VIII nerve response to click stimuli in normal and pathological cochleas

A flat 30--50 dB hearing loss was established in chinchillas following a 5 day exposure to an octave band of noise (354--708 Hz, 95 dB SPL). After exposure, single auditory nerve fiber recordings were obtained using click and tone burst stimuli. The thresholds of units from the noise-treated animals were elevated 30--70 dB and the tuning curves were abnormally broad. At the threshold for click stimulation, the fiber latencies were shorter in the noise-treated animals than those in normal animals. However, the latencies for the two groups were similar when stimulated at the same intensities. As indicated by the number of peaks in the PST histograms obtained with clicks, the units from the noise-treated animals showed considerably more damping in the neural response than those from normal units. The temporal spacing between the peaks in the histograms for units of similar CF was the same in the normal and noise-treated groups, although this cannot be taken to infer that an individual unit PST histogram would remain the same after noise exposure as before. These limited neural data therefore show changes in the same direction as those in the transient mechanical response of the basilar membrane reported by Robles et al.