Auditory Neurophysiology publications

Cisplatin (CDDP), a widely used chemotherapeutic agent, is limited by severe ototoxicity side effects. Local drug delivery via the middle ear represents the most effective approach for treating inner ear disease. However, therapeutic efficacy is constrained by poor middle ear retention and limited permeability across the round window membrane (RWM). Quercetin (QU) exhibits potent activity against CDDP-induced cytotoxicity but suffers from delivery challenges. To address this, we developed amino-functionalized mesoporous silica nanoparticles (NH2-MSNs) loaded with QU (QU-N-MSNs), leveraging the permselective properties of the RWM. This system was noninvasively administered to the cochlea via trans-tympanic delivery. The synthesized QU-N-MSNs demonstrated a uniform particle size of approximately 116–124 nm, positive charge, and sustained drug release properties. Compared to free QU, QU-N-MSNs demonstrated significantly enhanced antiapoptotic and cytoprotective activities in vitro. In vivo studies confirmed nanoparticle retention within the inner ear for ≥14 days post administration and efficient RWM penetration. Pretreatment with QU-N-MSNs prior to CDDP exposure in murine models substantially mitigated ototoxicity, as evidenced by reduced hearing threshold shifts across multiple frequencies, preservation of hair cells (HCs) and spiral ganglion neurons (SGNs), and attenuation of mitochondrial-mediated SGN apoptosis. These findings establish QU-N-MSNs as an effective RWM-penetrating delivery platform, offering a promising strategy to enhance hydrophobic drug bioavailability in the inner ear and prevent CDDP-induced ototoxicity.

Background: Millions of older adults have age-related hearing loss (ARHL), a disorder where potassium-secreting cells in the cochlea's lateral wall often degenerate. The degeneration reduces the force that drives ions into the sensory cells during sound stimulation, which is traditionally thought to explain the loss of hearing. Here we describe previously unknown mechanisms underlying this metabolic form of ARHL. Methods: Fluorescence spectroscopy and live-cell imaging was used in Dunkin-Hartley guinea pigs of either sex to investigate the effects of lateral wall dysfunction on the hearing organ and its accessory structures. Critical findings were confirmed by studying samples of human temporal bones. Findings: Lateral wall dysfunction caused calcium levels in the inner ear to decline, a change that was most pronounced in the tectorial membrane, an accessory structure crucial for transmitting acoustic stimuli to sensory cell stereocilia. This calcium depletion deprived the sensory cells of an essential ion. Additionally, the tectorial membrane detached from stereocilia, significantly impairing their ability to respond to sound. Sound-evoked responses were further decreased by sustained contraction of the entire hearing organ. Interpretation: These findings establish the tectorial membrane as a key factor in metabolic ARHL, which needs to be considered when developing better diagnostic tools or treatments. Funding: Swedish Research Council grants 2017-06092 and 2022-00548, Swedish Brain foundation grant FO2023-0171 and US National Institutes of Health grant R01DC000141-44.

Speech perception relies on two fundamental acoustic components: spectral and temporal. While spectral information is known to be represented in the auditory cortex through tonotopy, how temporal features are organized has remained unclear. Here, by varying onset rise-ramp steepness and frequencies, we reveal that temporal envelope steepness-a critical cue for phoneme discrimination and sound source perception-is systematically mapped in the mouse auditory cortex. Using widefield calcium imaging, we discovered that the envelope steepness is represented orthogonally to the tonotopic axis, forming a two-dimensional cortical map that mirrors the dual structure of sounds. This organization was observed in primary-like areas but not in higher-order-like areas, indicating distinct auditory processing streams. These findings uncover a principle of cortical organization, suggesting that the auditory cortex encodes sound along two independent axes and thereby provides a neural basis for the parallel processing of complex sounds such as speech and natural acoustic environments.

We recorded responses of single neurons in core and belt auditory cortical fields in alert macaque monkeys to frequency-modulated sounds, key feature of many natural sounds, including speech and vocalizations. We found that the timing of neural activity, rather than its magnitude, explained decoding accuracy in all cortical areas, and we did not observe compelling evidence of improved discriminability in core or belt fields beyond that seen in the primary auditory cortex.

