Analysis of the click-evoked brainstem potentials in humans using high-pass noise masking. II. Effect of click intensity.

Derived narrow-band brainstem responses were obtained for click levels of 10--60 dB SL in normal hearing subjects. The amplitudes and latencies of the wave I, wave III, and wave V components in the derived BSER were studied as a function of click intensity. Characteristic differences were found between the input-output behavior of waves I and III on one hand and wave V on the other hand, especially for the low-frequency narrow bands (center frequencies of 0.5 and 1.0 kHz). While the wave I and wave III (peak-to-succeeding trough) amplitude showed a small (20--30 dB) dynamic range with saturation effects, the wave V amplitude continued to increase across the intensity range studied. At the high-frequency end (narrow-band center frequencies of 4 and 8 kHz), wave V also showed saturation. It is suggested that this difference across center frequency (place of origin along the cochlear partition) is responsible for the dominance of wave V at low-frequency stimulation (e.g., with tonebursts). The latencies of the three waves studied maintained their constant interwave delays across the observed intensity range in each narrow band. Quite large (up to 3.5 ms) increases in the narrow-band latencies were found for decreasing click levels; this is comparable in value with those for the unmasked BSER although the mechanism seems to be different. The major contribution to the BSER which determines its latencies, originates at 60 dB SL from the 8-kHz region but at low SL (10 and 20 dB) from the 2-kHz region. At these low intensity levels, the contribution from the apical part of the cochlea, however, is still of the same size as that from the high-frequency end.