Model for production of cochlear microphonics and their relationship to basilar membrane displacement

A three-dimensional model of cochlear voltage sources and current pathways has been developed from a series of electrical resistance and impedance measurements made in the first turn of the Guinea pig cochlea. The model is validated by reproducing and thereby explaining a wide variety of cochlear microphonic (CM) data from the literature, including the difference in interscala CM phase behavior between the hook region and the rest of the cochlea. The model predicts that an electrode pair recording microphonics has a “field of view” which is frequency dependent. The width of this field of view gets narrower with frequency, enabling an electrode pair to record CM without interference from out-of-phase hair cells up to frequencies as high as 9 kHz in the first turn of the cochlea. The model also predicts that the known discrepancies between measured basilar membrane displacement and CM amplitude for frequencies below 9 kHz are due primarily to capacitive effects in the walls of scala media. Thus with the proper amplitude correction factor, CM can be used to measure basilar membrane displacement up to about 9 kHz in the first turn of the cochlea. Data from the literature supporting these conclusions will be presented.