Near-field responses from the round window, inferior colliculus, and auditory cortex of the unanesthetized chinchilla: manipulations of noiseburst level and rate.

Few studies have compared the response properties of near-field potentials from multiple levels of the auditory nervous system of unanesthetized animals. The purpose of this study was to investigate the effects of brief-duration noisebursts on neural responses recorded from electrodes chronically implanted at the round window, inferior colliculus and auditory cortex of chinchillas. Responses were obtained from seven unanesthetized chinchillas to a noiseburst-level and noiseburst-rate series. For the noiseburst-rate series, a 70 dB pSPL noiseburst was varied in rate from 10 to 100 Hz using conventional averaging procedures, and from 100 to 500 Hz using pseudorandom pulse trains called maximum length sequences (MLSs). Response thresholds were similar for the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP). With decreasing noiseburst level, there were decreases in the amplitudes and increases in the latencies of the CAP, ICP and ACP. The shapes of the mean normalized amplitude input/output (I/O) functions were similar for the ICP and ACP, while the normalized I/O functions for the first positive peak (P1) and first negative peak (N1) of the CAP differed from each other and from the ICP and ACP. The slopes of the latency/intensity functions were shallowest for the CAP, intermediate for the ICP, and steepest for the ACP. With increasing rate, the latency shift was least for the CAP, intermediate for the ICP and greatest for the ACP. The amplitude of P1 of the CAP varied little with rate. All other potentials showed a pronounced decrease in amplitude at high stimulation rates. Excluding CAP P1, proportional amplitude decrease with rate was greatest for the ACP, intermediate for N1 of the CAP and least for the ICP. Responses were present in most animals at all recording sites, even for the highest rate (500 Hz) used in this study. For all potentials, the MLS procedure allowed the collection of a response at rates well above those where sequential responses would have overlapped using conventional averaging procedures.