Effects of noise burst rise time and level on bottlenose dolphin (Tursiops truncatus) auditory brainstem responses.

Although the auditory brainstem response (ABR) is known to be an onset response, specific features of acoustic stimuli that affect the morphology of the ABR are not well understood. In this study, the effects of stimulus onset properties were investigated by measuring ABRs in seven bottlenose dolphins while systematically manipulating stimulus rise time and the amplitude of the sound pressure temporal envelope plateau. Stimuli consisted of spectrally pink (i.e., equal mean-square pressure in proportional frequency bands) noise bursts with linear rise (and fall) envelopes and frequency content from 10 to 160 kHz. Noise burst rise times varied from 32 μs to 4 ms and plateau sound pressure levels varied from 96 to 150 dB re 1 μPa. ABR peak latency was found to be a function of the rate of change of the sound pressure envelope, while ABR peak amplitude was a function of the envelope sound pressure at the end of a fixed integration window. The data support previous single-unit and nearfield response data from terrestrial mammals and a model where the rate of change of envelope sound pressure is integrated across a time window aligned with stimulus onset.