Auditory Neurophysiology publications

This dataset was collected to study the functional consequences of age-related hearing loss for the auditory nerve, which carries acoustic information from the periphery to the central auditory system. Using high-impedance glass electrodes, raw voltage traces and spike times were recorded from more than one thousand single fibres of the auditory nerve of young-adult, middle-aged, and old Mongolian gerbils raised in a quiet environment. The dataset contains not only responses to simple acoustic stimuli to characterize the fibres, but also to more complex stimuli, such as speech logatomes in background noise and Schroeder-phase stimuli. A software toolbox is provided to search through the dataset, to plot various analysed outcomes, and to give insight into the analyses. This dataset may serve as a valuable resource to test further hypotheses about age-related hearing loss. Additionally, it can aid in optimizing available computational models of the auditory system, which can contribute to, or eventually even fully replace, animal experiments.

Autism spectrum disorder (ASD) is often associated with social communication impairments and specific sound processing deficits, for example, problems in following speech in noisy environments. To investigate underlying neuronal processing defects located in the auditory cortex (AC), we performed two-photon Ca 2+ imaging in FMR1 ( fragile X messenger ribonucleoprotein 1 ) knock-out (KO) mice, a model for fragile X syndrome (FXS), the most common cause of hereditary ASD in humans. For primary AC (A1) and the anterior auditory field (AAF), topographic frequency representation was less ordered compared with control animals. We additionally analyzed ensemble AC activity in response to various sounds and found subfield-specific differences. In A1, ensemble correlations were lower in general, while in secondary AC (A2), correlations were higher in response to complex sounds, but not to pure tones. Furthermore, sound specificity of ensemble activity was decreased in AAF. Repeating these experiments 1 week later revealed no major differences regarding representational drift. Nevertheless, we found subfield- and genotype-specific changes in ensemble correlation values between the two times points, hinting at alterations in network stability in FMR1 KO mice. These detailed insights into AC network activity and topography in FMR1 KO mice add to the understanding of auditory processing defects in FXS.

Previous studies on the advantages of binaural hearing have long been focused on sound localization and spatial stream segregation. The binaural advantages have also been observed in speech perception in reverberation. Both human speech and animal vocalizations contain temporal features that are critical for speech perception and animal communication. However, whether there are binaural advantages for sound temporal information processing in the central auditory system has not been elucidated. Gap detection threshold (GDT), the ability to detect the shortest silent interval in a sound, has been widely used to measure the auditory temporal resolution. In the present study, we determined GDTs of rat inferior collicular neurons under both monaural and binaural hearing conditions. We found that the majority of the inferior collicular neurons in adult rats exhibited binaural advantages in gap detection, i.e., better neural gap detection ability in binaural hearing conditions compared to monaural hearing condition. However, this binaural advantage in sound temporal information processing was not significant in the inferior collicular neurons of P14-21 and P22-30 rats. Additionally, we also observed age-related changes in neural temporal acuity in the rat inferior colliculus. These results demonstrate a new advantage of binaural hearing (i.e., binaural advantage in temporal processing) in the central auditory system in addition to sound localization and spatial stream segregation.

In ethological behaviors like parenting, animals innately follow stereotyped patterns of choices to decide between uncertain outcomes but can learn to modify their strategies to incorporate new information. For example, female mice in a T-maze instinctively use spatial-memory to search for pups where they last found them but can learn more efficient strategies employing pup-associated acoustic cues. We uncovered neural correlates for transitioning between these innate and learned strategies. Auditory cortex (ACx) was required during learning. ACx firing at the nest increased with learning and correlated with subsequent search speed but not outcome. Surprisingly, ACx suppression rather than facilitation during search was more prognostic of correct sound-cued outcomes – even before adopting a sound-cued strategy. Meanwhile medial prefrontal cortex encoded the last pup location, but this decayed as the spatial-memory strategy declined. Our results suggest a neural competition between a weakening spatial-memory and strengthening sound-cued neural representation to mediate strategy switches.

