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Auditory sensation is based in nanoscale vibration of the sensory tissue of the cochlea, the organ of Corti complex (OCC). Motion within the OCC is now observable due to optical coherence tomography. In a previous study (Cooper et al., 2018), the region that includes the electro-motile outer hair cells (OHC) and Deiters cells (DC) was observed to move with larger amplitude than the basilar membrane (BM) and surrounding regions and was termed the "hotspot." In addition to this quantitative distinction, the hotspot moved qualitatively differently than the BM, in that its motion scaled nonlinearly with stimulus level at all frequencies, evincing sub-BF activity. Sub-BF activity enhances non-BF motion; thus the frequency tuning of the OHC/DC region was reduced relative to the BM. In this work we further explore the motion of the gerbil basal OCC and find that regions that lack significant sub-BF activity include the BM, the medial and lateral OCC, and the reticular lamina (RL) region. The observation that the RL region does not move actively sub-BF (already observed in Cho and Puria 2022), suggests that hair cell stereocilia are not exposed to sub-BF activity in the cochlear base. The observation that the lateral and RL regions move approximately linearly sub-BF indicates that linear forces dominate non-linear OHC-based forces on these components at sub-BF frequencies. A complex difference analysis was performed to reveal the internal motion of the OHC/DC region and showed that amplitude structure and phase shifts in the directly measured OHC/DC motion emerge due to the internal OHC/DC motion destructively interfering with BM motion., Competing Interests: Declaration of competing interest The authors declare no financial interests or conflicts of interest., (Copyright © 2024 Elsevier B.V. All rights reserved.)

In their article, "Measurement of cochlear power gain in the sensitive gerbil ear," Ren et al. (Nat Commun 2:216, 2011) claim to provide "the first direct experimental evidence of power amplification in the sensitive living cochlea." While we recognize the technical challenges of the experiments and appreciate the beauty of the data, the authors' analysis and interpretation of the measurements are invalid. We review the concept of impedance (i.e., the ratio of pressure to velocity) as it applies to cochlear mechanics and show that Ren et al. mistakenly equate the impedances near the basilar membrane and stapes with the impedance characteristic of an infinite, uniform tube of fluid. As a consequence of this error, Ren et al.'s measurements and analysis provide no evidence for power amplification in the cochlea. Compelling evidence for power amplification has, however, been previously provided by others.

Many cochlear models assign zero longitudinal coupling in the cochlea. Although this is consistent with the transverse basilar membrane (BM) fibers, the cochlear partition contains cellular longitudinal coupling. In cochlear models, longitudinal coupling diminishes passive BM tuning; however, it has recently been employed in theories of active mechanics to enhance tuning. Our goal in this study was to probe passive longitudinal coupling by comparing BM responses in damaged cochleae with passive responses in normal cochleae. The cochleae of gerbils were damaged with intratympanic neomycin followed by a waiting period to ensure that all of the cells of the partition were missing or severely disrupted. We then measured BM motion and examined the cochleae histologically. In comparison with passive responses in normal cochleae, we observed a downward shift in characteristic frequency, an expected consequence of reduced stiffness from cellular damage. However, we did not observe enhanced passive tuning in the damaged cochleae, as would be expected if longitudinal coupling were substantially greater in the normal cochleae. Thus, we conclude that cell-based longitudinal coupling is not large enough to influence passive cochlear mechanics. This finding constrains theories of active mechanics., (Copyright © 2011 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

The mechanism for passive cochlear tuning remains unsettled. Early models considered the organ of Corti complex (OCC) as a succession of spring-mass resonators. Later, traveling wave models showed that passive tuning could arise through the interaction of cochlear fluid mass and OCC stiffness without local resonators. However, including enough OCC mass to produce local resonance enhanced the tuning by slowing and thereby growing the traveling wave as it approached its resonant segment. To decide whether the OCC mass plays a role in tuning, the frequency variation of the wavenumber of the cochlear traveling wave was measured (in vivo, passive cochleae) and compared to theoretical predictions. The experimental wavenumber was found by taking the phase difference of basilar membrane motion between two longitudinally spaced locations and dividing by the distance between them. The theoretical wavenumber was a solution of the dispersion relation of a three-dimensional cochlear model with OCC mass and stiffness as the free parameters. The experimental data were only well fit by a model that included OCC mass. However, as the measurement position moved from a best-frequency place of 40 to 12 kHz, the role of mass was diminished. The notion of local resonance seems to only apply in the very high-frequency region of the cochlea.

A common way to measure submicroscopic motion of the organ of Corti is heterodyne interferometry. The depth over which vibration can be accurately measured with heterodyne interferometry is determined by both the optics, which controls to what extent light from nonfocal planes reaches the photodetectors, and demodulation electronics, which determines to what extent signal generated by out-of-focal-plane light influences the measurements. The influence of a second reflecting surface is investigated theoretically and experimentally. By reviewing the theory of FM demodulation and showing tests with a Revox FM demodulator, it is demonstrated that the influence of a secondary signal on a measurement depends on the modulation index. Both high- and low-modulation index signals are encountered in heterodyne interferometry of the cochlea. Using a He-Ne-like diode laser (lambda = 638 nm), the border between low- and high-modulation signals is at a displacement of about 25-100 nm. Confocal interferometry reduces the magnitude of out-of-focus signals, and therefore their effect on vibration measurement. The response of the confocal system to reflected signals from two surfaces separated by distances encountered within the cochlear partition is shown. The results underscore the benefit of steep optical sectioning for intracochlear measurements.

Intracochlear pressure was measured in vivo in the base of the gerbil cochlea. The measurements were made over a wide range of frequencies simultaneously in scalae vestibuli and tympani. Pressure was measured just adjacent to the stapes in scala vestibuli and at a number of positions spaced by tens of micrometers, including a position within several micrometers of the basilar membrane, in scala tympani. Two findings emerged from the basic results. First, the spatial variation in scala tympani pressure indicated that the pressure is composed of two modes, which can be identified with fast and slow waves. Second, at frequencies between 2 and 46 kHz (the upper frequency limit of the measurements) the scala vestibuli pressure adjacent to the stapes had a gain of approximately 30 dB with respect to the pressure in the ear canal, and a phase which decreased linearly with frequency. Thus, over these frequencies the middle ear and its termination in the cochlea operate as a frequency independent transmission line. A subset of the data was analyzed further to derive the velocity of the basilar membrane, the pressure difference across the organ of Corti complex (defined to include the tectorial and basilar membranes) and the specific acoustic impedance of the organ of Corti complex. The impedance was found to be tuned in frequency.