The effects of target range on a jitter discrimination task in echolocating bottlenose dolphins (Tursiops truncatus)

The dolphin's ability to resolve echo delay is a critical feature of its biosonar system, both to determine target range and to detect small changes in range. In this study, the dolphin's sensitivity to changes in echo delay was measured as a function of mean echo delay (target range) while holding echo level constant relative to the emitted click. The dolphins' task was to detect when phantom echoes with a fixed echo delay began to "jitter" by ±10 μs on successive presentations. Jitter detection was tested at 17 mean echo delays, simulating ranges from 2.5 to 20 m. Jitter detection sensitivity (d') decreased as the target range increased beyond 10 m and decreased below 5 m. To help understand the observed sensitivity changes, auditory evoked potentials (AEPs) to the emitted clicks and returning echoes were recorded. AEPs showed that decreased sensitivity below 5 m likely resulted from reduced echo detectability due to forward masking of the echo by the emitted click. At ranges beyond 10 m, decreased sensitivity occurred despite the progressive release from forward masking. These results suggest the dolphin's ability to integrate information across echoes declines when target range increases, likely due to the corresponding increase in inter-echo interval.