In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the “base” of the cochlea (near the stapes) and low-frequency waves approaching the “apex” of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the “cochlear amplifier.” This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

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Mechanics of the Mammalian Cochlea

The classical view of neural coding has emphasized the importance of information carried by the rate at which neurons discharge action potentials. More recent proposals that information may be carried by precise spike timing1,2,3,4,5 have been challenged by the assumption that these neurons operate in a noisy fashion—presumably reflecting fluctuations in synaptic input6—and, thus, incapable of transmitting signals with millisecond fidelity. Here we show that precisely synchronized action potentials can propagate within a model of cortical network activity that recapitulates many of the features of biological systems. An attractor, yielding a stable spiking precision in the (sub)millisecond range, governs the dynamics of synchronization. Our results indicate that a combinatorial neural code, based on rapid associations of groups of neurons co-ordinating their activity at the single spike level, is possible within a cortical-like network.

Stable propagation of synchronous spiking in cortical neural networks

Basilar-membrane responses to single tones were measured, using laser velocimetry, at a site of the chinchilla cochlea located 3.5 mm from its basal end. Responses to low-level ( 80 dB the largest responses are elicited by tones with frequency about 0.4–0.5 octave below CF. For stimulus frequencies well above CF, responses stop decreasing with increasing frequency: A plateau is reached. The compressive growth of responses to tones with frequency near CF is accompanied by intensity-dependent phase shifts. Death abolishes all nonlinearities, reduces sensitivity at CF by as much as 60–81 dB, and causes a relative phase lead at CF.

/pdf/basilar-membrane-responses-to-tones-at-the-base-of-the-4ht3urogkk.pdf

Basilar-membrane responses to tones at the base of the chinchilla cochlea

Peter Dallas Auditory Physiology Laboratory (The Hugh Knowles Center) and Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208 The cochlea is a hydromechanical frequency analyzer located in the inner ear (Fig. 1 a). Its principal role is to perform a real- time spectral decomposition of the acoustic signal in producing a spatial frequency map. The frequency analysis may be un- derstood with the aid of Figure 1 b, which shows a straightened cochlea with a snapshot of its basilar membrane displaced in response to a single-frequency sound (pure tone). Upon delivery of an acoustic signal into the fluid-filled cochlea, the basilar membrane undergoes an oscillatory motion at the frequency of the sound, resulting in a wave traveling toward its distal end. The drawing shows an instantaneous view ofthis traveling wave. The wave is spatially confined along the length of the basilar membrane, and the location of its maximum amplitude is re- lated to the frequency of the sound. The higher the frequency, the more restricted the disturbance to the proximal end. The vibrating membrane supports the sense organ of hearing, the spiral organ of Corti (Fig. 2) which is deformed maximally in the region of the peak of the traveling wave. In this location, the sensory receptor cells of the organ of Corti receive maximal mechanical stimulation, transduce it into maximal electrical signals, and thus produce maximal afferent sensory outflow from the cochlea. Thus, mechanical frequency analysis is performed by matching particular frequencies with particular groups of auditory receptor cells and their nerve fibers. Understanding of frequency analysis in the inner ear pro- gressed through three main epochs. The first was dominated by Helmholtz’s suggestions that lightly damped, spatially ordered, mechanically resonant elements in the cochlea perform the spec- tral analysis (this period was reviewed by Wever, 1949). The second epoch, lasting from the late 1940s to the early 1970s was dominated by von Bekesy’s description of the traveling wave (von BtkCsy, 1960). We are now in the third epoch (an overview of its evolution is given in Dallos, 1988) during which a fundamentally different paradigm has emerged. According to this paradigm, von Btktsy’s traveling wave is boosted by a local electromechanical amplification process in which one of the mammalian ear’s sensory cell groups, outer hair cells, function as both sensors and mechanical feedback elements. The ideas that the operation of a sense organ is dependent upon local mechanical feedback modification by what resembles a sensory receptor cell, that such mechanical feedback may operate at audio frequencies utilizing some novel cellular motor, and that

https://www.jneurosci.org/content/jneuro/12/12/4575.full.pdf

The active cochlea.