Objective: Screen for differential proteins in the cochlea of mice associated with Age-Related Hearing Loss (ARHL), analyze and validate the expression of specific differential proteins and genes in the cochlea and auditory cortex of ARHL mice, and preliminarily explore their potential mechanisms of action. Methods: ABR (Auditory Brainstem Response) hearing tests were conducted to select 15-month-old C57BL/6 mice with significantly decreased hearing as the experimental group and 2-month-old mice with normal hearing as the control group. Cochleae were dissected, and unlabeled quantitative proteomics was employed to identify and analyze differentially expressed proteins in the inner ear of the two groups of mice. Key node proteins were selected via the STRING database and Cytoscape analysis. The expression of two selected proteins, ApoE and Spp1, in the cochlea was detected using qRT-PCR, Western blot, and immunofluorescence techniques, and their expression in the auditory cortex of the brain was further explored. Results: Label-free quantitative proteomics identified 115 differentially expressed proteins in the cochlea of 15-month-old ARHL mice compared to 2-month-old hearing-normal mice, including 42 upregulated and 73 downregulated proteins. GO and KEGG enrichment analyses revealed significant enrichment of differentially expressed proteins in functions and signaling pathways associated with neurodegenerative diseases and neurotransmission. Protein-Protein Interaction (PPI) analysis using the STRING database and Cytoscape selection identified ApoE and Spp1 as key hub proteins. Validation via qRT-PCR, Western blot, and immunofluorescence demonstrated that both ApoE and Spp1 were highly expressed in the cochlea and auditory cortex of the ARHL mice compare to 2-month-old hearing-normal mice. Conclusion: ApoE and Spp1 are upregulated in the cochlea of ARHL mice, particularly in spiral ganglion neurons, and in the auditory cortex, suggesting their potential involvement in the pathogenesis and progression of ARHL through the modulation of auditory neural conduction systems. Level of evidence: Level 2.

Previous studies based on layer specificity suggest that ascending signals from the thalamus to the sensory neocortex preserve spatially organized information, but it remains unknown whether sensory information descending from sensory neocortex to the thalamus also maintains such spatial organization pattern. By focusing on projection specificity, we mapped the tone response properties of two groups of cortical neurons in the primary auditory cortex (A1), based on the relationship between their specific connections to other regions and their function in ascending (thalamocortical recipient [TR] neurons) or descending (corticothalamic [CT] neurons) auditory information. A clear tonotopic gradient was observed among TR neurons, but not CT neurons. Additionally, CT neurons exhibited markedly higher heterogeneity in their frequency tuning and had broader bandwidth than TR neurons. These results reveal that the information flow descending from A1 to the thalamus via CT neurons is not arranged tonotopically, suggesting that the descending information flow possibly contributes to higher-order feedback processing of diverse auditory inputs.

Early studies on orientation selectivity in the visual cortex have suggested that sensory systems generate new feature representations at specific processing stages. Many observations challenge this view, but in the absence of systematic, multistage measurements, the logic of how feature tuning emerges remains elusive. Here, using a generic approach based on representational similarity analysis with a noise-corrected population metric, we demonstrate in the mouse auditory system that feature representations evolve gradually with, in some cases, major, feature-specific improvements at particular stages. We observe that single frequency tuning is already fully developed in the cochlear nucleus, the first stage of processing, while tuning to higher-order features improves up to the auditory cortex, with major steps in the inferior colliculus for amplitude modulation frequency or noise bandwidth tuning and in the cortex for frequency modulation direction and for complex sound identity or direction. Moreover, we observe that intensity tuning is established in a feature-dependent manner, earlier for pure frequencies than for more complex sounds. This indicates that auditory feature computations are a mix of stepwise and gradual processes which together contribute to decorrelate sound representations.

We describe here a mode of activity of neurons in auditory cortex of freely moving rats consisting of large, slow firing rate modulations, which were not driven by sounds, were larger than the sound evoked responses, and were locked to specific time points during the task, similar to responses of hippocampal time-sensitive neurons. This activity mode had important functional consequences to sound processing during task performance. We show that the slow firing rate modulations caused the ongoing activity just before sound presentations to be higher during task performance than during passive listening. Concurrently, during task performance, the responses to target stimuli were weaker but more informative about the task-relevant sounds. In a model, higher ongoing activity caused more synaptic depression of the cortico-cortical synapses, reducing the tendency of the network to produce population spikes and resulting in weaker but more informative responses to sounds.

