The effects of click and masker spectrum on the auditory brainstem response of bottlenose dolphins (Tursiops truncatus)

Two experiments were performed that investigated the effects of (1) click level and (2) continuous broadband noise on the binaural auditory brainstem response (ABR) of normal-hearing and hearing-impaired bottlenose dolphins (Tursiops truncatus). In addition to spectrally uncompensated clicks and noise, stimuli were digitally compensated to achieve "white" spectra (flat spectral density level) or "pink" spectra (spectral density level rolling off at -3 dB/octave). For experiment 1, in all spectral conditions, ABR peak latencies increased and peak amplitudes decreased with decreasing click level, but interwave intervals changed little. Latency-intensity function (LIF) slopes ranged from -3 to -11 μs/dB. The LIF slopes of ABR peaks evoked by uncompensated clicks were steeper in dolphins with hearing loss. Click level was held constant during experiment 2, and the effect of bilaterally delivered broadband masking noise on the ABR was investigated. Clicks and noise were filtered to create a pink click/noise condition and a white click/noise condition. With increasing levels of masking noise, peak latencies increased (although only P1-P4 white reached significance), peak amplitudes decreased, and interpeak intervals increased (although not significantly). These effects are compared to results reported for terrestrial mammals, and implications for auditory health assessment and biosonar function are discussed.