Introduction Neuronal hyperactivity has been associated with many brain diseases. In the auditory system, hyperactivity has been linked to hyperacusis and tinnitus. Previous research demonstrated the development of hyperactivity in inferior colliculus (IC) neurons after sound overexposure, but the underlying mechanism of this hyperactivity remains unclear. The main goal of this study was to determine the mechanism of this hyperactivity. Methods Experiments were performed on CBA/CaJ mice in a restrained, unanesthetized condition using intracellular recordings with sharp microelectrodes. Recordings were obtained from control (unexposed) and unilaterally sound overexposed groups of mice. Results Our data suggest that sound exposure-induced hyperactivity was due to a depolarizing shift of the resting membrane potential (RMP) in the hyperactive neurons. The half width of action potentials in these neurons was also decreased after sound exposure. Surprisingly, we also found an RMP gradient in which neurons have more hyperpolarized RMPs with increasing depth in the IC. This gradient was altered in the overexposed animals.

Coordinated functioning of the two cortical hemispheres is crucial for perception. The human auditory cortex (ACx) shows functional lateralization with the left hemisphere specialized for processing speech, whereas the right analyzes spectral content. In mice, virgin females demonstrate a left-hemisphere response bias to pup vocalizations that strengthens with motherhood. However, how this lateralized function is established is unclear. We developed a widefield imaging microscope to simultaneously image both hemispheres of mice to bilaterally monitor functional responses. We found that global ACx topography is symmetrical and stereotyped. In both male and virgin female mice, the secondary auditory cortex (A2) in the left hemisphere shows larger responses than right to high-frequency tones and adult vocalizations; however, only virgin female mice show a left-hemisphere bias in A2 in response to adult pain calls. These results indicate hemispheric bias with both sex-independent and -dependent aspects. Analyzing cross-hemispheric functional correlations showed that asymmetries exist in the strength of correlations between DM-AAF and A2-AAF, while other ACx areas showed smaller differences. We found that A2 showed lower cross-hemisphere correlation than other cortical areas, consistent with the lateralized functional activation of A2. Cross-hemispheric activity correlations are lower in deaf, otoferlin knockout (OTOF −/− ) mice, indicating that the development of functional cross-hemispheric connections is experience dependent. Together, our results reveal that ACx is topographically symmetric at the macroscopic scale but that higher-order A2 shows sex-dependent and independent lateralized responses due to asymmetric intercortical functional connections. Moreover, our results suggest that sensory experience is required to establish functional cross-hemispheric connectivity.

The auditory cortex (AC) modulates the activity of upstream pathways in the auditory brainstem via descending (corticofugal) projections. This feedback system plays an important role in the plasticity of the auditory system by shaping response properties of neurons in many subcortical nuclei. The majority of layer (L) 5 corticofugal neurons project to the inferior colliculus (IC). This corticocollicular (CC) pathway is involved in processing of complex sounds, auditory-related learning, and defense behavior. Partly due to their location in deep cortical layers, CC neuron population activity patterns within neuronal AC ensembles remain poorly understood. We employed two-photon imaging to record the activity of hundreds of L5 neurons in anesthetized as well as awake animals. CC neurons are broader tuned than other L5 pyramidal neurons and display weaker topographic order in core AC subfields. Network activity analyses revealed stronger clusters of CC neurons compared to non-CC neurons, which respond more reliable and integrate information over larger distances. However, results obtained from secondary auditory cortex (A2) differed considerably. Here CC neurons displayed similar or higher topography, depending on the subset of neurons analyzed. Furthermore, specifically in A2, CC activity clusters formed in response to complex sounds were spatially more restricted compared to other L5 neurons. Our findings indicate distinct network mechanism of CC neurons in analyzing sound properties with pronounced subfield differences, demonstrating that the topography of sound-evoked responses within AC is neuron-type dependent.

