Developmental hearing loss disrupts synaptic inhibition: implications for auditory processing

Expression of group II mGluRs in the inferior colliculus, medial geniculate body, and auditory cortex increases with age

Metabotropic glutamate receptors (mGluRs) are widely expressed throughout the central nervous system. They are linked to G-protein coupled receptors and are known to modulate synaptic transmission. The data regarding their expression in auditory structures are not systematic and mainly originate from physiological studies where expression was used to support physiological findings. MGluRs are classified into three groups based on their sequence homology, G protein-coupling, and ligand selectivity. Our recent physiological findings made us focus on the group II mGluRs. The objective of this study was to characterize group II mGluR expression, and whether it changes in aged brains, in three central auditory structures: inferior colliculus (IC), medial geniculate body (MG), and auditory cortex (AC). We performed immunostaining experiments followed by optical density quantification in young and old mice. We found group II mGluR staining in the IC, MG, and AC. However, the intensity of this staining was not uniform within these auditory structures. In the IC expression was more intense in the dorsal compared to ventral part. Contrarily, the staining was more pronounced in the ventral part of the MG. In the AC, the staining was more intense near the surface and diminished toward white matter. We also found that the overall expression level of mGluR2/3 was increased significantly in aged animals in all auditory structures tested. Collectively, our detailed immunostaining findings suggest that the group II mGluRs are widely expressed throughout the central auditory system and their contribution to auditory processing increases with age.

Cortical sensory gating and reactions to dynamic speech-in-noise in older normal-hearing and hearing-impaired adults

OBJECTIVE

To examine whether cortical sensory gating predicts how older adults with and without hearing loss perform the Tracking of Noise Tolerance (TNT) test.

DESIGN

Single-blind mixed design. TNT performance was defined by average tolerated noise relative to speech levels (TNTAve) and by an average range of noise levels over a two-minute trial (excursion). Sensory gating of P1-N1-P2 components was measured using pairs of 1 kHz tone pips.

STUDY SAMPLE

Twenty-three normal-hearing (NH) and 16 hearing-impaired (HI) older adults with a moderate-to-severe degree of sensorineural hearing loss.

RESULTS

NH listeners tolerated significantly more noise than HI listeners, but the two groups did not differ in their excursion. Both NH and HI listeners exhibited significant gating of P1 amplitudes and N1P2 peak-to-peak amplitudes with no difference in gating magnitudes between listener groups. Sensory gating magnitudes of P1 and N1P2 did not predict TNTAve scores, but N1P2 gating negatively predicted excursion after accounting for listener age and hearing thresholds.

CONCLUSIONS

Listeners' reactivity to a roving noise (excursion), but not their average noise tolerance (TNTAve), was predicted by sensory gating at N1P2 generators. These results suggest that temporal aspects of speech-in-noise processing may be affected by declines in the central inhibition of older adults.

Neural hyperactivity and altered envelope encoding in the central auditory system: Changes with advanced age and hearing loss

Temporal modulations are ubiquitous features of sound signals that are important for auditory perception. The perception of temporal modulations, or temporal processing, is known to decline with aging and hearing loss and negatively impact auditory perception in general and speech recognition specifically. However, neurophysiological literature also provides evidence of exaggerated or enhanced encoding of specifically temporal envelopes in aging and hearing loss, which may arise from changes in inhibitory neurotransmission and neuronal hyperactivity. This review paper describes the physiological changes to the neural encoding of temporal envelopes that have been shown to occur with age and hearing loss and discusses the role of disinhibition and neural hyperactivity in contributing to these changes. Studies in both humans and animal models suggest that aging and hearing loss are associated with stronger neural representations of both periodic amplitude modulation envelopes and of naturalistic speech envelopes, but primarily for low-frequency modulations (<80 Hz). Although the frequency dependence of these results is generally taken as evidence of amplified envelope encoding at the cortex and impoverished encoding at the midbrain and brainstem, there is additional evidence to suggest that exaggerated envelope encoding may also occur subcortically, though only for envelopes with low modulation rates. A better understanding of how temporal envelope encoding is altered in aging and hearing loss, and the contexts in which neural responses are exaggerated/diminished, may aid in the development of interventions, assistive devices, and treatment strategies that work to ameliorate age- and hearing-loss-related auditory perceptual deficits.

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Impact of Prematurity on Auditory Processing in Children

Prematurity is one of the most crucial risk factors negatively affecting the maturation of the auditory system. Children born preterm demonstrate high rates of hearing impairments. Auditory processing difficulties in preterm children might be a result of disturbances in the central auditory system development and/or sensory deprivation due to peripheral hearing loss. To investigate auditory processing in preterm children, we utilized a set of psychoacoustic tests to assess temporal processing and speech intelligibility. A total of 241 children aged 6-11 years old (136 born preterm and 105 healthy full-term children forming the control group) were assessed. The preterm children were divided into three groups based on their peripheral hearing status: 74 normal hearing (NH group); 30 children with bilateral permanent sensorineural hearing loss (SNHL group) and 32 children with bilateral auditory neuropathy spectrum disorder (ANSD group). The results showed significantly worse performance in all tests in premature children compared with full-term children. NH and SNHL groups showed significant age-related improvement in speech recognition thresholds in noise that might signify a "bottom-up" auditory processing maturation effect. Overall, all premature children had signs of auditory processing disorders of varying degrees. Analyzing and understanding the auditory processing specificity in preterm children can positively contribute to the more effective implementation of rehabilitation programs.

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

Increased Central Auditory Gain and Decreased Parvalbumin-Positive Cortical Interneuron Density in the Df1/+ Mouse Model of Schizophrenia Correlate With Hearing Impairment

Background

Hearing impairment is a risk factor for schizophrenia. Patients with 22q11.2 deletion syndrome have a 25% to 30% risk of schizophrenia, and up to 60% also have varying degrees of hearing impairment, primarily from middle-ear inflammation. The Df1/+ mouse model of 22q11.2 deletion syndrome recapitulates many features of the human syndrome, including schizophrenia-relevant brain abnormalities and high interindividual variation in hearing ability. However, the relationship between brain abnormalities and hearing impairment in Df1/+ mice has not been examined.

Methods

We measured auditory brainstem responses, cortical auditory evoked potentials, and/or cortical parvalbumin-positive (PV+) interneuron density in over 70 adult mice (32 Df1/+, 39 wild-type). We also performed longitudinal auditory brainstem response measurements in an additional 20 animals (13 Df1/+, 7 wild-type) from 3 weeks of age.

Results

Electrophysiological markers of central auditory excitability were elevated in Df1/+ mice. PV+ interneurons, which are implicated in schizophrenia pathology, were reduced in density in the auditory cortex but not the secondary motor cortex. Both auditory brain abnormalities correlated with hearing impairment, which affected approximately 60% of adult Df1/+ mice and typically emerged before 6 weeks of age.

