Homodyne interferometer for basilar membrane vibration measurements. II. Hardware and techniques

Cochlear mechanics: new insights from vibrometry and Optical Coherence Tomography

The cochlea is a complex biological machine that transduces sound-induced mechanical vibrations to neural signals. Hair cells within the sensory tissue of the cochlea transduce vibrations into electrical signals, and exert electromechanical feedback that enhances the passive frequency separation provided by the cochlea's traveling wave mechanics; this enhancement is termed cochlear amplification. The vibration of the sensory tissue has been studied with many techniques, and the current state of the art is optical coherence tomography (OCT). The OCT technique allows for motion of intra-organ structures to be measured in vivo at many layers within the sensory tissue, at several angles and in previously under-explored species. OCT-based observations are already impacting our understanding of hair cell excitation and cochlear amplification.

Timing of the reticular lamina and basilar membrane vibration in living gerbil cochleae

Auditory sensory outer hair cells are thought to amplify sound-induced basilar membrane vibration through a feedback mechanism to enhance hearing sensitivity. For optimal amplification, the outer hair cell-generated force must act on the basilar membrane at an appropriate time at every cycle. However, the temporal relationship between the outer hair cell-driven reticular lamina vibration and the basilar membrane vibration remains unclear. By measuring sub-nanometer vibrations directly from outer hair cells using a custom-built heterodyne low-coherence interferometer, we demonstrate in living gerbil cochleae that the reticular lamina vibration occurs after, not before, the basilar membrane vibration. Both tone- and click-induced responses indicate that the reticular lamina and basilar membrane vibrate in opposite directions at the cochlear base and they oscillate in phase near the best-frequency location. Our results suggest that outer hair cells enhance hearing sensitivity through a global hydromechanical mechanism, rather than through a local mechanical feedback as commonly supposed.

What is the quietest sound the ear can detect? All sounds begin as vibrating air molecules, which enter the ear and cause the eardrum to vibrate. We can detect vibrations that move the eardrum by a distance of less than one picometer. That’s one thousandth of a nanometer, or about 100 times smaller than a hydrogen atom. But how does the ear achieve this level of sensitivity?

Vibrations of the eardrum cause three small bones within the middle ear to vibrate. The vibrations then spread to the cochlea, a fluid-filled spiral structure in the inner ear. Tiny hair cells lining the cochlea move as a result of the vibrations. There are two types of hair cells: inner and outer. Outer hair cells amplify the vibrations. It is this amplification that enables us to detect such small movements of the eardrum. Inner hair cells then convert the amplified vibrations into electrical signals, which travel via the auditory nerve to the brain.

The bases of outer hair cells are connected to a structure called the basilar membrane, while their tops are anchored to a structure called the reticular lamina. It was generally assumed that outer hair cells amplify vibrations of the basilar membrane via a local positive feedback mechanism that requires the hair cells to vibrate first. But by comparing the timing of reticular lamina and basilar membrane vibrations in gerbils, He et al. show that this is not the case. Outer hair cells vibrate after the basilar membrane, not before. This indicates that outer hair cells use a mechanism other than commonly assumed local feedback to amplify sounds.

The results presented by He et al. change our understanding of how the cochlea works, and may help bioengineers to design better hearing aids and cochlea implants. Millions of patients worldwide who suffer from hearing loss may ultimately stand to benefit.

Laser Doppler velocimetry of basilar membrane vibration

A method is described for the measurement of basilar membrane (BM) vibration velocimeter (LDV). The instrumentation was coupled to a compound microscope which served to visualize reflective glass microbeads placed on the BM. The laser beam of the LDV was focused in the microscope object plane and positioned over the reflective bead. We show examples of frequency tuning curves and displacement input/output intensity functions obtained with the technique.

Interferometric measurement of the amplitude and phase of tympanic membrane vibrations in cat

The amplitude and phase of the tympanic membrane and malleus vibrations were measured over a wide frequency range with a homodyne interferometer. When sound pressure was maintained constant near the tympanic membrane, the malleus frequency response followed the typical pattern up to 10 kHz as measured by previous investigators. At higher frequencies the response changes dramatically. Instead of decreasing with frequency, between 10 and 20 kHz the vibration amplitude oscillates around a value which is only about 20 dB lower than the low frequency plateau level. Measurements of malleus vibration at several points along its length indicate that its mode of vibration changes at high frequencies, and no longer consists of a simple rotational component. All points on the tympanic membrane vibrate in phase with the malleus up to a frequency of 1 kHz. Above 5 kHz discrete resonances are observed, and the response varies strongly with position on the tympanic membrane.

Homodyne interferometer for basilar membrane measurements

Techniques available for measuring the mechanical response of the inner ear are compared. These include capacitive probe, Mössbauer and interferometric methods. The theory of a homodyne interferometer utilized for inner ear measurements is given. Experimental apparatus built to test the interferometer performance is described. Experimental results show that the measuring system can detect vibrations as low as 3 X 10(-12) cm. Its frequency response is flat within 1 dB from 0.1 to 40 kHz. It has a linear dynamic range of over 90 dB. Immunity of the interferometer to various disturbances is demonstrated.

Measurement of basilar membrane vibrations and evaluation of the cochlear condition

The experimental procedure for measuring basilar membrane responses to acoustic signals is described. The surgical procedure developed for opening the cochlea with minimal trauma is presented. Each experiment included sound pressure level measurements to define the input signal, cochlear microphonic (CM) measurements to monitor the cochlear condition, interferometric measurements and histological evaluation of the cochleas. The characteristic frequency (CF) and the sensitivity at CF for the basilar membrane response is correlated with the change of CM response observed in six animals. It is demonstrated that both tuning characteristics are extremely sensitive to cochlear trauma as evidenced by changes of CM.

Relationship between basilar membrane tuning and hair cell condition

Basilar membrane tuning characteristics were measured in 15 cats using laser interferometry. The experimental procedures introduced varying degrees of cochlear trauma. Variability from animal to animal was also observed in the characteristic frequency (CF) of tuning, the sensitivity at CF and lower frequencies, and the sharpness of tuning. The changes in the tuning characteristics of the basilar membrane are correlated with the extent of outer hair cell (OHC) damage. These observations lead to the conclusion that the tuning properties in the CF region are predominantly determined by the mechanical properties of the OHC and not the basilar membrane.