Hair cell regeneration and recovery of function in the avian auditory system.

Chickens were exposed to an intense pure tone that destroyed the hair cells and tectorial membrane in a crescent shaped patch along the abneural edge of the basilar papilla. During the following weeks, when the hair cells and tectorial membrane were regenerating, psychophysical and electrophysiological measures were obtained to assess the time course and degree of recovery. Immediately after the exposure, the behavioral thresholds were elevated 30-40 dB and auditory temporal integration was greatly reduced; however, both measures fully recovered by 28 days post-exposure. In addition, tone-on-tone masking patterns recovered to normal. Immediately after the exposure, the thresholds of single cochlear ganglion neurons were elevated more than 30 dB, tuning curves were broader than normal, two-tone rate suppression (TTRS) boundary slopes were shallower than normal and spontaneous activity was reduced. Threshold and spontaneous discharge rate fully recovered after the exposure. Tuning and TTRS also recovered significantly in most neurons; however, some units with characteristic frequencies (CFs) near the exposure frequency showed abnormal tuning and TTRS suppression. The regeneration of the hair cells and lower honeycomb layer of the tectorial membrane is associated with considerable recovery of function; however, the incomplete recovery of tuning and TTRS in some neurons may be linked to the incomplete regeneration of the tectorial membrane.