Developmental fine-tuning of medial superior olive neurons mitigates their predisposition to contralateral sound sources.

Having two ears enables us to localize sound sources by exploiting interaural time differences (ITDs) in sound arrival. Principal neurons of the medial superior olive (MSO) are sensitive to ITD, and each MSO neuron responds optimally to a best ITD (bITD). In many cells, especially those tuned to low sound frequencies, these bITDs correspond to ITDs for which the contralateral ear leads, and are often larger than the ecologically relevant range, defined by the ratio of the interaural distance and the speed of sound. Using in vivo recordings in gerbils, we found that shortly after hearing onset the bITDs were even more contralaterally leading than found in adult gerbils, and travel latencies for contralateral sound-evoked activity clearly exceeded those for ipsilateral sounds. During the following weeks, both these latencies and their interaural difference decreased. A computational model indicated that spike timing-dependent plasticity can underlie this fine-tuning. Our results suggest that MSO neurons start out with a strong predisposition toward contralateral sounds due to their longer neural travel latencies, but that, especially in high-frequency neurons, this predisposition is subsequently mitigated by differential developmental fine-tuning of the travel latencies.

An inhibitory glycinergic projection from the cochlear nucleus to the lateral superior olive.

Auditory brainstem neurons in the lateral superior olive (LSO) receive excitatory input from the ipsilateral cochlear nucleus (CN) and inhibitory transmission from the contralateral CN via the medial nucleus of the trapezoid body (MNTB). This circuit enables sound localization using interaural level differences. Early studies have observed an additional inhibitory input originating from the ipsilateral side. However, many of its details, such as its origin, remained elusive. Employing electrical and optical stimulation of afferents in acute mouse brainstem slices and anatomical tracing, we here describe a glycinergic projection to LSO principal neurons that originates from the ipsilateral CN. This inhibitory synaptic input likely mediates inhibitory sidebands of LSO neurons in response to acoustic stimulation.

Somatic Integration of Incoherent Dendritic Inputs in the Gerbil Medial Superior Olive.

The medial superior olive (MSO) is a binaural nucleus that is specialized in detecting the relative arrival times of sounds at both ears. Excitatory inputs to its neurons originating from either ear are segregated to different dendrites. To study the integration of synaptic inputs both within and between dendrites, we made juxtacellular and whole-cell recordings from the MSO in anesthetized female gerbils, while presenting a "double zwuis" stimulus, in which each ear received its own set of tones, which were chosen in a way that all second-order distortion products (DP2s) could be uniquely identified. MSO neurons phase-locked to multiple tones within the multitone stimulus, and vector strength, a measure for spike phase-locking, generally depended linearly on the size of the average subthreshold response to a tone. Subthreshold responses to tones in one ear depended little on the presence of sound in the other ear, suggesting that inputs from different ears sum linearly without a substantial role for somatic inhibition. The "double zwuis" stimulus also evoked response components in the MSO neuron that were phase-locked to DP2s. Bidendritic subthreshold DP2s were quite rare compared with bidendritic suprathreshold DP2s. We observed that in a small subset of cells, the ability to trigger spikes differed substantially between both ears, which might be explained by a dendritic axonal origin. Some neurons that were driven monaurally by only one of the two ears nevertheless showed decent binaural tuning. We conclude that MSO neurons are remarkably good in finding binaural coincidences even among uncorrelated inputs.SIGNIFICANCE STATEMENT Neurons in the medial superior olive are essential for precisely localizing low-frequency sounds in the horizontal plane. From their soma, only two dendrites emerge, which are innervated by inputs originating from different ears. Using a new sound stimulus, we studied the integration of inputs both within and between these dendrites in unprecedented detail. We found evidence that inputs from different dendrites add linearly at the soma, but that small increases in somatic potentials could lead to large increases in the probability of generating a spike. This basic scheme allowed the MSO neurons to detect the relative arrival time of inputs at both dendrites remarkably efficient, although the relative size of these inputs could differ considerably.

Characterization of the superior olivary complex of chimpanzees (Pan troglodytes) in comparison to humans.

