Auditory Research publications

Like mammals, some reptiles possess sensitive and frequency-selective hearing at mid- to high-kilohertz frequencies. Both groups employ a place-frequency map within the inner ear to encode sound stimuli, but the mechanisms by which they achieve tuned nerve activation along the cochlea are believed to be distinct. To investigate the mechanical origins of auditory sensation in the reptiles, we measured sound-evoked displacement responses in the hearing organ of the tokay gecko ( Gekko gecko ), known as the basilar papilla. Using optical coherence tomography, we were able to resolve sub-nanometer-scale vibrations throughout the organ both in vivo and ex vivo . We saw that tuning was not present in the tissue-scale mechanics at any position within the organ; it instead exhibits an untuned rotational motion. We developed a mathematical model which predicted that this motion induces hair-bundle-stimulating fluid displacement, and is due to an anatomical asymmetry seen in many reptile species. These results are consistent with the theory that hair-bundle-level mechanics are primarily responsible for tuning in the tokay gecko cochlea, and we argue that our proposed mechanisms generalize to many other species across the Reptilia class. SIGNIFICANCE We present the most comprehensive picture of tissue-scale mechanics in a reptile hearing organ to-date: sound-evoked displacement responses within the basilar papilla of the tokay gecko ( Gekko gecko ) both ex vivo and in vivo , across the animal’s auditory frequency range, and along all three spatial dimensions. We observed that place-frequency tuning is not present in the mechanics of the tissue as it is in mammals. Instead, this organ relies on an untuned rocking mechanism to stimulate hair bundles. A mathematical model shows that this may be at play in other reptile species, indicating a unifying mechanism of hearing in the class Reptilia in which hair bundle mechanics play the leading role in frequency tuning.

Purpose: In anesthesia research and clinical practice, the level of consciousness is often assessed by stimulating subjects and observing their responses. It is possible that such stimulation alters the very state of consciousness being assessed. Whether this phenomenon is accompanied by specific cortical activities remains unknown. Here, we tested the effects of behavioral assessments on neuronal activity in the rat primary auditory cortex (A1) and perirhinal cortex (PrC). Methods: Neuronal spiking activity was recorded from microwire arrays targeting A1 and/or PrC in rats (n = 9), whilst presenting 40 Hz click-trains and gradually increasing infusion rates of propofol, remifentanil, ketamine or dexmedetomidine. We assessed righting reflex, pain withdrawal, whisking reflex, and corneal reflex every 12 minutes (every 360 auditory trials). We searched for significant changes in baseline firing rate and stimulus-evoked activities (where present) due to the behavioral assessments, by comparing 4 min intervals before and after the behavioral tests. Results: Out of 1004 units in 20 sessions, we found significantly elevated activity (p < 0.01) following behavioral tests in 32% and 20% of A1 auditory-responsive units and 8.6% and 5.3% of auditory-unresponsive units under propofol and dexmedetomidine, respectively, but not under remifentanil or ketamine. A higher proportion of units modulated their firing following behavioral tests in A1 compared to PrC, with median gains ranging from +0.23 to +1.06, depending on the specific response feature. Conclusion: Behavioral testing increases ongoing firing rate and responses to sounds in some cortical neurons. This increase is more prevalent in A1 than in PrC, mirroring clinical observations that patients exhibit basic responses under sedation, while complex processing, such as memory encoding, is impaired. The degree of neuronal modulation due to behavioral testing varies between anesthetic agents, mirroring their clinical phenotypes.

Stimulus-specific adaptation is a key feature of sensory systems, where the repeated presentation of a stimulus results in diminishing neuronal responses specifically to that stimulus, but not to a different stimulus. Cortical neurons display varying degrees of stimulus-specific adaptation, with cortical circuits shaping the dynamics of these responses. Here, we use calcium imaging and optogenetic approaches to assess the contributions of SOM and PV neurons to stimulus-specific adaptation in the mouse auditory cortex. We find opposing and complementary effects of SOM and PV neurons on the adaptation of principal neurons which manifest as context-dependent SOM neuron-mediated enhancement and PV-neuron mediated suppression of responses that depend on the temporal repetition rate of the sounds. Our results suggest that SOM and PV neurons can engage in multiple operating regimes that exist in a continuum during sensory processing.

