A comparison of N1 of the whole nerve action potential and wave i of the brain-stem auditory evoked response in Mongolian gerbil

The present study seeks to provide empirical support for the assumption that wave i of the gerbil brain-stem auditory evoked response (BAER) corresponds to N1 of the whole nerve action potential (WNAP) by comparing the latency and amplitude of BAER wave i and WNAP N1. Fourteen 3-month old gerbils were anesthetized with Nembutal and Urethane-Dial. Normothermia was maintained by a homeothermic blanket system. BAERs were recorded with Grass needle electrodes placed subdermally. The WNAP was recorded with a silver wire placed in the round window niche. WNAP and BAER were simultaneously recorded with a passband of 100–10 000 Hz. Responses consisted of 500 sweeps, and two responses were obtained for each condition. Clicks were 25-μs electrical pulses. Tonebursts were shaped with a Hanning window, with 1-ms rise and fall times. Toneburst frequencies included 1, 2, 4, 8, and 16 kHz. For each stimulus, responses were obtained at levels of 30, 50, 70, and 90 dB pSPL. SPL was measured near the entrance to the ear canal with an Etymotic ER-7C probe microphone. Dependent variables were the latency and amplitude of N1 of the WNAP and wave i of the BAER. The latencies of wave i and N1 were very similar. Mean (across animal) latencies of N1 and wave i were within 70 μs for all six stimuli (clicks, tonebursts) and all four levels. Latency/intensity function slopes for N1 and wave i were also very similar, with both dependent variables showing an increasing latency/intensity function slope with decreasing toneburst frequency. The N1/wave i amplitude ratio was computed. Interestingly, this amplitude ratio appears to increase with increasing stimulus level. In addition, stimulus spectrum influenced the N1/wave i amplitude ratio, but this pattern was dependent on whether peak amplitudes were measured from peak-to-following-trough, or from baseline to peak. This suggests that cochlear regions may be differentially weighted for the far-field and near-field recordings. The similarity of N1 and wave i peak latencies supports the assumption that wave i of the BAER is a far-field reflection of N1 of the WNAP.