Bottlenose dolphin (Tursiops truncatus) auditory brainstem responses to frequency-modulated "chirp" stimuli

Previous studies have demonstrated that increasing-frequency chirp stimuli (up-chirps) can enhance human auditory brainstem response (ABR) amplitudes by compensating for temporal dispersion occurring along the cochlear partition. In this study, ABRs were measured in two bottlenose dolphins (Tursiops truncatus) in response to spectrally white clicks, up-chirps, and decreasing-frequency chirps (down-chirps). Chirp durations varied from 125 to 2000 μs. For all stimuli, frequency bandwidth was constant (10-180 kHz) and peak-equivalent sound pressure levels (peSPLs) were 115, 125, and 135 dB re 1 μPa. Up-chirps with durations less than ∼1000 μs generally increased ABR peak amplitudes compared to clicks with the same peSPL or energy flux spectral density level, while down-chirps with durations from above ∼250 to 500 μs decreased ABR amplitudes relative to clicks. The findings generally mirror those from human studies and suggest that the use of chirp stimuli may be an effective way to enhance broadband ABR amplitudes in larger marine mammals.