Modulation rate transfer functions in three species of cetaceans

Abstract

A modulation rate transfer function (MRTF) is a quantitative description of the ability of a system to follow the envelope of a stimulating waveform. In this study MRTFs were obtained using auditory-evoked potentials in three species: false killer whale, Psuedorca; beluga, Delphinapterus; and bottlenose dolphin, Tursiops. MRTFs were obtained from nonsedated animals using two-tone stimuli consisting of frequencies at f1 and f2. Frequency f1 was held constant and f2 was varied such that difference frequencies ( f2−f1=f2,1) ranged from 18 Hz to 5.2 kHz; in all cases f2≳f1. In the study f1’s used were 0.5, 0.8, 1.0, 4.0, and 10.0 kHz. Evoked responses were recorded differentially from the scalp surface using suction cup electrodes. Responses to 350 stimulus presentations were averaged to yield the final response. Averaged responses were Fourier transformed. The magnitude and phase of the Fourier component corresponding to f2,1 was the dependent variable. The magnitude of the response at f2,1 as a function of modulation frequency was generally low-pass in shape with high-frequency corners of approximately 1.1, 1.5, and 2.2 kHz for Psuedorca, Tursiops, and Delphinapterus, respectively. Neither the amplitude nor corner frequency of the response exhibited a dependency on primary frequency over the range tested. The group delay, calculated from the response phase, indicated three distinct latencies: for 0 <50 Hz a delay of ∼18 ms; for 50 <300 Hz a delay of ∼4 ms; for f2,1≳300 Hz a delay of ∼0.5 ms. These latencies presumably indicate different neural generators for the responses to various modulation frequencies. [Work supported by ONR.]