392
w.w.L. Au
All of the targets used were examined acoustically in a test pool using
simulated dolphin sonar signals. From a visual inspection of the echo structures we can see that all of the targets have different arrival times for the
secondary echo components and, therefore, different echo structures. We
suggested that the predominant cue used by the dolphin in discriminating
the various cylinders was probably time separation pitch (TSP) generated
by the presence of the first and second echo components or highlights.
Arrival time differences in the highlights may be perceived as TSP, especially if the echo components are highly correlated. Humans, when presented with a correlated pair of sound pulses, perceive a pitch that is equal
to liT, where T is the separation time between pulses (Small and McClellan 1963; McClellan and Small 1965). Signals with ripples in the frequency
domain or highlights in the time domain can produce TSP in the human
auditory system.
The glass and aluminum cylinders described in Table 9.1 were next used
by Schusterman et al. (1980) in a two-alternative forced-choice discrimination experiment involving the same T. truncatus used in the Hammer and
Au (1980) study. Targets were located at a range of 6m from the front of
the experimental pen and were submerged in a vertical orientation at a
depth of 1 m. A trial consisted of having either a glass or aluminum cylinder presented and the animal was required to perform a sonar scan and
respond to paddle A if it thought that the target was either of the two aluminum cylinders or paddle B for the glass cylinders. After 30 sessions, the
dolphin could perfectly discriminate between the small (3.61-cm outer
diameter) aluminum and glass cylinders. However, the animal was never
able to discriminate between the large (7.62-cm outer diameter) aluminum
and glass cylinders.
Au and Turl (1991) performed a material composition experiment
with cylinders to test if there existed aspect-independent cues that
the dolphin could utilize to discriminate between cylinders. An aluminum cylinder with a 7.62cm outer diameter, 0.40cm wall thickness, and
17.1 cm length was used as the standard target. Two comparison targets,
a hollow stainless steel cylinder having the same dimensions as the standard and a coral rock encapsulated in degassed epoxy cylinder were used.
The standard and a comparison target were supported on the rotor
bar with their longitudinal axis parallel to the bar and the horizon. The
dolphin was required to swim into a hoop, begin echolocating when
the trainer presented a tone, and respond to a specific paddle if the
target was the aluminum standard and another paddle for the comparison
targets.
The dolphin was trained to discriminate between two targets at the
baseline aspect of 0°, 45°, and 90°, where 0° was the broadside aspect
(longitudinal axis perpendicular to the animal). After the dolphin performed tIle discrimination at a level close to 90% correct for the baseline
aspects, probe trials at novel aspect angles were introduced and the animal's
w.w.L. Au
All of the targets used were examined acoustically in a test pool using
simulated dolphin sonar signals. From a visual inspection of the echo structures we can see that all of the targets have different arrival times for the
secondary echo components and, therefore, different echo structures. We
suggested that the predominant cue used by the dolphin in discriminating
the various cylinders was probably time separation pitch (TSP) generated
by the presence of the first and second echo components or highlights.
Arrival time differences in the highlights may be perceived as TSP, especially if the echo components are highly correlated. Humans, when presented with a correlated pair of sound pulses, perceive a pitch that is equal
to liT, where T is the separation time between pulses (Small and McClellan 1963; McClellan and Small 1965). Signals with ripples in the frequency
domain or highlights in the time domain can produce TSP in the human
auditory system.
The glass and aluminum cylinders described in Table 9.1 were next used
by Schusterman et al. (1980) in a two-alternative forced-choice discrimination experiment involving the same T. truncatus used in the Hammer and
Au (1980) study. Targets were located at a range of 6m from the front of
the experimental pen and were submerged in a vertical orientation at a
depth of 1 m. A trial consisted of having either a glass or aluminum cylinder presented and the animal was required to perform a sonar scan and
respond to paddle A if it thought that the target was either of the two aluminum cylinders or paddle B for the glass cylinders. After 30 sessions, the
dolphin could perfectly discriminate between the small (3.61-cm outer
diameter) aluminum and glass cylinders. However, the animal was never
able to discriminate between the large (7.62-cm outer diameter) aluminum
and glass cylinders.
Au and Turl (1991) performed a material composition experiment
with cylinders to test if there existed aspect-independent cues that
the dolphin could utilize to discriminate between cylinders. An aluminum cylinder with a 7.62cm outer diameter, 0.40cm wall thickness, and
17.1 cm length was used as the standard target. Two comparison targets,
a hollow stainless steel cylinder having the same dimensions as the standard and a coral rock encapsulated in degassed epoxy cylinder were used.
The standard and a comparison target were supported on the rotor
bar with their longitudinal axis parallel to the bar and the horizon. The
dolphin was required to swim into a hoop, begin echolocating when
the trainer presented a tone, and respond to a specific paddle if the
target was the aluminum standard and another paddle for the comparison
targets.
The dolphin was trained to discriminate between two targets at the
baseline aspect of 0°, 45°, and 90°, where 0° was the broadside aspect
(longitudinal axis perpendicular to the animal). After the dolphin performed tIle discrimination at a level close to 90% correct for the baseline
aspects, probe trials at novel aspect angles were introduced and the animal's