Elsevier

Hearing Research

Volume 151, Issues 1–2, January 2001, Pages 48-60
Hearing Research

Basilar membrane vibration in the basal turn of the sensitive gerbil cochlea

https://doi.org/10.1016/S0378-5955(00)00211-2Get rights and content

Abstract

The basal membrane (BM) velocity responses to pure tones were measured using a newly developed laser interferometer microscope that does not require placing a reflecting object on the BM. It was demonstrated that the instrument is able to measure sub-nanometer vibration from the cochlear partition in the basal turn of the gerbil. The overall shape of the amplitude spectra shows typical tuning features. The ‘best’ frequencies (BFs) for the BM locations studied were between 14 kHz and 27 kHz, depending on the longitudinal position. For a given BM location, tuning sharpness was input level dependent, indicated by the Q10dB, which varied from approximately 3 at low stimulus levels to near 1.5 at high input levels. At frequencies below BF, parallel amplitude/frequency curves across stimulus levels indicate a linear growth function. However, at frequencies near BF, the velocity increased linearly at low levels (<40 dB SPL) and became compressed between 40 and 50 dB SPL. Although the velocity gain for the frequency range below BF was a function of frequency, for a given frequency the gains were approximately constant across different levels. At frequencies near BF, the velocity gain at low sound pressure level was greater than that at a high sound pressure level, indicating a nonlinear negative relationship to stimulus level. The data also showed that the BF shifts toward the low frequencies with stimulus intensity increase. The phase spectra showed two important features: (1) at frequencies about half octave below the BF, phase slope is very small, indicating an extremely short delay; (2) the greatest phase lag occurs at frequencies near the BF, indicating a significant delay near this frequency range.

Introduction

The most straightforward technique for studying cochlear mechanics is to measure cochlear partition vibration directly. Because it is more easily accessible than the organ of Corti and the tectorial membrane, the basilar membrane (BM) has been the most commonly used location for vibration measurement in the cochlea. Measurements of BM vibration have been accomplished with a variety of techniques in a number of different animal species.

Georg von Békésy’s BM vibration experiments in the early part of the last century were accomplished with great success using light microscopy (von Bekesy, 1960). However, a major problem with light microscopy techniques is low sensitivity, which essentially limits the observations to vibration amplitudes greater than about 1 μm (Tonndorf, 1977). Although the capacitive probe method provides better sensitivity than light microscopy (Wilson, 1973), the requirement of removal of perilymph from the scala tympani may alter cochlear mechanical properties (Robertson, 1974). Use of the Mössbauer technique for BM measurements was a major technical improvement, leading to a series of very important findings (Johnstone and Boyle, 1967, LePage, 1987, Rhode, 1971, Sellick et al., 1982). The main disadvantages of this approach are its nonlinearity, low signal to noise ratio, the load on the BM, and possible radioactive damage (Kliauga and Khanna, 1983). LePage (1989) and Xue et al. (1996) applied fiber optic displacement sensors to measure BM vibration using incoherent light from a light-emitting diode. The difficulties in positioning the sensor close to the BM without damage and the load on the BM by the reflector and the probe limit application of this method.

Currently, a widely used method for cochlear mechanical measurement is laser interferometry (Khanna and Leonard, 1982, Willemin et al., 1988), due to its high sensitivity and ease of use. High sensitivity and the linearity of the laser interferometer give this technique a wide dynamic range and high signal to noise ratio, both of which are necessary for BM measurement. However, the reflection coefficient of the cochlear partition is extremely low, approximately 0.0039–0.033% (Khanna et al., 1989). Most measurements using laser interferometry require application of reflective objects (gold crystals or beads of various materials) on the cochlear partition (Cooper and Rhode, 1995, Gummer et al., 1996, Khanna and Leonard, 1982, Mammano and Ashmore, 1993, Nuttall et al., 1991, Nuttall et al., 1999, Rhode and Cooper, 1993, Rhode and Cooper, 1996, Ruggero and Rich, 1991a, Ruggero et al., 1997). These measurements have provided considerable data for understanding cochlear mechanics. Nevertheless, and in spite of Cooper’s (1999) report that placing a bead did not change the mechanical response of the BM, there still remains a concern regarding possible effects of the reflective object on the mechanical properties of the cochlear partition. It has been suggested that a reflective object or a Mössbauer source placed on the BM may cause a loss in sensitivity (Khanna and Leonard, 1982, Xue et al., 1996). Additionally, the load of the reflective object may alter the motion pattern of the cochlear partition (Sellick et al., 1983b). Moreover, it is uncertain whether the mechanical coupling between the cochlear partition and the reflective object contributes to results (Khanna et al., 1998). These concerns and the desire for multiple-point measurement have motivated researchers to develop a technique for measurement of cochlear partition vibration in the sensitive cochlea without placing reflective objects (Cooper, 1999, Khanna and Hao, 1999, Russell and Nilsen, 1997).

Although the gerbil is one of the most common animals for auditory research, data on BM mechanics in this species are limited. The aim of this study is to develop a method of measuring BM vibration of the first turn in sensitive gerbil cochleae without placing a reflective object. To visualize the cochlear partition of the first turn, a hole was made in the bony capsule. Using a modified heterodyne laser interferometer microscope, the BM velocity in response to an acoustical tone was measured at different frequencies and levels.

Section snippets

Animals and surgical approach

Thirty healthy young Mongolian gerbils (45–90 g) were used in this study. Animals were housed in American Association for Accreditation of Laboratory Animal Care-approved facilities. Experimental protocols were approved by the Oregon Health Sciences University Committee on Use and Care of Animals.

The general animal preparation and surgical approach for opening the bulla were the same as in a previous study (Ren, 1996). The initial anesthesia was induced by intraperitoneal injection of ketamine

Results

The main factor limiting productivity of this experiment is preparation-induced cochlear damage. Of the 30 animals used in this study, two died during the experiment. Although 28 animals survived from the surgery and showed normal cardiovascular condition, most of them had different degrees of the hearing loss resulting from the preparation procedures. Only five animals had less than a 10 dB threshold increase at 12 and 18 kHz. Data from three of these relatively sensitive cochleae are reported

Mechanical frequency tuning curves

Frequency tuning curves have been used to describe frequency selectivity in different systems. In this study, the mechanical frequency tuning curves (FTCs) at different input levels were measured and produced by plotting the amplitude of the BM vibration velocity amplitude as a function of stimulus frequency for a given sound pressure level in the external ear canal (Fig. 2A). For the region of the BM accessible to measurement in this study, the FTCs at low sound pressure levels (below 40 dB

Summary

Using a newly developed laser interferometer, velocity responses of the BM to different levels of acoustical tones were measured, without a reflective bead, from the basal cochlear turn in sensitive and post-mortem cochleae. This study provides basic information on BM mechanical properties at the basal turn of the gerbil cochlea. It was found that the peak velocity response, indicating best frequency, is level dependent, with a half octave or more shift toward low frequencies with stimulus

Acknowledgements

Supported in part by research grants from the National Institute of Deafness and Other Communication Disorders (R01 DC000105, PO1 0078 and R03 DC033642), the National Institutes of Health, the Research Fund of the American Otological Society, the Medical Research Foundation of Oregon, and VA RR and D Center Grant RCTR-597-0160, Portland, VAMC.

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