Intrinsically generated neural activity propagates through the developing auditory system to promote maturation and refinement of sound processing circuits prior to hearing onset. This early patterned activity is induced by non-sensory supporting cells in the organ of Corti, which are highly interconnected through gap junctions containing connexin 26 ( Gjb2 ). Although loss of function mutations in Gjb2 impair cochlear development and are the most common cause of congenital deafness, it is not known if these variants disrupt spontaneous activity and the developmental trajectory of sound processing circuits in the brain. Here, we show in a new mouse model of Gjb2- mediated congenital deafness that cochlear supporting cells adjacent to inner hair cells (IHCs) unexpectedly retain intercellular coupling and the capacity to generate spontaneous activity, exhibiting only modest deficits prior to hearing onset. Supporting cells lacking Gjb2 elicited coordinated activation of IHCs, leading to coincident bursts of activity in central auditory neurons that will later process similar frequencies of sound. Despite alterations in the structure of the sensory epithelium, hair cells within the cochlea of Gjb2 -deficient mice were intact and central auditory neurons could be activated within appropriate tonotopic domains by loud sounds at hearing onset, indicating that early maturation and refinement of auditory circuits was preserved. Only after cessation of spontaneous activity following hearing onset did progressive hair cell degeneration and enhanced auditory neuron excitability manifest. This preservation of cochlear spontaneous neural activity in the absence of connexin 26 may increase the effectiveness of early therapeutic interventions to restore hearing.

Listening in complex sound environments requires rapid segregation of different sound sources e.g., speakers from each other, speakers from other sounds, or different instruments in an orchestra, and also adjust auditory processing on the prevailing sound conditions. Thus, fast encoding of inputs and identifying and adapting to reoccurring sounds are necessary for efficient and agile sound perception. This adaptation process represents an early phase of developing implicit learning of sound statistics and thus represents a form of auditory memory. The auditory cortex (ACtx) is known to play a key role in this encoding process but the underlying circuits and if hierarchical processing exists are not known. To identify ACtx regions and cells involved in this process, we simultaneously imaged population of neurons in different ACtx subfields using in vivo 2-photon imaging in awake mice. We used an experimental stimulus paradigm adapted from human studies that triggers rapid and robust implicit learning to passively present complex sounds and imaged A1 Layer 4 (L4), A1 L2/3, and A2 L2/3. In this paradigm, a frozen spectro-temporally complex ‘ Target ’ sound would be randomly re-occurring within a stream of random other complex sounds. We find distinct groups of cells that are specifically responsive to complex acoustic sequences across all subregions indicating that even the initial thalamocortical input layers (A1 L4) respond to complex sounds. Cells in all imaged regions showed decreased response amplitude for reoccurring Target sounds indicating that a memory signature is present even in the thalamocortical input layers. On the population level we find increased synchronized activity across cells to the Target sound and that this synchronized activity was more consistent across cells regardless of the duration of frozen token within Target sounds in A2, compared to A1. These findings suggest that ACtx and its input layers play a role in auditory memory for complex sounds and suggest a hierarchical structure of processes for auditory memory.

Detecting patterns, and noticing unexpected pattern changes, in the environment is a vital aspect of sensory processing. Adaptation and prediction error responses are two components of neural processing related to these tasks, and previous studies in the auditory system in rodents show that these two components are partially dissociable in terms of the topography and latency of neural responses to sensory deviants. However, many previous studies have focused on repetitions of single stimuli, such as pure tones, which have limited ecological validity. In this study, we tested whether the auditory cortical activity shows adaptation to repetition of more complex sound patterns (bisyllabic pairs). Specifically, we compared neural responses to violations of sequences based on single stimulus probability only, against responses to more complex violations based on stimulus order. We employed an auditory oddball paradigm and monitored the auditory cortex (ACtx) activity of awake mice (N=8) using wide-field calcium imaging. We found that cortical responses were sensitive both to single stimulus probabilities and to more global stimulus patterns, as mismatch signals were elicited following both substitution deviants and transposition deviants. Notably, A2 area elicited larger mismatch signaling to those deviants than primary ACtx (A1), which suggests a hierarchical gradient of prediction error signaling in the auditory cortex. Such a hierarchical gradient was observed for late but not early peaks of calcium transients to deviants, suggesting that the late part of the deviant response may reflect prediction error signaling in response to more complex sensory pattern violations.