Hearing relies upon speedy synaptic transmission of sound information from inner hair cells (IHCs) to spiral ganglion neurons. To accomplish this, IHCs use a sophisticated presynaptic machinery including the multi-C 2 domain protein otoferlin that is affected by human deafness mutations. Otoferlin is essential for IHC exocytosis, but how it binds Ca 2+ and the target membrane to serve synaptic vesicle (SV) tethering, docking, and fusion remained unclear. Here, we obtained cryo–electron microscopy structures of otoferlin and employed molecular dynamics simulations of membrane binding. We show that membrane binding by otoferlin involves C 2 B-C 2 G domains and repositions C 2 F and C 2 G domains. Disruption of Ca 2+ -binding sites of the C 2 D domain in mice altered synaptic sound encoding and eliminated the Ca 2+ cooperativity of IHC exocytosis, indicating that it requires the binding of several Ca 2+ -ions by otoferlin. Together, our findings elucidate molecular mechanisms underlying otoferlin-mediated SV docking and support the role of otoferlin as Ca 2+ sensor of SV fusion in IHCs.

Fragile X Syndrome (FXS) is a leading genetic cause of intellectual disability and autism-like behaviors. Glutamatergic mGluR5 receptors and matrix metalloproteinase-9 (MMP-9) are therapeutic targets to treat FXS, but clinical trials targeting each of these pathways have not been successful. Here, we tested if the electroencephalography (EEG) phenotypes associated with FXS are reversed with a novel combination of treatments affecting the two pathways. Fmr1 knockout (KO) mice were given 10 days of CTEP (mGluR5 antagonist) alone or in combination with minocycline (MMP-9 inhibitor). EEG was recorded during resting (no acoustic stimulation) and during sound presentations (to produce sound-evoked EEG) at 1 day and 10 days after the beginning of treatment administration to test acute effects and potential tachyphylaxis. In pre-treatment WT and KO mice comparisons, we replicated previously published Fmr1 KO mouse EEG phenotypes including elevated power in the resting gamma band, elevated single trial power, and reduced phase-locking to spectrotemporally dynamic auditory stimuli. We found that CTEP treatment alone did not show any benefit compared to vehicle in Fmr1 KO mice after either 1 or 10 days of treatment. CTEP + minocycline reduced resting gamma band power in the Fmr1 KO mice to a greater extent than vehicle at both treatment time points. There were no effects on sound-evoked responses. These data suggest that combined CTEP and minocycline treatment alters resting EEG measures while each treatment administered separately does not yield similar changes. High power in broadband gamma frequency correlates with irritability, stereotyped behaviors, and hyperactivity in FXS patients, suggesting a combination of drugs that reduce mGluR5 and MMP-9 activity may be beneficial in FXS.

Adolescence is a developmental period characterized by heightened plasticity. Yet, how ongoing development affects sensory processing and cognitive function is unclear. We investigated how adolescent (postnatal day 20–42) and adult (postnatal day 60–82) mice differ in performance on a pure tone Go/No-Go auditory discrimination task of varying difficulty. Using dense electrophysiological recordings, we measured spiking activity at single neuron resolution in the auditory cortex while mice were engaged in the task. As compared to adults, adolescent mice showed lower auditory discrimination performance in a difficult task. This difference in performance was due to higher response variability and weaker cognitive control expressed as higher lick bias. Adolescent and adult neuronal responses differed only slightly in representations of pure tones when measured outside the context of learning and the task. However, cortical representations after learning within the context of the task were markedly different. We found differences in stimulus- and choice-related activity at the single neuron level representations, as well as lower population-level decoding of the difficult task in adolescents. Overall, cortical decoding in adolescents was lower and slower, especially for difficult sound discrimination, reflecting immature cortical representations of sounds and choices. Notably, we found age-related differences, which were more pronounced after learning, reflecting the combined impact of age and learning. Our findings highlight distinct neurophysiological and behavioral profiles in adolescence, underscoring the ongoing development of cognitive control mechanisms and cortical plasticity during this sensitive developmental period.

