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

This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

This tutorial reviews various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations. The techniques that are discussed and compared include intensity-based sensing, triangulation, time-of-flight sensing, confocal sensing, Doppler sensing, and various kinds of interferometric sensing with both high- and low-coherence sources.

Optical methods for distance and displacement measurements

We have constructed and tested a laser interferometer transducer for a long, wideband, laser-linked gravitational radiation antenna. Photon-noise-limited performance was achieved using 80 microW from a single mode Spectra-Physics 119 laser in a modified Michelson interferometer on a vibration isolation table in a quiet room. A piezoelectric driver on one of the interferometer mirrors was used to generate subangstrom (3 x 10(-14)-m) vibrations of known amplitude. The measured displacement sensitivity of the system in the kilohertz region was 1.3 x 10(-14)m/Hz(1/2), which compares well with the calculated photon noise limit of 1.06 x 10(-14) m/Hz(1/2). This is the smallest vibrational displacement measured directly with a laser to date.

Photon-noise-limited laser transducer for gravitational antenna

A system for measuring nanometer motions of microscopic structures is demonstrated. Stop-action images of a target are obtained with a light microscope, CCD camera, and stroboscopic illuminator. Mo- tions are determined directly from measured images using algorithms from computer vision. The accuracy of motion measurements using the system is assessed using a moving target with calibrated displacements. Accuracy is determined for specimens viewed under our most optimal conditions as well as for a number of suboptimal conditions that illustrate important degradation mechanisms. Measured errors are compared to predictions based on computer simulations of theoretical models. Re- sults show that the most important hardware factors include substrate vibrations and camera imperfections. Measurement errors for the most optimal hardware conditions are primarily due to systematic bias in the computer vision algorithms. For our most optimal conditions, the system can resolve motions as small as nanometers. Thus, errors in motion measurements are small compared to both the wavelength of the light used to obtain the images and the pixel spacing of the video microscope. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)03904-X) Subject terms: video microscopy; stroboscopic illumination; motion measure- ment; fixed-pattern noise; shot noise; substrate vibration.

Using a light microscope to measure motions with nanometer accuracy

Intrinsic performance limits of noncontacting fiber lever displacement measuring systems are quantitatively described. Generalized relationships linking displacement detection limit, frequency response, dynamic range, linearity, and working distance to fiber diameter, illumination irradiance and coupling angle, photo-detector characteristics, and reflection and transmission losses were obtained by analysis and confirmed by measurement. Both procedures showed performance limits to be functions of the square root of the flux density coupled into the target-illuminating fiber(s) by the electroluminescent source. Displacement detection and bandwidth limits achievable with tungsten or LED sources were in the 2 × 10−11 to 2×10-12m/Hz and MHz, range respectively. A basis for optimizing levers for different applications and determination of intrinsic performance limits is provided.

R. O. Cook

Papers

Fiber optic lever displacement transducer.