324
W.E Dolphin
insensitivity to stimulus intensity. Unlike the effects seen in humans, gerbils,
cats, and other mammals, changes in stimulus intensity results in very little
increase in ABR latency. Similar results are obtained with increasing stimulus rate. Thus, timing information contained in an acoustic signal is preserved over a wide dynamic range. Such capabilities would be potentially
extremely valuable in echolocation tasks where timing information is critical (e.g., in the estimation of target distance and relative velocity).
The temporal resolution capabilities of cetaceans also appears to surpass
that found in other mammalian species. Results of double-click experiments
as well as the obtained modulation rate transfer functions indicate temporal integration times on the order of 250 to 300/ls. These results match well
with behavioral studies using similar stimulus conditions. The advantage of
such high temporal resolution to an echolocating animal is clear. The ability
to detect a single event in a stream of extremely rapid acoustic events,
coupled with the capability to perform temporal and spectral analysis on
this detected event, would be highly useful in making fine target distance
and position estimates as well as in the characterization and identification
of the target.
There is potentially a tremendous amount of information that may be
extracted from the temporal envelope of a target echo. Although the
echolocation signal emitted by a dolphin is relatively simple in temporal
structure, consisting of a brief (microsecond duration) broadband pulse or
click, the returned echo is likely to be several milliseconds in duration and
highly amplitude-modulated. The temporal envelop of a returned echo
contains nonspectral information (e.g., target size, spatial orientation, and
material composition) that could be of considerable use to the animals.
The results of the MTF and multienvelope experiments indicate that the
auditory system of the cetaceans is capable of simultaneously extracting and
following multiple, high-frequency envelope amplitude fluctuations, clearly
demonstrating the minimum capability required to use the abundance of
information contained in a signal envelope. Thus, the auditory systems of
cetaceans possess the potential to utilize the information carried in the
time-varying structure of an acoustic signal, whether it be a relatively lowfrequency, narrowband communication call or a more broadband, highfrequency biosonar target echo. Additionally, the ability to extract and
follow multiple envelope components across multiple neural frequency
channels may be important in the formation of "auditory objects", allowing an animal to isolate and follow a particular sound source in a mixture
of competing background sounds overlapping in both time and frequency.
In addition to the high temporal processing capabilities of the cetaceans, they also appear to posses remarkable capabilities for processing
spectral information. Many of the cetacean species examined have audiograms that extend from a few hundred Hz to the 150kHz range. Fascinatingly, especially in light of the extreme temporal resolving capabilities
of these animals, the cetaceans that have been examined have
W.E Dolphin
insensitivity to stimulus intensity. Unlike the effects seen in humans, gerbils,
cats, and other mammals, changes in stimulus intensity results in very little
increase in ABR latency. Similar results are obtained with increasing stimulus rate. Thus, timing information contained in an acoustic signal is preserved over a wide dynamic range. Such capabilities would be potentially
extremely valuable in echolocation tasks where timing information is critical (e.g., in the estimation of target distance and relative velocity).
The temporal resolution capabilities of cetaceans also appears to surpass
that found in other mammalian species. Results of double-click experiments
as well as the obtained modulation rate transfer functions indicate temporal integration times on the order of 250 to 300/ls. These results match well
with behavioral studies using similar stimulus conditions. The advantage of
such high temporal resolution to an echolocating animal is clear. The ability
to detect a single event in a stream of extremely rapid acoustic events,
coupled with the capability to perform temporal and spectral analysis on
this detected event, would be highly useful in making fine target distance
and position estimates as well as in the characterization and identification
of the target.
There is potentially a tremendous amount of information that may be
extracted from the temporal envelope of a target echo. Although the
echolocation signal emitted by a dolphin is relatively simple in temporal
structure, consisting of a brief (microsecond duration) broadband pulse or
click, the returned echo is likely to be several milliseconds in duration and
highly amplitude-modulated. The temporal envelop of a returned echo
contains nonspectral information (e.g., target size, spatial orientation, and
material composition) that could be of considerable use to the animals.
The results of the MTF and multienvelope experiments indicate that the
auditory system of the cetaceans is capable of simultaneously extracting and
following multiple, high-frequency envelope amplitude fluctuations, clearly
demonstrating the minimum capability required to use the abundance of
information contained in a signal envelope. Thus, the auditory systems of
cetaceans possess the potential to utilize the information carried in the
time-varying structure of an acoustic signal, whether it be a relatively lowfrequency, narrowband communication call or a more broadband, highfrequency biosonar target echo. Additionally, the ability to extract and
follow multiple envelope components across multiple neural frequency
channels may be important in the formation of "auditory objects", allowing an animal to isolate and follow a particular sound source in a mixture
of competing background sounds overlapping in both time and frequency.
In addition to the high temporal processing capabilities of the cetaceans, they also appear to posses remarkable capabilities for processing
spectral information. Many of the cetacean species examined have audiograms that extend from a few hundred Hz to the 150kHz range. Fascinatingly, especially in light of the extreme temporal resolving capabilities
of these animals, the cetaceans that have been examined have