The impulse response of the organ of Corti at the apex of the intact cochlea.

Georg von Békésy's initial observations of inner-ear vibrations were carried out at the apex of human-cadaver cochleae but contemporary studies have focused on responses at the base of the cochlea of live rodents. Apical cochlear recordings have proven difficult to achieve because opening the otic capsule often produces surgical injury and experimental artifacts. Using optical coherence tomography, I recorded mechanical responses to click stimuli at the organ of Corti (OoC) in the intact chinchilla cochlea at sites with characteristic frequencies (CFs) around 500 Hz. In general, OoC velocity responses to clicks consist of two segments: an initial component with a band-pass tuning centered at around 500 Hz, and a later component with low-pass characteristics. Spectral analysis of the transient responses revealed that responses had greater amplitudes and wider frequency bandwidth near the Hensen's cells region than in the proximity of the basilar membrane, their frequency selectivity increased with stimulus level and death, and CF changes were negligible postmortem. Noise analysis, using Wiener kernels, indicates that click responses contain nonlinearities absent in first-order Wiener kernels. Click responses were well approximated by a frequency independent delay followed by a minimum-phase filter. The average length of the delay was approximately 1.1 ms.