Classification of simulated two-highlight echoes based on time separation in the bottlenose dolphin (Tursiops truncatus)

Abstract

Previous studies have suggested that dolphins perceive echo spectral features on both large (macrospectrum) and small (microspectrum) scales. The current study was based on a finding that these percepts are—to some degree—dependent on the dolphin’s ∼250-μs “critical interval” of echolocation. Two dolphins were trained to provide a behavioral response upon detecting a passively presented, simulated two-highlight echo. This “target” had consistent spectral features according to a 120-μs inter-highlight interval (IHI). The target was presented among distractor echoes with various macrospectra and IHIs from 50 to 500 μs (i.e., various microspectra). Following acquisition of this discrimination task, probe stimuli with the macrostructure of the target but IHIs from 50 to 500 μs were presented. Both dolphins responded more frequently to probes with IHIs of 80 to 200 μs during initial presentations. Response strategies diverged, however, with increasing probe presentations: one dolphin progressively responded to a narrower range of probe IHIs, while the second increased response rates for probes with IHIs >250 μs. These results support previous conclusions that perception of macrostructure for complex echoes is non-constant as echo IHI decreases below approximately 80 μs, but results near 250 μs (i.e., the critical interval upper limit) were more ambiguous. [Work funded by ONR.]