Conclusions

In the Df1/+ mouse model of 22q11.2 deletion syndrome, abnormalities in central auditory excitability and auditory cortical PV+ immunoreactivity correlate with hearing impairment. This is the first demonstration of cortical PV+ interneuron abnormalities correlating with hearing impairment in a mouse model of either schizophrenia or middle-ear inflammation.

Auditory Deprivation during Development Alters Efferent Neural Feedback and Perception

Auditory experience plays a critical role in hearing development. Developmental auditory deprivation because of otitis media, a common childhood disease, produces long-standing changes in the central auditory system, even after the middle ear pathology is resolved. The effects of sound deprivation because of otitis media have been mostly studied in the ascending auditory system but remain to be examined in the descending pathway that runs from the auditory cortex to the cochlea via the brainstem. Alterations in the efferent neural system could be important because the descending olivocochlear pathway influences the neural representation of transient sounds in noise in the afferent auditory system and is thought to be involved in auditory learning. Here, we show that the inhibitory strength of the medial olivocochlear efferents is weaker in children with a documented history of otitis media relative to controls; both boys and girls were included in the study. In addition, children with otitis media history required a higher signal-to-noise ratio on a sentence-in-noise recognition task than controls to achieve the same criterion performance level. Poorer speech-in-noise recognition, a hallmark of impaired central auditory processing, was related to efferent inhibition, and could not be attributed to the middle ear or cochlear mechanics.SIGNIFICANCE STATEMENT Otitis media is the second most common reason children go to the doctor. Previously, degraded auditory experience because of otitis media has been associated with reorganized ascending neural pathways, even after middle ear pathology resolved. Here, we show that altered afferent auditory input because of otitis media during childhood is also associated with long-lasting reduced descending neural pathway function and poorer speech-in-noise recognition. These novel, efferent findings may be important for the detection and treatment of childhood otitis media.

Peripheral Fragile X messenger ribonucleoprotein is required for the timely closure of a critical period for neuronal susceptibility in the ventral cochlear nucleus

Alterations in neuronal plasticity and critical periods are common across neurodevelopmental diseases, including Fragile X syndrome (FXS), the leading single-gene cause of autism. Characterized with sensory dysfunction, FXS is the result of gene silencing of Fragile X messenger ribonucleoprotein 1 (FMR1) and loss of its product, Fragile X messenger ribonucleoprotein (FMRP). The mechanisms underlying altered critical period and sensory dysfunction in FXS are obscure. Here, we performed genetic and surgical deprivation of peripheral auditory inputs in wildtype and Fmr1 knockout (KO) mice across ages and investigated the effects of global FMRP loss on deafferentation-induced neuronal changes in the ventral cochlear nucleus (VCN) and auditory brainstem responses. The degree of neuronal cell loss during the critical period was unchanged in Fmr1 KO mice. However, the closure of the critical period was delayed. Importantly, this delay was temporally coincidental with reduced hearing sensitivity, implying an association with sensory inputs. Functional analyses further identified early-onset and long-lasting alterations in signal transmission from the spiral ganglion to the VCN, suggesting a peripheral site of FMRP action. Finally, we generated conditional Fmr1 KO (cKO) mice with selective deletion of FMRP in spiral ganglion but not VCN neurons. cKO mice recapitulated the delay in the VCN critical period closure in Fmr1 KO mice, confirming an involvement of cochlear FMRP in shaping the temporal features of neuronal critical periods in the brain. Together, these results identify a novel peripheral mechanism of neurodevelopmental pathogenesis.

Developmental changes in electrophysiological properties of auditory cortical neurons in the Cntnap2 knockout rat

Disruptions in the CNTNAP2 gene are known to cause language impairments and symptoms associated with autism spectrum disorder (ASD). Importantly, knocking out this gene in rodents results in ASD-like symptoms that include auditory processing deficits. This study used in vitro patch-clamp electrophysiology to examine developmental alterations in auditory cortex pyramidal neurons of Cntnap2-/- rats, hypothesizing that CNTNAP2 is essential for maintaining intrinsic neuronal properties and synaptic wiring in the developing auditory cortex. Whole cell patch-clamp recordings were conducted in wildtype and Cntnap2-/- littermates at three postnatal age ranges (P8-12, P18-21, and P70-90). Consistent changes across age were seen in all measures of intrinsic membrane properties and spontaneous synaptic input. Intrinsic cell properties such as action potential half-widths, rheobase, and action-potential firing frequencies were different between wildtype and Cntnap2-/- rats predominantly during the juvenile stage (P18-21), whereas adult Cntnap2-/- rats showed higher frequencies of spontaneous and mini postsynaptic currents (sPSCs; mPSCs), with lower sPSC amplitudes. These results indicate that intrinsic cell properties are altered in Cntnap2-/- rats during the juvenile age, leading to a hyperexcitable phenotype during this stage of synaptic remodeling and refinement. Although intrinsic properties eventually normalize by reaching adulthood, changes in synaptic input, potentially caused by the differences in intrinsic membrane properties, seem to manifest in the adult age and are presumably responsible for the hyperreactive behavioral phenotype. In conjunction with a previous study, the present results also indicate a large influence of breeding scheme, i.e., pre- or postnatal environment, on the impact of Cntnap2 on cellular physiology.NEW & NOTEWORTHY This study shows that neurons in the auditory cortex of Cntnap2 knockout rats are hyperexcitable only during the juvenile age, whereas resulting changes in synaptic input persist in the adult. In conjunction with a previous study, the present results indicate that it is not the genes alone, but also the influence of pre- and postnatal environment, that shape neuronal function, highlighting the importance of early intervention in neurodevelopmental disorders.

Sound source localization patterns and bilateral cochlear implants: Age at onset of deafness effects

The ability to determine a sound’s location is critical in everyday life. However, sound source localization is severely compromised for patients with hearing loss who receive bilateral cochlear implants (BiCIs). Several patient factors relate to poorer performance in listeners with BiCIs, associated with auditory deprivation, experience, and age. Critically, characteristic errors are made by patients with BiCIs (e.g., medial responses at lateral target locations), and the relationship between patient factors and the type of errors made by patients has seldom been investigated across individuals. In the present study, several different types of analysis were used to understand localization errors and their relationship with patient-dependent factors (selected based on their robustness of prediction). Binaural hearing experience is required for developing accurate localization skills, auditory deprivation is associated with degradation of the auditory periphery, and aging leads to poorer temporal resolution. Therefore, it was hypothesized that earlier onsets of deafness would be associated with poorer localization acuity and longer periods without BiCI stimulation or older age would lead to greater amounts of variability in localization responses. A novel machine learning approach was introduced to characterize the types of errors made by listeners with BiCIs, making them simple to interpret and generalizable to everyday experience. Sound localization performance was measured in 48 listeners with BiCIs using pink noise trains presented in free-field. Our results suggest that older age at testing and earlier onset of deafness are associated with greater average error, particularly for sound sources near the center of the head, consistent with previous research. The machine learning analysis revealed that variability of localization responses tended to be greater for individuals with earlier compared to later onsets of deafness. These results suggest that early bilateral hearing is essential for best sound source localization outcomes in listeners with BiCIs.

