Responses of auditory nerve fibers of the unanesthetized decerebrate cat to click pairs as simulated echoes

1. To elucidate the peripheral contribution to 'echo' processing in the auditory system, we examined the characteristics of auditory nerve responses to click-pair stimuli in unanesthetized, decerebrate cats. We used equilevel click pairs at peak levels of 45, 65, and 85 dB SPL re 20 μPa. The interclick intervals ranged from 1 to 32 ms. This study reports results from 78 auditory nerve fibers in 7 cats. The fibers were divided into 2 groups: 33 low- and 45 high-spontaneous rate (SR), with SRs less than and ≤20 spikes/s, respectively. A method was introduced to quantify the second-click response, and its recovery was examined as a function of the interclick interval. 2. In general, auditory nerve fibers showed a gradual recovery of the second-click response as interclick interval was increased. Noticeable differences in the second-click response recovery functions emerged among fiber populations that were related to the SR. Low-SR fibers showed little change in the recovery functions of the second-click response as the click level was increased from 45 to 85 dB SPL. In contrast, high-SR fibers showed slower recoveries with increasing click level from 45 to 85 dB SPL. At 45 and 65 dB SPL, the recovery functions of the two SR groups were similar. At 85 dB SPL, high SR fibers exhibited slower recovery than low-SR fibers, regardless of fiber characteristic frequency. The interclick intervals at 50% second-click response ranged from 1 to 6 ms (mean, 1.4 ms) among low-SR fibers. The interclick intervals at 50% second-click response for high SR fibers, whereas similar to those for the low SR fibers at 45 and 65 dB SPL, ranged from 2 to 16 ms (mean, 3 ms) for high SR fibers, at 85 dB SPL. 3. We also examined auditory nerve compound action potentials (CAPs) evoked by click-pair stimuli for various interclick intervals and click levels. With increasing interclick interval, the amplitude of the second-click CAP increased, and with increasing level, the second-click CAP showed slower recovery. At 45 dB SPL, the recovery functions of the second-click CAP were similar to those of the high- and low-SR fibers. At higher levels, the CAP exhibited lower second- click response values than both high- and low-SR fiber populations for interclick intervals <4-8 ms. At 85 dB SPL, as interclick interval increased, between 8 and 16 ms, the CAP second-click response converged with that of the high-SR fibers, and by 32 ms, the second-click response values were similar for the CAP, high- and low-SR fibers. 4. The present results are consistent with those of forward masking studies at the level of the auditory nerve in that both demonstrate a short-term reduction of the neural responses. However, the two results differ in that we observed that high-SR fibers exhibited slower recovery than low-SR fibers in response to click-pair stimuli, opposite of the trend observed in the forward masking studies of responses to pure-tone bursts. 5. The present results on auditory nerve tiber responses to click pair stimuli provide a reference for comparison with responses of central auditory neurons to similar stimuli. This information should serve to elucidate the peripheral contribution to the processing of echoes in the auditory system.