Background Peripheral hearing loss is associated with auditory hallucinations and increased risk for psychotic disorder, particularly in genetically vulnerable individuals. Moreover, both hearing loss and schizophrenia disrupt auditory temporal acuity, a sensitive measure of auditory brain function. Here, we used mouse models of hearing loss and schizophrenia to reveal how neural mechanisms of auditory cortical temporal acuity depend on sensory and genetic risk factors for psychosis. Methods We quantified auditory cortical temporal acuity in mice with or without hearing loss and with or without the 22q11.2 deletion — one of the strongest known genetic risk factors for schizophrenia. Cortical single-unit and population activity were recorded in awake mice (N = 23) during presentations of loudness-adjusted gap-in-noise stimuli with varying durations of silent gap, which are commonly used to assess auditory temporal acuity in humans. Results Both hearing loss and the 22q11.2 deletion disrupted auditory cortical temporal acuity, but through distinct mechanisms. Hearing loss broadly degraded temporal acuity at the level of single-unit responses and neural population activity. In contrast, the 22q11.2 deletion selectively impaired gap duration thresholds in regular-spiking (putative excitatory) but not fast-spiking (putative inhibitory) neurons. Mice with comorbid hearing loss and 22q11.2 deletion exhibited both abnormalities in auditory cortical temporal acuity. Conclusions Sensory and genetic risk factors for psychosis can disrupt auditory cortical temporal acuity via distinct mechanisms that remain partially dissociable even under comorbid conditions. These findings underscore the importance of accounting for hearing loss comorbidity when interpreting auditory cortical dysfunction in psychotic disorder.

Purpose Sound pressure recordings at different positions at the head such as ear canal, nasal cavity, or on the skin are effective tools to verify, fit, or follow up the output of bone conduction devices (BCDs) intra- and post-operatively. Here, we investigated the possibility of using a surface microphone (SM) as a non-invasive alternative to laser Doppler vibrometry (LDV) measuring cochlear promontory (CP) vibrations in human heads. Methods A percutaneous BCD (Ponto system) was implanted at the standard position in five human (four males/one female) cadaver heads (ten ears). CP vibration was measured using LDV in response to the BCD stimulation. Simultaneously, the sound pressure level (SPL) emitted by the skin was measured by the SM attached to the forehead of the specimens. A linear regression model estimated the vibration amplitudes based on the measured SPL. Results A frequency-independent linear regression between recorded SM SPL and CP velocity showed a significant correlation (slope = 1.012; r 2 = 0.535, p < 0.001). An enforced fixed slope (constant = 166.7 dB) of one resulted in a mean absolute error of MAE = 7.7 ± 2.7 dB across frequencies. Although the initial linear model with a fixed slope of one showed frequency-dependent deviations, applying a frequency-specific correction significantly improved the prediction accuracy ( r 2 = 0.557, MAE = 6.5 ± 1.5 dB). Conclusion Microphone-based recording of acoustic surface emissions offers a non-invasive alternative to LDV to assess BCD output.

To investigate auditory acclimatization to high-altitude hypobaric hypoxia, we conducted longitudinal transcriptomic analyses in Wistar rats rapidly transported from 500 m to 3,700 m. Auditory brainstem response (ABR) testing and hematoxylin and eosin staining revealed progressive changes, with hearing impairment at 30 days (D30) followed by acclimatization at 60 days (D60). RNA-seq of cochlear tissues identified 1,329 differentially expressed genes (DEGs) during impairment and 1,441 DEGs during acclimatization. STEM analysis linked these temporal patterns to specific biological processes: peripheral nervous system development (acclimatization) and myotube differentiation (impairment), as annotated by GO/KEGG and Metascape. STEM analysis revealed gene expression profiles that significantly correlated with ABR threshold shifts, encompassing biological processes such as sensory organ morphogenesis and gravity-sensing pathways. Subsequent validation through RT-PCR and immunofluorescence staining confirmed the involvement of key candidate genes, including morphogenesis-associated factors (Aqp1, Rbp4), the mechanosensitive hormone Bglap, and critical transcriptional regulators (Maz, Tfap4, Nf1) in the acclimatization response. This study elucidates key transcriptional mechanisms governing both the initial hearing impairment and subsequent acclimatization processes during high-altitude adaptation.

