The modulation rate transfer function of a Rissos dolphin, Grampus griseus, using the envelope following response.

Abstract

Auditory evoked potentials (AEPs) were used to obtain a modulation rate transfer function (MRTF) of a young, stranded Rissos dolphin (Grampus griseus). The animal’s neurological response to sound was stimulated by triangle-shaped broadband clicks, 1/2 ms in duration, played at presentation rates from 100 to 2000 Hz. These stimuli were played from a transducer 1 m in front of the subject, within a calibrated rehabilitation pool. The AEP responses were recorded using human EEG sensors attached by two soft suction-cups, placed just behind the blowhole and in front of the dorsal fin, respectively. By playing the stimuli at different rates, from low to high frequency, it was possible to determine the maximum and most efficient modulation rate, to be used in an AEP audiogram. The MRTF was similar to other cetaceans as it is low-pass in shape. Corner frequencies were somewhat lower than other published marine mammal MRTFs, dropping steeply after 1000 Hz. At this frequency, the evoked potential was highest, 230 nV, compared to noise levels in the tens of nV. Thus, 1000 Hz was determined to be the modulation rate used in determining the animals AEP audiogram.