Auditory brainstem response in dolphins.

We recorded the auditory brainstem response (ABR) in four dolphins (Tursiops truncatus and Delphinus delphis). The ABR evoked by clicks consists of seven waves within 10 msec; two waves often contain dual peaks. The main waves can be identified with those of humans and laboratory mammals; in spite of a much longer path, the latencies of the peaks are almost identical to those of the rat. The dolphin ABR waves increase in latency as the intensity of a sound decreases by only 4 microseconds/decibel(dB) (for clicks with peak power at 66 kHz) compared to 40 microseconds/dB in humans (for clicks in the sonic range). Low-frequency clicks (6-kHz peak power) show a latency increase about 3 times (12 microseconds/dB) as great. Although the dolphin brainstem tracks individual clicks to at least 600 per sec, the latency increases and amplitude decreases with increasing click rates. This effect varies among different waves of the ABR; it is around one-fifth the effect seen in man. The dolphin brain is specialized for handling brief, frequent clicks. A small latency difference is seen between clicks 180 degrees different in phase--i.e., with initial compression vs. initial rarefaction. The ABR can be used to test theories of dolphin sonar signal processing. Hearing thresholds can be evaluated rapidly. Cetaceans that have not been investigated can now be examined, including the great whales, a group for which data are now completely lacking.