The pre‐hearing mouse cochlea undergoes critical periods of spontaneous Ca 2+ ‐dependent activity that spreads across non‐sensory supporting cells and inner hair cells (IHCs). These signals have been shown to regulate not only the refinement of neural circuits along the auditory pathway towards functional maturity, but also the maturation of the hair cells into sensory receptors. Although the origin and interplay of these Ca 2+ signals during cochlear development have recently been investigated in live mice, the impact of anaesthesia on in vivo functional measurements was not explored. Here, we investigate the effects of different anaesthetic regimes (ketamine and xylazine; 2.5% isoflurane; and 1.0%–1.5% isoflurane with the sedative acepromazine) that provided an effective unconsciousness to perform the surgery and Ca 2+ ‐imaging recordings from the intact cochlea of live mice. The IHCs, supporting cells and spiral ganglion neuron terminals onto the IHCs showed spontaneous Ca 2+ ‐dependent activity under all anaesthetic regimes, with a few significant differences observed between conditions. Calcium waves from supporting cells synchronized the activity of IHCs. Moreover, we found that the endocochlear potential, which is crucial for cochlear function, was unaffected by the different anaesthetics. However, low concentrations of isoflurane produced the most stable recordings of vital physiological signs in mice, including heart rate and breathing rate. Although all anaesthetic regimes tested appeared to be suitable for performing Ca 2+ imaging from the cochlea of pre‐hearing live mice, a low concentration of isoflurane (1.0%–1.5%), combined with the pre‐anaesthetic sedative acepromazine and oxygenation, represents the most suitable approach to maintain a stable and long‐lasting depth of anaesthesia.

Discrimination of sensory stimuli is fundamentally constrained by the information encoded in neuronal responses. In the barn owl, interaural time difference (ITD) serves as a primary cue for azimuthal sound localization and is represented topographically in the midbrain auditory space map in the external nucleus of the inferior colliculus (ICx). While prior studies have demonstrated a correspondence between spatial tuning and behavioral acuity, it remains unclear how changes in sensory reliability influence this relationship. Here, we examined how behavioral and neuronal ITD discrimination thresholds vary with binaural correlation (BC), which manipulates ITD cue reliability. Using the pupil dilation response as a behavioral metric in head-fixed owls of either sex, we found that ITD just-noticeable differences increased exponentially as BC decreased. In contrast, the widths of ICx ITD tuning curves increased more modestly, indicating that tuning resolution alone does not account for behavioral discrimination performance. By computing the Fisher information from ICx neuronal responses, we showed that the average neuronal discriminability predicts behavioral thresholds across BC values. A habituation-based model incorporating BC-dependent changes in tuning width, firing rate, and response variability successfully accounted for both direction and ITD discrimination. These findings support a model in which perceptual acuity is governed by the combined influence of neuronal tuning and variability and provide a unified framework for understanding how midbrain auditory representations underlie adaptive spatial hearing.

Exposure to loud and/or prolonged noise damages the cochlea and triggers downstream brain changes, resulting in hearing loss and altered speech comprehension. It remains unclear however whether noise exposure also compromises the cochlear efferent system; a feedback pathway originates in the brain that fine-tunes hearing sensitivity in the cochlea. Recent evidence suggests that lateral olivocochlear (LOC) efferent system supports hearing recovery by sustaining auditory nerve activity for several weeks after acoustic trauma (Sitko et al., 2025); however, the underlying neural mechanisms have not been fully elucidated. To address this gap, we examined the long-term effects of noise-induced hearing loss (NIHL) on the spontaneous action potential (AP) firing pattern in mouse LOC neurons of either sex. Under normal conditions, these neurons display a characteristic burst firing pattern driven by Ca 2+ channel-mediated membrane voltage oscillations. One week following noise exposure, hearing thresholds were significantly elevated, and the duration of AP bursts was increased, as a result of an enhanced Ca 2+ current. Moreover, LOC neurons exhibit Ca 2+ -dependent inactivation (CDI) of Ca 2+ currents—a key process that shapes the duration of voltage oscillations—whose properties were also modified following noise exposure. Interestingly, our data suggest that the interplay between Ca 2+ channel activation, CDI, and outward leak currents is sufficient to sustain the oscillating behavior of LOC neurons. We propose that NIHL increases efferent activity, thereby enhancing the release of neurotransmitters and neuropeptides, which may support long-lasting restoration of sensory coding in the damaged cochlea.

