Auditory Weighting Functions and Frequency-Dependent Effects of Sound in Bottlenose Dolphins (Tursiops Truncatus)

The long term goal of this effort is to develop meaningful auditory weighting functions for marine mammals. These weighting functions would improve assessments of the effects of anthropogenic sound by emphasizing frequencies to which animals are most sensitive and de-emphasizing those to which they are not. The objective of this effort is to develop auditory weighting functions for bottlenose dolphins with normal hearing and high-frequency hearing loss. The weighting functions would be defined by measuring subjective loudness and temporary threshold shift (TTS) as functions of the sound frequency. The specific objectives for FY08 were to (1) define TTS onset/growth at 3 kHz in a bottlenose dolphin with good high-frequency hearing, (2) train the same dolphin to participate in equal loudness tests, and (3) define TTS-onset at 20 kHz in a second bottlenose dolphin. TTS is defined as the difference between hearing thresholds measured before and after an intense (fatiguing) sound exposure. Hearing thresholds are estimated using either a behavioral response paradigm, where the subject is trained to perform a specific action when it hears a test tone, or an electrophysiological method, where auditory evoked potentials (AEPs) are measured in response to test tones. Behavioral methods developed at the Navy Marine Mammal Program (MMP) allow thresholds to be obtained within four minutes of intense sound exposures. This is accomplished using computer-controlled stimulus presentations, recording acoustic responses emitted by the subject in response to those stimuli, and presenting multiple trials before subject reinforcement. Dolphins typically produce an acoustic response (a whistle) within a few hundred milliseconds of tone onset, allowing a rapid pace of stimulus presentation and fast threshold estimates. A modified up/down descending staircase technique is used to adjust the stimulus level in an adaptive fashion from one trial to the next and bracket the threshold.