Auditory Sensory Gating in Children With Cochlear Implants: A P50-N100-P200 Study

Background: While a cochlear implant (CI) can restore access to audibility in deaf children, implanted children may still have difficulty in concentrating. Previous studies have revealed a close relationship between sensory gating and attention. However, whether CI children have deficient auditory sensory gating remains unclear.

Methods: To address this issue, we measured the event-related potentials (ERPs), including P50, N100, and P200, evoked by paired tone bursts (S1 and S2) in CI children and normal-hearing (NH) controls. Suppressed amplitudes for S2 compared with S1 in these three ERPs reflected sensory gating during early and later phases, respectively. A Swanson, Nolan, and Pelham IV (SNAP-IV) scale was performed to assess the attentional performance.

Results: Significant amplitude differences between S1 and S2 in N100 and P200 were observed in both NH and CI children, indicating the presence of sensory gating in the two groups. However, the P50 suppression was only found in NH children and not in CI children. Furthermore, the duration of deafness was significantly positively correlated with the score of inattention in CI children.

Conclusion: Auditory sensory gating can develop but is deficient during the early phase in CI children. Long-term auditory deprivation has a negative effect on sensory gating and attentional performance.

Visuo-Acoustic Stimulation's Role in Synaptic Plasticity: A Review of the Literature

Brain plasticity is the capacity of cerebral neurons to change, structurally and functionally, in response to experiences. This is an essential property underlying the maturation of sensory functions, learning and memory processes, and brain repair in response to the occurrence of diseases and trauma. In this field, the visual system emerges as a paradigmatic research model, both for basic research studies and for translational investigations. The auditory system remains capable of reorganizing itself in response to different auditory stimulations or sensory organ modification. Acoustic biofeedback training can be an effective way to train patients with the central scotoma, who have poor fixation stability and poor visual acuity, in order to bring fixation on an eccentrical and healthy area of the retina: a pseudofovea. This review article is focused on the cellular and molecular mechanisms underlying retinal sensitivity changes and visual and auditory system plasticity.

Impact of cochlear ablation on calbindin and synaptophysin in the gerbil medial nucleus of the trapezoid body before hearing onset

Spontaneous bursting activity is already generated in the cochlea before hearing onset and represents an important condition of the functional and anatomical organization of auditory brainstem nuclei. In the present study, cochlea ablation induced changes were characterized in auditory brainstem nuclei indirectly innervated by auditory nerve fibers before hearing onset. In Meriones unguiculatus immunohistochemical labeling of calbindin-D28k (CB) and synaptophysin (SYN) were performed. The influence of cochlea-ablation on CB or SYN was analyzed by considering their differential immunoreaction during development. During the normal postnatal development, CB was first detected in somata of the medial nucleus of the trapezoid body (MNTB) at postnatal day (P)4. The immunoreaction increased gradually in parallel to the appearance of CB-immunoreactive terminal fields in distinct superior olivary complex (SOC) nuclei. Cochlear removal at P5 or P9 in animals with 24 and 48 h survival times resulted in an increase in somatic CB-labeling in the lesioned MNTB including terminal fields compared to the non-lesioned MNTB. SYN-immunolabeling was first detected at P0 and began to strongly encircle the MNTB neurons at P4. A further progression was observed with age. Cochlear ablation resulted in a significant reduction of SYN-labeled MNTB areas of P5-cochlea-ablated gerbils after 48 h post-lesion. In P9 cochlea-ablated gerbils, a redistribution of SYN-positive terminals was seen after 24 and 48 h. Taken together, the destruction of cochlea differentially influences CB- and SYN-labeling in the MNTB, which should be considered in association with different critical periods before hearing onset.

Hearing loss and brain plasticity: the hyperactivity phenomenon

Many aging adults experience some form of hearing problems that may arise from auditory peripheral damage. However, it has been increasingly acknowledged that hearing loss is not only a dysfunction of the auditory periphery but also results from changes within the entire auditory system, from periphery to cortex. Damage to the auditory periphery is associated with an increase in neural activity at various stages throughout the auditory pathway. Here, we review neurophysiological evidence of hyperactivity, auditory perceptual difficulties that may result from hyperactivity, and outline open conceptual and methodological questions related to the study of hyperactivity. We suggest that hyperactivity alters all aspects of hearing-including spectral, temporal, spatial hearing-and, in turn, impairs speech comprehension when background sound is present. By focusing on the perceptual consequences of hyperactivity and the potential challenges of investigating hyperactivity in humans, we hope to bring animal and human electrophysiologists closer together to better understand hearing problems in older adulthood.

Early Sensory Deprivation Leads to Differential Inhibitory Changes in the Striatum During Learning

The corticostriatal circuit has been identified as a vital pathway for associative learning. However, how learning is implemented when the sensory striatum is permanently impaired remains unclear. Using chemogenetic techniques to suppress layer five auditory cortex (AC) input to the auditory striatum, learning of a sound discrimination task was significantly impacted in freely moving Mongolian gerbils, in particular when this suppression occurs early on during learning. Whole-cell recordings sampled throughout learning revealed a transient reduction in postsynaptic (GABAA) inhibition in both striatal D1 and D2 cells in normal-hearing gerbils during task acquisition. In contrast, when the baseline striatal inhibitory strengths and firing rates were permanently reduced by a transient period of developmental sensory deprivation, learning was accompanied by augmented inhibition and increased firing rates. Direct manipulation of striatal inhibition in vivo and in vitro revealed a key role of the transient inhibitory changes in task acquisition. Together, these results reveal a flexible corticostriatal inhibitory synaptic plasticity mechanism that accompanies associative auditory learning.

Profiles and predictors of auditory functioning in abacus-trained children

OBJECTIVES

The present study explored the auditory benefits of abacus-training using a battery of tests (auditory acuity, clarity, and cognition). The study also aimed to identify the relative contributions of auditory processing tests that are most sensitive to the effects of abacus-training.

MATERIALS AND METHODS

The study was conducted on 60 children aged between 9 - 14 years. These participants were divided into two groups (abacus trained and untrained) of 30 each, who underwent a series of auditory functioning tests. The battery of tests included: auditory acuity (frequency, intensity, temporal, binaural and spatial resolution), auditory clarity (speech perception in noise), and auditory cognition (working digit and syllable memory).

RESULTS

Statistically (t-test and Mann Whitney U test), significant changes were observed in the spatial resolution, auditory clarity, and cognition tests, suggestive of positive outcomes of abacus training at the higher-order auditory processing. This finding was complemented by the discriminant function (DF) analyses, which showed that clarity and cognitive measures helped for effective group segregation (abacustrained and untrained). These measures had significantly higher contributions to the DF.