Objective: This study aims to establish an economically viable and easily accessible adult animal model for optogenetic activation of auditory neurons using adeno-associated viruses (AAVs) carrying ChR2(H134R) to explore the potential of cochlear optogenetics as a hearing restoration technology. Methods: Healthy adult guinea pigs were used in the experiments. The viral vector AAV2/8-ChR2(H134R)-hSyn-eYFP was administered to the right cochlea via the round window membrane. The confocal microscopy and reverse transcription polymerase chain reaction (RT-PCR) were utilized to analyze the ChR2(H134R) expression localized to spiral ganglion neurons (SGNs). The auditory pathway activation was assessed by recording the optical compound action potential (oCAP) and acoustic compound action potential (aCAP) at various laser intensities. Results: The ChR2(H134R)-eYFP expression was confirmed in 90% of the tested animals, localized to the SGNs of the injected ear. Higher mRNA levels of ChR2(H134R) and eYFP were observed in the injected ear compared to the non-injected ear, while actin (Actb) mRNA levels were not significantly different. The oCAP was successfully elicited by a 470 nm blue light laser stimulus, with similar amplitudes and latency periods to those of aCAPs when the oCAP was evoked by 5.80 mW blue light and the aCAP was evoked by a 40 dB SPL click. The amplitudes of oCAPs increased with increasing laser intensity. Conclusion: This study demonstrates the viability of optogenetic activation of the auditory system in adult guinea pigs through the transduction of AAV-ChR2(H134R) in SGNs. Cochlear optogenetics demonstrates potential as a hearing restoration technology, providing a basis for further clinical research and opening new avenues for investigation.

Cross-modal responses are common in primary sensory cortex, yet a recent study showed that signals in primary visual cortex (V1) once labeled multisensory were instead more closely related to face movements. To what extent can multisensory processing in other modalities also be explained by face movements? We measured face movements and neural activity in awake mouse primary auditory cortex (A1) and V1 in response to visual and auditory stimulation as well as during spontaneous activity. We find that visual stimuli rarely evoke face movements, A1 responses to visual input remain robust even in the absence of face movement, and optogenetic silencing of V1 reduces A1 visual responses. Taken together, these results confirm that cross-modal responses in the auditory system are primarily sensory, not state driven, in origin.

Humans live in an environment that contains rich auditory stimuli, which must be processed efficiently. The entrainment of neural oscillations to acoustic inputs may support the processing of simple and complex sounds. However, the characteristics of this entrainment process have been shown to be inconsistent across species and experimental paradigms. It is imperative to establish whether neural activity in response to speech is a result of combination of simple evoked responses or of entrainment of neural oscillations in human participants. In this study, 12 participants with intracranial electrodes listened to natural speech and neural entrainment as evidenced by oscillatory activity persisting beyond the evoked responses was assessed. Neural activity was recorded from 165 contacts in Heschl's gyrus and superior temporal gyrus. First, acoustic edges in the speech envelope induced coherence between speech and auditory cortex activity. Further, entrainment in the theta-alpha band outlasted the acoustic stimulation. This activity exceeded what could be expected from a simple evoked response. These findings suggest that speech has the potential to entrain neural oscillations in the human auditory cortex.

