Nonlinear cochlear mechanics without direct vibration-amplification feedback

Abstract

Recent in vivo recordings from the mammalian cochlea indicate that although the motion of the basilar membrane appears actively amplified and nonlinear only at frequencies relatively close to the peak of the response, the internal motions of the organ of Corti display these same features over a much wider range of frequencies. These experimental findings are not easily explained by the textbook view of cochlear mechanics, in which cochlear amplification is controlled by the motion of the basilar membrane (BM) in a tight, closed-loop feedback configuration. This study shows that a simple phenomenological model of the cochlea inspired by the work of Zweig [J. Acoust. Soc. Am. 138, 1102 (2015)] can account for recent data in mouse and gerbil. In this model, the active forces are regulated indirectly, through the effect of BM motion on the pressure field across the cochlear partition, rather than via direct coupling between active-force generation and BM vibration. The absence of strong vibration-amplification feedback in the cochlea also provides a compelling explanation for the observed intensity invariance of fine time structure in the BM response to acoustic clicks.

Mechanical Frequency Tuning by Sensory Hair Cells, the Receptors and Amplifiers of the Inner Ear

“…This situation is relevant to the cochlea, in which energy is pumped into the traveling wave basal to the position where the amplitude of vibration peaks (115,116). The local response of the cochlea to weak sounds of varying frequency can actually be described by that of phenomenological oscillators obeying Equation 9, in which the stimulus � ( ) corresponds-remarkably enough-to a combination of the pressure difference transverse to the basilar membrane and its weighted time derivative (117,118).…”

The cochlear ear horn: geometric origin of tonotopic variations in auditory signal processing

“…We elucidate this phenomenon further using an active model of the CP admittance (see Supplementary Appendix C; see also Supplementary Appendix G for the effects of active amplification on cochlear hydrodynamics) 38,39 . For purposes of illustration, and in order to highlight the role of macromechanical processes, we stipulate that the sharpness of "micromechanical" tuning in the model be everywhere identical.…”

The outer-hair-cell RC time constant: A feature, not a bug, of the mammalian cochlea

“…(d) In analogy with the signal amplifier of panel (b), OHC forces are AC coupled to the traveling wave on the BM. This panel compares the magnitude of OHC forces and amplification of BM response, quantified as the ratio between BM displacement in both the presence and absence of active forces (i.e., postmortem) in a 3-D model of the mouse cochlea ( Altoè and Shera, 2020b ). Whereas OHC forces are broadband, BM amplification retains a prominent high-pass characteristic, so that low-frequency OHC activity has a negligible effect on the BM response.…”

“…Furthermore, we have recently demonstrated ( Altoè and Shera, 2020b ) that significant level-dependencies of the partition damping and stiffness cannot easily be reconciled with the approximate level-invariance of the fine-time structure of the BM click-response and the biological robustness of this phenomenon (for a review, see Shera, 2001 ; Zweig, 2016 ). Indeed, the problem would be particularly acute were the level-dependence to mirror OHC activity and occur in extended regions basal to the peak of the traveling wave (see Appendix A of Altoè and Shera, 2020b ).…”

“…The coherently emitted 4 power accumulates spatially and boosts the amplitude of the traveling pressure wave and thereby the motion of the BM. Coherent wave amplification is most effective in the so-called “short-wave region” that extends roughly 0.5–1 octave basal to CF, depending on species and tonotopic location (see Altoè and Shera, 2020b , and Fig. 3d).…”