The ability to localize a sound source in complex environments is essential for communication and navigation. Spatial hearing relies predominantly on the comparison of differences in the arrival time of sound between the two ears, the interaural time differences (ITDs). Hearing impairments are highly detrimental to sound localization. While cochlear implants (CIs) have been successful in restoring many crucial hearing capabilities, sound localization via ITD detection with bilateral CIs remains poor. The underlying reasons are not well understood. Neuronally, ITD sensitivity is generated by coincidence detection between excitatory and inhibitory inputs from the two ears performed by specialized brainstem neurons. Due to the lack of electrophysiological brainstem recordings during CI stimulation, it is unclear to what extent the apparent deficits are caused by the binaural comparator neurons or arise already on the input level. Here, we use a bottom-up approach to compare response features between electric and acoustic stimulation in an animal model of CI hearing. Conducting extracellular single neuron recordings in gerbils, we find severe hyper-precision and moderate hyper-entrainment of both the excitatory and inhibitory brainstem inputs to the binaural comparator neurons during electrical pulse-train stimulation. This finding establishes conclusively that the binaural processing stage must cope with highly altered input statistics during CI stimulation. To estimate the consequences of these effects on ITD sensitivity, we used a computational model of the auditory brainstem. After tuning the model parameters to match its response properties to our physiological data during either stimulation type, the model predicted that ITD sensitivity to electrical pulses is maintained even for the hyper-precise inputs. However, the model exhibits severely altered spatial sensitivity during electrical stimulation compared to acoustic: while resolution of ITDs near midline was increased, more lateralized adjacent source locations became inseparable. These results directly resemble recent findings in rodent and human CI listeners. Notably, decreasing the phase-locking precision of inputs during electrical stimulation recovered a wider range of separable ITDs. Together, our findings suggest that a central problem underlying the diminished ITD sensitivity in CI users might be the temporal hyper-precision of inputs to the binaural comparator stage induced by electrical stimulation.

From myriads of ongoing stimuli, the brain creates a fused percept of the environment. This process, which culminates in perceptual binding, is presumed to occur through the operations of multisensory neurons that occur throughout the brain. However, because different brain areas receive different inputs and have different cytoarchitechtonics, it would be expected that local multisensory features would also vary across regions. The present study investigated that hypothesis using multiple single-unit recordings from anesthetized cats in response to controlled, electronically-generated separate and combined auditory, visual, and somatosensory stimulation. These results were used to compare the multisensory features of neurons in cat primary auditory cortex (A1) with those identified in the nearby higher-order auditory region, the Dorsal Zone (DZ). Both regions exhibited the same forms of multisensory neurons, albeit in different proportions. Multisensory neurons exhibiting excitatory or inhibitory properties occurred in similar proportions in both areas. Also, multisensory neurons in both areas expressed similar levels of multisensory integration. Because responses to auditory cues alone were so similar to those that included non-auditory stimuli, it is proposed that this effect represents a mechanism by which multisensory neurons subserve the process of perceptual binding.

Intrinsically generated neural activity propagates through the developing auditory system to promote maturation and refinement of sound processing circuits prior to hearing onset. This early patterned activity is induced by non-sensory supporting cells in the organ of Corti, which are highly interconnected through gap junctions containing connexin 26 ( Gjb2 ). Although loss of function mutations in Gjb2 impair cochlear development and are the most common cause of congenital deafness, it is not known if these mutations disrupt spontaneous activity and the developmental trajectory of sound processing circuits in the brain. Here, we show in a new mouse model of Gjb2- mediated congenital deafness that cochlear supporting cells unexpectedly retained intercellular coupling and the capacity to generate spontaneous activity, exhibiting only modest deficits prior to hearing onset. This coordinated activation of IHCs led to coincident bursts of activity in central auditory neurons that will later process similar frequencies of sound. Despite alterations in the structure of the sensory epithelium, hair cells within the cochlea of Gjb2 deficient mice were intact and central auditory neurons could be activated within appropriate tonotopic domains by loud sounds at hearing onset, indicating that early maturation and refinement of auditory circuits was preserved. Only after cessation of spontaneous activity following hearing onset did progressive hair cell degeneration and enhanced auditory neuron excitability manifest. This preservation of cochlear spontaneous activity in the absence of connexin 26 may increase the effectiveness of early therapeutic interventions to restore hearing.

