6. Auditory eNS of Dolphins
287
those of some smaller delphinids. It has been suggested (Ridgway 1986) that
BIT in this large brain may be kept short by increased fiber diameters in
the auditory pathway.
Szymanski and colleagues (1995, 1998) found that ABR wave IV latency
was considerably longer than that in T truncatus (Ridgway et at. 1981;
Popov and Supin 1990)-7 to 8ms compared to 3.5 to 4.5ms. Comparison
of BIT values revealed latencies that were more similar. I compared wave
II to IV latency presented by Szymanski et at. (1998) to data from earlier
studies of T truncatus (Ridgway et at. 1981). This comparison reveals a wave
II to IV latency of 1.7 to 2.2ms in 0. orca compared with 1.3 to 1.5ms in
T truncatus for ABRs of similar amplitude (Fig. 6.8). Although De Graaf
(1967) did comment on the large size of the inferior colliculus, the lateral
lemniscus and other auditory structures of 0. orca, he did not make fiber
diameter measurements in the species. The relatively similar wave II to IV
latency despite a brain four times larger suggests that myelinated auditory
fibers of 0. orca are enlarged compared to its odontocete, delphinoid family
member T truncatus to keep BIT fast.
8. Comparative Studies of Auditory Processing
The above findings suggest that the dolphin brain is specialized for rapid
processing of auditory stimuli (see also Bullock et at. 1968; Bullock and
Ridgway 1972). If given enough time, the human auditory system seems to
perform as well on some echolocation tasks (Fish et at. 1976). Instrumented
divers projected pulses similar to dolphin echolocation pulses at targets. The
received echoes were stretched 128 times (tantamount to a slowed-down
tape recording and therefore reduced equivalently in frequency), and
human divers performed with as few errors as T truncatus in distinguishing metal targets of copper, brass, or aluminum and geometrical aluminum
shapes covered with neoprene rubber (Fish et at. 1976).
Studies with static metal targets may give only limited detail of the
dolphin's echolocation processing ability. For example, the unnatural
targets could well be a factor. However, if the findings of Fish et at. (1976)
are a fair comparison of echolocation ability and based on a sonar discrimination task that is difficult for dolphins,I we must conclude that the
major accomplishment in the sonar processing component of the dolphin
auditory system is the ability to process sound rapidly.
I I have some doubt that they are. For example, an analogy might be a track-andfield high jump competition. If we placed the bar at 1 m, millions of people could
jump it as well as Olympic athletes. If we moved the bar to 2 m, only good athletes
could do it but perhaps thousands would by this criteria be equal. However, if the
bar were at 2.3 m, only the best Olympic athletes could jump it.
287
those of some smaller delphinids. It has been suggested (Ridgway 1986) that
BIT in this large brain may be kept short by increased fiber diameters in
the auditory pathway.
Szymanski and colleagues (1995, 1998) found that ABR wave IV latency
was considerably longer than that in T truncatus (Ridgway et at. 1981;
Popov and Supin 1990)-7 to 8ms compared to 3.5 to 4.5ms. Comparison
of BIT values revealed latencies that were more similar. I compared wave
II to IV latency presented by Szymanski et at. (1998) to data from earlier
studies of T truncatus (Ridgway et at. 1981). This comparison reveals a wave
II to IV latency of 1.7 to 2.2ms in 0. orca compared with 1.3 to 1.5ms in
T truncatus for ABRs of similar amplitude (Fig. 6.8). Although De Graaf
(1967) did comment on the large size of the inferior colliculus, the lateral
lemniscus and other auditory structures of 0. orca, he did not make fiber
diameter measurements in the species. The relatively similar wave II to IV
latency despite a brain four times larger suggests that myelinated auditory
fibers of 0. orca are enlarged compared to its odontocete, delphinoid family
member T truncatus to keep BIT fast.
8. Comparative Studies of Auditory Processing
The above findings suggest that the dolphin brain is specialized for rapid
processing of auditory stimuli (see also Bullock et at. 1968; Bullock and
Ridgway 1972). If given enough time, the human auditory system seems to
perform as well on some echolocation tasks (Fish et at. 1976). Instrumented
divers projected pulses similar to dolphin echolocation pulses at targets. The
received echoes were stretched 128 times (tantamount to a slowed-down
tape recording and therefore reduced equivalently in frequency), and
human divers performed with as few errors as T truncatus in distinguishing metal targets of copper, brass, or aluminum and geometrical aluminum
shapes covered with neoprene rubber (Fish et at. 1976).
Studies with static metal targets may give only limited detail of the
dolphin's echolocation processing ability. For example, the unnatural
targets could well be a factor. However, if the findings of Fish et at. (1976)
are a fair comparison of echolocation ability and based on a sonar discrimination task that is difficult for dolphins,I we must conclude that the
major accomplishment in the sonar processing component of the dolphin
auditory system is the ability to process sound rapidly.
I I have some doubt that they are. For example, an analogy might be a track-andfield high jump competition. If we placed the bar at 1 m, millions of people could
jump it as well as Olympic athletes. If we moved the bar to 2 m, only good athletes
could do it but perhaps thousands would by this criteria be equal. However, if the
bar were at 2.3 m, only the best Olympic athletes could jump it.