An analog electronic cochlea has been built in CMOS VLSI technology using micropower techniques. The key point of the model and circuit is that a cascade of simple, nearly linear, second-order filter stages with controllable Q parameters suffices to capture the physics of the fluid-dynamic traveling-wave system in the cochlea, including the effects of adaptation and active gain involving the outer hair cells. Measurements on the test chip suggest that the circuit matches both the theory and observations from real cochleas. >

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An analog electronic cochlea

We claim that speech analysis algorithms should be based on computational models of human audition, starting at the ears. While much is known about how hearing works, little of this knowledge has been applied in the speech analysis field. We propose models of the inner ear, or cochlea, which are expressed as time- and place-domain signal processing operations; i.e. the models are computational expressions of the important functions of the cochlea. The main parts of the models concern mechanical filtering effects and the mapping of mechanical vibrations into neural representation. Our model cleanly separates these effects into time-invariant linear filtering based on a simple cascade/parallel filterbank network of second-order sections, plus transduction and compression based on half-wave rectification with a nonlinear coupled automatic gain control network. Compared to other speech analysis techniques, this model does a much better job of preserving important detail in both time and frequency, which is important for robust sound analysis. We discuss the ways in which this model differs from more detailed cochlear models.

A computational model of filtering, detection, and compression in the cochlea

Current models of evoked otoacoustic emissions explain the striking periodicity in their frequency spectra by suggesting that it originates through the reflection of forward‐traveling waves by a corresponding spatial corrugation in the mechanics of the cochlea. Although measurements of primate cochlear anatomy find no such corrugation, they do indicate a considerable irregularity in the arrangement of outer hair cells. It is suggested that evoked emissions originate through a novel reflection mechanism, representing an analogue of Bragg scattering in nonuniform, disordered media. Forward‐traveling waves reflect off random irregularities in the micromechanics of the organ of Corti. The tall, broad peak of the traveling wave defines a localized region of coherent reflection that sweeps along the organ of Corti as the frequency is varied monotonically. Coherent scattering occurs off irregularities within the peak with spatial period equal to half the wavelength of the traveling wave. The phase of the net ref...

http://web.mit.edu/apg/papers/shera-zweig-origin-JASA95.pdf

The origin of periodicity in the spectrum of evoked otoacoustic emissions.

A class of cochlear models which account for much of the characteristic variation with frequency of human otoacoustic emissions and hearing threshold microstructure is presented. The models are based upon wave reflections via distributed spatial cochlear inhomogeneities and tall and broad cochlear activity patterns, as suggested by Zweig and Shera [J. Acoust. Soc. Am. 98, 2018–2047 (1995)]. They successfully describe in particular the following features: (1) the characteristic quasiperiodic frequency variations (fine structures) of the hearing threshold, synchronous and click-evoked emissions, distortion-product emissions, and spontaneous emissions; (2) the relationships between these fine structures; and (3) the distortion product emission filter shape. All of the characteristic frequency spacings are approximately the same (0.4 bark) and are mainly determined by the phase behavior of the apical reflection function. The frequency spacings for spontaneous emissions and threshold microstructure are predicted to be the same, but some deviations from these values are predicted for synchronous and click-evoked and distortion-product emissions. The analysis of models is aided considerably by the use of the solutions of apical, and basal, moving solutions (basis functions) of the cochlear wave equation in the absence of inhomogeneities.

Modeling otoacoustic emission and hearing threshold fine structures

5 1 INTRODUCTION The human auditory system has an amazing ability to separate and understand sounds. We believe that temporal information plays a key role in this ability, more important than the spectral information that is traditionally emphasized in hearing science. In many hearing tasks, such as describing or classifying single sound sources, the underlying mathematical equivalence makes the temporal versus spectral argument moot. We show how the nonlinear-ity of the auditory system breaks this equivalence, and is especially important in analyzing complex sounds from multiple sources of different characteristics. The auditory system is inherently nonlinear. In a linear system, the component frequencies of a signal are unchanged, and it is easy to characterize the amplitude and phase changes caused by the system. The cochlea and the neural processing that follow are more interesting. The bandwidth of a cochlear " filter " changes at different sound levels, and neurons change their sensitivity as they adapt to sounds. Inner Hair Cells (IHC) produce nonlinear rectified versions of the sound, generating new frequencies such as envelope components. All of these changes make it difficult to describe auditory perception in terms of the spectrum or Fourier transform of a sound. One characteristic of an auditory signal that is undisturbed by most nonlinear transformations is the periodicity information in the signal. Even if the bandwidth, amplitude, and phase characteristics of a signal are changing, the repetitive characteristics do not. In addition, it is very unlikely that a periodic signal could come from more than one source. Thus the auditory system can safely assume that sound fragments with a consistent periodicity can be combined and assigned to a single source. Consider, for example, a sound formed by opening and closing the glottis four times and filtering the resulting puffs of air with the vocal resonances. After nonlinear processing the lower auditory nervous system will still detect four similar events which will be heard and integrated as coming from a voice. The duplex theory of pitch perception, proposed by Licklider in 1951 [11] as a unifying model of pitch perception, is even more useful as a model for the extraction and representation of temporal structure for both periodic and non-periodic signals. This theory produces a movie-like image of sound which is called a correlogram. We believe that the correlogram, like other representations that summarize the temporal information in a signal, is an important tool for understanding …

On the importance of time—a temporal representation of sound

It is conjectured that the design of the cochlea is influenced by two conflicting requirements: (1) the cochlea should act as a precise frequency analyzer and (2) waves propagating along the basilar membrane should be transmitted without reflections. Accurate frequency analysis is possible only if the mechanical properties of the cochlea change rapidly with distance along the basilar membrane. Reflections of waves traveling on the basilar membrane will be negligible, however, only if these same mechanical properties change slowly. A compromise between these two requirements is possible if a loss constant δ related to the sharpness of response of the basilar membrane to a pure tone is related to the number N of wavelengths of the wave on the basilar membrane [N/(δ)1/2?1]. Furthermore, if sizable changes in the displacement occur only over distances larger than the width of a hair cell, then δ must be larger than the ratio of the width w of a hair cell to the distance d along the basilar membrane over which...

J. R. Pierce

Papers

The cochlear compromise