The superior olivary complex (SOC) is a collection of nuclei in the hindbrain of mammals with numerous roles in hearing, including localization of sound sources in the environment, encoding temporal and spectral elements of sound, and descending modulation of the cochlea. While there have been several investigations of the SOC in primates, there are discrepancies in the descriptions of nuclear borders and even the presence of certain cell groups among studies and species. Herein, we aimed to clarify some of these issues by characterizing the SOC from chimpanzees using Nissl staining, quantitative morphometry and immunohistochemistry. We found the medial superior olive (MSO) to be the largest of the SOC nuclei and the arrangement of its neurons and peri-MSO to be very similar to humans. Additionally, we found neurons in the medial nucleus of the trapezoid body (MNTB) to be immunopositive for the calcium binding protein calbindin. Further, most neurons in the MNTB, and some neurons in the lateral nucleus of the trapezoid body were associated with large, calretinin-immunoreactive calyx terminals. Together, these findings indicate the organization of the SOC of chimpanzees is organized very similar to the SOC in humans and suggests modifications to this region among species consistent with differences in head/body size, restricted hearing range and sensitivity to low frequency sounds.

Tonotopic distribution and inferior colliculus projection pattern of inhibitory and excitatory cell types in the lateral superior olive of Mongolian gerbils.

Sound localization critically relies on brainstem neurons that compare information from the two ears. The conventional role of the lateral superior olive (LSO) is extraction of intensity differences; however, it is increasingly clear that relative timing, especially of transients, is also an important function. Cellular diversity within the LSO that is not well understood may underlie its multiple roles. There are glycinergic inhibitory and glutamatergic excitatory principal neurons in the LSO, however, there is some disagreement regarding their relative distribution and projection pattern. Here we employ in situ hybridization to definitively identify transmitter types combined with retrograde labeling of projections to the inferior colliculus (IC) to address these questions. Excitatory LSO neurons were more numerous (76%) than inhibitory ones. A smaller proportion of inhibitory neurons were IC-projecting (45% vs. 64% for excitatory) suggesting that inhibitory LSO neurons may have more projections to other regions such the lateral lemniscus or more distributed IC projections. Inhibitory LSO neurons almost exclusively projected ipsilaterally making up a sizeable proportion (41%) of the transmitter type-labeled ipsilateral IC projection from LSO and exhibited a moderate low frequency bias (10% difference H-L). Two thirds of excitatory neurons projected contralaterally and had a slight high frequency bias (4%). One third of excitatory LSO neurons projected ipsilaterally to the IC and these cells were strongly biased toward the low frequency limb of the LSO (37%). This projection appears to be species specific in animals with good low frequency hearing suggesting that it may be a specialization for such ability.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex.

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Robustness of neuronal tuning to binaural sound localization cues against age-related loss of inhibitory synaptic inputs.

Sound localization relies on minute differences in the timing and intensity of sound arriving at both ears. Neurons of the lateral superior olive (LSO) in the brainstem process these interaural disparities by precisely detecting excitatory and inhibitory synaptic inputs. Aging generally induces selective loss of inhibitory synaptic transmission along the entire auditory pathways, including the reduction of inhibitory afferents to LSO. Electrophysiological recordings in animals, however, reported only minor functional changes in aged LSO. The perplexing discrepancy between anatomical and physiological observations suggests a role for activity-dependent plasticity that would help neurons retain their binaural tuning function despite loss of inhibitory inputs. To explore this hypothesis, we use a computational model of LSO to investigate mechanisms underlying the observed functional robustness against age-related loss of inhibitory inputs. The LSO model is an integrate-and-fire type enhanced with a small amount of low-voltage activated potassium conductance and driven with (in)homogeneous Poissonian inputs. Without synaptic input loss, model spike rates varied smoothly with interaural time and level differences, replicating empirical tuning properties of LSO. By reducing the number of inhibitory afferents to mimic age-related loss of inhibition, overall spike rates increased, which negatively impacted binaural tuning performance, measured as modulation depth and neuronal discriminability. To simulate a recovery process compensating for the loss of inhibitory fibers, the strength of remaining inhibitory inputs was increased. By this modification, effects of inhibition loss on binaural tuning were considerably weakened, leading to an improvement of functional performance. These neuron-level observations were further confirmed by population modeling, in which binaural tuning properties of multiple LSO neurons were varied according to empirical measurements. These results demonstrate the plausibility that homeostatic plasticity could effectively counteract known age-dependent loss of inhibitory fibers in LSO and suggest that behavioral degradation of sound localization might originate from changes occurring more centrally.

