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

Previous studies suggested that dolphins perceive echo spectral features on coarse (macrospectrum) and fine (microspectrum) scales. This study was based on a finding that these auditory percepts are, to some degree, dependent on the dolphin's ∼250-μs auditory temporal window (i.e., "critical interval"). Here, two dolphins were trained to respond on passively detecting a simulated "target" echo complex [a pair of echo "highlights" with a characteristic 120-μs inter-highlight interval (IHI)]. This target had unique micro- and macrospectral features and was presented among "distractor" echoes with IHIs from 50 to 500 μs (i.e., microspectra) and various highlight durations (i.e., macrospectra). Following acquisition of this discrimination task, probe echo complexes with the macrospectrum of the target but IHIs matching the distractors were infrequently presented. Both dolphins initially responded more often to probes with IHIs of 80-200 μs. Response strategies diverged with increasing probe presentations; one dolphin responded to a progressively 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 macrospectra for complex echoes is nonconstant as the IHI decreases below ∼100 μs, but results approaching and exceeding 250 μs-the temporal window upper boundary-were more ambiguous.