8. Psychoacoustic Studies of Dolphins and Whales
347
These observations may overestimate the minimum audible angle,
limited by the imprecision of the technique used to measure them. Measurement of small angles depends on precise specification of the location
of the animal's head at the time the sound is presented, which is difficult to
assess in a free-swimming animal. A more tightly controlled study with
T truncatus found smaller minimum audible angles. Renaud and Popper
(1975) measured the minimum audible angle of a bottlenose dolphin in
both the horizontal and the vertical planes. The position of the dolphin was
rigidly controlled during presentation of the stimulus by having it bite an
acoustically "transparent" Plexiglas bar. The center line of the apparatus
was marked by a buzzer, which also served as a warning signal. The sound
of the buzzer indicated the start of a trial. The test sound was then presented randomly from either the right or left of the dolphin and the buzzer.
At the termination of the signal, the dolphin was to swim to a response
paddle located on the same side of the dolphin as the tone signal.
Renaud and Popper (1975) used a staircase procedure to determine the
minimum audible angle. A session began with a large angle of at least 6°.
Following two correct responses the angle was reduced by a preset amount.
Following one incorrect response the angle was increased. The step size was
0.5° in some experiments and 0.7° in others.
Pulsed pure tones and clicks were used as stimuli in separate experiments.
The pure tones were presented in the form of 80 sinusoid pulses, each of 3ms duration with a 0.25-ms rise and 0.25-ms decay. The pure tone pulses
had relatively long rise and decay times to prevent the introduction of clicklike transients produced as an artifact of the transducers. All stimuli were
presented at amplitudes at least 40dB above Johnson's (1966) threshold
levels.
The minimum audible angle was found to be a roughly U-shaped function of frequency. The minimum audible angle was smallest with 20 kHz
signals (2.1°). The minimum angle increased at lower and higher frequencies with a possible secondary dip at 50 kHz. The minimum audible angle
was much lower when stimuli simulating dolphin echolocation clicks were
used. The signal consisted of a click train of 166 35-J..Ls clicks presented every
3ms for a total duration of 498ms. Each click had a center frequency of
64.35 kHz. Using these click trains, the minimum audible angle was found
to be 0.7° in the horizontal plane.
The ability of the dolphin to discriminate audible angles in the vertical
plane was also studied by having the dolphin position itself on its side biting
a vertically oriented bite plate. Stimuli presented on the left and right sides
of the pen, were now positioned "above" and "below" the dolphin's head.
The dolphin's ability to localize the tone pulses and clicks in the vertical
plane (relative to its body orientation) was about as good as its ability to
localize them in the horizontal plane. As in the horizontal localization task,
the minimum audible angle for click stimuli (0.9°) was substantially below
that for tones.
347
These observations may overestimate the minimum audible angle,
limited by the imprecision of the technique used to measure them. Measurement of small angles depends on precise specification of the location
of the animal's head at the time the sound is presented, which is difficult to
assess in a free-swimming animal. A more tightly controlled study with
T truncatus found smaller minimum audible angles. Renaud and Popper
(1975) measured the minimum audible angle of a bottlenose dolphin in
both the horizontal and the vertical planes. The position of the dolphin was
rigidly controlled during presentation of the stimulus by having it bite an
acoustically "transparent" Plexiglas bar. The center line of the apparatus
was marked by a buzzer, which also served as a warning signal. The sound
of the buzzer indicated the start of a trial. The test sound was then presented randomly from either the right or left of the dolphin and the buzzer.
At the termination of the signal, the dolphin was to swim to a response
paddle located on the same side of the dolphin as the tone signal.
Renaud and Popper (1975) used a staircase procedure to determine the
minimum audible angle. A session began with a large angle of at least 6°.
Following two correct responses the angle was reduced by a preset amount.
Following one incorrect response the angle was increased. The step size was
0.5° in some experiments and 0.7° in others.
Pulsed pure tones and clicks were used as stimuli in separate experiments.
The pure tones were presented in the form of 80 sinusoid pulses, each of 3ms duration with a 0.25-ms rise and 0.25-ms decay. The pure tone pulses
had relatively long rise and decay times to prevent the introduction of clicklike transients produced as an artifact of the transducers. All stimuli were
presented at amplitudes at least 40dB above Johnson's (1966) threshold
levels.
The minimum audible angle was found to be a roughly U-shaped function of frequency. The minimum audible angle was smallest with 20 kHz
signals (2.1°). The minimum angle increased at lower and higher frequencies with a possible secondary dip at 50 kHz. The minimum audible angle
was much lower when stimuli simulating dolphin echolocation clicks were
used. The signal consisted of a click train of 166 35-J..Ls clicks presented every
3ms for a total duration of 498ms. Each click had a center frequency of
64.35 kHz. Using these click trains, the minimum audible angle was found
to be 0.7° in the horizontal plane.
The ability of the dolphin to discriminate audible angles in the vertical
plane was also studied by having the dolphin position itself on its side biting
a vertically oriented bite plate. Stimuli presented on the left and right sides
of the pen, were now positioned "above" and "below" the dolphin's head.
The dolphin's ability to localize the tone pulses and clicks in the vertical
plane (relative to its body orientation) was about as good as its ability to
localize them in the horizontal plane. As in the horizontal localization task,
the minimum audible angle for click stimuli (0.9°) was substantially below
that for tones.