Otoacoustic emissions (OAEs) of all types are widely assumed to arise by a common mechanism: nonlinear electromechanical distortion within the cochlea. In this view, both stimulus-frequency (SFOAEs) and distortion-product emissions (DPOAEs) arise because nonlinearities in the mechanics act as "sources" of backward-traveling waves. This unified picture is tested by analyzing measurements of emission phase using a simple phenomenological description of the nonlinear re-emission process. The analysis framework is independent of the detailed form of the emission sources and the nonlinearities that produce them. The analysis demonstrates that the common assumption that SFOAEs originate by nonlinear distortion requires that SFOAE phase be essentially independent of frequency, in striking contradiction with experiment. This contradiction implies that evoked otoacoustic emissions arise by two fundamentally different mechanisms within the cochlea. These two mechanisms (linear reflection versus nonlinear distortion) are described and two broad classes of emissions--reflection-source and distortion-source emissions--are distinguished based on the mechanisms of their generation. The implications of this OAE taxonomy for the measurement, interpretation, and clinical use of otoacoustic emissions as noninvasive probes of cochlear function are discussed.

http://web.mit.edu/apg/papers/shera-guinan-taxonomy-JASA99.pdf

Evoked otoacoustic emissions arise by two fundamentally different mechanisms: A taxonomy for mammalian OAEs

Alternating current delivered into the scala media of the gerbil cochlea modulates the amplitude of a test tone measured near the eardrum. Variations in the electromechanical effect with acoustic stimulus parameters and observed physiological vulnerability suggest that cochlear hair cells are the biophysical origin of the process. Cochlear hair cells have traditionally been thought of as passive receptor cells, but they may play an active role in cochlear micromechanics.

Alternating current delivered into the scala media alters sound pressure at the eardrum

A two-mode model of the cochlea that uses active intermode feedback has been developed that quantitatively accounts for the motion of the basilar membrane in response to single tones and qualitatively accounts for cochlear emission phenomena. In contrast to existing single-mode models, this model amplifies the mechanical traveling wave in spatially localized cochlear regions where an approximate match occurs between the traveling-wave velocities of each of the two traveling-wave lines or modes.

https://science.sciencemag.org/content/sci/259/5091/68.full.pdf

A traveling-wave amplifier model of the cochlea

An apparatus for performing assays includes an axially rotatable substrate including a plurality of layers of a semiconductor material and numerous radially-arrayed reaction sites. The apparatus further includes a rotary stepper motor which rotates the substrate at an adjustable and substantially continuous speed and controls the rotation of the substrate by adjusting the speed and a direction of rotation. In addition, the apparatus includes a dual function head which has a fluid dispenser that has a fluid dispenser outlet and delivers a fluid to a reaction site and also has a readout device that has a sensor which receives an identifying signal from the reaction site on the substrate or scans the substrate to read identifying marks at the reaction site. Moreover, the apparatus may be aligned by a computer having a memory for storing a start location for the dispenser outlet on the substrate and additional electronics. The computer provides movement signals to the rotary stepper motor and a linear stepper motor on which the dispenser outlet is mounted, whereby the motors align the dual function head over the substrate, such that the dispenser outlet is aligned over the reaction site.

Apparatus for performing assays at reaction sites

Mammalian outer hair cells (OHC) are believed to increase cochlear sensitivity and frequency selectivity via electromechanical feedback. A simple piezoelectric model of outer hair cell function is presented which integrates existing data from isolated OHC experiments. The model predicts maximum OHC force production to equal 1.25 nN/mV. The model also predicts that the maximum velocity of OHC contraction in situ to be 800 μm/s. These predictions are compared to available experimental data and are found to be in good agreement. The good agreement between the predicted and experimental results suggests that, at the characteristic frequency of a given cochlear location, the OHC receptor current is very efficiently converted into basilar membrane motion.

Allyn E. Hubbard

Papers

Alternating current delivered into the scala media alters sound pressure at the eardrum

A traveling-wave amplifier model of the cochlea

Apparatus for performing assays at reaction sites

A piezoelectric model of outer hair cell function.

Rapid force production in the cochlea