The medial olivocochlear reflex strength is modulated during a visual working memory task.

Top-down modulation of sensory responses to distracting stimuli by selective attention has been proposed as an important mechanism by which our brain can maintain relevant information during working memory tasks. Previous works in visual working memory (VWM) have reported modulation of neural responses to distracting sounds at different levels of the central auditory pathways. Whether these modulations occur also at the level of the auditory receptor is unknown. Here, we hypothesize that cochlear responses to irrelevant auditory stimuli can be modulated by the medial olivocochlear system during VWM. Twenty-one subjects (13 males, mean age 25.3 yr) with normal hearing performed a visual change detection task with different VWM load conditions (high load = 4 visual objects; low load = 2 visual objects). Auditory stimuli were presented as distractors and allowed the measurement of distortion product otoacoustic emissions (DPOAEs) and scalp auditory evoked potentials. In addition, the medial olivocochlear reflex strength was evaluated by adding contralateral acoustic stimulation. We found larger contralateral acoustic suppression of DPOAEs during the visual working memory period (n = 21) compared with control experiments (n = 10), in which individuals were passively exposed to the same experimental conditions. These results show that during the visual working memory period there is a modulation of the medial olivocochlear reflex strength, suggesting a possible common mechanism for top-down filtering of auditory responses during cognitive processes.NEW & NOTEWORTHY The auditory efferent system has been proposed to function as a biological filter of cochlear responses during selective attention. Here, we recorded electroencephalographic activity and otoacoustic emissions in response to auditory distractors during a visual working memory task in humans. We found that the olivocochlear efferent activity is modulated during the visual working memory period suggesting a common mechanism for suppressing cochlear responses during selective attention and working memory.

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review.

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

Ambient noise exposure induces long-term adaptations in adult brainstem neurons.

To counterbalance long-term environmental changes, neuronal circuits adapt the processing of sensory information. In the auditory system, ongoing background noise drives long-lasting adaptive mechanism in binaural coincidence detector neurons in the superior olive. However, the compensatory cellular mechanisms of the binaural neurons in the medial superior olive (MSO) to long-term background changes are unexplored. Here we investigated the cellular properties of MSO neurons during long-lasting adaptations induced by moderate omnidirectional noise exposure. After noise exposure, the input resistance of MSO neurons of mature Mongolian gerbils was reduced, likely due to an upregulation of hyperpolarisation-activated cation and low voltage-activated potassium currents. Functionally, the long-lasting adaptations increased the action potential current threshold and facilitated high frequency output generation. Noise exposure accelerated the occurrence of spontaneous postsynaptic currents. Together, our data suggest that cellular adaptations in coincidence detector neurons of the MSO to continuous noise exposure likely increase the sensitivity to differences in sound pressure levels.

Medial olivocochlear reflex effects on amplitude growth functions of long- and short-latency components of click-evoked otoacoustic emissions in humans.

Functional outcomes of medial olivocochlear reflex (MOCR) activation, such as improved hearing in background noise and protection from noise damage, involve moderate to high sound levels. Previous noninvasive measurements of MOCR in humans focused primarily on otoacoustic emissions (OAEs) evoked at low sound levels. Interpreting MOCR effects on OAEs at higher levels is complicated by the possibility of the middle-ear muscle reflex and by components of OAEs arising from different locations along the length of the cochlear spiral. We overcame these issues by presenting click stimuli at a very slow rate and by time-frequency windowing the resulting click-evoked (CE)OAEs into short-latency (SL) and long-latency (LL) components. We characterized the effects of MOCR on CEOAE components using multiple measures to more comprehensively assess these effects throughout much of the dynamic range of hearing. These measures included CEOAE amplitude attenuation, equivalent input attenuation, phase, and slope of growth functions. Results show that MOCR effects are smaller on SL components than LL components, consistent with SL components being generated slightly basal of the characteristic frequency region. Amplitude attenuation measures showed the largest effects at the lowest stimulus levels, but slope change and equivalent input attenuation measures did not decrease at higher stimulus levels. These latter measures are less commonly reported and may provide insight into the variability in listening performance and noise susceptibility seen across individuals.NEW & NOTEWORTHY The auditory efferent system, operating at moderate to high sound levels, may improve hearing in background noise and provide protection from noise damage. We used otoacoustic emissions to measure these efferent effects across a wide range of sound levels and identified level-dependent and independent effects. Previous reports have focused on level-dependent measures. The level-independent effects identified here may provide new insights into the functional relevance of auditory efferent activity in humans.

