A Biophysical Model of the Role of the Outer Hair Cell in Cochlear Nonlinearity
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It has been observed that the response characteristics of the basilar membrane in normal living cochleae are both frequency and level-sensitive (Robles & Ruggero 2001). The quality factor of the tuning curve is large at low sound levels and decreases as the sound level increases, and the peak of the tuning curve moves towards lower frequencies as the sound level increases. The current study proposes a nonlinear cochlear model that responds adaptively to the incoming sounds via feedback control arising from the mechanical attributes of the cochlear partition. These attributes are dependent on the membrane potential of the outer hair cells (He & Dallos 1999, Santos-Sacchi 1992). A parallel resistor-capacitor circuit analogy of the outer hair cell with related perilymph and endolymph potentials is designed to simulate sound-evoked changes in the outer hair cell membrane potential. Nonlinear responses of the cochlea, such as compression and two tone suppression, can be explained using this model. Furthermore, it has been shown that the basilar membrane response to pure tone stimuli is attenuated by directly stimulating the medial olivo-cochlear bundle using electrical shocks (Cooper & Guinan 2006). Basilar membrane responses in the presence of efferent stimulation can be demonstrated using the same model, through modulation of the outer hair cell rnembrane potential. The proposed model provides a unified account of the combined effect of sounds and efferent stimulation on cochlear responses.
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