TUNING, SPONTANEOUS ACTIVITY AND TONOTOPIC MAP IN CHICKEN COCHLEAR GANGLION NEURONS FOLLOWING SOUND-INDUCED HAIR CELL LOSS AND REGENERATION

Adult chickens were exposed for 48 h to a 525 Hz, 120 dB SPL tone that destroyed the hair cells and tectorial membrane in a crescent-shaped patch along the abneural edge of the basilar papilla. Single-unit reco rdings were obtained from cochlear ganglion neurons 0-1, 5, 14 and 28 days post-exposure to determine what effect the cochlear lesion had on neural discharge patterns and if the discharge patterns fully recover ed. Immediately after exposure, the tuning curves were extremely broad and CF thresholds were elevated by 30-40 dB. In addition, the average spontaneous rate and percentage of neurons with interspike interval h istograms with preferred intervals were greatly reduced. Tuning curves and spontaneous activity started to recover by 5 days post-exposure; however, some W-shaped tuning curves with two distinct tips and a hype rsensitive tail were observed at this time. W-shaped tuning curves dis appeared and spontaneous activity recovered to normal levels 14-28 day s post-exposure. However, the CF thresholds of the most sensitive neur ons were still slightly elevated, tuning curve slopes below CF were sh allower than normal, and thresholds in the low-frequency tail of the t uning curves were often hypersensitive. These functional deficits were most closely associated with residual damage to the upper fibrous lay er of the tectorial membrane. To determine if the cochlear frequency-p lace map was altered by the cochlear lesion, four physiologically char acterized neurons were labeled with biocytin at 5 days post-exposure. The CFs of the labeled neurons were consistent with the normal frequen cy-place map (Chen et al. (1994) Hearing Research 81, 130-136) indicat ing that the tonotopic map was not altered.