Dolphin biosonar detection of phase changes in complex echo highlights

The dolphin's ability to detect changes in the phase of complex echo highlights was investigated by training two bottlenose dolphins to detect a phase "jitter" applied to the second highlight of a two-highlight "phantom" echo. One dolphin had normal hearing and the other had high-frequency hearing loss. The phase jitter changed the complex echo temporal and spectral fine structure without altering the echo energy, temporal and spectral envelopes, or spectral notch spacing. Over the course of testing, the inter-highlight interval (IHI) and frequency content of the echoes were varied. When echo IHIs were less than 300 μs, phase jitter thresholds for the two dolphins were equal, despite large differences in high-frequency hearing ability. At larger time separations, the perceptual cue appeared to change, presumably from spectral to temporal. High-pass filtering of echoes suggested that the lower range of echolocation frequencies are most useful for detecting echo highlight phase shifts. Simple models for across-channel spectral profile differences and differences in repetition pitch provided mixed results: both models matched the experimental data well for some conditions but poorly for other conditions, especially as the IHI and high-pass cutoff frequencies increased.