Because at 66 kHz one-half of a cycle is about 7.5 usec in duration, the 40 μsec difference in latency seen is over 5 times what would be expected from simply the difference in arrival time of the excitatory rarefaction. These data are evidence for phase detection by the dolphin auditory system at least to 66 kHz.
The above findings suggest that the dolphin brain is specialized for rapid processing of auditory stimuli (see also Bullock et al., 1968; Bullock & Ridgway , 1972). If given enough time, the human auditory system seems to perform as well on some echolocation tasks ( Fish, Johnson, & Ljungblad, 1976). Indeed, when pulses similar to dolphin echolocation pulses were projected at targets by instrumented divers and the received echoes stretched (tantamount to a slowed down tape recording and therefore reduced equivalently in frequency) 128 times, human divers performed with as few errors as Tursiops in distinguishing metal targets of copper, brass, or aluminum and geometrical aluminum shapes covered with neoprene rubber ( Fish et al., 1976).
Studies with static metal targets may give only limited detail of the dolphin's echolocation processing ability. However, if the findings of Fish et al. ( 1976) are a fair comparison of echolocation ability and based on a sonar discrimination task that is difficult for dolphins, we must conclude that the major accomplishment in the sonar processing component of the dolphin auditory system is the ability to process sound rapidly. In this paper I have reviewed a number of other findings, for example, the large diameter of auditory nerve fibers, the short latency and the relatively flat latency-intensity function of the dolphin ABR waves, and the rapid temporal resolution of successive sounds, that lend support to my view that much of the great hypertrophy of the dolphin auditory system--and perhaps the entire cerebrum--results from the animal's need for great precision and speed in processing sound.
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