Auditory evoked responses undergo notable changes during childhood, likely reflecting modifications in synaptic signaling in the auditory cortex. Particularly robust response, observed around 200 to 300 ms post stimulus (N/M250), has been consistently reported in children but is absent in adults. This long-latency response, evoked even in passive listening conditions, may indicate heightened sensory pathway responsiveness, facilitating experience-driven cortical plasticity. However, it remains unclear whether this delayed activation pattern is an intrinsic, species-general feature of brain development. We recorded cortical auditory evoked responses to monaural sine-wave tones/click sounds in 3 age groups (preadolescents, adolescents, and young adults) of human subjects and rats. Following short-lived early responses, both species exhibited a long-latency (150 to 450 ms) response in the auditory cortex. In both species, the relative amplitude of the long-latency response, compared to early responses, was increased in younger individuals. In human children, single-trial analysis demonstrated more consistent trial-by-trial timing of the response in this later time window than in the adult-typical 100-ms response in the earlier time window. Given its emergence in purely passive conditions, and across species, the robust current activity in late time window could represent a distinct synaptic event and may serve as a marker of the maturational stage, particularly in GABAergic cortical circuits.

Background DFNB16, the second most common genetic cause of hearing loss, is caused by mutations of the STRC gene encoding stereocilin, a protein essential for the effective functioning of outer hair cells (OHCs) as cochlear amplifiers. Strc −/− mice, which lack stereocilin, display severe to profound deafness and constitute a relevant preclinical model for DFNB16. Methods Using Strc − / − mice, we developed a gene therapy strategy based on the use of dual AAV9‐PHP.eB vectors to deliver the full‐length Strc cDNA. Therapeutic efficacy was assessed by evaluating stereocilin expression, OHC bundle architecture, and their attachment to the tectorial membrane, together with functional recovery using distortion product otoacoustic emissions (DPOAEs), auditory brainstem responses (ABR) measurements and Go/No‐Go behavioral testing with psychometric analysis. Results Dual‐AAV–mediated Strc gene delivery restored stereocilin expression, OHC bundle architecture and their attachment to the tectorial membrane, leading to the recovery of cochlear amplification and hearing to near normal thresholds, as confirmed by distortion product otoacoustic emission (DPOAE) and auditory brainstem response measurements. Behavioural assessment showed that treated Strc −/− mice regained normal frequency discrimination, indicating a restoration of higher‐order auditory processing, up to 100 days post‐treatment. Conclusion These findings provide the first proof‐of‐principle that peripheral gene therapy can restore OHC function, cochlear amplification and central auditory perception in a DFNB16 model. Key points Dual AAV‐mediated gene delivery restored peripheral hearing in a DFNB16 preclinical mouse model. The same treatment also restored central auditory processing. AAV‐mediated gene therapy represents a promising curative strategy for DFNB16. These results reinforce the translational potential for treating human genetic deafness.

Purpose Although human temporal bones suggest that cochlear synapse numbers decline with age and noise exposure, no validated diagnostic method exists. In animal models, cochlear synaptopathy is associated with reduced auditory brainstem response (ABR) wave 1 amplitude and envelope following response (EFR) magnitude for a sinusoidally amplitude modulated (SAM) tone. However, measuring SAM EFR at the optimal modulation frequency (1000 Hz) is difficult in humans. A rectangular amplitude modulated (RAM) tone may be more sensitive to synaptopathy, but this has not been validated in animals. In addition, because synaptopathy likely co-occurs with outer hair cell dysfunction (OHC), a diagnostic assay needs to be robust to abnormal auditory thresholds. The objective of this study was to evaluate the relative ability of ABR and EFR measures to predict synapse numbers in mice with varying degrees of synaptopathy and OHC dysfunction. Methods Distortion product otoacoustic emissions (DPOAEs), ABR, SAM EFR, and RAM EFR were recorded from 57 mice with a range of auditory thresholds and degrees of synaptopathy. Cross-validation was used to compare the relative ability of linear regression models incorporating different measures to predict synapse numbers. Predictions were confirmed histologically. Results RAM EFR modulated at 1000 Hz was the single best predictor of synapse numbers for broad synapse loss across frequency, while combining RAM EFR with ABR further improved predictions. In contrast, ABR best predicted focal synaptopathy. Incorporating DPOAEs improved predictions for EFR, but not ABR. Conclusion RAM EFR, ABR, and DPOAEs should be used in the future when predicting synapse numbers.

