Gerbil brain-stem auditory-evoked responses to maximum length sequences

Abstract

This study evaluated the feasibility of obtaining brain-stem auditory-evoked responses (BAERs) to trains of clicks with average rates of up to 500 clicks per second in the gerbil, in order to evaluate BAER behavior at rates higher than those possible in conventional averaging, and to evaluate whether these higher rates allow obtaining BAERs more efficiently. Gerbils were anesthetized with Nembutal, and normothermia was maintained with a homeothermic blanket system. Pseudorandom pulse trains, called maximum length sequences (MLSs), were generated by an Ariel DSP-16 board housed in an IBM-AT. Electrical activity was recorded with Grass needle electrodes placed subdermally. This activity was amplified and filtered by a Nicolet Compact Four, and digitized by the A/D of the Ariel DSP-16. Clicks were presented at levels of 50, 60, 70, 80, and 90 dB pSPL. Conventional BAERs were obtained at a rate of 30 Hz. MLS BAERs were obtained by presenting MLS pulse trains which were comprised of 64 pulses. Following collection of the response to MLS pulse trains, the MLS BAER was obtained by cross correlating the response with a recovery sequence. The minimum time between pulses is called the minimum pulse interval (MPI). MLS BAERs were obtained for MPIs of 6, 4, 2, and 1 ms. BAERs were obtained in ten young adult gerbils for all rate by click-level conditions. BAERs were observed in all animals for all click levels and rates. Generally, with increasing click level there were decreases in peak latencies and increases in peak amplitudes. With increasing rate (decreasing MPI), there were increases in peak latencies, increases in the i-iv interval, and decreases in peak amplitudes. Computations suggested the the most efficient rate (i.e., the rate producing a constant signal-to-noise ratio response in the least amount of time) is not necessarily the fastest rate, but depends on the BAER peak and click level.