Single auditory nerve fiber and action potential latencies in normal and noise-treated chinchillas

Latency-intensity functions for the auditory nerve action potential (AP) and single auditory nerve fibers were measured in a group of normal and a group of noise-treated chinchillas using click stimuli. The noise-treated group was exposed for 5 days to an octave band of noise centered at 0.5 kHz and having an SPL of 95 dB. The AP visual detection level (VDL) and the single neuron thresholds were approximately 40-50 dB higher in the noise-treated group than in normals. At the VDL, the latency for AP was shorter in the noise-treated group than in the normal group; however, for the same intensity the latency was the same for both groups. Similar results were obtained from single fibers of similar characteristics frequency (CF). At threshold, the fiber latencies were shorter for the noise-treated group than for normal animals, but at the same intensity, the latencies for the two groups were similar. The results indicate that AP and single fiber latencies are a function of absolute intensity rather than intensity relative to a 'neural threshold'. A comparison of poststimulus time (PST) histograms made at the same intensity revealed fewer peaks in the units from noise-treated animals than in normal units; however, the width of the peaks and the time between peaks appeared normal.