7. Electrophysiological Techniques
311
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10 1 2 3 4
10 2 2 3 4
10 3 2 3 4
Modulation Frequency (Hz)
FIGURE 7.10. Modulation rate transfer functions from three cetacean species (DI,
Delphinapterus lecuas; Pc, Pseudorca crassidens; Tt, Tursiops truncatus), humans
(Hs, Homo sapiens), and Mongolian gerbils (Mu, Meriones unguiculatus). Gerbils'
responses are scaled against the right abscissa. Response magnitude is the magnitude of the Fourier component corresponding to the stimulus envelope frequency.
(Cetacean data from Dolphin et al. 1995; gerbil data from Dolphin et al. 1994.)
plotted in Figure 7.1O. In all cases the transfer functions are roughly lowpass
in shape. In humans, MTFs obtained using the EFR have corner frequencies (i.e., the -3dB corner) of 50 to 70Hz, which shows close agreement to
the psychophysically obtained cutoff frequencies (e.g., Viemeister 1979;
Eddins 1993). In contrast, all cetaceans tested to date have had corner
frequencies between 1.7 and 2.5 kHz. In many instances, good responses
(i.e., response magnitude greater than 3dB above the EEG noise floor)
were still obtainable at modulation frequencies close to 4 kHz. These
responses are an order of magnitude or more higher than human or gerbil
cutoff frequencies.
Supin and Popov (1995c) reported results of a similar study to that of
Dolphin and colleagues using sinusoidally amplitude-modified signals,
again with T truncatus. In most regards, the results of the two studies
showed good agreement, however, in one respect significant differences
were reported. In contrast to the low-pass characteristics of the MTFs
reported by Dolphin's group, Supin and Popov obtained MTFs that were
distinctly bandpass in shape, with little energy in the response for modulation frequencies below 250 Hz. The reasons for these differences are
311
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Q)
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--e-- Hs
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Q)
----+-- Mu
a.
a:::
10
(/)
Q)
0
a:::
10 1 2 3 4
10 2 2 3 4
10 3 2 3 4
Modulation Frequency (Hz)
FIGURE 7.10. Modulation rate transfer functions from three cetacean species (DI,
Delphinapterus lecuas; Pc, Pseudorca crassidens; Tt, Tursiops truncatus), humans
(Hs, Homo sapiens), and Mongolian gerbils (Mu, Meriones unguiculatus). Gerbils'
responses are scaled against the right abscissa. Response magnitude is the magnitude of the Fourier component corresponding to the stimulus envelope frequency.
(Cetacean data from Dolphin et al. 1995; gerbil data from Dolphin et al. 1994.)
plotted in Figure 7.1O. In all cases the transfer functions are roughly lowpass
in shape. In humans, MTFs obtained using the EFR have corner frequencies (i.e., the -3dB corner) of 50 to 70Hz, which shows close agreement to
the psychophysically obtained cutoff frequencies (e.g., Viemeister 1979;
Eddins 1993). In contrast, all cetaceans tested to date have had corner
frequencies between 1.7 and 2.5 kHz. In many instances, good responses
(i.e., response magnitude greater than 3dB above the EEG noise floor)
were still obtainable at modulation frequencies close to 4 kHz. These
responses are an order of magnitude or more higher than human or gerbil
cutoff frequencies.
Supin and Popov (1995c) reported results of a similar study to that of
Dolphin and colleagues using sinusoidally amplitude-modified signals,
again with T truncatus. In most regards, the results of the two studies
showed good agreement, however, in one respect significant differences
were reported. In contrast to the low-pass characteristics of the MTFs
reported by Dolphin's group, Supin and Popov obtained MTFs that were
distinctly bandpass in shape, with little energy in the response for modulation frequencies below 250 Hz. The reasons for these differences are