A mechanism for active hearing

A finite element model to predict the consequences of endolymphatic hydrops in the basilar membrane

Ménière's disease is an inner ear disorder, associated with episodes of vertigo, fluctuant hearing loss, tinnitus, and aural fullness. Ménière's disease is associated with endolymphatic hydrops. Clinical evidences show that this disease is often incapacitating, negatively affecting the patients' everyday life. The pathogenesis of Ménière's disease is still not fully understood and remains unclear. Previous numerical studies available in the literature related with endolymphatic hydrops, are very scarce. The present work applies the finite element method to investigate the consequences of endolymphatic hydrops in the normal hearing, associated with the Ménière's disease. The obtained results for the steady state dynamics analysis are in accordance with clinical evidences. The results show that the basilar membrane is not affected in the same intensity along its length and that the lower frequencies are more affected by the endolymphatic hydrops. From a clinical point of view, this work shows the relationship between the increasing of the endolymphatic pressure and the development of hearing loss.

Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity.

Noninvasive fMRI investigation of interaural level difference processing in the rat auditory subcortex

Objective

Interaural level difference (ILD) is the difference in sound pressure level (SPL) between the two ears and is one of the key physical cues used by the auditory system in sound localization. Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC). Noninvasive brain imaging enables studying ILD processing in multiple structures simultaneously.

Methods

In this study, blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is used for the first time to measure changes in the hemodynamic responses in the adult Sprague-Dawley rat subcortex during binaural stimulation with different ILDs.

Results and Significance

Consistent responses are observed in the CN, SOC, LL, and IC in both hemispheres. Voxel-by-voxel analysis of the change of the response amplitude with ILD indicates statistically significant ILD dependence in dorsal LL, IC, and a region containing parts of the SOC and LL. For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus. These findings are supported by region of interest analysis. fMRI shows that ILD dependence occurs in both hemispheres and multiple subcortical levels of the auditory system. This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

Effects of Stimulus Intensity on Low-Frequency Toneburst Cochlear Microphonic Waveforms

This study investigates changes in amplitude and delays in low-frequency toneburst cochlear microphonic (CM) waveforms recorded at the ear canal in response to different stimulus intensities. Ten volunteers aged 20-30 were recruited. Low-frequency CM waveforms at 500 Hz in response to a 14-ms toneburst were recorded from an ear canal electrode using electrocochleography techniques. The data was statistically analyzed in order to confirm whether the differences were significant in the effects of stimulus intensity on the amplitudes and delays of the low-frequency CM waveforms. Electromagnetic interference artifacts can jeopardize CM measurements but such artifacts can be avoided. The CM waveforms can be recorded at the ear canal in response to a toneburst which is longer than that used in ABR measurements. The CM waveforms thus recorded are robust, and the amplitude of CM waveforms is intensity-dependent. In contrast, the delay of CM waveforms is intensity-independent, which is different from neural responses as their delay or latency is intensity-dependent. These findings may be useful for development of the application of CM measurement as a supplementary approach to otoacoustic emission (OAE) measurement in the clinic which is severely affected by background acoustic noise. The development of the application in the assessment of low-frequency cochlear function may become possible if a further series of studies can verify the feasibility, but it is not meant to be a substitute for audiometry or OAE measurements. The measurement of detection threshold of CM waveform responses using growth function approach may become possible in the clinic. The intensity-independent nature of CMs with regards to delay measurements may also become an impacting factor for differential diagnoses and for designing new research studies.

Probing the cochlear amplifier by immobilizing molecular motors of sensory hair cells

The location and longitudinal extent of cochlear biomechanical amplification has been an open question. In this issue of Neuron, Fisher et al. (2012) demonstrate that sound-induced vibration is amplified over a short region-about one wavelength-prior to the response peak.

Active touch sensing

Active sensing systems are purposive and information-seeking sensory systems. Active sensing usually entails sensor movement, but more fundamentally, it involves control of the sensor apparatus, in whatever manner best suits the task, so as to maximize information gain. In animals, active sensing is perhaps most evident in the modality of touch. In this theme issue, we look at active touch across a broad range of species from insects, terrestrial and marine mammals, through to humans. In addition to analysing natural touch, we also consider how engineering is beginning to exploit physical analogues of these biological systems so as to endow robots with rich tactile sensing capabilities. The different contributions show not only the varieties of active touch--antennae, whiskers and fingertips--but also their commonalities. They explore how active touch sensing has evolved in different animal lineages, how it serves to provide rapid and reliable cues for controlling ongoing behaviour, and even how it can disintegrate when our brains begin to fail. They demonstrate that research on active touch offers a means both to understand this essential and primary sensory modality, and to investigate how animals, including man, combine movement with sensing so as to make sense of, and act effectively in, the world.

Measurement of cochlear power gain in the sensitive gerbil ear

The extraordinary sensitivity of the mammalian ear is commonly attributed to the cochlear amplifier, a cellular process thought to locally boost responses of the cochlear partition to soft sounds. However, cochlear power gain has not been measured directly. Here we use a scanning laser interferometer to determine the volume displacement and volume velocity of the cochlear partition by measuring its transverse vibration along and across the partition. We show the transverse displacement at the peak-response location can be >1,000 times greater than the displacement of the stapes, whereas the volume displacement of an area centred at this location is approximately tenfold greater than that of the stapes. Using the volume velocity and cochlear-fluid impedance, we discover that power at the peak-response area is >100-fold greater than that at the stapes. These results demonstrate experimentally that the cochlea amplifies soft sounds, offering insight into the mechanism responsible for the cochlear sensitivity.

