Active control of transverse viscoelastic damping in the tectorial membrane: A second mechanism for traveling-wave amplification?

Observations from optical coherence tomography (OCT) have revealed a velocity gradient across the reticular lamina in response to sounds (Cho and Puria, 2022). Since viscoelastic forces depend on velocity gradients, this finding suggests that OHC activity may influence viscous loss in the cochlea. Here, we propose a candidate mechanism for regulating traveling-wave viscous dissipation which involves the tectorial membrane (TM). We hypothesize that the velocity gradient generated in the OHC region, combined with TM structural properties, can reduce transverse deformations in the TM and, subsequently, transverse viscous damping. Based on this hypothesis and a simplified mechanical model, we derive a formula for an equivalent basilar membrane (BM) admittance in both passive and active scenarios. We use the WKB approximation to simulate traveling waves in response to tones at different stimulation levels. The calibration of the model is based on OCT data from mice, including data on TM motion. Our simulations show that modulating the viscous load affects the traveling wave in the peak region, with changes in BM velocity magnitude of up to 10 dB. The inclusion of a more classical anti-damping term is necessary to capture the full dynamic range of the response gain. With the textbook view of OHCs acting directly on the BM under re-evaluation in light of recent OCT data, the control of viscous damping in the TM emerges as a viable candidate for a second mechanism governing traveling-wave amplification.