IC) located ipsilateral or contralateral to the projection pathway from one ear. Absolute thresholds for the detection of a broad-band noise burst were compared for rats with a profound conductive hearing loss in one ear and a lesion placed either ipsilateral or contralateral to the normally functioning ear. The rats were trained to make withdrawal responses to avoid a shock when they detected the presence of a noise burst. Sound pressure level was systematically lowered to obtain psychophysical curves from which absolute thresholds could be determined. Complete lesions of the contralateral IC resulted in substantial elevations in absolute threshold relative to normal whereas equivalent lesions of the ipsilateral IC produced relatively little elevation. In neither case did unilateral destruction of the IC produce a total inability to respond to sound. Contralateral IC lesions that included the dorsal nucleus of the lateral lemniscus (DNLL) produced a significantly greater elevation in behavioral thresholds than complete lesions limited to the IC. The results indicate a predominance of the contralateral over the ipsilateral pathway to IC for maintaining normal thresholds. They also indicate that other pathways that bypass the IC are likely involved in detecting the presence of a sound.

Neural circuits underlying adaptation and learning in the perception of auditory space

Sound localization mechanisms are particularly plastic during development, when the monaural and binaural acoustic cues that form the basis for spatial hearing change in value as the body grows. Recent studies have shown that the mature brain retains a surprising capacity to relearn to localize sound in the presence of substantially altered auditory spatial cues. In addition to the long-lasting changes that result from learning, behavioral and electrophysiological studies have demonstrated that auditory spatial processing can undergo rapid adjustments in response to changes in the statistics of recent stimulation, which help to maintain sensitivity over the range where most stimulus values occur. Through a combination of recording studies and methods for selectively manipulating the activity of specific neuronal populations, progress is now being made in identifying the cortical and subcortical circuits in the brain that are responsible for the dynamic coding of auditory spatial information.

Evaluating the perceptual and pathophysiological consequences of auditory deprivation in early postnatal life: a comparison of basic and clinical studies

Decades of clinical and basic research in visual system development have shown that degraded or imbalanced visual inputs can induce a long-lasting visual impairment called amblyopia. In the auditory domain, it is well established that inducing a conductive hearing loss (CHL) in young laboratory animals is associated with a panoply of central auditory system irregularities, ranging from cellular morphology to behavior. Human auditory deprivation, in the form of otitis media (OM), is tremendously common in young children, yet the evidence linking a history of OM to long-lasting auditory processing impairments has been equivocal for decades. Here, we review the apparent discrepancies in the clinical and basic auditory literature and provide a meta-analysis to show that the evidence for human amblyaudia, the auditory analog of amblyopia, is considerably more compelling than is generally believed. We argue that a major cause for this discrepancy is the fact that most clinical studies attempt to link central auditory deficits to a history of middle ear pathology, when the primary risk factor for brain-based developmental impairments such as amblyopia and amblyaudia is whether the afferent sensory signal is degraded during critical periods of brain development. Accordingly, clinical studies that target the subset of children with a history of OM that is also accompanied by elevated hearing thresholds consistently identify perceptual and physiological deficits that can endure for years after peripheral hearing is audiometrically normal, in keeping with the animal studies on CHL. These studies suggest that infants with OM severe enough to cause degraded afferent signal transmission (e.g., CHL) are particularly at risk to develop lasting central auditory impairments. We propose some practical guidelines to identify at-risk infants and test for the positive expression of amblyaudia in older children.

Feature-dependent sensitive periods in the development of complex sound representation

Simple tonal stimuli can shape spectral tuning of cortical neurons during an early epoch of brain development. The effects of complex sound experience on cortical development remain to be determined. We exposed rat pups to a frequency-modulated (FM) sweep in different time windows during early development, and examined the effects of such sensory experience on sound representations in the primary auditory cortex (AI). We found that early exposure to a FM sound resulted in altered characteristic frequency representations and broadened spectral tuning in AI neurons, whereas later exposure to the same sound only led to greater selectivity for the sweep rate and direction of the experienced FM sound. These results indicate that cortical representations of different acoustic features are shaped by complex sounds in a series of distinct sensitive periods.

