Tuning the cochlea: wave-mediated positive feedback between cells

Abstract

Frequency analysis by the mammalian cochlea is traditionally thought to occur via a hydrodynamically coupled 'travelling wave' along the basilar membrane. A persistent difficulty with this picture is how sharp tuning can emerge. This paper proposes, and models, a supplementary or alternative mechanism: it supposes that the cochlea analyses sound by setting up standing waves between parallel rows of outer hair cells. In this scheme, multiple cells mutually interact through positive feedback of wave-borne energy. Analytical modelling and numerical evaluation presented here demonstrate that this can provide narrow-band frequency analysis. Graded cochlear tuning will then rely on the distance between rows becoming greater as distance from the base increases (as exhibited by the actual cochlea) and on the wave's phase velocity becoming slower. In effect, tuning is now a case of varying the feedback delay between the rows, and a prime candidate for a wave exhibiting suitably graded phase velocity-a short-wavelength 'squirting wave'-is identified and used in the modelling. In this way, resonance between rows could supply both amplification and high Q, characteristics underlying the 'cochlear amplifier'-the device whose action has long been evident to auditory science but whose anatomical basis and mode of operation are still obscure.

Sensors, motors, and tuning in the cochlea: interacting cells could form a surface acoustic wave resonator

“…The structural parallels are suggestive, but does the cochlea really work that way? Earlier we pointed to some initial modelling results (Bell 2007a) indicating that, despite a tendency towards unrealistically sharp tuning (high Q values), the SAW model showed some attractive features, particularly in regard to phase responses. Because the feedback is wave mediated, phase jumps of 2π are a natural property of the model, whereas it is difficult to account for them on the standard travelling wave picture.…”

“…The tendency of the SAW model is to produce high Q responses, whereas the travelling wave is inherently low Q. The modelling of Bell (2007a) suggests a possible interplay between the two aspects, leading perhaps to the moderately tuned responses actually observed. Provided the gain and Q of the SAW model can be independently controlled, realistic amplitude and phase responses might be achieved.…”

“…The proximity of motors (OHC bodies) to sensors (OHC stereocilia) invites feedback, and if the phase delay of the wave reaches 360 • , oscillation between the rows of OHCs will occur. Using a simplified model and analytical wave equations, Bell (2007a) has shown that, with polarity of the input signal alternating, SAW-like, from one row to the next, a standing wave will appear as a whole wavelength between the outermost rows (OHC1 and OHC3). At the same time, a progressive, but attenuating, wave will move towards the inner hair cells (IHCs), the ear's detectors.…”

A Resonance Approach to Cochlear Mechanics

“…As well, there could be appreciable phase delays between the motion of outer and inner hair cells, and one model of this process calculated that the delays here could amount to several cycles (Fig. 12 of [92]). …”

Reptile Ears and Mammalian Ears: Hearing Without A Travelling Wave

“…In this alternative to the conventional picture, the fast pressure wave signal causes standing wave resonance between rows of outer hair cells, creating a set of nodes and antinodes, in many ways like its electronic equivalent, the surface acoustic wave (SAW) resonator [10,11,20,26]. In this SAW model, which relies on the slow propagation and cycle-by-cycle feedback of fluid-structure waves, outer hair cells themselves are stimulated by the fast pressure wave (and also by feedback, via their stereocilia, from OHCs in neighbouring rows).…”