CONCLUSIONS

The findings of the study provide evidence of the multi-component benefits of abacus training in children and the transferability of learning effects to the auditory modality.

Linking anatomical and physiological markers of auditory system degeneration with behavioral hearing assessments in a mouse (Mus musculus) model of age-related hearing loss

Age-related hearing loss is a very common sensory disability, affecting one in three older adults. Establishing a link between anatomical, physiological, and behavioral markers of presbycusis in a mouse model can improve the understanding of this disorder in humans. We measured age-related hearing loss for a variety of acoustic signals in quiet and noisy environments using an operant conditioning procedure and investigated the status of peripheral structures in CBA/CaJ mice. Mice showed the greatest degree of hearing loss in the last third of their lifespan, with higher thresholds in noisy than in quiet conditions. Changes in auditory brainstem response thresholds and waveform morphology preceded behavioral hearing loss onset. Loss of hair cells, auditory nerve fibers, and signs of stria vascularis degeneration were observed in old mice. The present work underscores the difficulty in ascribing the primary cause of age-related hearing loss to any particular type of cellular degeneration. Revealing these complex structure-function relationships is critical for establishing successful intervention strategies to restore hearing or prevent presbycusis.

Dynamics of the fragile X mental retardation protein correlates with cellular and synaptic properties in primary auditory neurons following afferent deprivation

Afferent activity dynamically regulates neuronal properties and connectivity in the central nervous system. The Fragile X mental retardation protein (FMRP) is an RNA-binding protein that regulates cellular and synaptic properties in an activity-dependent manner. Whether and how FMRP level and localization are regulated by afferent input remains sparsely examined and how such regulation is associated with neuronal response to changes in sensory input is unknown. We characterized changes in FMRP level and localization in the chicken nucleus magnocellularis (NM), a primary cochlear nucleus, following afferent deprivation by unilateral cochlea removal. We observed rapid (within 2 hr) aggregation of FMRP immunoreactivity into large granular structures in a subset of deafferented NM neurons. Neurons that exhibited persistent FMRP aggregation at 12-24 hr eventually lost cytoplasmic Nissl substance, indicating cell death. A week later, FMRP expression in surviving neurons regained its homeostasis, with a slightly reduced immunostaining intensity and enhanced heterogeneity. Correlation analyses under the homeostatic status (7-14 days) revealed that neurons expressing relatively more FMRP had a higher capability of maintaining cell body size and ribosomal activity, as well as a better ability to detach inactive presynaptic terminals. Additionally, the intensity of an inhibitory postsynaptic protein, gephyrin, was reduced following deafferentation and was positively correlated with FMRP intensity, implicating an involvement of FMRP in synaptic dynamics in response to reduced afferent inputs. Collectively, this study demonstrates that afferent input regulates FMRP expression and localization in ways associated with multiple types of neuronal responses and synaptic rearrangements.

Regulation of auditory plasticity during critical periods and following hearing loss

Sensory input has profound effects on neuronal organization and sensory maps in the brain. The mechanisms regulating plasticity of the auditory pathway have been revealed by examining the consequences of altered auditory input during both developmental critical periods—when plasticity facilitates the optimization of neural circuits in concert with the external environment—and in adulthood—when hearing loss is linked to the generation of tinnitus. In this review, we summarize research identifying the molecular, cellular, and circuit-level mechanisms regulating neuronal organization and tonotopic map plasticity during developmental critical periods and in adulthood. These mechanisms are shared in both the juvenile and adult brain and along the length of the auditory pathway, where they serve to regulate disinhibitory networks, synaptic structure and function, as well as structural barriers to plasticity. Regulation of plasticity also involves both neuromodulatory circuits, which link plasticity with learning and attention, as well as ascending and descending auditory circuits, which link the auditory cortex and lower structures. Further work identifying the interplay of molecular and cellular mechanisms associating hearing loss-induced plasticity with tinnitus will continue to advance our understanding of this disorder and lead to new approaches to its treatment.

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During CPs, brain plasticity is enhanced and sensitive to acoustic experience.

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Enhanced plasticity can be reinstated in the adult brain following hearing loss.

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Molecular, cellular, and circuit-level mechanisms regulate CP and adult plasticity.

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Plasticity resulting from hearing loss may contribute to the emergence of tinnitus.

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Modifying plasticity in the adult brain may offer new treatments for tinnitus.

Simulation of the efficiency of inner hair cell secretion in the auditory pathway

Sound coding involves several stages of processing along the auditory path. Specifically, the Inner Hair Cells (IHC) act as sensory receptors and transduce acoustic information -frequency, intensity and duration of the stimulus- into neuronal signals. In this work, a stochastic model was implemented to achieve a better understanding of the IHC-auditory nerve synapse, specifically, the process of Ready Releasable Pool (RRP) vesicle exocytosis, a complicated process to study experimentally because current protocols do not provide adequate temporal resolution, in the order of milliseconds. The presented model allows predicting the efficiency of glutamate release towards explaining maturation changes or disease impacts in the auditory pathway.

Unilateral hearing loss: benefit of amplification in sound localization, temporal ordering and resolution

PURPOSE

To assess the hearing abilities of temporal ordering, temporal resolution and sound localization before and after the fitting of a hearing aid (HA) in individuals with unilateral hearing loss (UHL).

METHODS

There were evaluated 22 subjects, aged 18 to 60 years, diagnosed with sensorineural or mixed UHL, from mild to severe degrees. The study was divided into two stages: the pre and post-adaptation of HA. In both phases, subjects performed an interview, application of Questionnaire for Disabilities Associated with Impaired Auditory Localization, auditory processing screening protocol (APSP) and Random Gap Detection Test (RGDT).

RESULTS

This study found no statistically significant difference in sound localization and memory evaluations for verbal sounds in sequence, in RGDT and Questionnaire for Disabilities Associated with Impaired Auditory Localization.

CONCLUSION

With the effective use of hearing aids, individuals with UHL showed improvement in the auditory abilities of sound localization, ordering and temporal resolution.

