A study on time and frequency analysis of a double-click by the dolphin Tursiops truncatus is reported. By simple modification of mirror image double-clicks, which were first used in experiments with human subjects by Ronken (1970), I designed mirror-image double-clicks which differed in interclick intervals, but had the same long term amplitude spectrum and practically the same phase spectrum within dolphin's hearing range. The bottlenose dolphin was found to distinguish such stimuli at approximately a 10% difference in the interclick intervals. The shortest interclick interval of about 25 µms at which the dolphin could discriminate between the double- clicks can be considered as the estimate of the actual time resolution of its hearing. Frequency domain representation of a double-click is supposed to be based on some sort of interaction between the first and the second click, similar to an interference between the clicks in frequency filters. In order to measure the recovery time for dolphin hearing filters I used double-clicks which differed in amplitude spectrum but had the same interclick intervals. At equal interpulse intervals, the amplitude spectra of the pairs are rippled with the same period, but the maxima of one spectrum corresponds to the minima of the other. The largest interclick interval at which two dolphins were able to discriminate the pairs was found to be 100-110 µs. Thus, the representation of a double-click in the dolphin's auditory system can be described by its extremely high time resolution of less than 25-30 µs, and frequency filter's recovery time of 100-110 µs. The real time resolution of the dolphin hearing proved to be practically equal to the theoretical time resolution of sonar's clicks. The 'recovery time seems to represent the dolphin's limit for frequency analysis of double-clicks at both audio and echolocation frequencies. The sonar's high time resolution suggests that target discrimination cues are available for the dolphin in the time domain. Still, at least in experimental conditions it is possible to force the dolphin to process double-clicks in the frequency domain, even the ones which it normally processes in time domain, provided the conditions for frequency analysis are fulfilled. Experimental data were collected at the Karadag Department of Institute of Biology of Southern Seas, Crimea, USSR in 1978-1982.
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