Arrangement of Excitatory Synaptic Inputs on Dendrites of the Medial Superior Olive.

Neurons in the medial superior olive (MSO) detect 10 µs differences in the arrival times of a sound at the two ears. Such acuity requires exquisitely precise integration of binaural synaptic inputs. There is substantial understanding of how neuronal phase locking of afferent MSO structures, and MSO membrane biophysics subserve such high precision. However, we still lack insight into how the entirety of excitatory inputs is integrated along the MSO dendrite under sound stimulation. To understand how the dendrite integrates excitatory inputs as a whole, we combined anatomic quantifications of the afferent innervation in gerbils of both sexes with computational modeling of a single cell. We present anatomic data from confocal and transmission electron microscopy showing that single afferent fibers follow a single dendrite mostly up to the soma and contact it at multiple (median 4) synaptic sites, each containing multiple independent active zones (the overall density of active zones is estimated as 1.375 per µm2). Thus, any presynaptic action potential may elicit temporally highly coordinated synaptic vesicle release at tens of active zones, thereby achieving secure transmission. Computer simulations suggest that such an anatomic arrangement boosts the amplitude and sharpens the time course of excitatory postsynaptic potentials by reducing current sinks and more efficiently recruiting subthreshold potassium channels. Both effects improve binaural coincidence detection compared with single large synapses at the soma. Our anatomic data further allow for estimation of a lower bound of 7 and an upper bound of 70 excitatory fibers per dendrite.SIGNIFICANCE STATEMENT Passive dendritic propagation attenuates the amplitude of postsynaptic potentials and widens their temporal spread. Neurons in the medial superior olive, with their large bilateral dendrites, however, can detect coincidence of binaural auditory inputs with submillisecond precision, a computation that is in stark contrast to passive dendritic processing. Here, we show that dendrites can counteract amplitude attenuation and even decrease the temporal spread of postsynaptic potentials, if active subthreshold potassium conductances are triggered in temporal coordination along the whole dendrite. Our anatomic finding that axons run in parallel to the dendrites and make multiple synaptic contacts support such coordination since incoming action potentials would depolarize the dendrite at multiple sites within a brief time interval.

Volatile Anesthetics Activate a Leak Sodium Conductance in Retrotrapezoid Nucleus Neurons to Maintain Breathing during Anesthesia in Mice.

BACKGROUND: Volatile anesthetics moderately depress respiratory function at clinically relevant concentrations. Phox2b-expressing chemosensitive neurons in the retrotrapezoid nucleus, a respiratory control center, are activated by isoflurane, but the underlying mechanisms remain unclear. The hypothesis of this study was that the sodium leak channel contributes to the volatile anesthetics-induced modulation of retrotrapezoid nucleus neurons and to respiratory output. METHODS: The contribution of sodium leak channels to isoflurane-, sevoflurane-, and propofol-evoked activity of Phox2b-expressing retrotrapezoid nucleus neurons and respiratory output were evaluated in wild-type and genetically modified mice lacking sodium leak channels (both sexes). Patch-clamp recordings were performed in acute brain slices. Whole-body plethysmography was used to measure the respiratory activity. RESULTS: Isoflurane at 0.42 to 0.50 mM (~1.5 minimum alveolar concentration) increased the sodium leak channel-mediated holding currents and conductance from -75.0 ± 12.9 to -130.1 ± 34.9 pA (mean ± SD, P = 0.002, n = 6) and 1.8 ± 0.5 to 3.6 ± 1.0 nS (P = 0.001, n = 6), respectively. At these concentrations, isoflurane increased activity of Phox2b-expressing retrotrapezoid nucleus neurons from 1.1 ± 0.2 to 2.8 ± 0.2 Hz (P < 0.001, n = 5), which was eliminated by bath application of gadolinium or genetic silencing of sodium leak channel. Genetic silencing of sodium leak channel in the retrotrapezoid nucleus resulted in a diminished ventilatory response to carbon dioxide in mice under control conditions and during isoflurane anesthesia. Sevoflurane produced an effect comparable to that of isoflurane, whereas propofol did not activate sodium leak channel-mediated holding conductance. CONCLUSIONS: Isoflurane and sevoflurane increase neuronal excitability of chemosensitive retrotrapezoid nucleus neurons partly by enhancing sodium leak channel conductance. Sodium leak channel expression in the retrotrapezoid nucleus is required for the ventilatory response to carbon dioxide during anesthesia by isoflurane and sevoflurane, thus identifying sodium leak channel as a requisite determinant of respiratory output during anesthesia of volatile anesthetics.