Preserving Inhibition during Developmental Hearing Loss Rescues Auditory Learning and Perception

Transient periods of childhood hearing loss can induce deficits in aural communication that persist long after auditory thresholds have returned to normal, reflecting long-lasting impairments to the auditory CNS. Here, we asked whether these behavioral deficits could be reversed by treating one of the central impairments: reduction of inhibitory strength. Male and female gerbils received bilateral earplugs to induce a mild, reversible hearing loss during the critical period of auditory cortex development. After earplug removal and the return of normal auditory thresholds, we trained and tested animals on an amplitude modulation detection task. Transient developmental hearing loss induced both learning and perceptual deficits, which were entirely corrected by treatment with a selective GABA reuptake inhibitor (SGRI). To explore the mechanistic basis for these behavioral findings, we recorded the amplitudes of GABA_A and GABA_B receptor-mediated IPSPs in auditory cortical and thalamic brain slices. In hearing loss-reared animals, cortical IPSP amplitudes were significantly reduced within a few days of hearing loss onset, and this reduction persisted into adulthood. SGRI treatment during the critical period prevented the hearing loss-induced reduction of IPSP amplitudes; but when administered after the critical period, it only restored GABA_B receptor-mediated IPSP amplitudes. These effects were driven, in part, by the ability of SGRI to upregulate α1 subunit-dependent GABA_A responses. Similarly, SGRI prevented the hearing loss-induced reduction of GABA_A and GABA_B IPSPs in the ventral nucleus of the medial geniculate body. Thus, by maintaining, or subsequently rescuing, GABAergic transmission in the central auditory thalamocortical pathway, some perceptual and cognitive deficits induced by developmental hearing loss can be prevented.SIGNIFICANCE STATEMENT Even a temporary period of childhood hearing loss can induce communication deficits that persist long after auditory thresholds return to normal. These deficits may arise from long-lasting central impairments, including the loss of synaptic inhibition. Here, we asked whether hearing loss-induced behavioral deficits could be reversed by reinstating normal inhibitory strength. Gerbils reared with transient hearing loss displayed both learning and perceptual deficits. However, when animals were treated with a selective GABA reuptake inhibitor during or after hearing loss, behavioral deficits were entirely corrected. This behavioral recovery was correlated with the return of normal thalamic and cortical inhibitory function. Thus, some perceptual and cognitive deficits induced by developmental hearing loss were prevented with a treatment that rescues a central synaptic property.

Neural ITD Sensitivity and Temporal Coding with Cochlear Implants in an Animal Model of Early-Onset Deafness

Users of cochlear implant (CI) face challenges in everyday situations such as understanding conversations in noise, even with CIs in both ears. These challenges are related to difficulties with tasks that require fine temporal processing such as discrimination of pulse rates or interaural time differences (ITD), a major cue for sound localization. The degradation in temporal processing and ITD sensitivity are especially acute in those who lost hearing in early childhood. Here, we characterized temporal coding and ITD sensitivity of single neurons in a novel animal model of early-onset deafness. Rabbits were deafened as neonates and deprived of auditory stimulation until they reached adult age when single-unit recordings from the auditory midbrain were made chronically using an unanesthetized preparation. The results are compared to measurements from adult-deafened rabbits with normal auditory development to understand the effect of early-onset deafness on neural temporal coding and ITD sensitivity. Neurons in the inferior colliculus (IC) of early-deafened rabbits were less likely to show sustained, excitatory responses to pulse train stimulation and more likely to show suppressive responses compared to neurons in adult-deaf animals. Fewer neurons showed synchronized responses to pulse trains at any rate in the early-deaf group. In addition, fewer neurons showed significant ITD sensitivity in their overall firing rate in the early-deaf group compared to adult-deaf animals. Neural ITD discrimination thresholds in the early-deaf group were poorer than thresholds in adult-deaf group, especially at high pulse rates. The overall degradation in neural ITD sensitivity is consistent with the difficulties encountered by human CI users with early-onset hearing loss. These results lay the groundwork for investigating whether the degradations in temporal coding and ITD sensitivity observed in early-deaf animals can be reversed by appropriate CI stimulation during development.

Aging alters envelope representations of speech-like sounds in the inferior colliculus

Hearing impairment in older people is thought to arise from impaired temporal processing in auditory circuits. We used a systems-level (scalp recordings) and a microcircuit-level (extracellular recordings) approach to investigate how aging affects the sensitivity to temporal envelopes of speech-like sounds in rats. Scalp-recorded potentials suggest an age-related increase in sensitivity to temporal regularity along the ascending auditory pathway. The underlying cellular changes in the midbrain were examined using extracellular recordings from inferior colliculus neurons. We observed an age-related increase in sensitivity to the sound's onset and temporal regularity (i.e., periodicity envelope) in the spiking output of inferior colliculus neurons, relative to their synaptic inputs (local field potentials). This relative enhancement for aged animals was most prominent for multi-unit (relative to single-unit) spiking activity. Spontaneous multi-unit, but not single-unit, activity was also enhanced in aged compared with young animals. Our results suggest that aging is associated with altered sensitivity to a sound's temporal regularities, and that these effects may be due to increased gain of neural network activity in the midbrain.

A Computational Model of Thalamocortical Dysrhythmia in People With Tinnitus

Tinnitus is a problem that affects a diverse range of people. One common trait amongst people with tinnitus is the presence of hearing loss, which is apparent in over 90% of the cohort. It is postulated that the remainder of people with tinnitus have hidden hearing loss in the form of cochlear synaptopathy. The loss of hearing sensation is thought to cause a reduction in the bottom-up excitatory signals of the auditory pathway leading to a change in the frequency of thalamocortical oscillations known as thalamocortical dysrhythmia (TCD). The downward shift in oscillatory behavior, characteristic of TCD, has been recorded experimentally but the underlying mechanisms responsible for TCD in tinnitus subjects cannot be directly observed. This paper investigates these underlying mechanisms by creating a biologically faithful model of the auditory periphery and thalamocortical network, called the central auditory processing (CAP) model. The proposed model replicates tinnitus related activity in the presence of hearing loss and hidden hearing loss in the form of cochlear synaptopathy. The results of this paper show that, both the bottom-up and top-down changes are required in the auditory system for tinnitus related hyperactivity to coexist with TCD, contrary to the theoretical model for TCD. The CAP model provides a novel modeling approach to account for tinnitus related activity with and without hearing loss. Moreover, the results provide additional clarity to the understanding of TCD and tinnitus and provide direction for future approaches to treating tinnitus.

Neural Processing of Acoustic and Electric Interaural Time Differences in Normal-Hearing Gerbils

Bilateral cochlear implants (CIs) provide benefits for speech perception in noise and directional hearing, but users typically show poor sensitivity to interaural time differences (ITDs). Possible explanations for this deficit are deafness-induced degradations in neural ITD sensitivity, between-ear mismatches in electrode positions or activation sites, or differences in binaural brain circuits activated by electric versus acoustic stimulation. To identify potential limitations of electric ITD coding in the normal-hearing system, responses of single neurons in the dorsal nucleus of the lateral lemniscus and in the inferior colliculus to ITDs of electric (biphasic pulses) and acoustic (noise, clicks, chirps, and tones) stimuli were recorded in normal-hearing gerbils of either sex. To maintain acoustic sensitivity, electric stimuli were delivered to the round window. ITD tuning metrics (e.g., best ITD) and ITD discrimination thresholds for electric versus transient acoustic stimuli (clicks, chirps) obtained from the same neurons were not significantly correlated. Across populations of neurons with similar characteristic frequencies, however, ITD tuning metrics and ITD discrimination thresholds were similar for electric and acoustic stimuli and largely independent of the spectrotemporal properties of the acoustic stimuli when measured in the central range of ITDs. The similarity of acoustic and electric ITD coding on the population level in animals with normal hearing experience suggests that poorer ITD sensitivity in bilateral CI users compared with normal-hearing listeners is likely due to deprivation-induced changes in neural ITD coding rather than to differences in the binaural brain circuits involved in the processing of electric and acoustic ITDs.SIGNIFICANCE STATEMENT Small differences in the arrival time of sound at the two ears (interaural time differences, ITDs) provide important cues for speech understanding in noise and directional hearing. Deaf subjects with bilateral cochlear implants obtain only little benefit from ITDs. It is unclear whether these limitations are due to between-ear mismatches in activation sites, differences in binaural brain circuits activated by electric versus acoustic stimulation, or deafness-induced degradations in neural ITD processing. This study is the first to directly compare electric and acoustic ITD coding in neurons of known characteristic frequencies. In animals with normal hearing, populations of auditory brainstem and midbrain neurons demonstrate general similarities in electric and acoustic ITD coding, suggesting similar underlying central auditory processing mechanisms.

