Discrimination of double-click synthetic echoes by bottlenose dolphins: Effects of inter-highlight interval and phasea

Two bottlenose dolphins (Tursiops truncatus) were trained to discriminate double-click synthetic “echoes” differing in inter-highlight interval (IHI). In the first experimental task, dolphins passively listened to background (S−) stimuli with constant IHI and responded on discriminating a change to target (S+) stimuli with a slightly increased IHI. The second task was similar, but the highlights were assigned random, frequency-independent phase angles. This phase randomization was designed to interfere with potential auditory cues from characteristic spectral interference patterns linked to IHI changes. Discrimination thresholds were higher with randomized phase when the S− stimuli had IHIs of 50–250 μs. Thresholds were highest and comparable at the longest S− IHIs of 375 and 500 μs and were independent of phase condition. Although dolphin detection of changes in highlight envelope timing can explain threshold patterns at 375 and 500 μs, this cue did not explain performance at IHIs less than the dolphin auditory temporal window of ∼250 μs. Modeling results suggested that phase manipulations eliminated the availability of a simple difference in spectral magnitudes at the shortest IHIs, but the perception of a time separation pitch cue may still explain the dolphins' observed threshold patterns.