Acoustic pathways of hearing in the bottlenose dolphin, Tursiops truncatus

The acoustic pathways for the reception of sound in the Atlantic bottlenose dolphin were studied by measuring auditory brain-stem-evoked potential responses as a specially designed suction cup hydrophone was placed at different areas of the animal’s head. The dolphin was trained to come out of the water and beach on a rubber mat. Evoked responses were recorded differentially with suction cup surface electrodes placed between the parietal (noninverting) just posterior to the blowhole and mastoid (inverting). A transient signal was used as the stimulus and the amplitude of the stimulus was lowered in fixed dB steps until the evoked response reached a specific level close to the threshold level. The results indicate that the most sensitive portion of the lower jaw is slightly forward of the pan-bone and about 3/4 of the way toward the front of the lower jaw and along certain spots on the bottom midline of the lower jaw. The relative sensitivity of different areas of the lower jaw was also determined. The measurement results will be compared with results from calculations based on numerically solving the three-dimensional wave equation for wave propagation into the lower jaw of the dolphin. [Work supported in part by ONR, Grant No. N0001496WX30364.]