What can be expected from a late cochlear implantation?

OBJECTIVE

Verify if late cochlear implantation allows pre-lingual deafs to convert from visual to oral communication mode only.

METHOD

Thirteen pre-lingual profoundly deaf patients implanted the ages of 8 and 22 years were included in the study. Before cochlear implantation, none of the patients used the oral language. Six patients used cued speech and seven used the sign language to communicate. Evaluations were made with measures of hearing thresholds, phoneme identification, categories of auditory performance and rating of the intelligibility of speech before and after implantation. Changes in principal mode of communication (i.e. oral, cued speech or sign language) were also monitored.

RESULTS

The former users of cued speech benefited significantly more from cochlear implantation than the sign language users for phoneme identification and categories of auditory performance, although all had similar hearing thresholds before and after cochlear implantation. After a mean implant use of 4.5 years, four out of six cued speech users converted to exclusive use of the oral language, while only one out of seven former users of the sign language converted to the use of the oral language.

DISCUSSION

It is possible for pre-lingual or congenital deafs to convert totally from a visual to an oral communication mode even in case of late cochlear implantation. Previous awareness of the structure of the oral language, even without hearing (e.g. via cued speech) influences positively the outcome of delayed implantations. We recommend the adoption of oral communication with the cued speech code in cases where a late cochlear implantation is envisioned.

The acoustical cues to sound location in the rat: measurements of directional transfer functions

The acoustical cues for sound location are generated by spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although rats have been a common model system for anatomy, physiology, and psychophysics of localization, there have been few studies of the acoustical cues available to rats. Here, directional transfer functions (DTFs), the directional components of the head-related transfer functions, were measured in six adult rats. The cues to location were computed from the DTFs. In the frontal hemisphere, spectral notches were present for frequencies from approximately 16 to 30 kHz; in general, the frequency corresponding to the notch increased with increases in source elevation and in azimuth toward the ipsilateral ear. The maximum high-frequency envelope-based interaural time differences (ITDs) were 130 mus, whereas low-frequency (<3.5 kHz) fine-structure ITDs were 160 mus; both types of ITDs were larger than predicted from spherical head models. Interaural level differences (ILDs) strongly depended on location and frequency. Maximum ILDs were <10 dB for frequencies <8 kHz and were as large as 20-40 dB for frequencies >20 kHz. Removal of the pinna eliminated the spectral notches, reduced the acoustic gain and ILDs, altered the acoustical axis, and reduced the ITDs.

Auditory brainstem activity in children with 9–30 months of bilateral cochlear implant use

Bilateral cochlear implants aim to restore binaural processing along the auditory pathways in children with bilateral deafness. We assessed auditory brainstem activity evoked by single biphasic pulses delivered by an apical or basal electrode from the left, right and both cochlear implants in 13 children. Repeated measures were made over the first 9-30 months of bilateral implant use. In children with short or long periods of unilateral implant use prior to the second implantation, Wave eV of the auditory brainstem response was initially prolonged when evoked by the naïve versus experienced side. These differences tended to resolve in children first implanted <3 years of age but not in children implanted at older ages with long delays between implants. Latency differences were projected to persist for longer periods in children with long delays between implants compared with children with short delays. No differences in right versus left evoked eV latency were found in 2 children receiving bilateral implants simultaneously and their response latencies decreased over time. Binaural interaction responses showed effects of stimulating electrode position (responses were more detectable when evoked by an apical than basal pair of implant electrodes), and duration of delay between implants (measured by latency delays). The trends shown here suggest a negative impact of unilateral implant use on bilateral auditory brainstem plasticity.

