Effects on auditory-nerve fibers of opening the otic capsule at the apex of the chinchilla cochlea

Vibration responses to clicks measured at the apex of chinchilla cochleae with open otic capsules have onsets much shorter than those of responses of auditory-nerve fibers (ANFs) corrected for synaptic and neural delays. Apical vibration responses to tones in open cochleae also differ in other respects from the responses to tones of ANFs with low characteristic frequency (CF) in normal chinchilla cochleae. To further specify the origin(s) of these differences, we recorded from chinchilla ANFs after delicately opening a small hole in the otic capsule overlying scala vestibuli in the cochlear apex. In those cochleae, the earliest ANF responses to clicks are often evoked by condensation (rather than rarefaction) clicks and responses to tones often exhibit level-dependent phase changes different from those in normal cochleae. These findings are largely consistent with, and seem to account for, apical vibration responses of cochleae with open otic capsules. An unexpected finding is that the tuning curves of ANFs with moderately high CF and normal CF thresholds often had hypersensitive tails.