320
W.E Dolphin
is thus a plot of the auditory threshold over a range of frequencies. Techniques for obtaining behavioral audiograms from a number of cetacean
species have been discussed by Moore (1997).
Behavioral audiograms, obtained for cetaceans as well as other nonhuman animals, are extremely time consuming, often requiring many
months of trainer and animal time to obtain. The use of electrophysiological techniques offers a potentially much more rapid and objective approach
to acquisition of audiograms. When ABR testing is performed to estimate
hearing status, click or tone burst stimuli are presented at different
intensity levels to determine which levels elicit detectable responses (e.g.,
Popov and Supin 1990a). As in the tone-on-tone masking experiments in
the determination of frequency tuning curves described previously, the
presence or absence of a response is based on visual inspection of ABR
waveforms.
Audiograms obtained using tone burst ABRs have shown generally
good agreement with behaviorally obtained audiograms for the higher frequencies. It must be kept in mind that the ABR is an onset response
that is elicited by the leading edge of the acoustic signal. An abrupt signal
onset results in a high degree of synchrony in the discharge of neural fibers,
which, in turn, results in a more clearly defined ABR. Abrupt onset acoustic
clicks are for this reason the most widely used stimulus for acquisition of
an ABR. However, a rapid stimulus rise time results in a broad stimulus
spectrum and, therefore, a loss of frequency specificity, particularly for
the lower-frequency signals. For this reason, tone burst stimuli are usually
restricted to the higher frequencies. In order to insure a tonal stimulus
(i.e., minimization of spectral splatter) several cycles of the stimulus must
be presented. Consequently, for frequencies below a few kilohertz the
stimuli must be long, often several milliseconds in duration, and hence there
is a reduced synchrony of neural discharge as compared to that elicited
by a click stimulus. The result is that poorly defined ABR waveforms
are obtained from tone burst stimuli, even at relatively high intensity
levels.
Although several alternative ABR techniques have been attempted to
ensure the frequency specificity of the test stimulus while maximizing the
likelihood of a response at low intensities (including simultaneous highpass
and notched noise masking (e.g., Stapells et al. 1990)), none have been
particularly successful. However, another class of AEPs, namely the EFR
described in Section 3.1.2.4. offer considerable promise in this area. The presentation of long duration tonal stimuli minimizes spectral splatter. As was
demonstrated in Figure 7.10, amplitude-modulated signals are very frequency specific, having energy only at the stimulus frequencies. Like the
ABR, the EFR reflects the synchronized discharge of auditory neurons;
however, instead of exciting neurons across a broad frequency range as
with a transient signal, only those neurons tuned to the narrow stimulus
frequency band are excited. The discharge of neurons within the stimulus
W.E Dolphin
is thus a plot of the auditory threshold over a range of frequencies. Techniques for obtaining behavioral audiograms from a number of cetacean
species have been discussed by Moore (1997).
Behavioral audiograms, obtained for cetaceans as well as other nonhuman animals, are extremely time consuming, often requiring many
months of trainer and animal time to obtain. The use of electrophysiological techniques offers a potentially much more rapid and objective approach
to acquisition of audiograms. When ABR testing is performed to estimate
hearing status, click or tone burst stimuli are presented at different
intensity levels to determine which levels elicit detectable responses (e.g.,
Popov and Supin 1990a). As in the tone-on-tone masking experiments in
the determination of frequency tuning curves described previously, the
presence or absence of a response is based on visual inspection of ABR
waveforms.
Audiograms obtained using tone burst ABRs have shown generally
good agreement with behaviorally obtained audiograms for the higher frequencies. It must be kept in mind that the ABR is an onset response
that is elicited by the leading edge of the acoustic signal. An abrupt signal
onset results in a high degree of synchrony in the discharge of neural fibers,
which, in turn, results in a more clearly defined ABR. Abrupt onset acoustic
clicks are for this reason the most widely used stimulus for acquisition of
an ABR. However, a rapid stimulus rise time results in a broad stimulus
spectrum and, therefore, a loss of frequency specificity, particularly for
the lower-frequency signals. For this reason, tone burst stimuli are usually
restricted to the higher frequencies. In order to insure a tonal stimulus
(i.e., minimization of spectral splatter) several cycles of the stimulus must
be presented. Consequently, for frequencies below a few kilohertz the
stimuli must be long, often several milliseconds in duration, and hence there
is a reduced synchrony of neural discharge as compared to that elicited
by a click stimulus. The result is that poorly defined ABR waveforms
are obtained from tone burst stimuli, even at relatively high intensity
levels.
Although several alternative ABR techniques have been attempted to
ensure the frequency specificity of the test stimulus while maximizing the
likelihood of a response at low intensities (including simultaneous highpass
and notched noise masking (e.g., Stapells et al. 1990)), none have been
particularly successful. However, another class of AEPs, namely the EFR
described in Section 3.1.2.4. offer considerable promise in this area. The presentation of long duration tonal stimuli minimizes spectral splatter. As was
demonstrated in Figure 7.10, amplitude-modulated signals are very frequency specific, having energy only at the stimulus frequencies. Like the
ABR, the EFR reflects the synchronized discharge of auditory neurons;
however, instead of exciting neurons across a broad frequency range as
with a transient signal, only those neurons tuned to the narrow stimulus
frequency band are excited. The discharge of neurons within the stimulus