In the healthy mammalian cochlea, vibrations evoked by tones exhibit characteristics that vary with sound intensity level. At behavior thresholds, the gain of basilar membrane (BM) displacement at the characteristic place is at least 30 dB higher than for high stimulus intensities (Ruggero et al., 1997, Ren et al., 2016, Lee et al., 2016, Cooper et al., 2018, Dewey et al., 2019, He et al., 2022). This gain decrease has alternately been interpreted as indicative of the amplification of low-intensity traveling waves (e.g., Dewey et al., 2019, Dewey et al., 2021), or as evidence of the compression of higher-intensity waves (van der Heijden and Vavakou, 2022). The exact mechanisms involved in this nonlinear process — also called the “active process” of the cochlea — are partly unknown or still under debate, although it is widely accepted that the electromotile outer hair cells (OHCs) play a critical role (Dewey et al., 2021, Hudspeth, 2008, Reichenbach and Hudspeth, 2014, Strimbu et al., 2020). A commonly cited mechanism underpinning the active process is the push-pull action of OHCs. This mechanism suggests that the cell bodies of the OHCs contract when the BM goes up, in response to well-timed hair cell deflections caused by shear motion between the tectorial membrane (TM) and the reticular lamina (RL). As a result, OHCs would exert a pulling force on the basilar membrane (BM), amplifying its upward motion (Reichenbach and Hudspeth, 2014, Guinan et al., 2012). However, recent experimental data, including observations from optical coherence tomography (OCT), have revealed unexpected motion patterns in the organ of Corti, with movement in the OHC region showing more independence from BM motion than previously generally assumed (Ren et al., 2016, Cooper et al., 2018, Zhou et al., 2022). OCT data has also raised questions about the timing of the OHCs to operate at the correct phase (Guinan, 2022, Altoè et al., 2022), since new measurements indicate a slight phase lead of the RL over BM at frequencies below the characteristic frequency (Cho and Puria, 2022, Strimbu et al., 2024), rather than the quarter-cycle phase lag predicted by the push-pull hypothesis (Altoè et al., 2022). These patterns appear at odds with the apparent simplicity of the push-pull action of OHCs, leaving room for alternative, less direct explanations of the active process (Guinan, 2022, Altoè et al., 2022).

This paper introduces the hypothesis of an additional role for OHCs in controlling transverse viscoelastic damping occurring in the TM. The existence of a ‘friction control’ mechanism capable of modulating cochlear gain has already been proposed by some authors (van der Heijden and Vavakou, 2022, van der Heijden and Versteegh, 2015). The hypothesis presented here, however, originates from two other recent studies. The first is a modeling study that described the opposing roles of two hydrodynamic effects — ‘fluid focusing’ and fluid viscous dissipation — on the traveling wave (Sisto et al., 2021, Sisto et al., 2023). Both effects are wavelength-dependent and increase in strength in the short-wave region, where the traveling wave in response to a tone reaches its peak amplitude. This work highlighted the potential role of viscosity in stabilizing cochlear amplification and provided a simple mathematical framework to include viscous dissipation in simulations of cochlear pressure waves using the WKB approximation. The second study is a report of OCT data showing the existence of a velocity gradient in the OHC region during auditory stimulation in live gerbils (Cho and Puria, 2022). The authors showed that the part of the RL above the innermost row of OHCs moved with lower amplitude than for the two other rows by a factor of about 10 dB. This observation was valid on a wide range of frequencies around the characteristic frequency (CF).

Our premise is that the radial gradient of transverse velocity observed in the OHC region can modulate transverse deformation in the TM, thus affecting viscoelastic loss in the TM mechanically coupled to the organ of Corti and BM. In this paper, we explore the implications of this hypothesis through a formal analysis carried out under simplifying assumptions, followed by numerical simulations of tone-evoked cochlear waves using the WKB method. Our approach focuses on the traveling wave, assuming that its critical element is the BM moving transversely. In particular, the complex interplay between the active process, TM (radial) load, and hair cell deflection, is beyond the scope of this study. This simplified modeling approach allows us to present an overview of important ideas related to the proposed viscous undamping mechanism, without addressing all the micromechanical or biophysical details of how it could be implemented in the cochlea.