Discrimination of two-highlight versus single-highlight phantom echoes by bottlenose dolphins (Tursiops truncatus)

Range resolution defines the ability to discriminate multiple closely spaced surfaces from a single surface and is a fundamental determinant of dolphin biosonar performance. In this study, three bottlenose dolphins were trained to discriminate "phantom" echoes with a single highlight from those with two highlights. The inter-highlight interval of the two-highlight echo was varied to determine the discrimination threshold. Threshold measurements were conducted at echo delays corresponding to simulated target ranges of 2.5-80 m. Discrimination thresholds were 2-3 μs (∼2 mm) for two dolphins with a full-bandwidth of hearing. A third dolphin with high-frequency hearing loss had higher thresholds, between 3 and 5 μs (∼2-4 mm). Thresholds were independent of simulated range, which contrasts with previous reports that the dolphin's accuracy of range determination-as opposed to range resolution-is reduced beyond 10-20 m. Dolphin discriminations in the current study appeared to be based on a spectral cue that consisted of a low-pass filter effect at the highest audible frequencies in the two-highlight echoes. It is unknown if dolphins perceive highlight-dependent spectral interference patterns as a spatial separation of target features along a range axis.