The effect of monaural middle ear destruction on response properties of neurons in the auditory midbrain of juvenile and adult mice

This article reviews our studies of the effect of monaural middle ear destruction on midbrain auditory response properties of the laboratory mouse, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice and the auditory sensitivity of neurons in the midbrain inferior colliculus (IC) ipsilateral and contralateral to the intact ear was examined 4 weeks later. When stimulated with sound pulses, IC neurons of the control mice typically had lower minimum threshold, larger dynamic range, and sharper frequency tuning curve than IC neurons of the experimental juvenile and adult mice. In the experimental mice, neurons in the ipsilateral IC had significantly longer latency, higher minimum threshold, and smaller dynamic range than neurons in the contralateral IC. When determined at two sound directions (ipsilateral 40 degrees and contralateral 40 degrees to the recording site), IC neurons of the control mice had higher minimum threshold, sharper frequency tuning curve but smaller dynamic range at I-40 degrees than at C-40 degrees . However, these direction-dependent response properties were not observed for IC neurons of the experimental juvenile and adult mice. Clear tonotopic organization was only observed in the IC of the control mice and experimental adult mice but not in the IC of experimental juvenile mice. These different response properties are discussed in relation to the effect of monaural middle ear destruction.

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.

Monaural middle ear destruction in juvenile and adult mice: effects on responses to sound direction in the inferior colliculus ipsilateral to the intact ear

This study examined the effect of monaural middle ear destruction on auditory responses to sound direction in the inferior colliculus (IC) of the laboratory mice, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice (the experimental mice). Auditory response properties of neurons to ipsilateral and contralateral sounds (I-40 degrees and C-40 degrees ) were examined in the IC ipsilateral to the intact ear 4 weeks later. IC neurons of control mice had higher minimum thresholds (MTs), larger Q(n) (Q(10), Q(30)) values but smaller dynamic ranges at I-40 degrees than at C-40 degrees. These direction-dependent response properties were not observed for IC neurons of experimental juvenile and adult mice. However, Q(n) values of IC neurons were significantly smaller in experimental juvenile than in control and experimental adult mice. Normal tonotopic organization in terms of positive correlation between recording depth and best frequency (BF) was observed in the IC of control and experimental adult mice at both sound directions but not in the IC of experimental juvenile mice. A positive correlation of increasing MT with BF was only observed for IC neurons in control mice but not in both experimental mice. Possible mechanisms for these different response properties are discussed.

Conductive hearing loss results in changes in cytochrome oxidase activity in gerbil central auditory system

Conductive hearing loss (CHL) restricts auditory input to an intact peripheral auditory system. Effects of deprivation on the central auditory system (CAS) have been debated, although a number of studies support the hypothesis that CHL can cause modification of CAS structure and function. The present study was designed to test the hypothesis that unilateral CHL results in a decrease in cytochrome oxidase (CO) activity in CAS nuclei that receive major afferent input from the affected ear. Gerbils at postnatal day 12 (P21) or 6-8 weeks underwent left unilateral CHL (malleus removal), cochlear ablation, or a sham surgical procedure. After a survival time of 48 hours or 3 weeks, animals were sacrificed and tissue was processed for cytochrome oxidase histochemistry. Optical density (OD) measurements were made from individual neurons in the anteroventral cochlear nucleus (AVCN) and from medial and lateral dendritic fields in the medial superior olivary nucleus (MSO), the lateral superior olivary nucleus, and the inferior colliculus. The width of the CO-stained neuropil in MSO was also measured as an estimate of dendritic length. OD measures were corrected to neutral areas of the brain. Cochlear ablation caused significant decreases in CO activity in left lower brainstem nuclei, particularly in adult animals. Following CHL, a significant decrease in CO activity was observed in the ipsilateral AVCN and a significant increase was observed in the contralateral AVCN. Cochlear ablation resulted in decreased width of MSO neuropil containing dendrites that receive primary input from the ablated ear. CHL resulted in a significant increase in the width of MSO neuropil on both sides of the brain in the P21 animals that survived 3 weeks but not in P21 animals that survived only 48 hours or in the adult animals. Unilateral CHL is associated with changes in CO activity in the AVCN and may affect MSO dendritic length in younger animals.

[Implantation of a program for the early detection of neonatal hearing loss]

Permanent childhood hearing impairment is a serious public health problem. Identification by screening in the first few months of life has the potential to improve affected children development. Neonatal hearing screening programs endorse the WHO requirement for a cost efficient screening. This paper shows the implementation of a medium-size hospital-based universal newborn hearing screening program using transient evoked otoacoustic emissions. Operational procedures and requirements are discussed. Quality control and results are showed.

