Abstract
The auditory brainstem response (ABR) to stimulus onset has been extensively used to investigate dolphin hearing. The mechanisms underlying this onset response have been thoroughly studied in mammals. In contrast, the ABR evoked by sound offset has received relatively little attention. To build upon previous observations of the dolphin offset ABR, a series of experiments was conducted to (1) determine the cochlear places responsible for response generation and (2) examine differences in response morphologies when using toneburst versus noiseburst stimuli. Measurements were conducted with seven bottlenose dolphins (Tursiops truncatus) using tonebursts and spectrally "pink" broadband noisebursts, with highpass noise used to limit the cochlear regions involved in response generation. Results for normal-hearing and hearing-impaired dolphins suggest that the offset ABR contains contributions from at least two distinct responses. One type of response (across place) might arise from the activation of neural units that are shifted basally relative to stimulus frequency and shares commonalities with the onset ABR. A second type of response (within place) appears to represent a "true" offset response from afferent centers further up the ascending auditory pathway from the auditory nerve, and likely results from synchronous activity beginning at or above the cochlear nucleus.
Highlights
University of California Santa Cruz, Santa Cruz, CA 95064, USA.bottlenose dolphins (Tursiops truncatus) behave to those in terrestrial mammals; offset auditory brainstem response (ABR) amplitudes are typically smaller than those for onset ABRs and increase with decreasing rise-fall time (RFT) and increasing stimulus level (Burkard et al, 2020)
Measurements were conducted with seven bottlenose dolphins (Tursiops truncatus) using tonebursts and spectrally “pink” broadband noisebursts, with highpass noise used to limit the cochlear regions involved in response generation
The results provide insight into the mechanisms responsible for generating offset responses in dolphins, and the findings are contrasted with the more widely studied onset ABR
Summary
Bottlenose dolphins (Tursiops truncatus) behave to those in terrestrial mammals; offset ABR amplitudes are typically smaller than those for onset ABRs and increase with decreasing rise-fall time (RFT) and increasing stimulus level (Burkard et al, 2020). Results with stimuli of varying frequency content and for dolphins with varying degrees of high-frequency hearing impairment suggested some difference in the specific cochlear places for onset and offset responses. This is similar to results with mice, where offset ABRs to tones appear to be generated by cochlear frequency regions near those responsible for the onset ABR, with more limited contributions from the stimulus center frequency (Henry, 1985a,b). Dolphin offset ABRs for toneburst stimuli were sufficiently different from those for noiseburst stimuli to suggest potentially different mechanisms in underlying response generation [see Brinkmann and
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