In the post-natal mouse cochlea, type II spiral ganglion neurons (SGNs) innervating the electromotile outer hair cells (OHCs) of the ‘cochlear amplifier' selectively express the type III intermediate filament peripherin gene ( Prph) . Immunolabeling showed that Prph knockout (KO) mice exhibited disruption of this (outer spiral bundle) afferent innervation, while the radial fiber (type I SGN) innervation of the inner hair cells (~95% of the SGN population) was retained. Functionality of the medial olivocochlear (MOC) efferent innervation of the OHCs was confirmed in the Prph KO, based on suppression of distortion product otoacoustic emissions (DPOAEs) via direct electrical stimulation. However, “contralateral suppression” of the MOC reflex neural circuit, evident as a rapid reduction in cubic DPOAE when noise is presented to the opposite ear in wildtype mice, was substantially disrupted in the Prph KO. Auditory brainstem response (ABR) measurements demonstrated that hearing sensitivity (thresholds and growth-functions) were indistinguishable between wildtype and Prph KO mice. Despite this comparability in sound transduction and strength of the afferent signal to the central auditory pathways, high-intensity, broadband noise exposure (108 dB SPL, 1 h) produced permanent high frequency hearing loss (24–32 kHz) in Prph KO mice but not the wildtype mice, consistent with the attenuated contralateral suppression of the Prph KO. These data support the postulate that auditory neurons expressing Prph contribute to the sensory arm of the otoprotective MOC feedback circuit.

The functional organization of sensory cortices is modulated by both extrinsic events and intrinsic states. The present study aimed to assess the mouse auditory cortex (AC) responses under varying conditions of stimulus complexity and brain state. Using wide-field calcium imaging, our results suggest a complete outline of topographic maps of frequency and FM rate as well as highly responsive regions to mouse ultrasonic vocalizations (USVs) in both awake and anesthetized states. Three new regions responsive to high-frequency tones and four new gradients responsive to frequency modulations (FMs) were identified. These maps are highly replicable across weeks and between animals. In awake versus anesthetized states, cortical responsiveness to pure tones was stronger, and regions that preferentially responded to slow rate FMs were smaller. In both states, fast FM regions showed the greatest contribution to the processing of USVs. Finally, our modeling of how best tone frequency or FM rate changes as a function of distance along a topographic gradient resulted in a sigmoid function. Together, our findings provide a better understanding of mouse AC functional organization and how this organization is modulated by changes in stimulus complexity and brain state. The function of newly identified regions in higher-order auditory/vocal processing and animal behavior should be considered in future research.

Cortical feedback has long been considered crucial for the modulation of sensory perception and recognition. However, previous studies have shown varying modulatory effects of the primary auditory cortex (A1) on the auditory response of subcortical neurons, which complicate interpretations regarding the function of A1 in sound perception and recognition. This has been further complicated by studies conducted under different brain states. In the current study, we used cryo-inactivation in A1 to examine the role of corticothalamic feedback on medial geniculate body (MGB) neurons in awake marmosets. The primary effects of A1 inactivation were a frequency-specific decrease in the auditory response of most MGB neurons coupled with an increased spontaneous firing rate, which together resulted in a decrease in the signal-to-noise ratio. In addition, we report for the first time that A1 robustly modulated the long-lasting sustained response of MGB neurons, which changed the frequency tuning after A1 inactivation, e.g. some neurons are sharper with corticofugal feedback and some get broader. Taken together, our results demonstrate that corticothalamic modulation in awake marmosets serves to enhance sensory processing in a manner similar to center-surround models proposed in visual and somatosensory systems, a finding which supports common principles of corticothalamic processing across sensory systems.

