6. Auditory eNS of Dolphins
283
dolphin, bats reveal to the histologist a more primitive level of cortical
development than, for example, carnivores or primates (Morgane et al.
1986; Glezer et al. 1988). The extent of auditory cortex in dolphins may be
greater than that indicated in the Soviet mapping experiments (Ridgway
1990). A tonotopic map of the cochlear projections on the dolphin cortex
has not been created. Complex stimuli of different delays or rise times or
other acoustic characteristics have not been used in attempts to find areas
of cortex specialized for specific auditory information resembling the
complex sound processing found in areas of the bat cortex (Suga 1984).
Therefore, it is entirely possible that further auditory projection areas will
be found in temporal cortex (Fig. 6.3), which, in dolphins, is less accessible
than the dorsal area that has been mapped (Supin et al. 1978; Bullock and
Gurevich 1979).
6. Echolocation
Research on dolphin echolocation done since the late 1950s has confirmed
an earlier suspicion. Those who studied the first collected colony of T. truncatus at Marine Studios in Florida in the late 1930s and 1940s suspected
that these animals possessed some sort of sonar or echolocation ability
(McBride 1956). Subsequent studies (Kellogg 1958; Norris et a1.1961; Evans
and Powell 1967) have shown that blindfolded T. truncatus can make
extremely fine discriminations. They can make fine discriminations over
underwater distances up to at least 100m (Au 1980) by employing trains of
high-frequency clicks emitted from the nasal system and directed forward
in a fairly narrow beam (Au and Moore 1984; Au et al. 1986; Au 1993,
Chapter 9).
7. Processing the Auditory Input in the Brain
7.1 Specialized Auditory Analysis Systems Within
the Brain
Electrophysiological experiments with dolphins have shown that temporal
resolution of successive sounds is extremely rapid and that very small
changes in the frequency of a stimulus altered evoked potential (EP) amplitude and waveform (Bullock et al. 1968). The typical midbrain EP of T. truncatus appears specialized for ultrasonic, ultrabrief, fast-rising, closely spaced
sounds like the echolocation clicks. However, at several cerebral locations
(mainly in the posterior lateral temporal cortex) long latency, long duration, slowly recovering EPs have been evoked by lower frequencies, with
either fast or slowly rising acoustic envelopes (Bullock and Ridgway 1972).
This suggests dual analysis systems, one specialized for the ultrasonic clicks
283
dolphin, bats reveal to the histologist a more primitive level of cortical
development than, for example, carnivores or primates (Morgane et al.
1986; Glezer et al. 1988). The extent of auditory cortex in dolphins may be
greater than that indicated in the Soviet mapping experiments (Ridgway
1990). A tonotopic map of the cochlear projections on the dolphin cortex
has not been created. Complex stimuli of different delays or rise times or
other acoustic characteristics have not been used in attempts to find areas
of cortex specialized for specific auditory information resembling the
complex sound processing found in areas of the bat cortex (Suga 1984).
Therefore, it is entirely possible that further auditory projection areas will
be found in temporal cortex (Fig. 6.3), which, in dolphins, is less accessible
than the dorsal area that has been mapped (Supin et al. 1978; Bullock and
Gurevich 1979).
6. Echolocation
Research on dolphin echolocation done since the late 1950s has confirmed
an earlier suspicion. Those who studied the first collected colony of T. truncatus at Marine Studios in Florida in the late 1930s and 1940s suspected
that these animals possessed some sort of sonar or echolocation ability
(McBride 1956). Subsequent studies (Kellogg 1958; Norris et a1.1961; Evans
and Powell 1967) have shown that blindfolded T. truncatus can make
extremely fine discriminations. They can make fine discriminations over
underwater distances up to at least 100m (Au 1980) by employing trains of
high-frequency clicks emitted from the nasal system and directed forward
in a fairly narrow beam (Au and Moore 1984; Au et al. 1986; Au 1993,
Chapter 9).
7. Processing the Auditory Input in the Brain
7.1 Specialized Auditory Analysis Systems Within
the Brain
Electrophysiological experiments with dolphins have shown that temporal
resolution of successive sounds is extremely rapid and that very small
changes in the frequency of a stimulus altered evoked potential (EP) amplitude and waveform (Bullock et al. 1968). The typical midbrain EP of T. truncatus appears specialized for ultrasonic, ultrabrief, fast-rising, closely spaced
sounds like the echolocation clicks. However, at several cerebral locations
(mainly in the posterior lateral temporal cortex) long latency, long duration, slowly recovering EPs have been evoked by lower frequencies, with
either fast or slowly rising acoustic envelopes (Bullock and Ridgway 1972).
This suggests dual analysis systems, one specialized for the ultrasonic clicks