Effects of rise-fall time, frequency, and intensity on the early/middle evoked response

Auditory evoked responses to tone pips were recorded on 10 normally hearing adults. Tone pips centered at 500 and 2000 Hz with 1, 2, and 4 ms rise-fall times were presented at intensities of 40, 30, 20, and 10 dB nHL. The band-pass of the recording-amplifier system was set to 55 and 3000 Hz. Responses were measured during the first 25 ms following the onset of the stimulus and the first three prominent waves were labeled P10, N15, and P20. The results indicated that varying rise-fall times from 1 to 4 ms had little effect on the detectability of these waves. Consequently, the 4-ms rise-fall time was recommended because of its greater frequency specificity. The number of identifiable responses was similar for both 500 and 2000 Hz for waves P10, N15, and P20. The similarity in the number of detectable responses suggests that any of these waves may be used as a threshold indicator. The acoustic/physiologic mechanisms underlying the latency changes are discussed.