Computational principles of neural adaptation for binaural signal integration.

Adaptation to statistics of sensory inputs is an essential ability of neural systems and extends their effective operational range. Having a broad operational range facilitates to react to sensory inputs of different granularities, thus is a crucial factor for survival. The computation of auditory cues for spatial localization of sound sources, particularly the interaural level difference (ILD), has long been considered as a static process. Novel findings suggest that this process of ipsi- and contra-lateral signal integration is highly adaptive and depends strongly on recent stimulus statistics. Here, adaptation aids the encoding of auditory perceptual space of various granularities. To investigate the mechanism of auditory adaptation in binaural signal integration in detail, we developed a neural model architecture for simulating functions of lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB) composed of single compartment conductance-based neurons. Neurons in the MNTB serve as an intermediate relay population. Their signal is integrated by the LSO population on a circuit level to represent excitatory and inhibitory interactions of input signals. The circuit incorporates an adaptation mechanism operating at the synaptic level based on local inhibitory feedback signals. The model's predictive power is demonstrated in various simulations replicating physiological data. Incorporating the innovative adaptation mechanism facilitates a shift in neural responses towards the most effective stimulus range based on recent stimulus history. The model demonstrates that a single LSO neuron quickly adapts to these stimulus statistics and, thus, can encode an extended range of ILDs in the ipsilateral hemisphere. Most significantly, we provide a unique measurement of the adaptation efficacy of LSO neurons. Prerequisite of normal function is an accurate interaction of inhibitory and excitatory signals, a precise encoding of time and a well-tuned local feedback circuit. We suggest that the mechanisms of temporal competitive-cooperative interaction and the local feedback mechanism jointly sensitize the circuit to enable a response shift towards contra-lateral and ipsi-lateral stimuli, respectively.

Physiological and anatomical development of glycinergic inhibition in the mouse superior paraolivary nucleus following hearing onset.

Neural circuits require balanced synaptic excitation and inhibition to ensure accurate neural computation. Our knowledge about the development and maturation of inhibitory synaptic inputs is less well developed than that concerning excitation. Here we describe the maturation of an inhibitory circuit within the mammalian auditory brainstem where counterintuitively, inhibition drives action potential firing of principal neurons. With the use of combined anatomical tracing and electrophysiological recordings from mice, neurons of the superior paraolivary nucleus (SPN) are shown to receive converging glycinergic input from at least four neurons of the medial nucleus of the trapezoid body (MNTB). These four axons formed 30.71 ± 2.72 (means ± SE) synaptic boutons onto each SPN neuronal soma, generating a total inhibitory conductance of 80 nS. Such strong inhibition drives the underlying postinhibitory rebound firing mechanism, which is a hallmark of SPN physiology. In contrast to inhibitory projections to the medial and lateral superior olives, the inhibitory projection to the SPN does not exhibit experience-dependent synaptic refinement following the onset of hearing. These findings emphasize that the development and function of neural circuits cannot be inferred from one synaptic target to another, even if both originate from the same neuron.NEW & NOTEWORTHY Neuronal activity regulates development and maturation of neural circuits. This activity can include spontaneous burst firing or firing elicited by sensory input during early development. For example, auditory brainstem circuits involved in sound localization require acoustically evoked activity to form properly. Here we show, that an inhibitory circuit, involved in processing sound offsets, gaps, and rhythmically modulated vocal communication signals, matures before the onset of acoustically evoked activity.

Preventing presbycusis in mice with enhanced medial olivocochlear feedback.