Aging Affects Adaptation to Sound-Level Statistics in Human Auditory Cortex

Optimal perception requires efficient and adaptive neural processing of sensory input. Neurons in nonhuman mammals adapt to the statistical properties of acoustic feature distributions such that they become sensitive to sounds that are most likely to occur in the environment. However, whether human auditory responses adapt to stimulus statistical distributions and how aging affects adaptation to stimulus statistics is unknown. We used MEG to study how exposure to different distributions of sound levels affects adaptation in auditory cortex of younger (mean: 25 years; n = 19) and older (mean: 64 years; n = 20) adults (male and female). Participants passively listened to two sound-level distributions with different modes (either 15 or 45 dB sensation level). In a control block with long interstimulus intervals, allowing neural populations to recover from adaptation, neural response magnitudes were similar between younger and older adults. Critically, both age groups demonstrated adaptation to sound-level stimulus statistics, but adaptation was altered for older compared with younger people: in the older group, neural responses continued to be sensitive to sound level under conditions in which responses were fully adapted in the younger group. The lack of full adaptation to the statistics of the sensory environment may be a physiological mechanism underlying the known difficulty that older adults have with filtering out irrelevant sensory information.SIGNIFICANCE STATEMENT Behavior requires efficient processing of acoustic stimulation. Animal work suggests that neurons accomplish efficient processing by adjusting their response sensitivity depending on statistical properties of the acoustic environment. Little is known about the extent to which this adaptation to stimulus statistics generalizes to humans, particularly to older humans. We used MEG to investigate how aging influences adaptation to sound-level statistics. Listeners were presented with sounds drawn from sound-level distributions with different modes (15 vs 45 dB). Auditory cortex neurons adapted to sound-level statistics in younger and older adults, but adaptation was incomplete in older people. The data suggest that the aging auditory system does not fully capitalize on the statistics available in sound environments to tune the perceptual system dynamically.

Brief Stimulus Exposure Fully Remediates Temporal Processing Deficits Induced by Early Hearing Loss

In childhood, partial hearing loss can produce prolonged deficits in speech perception and temporal processing. However, early therapeutic interventions targeting temporal processing may improve later speech-related outcomes. Gap detection is a measure of auditory temporal resolution that relies on the auditory cortex (ACx), and early auditory deprivation alters intrinsic and synaptic properties in the ACx. Thus, early deprivation should induce deficits in gap detection, which should be reflected in ACx gap sensitivity. We tested whether earplugging-induced, early transient auditory deprivation in male and female Mongolian gerbils caused correlated deficits in behavioral and cortical gap detection, and whether these could be rescued by a novel therapeutic approach: brief exposure to gaps in background noise. Two weeks after earplug removal, animals that had been earplugged from hearing onset throughout auditory critical periods displayed impaired behavioral gap detection thresholds (GDTs), but this deficit was fully reversed by three 1 h sessions of exposure to gaps in noise. In parallel, after earplugging, cortical GDTs increased because fewer cells were sensitive to short gaps, and gap exposure normalized this pattern. Furthermore, in deprived animals, both first-spike latency and first-spike latency jitter increased, while spontaneous and evoked firing rates decreased, suggesting that deprivation causes a wider range of perceptual problems than measured here. These cortical changes all returned to control levels after gap exposure. Thus, brief stimulus exposure, perhaps in a salient context such as the unfamiliar placement into a testing apparatus, rescued impaired gap detection and may have potential as a remediation tool for general auditory processing deficits.SIGNIFICANCE STATEMENT Hearing loss in early childhood leads to impairments in auditory perception and language processing that can last well beyond the restoration of hearing sensitivity. Perceptual deficits can be improved by training, or by acoustic enrichment in animal models, but both approaches involve extended time and effort. Here, we used a novel remediation technique, brief periods of auditory stimulus exposure, to fully remediate cortical and perceptual deficits in gap detection induced by early transient hearing loss. This technique also improved multiple cortical response properties. Rescue by this efficient exposure regime may have potential as a therapeutic tool to remediate general auditory processing deficits in children with perceptual challenges arising from early hearing loss.

Neural Coding of Interaural Time Differences with Bilateral Cochlear Implants in Unanesthetized Rabbits

Although bilateral cochlear implants (CIs) provide improvements in sound localization and speech perception in noise over unilateral CIs, bilateral CI users' sensitivity to interaural time differences (ITDs) is still poorer than normal. In particular, ITD sensitivity of most CI users degrades with increasing stimulation rate and is lacking at the high carrier pulse rates used in CI processors to deliver speech information. To gain a better understanding of the neural basis for this degradation, we characterized ITD tuning of single neurons in the inferior colliculus (IC) for pulse train stimuli in an unanesthetized rabbit model of bilateral CIs. Approximately 73% of IC neurons showed significant ITD sensitivity in their overall firing rates. On average, ITD sensitivity was best for pulse rates near 80-160 pulses per second (pps) and degraded for both lower and higher pulse rates. The degradation in ITD sensitivity at low pulse rates was caused by strong, unsynchronized background activity that masked stimulus-driven responses in many neurons. Selecting synchronized responses by temporal windowing revealed ITD sensitivity in these neurons. With temporal windowing, both the fraction of ITD-sensitive neurons and the degree of ITD sensitivity decreased monotonically with increasing pulse rate. To compare neural ITD sensitivity to human performance in ITD discrimination, neural just-noticeable differences (JNDs) in ITD were computed using signal detection theory. Using temporal windowing at lower pulse rates, and overall firing rate at higher pulse rates, neural ITD JNDs were within the range of perceptual JNDs in human CI users over a wide range of pulse rates.

