Aural discrimination of target echoes by human observers using broadband sonar pulses

Abstract

Echo discrimination experiments were conducted with human observers, using targets employed in a dolphin echo-recognition study. Digital recordings of the target echoes were obtained using dolphin-like signals, and were played back to five subjects at 1/50 of the original sample rate. Targets included hollow cylinders of aluminum and glass having diameters of 3.81 and 7.62 cm, and a solid 7.62-cm aluminum cylinder. Echo amplitudes were adjusted so that stimulus intensity was never a cue. The subjects discriminated between (1) solid and hollow, (2) large and small aluminum cylinders with correct responses of 98% and 92%, respectively, after one session per task. The aluminum-glass discriminations were performed in five categories and after 30 sessions per subject, the correct responses varied between 73% and 97%. The dolphin discriminated between the 3.81-cm aluminum and glass cylinders, but failed to learn the task with the 7.62-cm targets. Discrimination cues were time-separation pitch associated with echo highlights, and echo duration. Duration may not have been a cue to the dolphin, while becoming apparent to the human subjects because of pulse stretching.