Bottlenose dolphins reliably discriminate single- versus two-highlight echoes independent of highlight timing and amplitude

Previous experiments have demonstrated that big brown bats deconstruct echo-highlight timing onto a range axis to perceive the target shape. These experiments have also suggested that manipulation of echo amplitude changes the perceived target range; increased delay of auditory neural responses with decreasing echo amplitude leads to an apparent increase in target range (i.e., an amplitude-latency trading effect). These findings motivated the current series of experiments with bottlenose dolphins. Three dolphins were trained in a two-alternative forced-choice task comparing phantom targets with mean simulated ranges of 5 m. The S+ was a two-highlight echo with fixed highlight delays, and the S− was a single-highlight echo that varied in delay and amplitude. In the previous bat experiments, more errors (“error peaks”) occurred when the S− delay was equal to one of the two S+ highlight delays, and shifts in the error peaks were noted when the amplitude of the S− echo varied. Unlike the bat experiments, dolphin performance was above 90% correct independent of S− and S+ highlight timing overlap and no error peaks arose out of S− amplitude manipulations. Dolphins apparently use different processes to determine target spatial features, with perceived range unaffected by changes in echo amplitude. [Work funded by ONR.]