William F. Dolphin
Dept. of Biomed. Eng., Boston Univ., Boston, MA 02215
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 f[sub 1] and f[sub 2]. Frequency f[sub 1]
was held constant and f[sub 2] was varied such that difference frequencies (
f[sub 2]-f[sub 1]=f[sub 2,1]) ranged from 18 Hz to 5.2 kHz; in all cases f[sub
2]>f[sub 1]. In the study f[sub 1]'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 f[sub 2,1] was
the dependent variable. The magnitude of the response at f[sub 2,1] 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