The Cochlear Microphonic Potential Does Not Reflect the Passive Basilar Membrane Traveling Wave

Abstract

Understanding the cochlear microphonic potential (CM) can contribute to its clinical usefulness, e.g. in auditory neuropathy where the CM is present, while auditory evoked neural potentials are absent. The space-time pattern of the basilar membrane traveling wave in the cochlea is thought to be responsible for generation of the CM and its pattern along the cochlea. This relationship was studied in two experiments. EXPERIMENT I: The threshold of the CM was measured in animals before and after drilling a hole in the wall of the inner ear vestibule. Such a hole likely reduces the magnitude of the basilar membrane traveling wave. The hole did not cause a change in CM threshold. This is evidence that the passive basilar membrane traveling wave described by von Bekesy in cadavers in response to high intensity stimulation may not be the trigger for cochlear activation and CM generation at low intensities. EXPERIMENT II: A saline filled tube provided fluid coupling between the cochlea of one animal and that of a second through the perforated round windows of their cochleae. In response to sound stimulation of animal 1, CM (and ABR) could be recorded in animal 2, even when animal 1 was no longer living. It is highly unlikely that a basilar membrane traveling wave could have been induced in cochlea 2 in these conditions. It is therefore suggested that at low sound intensities, the initial event activating the cochlea in general and the CM in particular is the fluid pressures (condensations/rarefactions) induced in the cochlear fluids by sound-induced stapes footplate vibrations.