D. O. Kim

Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity

We confirm the report of Mountain (Mountain, D.C. (1980): Science 210, 77-72) that stimulating the crossed olivocochlear bundle (COCB) can change the magnitude of the distortion product (f2-f1) in the ear-canal sound pressure. Our results are extended to include (2f1-f2) as well as (f2-f1) from anesthetized chinchillas with both middle-ear muscles sectioned. In contrast to Mountains report, the polarity of the change can be either positive, negative or absent, depending on the choice of two-tone stimulus frequencies. The influence of two-tone stimulus level is also complex, but we have not yet seen the polarity of the COCB effect change with stimulus level. The magnitude and polarity of the change in (2f1-f2) are not simply related to those for (f2-f1). The effect of COCB stimulation is abolished when scala tympani is perfused with artificial perilymph containing 10(-5) M d-tubocurarine. These results demonstrate that the COCB effect is postsynaptic, probably mediated by outer hair cells. We suggest that the normal cochlea contains an active biomechanical mechanism which reduces the damping of the cochlear-partition motion and is modulated by activating the efferents. It is thus possible that the central nervous system may be able to control the dynamics of the motion of the cochlear partition.

A model for active elements in cochlear biomechanics

A linear, mathematical model of cochlear biomechanics is presented in this paper. In this model, active elements are essential for simulating the high sensitivity and sharp tuning characteristic of the mammalian cochlea. The active elements are intended to represent the motile action of outer hair cells; they are postulated to be mechanical force generators that are powered by electrochemical energy of the cochlear endolymph, controlled by the bending of outer hair cell stereocilia, and bidirectionally coupled to cochlear partition mechanics. The active elements are spatially distributed and function collectively as a cochlear amplifier. Excessive gain in the cochlear amplifier causes spontaneous oscillations and thereby generates spontaneous otoacoustic emissions.

Cochlear mechanics: Nonlinear behavior in two‐tone responses as reflected in cochlear‐nerve‐fiber responses and in ear‐canal sound pressure

Over the last five years, we have studied spatial distributions of cochlear responses to single‐ and two‐tone stimuli by recording sequentially from as many as 418 cochlear nerve fibers in each cat and obtaining plots of amplitude and phase of response components at primary and distortion frequencies. We have observed: (1) that such responses to two‐tone stimuli of sound pressure levels (SPL) as low as 34 dB re 20 μN/m2 rms show noticeable deviations from linear behavior; (2) that two major forms of nonlinear behavior in responses to two‐tone stimuli are distortion products and two‐tone suppression; (3) that the spatial distributions of amplitude and phase of (2f1‐f2) and (f2‐f1) components in response to two‐tone stimuli are similar, in the region near and apical to the distortion‐frequency place, to those of the fs component in response to a single‐tone stimulus whose frequency fs is equal to the particular distortion frequency, and dissimilar in the more basal region; and (4) that each of the f1 and f2...

An active cochlear model showing sharp tuning and high sensitivity

Recent in vivo measurements of cochlear-partition motion indicate very high sensitivity and sharp mechanical tuning similar to the tuning of single cochlear nerve fibers. Our experience with mathematical models of the cochlea leads us to believe that this type of mechanical response requires the presence of active elements in the cochlea. We have developed an active cochlear model which incorporates negative damping components; this model produces partition displacement in good agreement with many of the mechanical and neural tuning characteristics which have been observed in vivo by other researchers. We suggest that the negative damping components of our model may represent an active mechanical behavior of the outer hair cells, functioning in the electromechanical environment of the normal cochlea.

Cochlear mechanics: Implications of electrophysiological and acoustical observations ☆

Implications of the spatial distribution of distortion products (2f1--f2) and (f2--f1) observed from populations of cochlear nerve fibers for cochlear mechanics are reviewed (the terms f1 and f2 represent the primary stimulus frequencies; f1 < f2). Characteristics of the distortion products (2f1--f2) and (f2--f1) in the ear-canal sound pressure of the cat and the chinchilla are investigated. Physiological origin of the acoustic distortion product (2f1--f2) is supported by demonstrations of the vulnerability of the distortion product to anoxia, to overstimulation and to cyanide perfusion of the cochlea. Observations are presented describing the dependence of levels of acoustic distortion products (2f1--f2) and (f2--f1): (1) on primary levels; (2) on f2 with iso-f1; and (3) on f1 and f2 with iso-(2f1--f2). Observations and interpretations are discussed in support of the conclusions: (1) that the distortion product (2f1--f2) in the ear-canal sound pressure observed in our studies is not generated in the experimental apparatus, in the eardrum, or in the middle ear but in the primary-frequency region of the cochlea; (2) that the distortion-product generation requires normal physiological processes in the cochlear sensory apparatus but not the neural activity; and (3) that the distortion-product is mechanically propagated from the generation region in the cochlea toward the distortion-frequency place and toward the stapes, through the middle ear, and into the ear canal involving gross motions of the cochlear partition and the middle-ear ossicles. It is now inevitable that we accept the notion that, in a normal ear, manifestations of significant nonlinear behavior are present in the mechanical response of the middle ear and the cochlea at most of the physiologically normal sound pressure levels.