"Growing old" is the most common cause of hearing loss. Age-related hearing loss (ARHL) (presbycusis) first affects the ability to understand speech in background noise, even when auditory thresholds in quiet are normal. It has been suggested that cochlear denervation ("synaptopathy") is an early contributor to age-related auditory decline. In the present work, we characterized age-related cochlear synaptic degeneration and hair cell loss in mice with enhanced α9α10 cholinergic nicotinic receptors gating kinetics ("gain of function" nAChRs). These mediate inhibitory olivocochlear feedback through the activation of associated calcium-gated potassium channels. Cochlear function was assessed via distortion product otoacoustic emissions and auditory brainstem responses. Cochlear structure was characterized in immunolabeled organ of Corti whole mounts using confocal microscopy to quantify hair cells, auditory neurons, presynaptic ribbons, and postsynaptic glutamate receptors. Aged wild-type mice had elevated acoustic thresholds and synaptic loss. Afferent synapses were lost from inner hair cells throughout the aged cochlea, together with some loss of outer hair cells. In contrast, cochlear structure and function were preserved in aged mice with gain-of-function nAChRs that provide enhanced olivocochlear inhibition, suggesting that efferent feedback is important for long-term maintenance of inner ear function. Our work provides evidence that olivocochlear-mediated resistance to presbycusis-ARHL occurs via the α9α10 nAChR complexes on outer hair cells. Thus, enhancement of the medial olivocochlear system could be a viable strategy to prevent age-related hearing loss.

The association between subcortical and cortical fMRI and lifetime noise exposure in listeners with normal hearing thresholds.

In animal models, exposure to high noise levels can cause permanent damage to hair-cell synapses (cochlear synaptopathy) for high-threshold auditory nerve fibers without affecting sensitivity to quiet sounds. This has been confirmed in several mammalian species, but the hypothesis that lifetime noise exposure affects auditory function in humans with normal audiometric thresholds remains unconfirmed and current evidence from human electrophysiology is contradictory. Here we report the auditory brainstem response (ABR), and both transient (stimulus onset and offset) and sustained functional magnetic resonance imaging (fMRI) responses throughout the human central auditory pathway across lifetime noise exposure. Healthy young individuals aged 25-40 years were recruited into high (n = 32) and low (n = 30) lifetime noise exposure groups, stratified for age, and balanced for audiometric threshold up to 16 kHz fMRI demonstrated robust broadband noise-related activity throughout the auditory pathway (cochlear nucleus, superior olivary complex, nucleus of the lateral lemniscus, inferior colliculus, medial geniculate body and auditory cortex). fMRI responses in the auditory pathway to broadband noise onset were significantly enhanced in the high noise exposure group relative to the low exposure group, differences in sustained fMRI responses did not reach significance, and no significant group differences were found in the click-evoked ABR. Exploratory analyses found no significant relationships between the neural responses and self-reported tinnitus or reduced sound-level tolerance (symptoms associated with synaptopathy). In summary, although a small effect, these fMRI results suggest that lifetime noise exposure may be associated with central hyperactivity in young adults with normal hearing thresholds.

Intrinsic and Miniature Postsynaptic Current Changes in Rat Principal Neurons of the Lateral Superior Olive after Unilateral Auditory Deprivation at an Early Age.

Unilateral auditory deprivation results in lateralization changes in the central auditory system, interfering with the integration of binaural information and thereby leading to a decrease in binaural auditory functions such as sound localization. Principal neurons of the lateral superior olive (LSO) are responsible for computing the interaural intensity differences that are critical for sound localization in the horizontal plane. To investigate changes caused by unilateral auditory deprivation, electrophysiological activity was recorded from LSO principal neurons in control rats and rats with unilateral cochlear ablation. At one week after unilateral cochlear ablation, the excitability of LSO principal neurons on the side ipsilateral to the ablation (the ablated side) was greater than that on the side contralateral to the ablation (the intact side); however, the input resistance increased on both sides. Furthermore, by analysing the miniature inhibitory postsynaptic currents and miniature excitatory postsynaptic currents, we found that unilateral auditory deprivation weakened the inhibitory driving force on the intact side, whereas it strengthened the excitatory driving force on the ablated side. In summary, asymmetric changes in the electrophysiological activity of LSO principal neurons were found on both sides at postnatal day 19, one week after unilateral cochlear ablation.

