7. Electrophysiological Techniques
295
and based on totally objective acceptance criteria, and (4) tests are noninvasive and therefore amenable to examination of protected species.
Auditory evoked potentials have now been obtained from a wide range
of species spanning several orders. The availability of such data greatly
facilitates comparative studies of hearing and auditory function. The
cetaceans-at least the small whales that have been tested to date-have
proven to be excellent subjects for the use of evoked response audiometry
techniques. From a practical perspective, cetaceans make excellent subjects
for the recording of AEPs for a number of reasons: (1) due to the considerable hypertrophy of the cetacean auditory system, relatively large potentials are measurable from the scalp surface which, in turn, minimizes the
amount of averaging and therefore total test time required to obtain a reliable response; (2) lack of body hair allows low impedance electrode-skin
contact thereby minimizing noise contamination of the signal and so speeding response acquisition; and (3) cetaceans are easily trained, making possible acquisition of large amounts of data in relatively brief testing sessions
from conscious, behaving subjects which, very importantly, allows for the
comparison of AEP results with analogous psychophysical results in the
same animal. More importantly, as is clear from this and other chapters in
this volume, the cetaceans are extraordinary hearers. The phenomenal auditory capabilities of these animals alone makes them worthy of study. By
examining these highly specialized animals we may learn a great deal about
the mechanisms and computational algorithms employed in auditory processing in mammals; much of this knowledge may have direct human clinical applications as well as application in human engineered sonar systems.
2. Auditory Evoked Potentials as Electrophysiological
Measures of Auditory Function
2.1 Evoked Responses to Transient Stimuli: The Auditory
Brain Stem Response
When presented with a suprathreshold acoustic stimulus, cells within the
auditory pathway of a listener are excited. This excitation, which spreads
from the peripheral to more central auditory structures, results in the
discharge of large numbers of neurons within the pathway. If this neural
activity is time-locked to an acoustic stimulus, the synchronized discharge
of large neuronal assemblies produces electrical deflections that can be
recorded from the far-field from an animal's scalp surface. These perturbations in the ongoing electroencephalogram (EEG), synchronized to an
acoustic stimulus, have been termed auditory evoked potentials and reflect
the massed activity patterns of populations of neurons within the auditory
pathway. AEPs can provide a "window" into the mechanisms underlying
auditory processing.
295
and based on totally objective acceptance criteria, and (4) tests are noninvasive and therefore amenable to examination of protected species.
Auditory evoked potentials have now been obtained from a wide range
of species spanning several orders. The availability of such data greatly
facilitates comparative studies of hearing and auditory function. The
cetaceans-at least the small whales that have been tested to date-have
proven to be excellent subjects for the use of evoked response audiometry
techniques. From a practical perspective, cetaceans make excellent subjects
for the recording of AEPs for a number of reasons: (1) due to the considerable hypertrophy of the cetacean auditory system, relatively large potentials are measurable from the scalp surface which, in turn, minimizes the
amount of averaging and therefore total test time required to obtain a reliable response; (2) lack of body hair allows low impedance electrode-skin
contact thereby minimizing noise contamination of the signal and so speeding response acquisition; and (3) cetaceans are easily trained, making possible acquisition of large amounts of data in relatively brief testing sessions
from conscious, behaving subjects which, very importantly, allows for the
comparison of AEP results with analogous psychophysical results in the
same animal. More importantly, as is clear from this and other chapters in
this volume, the cetaceans are extraordinary hearers. The phenomenal auditory capabilities of these animals alone makes them worthy of study. By
examining these highly specialized animals we may learn a great deal about
the mechanisms and computational algorithms employed in auditory processing in mammals; much of this knowledge may have direct human clinical applications as well as application in human engineered sonar systems.
2. Auditory Evoked Potentials as Electrophysiological
Measures of Auditory Function
2.1 Evoked Responses to Transient Stimuli: The Auditory
Brain Stem Response
When presented with a suprathreshold acoustic stimulus, cells within the
auditory pathway of a listener are excited. This excitation, which spreads
from the peripheral to more central auditory structures, results in the
discharge of large numbers of neurons within the pathway. If this neural
activity is time-locked to an acoustic stimulus, the synchronized discharge
of large neuronal assemblies produces electrical deflections that can be
recorded from the far-field from an animal's scalp surface. These perturbations in the ongoing electroencephalogram (EEG), synchronized to an
acoustic stimulus, have been termed auditory evoked potentials and reflect
the massed activity patterns of populations of neurons within the auditory
pathway. AEPs can provide a "window" into the mechanisms underlying
auditory processing.