No pharmacological interventions exist that can restore or preserve auditory function in the mammalian cochlea. Auditory hair cells (HCs) do not spontaneously regenerate, leading to permanent hearing loss. In non-mammalian vertebrates, HC regeneration happens through proliferation and differentiation of their clonally related supporting cells (SCs). The present study supports the potential of quinoxaline (Qx), a nonsteroidal anti-inflammatory compound, to stimulate SC proliferation in the auditory sensory epithelium, a process that may prime the tissue for future HC regeneration. We synthesized a series of Qx derivatives by introducing various substitutions, ranging from hydrophilic to lipophilic. Seventy analogs were generated and tested in vitro and in vivo. Among those, only one (Qx-100) exhibited the best medicinal chemistry profile and was further modified to expand the structure–activity relationship of the chemotype, develop additional analogs, and optimize potency, bioavailability, and in vivo efficacy. Ten new lead variants were generated. Of those, Qx-294 and Qx-301 demonstrated promising in vitro Absorption, Distribution, Metabolism, and Excretion (ADME) profiles and were selected for further testing. Overall, both compounds were rapidly absorbed in zebrafish and mice and promoted cell proliferation in vitro and in vivo without signs of apoptosis, supporting their potential for sensory HC regeneration.

During wake, sound‐evoked and spontaneous neural activity of the auditory cortex evolves in distinct subspaces, whereas anesthesia disrupts sound responses and merges these spaces. To evaluate if similar modifications of sound representation geometry explain sensory disconnection during sleep, large neural populations of the mouse auditory cortex are followed across slow‐wave sleep and wakefulness. It is observed that sleep dampens sound responses but preserves the geometry of sound representations such that they remain separate from spontaneous activity. Moreover, response dampening is strongly coordinated across neurons and varied throughout sleep, spanning from fully preserved response patterns to population response failures on a fraction of sound presentations. These failures rarely occurred in wakefulness and are more common during high spindle‐band activity. Therefore, in sleep, the auditory system preserves sound feature selectivity up to the cortex for detailed acoustic surveillance but concurrently implements an intermittent gating mechanism leading to local sensory disconnections.

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.

The locus coeruleus–norepinephrine (LC-NE) system regulates arousal and awakening; however, it remains unclear whether the LC does this in a global or circuit-specific manner. We hypothesized that sensory-evoked awakenings are predominantly regulated by specific LC-NE efferent pathways. Anatomical, physiological, and functional modularities of LC-NE pathways involving the mouse basal forebrain (BF) and pontine reticular nucleus (PRN) were tested. We found partial anatomical segregation between the LC → PRN and LC → BF circuits. Extracellular NE dynamics in BF and PRN exhibited distinct sound-evoked activation during sleep, including a fast sound-evoked NE peak specific to PRN. Causal optogenetic interrogation of LC efferent pathways, by retro-channelrhodopsin (ChR2) activation or Platynereis dumerilii ciliary opsin (PdCO) silencing of synapses in target regions, revealed a role for early LC → PRN activity in driving arousal and sound-evoked awakenings. Together, our results uncover a role for early LC-NE PRN activity in connecting sensory and arousal pathways and establish LC heterogeneity in regulating arousal.

Early sensory experience can exert lasting perceptual consequences. For example, a brief period of auditory deprivation early in life can lead to persistent spatial hearing deficits. Some forms of hearing loss (i.e., conductive; CHL) can distort acoustical cues needed for spatial hearing, which depend on inputs from both ears. We hypothesize that asymmetric acoustic input during development disrupts auditory circuits that integrate binaural information. Here, we identify prolonged maturation of the binaural auditory brainstem in the guinea pig by tracking auditory evoked potentials across development. Using this age range, we induce a reversible unilateral CHL and ask whether behavioral and neural maturation are disrupted. We find that developmental CHL is associated with alterations in a brainstem readout of binaural function, an effect that was not observed in a separate cohort with adult-onset CHL. Startle-based behavioral measures suggest that Early CHL animals exhibit reduced spatial resolution for high-frequency sound sources. Finally, single-unit recordings of auditory midbrain neurons reveal significantly poorer neural acuity to a sound location cue that largely depends on high-frequency sounds. Thus, these findings show that unilateral deprivation can disrupt developing auditory circuits that integrate binaural information and may give rise to lingering spatial hearing deficits.

