Volume 103, Issue 2, February 1998
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
A descriptive model of the receptor potential nonlinearities generated by the hair cell mechanoelectrical transducer103(1998); http://dx.doi.org/10.1121/1.421214View Description Hide Description
This paper describes a model for generating the hair cell receptor potential based on a second-order Boltzmann function. The model includes only the resistive elements of the hair cellmembranes with batteries across them and the series resistance of the external return path of the transducer current through the tissue of the cochlea. The model provides a qualitative description of signal processing by the hair celltransducer and shows that the nonlinearity of the hair celltransducer can give rise to nonlinear phenomena, such as intermodulation distortion products and two-tone suppression with patterns similar to those which have been recorded from the peripheral auditory system. Particular outcomes of the model are the demonstration that two-tone suppression depends not on the saturation of the receptor current, but on the behaviour of the hair celltransducer function close to the operating point. The model also shows that there is non-monotonic growth and phase change for any spectral component, but not for the fundamental of the receptor potential.
103(1998); http://dx.doi.org/10.1121/1.421215View Description Hide Description
This study varied stimulus frequency and recorded distortion product otoacoustic emissions (DPOAEs) in human newborns and adults. Because of outer and middle ear acoustics, the same auditory input resulted in higher newborn stimulus sound pressure levels across a broad frequency range in the occluded outer ear canal. Noise levels in the canal were 5–15 dB lower for adults at frequencies less than about 3 kHz. The DPOAE was the most reliably recorded DPOAE except at the lowest frequencies assessed. At the lowest frequencies the DPOAE was more frequently recorded than any other DPOAE. There were no striking developmental differences in the kinds of DPOAEs that were recorded. The amplitudes of consecutively recorded DPOAEs were generally within 1.5 dB of each other for all age groups (slightly better reproducibility for adults than newborns). The phases of consecutively recorded DPOAEs were generally within 15 degrees of each other (often less than 10 and 5 degrees for newborns and adults respectively). At the highest frequencies assessed all subjects had similar amplitude DPOAEs. At lower frequencies adult amplitudes were significantly less than those of newborns. At the lowest frequencies reliably assessed term newborns had significantly larger DPOAEs than preterm newborns. Newborn and adult DPOAE amplitude functions were quite similar although there were reliable differences. Age related differences in the outer and middle ears may explain some of the differences in DPOAEs that were observed.
103(1998); http://dx.doi.org/10.1121/1.421246View Description Hide Description
This study varied the levels of the primaries and recorded distortion product otoacoustic emissions (DPOAEs) from human newborns and adults. Preterm as well as term newborns were tested. The DPOAE was the most reliably recorded DPOAE, especially at low levels of the primaries. Amplitude and phase reproducibility deteriorated with decreasing level of the primaries. Newborn DPOAEs were slightly less reproducible than adult DPOAEs. The underlying DPOAE I/O functions were nonmonotonic for both newborns and adults. Unity gain characterized the initial increasing portion of those functions in most subjects. Although newborn and adult I/O functions were similar, they did differ. In particular, adult functions tended to be more linear with saturation at higher primary levels. Some of the newborn functions saturated at very low stimulus levels. Although differences in cochlear mechanics may explain developmental difference in DPOAE I/O functions, developmental differences in the resonancecharacteristics of the outer and middle ears may also be involved.
103(1998); http://dx.doi.org/10.1121/1.421216View Description Hide Description
The in-plane and bending stiffnesses of the outer hair cell wall are characteristics crucial to the understanding of force and energy transmission between cochlear elements. A mathematical interpretation is given of the micropipet experiment directed at determining the outer hair cell wall stiffness. On the basis of the microstructural observation of the wall showing that it comprises protein networks with different elastic properties, the model of an orthotropic cylindrical shell is used. The boundary-value problem is analyzed corresponding to the stress–strain state of the wall (shell) caused by the action of the micropipet. The solution is given in terms of Fourier series with respect to the circumferential coordinate. An asymptotic analysis of the solution is developed and an approximate formula for the length of the tongue aspirated within the pipet is derived. This leads to an analytical expression for the stiffness parameter measured in the micropipet experiment in terms of Young’s moduli and Poisson’s ratios of the wall. This expression is an important part of the estimation of the elastic constants of the wall.
103(1998); http://dx.doi.org/10.1121/1.421217View Description Hide Description
The outer hair cell makes both passive and active contributions to basilar membrane mechanics. The outer hair cell mechanics is strongly coupled to the elastic properties of the cell lateral wall. The lateral wall experiences both in-plane deformations and bending under physiological and experimental conditions. To characterize the outer hair cell wall, the model of an orthotropic cylindrical shell is used. The elastic constants of the wall are estimated by solving a set of three equations based on the analyses of three independent experiments. The first equation is derived from a new interpretation of the micropipet experiment; the other two are obtained from the axial loading and the osmotic challenge experiments. The two Young’s moduli corresponding to the longitudinal and circumferential directions and two Poisson’s ratios are estimated. The longitudinal, circumferential, and mixed modes of the bending stiffness are also estimated. The sensitivity of the derived constants to the variation of the cell axial stiffness, which has been measured by several independent groups, is examined. The new estimates are also compared with results obtained by using the assumption of the wall isotropy.