SIGNIFICANCE STATEMENT

Many profoundly deaf people wearing cochlear implants (CIs) still face challenges in everyday situations, such as understanding conversations in noise. Even with CIs in both ears, they have difficulty making full use of subtle differences in the sounds reaching the two ears [interaural time difference (ITD)] to identify where the sound is coming from. This problem is especially acute at the high stimulation rates used in clinical CI processors. This study provides a better understanding of ITD processing with bilateral CIs and shows a parallel between human performance in ITD discrimination and neural responses in the auditory midbrain. The present study is the first report on binaural properties of auditory neurons with CIs in unanesthetized animals.

Neurophysiology and Regulation of the Balance Between Excitation and Inhibition in Neocortical Circuits

Brain function relies on the ability of neural networks to maintain stable levels of activity, while experiences sculpt them. In the neocortex, the balance between activity and stability relies on the coregulation of excitatory and inhibitory inputs onto principal neurons. Shifts of excitation or inhibition result in altered excitability impaired processing of incoming information. In many neurodevelopmental and neuropsychiatric disorders, the excitability of local circuits is altered, suggesting that their pathophysiology may involve shifts in synaptic excitation, inhibition, or both. Most studies focused on identifying the cellular and molecular mechanisms controlling network excitability to assess whether they may be altered in animal models of disease. The impact of changes in excitation/inhibition balance on local circuit and network computations is not clear. Here we report findings on the integration of excitatory and inhibitory inputs in healthy cortical circuits and discuss how shifts in excitation/inhibition balance may relate to pathological phenotypes.

Ageing affects dual encoding of periodicity and envelope shape in rat inferior colliculus neurons

Extracting temporal periodicities and envelope shapes of sounds is important for listening within complex auditory scenes but declines behaviorally with age. Here, we recorded local field potentials (LFPs) and spikes to investigate how ageing affects the neural representations of different modulation rates and envelope shapes in the inferior colliculus of rats. We specifically aimed to explore the input-output (LFP-spike) response transformations of inferior colliculus neurons. Our results show that envelope shapes up to 256-Hz modulation rates are represented in the neural synchronisation phase lags in younger and older animals. Critically, ageing was associated with (i) an enhanced gain in onset response magnitude from LFPs to spikes; (ii) an enhanced gain in neural synchronisation strength from LFPs to spikes for a low modulation rate (45 Hz); (iii) a decrease in LFP synchronisation strength for higher modulation rates (128 and 256 Hz) and (iv) changes in neural synchronisation strength to different envelope shapes. The current age-related changes are discussed in the context of an altered excitation-inhibition balance accompanying ageing.

Developmental exposure to PCBs alters the activation of the auditory cortex in response to GABAA antagonism

Developmental exposure of rats to polychlorinated biphenyls (PCBs) causes impairments in hearing and in the functioning of peripheral and central auditory structures. Additionally, recent work from our laboratory has demonstrated an increase in audiogenic seizures. The current study aimed to further characterize the effects of PCBs on auditory brain structures by investigating whether developmental exposure altered the magnitude of activation in the auditory cortex (AC) in response to electrical stimulation of thalamocortical afferents. Long-Evans female rats were fed cookies containing either 0 or 6mg/kg of an environmental PCB mixture daily from 4 weeks prior to breeding until postnatal day 21. Brain slices containing projections from the thalamus to the AC were collected from adult female offspring and were bathed in artificial cerebrospinal fluid (aCSF) alone, aCSF containing a gamma-aminobutyric acid (GABA) receptor antagonist (200nM SR95531), and aCSF containing an and N-methyl-d-aspartate (NMDA) receptor antagonist (50μM AP5). During each of these drug conditions, electrical stimulations ranging from 25 to 600μA were delivered to the thalamocortical afferents. Activation of the AC was measured using flavoprotein autofluorescence imaging. Although there were no differences seen between treatment groups in the aCSF condition, there were significant increases in the ratio of aCSF/SR95531 activation in slices from PCB-exposed animals compared to control animals. This effect was seen in both the upper and lower layers of the AC. No differences in activation were noted between treatment groups when slices were exposed to AP5. These data suggest that developmental PCB exposure leads to increased sensitivity to antagonism of GABA_A receptors in the AC without a change in NMDA-mediated intrinsic excitability.

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

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

Ageing-related changes in GABAergic inhibition in mouse auditory cortex, measured using in vitro flavoprotein autofluorescence imaging

KEY POINTS

Ageing is associated with hearing loss and changes in GABAergic signalling in the auditory system. We tested whether GABAergic signalling in an isolated forebrain preparation also showed ageing-related changes. A novel approach was used, whereby population imaging was coupled to quantitative pharmacological sensitivity. Sensitivity to GABAA blockade was inversely associated with age and cortical thickness, but hearing loss did not independently contribute to the change in GABAA ergic sensitivity. Redox states in the auditory cortex of young and aged animals were similar, suggesting that the differences in GABAA ergic sensitivity are unlikely to be due to differences in slice health. To examine ageing-related changes in the earliest stages of auditory cortical processing, population auditory cortical responses to thalamic afferent stimulation were studied in brain slices obtained from young and aged CBA/CAj mice (up to 28 months of age). Cortical responses were measured using flavoprotein autofluorescence imaging, and ageing-related changes in inhibition were assessed by measuring the sensitivity of these responses to blockade of GABAA receptors using bath-applied SR95531. The maximum auditory cortical response to afferent stimulation was not different between young and aged animals under control conditions, but responses to afferent stimulation in aged animals showed a significantly lower sensitivity to GABA blockade with SR95531. Cortical thickness, but not hearing loss, improved the prediction of all imaging variables when combined with age, particularly sensitivity to GABA blockade for the maximum response. To determine if the observed differences between slices from young and aged animals were due to differences in slice health, the redox state in the auditory cortex was assessed by measuring the FAD+/NADH ratio using fluorescence imaging. We found that this ratio is highly sensitive to known redox stressors such as H2 O2 and NaCN; however, no difference was found between young and aged animals. By using a new approach to quantitatively assess pharmacological sensitivity of population-level cortical responses to afferent stimulation, these data demonstrate that auditory cortical inhibition diminishes with ageing. Furthermore, these data establish a significant relationship between cortical thickness and GABAergic sensitivity, which had not previously been observed in an animal model of ageing.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Limitations on Monaural and Binaural Temporal Processing in Bilateral Cochlear Implant Listeners

Monaural rate discrimination and binaural interaural time difference (ITD) discrimination were studied as functions of pulse rate in a group of bilaterally implanted cochlear implant users. Stimuli for the rate discrimination task were pulse trains presented to one electrode, which could be in the apical, middle, or basal part of the array, and in either the left or the right ear. In each two-interval trial, the standard stimulus had a rate of 100, 200, 300, or 500 pulses per second and the signal stimulus had a rate 35 % higher. ITD discrimination between pitch-matched electrode pairs was measured for the same standard rates as in the rate discrimination task and with an ITD of +/− 500 μs. Sensitivity (d′) on both tasks decreased with increasing rate, as has been reported previously. This study tested the hypothesis that deterioration in performance at high rates occurs for the two tasks due to a common neural basis, specific to the stimulation of each electrode. Results show that ITD scores for different pairs of electrodes correlated with the lower rate discrimination scores for those two electrodes. Statistical analysis, which partialed out overall differences between listeners, electrodes, and rates, supports the hypothesis that monaural and binaural temporal processing limitations are at least partly due to a common mechanism.

