Discrimination of simulated two-highlight echoes including phase manipulations by bottlenose dolphins (Tursiops truncatus)

Abstract

Dolphins potentially use spectral cues to discriminate inter-highlight interval (IHI) for passively presented, simulated two-highlight echo stimuli. To investigate this potential, dolphins were trained to listen to repetitive two-highlight “background” echoes and respond upon a change to “target” echoes with increased IHI. In the first experimental task, all highlights had the same phase, and the target differed from the background only in terms of IHI. In the second task, highlights within both background and target echoes had random phase angles. This resulted in a spectral interference pattern for the two-highlight echo with the same notch spacing as the first condition, but different absolute positions of the notches along the frequency axis. Discrimination thresholds for the constant-phase task were lower than those for the random-phase task at IHIs of 250 μs and lower. Thresholds for both tasks increased with increasing background IHI and were comparable at 375 and 500 μs. Within the dolphin auditory temporal window of ∼250 μs, IHI discrimination is therefore likely related to—but not strictly based on—the spacing of notches in the complex echo spectrum. Above 250 μs, IHI is likely determined through direct encoding of highlight timing in the auditory periphery. [Work funded by ONR.]