Neural phase-locking properties in the absence of cochlear outer hair cells.

A combined regimen of kanamycin sulfate treatment (175 mg/kg/day) and behavioral evaluation of resulting audiometric threshold shifts was used to produce selective outer hair cells (OHC) loss in chinchillas. This protocol resulted in a 3-7 mm region in the cochlear base in which OHCs were completely absent and inner hair cells (IHCs) were largely resent and normal at both light and electron microscopic levels. Partial OHC loss was associated with audiometric threshold shifts in excess of 15 dB, while complete OHC loss was associated with audiometric threshold shifts in excess of 40 dB. After recovery periods of at least three weeks, phase-locking was examined across frequency for auditory nerve (VIIIth nerve) and ventral cochlear nucleus (VCN) neurons. The frequency range for neural phase-locking in normal subjects extended up to approximately 4 kHz for VIIIth nerve fibers and 3 kHz for VCN neurons. Following kanamycin intoxication, however, the frequency range for neural phase-locking in both of these auditory regions varied with characteristic frequency (CF): neurons whose CF corresponded to normal cochlear regions exhibited phase-locking throughout the normal frequency range; neurons whole CF corresponded to cochlear regions with selective OHC loss exhibited a marked reduction in the frequency range over which they could phase-lock.