Micromechanical responses to tones in the auditory fovea of the greater mustached bat's cochlea

An extended region of the greater mustached bat's cochlea, the sparsely inn ervated (SI) zone, is located just basally to the frequency place of the do minant 61-kHz component of the echolocation signal (CF2). Anatomic adaptati ons in the SI zone are thought to provide the basis for cochlear resonance to the CF2 echoes and for the extremely sharp tuning throughout the auditor y system that allows these bats to detect Doppler shifts in the echoes caus ed by insect wing brat. We measured basilar membrane (BM) displacements in the SI zone with a laser interferometer and recorded acoustic distortion pr oducts at the ear drum at frequencies represented in the SI zone. The basil ar membrane in the SI region was tuned both to its characteristic frequency (62-72 kHz) and to the resonance frequency (61-62 kHz). With increasing st imulus levels, the displacement growth functions are compressive curves wit h initial slopes close to unity, and their properties are consistent with t he mammalian cochlear amplifier working at high sound frequencies. The shar p basilar membrane resonance is associated with a phase lag of 180 degrees and with a shift of the peak resonance to lower frequencies for high stimul us levels. Within the range of the resonance, the distortion product otoaco ustic emissions, which have been attributed to the resonance of the tectori al membrane in the SI region, are associated with an abrupt phase change of 360 degrees. It is proposed that a standing wave resonance of the tectoria l membrane drives the BM in the SI region and that the outer hair cells enh ance, fine tune, and control the resonance. In the SI region, cochlear micr omechanics appear to be able to work in two different modes: a conventional traveling wave leads to shear displacement between basilar and tectorial m embrane and to neuronal excitation for 62-70 kHz. In addition, the SI regio n responds to 61-62 kHz with a resonance based on standing waves and thus p reprocesses signals which are represented more apically in the CF2 region o f the cochlea.