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 mu Pa. The interclick intervals ranged from 1 to 32 ms. This stu dy 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 greater than or equal to 20 spikes/s, respect ively. 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 th e second-click response as interclick interval was increased. Noticeab le differences in the second-click response recovery functions emerged among fiber populations that were related to the SR. Low-SR fibers sh owed Little change in the recovery functions of the second-click respo nse 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 t he 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 rang ed from 1 to 6 ms (mean, 1.4 ms) among low-SR fibers. The interclick i ntervals at 50% second-click response for high-SR fibers, whereas simi lar 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 exa mined 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 slow er 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 le vels, the CAP exhibited lower second-click response values than both h igh- 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 CA P, high- and low-SR fibers. 4. The present results are consistent with those of forward masking studies at the level of the auditory nerve i n that both demonstrate a short-term reduction of the neural responses . However, the two results differ in that we observed that high-SR fib ers exhibited slower recovery than low-SR fibers in response to click- pair stimuli, opposite of the trend observed in the forward masking st udies of responses to pure-tone bursts. 5. The present results on audi tory nerve fiber responses to click-pair stimuli provide a reference f or comparison with responses of central auditory neurons to similar st imuli. This information should serve to elucidate the peripheral contr ibution to the processing of echoes in the auditory system.