Mechanosensation – by which mechanical stimuli are converted into a neuronal signal – is the basis for the sensory systems of hearing, balance, and touch. Mechanosensation is unmatched in speed and its diverse range of sensitivities, reaching its highest temporal limits with the sense of hearing; however, hair cells (HCs) and the auditory nerve (AN) serve as obligatory bottlenecks for sounds to engage the brain. Like other sensory neurons, auditory neurons use the canonical pathway for neurotransmission and millisecond-duration action potentials (APs). How the auditory system utilizes the relatively slow transmission mechanisms to achieve ultrafast speed, and high audio-frequency hearing remains an enigma. Here, we address this paradox and report that the mouse, and chinchilla, AN are mechanically sensitive, and minute mechanical displacement profoundly affects its response properties. Sound-mimicking sinusoidal mechanical and electrical current stimuli affect phase-locked responses. In a phase-dependent manner, the two stimuli can also evoke suppressive responses. We propose that mechanical sensitivity interacts with synaptic responses to shape responses in the AN, including frequency tuning and temporal phase locking. Combining neurotransmission and mechanical sensation to control spike patterns gives the mammalian AN a secondary receptor role, an emerging theme in primary neuronal functions.

Cell cycle associated protein 1 (Caprin1) is an RNA-binding protein that can regulate the cellular post-transcriptional response to stress. It is a component of both stress granules and neuronal RNA granules and is implicated in neurodegenerative disease, synaptic plasticity and long-term memory formation. Our previous work suggested that Caprin1 also plays a role in the response of the cochlea to stress. Here, targeted inner ear-deletion of Caprin1 in mice leads to an early onset, progressive hearing loss. Auditory brainstem responses from Caprin1 -deficient mice show reduced thresholds, with a significant reduction in wave-I amplitudes compared to wildtype. Whilst hair cell structure and numbers were normal, the inner hair cell-spiral ganglion neuron (IHC-SGN) synapse revealed abnormally large post-synaptic GluA2 receptor puncta, a defect consistent with the observed wave-I reduction. Unlike wildtype mice, mild-noise-induced hearing threshold shifts in Caprin1 -deficient mice did not recover. Oxidative stress triggered TIA-1/HuR-positive stress granule formation in ex-vivo cochlear explants from Caprin1- deficient mice, showing that stress granules could still be induced. Taken together, these findings suggest that Caprin1 plays a key role in maintenance of auditory function, where it regulates the normal status of the IHC-SGN synapse.

Binaural hearing is critically important for the perception of sound spatial locations. The primary auditory cortex (AI) has been demonstrated to be necessary for sound localization. However, after hearing onset, how the processing of binaural cues by AI neurons develops, and how the binaural processing of AI neurons is affected by reversible unilateral conductive hearing loss (RUCHL), are not fully elucidated. Here, we determined the binaural processing of AI neurons in four groups of rats: postnatal day (P) 14–18 rats, P19–30 rats, P57–70 adult rats, and RUCHL rats (P57–70) with RUCHL during P14–30. We recorded the responses of AI neurons to both monaural and binaural stimuli with variations in interaural level differences (ILDs) and average binaural levels. We found that the monaural response types, the binaural interaction types, and the distributions of the best ILDs of AI neurons in P14–18 rats are already adult-like. However, after hearing onset, there exist developmental refinements in the binaural processing of AI neurons, which are exhibited by the increase in the degree of binaural interaction, and the increase in the sensitivity and selectivity to ILDs. RUCHL during early hearing development affects monaural response types, decreases the degree of binaural interactions, and decreases both the selectivity and sensitivity to ILDs of AI neurons in adulthood. These new evidences help us to understand the refinements and plasticity in the binaural processing of AI neurons during hearing development, and might enhance our understanding in the neuronal mechanism of developmental changes in auditory spatial perception.

Hearing is one of the five sensory organs that allows us to interact with society and our environment. However, one in eight Americans suffers from sensorineural hearing loss that is great enough to adversely impact their daily life. There is an urgent need to identify what part/degree of the auditory pathway (sensory or neural) is compromised so that appropriate treatment/intervention can be implemented. Single- or two-tone evoked potentials, the electrocochleography (eCochG), were measured along the auditory pathway, i.e., at the round window and remotely at the vertex, with simultaneous recordings of ear canal distortion product otoacoustic emissions. Sensory (cochlear) and neural components in the (remote-) eCochG responses showed distinct level- and frequency-dependent features allowing to be differentiated from each other. Specifically, the distortion products in the (remote-)eCochGs can precisely localize the sensory damage showing that they are effective to determine the sensory or neural damage along the auditory pathway.