260
Marine Mammal Physiology: Requisites for Ocean Living
ability to detect the sound. Procedures involve presenting a sound to an animal that is
trained to station itself in a precise location within a calibrated sound field, or in the
case of some pinnipeds, to wear headphones. If the animal hears the tone, it provides a
response, such as producing a sound (e.g., whistle) or performing a behavior (e.g., touching a paddle). If no sound is heard, the animal either provides a different response (e.g.,
touches a different paddle) or remains quiet. In staircase procedures, designed to determine hearing thresholds, correct responses to sound detection result in the sound level,
the physical quantity under study, being reduced before it is presented again. If sounds
are presented, but are not detected, then sound levels are typically increased. Through
this titration process, the minimum level of sound that is detectable, which is the threshold of detection, can be determined. If this process is performed across a broad range of
frequencies, at least enough to cover the range of hearing, then a curve defining hearing sensitivity as a function of frequency can be derived. This is the audiogram, which
provides the most fundamental piece of sensory information necessary to begin understanding marine mammal auditory perception and ecology, as well as the potential for
anthropogenic sound to affect marine mammals.
There are many standardized psychoacoustic procedures that can be performed
and a detailed discussion of the psychophysical procedures used in marine mammal
research is beyond the scope of this chapter. Nevertheless, it is important to be aware
that the use of these procedures is critical to exploring both sensory capability and
inferring physiological function. Examples of auditory system information obtained
with psychophysical methods include: quantification of hearing thresholds; interaural
time and intensity differences (Moore et al. 1995); angular discrimination (Branstetter
et al. 2003); minimum detectable frequency and amplitude differences (Herman and
Arbeit 1972); auditory filter shape, critical ratios, and critical bands (Finneran et al. 2002;
Lemonds et al. 2012); temporal auditory summation (Terhune 1988; Kastelein et al. 2010;
Holt et al. 2012); receiving beam patterns (i.e., directionality of hearing; Au and Moore
1984); auditory masking (Branstetter et al. 2013); and others (e.g., see references from the
Order Carnivora section).
Electrophysiological studies of marine mammal hearing dates back to the 1960s,
when it was performed by a relatively small group of researchers (see Supin et al. 2001).
However, with modern advances in computational capabilities and the miniaturization
of technological components utilized for electrophysiological research, the ability to perform electrophysiological research with marine mammals has become more common
both in the laboratory and in the field. Unlike psychophysical approaches to studying
sensory capabilities, electrophysiological approaches to studying hearing monitor the
neural responses of the auditory system to acoustic stimuli, either as the voltages generated by single units or as a conglomerate (summed) response. The presence and magnitude of neural responses can correlate with some psychophysical functions; indeed, one
of the most common electrophysiological procedures performed in studying hearing
sensitivity in marine mammals is the recording of the auditory evoked potential (AEP).
Auditory evoked potentials are voltages generated by the brain in response to the detection of an acoustic stimulus, and the threshold of acoustic stimulation required to produce a measureable response has been shown to correlate with hearing sensitivity. The
use of AEP methods to study hearing has greatly advanced the number of marine mammal subjects for which hearing tests have been performed (Figure 11.5) (Houser and
Finneran 2006), and has permitted a number of species not maintained under human
care to be tested when stranded or when rehabilitating (Nachtigall et al. 2005; Cook et al.
2006; Finneran et al. 2009).
Marine Mammal Physiology: Requisites for Ocean Living
ability to detect the sound. Procedures involve presenting a sound to an animal that is
trained to station itself in a precise location within a calibrated sound field, or in the
case of some pinnipeds, to wear headphones. If the animal hears the tone, it provides a
response, such as producing a sound (e.g., whistle) or performing a behavior (e.g., touching a paddle). If no sound is heard, the animal either provides a different response (e.g.,
touches a different paddle) or remains quiet. In staircase procedures, designed to determine hearing thresholds, correct responses to sound detection result in the sound level,
the physical quantity under study, being reduced before it is presented again. If sounds
are presented, but are not detected, then sound levels are typically increased. Through
this titration process, the minimum level of sound that is detectable, which is the threshold of detection, can be determined. If this process is performed across a broad range of
frequencies, at least enough to cover the range of hearing, then a curve defining hearing sensitivity as a function of frequency can be derived. This is the audiogram, which
provides the most fundamental piece of sensory information necessary to begin understanding marine mammal auditory perception and ecology, as well as the potential for
anthropogenic sound to affect marine mammals.
There are many standardized psychoacoustic procedures that can be performed
and a detailed discussion of the psychophysical procedures used in marine mammal
research is beyond the scope of this chapter. Nevertheless, it is important to be aware
that the use of these procedures is critical to exploring both sensory capability and
inferring physiological function. Examples of auditory system information obtained
with psychophysical methods include: quantification of hearing thresholds; interaural
time and intensity differences (Moore et al. 1995); angular discrimination (Branstetter
et al. 2003); minimum detectable frequency and amplitude differences (Herman and
Arbeit 1972); auditory filter shape, critical ratios, and critical bands (Finneran et al. 2002;
Lemonds et al. 2012); temporal auditory summation (Terhune 1988; Kastelein et al. 2010;
Holt et al. 2012); receiving beam patterns (i.e., directionality of hearing; Au and Moore
1984); auditory masking (Branstetter et al. 2013); and others (e.g., see references from the
Order Carnivora section).
Electrophysiological studies of marine mammal hearing dates back to the 1960s,
when it was performed by a relatively small group of researchers (see Supin et al. 2001).
However, with modern advances in computational capabilities and the miniaturization
of technological components utilized for electrophysiological research, the ability to perform electrophysiological research with marine mammals has become more common
both in the laboratory and in the field. Unlike psychophysical approaches to studying
sensory capabilities, electrophysiological approaches to studying hearing monitor the
neural responses of the auditory system to acoustic stimuli, either as the voltages generated by single units or as a conglomerate (summed) response. The presence and magnitude of neural responses can correlate with some psychophysical functions; indeed, one
of the most common electrophysiological procedures performed in studying hearing
sensitivity in marine mammals is the recording of the auditory evoked potential (AEP).
Auditory evoked potentials are voltages generated by the brain in response to the detection of an acoustic stimulus, and the threshold of acoustic stimulation required to produce a measureable response has been shown to correlate with hearing sensitivity. The
use of AEP methods to study hearing has greatly advanced the number of marine mammal subjects for which hearing tests have been performed (Figure 11.5) (Houser and
Finneran 2006), and has permitted a number of species not maintained under human
care to be tested when stranded or when rehabilitating (Nachtigall et al. 2005; Cook et al.
2006; Finneran et al. 2009).