Topographic map refinement and synaptic strengthening of a sound localization circuit require spontaneous peripheral activity.

KEY POINTS: Loss of the calcium sensor otoferlin disrupts neurotransmission from inner hair cells. Central auditory nuclei are functionally denervated in otoferlin knockout mice (Otof KOs) via gene ablation confined to the periphery. We employed juvenile and young adult Otof KO mice (postnatal days (P)10-12 and P27-49) as a model for lacking spontaneous activity and deafness, respectively. We studied the impact of peripheral activity on synaptic refinement in the sound localization circuit from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO). MNTB in vivo recordings demonstrated drastically reduced spontaneous spiking and deafness in Otof KOs. Juvenile KOs showed impaired synapse elimination and strengthening, manifested by broader MNTB-LSO inputs, imprecise MNTB-LSO topography and weaker MNTB-LSO fibres. The impairments persisted into young adulthood. Further functional refinement after hearing onset was undetected in young adult wild-types. Collectively, activity deprivation confined to peripheral protein loss impairs functional MNTB-LSO refinement during a critical prehearing period. ABSTRACT: Circuit refinement is critical for the developing sound localization pathways in the auditory brainstem. In prehearing mice (hearing onset around postnatal day (P)12), spontaneous activity propagates from the periphery to central auditory nuclei. At the glycinergic projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) of neonatal mice, super-numerous MNTB fibres innervate a given LSO neuron. Between P4 and P9, MNTB fibres are functionally eliminated, whereas the remaining fibres are strengthened. Little is known about MNTB-LSO circuit refinement after P20. Moreover, MNTB-LSO refinement upon activity deprivation confined to the periphery is largely unexplored. This leaves a considerable knowledge gap, as deprivation often occurs in patients with congenital deafness, e.g. upon mutations in the otoferlin gene (OTOF). Here, we analysed juvenile (P10-12) and young adult (P27-49) otoferlin knockout (Otof KO) mice with respect to MNTB-LSO refinement. MNTB in vivo recordings revealed drastically reduced spontaneous activity and deafness in knockouts (KOs), confirming deprivation. As RNA sequencing revealed Otof absence in the MNTB and LSO of wild-types, Otof loss in KOs is specific to the periphery. Functional denervation impaired MNTB-LSO synapse elimination and strengthening, which was assessed by glutamate uncaging and electrical stimulation. Impaired elimination led to imprecise MNTB-LSO topography. Impaired strengthening was associated with lower quantal content per MNTB fibre. In young adult KOs, the MNTB-LSO circuit remained unrefined. Further functional refinement after P12 appeared absent in wild-types. Collectively, we provide novel insights into functional MNTB-LSO circuit maturation governed by a cochlea-specific protein. The central malfunctions in Otof KOs may have implications for patients with sensorineuronal hearing loss.

The Influence of Efferent Inhibition on Speech Perception in Noise: A Revisit Through Its Level-Dependent Function.

Purpose The study aimed to assess the relationship between the level-dependent function of efferent inhibition and speech perception in noise across different intensities of suppressor and across different signal-to-noise ratios (SNRs) of speech. Method Twenty-six young normal-hearing adults participated in the study. Contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) was measured for 3 levels of suppressor (40, 50, and 60 dB SPL). Speech identification score (SIS) was measured at 5 ipsilateral SNR conditions (quiet, 0, -5, -10, and -15 dB), with and without contralateral broadband noise at 3 levels (40, 50, and 60 dB SPL). Furthermore, SNR-50 was measured with and without the same 3 levels of contralateral broadband noise. Results The results showed that the suppression magnitude of TEOAE increased with an increase in suppressor level. However, neither SIS nor SNR-50 was influenced by the contralateral noise. In addition, SIS and SNR-50 did not show significant correlation with contralateral suppression of TEOAEs. This was true at all the SNRs and contralateral noise levels used in the study. Conclusions The findings suggest that the intensity of noise directly influences medial olivocochlear bundle-mediated efferent inhibition. However, the role of the medial olivocochlear bundle in regulating speech perception in noise needs to be revisited. Supplemental Material https://doi.org/10.23641/asha.9336353.