Active and nonlinear cochlear biomechanics and the role of outer-hair-cell subsystem in the mammalian auditory system

An increasing amount of support is accumulating for the hypothesis that the outer hair cells (OHC) of a mammalian cochlea give rise to an enhanced sensitivity and markedly sharp tuning of the mechanical response of the cochlear partition. The enhancing and sharpening effects of the OHCs are postulated to arise from a bidirectional transduction mechanism whereby not only a mechanical signal applied to the hair bundle is (forward) transduced into electrophysiological signals, but also an electrophysiological signal applied to the hair cell is (reverse) transduced into generation of mechanical forces and related displacements. This paper will review experimental evidence for the hypothesis and attempt to integrate results of various experimental and theoretical studies into a coherent framework.

Responses of DCN-PVCN neurons and auditory nerve fibers in unanesthetized decerebrate cats to AM and pure tones: Analysis with autocorrelation/power-spectrum

We investigated amplitude-modulated (AM) tone encoding behavior of dorsal and posteroventral cochlear-nucleus (DCN and PVCN) neurons and auditory nerve (AN) fibers in decerebrate unanesthetized cats. Some of the modulation transfer functions (MTFs) were narrowly-tuned band-pass functions; these included responses at moderate and high stimulus levels of DCN pause/build-type-III neurons and the following types of DCN and PVCN chopper neurons: chop-S and/or chop-type-I/III. Other MTFs were broad low-pass or complex functions. Chop-T neurons of the DCN and PVCN tended to exhibit low-pass or flat MTFs. The band-pass MTF neurons exhibited intrinsic oscillations (IOs) in responses to AM or pure tones. The IOs, which were detected in autocorrelation functions and power spectra, were closely correlated (r = 0.863) with the best envelope frequency (BEF). All of the AN fibers showed broad low-pass MTFs with some showing a rudimentary peak in the MTF. The MTFs of DCN-PVCN neurons and AN fibers showed, respectively: (1) BEFs ranging 50-500 Hz, and 400-1300 Hz; (2) upper cut-off frequencies ranging 200-1200 Hz, and 1600-3200 Hz. At stimulus levels of 60-85 dB SPL, maximum modulation gains were as high as 12 dB for DCN-PVCN neurons but were limited to below about 0 dB for AN fibers. The median dynamic ranges of DCN and PVCN neurons (51 and 42 dB, respectively) were substantially wider than those of the low and high spontaneous rate AN fibers (30 and 31 dB, respectively). The observation of higher modulation gain, wider dynamic range, and more narrowly-tuned MTF of DCN-PVCN neurons than AN fibers supports the concept that the capabilities to encode dynamic signals are enhanced in DCN-PVCN neurons compared with AN fibers.

The behavior of acoustic distortion products in the ear canals of chinchillas with normal or damaged ears

Acoustic intermodulation distortion products were measured in 15 ear canals of chinchillas with normal or damaged ears. Pretreatment results showed that when two primary tones at frequencies f1 and f2, f1 less than f2, were presented at levels from 30 to 90 dB SPL, acoustic distortion products at 2f1-f2 and 2f2-f1 were 30 to 50 dB below primary-tone levels. Noise exposures that caused temporary or permanent hearing loss produced corresponding temporary or permanent reductions in distortion-product levels. Mechanical damage to the cochlea or middle ear reduced the distortion-product levels to below the noise floor of the measurement system. Comparisons of distortion-product level with behaviorally measured threshold shift and cochlear histopathology suggest that, in the absence of conductive impairment, the level of 2f1-f2 or 2f2-f1 can be used as a sensitive indicator of hearing sensitivity and the condition of the cochlea.

Cochlear nerve fiber responses: Distribution along the cochlear partition

Fourier analysis of discharge patterns in response to sinusoidal acoustic stimulation provides a consistent and repeatable measure of response phase and amplitude. The distribution of the fundamental components of response for large populations of fibers as a function of their characteristic frequency provides a link between the spatiotemporal characteristics of basilar membrane vibration and single fiber response.Subject Classification: 65.42, 65.40.

Response Patterns of Single Cochlear Nerve Fibers to Click Stimuli: Descriptions for Cat

Response patterns to click stimulation of 907 single cochlear nerve fibers, having characteristic frequencies below 2000 Hz, can be separated into two populations on the basis of salient features. Population I consists of approximately 93% of the fibers, and Population II consists of approximately 7% of the fibers. A statistical description of the correlation between properties of response patterns of Population I fibers and stimulus level and characteristic frequency is given. For the Population I fibers, with characteristic frequencies below 500 Hz, deviations from the precise interlacing of preferred times of spike discharges in response to rarefaction and condensation clicks, as well as some instances of biased response to condensation clicks are described. The features that set Population II fibers apart from those of Population I are given, and a correlation of these populations with anatomical details of innervation is suggested.