The effect of monaural middle ear destruction on postnatal development of auditory response properties of mouse inferior collicular neurons

This study examined the effect of monaural middle ear destruction on postnatal development of auditory response properties of inferior collicular (IC) neurons of the laboratory mouse, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice and the auditory response properties of neurons in both ICs were examined 4 weeks thereafter. IC neurons of control mice typically had lower minimum thresholds, larger dynamic ranges and greater Q(10) values than IC neurons of experimental juvenile and adult mice. In experimental mice, neurons in the ipsilateral IC (relative to the intact ear) typically had longer latencies, higher minimum thresholds, and smaller dynamic ranges than neurons in the contralateral IC. In experimental adult mice, neurons in the ipsilateral IC had sharper frequency tuning curves than neurons in the contralateral IC. Clear tonotopic organization was only observed in the IC of control mice and experimental adult mice. However, the correlation of increasing minimum threshold with best frequency was observed for IC neurons in control mice but not in experimental juvenile and adult mice. Possible mechanisms for these different response properties are discussed.

Early unilateral auditory deprivation increases 2-deoxyglucose uptake in contralateral auditory cortex of juvenile Mongolian gerbils

The effects of early onset, unilateral conductive hearing loss on tone-induced 2-deoxyglucose (2-DG) uptake in the auditory cortex of juvenile Mongolian gerbils (Meriones unguiculatus) were studied. Atresia of the left ear canal was induced at postnatal day 9 (P9) to achieve reversible auditory deprivation prior to onset of hearing (around P12). Atresia either persisted (ATR, n=4) or the canal was opened 15 min before the 2-DG experiments (RE, n=4) at P27. Control animals were either non-deprived (CON, n=4), or their left ears were plugged acutely (PAX, n=4). In PAX, 2-DG uptake in primary auditory cortex (AI) and anterior auditory field (AAF) was lower in right than in left AI and AAF. In contrast, in ATR and RE, uptake was significantly higher on the right side contralateral to the atresia. Hence, atresia during early development leads to plastic changes resulting in an interhemispheric imbalance of functional metabolism in favor of the auditory cortex contralateral to the manipulated ear. Distances between tone-induced 2-DG labeling in AI and AAF were increased in PAX, but smaller in ATR in the right compared to the left hemisphere, suggesting effects of atresia also on spatial relations in cortical tonotopic maps.

The efficacy of early identification and intervention for children with hearing impairment

From these findings, the inevitable conclusion is that identification of hearing loss by 6 months of age, followed by appropriate intervention, is the most effective strategy for the normal development of language in infants and toddlers with hearing loss. Identification of hearing loss by 6 months can only be accomplished through universal newborn hearing screening. Some questions that arise as a result of these studies include: What can one conclude from the finding that the language skills of children with mild hearing losses are no better than those with greater losses? If the finding holds up, it indicates a great need for investigations into biobehavior theories of language acquisition and into the part played by the prenatal 4 months of hearing. And it also shows a need for answering the question, When does a hearing loss begin?, because it certainly seems that all hearing losses are similar in their outcomes. Can the findings from these studies be used to benefit normally hearing children who are at risk for language delays as a result of limited language environments? Such children suffer from auditory deprivation just as surely as those with hearing losses. If the language skills of the latter children can be brought to normal range by early intervention, the same strategy may help high-risk populations. The efficacy of early intervention is just as valid for these children as it is for the children with hearing impairment. Now that the benefits of early identification of children with congenital hearing loss have been demonstrated, these benefits should be extended to all children who are at risk for language delays, with appropriate interventions applied immediately.

Perceptual consequences of peripheral hearing loss: do edge effects exist for abrupt cochlear lesions?

