Spectral and temporal cues used by echolocating bottlenose dolphins to discriminate changes in inter-highlight intervals

Abstract

Echolocating dolphins use spectral cues to discriminate complex echoes from targets with multiple reflective surfaces; however, the specific cues involved are not clear. This study investigated the role of spectral interference patterns on the dolphin’s ability to discriminate two-highlight echoes. In task one of the study, two dolphins were trained to echolocate, listen to returning electronic (“phantom”) echoes with two identical highlights, and produce a conditioned acoustic response if the inter-highlight interval (IHI) increased. The amount that the IHI increased varied across trials. Task two replicated task one but applied a random phase shift to all highlights. This varied the specific locations of notches along the frequency axis in the complex echo spectrum but preserved the notch spacing (equal to the inverse of the IHI). For IHIs less than 250 μs, discrimination thresholds increased with IHI and were larger with random phase shifts. For IHIs greater than 250 μs, thresholds plateaued and were similar for both tasks. Results agree with those from a previous study with passively listening dolphins. Namely, echolocating dolphins can use the spacing of notches in spectral interference patterns to detect IHI changes within the ∼250-μs temporal window, but temporal cues are used at longer IHIs. [Work supported by ONR]