Hearing hinges upon the ear’s ability to enhance its responsiveness by means of an energy-expending active process that amplifies the very mechanical inputs that it detects. This process is defined by four properties that, although seemingly unrelated, consistently occur together: amplification, sharp frequency tuning, compressive nonlinearity, and spontaneous otoacoustic emission. In nonmammal tetrapods, the active process is evident in individual hair cells. The hair bundles of the bullfrog, for example, exhibit all four attributes by operating near a Hopf bifurcation—a critical regime in which these properties naturally coalesce. In mammals, however, the delicate nature of the cochlea has restricted the evidence for an active process to studies in vivo, where it is generally attributed to the collective effort of the outer hair cells that energize the traveling wave along the cochlear spiral. As a result, the cellular mechanisms that underlie the properties of mammalian hearing remain contested, with uncertainty about whether criticality plays a role in the cochlea’s active process. Here we show that, when placed in a recording chamber that closely mimics the in vivo physiological environment, a segment of the mammalian cochlea ex vivo displays the features of the active process—amplification, frequency tuning, compressive nonlinearity, and the generation of distortion products. We show that this process operates locally, independently of traveling waves, and that the sensory epithelium achieves active amplification by operating near criticality at a Hopf bifurcation. The results reveal the existence of a unified biophysical principle that underlies auditory processing across species and even phyla.

Atypical sensory processing, particularly in the auditory domain, is one of the most common and quality-of-life affecting symptoms seen in autism spectrum disorders (ASD). Fragile X Syndrome (FXS) is a leading inherited cause of ASD and a majority of FXS individuals present with auditory processing alterations. While auditory hypersensitivity is a common phenotype observed in FXS and Fmr1 knockout (KO) rodent models, it is important to consider other auditory coding impairments that could contribute to sound processing difficulties and disrupted language comprehension in FXS. We have shown previously that a Fmr1 KO rat model of FXS exhibits heightened sound sensitivity that coincided with abnormal perceptual integration of stimulus bandwidth, indicative of altered spectral processing. Frequency discrimination is a fundamental aspect of sound encoding that is important for a range of auditory processes, such as source segregation and speech comprehension, and disrupted frequency coding could thus contribute to a range of auditory issues in FXS and ASD. Here we explicitly characterized spectral processing deficits in male Fmr1 KO rats using an operant conditioning tone discrimination assay and in vivo electrophysiological recordings from the auditory cortex and inferior colliculus. We found that Fmr1 KO rats exhibited poorer frequency resolution, which corresponded with neuronal hyperactivity and broader frequency tuning in auditory cortical but not collicular neurons. Using an experimentally informed population model, we show that these cortical physiological differences can recapitulate the observed behavior discrimination deficits, with decoder performance being tightly linked to differences in cortical tuning width and signal-to-noise ratios. Together, these findings indicate that cortical hyperexcitability in Fmr1 KO rats may act to preserve signal-to-noise ratios and signal detection threshold at the expense of sound sensitivity and fine feature discrimination, highlighting a potential mechanistic locus for a range of auditory behavioral phenotypes in FXS.

