306
W.E Dolphin
several species of cetaceans. The stimuli used in the experiments consisted
of noise bursts that varied from 20 Ils to 5 ms in duration. The rise time of
the noise burst was 20lls and the fall time was 50 to lOOlls. The minimum
duration of the noise bursts was limited by the equipment in that it was not
possible to produce signals less than 20 Ils in duration at a steady sound
pressure.
In these experiments the duration and intensity of the noise burst stimulus was varied and the amplitude of the resulting ABR was measured. At
high stimulus intensities the response had maximum amplitude even for the
shortest stimuli (20 to 50 Ils). At low stimulus intensities near threshold
values the response amplitude increased with increasing stimulus duration
up to approximately 0.5 ms; hence, this is the duration of temporal summation. Further increase in the duration of the noise burst resulted in no
further increase in the response amplitude. As shown in Figure 7.7a, the
more intense the stimulus, the shorter the time required to reach the summation point.
These same data are presented differently in Figure 7.7b. When ABR
amplitude is plotted as a function of stimulus duration over a range of stimulus intensities it is apparent that temporal summation is influenced by
stimulus intensity. The duration of the stimulus needed to elicit a response
is dependent on the stimulus intensity. Thus, for low-level stimuli of intensities close to threshold, a stimulus duration of approximately 0.5 ms was
required to elicit a response, whereas stimuli at approximately 6dB above
threshold required only a 50 Ils stimulus, and stimuli at 8 dB re threshold
required less than 20llS stimulus duration.
3.1.2.1 Temporal Resolution
Temporal resolution involves the ability of the system under study to
resolve or respond to as an isolated event a single acoustic event out of a
temporal series of such events. That is, if a stimulus such as a click is resolvable, each individual click in a series of clicks (i.e., a click train) may be
"heard out." The opposite of stimulus resolution is stimulus fusion, for
example, in a click train individual clicks are so closely spaced that each
separate click cannot be identified and the signal takes on a "buzzing" or
even a rough tonal characteristic instead of sounding like a series of individual clicks. Temporal resolution may be described in terms of temporal
integration time in that it describes the ability of the processor to isolate
individual acoustic events; longer temporal integration times correspond to
decreased temporal resolution.
From psychophysical experiments involving echolocation tasks (e.g., Au
et al. 1988) temporal integration times of 200 to 300 Ils have been found for
T. truncatus. Similarly, Dubrovskiy (1990) found, also in T. truncatus, that a
fusion of individual clicks occurred when interpulse intervals were less than
200 to 300lls. Supplying further agreement, backward masking experiments
also found similar 200 to 300llS integration times.
W.E Dolphin
several species of cetaceans. The stimuli used in the experiments consisted
of noise bursts that varied from 20 Ils to 5 ms in duration. The rise time of
the noise burst was 20lls and the fall time was 50 to lOOlls. The minimum
duration of the noise bursts was limited by the equipment in that it was not
possible to produce signals less than 20 Ils in duration at a steady sound
pressure.
In these experiments the duration and intensity of the noise burst stimulus was varied and the amplitude of the resulting ABR was measured. At
high stimulus intensities the response had maximum amplitude even for the
shortest stimuli (20 to 50 Ils). At low stimulus intensities near threshold
values the response amplitude increased with increasing stimulus duration
up to approximately 0.5 ms; hence, this is the duration of temporal summation. Further increase in the duration of the noise burst resulted in no
further increase in the response amplitude. As shown in Figure 7.7a, the
more intense the stimulus, the shorter the time required to reach the summation point.
These same data are presented differently in Figure 7.7b. When ABR
amplitude is plotted as a function of stimulus duration over a range of stimulus intensities it is apparent that temporal summation is influenced by
stimulus intensity. The duration of the stimulus needed to elicit a response
is dependent on the stimulus intensity. Thus, for low-level stimuli of intensities close to threshold, a stimulus duration of approximately 0.5 ms was
required to elicit a response, whereas stimuli at approximately 6dB above
threshold required only a 50 Ils stimulus, and stimuli at 8 dB re threshold
required less than 20llS stimulus duration.
3.1.2.1 Temporal Resolution
Temporal resolution involves the ability of the system under study to
resolve or respond to as an isolated event a single acoustic event out of a
temporal series of such events. That is, if a stimulus such as a click is resolvable, each individual click in a series of clicks (i.e., a click train) may be
"heard out." The opposite of stimulus resolution is stimulus fusion, for
example, in a click train individual clicks are so closely spaced that each
separate click cannot be identified and the signal takes on a "buzzing" or
even a rough tonal characteristic instead of sounding like a series of individual clicks. Temporal resolution may be described in terms of temporal
integration time in that it describes the ability of the processor to isolate
individual acoustic events; longer temporal integration times correspond to
decreased temporal resolution.
From psychophysical experiments involving echolocation tasks (e.g., Au
et al. 1988) temporal integration times of 200 to 300 Ils have been found for
T. truncatus. Similarly, Dubrovskiy (1990) found, also in T. truncatus, that a
fusion of individual clicks occurred when interpulse intervals were less than
200 to 300lls. Supplying further agreement, backward masking experiments
also found similar 200 to 300llS integration times.