There is a growing body of research that shows evidence of central neural reorganization in response to lesions in the auditory periphery, even if the lesions occur in maturity. This reorganization consists of an increased neural representation of frequencies corresponding to the edge frequency of the lesion. Data were collected to determine whether this over-representation might have consequences for human perception. The hypothesis was that increased central representation might increase acuity on some psychophysical tasks performed at the edge frequency. Tasks included frequency sweep detection (for tones), intensity discrimination (for 100-Hz-wide bands of noise and tones), gap detection and gap discrimination (both for 100-Hz-wide bands of noise). Results from observers with steeply sloping hearing losses were compared with results from normal-hearing observers performing these tasks with masking noise generated to simulate steeply sloping hearing loss. None of these data provide compelling evidence for the hypothesized edge effect. A 40-Hz following response to tone bursts was collected from a subset of the hearing-impaired observers in an attempt to confirm the animal physiology findings of neural over-representation of the edge frequency. No edge-frequency effect was noted in the results, though there was a non-significant tendency for one of the hearing-impaired observers to show shorter latency of response.

The consequences of deafness and chronic intracochlear electrical stimulation on the central auditory pathways

1. Auditory deprivation can result in significant morphological and physiological changes within the central auditory nervous system. These changes are generally more pronounced when the onset of deafness occurs early in development, as is the case with congenitally deaf children. 2. A review of the auditory plasticity literature suggests that some of these deafness-induced changes may be preventable or partially reversible by electrical stimulation of the auditory nerve. 3. With the increasing application of cochlear implants in congenitally deaf children, it is important to evaluate the consequences of introducing electrical stimulation to the developing auditory nervous system.

Single unit activity in the inferior colliculus of the rat elicited by electrical stimulation of the cochlea

The activity of single neurons (n = 182) of the central nucleus of the inferior colliculus (CIC) of the rat was recorded in response to unilateral electrical stimulation of the left cochlea and/or acoustical stimulation of the right ear. The probability of response to both modes of stimulation was comparable (90 per cent for contralateral and 60 per cent for ipsilateral presentation). Response patterns consisted predominantly of onset excitations. Response latencies to electrical stimuli ranged from 3 to 21 ms, with an average value of 9.7 ms (SD = 3.5 ms) in the ipsilateral CIC and 6.6 ms (SD = 3.4 ms) in the contralateral CIC. With respect to binaural inputs, the majority of units were excited by stimulation of either ear (EE; about 60 per cent) while about one third were influenced by one ear only (EO). Units excited by one ear and inhibited by the other (EI) were rare. The main difference between the present implanted rats and normal animals was the virtual absence here of inhibitory effects for both types of stimuli when they were delivered to the ipsilateral ear (very few EI units).

Binaural hearing in adults with histories of otitis media in childhood

There is evidence that children with recent histories of otitis media (OM) have poorer binaural hearing than children with no such histories. In order to investigate whether these deficits persist into adulthood, young adults were selected for auditory testing on the basis of responses to a questionnaire on childhood OM. Binaural performance measures were obtained for the binaural masking level difference, binaural loudness summation and lateralisation of binaural clicks. Although subjects with OM histories reported more auditory disabilities in adulthood, there were no significant differences in binaural performance between OM history subjects and controls. It is suggested that binaural deficits resulting from OM do not usually persist into adulthood.

Influence of auditory experience on the development of brain stem auditory-evoked potentials in mallard duck embryos and hatchlings

The development of brain stem auditory-evoked potentials (BAEP) was studied in devocalized ducklings reared in auditory isolation and in vocal ducklings exposed to enhanced species-specific auditory stimulation with embryonic contact-contentment calls (CTs). Thresholds and latency of BAEP wave P1 in the mute ducklings indicated that even short-term auditory deprivation affected the development of auditory sensitivity, substantially reducing the rate of decline in BAEP thresholds and latencies of P1, especially in the low-frequency (500 and 750 Hz) and high-frequency (above 2.0 kHz) ranges. The ducklings exposed to enhanced stimulation, on the other hand, showed an accelerated decline in BAEP thresholds and latencies of P1 across all test frequencies, with the most marked influence on low (below 1.5 kHz) and high (above 2.5 kHz) frequencies. The influence of species-specific auditory experience had its most potent effect during the embryonic critical period for postnatal auditory perceptual (behavioral) development, at which time the embryo produces and hears a special low-frequency (1.5-2.5 kHz) version of its own contact-contentment vocalization.

