EFFECTS OF STIMULUS RATE ON THE FELINE BRAIN-STEM AUDITORY-EVOKED RESPONSE DURING DEVELOPMENT .1. PEAK LATENCIES

The effects of stimulation rate on the brain-stem auditory evoked resp onse (BAER) of developing and adult cats were investigated. Age ranged from 10-post-natal days to young adults. Clicks were presented at lev els of 90 dB pSPL and 20 dB above each animal's click threshold (20 dB SI,). For all animals, a conventional BAER fate series was obtained a t rates of 5, 10, 30, 60, and 90 Hz. BAERs were also obtained using ps eudorandom pulse sequences called maximum length sequences (MLSs). The minimum time between pulses, the minimum pulse interval (MPI), includ ed 0.5, 1, 2, 4, and 6 ms, which correspond to average rates of 1000, 500, 250, 125, and 83 Hz, respectively. Dependent variables included t he latencies of the first four BAER peaks, labeled i through iv. MLS B AERs were indistinguishable from conventional BAERs at all ages studie d. In general, for both conventional and MLS BAERs, peak latencies and the i-iv interval increased with increasing stimulus rate. Although a bsolute peak latencies and the i-iv interval decreased systematically with age, the relative shift in latency and i-iv interval induced by i ncreasing stimulus rate decreased during development. The enhanced sen sitivity to stimulus rate observed at younger ages was not the consequ ence of the threshold improvement that occurs during development since similar observations were made when stimuli were presented at a const ant absolute level (90 dB pSPL) or at a constant level above threshold (20 dB SL). In addition, successive BAER peaks exhibited progressivel y larger latency shifts with increasing stimulus rate at all ages stud ied. These data suggest that higher stimulus rates produce greater neu ral adaptation resulting in prolonged BAER peak latencies. Furthermore , the effects of adaptation are cumulative across synapses, and the me chanisms responsible for the acquisition of adult-like adaptation prop erties develop during the early post-natal period in the cat. (C) 1996 Acoustical Society of America.