The electromechanical processes responsible for fine tuning in the cochlea are still a matter of controversy. When motility and force generation of independently acting outer hair cells (OHCs) are assumed to be the source of activity, the low-pass character of isolated hair cells clearly limits the degree to which waveforms on the cochlear partition (CP) can be sharpened. We address this problem in a mathematical model for actively-controlled waves on the CP with two degrees of freedom in a cross section representing the coupled transverse motions of the tectorial (TM) and basilar membrane (BM). We determined the space-frequency characteristics of the cochlear amplifier (CA) that enables the model to reproduce experimentally determined tuning curves, assuming only that OHCs are activated by the motion of the TM and exert forces on both the reticular lamina (RL) and BM. We found that OHCs must not only sense localized motions, but must also act synergistically to sense the local wavelength along the CP. Furthermore, we can account for this behavior with an electromechanical circuit containing resistive coupling between OHCs, supporting cells, and extracellular spaces, provided the coupling resistances are not too large. When this activity mechanism is used, very sharp wave forms can be computed, and the frequency selectivity of a specific location along the CP can attain a physiological value.
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