Maturation of auditory evoked potentials in young guinea pigs with binaural conductive hearing loss

A reversible conductive hearing loss produced during the first 4 weeks post partum caused marked alterations in the development of click-evoked auditory brainstem (ABR) and middle latency (MLR) responses in guinea pigs. The early component PI in the ABR in controls showed adult-like latencies at the time of birth, while the later ABR components PIII and PV and all components investigated in the MLR showed postnatal development characterized by a shortened latency that persisted for the whole period of investigation. The course of the ABR latencies showed the sharpest decrease during the first 2-3 weeks of life, while that for the MLR took place during the first 5 weeks. In addition to the increased ABR thresholds and lengthened ABR latencies due to the conductive hearing loss, development of the ABR inter-peak latencies (IPL) and MLR latencies was retarded. The IPL reached control values 1 week after the end of the deprivation phase, while the delay in MLR started later (4th week) and lasted longer than did that in the ABR. These findings showed that a long-lasting postnatal conductive hearing loss does not generate sustained impairment at the level of the brainstem but can evoke longer-lasting deficiencies at higher stations of the auditory pathway.

Auditory behaviour and brainstem histochemistry in adult rats with characterized ear damage after neonatal ossicle ablation or cochlear disruption

Binaural and monaural ossicle ablation in neonate rats before the time of onset of auditory input resulted in hearing deficits as detected by behavioural responses to sound stimuli in these rats as young adults. Cochlear disruption at the same neonatal age similarly resulted in the absence of startle reflexes in many of the rats. When the middle and inner ears of the rats were analysed postmortem in serial sections, it was observed that most ears after neonatal ossicle ablation contained only small remnants of the malleus-incus unit, separated from the stapes; in other ears an apparent continuity of ossicles had been restored. The rats with blind-ending ear canals and ossicle atrophy were those that had shown little response to sound stimuli. In the cochlear-disrupted rats, those with modiolar damage and loss of most spiral ganglion cells had shown substantial impairment of sound perception, even in some rats with only monaural modiolar loss. The chronic conduction deficit caused by neonatal ossicle removal did not result in detectable differences in relative cytochrome oxidase activity in the dorsal cochlear nuclei and central nucleus of the inferior colliculus. For monaurally ossicle-ablated rats, quantitation of the average intensity of enzyme reaction product in sections of dorsal or ventral cochlear nuclei, or central nucleus, did not reveal a difference between operated and non-operated sides. However, in binaurally ossicle-ablated rats, the relative enzyme activity in the anteroventral cochlear nuclei was reduced in comparison to this nucleus in control rats. The volume of the anteroventral cochlear nucleus in rats that had had neonatal binaural cochlear disruption was reduced relative to the volume in control rats or in rats that had had binaural ossicle ablation (P < 0.001); the latter procedure did not result in a statistically significant difference from controls in AVCN volume. In cochlear-operated rats with monaural modiolar damage, the AVCN contralateral to the damaged cochlea had a lower mean level of cytochrome oxidase activity in its neurons measured individually than that for neurons in the ipsilateral AVCN. These results suggested the importance during development of input from contralateral cochlear neurons.

The effect of otitis media with effusion on the masking-level difference and the auditory brainstem response

This study investigated the masking-level difference (MLD) and auditory brainstem response (ABR) in a group of children with a history of otitis media with effusion (OME) and a control group of children with no known history of ear disease. All children had normal hearing at the time of testing. The main goal of the study was to determine whether there was an association between a reduced MLD in the OME children and an abnormal ABR (in terms of prolonged absolute or interwave intervals, or interaural differences in the ABR waveforms). The results indicated that the group of children having a history of OME had significantly reduced MLDs and had significantly prolonged waves III and V, and I-III and I-V interwave intervals. The correlations between MLD and delays in absolute wave or interwave intervals were not significant. However, some correlations between interaural asymmetries of the interwave intervals and the MLD were significant. The results suggest that the reduction in MLD found in children having a history of OME may be related to abnormal brainstem processing.