Understanding speech in noisy, multi-talker environments is crucial for social communication but becomes increasingly challenging and frustrating as we age. Here, we simulated the acoustic challenges of multi-talker listening and found that adults over 50 years old (N = 76) recognized speech more slowly, less accurately, and less consistently than younger adults (N = 107). While peripheral hearing status accounted for average differences in speech intelligibility by age, it did not account for moment-to-moment variability in speed and accuracy - fluctuations central to the frustration experienced by older listeners in challenging environments. We hypothesized that age-related changes in brain arousal systems might account for the fluctuant “noise” in speech processing observed in older listeners. To isolate the contribution of arousal state independent of hearing status and cognitive load, we measured the pre-stimulus pupil-indexed arousal state (PPAS) immediately prior to speech onset. Older – but not younger - adults exhibited a striking inverted-U relationship between PPAS and speech recognition accuracy. Notably, pupil-indexed listening effort measured seconds later during speech encoding was not associated with trial-to-trial performance. Moreover, older adults exhibited altered arousal regulation, occupying a lower PPAS extremum not observed in younger listeners that was specifically associated with performance deficits and subjective listening difficulties reported in hearing health questionnaires. These findings show that age-related changes in central arousal states interact with peripheral hearing status to offer a more complete explanation for why older adults find speech processing in social setting so challenging. Significance Statement As we age, following a conversation in crowded environments becomes more difficult and frustrating, even when hearing tests appear normal. This study shows that variations in arousal level (measured through pre-stimulus pupil size) are tightly linked to fluctuations in the speed and accuracy of speech processing in adults over 50 years of age. Changes in arousal state regulation uniquely accounted for the moment-to-moment variations in speech intelligibility and were more closely associated with self-reported listening challenges than other conventional measures. These findings highlight the contribution of brain-wide arousal systems to real-world listening challenges and identify a straightforward measure that could be used to more comprehensively assess and potentially improve speech understanding in social settings.

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive method that has been used to treat various brain disorders. The modulatory effects of rTMS can be adjusted by changing the repetition patterns. Theta-burst magnetic stimulation (TBS) is a magnetic stimulation pattern that can induce long-lasting modulatory effects with a short stimulation period. However, its effects on auditory brain regions remain unclear because of a lack of animal studies in which invasive techniques allow for a detailed exploration of the underlying neural mechanisms. In the current study, we investigated the effects of TBS on the C57BL/6J mouse auditory cortex using a custom-built 7 mm magnetic stimulation coil. Extracellular recordings were made before, during, and after the application of intermittent TBS (iTBS), continuous TBS (cTBS), or sham stimulation. Local field potential amplitudes were increased for 5–20 min post-iTBS compared with the sham condition and were decreased at 10 min post-cTBS compared with the sham condition. The bidirectional modulatory effects observed in our study are consistent with previous findings from other brain regions. Additionally, multiunit activities were significantly altered in cortical layers 2/3 and 4 but not layer 5, indicating that the modulatory effects were localized to the surface region of the auditory cortex. Interestingly, in the iTBS group, the amplitude of average spike waveforms increased with a 15 min delay. Our findings provide physiological evidence of TBS modulation of the rodent auditory cortex and may guide future research seeking to optimize rTMS for modulating hearing abilities.

Purpose In the receptor organs of the inner ear, hair cells detect mechanical stimuli such as sounds and accelerations by deflection of their hair bundles. Myosin regulatory light chain (RLC) and non-muscle myosin II (NM2) are expressed at the apical surfaces of hair cells, and NM2 and the phosphorylation of RLC by myosin light chain kinase (MLCK) have earlier been shown to regulate the shapes of hair cells’ apical surfaces in rodents. The aim of our study was to elucidate the function of myosin molecules on hair cell physiology. Methods We investigated the expression of NM2 and RLC in the bullfrog’s saccule by immunostaining. Using NM2 and MLCK inhibitors, we measured the stiffness, spontaneous oscillation, and resting open probability of frog hair bundles. Six to ten saccules from pleural animals were used in each experiment. In addition, we recorded auditory brainstem responses in ten mice after transtympanic injection of an MLCK inhibitor. Results We confirmed the expression of NM2A/B and MYL9 on the apical surfaces of hair cells and of NM2A and MYL12A in hair bundles. We found that NM2 and MLCK inhibitors reduce the stiffness of hair bundles from the bullfrog's saccule. Moreover, MLCK inhibition inhibits the spontaneous oscillation of hair bundles and increases the resting open probability of transduction channels. In addition, MLCK inhibition elevates hearing thresholds in mice. Conclusion We conclude that NM2 and the phosphorylation of RLC modulate the physiological function of hair cells and thereby help to set the normal operating conditions of hair bundles.