Mechanical tuning and amplification within the apex of the guinea pig cochlea

Abstract

The popular notion of mammalian cochlear function is that auditory nerves are tuned to respond best to different sound frequencies because basilar membrane vibration is mechanically tuned to different frequencies along its length. However, this concept has only been demonstrated in regions of the cochlea tuned to frequencies >7 kHz, not in regions sensitive to lower frequencies where human speech is encoded. Here, we overcame historical technical limitations and non-invasively measured sound-induced vibrations at four locations distributed over the apical two turns of the guinea pig cochlea. In turn 3, the responses demonstrated low-pass filter characteristics. In turn 2, the responses were low-pass-like, in that they occasionally did have a slight peak near the corner frequency. The corner frequencies of the responses were tonotopically tuned and ranged from 384 to 668 Hz. Non-linear gain, or amplification of the vibrations in response to low-intensity stimuli, was found both below and above the corner frequencies. Post mortem, cochlear gain disappeared. The non-linear gain was typically 10-30 dB and was broad-band rather than sharply tuned. However, the gain did reach nearly 50 dB in turn 2 for higher stimulus frequencies, nearly the amount of gain found in basal cochlear regions. Thus, our data prove that mechanical responses do not match neural responses and that cochlear amplification does not appreciably sharpen frequency tuning for cochlear regions that respond to frequencies <2 kHz. These data indicate that the non-linear processing of sound performed by the guinea pig cochlea varies substantially between the cochlear apex and base.

Cochlear partition anatomy and motion in humans differ from the classic view of mammals

“…The longitudinally carried TM radial motion may be more important than TM resonances, but in vivo TM motion is poorly understood and is likely different from the classic view. In addition, recent experiments show that the reticular lamina moves much more than the BM, and there is differential motion between structures at the top of the OoC, such as rotation of the reticular lamina (2)(3)(4)(5)(6)(7)(8)46). All of the above indicate that the classic view in Fig.…”

“…Considering these observations, the overall pattern of OSL and bridge motion we have seen in the base is likely present throughout the human cochlea. This does not rule out there being differences in cochlear motions between base and apex as has been found in laboratory animals (5,6,8).…”

“…The classic view has been that BM motion is translated into a shearing motion at the top of the OoC between the reticular lamina and the tectorial membrane (TM), and this shearing deflects hair cell stereocilia (1). Recent studies that were able to measure motion near the reticular lamina, TM, and hair cells have shown that OoC motion is more complex (2)(3)(4)(5)(6)(7)(8). Despite the lack of a full detailed understanding of cochlear micromechanics, the belief persists that the basic pattern of CP motion is universal across mammalian species because BM and OoC anatomy are similar across mammals (9).…”

Organ of Corti vibration within the intact gerbil cochlea measured by volumetric optical coherence tomography and vibrometry

“…A strength of this study was that, using VOCTV, we could record vibratory responses within the OoC noninvasively, thus increasing the reliability of the recordings over previous approaches that required opening the cochlea (Cooper andRhode 1997, 1995;Dong and Cooper 2006;Khanna and Hao 1999;Zinn et al 2000). This has the benefit in that it removes the possibility of changing cochlear hydrodynamics because opening the cochlea creates a high-pass filter effect (Dong and Cooper 2006;Recio-Spinoso and Oghalai 2017). Another strength was that our approach permitted vibratory measurements to low-intensity stimuli, especially at turn 2 and 3, where cochlear amplification typically has the most impact.…”

Hearing at speech frequencies is different from what we thought

“…A breakthrough in measuring cochlear mechanical responses in the low‐frequency region has come from a new technique: vibration measurements using optical coherence tomography at wavelengths that penetrate bone. In this issue of The Journal of Physiology , Recio‐Spinoso & Oghalai (), using this technique, have shown in a never‐opened cochlea that the mechanical response is low‐pass‐like, and surprisingly, there are frequencies with gains of ∼300, which is similar to gains in high‐frequency regions. Not only is the low‐pass‐like tuning different from BM band‐pass tuning at high frequencies, the frequency distribution of the gain is different.…”