Brainstem auditory evoked potentials in young children before and after tympanostomy tube placement

To evaluate the effect of tympanostomy tube placement upon the hearing function of infants and young children, brainstem auditory evoked potentials (BAEP) were recorded in a group of young children (mean age 22 mos) receiving this treatment for otitis media with effusion (OME). For comparison, a group of healthy infants with normal behavioral audiometry were also tested with BAEP. Hearing loss was estimated for each ear using peak V latency-intensity curves. To evaluate the immediate effect of tube placement, 98 ears in 52 children were tested immediately before and after tube insertion. The 63 ears with effusion had prolonged peak latencies that decreased significantly (P < 0.001) immediately after tube placement and a mean hearing loss estimate of 22 dB that improved significantly (P < 0.0001) to 11 dB immediately after tube insertion. The 35 ears without effusion at myringotomy had a mean hearing loss estimate of 8 dB that did not change significantly after tube insertion. To evaluate the short-term effect of tube presence, 39 ears in 25 children were tested 3 weeks to 18 months after tube insertion. The 28 ears with dry tubes had a mean hearing loss estimate of 3 dB, and the 11 ears with otorrhea had a mean hearing loss estimate of 31 dB. The magnitude of mean hearing loss estimates in these young children with OME and the improvement in hearing function with tube placement is similar to that reported in older children studied with BAEP and audiometry. The study groups with a history of otitis media that had resolved by the time of testing had isolated prolongation of mean III-V interpeak latencies compared to normals (P < 0.01). These studies show that BAEP techniques are useful in estimating hearing loss in children with OME who are difficult to test by behavioral audiometry and show changes in rostral brainstem transmission in very young children with a history of OME.

Effects of recurrent otitis media in infancy on auditory perception and speech

INTRODUCTION

Recurrent otitis media is commonly encountered in children before 3 years of age. Conductive hearing loss up to 40 dB is frequently associated with suppurative otitis media. Good hearing is believed to be critical to the development of normal language. This study was undertaken to evaluate the effect of recurrent otitis media on auditory perception and speech.

PATIENTS AND METHODS

The experimental group consisted of 33 children with a history of at least three episodes of acute otitis media before 2 years of age. A control group of 29 children experienced one or fewer episodes of otitis media in the first 2 years of age. At the time of investigation, all children were 8 to 10 years of age and had attended similar pre-school, kindergarten, and elementary school programs. None showed evidence of mental retardation nor behavioral or emotional disorders. Speech ability was assessed by a battery of tests specific for Slovene language. Subtests of the Wechsler Intelligence Scale for Children were used as well as the Reading and Writing Test.

RESULTS

Auditory perception disorders were evident in 88% of children investigated who had a history of at least three episodes of otitis media by 2 years of age. Decreased auditory stimulation during the time of auditory maturation could prevent the development of these functions completely. No statistically significant differences were observed in the development of articulation. These observations underscore the importance of early management of recurrent otitis media during the first 2 years of age.

Maturation of binaural interaction components in auditory brainstem responses of young guinea pigs with monaural or binaural conductive hearing loss

Reversible conductive hearing loss created during the first 4 weeks post partum caused marked alterations in the maturation of binaural interaction components in the auditory brainstem responses of guinea pigs. In untreated control animals all three components investigated demonstrated postnatal development in terms of latency shortening that was completed during the first 3 weeks of life. Plugging of both external ear canals caused a significant delay in the maturation of the late component DN2, where latency values of the controls were reached only 2 weeks after the end of the treatment, i.e. after 6 weeks of life. Monaural deprivation likewise led to a retarded development of peak latencies during the phase of imbalanced sensory input. After the end of the one-sided conductive hearing loss the maturation process was markedly enhanced, even resulting in latency values for DN2 and DP1 that were significantly shorter than those of the controls. This phenomenon persisted until the end of the study period and was the case for both plugged and untreated ears in this group of animals. The time course of latencies in two other groups of experimental animals which were deprived in the same way as adults suggests that the effects observed are due to a sensitive period in the maturation process of the auditory pathway.