Introduction The posterior tail of the striatum receives dense inputs from sensory regions of cortex and thalamus, as well as midbrain dopaminergic innervation, providing a neural substrate for associative sensory learning. Previously, we have demonstrated that developmental hearing loss is associated with aberrant physiological states in striatal medium spiny neurons (MSNs). Methods Here we directly investigated auditory associative learning impairments in the striatum of adult Mongolian gerbils that underwent transient developmental hearing loss or sham hearing loss during the critical period of auditory development. We used electrophysiology to reveal significant changes to neuronal population responses in vivo and intrinsic and synaptic properties to medium spiny neurons in vitro as animals learned an appetitive “Go/No-Go” auditory discrimination task. For in vivo experiments a 64-channel electrode was implanted in the auditory region of the posterior tail of the striatum and neuronal recordings were carried out as animals learned the task. For in vitro experiments, corticostriatal slice preparations were made from animals on each day of training. Results In naïve animals from both groups there was limited to no phase locking to either auditory stimulus in vivo , and long term depression resulted from theta burst stimulation in vitro . Furthermore, intrinsic and synaptic properties in normal hearing animals were unaffected; however, the hearing loss group continued to show lowered synaptic inhibition, synaptic hyperexcitation, and suppressed intrinsic excitability in the hearing loss group. Starting around day 3–4 in both groups, the emergence of striatal medium spiny neuron phase locking to the auditory conditioning stimuli was observed in vivo . This occurred contemporaneous to an increased probability of theta burst induced LTP during MSN whole cell recording in vitro , and acquisition of the task as the correct rejection response significantly increased in the behaving animals. During the acquisition phase MSNs in the normal hearing group showed a significant decrease in synaptic inhibition and increase in synaptic excitation with no change to intrinsic excitability, while the MSNs in the hearing loss group showed a significant increase in synaptic inhibition, reduction of synaptic hyper excitability, and compensatory changes to intrinsic excitability that supported normal action potential generation. In both groups, synaptic properties were resolved to similar level of E/I balance that could be part of a conserved learning state. Discussion These changes to the intrinsic and synaptic properties likely support LTP induction in vivo and the strengthening of synapses between auditory inputs and MSNs that facilitate neuronal phase locking. These findings have significant implications for our understanding of striatal resilience to sensory impairments in early life, in addition to establishing a granular understanding of the striatal circuit changes that support reward driven stimulus–response learning.

Deep Brain Stimulation (DBS) of the superolateral medial forebrain bundle has shown promising long-term anti-depressant effects in treatment-resistant depression patients, although the mechanisms are not clear. The study explored medial forebrain bundle DBS mediated modulation of central noradrenaline transmission in a rodent depression model, the Flinders Sensitive Line (FSL), and in controls, Sprague Dawley (SD) rats. In vivo noradrenergic signaling in the prefrontal cortex (PFC) and nucleus accumbens (NAc) and ultrasonic vocalization were monitored during unilateral mfb-DBS across diverse stimulation parameters. The fiber amount, myelination status, and the activation of ascending projected noradrenergic cell groups (A1, A2, A6) were quantified. Moreover, stimulation induced changes in the parvalbumin-mediated feedforward microcircuitry and neuron activation at PFC and NAc were assessed. FSL rats showed decrease in NA fibers in mfb. Stimulation increased PFC noradrenergic signaling similarly across both groups compared to baseline, but in the NAc, the FSLs had notably higher signaling compared with SDs. FSLs demonstrated more positive affective ultrasonic vocalizations post-DBS than SDs. Brainstem nuclei A1 and A2 had similar noradrenergic neuron density across the experimental groups, and mfb DBS increased neuronal activation in both groups. FSLs had fewer noradrenergic neurons in the A6 nuclei, fewer unmyelinated noradrenergic fibers traversing the mfb, and decreased parvalbumin interneuron activity in both PFC and NAc. DBS normalized parvalbumin interneuron activity in the FSL rats. The study proposes that mfb DBS, via the modulation of the central NA system and the GABAergic inhibitory control of neural excitability, likely contributes to the anti-depressant therapeutic mechanisms reported in both clinical and experimental studies.

Layer 6 corticothalamic neurons (CTs) provide strong feedback input that is crucial to perception and cognition in normal and pathological states; however, the synaptic properties of this input remain largely unknown, especially in pathology. Here, we examined the synaptic properties of CT axon terminals in the medial geniculate body (MGB), the auditory thalamus, in normal hearing male and female mice and in a mouse model of noise-induced hearing loss (NIHL), also in male and female mice. In normal hearing mice, we found that the amplitude of CT-evoked excitatory postsynaptic current (EPSC) to the core-type ventral subdivision of the auditory thalamus (MGv), which mainly conveys rapid sensory information, is larger compared with the amplitude of CT-evoked EPSC to the matrix-type dorsal subdivision of the auditory thalamus (MGd), which likely conveys higher-order internal state information. This is due to higher axonal density and/or axonal recruitment in CT→MGv compared with CT→MGd synapses. After noise trauma, we observed enhanced short-term facilitation in CT→MGd but not CT→MGv synapses. Our findings reveal a previously unknown mechanism of short-term synaptic plasticity after NIHL via which CTs enhance the throughput of matrix-type thalamus, likely to improve perceptual recovery via higher-order contextual modulation.