1. Overview
33
centers of the auditory system (Wever et al. 1972). The eighth nerve in dolphins has many more fibers than humans and the fibers have diameters that
are about twice that of humans (Ridgway, Chapter 6). Discussion of the
auditory pathways and nuclei between the ear and higher centers of the
brain is presented by Ridgway (Chapter 6).
There are a variety of electrophysiological techniques that can be used
with animals. Some of these techniques are extremely invasive and would
eventually lead to the demise of the subject while other techniques are
totally noninvasive. In the United States, dolphins and whales are protected
by the Marine Mammal Protection Act and the Animal Welfare Act and
are therefore protected from extremely invasive techniques. Bullock et al.
(1968) were the first to use electrophysiology to examine the auidtory processing of acoustic signals by dolphins. They inserted electrodes into the
brain stem of their expiring subjects and measured the evoked potential
produced by various types of acoustic stimuli. The work of Bullock et al.
(1968) is a landmark that deserves serious and careful examination.
Auditory evoked potentials are classified into different categories
depending on the delay or latency between the presentation of the acoustic
stimulus and the occurrence of the evoked potential. Three different latency
regimes are generally used to categorize AEP, short-latency brain stem AEP
occurring within lOms, mid-latency AEP occurring between 10 and 50ms,
and long-latency AEP occurring more than 50ms after the onset of a brief
acoustic stimulus (Dolphin, Chapter 7). The short-latency AEP are referred
to as auditory brain stem responses or ABR, and are caused by the onset
or beginning portion of a brief acoustic signal. The mid-latency AEP is
thought to originate from the auditory cortex and is referred to as the auditory cortical response (ACR) by Supin and Popov (1990).
The dolphin ABRs are larger than most mammals and are about 10 times
larger than for humans (Ridgway 1983). They are relatively easy to measure
with a surface electrode placed about 6cm posterior of the blow hole (Supin
and Popov 1990) and is probably the most popular electrophysiological
technique used with dolphins. Measuring the relative difference in the ABR
of subjects, as the properties of the acoustic stimuli are manipulated, can be
very effective in studying the auditory system of dolphin. Dr. Alexander
Supin and his colleagues at the Severtsov Institute of Evolutionary Morphology in Moscow, Russia, have been at the forefront in the use of ABR
to study the auditory processes in dolphins. Some of the auditory processes
they have studied include frequency selectivity, interaural time delay,
interaural intensity difference, location of the auditory window, and hearing
sensitivity.
The ABR technique to measure auditory thresholds has been used by
many investigators (Dolphin, Chapter 7), however, extreme precaution
must be taken when interpreting the results and comparing with behaviorally obtained auditory thresholds. Behavioral auditory thresholds are
typically measured in terms of the root-mean-square (rms) acoustic pres-
33
centers of the auditory system (Wever et al. 1972). The eighth nerve in dolphins has many more fibers than humans and the fibers have diameters that
are about twice that of humans (Ridgway, Chapter 6). Discussion of the
auditory pathways and nuclei between the ear and higher centers of the
brain is presented by Ridgway (Chapter 6).
There are a variety of electrophysiological techniques that can be used
with animals. Some of these techniques are extremely invasive and would
eventually lead to the demise of the subject while other techniques are
totally noninvasive. In the United States, dolphins and whales are protected
by the Marine Mammal Protection Act and the Animal Welfare Act and
are therefore protected from extremely invasive techniques. Bullock et al.
(1968) were the first to use electrophysiology to examine the auidtory processing of acoustic signals by dolphins. They inserted electrodes into the
brain stem of their expiring subjects and measured the evoked potential
produced by various types of acoustic stimuli. The work of Bullock et al.
(1968) is a landmark that deserves serious and careful examination.
Auditory evoked potentials are classified into different categories
depending on the delay or latency between the presentation of the acoustic
stimulus and the occurrence of the evoked potential. Three different latency
regimes are generally used to categorize AEP, short-latency brain stem AEP
occurring within lOms, mid-latency AEP occurring between 10 and 50ms,
and long-latency AEP occurring more than 50ms after the onset of a brief
acoustic stimulus (Dolphin, Chapter 7). The short-latency AEP are referred
to as auditory brain stem responses or ABR, and are caused by the onset
or beginning portion of a brief acoustic signal. The mid-latency AEP is
thought to originate from the auditory cortex and is referred to as the auditory cortical response (ACR) by Supin and Popov (1990).
The dolphin ABRs are larger than most mammals and are about 10 times
larger than for humans (Ridgway 1983). They are relatively easy to measure
with a surface electrode placed about 6cm posterior of the blow hole (Supin
and Popov 1990) and is probably the most popular electrophysiological
technique used with dolphins. Measuring the relative difference in the ABR
of subjects, as the properties of the acoustic stimuli are manipulated, can be
very effective in studying the auditory system of dolphin. Dr. Alexander
Supin and his colleagues at the Severtsov Institute of Evolutionary Morphology in Moscow, Russia, have been at the forefront in the use of ABR
to study the auditory processes in dolphins. Some of the auditory processes
they have studied include frequency selectivity, interaural time delay,
interaural intensity difference, location of the auditory window, and hearing
sensitivity.
The ABR technique to measure auditory thresholds has been used by
many investigators (Dolphin, Chapter 7), however, extreme precaution
must be taken when interpreting the results and comparing with behaviorally obtained auditory thresholds. Behavioral auditory thresholds are
typically measured in terms of the root-mean-square (rms) acoustic pres-