Auditory brainstem responses to stimulus offset in the bottlenose dolphin (Tursiops truncatus)

Abstract

The auditory brainstem response (ABR) is typically associated with the onset of a sound stimulus; however, ABRs can also be produced at sound offset. Here, ABRs in response to stimulus offset were examined in normal hearing (NH) and hearing impaired (HI) bottlenose dolphins. Tests were conducted in San Diego Bay, with the dolphin positioned underwater in front of a piezoelectric sound projector. Scalp activity was amplified (94 dB) and filtered (300–3000 Hz) before synchronous, weighted averaging and digitization. Each ABR was the averaged response to 512 stimuli; two ABRs were obtained for each stimulus condition. Stimuli included spectrally pink noisebursts (bandwidth: 20–160 kHz) and tonebursts (40-kHz; 113-kHz for the NH dolphins only). Stimulus level, risetime, and duration were systematically manipulated across experimental trials. Regardless of onset/offset envelope (cosine or linear), large amplitude offset responses were observed to 40-kHz tonebusts presented at higher levels with faster rise times for the NH but not the HI animals. One interpretation of these findings is that the offset response may be arising from more basal regions of the cochlea than the onset response. [Work supported by US Navy Living Marine Resources Program.]