Central plasticity and dysfunction elicited by aural deprivation in the critical period

The acoustic signal is crucial for animals to obtain information from the surrounding environment. Like other sensory modalities, the central auditory system undergoes adaptive changes (i.e., plasticity) during the developmental stage as well as other stages of life. Owing to its plasticity, auditory centers may be susceptible to various factors, such as medical intervention, variation in ambient acoustic signals and lesion of the peripheral hearing organ. There are critical periods during which auditory centers are vulnerable to abnormal experiences. Particularly in the early postnatal development period, aural inputs are essential for functional maturity of auditory centers. An aural deprivation model, which can be achieved by attenuating or blocking the peripheral acoustic afferent input to the auditory center, is ideal for investigating plastic changes of auditory centers. Generally, auditory plasticity includes structural and functional changes, some of which can be irreversible. Aural deprivation can distort tonotopic maps, disrupt the binaural integration, reorganize the neural network and change the synaptic transmission in the primary auditory cortex or at lower levels of the auditory system. The regulation of specific gene expression and the modified signal pathway may be the deep molecular mechanism of these plastic changes. By studying this model, researchers may explore the pathogenesis of hearing loss and reveal plastic changes of the auditory cortex, facilitating the therapeutic advancement in patients with severe hearing loss. After summarizing developmental features of auditory centers in auditory deprived animals and discussing changes of central auditory remodeling in hearing loss patients, we aim at stressing the significant of an early and well-designed auditory training program for the hearing rehabilitation.

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

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

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

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

Balancing plasticity/stability across brain development

The potency of the environment to shape brain function changes dramatically across the lifespan. Neural circuits exhibit profound plasticity during early life and are later stabilized. A focus on the cellular and molecular bases of these developmental trajectories has begun to unravel mechanisms, which control the onset and closure of such critical periods. Two important concepts have emerged from the study of critical periods in the visual cortex: (1) excitatory-inhibitory circuit balance is a trigger; and (2) molecular "brakes" limit adult plasticity. The onset of the critical period is determined by the maturation of specific GABA circuits. Targeting these circuits using pharmacological or genetic approaches can trigger premature onset or induce a delay. These manipulations are so powerful that animals of identical chronological age may be at the peak, before, or past their plastic window. Thus, critical period timing per se is plastic. Conversely, one of the outcomes of normal development is to stabilize the neural networks initially sculpted by experience. Rather than being passively lost, the brain's intrinsic potential for plasticity is actively dampened. This is demonstrated by the late expression of brake-like factors, which reversibly limit excessive circuit rewiring beyond a critical period. Interestingly, many of these plasticity regulators are found in the extracellular milieu. Understanding why so many regulators exist, how they interact and, ultimately, how to lift them in noninvasive ways may hold the key to novel therapies and lifelong learning.

Shaping the aging brain: role of auditory input patterns in the emergence of auditory cortical impairments

Age-related impairments in the primary auditory cortex (A1) include poor tuning selectivity, neural desynchronization, and degraded responses to low-probability sounds. These changes have been largely attributed to reduced inhibition in the aged brain, and are thought to contribute to substantial hearing impairment in both humans and animals. Since many of these changes can be partially reversed with auditory training, it has been speculated that they might not be purely degenerative, but might rather represent negative plastic adjustments to noisy or distorted auditory signals reaching the brain. To test this hypothesis, we examined the impact of exposing young adult rats to 8 weeks of low-grade broadband noise on several aspects of A1 function and structure. We then characterized the same A1 elements in aging rats for comparison. We found that the impact of noise exposure on A1 tuning selectivity, temporal processing of auditory signal and responses to oddball tones was almost indistinguishable from the effect of natural aging. Moreover, noise exposure resulted in a reduction in the population of parvalbumin inhibitory interneurons and cortical myelin as previously documented in the aged group. Most of these changes reversed after returning the rats to a quiet environment. These results support the hypothesis that age-related changes in A1 have a strong activity-dependent component and indicate that the presence or absence of clear auditory input patterns might be a key factor in sustaining adult A1 function.

Hearing loss differentially affects thalamic drive to two cortical interneuron subtypes

Sensory deprivation, such as developmental hearing loss, leads to an adjustment of synaptic and membrane properties throughout the central nervous system. These changes are thought to compensate for diminished sound-evoked activity. This model predicts that compensatory changes should be synergistic with one another along each functional pathway. To test this idea, we examined the excitatory thalamic drive to two types of cortical inhibitory interneurons that display differential effects in response to developmental hearing loss. The inhibitory synapses made by fast-spiking (FS) cells are weakened by hearing loss, whereas those made by low threshold-spiking (LTS) cells remain strong but display greater short-term depression (Takesian et al. 2010). Whole-cell recordings were made from FS or LTS interneurons in a thalamocortical brain slice, and medial geniculate (MG)-evoked postsynaptic potentials were analyzed. Following hearing loss, MG-evoked net excitatory potentials were smaller than normal at FS cells but larger than normal at LTS cells. Furthermore, MG-evoked excitatory potentials displayed less short-term depression at FS cells and greater short-term depression at LTS cells. Thus deprivation-induced adjustments of excitatory synapses onto inhibitory interneurons are cell-type specific and parallel the changes made by the inhibitory afferents.

Central auditory maturation and behavioral outcome in children with auditory neuropathy spectrum disorder who use cochlear implants

OBJECTIVE

We examined cortical auditory development and behavioral outcomes in children with ANSD fitted with cochlear implants (CI).

DESIGN

Cortical maturation, measured by P1 cortical auditory evoked potential (CAEP) latency, was regressed against scores on the infant toddler meaningful auditory integration scale (IT-MAIS). Implantation age was also considered in relation to CAEP findings.

STUDY SAMPLE

Cross-sectional and longitudinal samples of 24 and 11 children, respectively, with ANSD fitted with CIs.

RESULTS

P1 CAEP responses were present in all children after implantation, though previous findings suggest that only 50-75% of ANSD children with hearing aids show CAEP responses. P1 CAEP latency was significantly correlated with participants' IT-MAIS scores. Furthermore, more children implanted before age two years showed normal P1 latencies, while those implanted later mainly showed delayed latencies. Longitudinal analysis revealed that most children showed normal or improved cortical maturation after implantation.

CONCLUSION

Cochlear implantation resulted in measureable cortical auditory development for all children with ANSD. Children fitted with CIs under age two years were more likely to show age-appropriate CAEP responses within six months after implantation, suggesting a possible sensitive period for cortical auditory development in ANSD. That CAEP responses were correlated with behavioral outcome highlights their clinical decision-making utility.

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.