Plasticity in membrane cholesterol contributes toward electrical maturation of hearing

Advances in refining the "fluid mosaic" model of the plasma membrane have revealed that it is wrought with an ordered lipid composition that undergoes remarkable plasticity during cell development. Despite the evidence that specific signaling proteins and ion channels gravitate toward these lipid microdomains, identification of their functional impact remains a formidable challenge. We report that in contrast to matured auditory hair cells, depletion of membrane cholesterol in developing hair cells produced marked potentiation of voltage-gated K(+) currents (I(Kv)). The enhanced magnitude of I(Kv) in developing hair cells was in keeping with the reduced cholesterol-rich microdomains in matured hair cells. Remarkably, potentiation of the cholesterol-sensitive current was sufficient to abolish spontaneous activity, a functional blueprint of developing and regenerating hair cells. Collectively, these findings provide evidence that developmental plasticity of lipid microdomains and the ensuing changes in K(+) currents are important determinants of one of the hallmarks in the maturation of hearing.

Distribution of frequencies of spontaneous oscillations in hair cells of the bullfrog sacculus

Under in vitro conditions, free-standing hair bundles of the bullfrog (Rana catesbeiana) sacculus have exhibited spontaneous oscillations. We used a high-speed complementary metal oxide semiconductor camera to track the active movements of multiple hair cells in a single field of view. Our techniques enabled us to probe for correlations between pairs of cells, and to acquire records on over 100 actively oscillating bundles per epithelium. We measured the statistical distribution of oscillation periods of cells from different areas within the sacculus, and on different epithelia. Spontaneous oscillations exhibited a peak period of 33 ms (+29 ms, -14 ms) and uniform spatial distribution across the sacculus.

Sensitive response to low-frequency cochlear distortion products in the auditory midbrain

During auditory stimulation with several frequency components, distortion products (DPs) are generated as byproduct of nonlinear cochlear amplification. After generated, DP energy is reemitted into the ear channel where it can be measured as DP otoacoustic emission (DPOAE), and it also induces an excitatory response at cochlear places related to the DP frequencies. We measured responses of 91 inferior colliculus (IC) neurons in the gerbil during two-tone stimulation with frequencies well above the unit's receptive field but adequate to generate a distinct distortion product (f2-f1 or 2f1-f2) at the unit's characteristic frequency (CF). Neuronal responses to DPs could be accounted for by the simultaneously measured DPOAEs for DP frequencies >1.3 kHz. For DP frequencies <1.3 kHz (n = 25), there was a discrepancy between intracochlear DP magnitude and DPOAE level, and most neurons responded as if the intracochlear DP level was significantly higher than the DPOAE level in the ear channel. In 12% of those low-frequency neurons, responses to the DPs could be elicited even if the stimulus tone levels were below the threshold level of the neuron at CF. High intracochlear f2-f1 and 2f1-f2 DP-levels were verified by cancellation of the neuronal DP response with a third phase-adjusted tone stimulus at the DP frequency. A frequency-specific reduction of middle ear gain at low frequencies is possibly involved in the reduction of DPOAE level. The results indicate that pitch-related properties of complex stimuli may be produced partially by high intracochlear f2-f1 distortion levels.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

The hearing gene Prestin reunites echolocating bats

The remarkable high-frequency sensitivity and selectivity of the mammalian auditory system has been attributed to the evolution of mechanical amplification, in which sound waves are amplified by outer hair cells in the cochlea. This process is driven by the recently discovered protein prestin, encoded by the gene Prestin. Echolocating bats use ultrasound for orientation and hunting and possess the highest frequency hearing of all mammals. To test for the involvement of Prestin in the evolution of bat echolocation, we sequenced the coding region in echolocating and nonecholocating species. The resulting putative gene tree showed strong support for a monophyletic assemblage of echolocating species, conflicting with the species phylogeny in which echolocators are paraphyletic. We reject the possibilities that this conflict arises from either gene duplication and loss or relaxed selection in nonecholocating fruit bats. Instead, we hypothesize that the putative gene tree reflects convergence at stretches of functional importance. Convergence is supported by the recovery of the species tree from alignments of hydrophobic transmembrane domains, and the putative gene tree from the intra- and extracellular domains. We also found evidence that Prestin has undergone Darwinian selection associated with the evolution of specialized constant-frequency echolocation, which is characterized by sharp auditory tuning. Our study of a hearing gene in bats strongly implicates Prestin in the evolution of echolocation, and suggests independent evolution of high-frequency hearing in bats. These results highlight the potential problems of extracting phylogenetic signals from functional genes that may be prone to convergence.

Silencing the cochlear amplifier by immobilizing prestin

Achieving the exquisite sensitivity and frequency selectivity of the mammalian ear requires active amplification of input sound. In this issue of Neuron, Dallos and colleagues demonstrate that the molecular motor prestin, which drives shape changes in the soma of mechanosensory hair cells, underlies mechanical feedback mechanisms for sound amplification in mammals.

Mouse models for human hereditary deafness

Hearing impairment is a frequent condition in humans. Identification of the causative genes for the early onset forms of isolated deafness began 15 years ago and has been very fruitful. To date, approximately 50 causative genes have been identified. Yet, limited information regarding the underlying pathogenic mechanisms can be derived from hearing tests in deaf patients. This chapter describes the success of mouse models in the elucidation of some pathophysiological processes in the auditory sensory organ, the cochlea. These models have revealed a variety of defective structures and functions at the origin of deafness genetic forms. This is illustrated by three different examples: (1) the DFNB9 deafness form, a synaptopathy of the cochlear sensory cells where otoferlin is defective; (2) the Usher syndrome, in which deafness is related to abnormal development of the hair bundle, the mechanoreceptive structure of the sensory cells to sound; (3) the DFNB1 deafness form, which is the most common form of inherited deafness in Caucasian populations, mainly caused by connexin-26 defects that alter gap junction communication between nonsensory cochlear cells.