Bottlenose dolphin (Tursiops truncatus) detection of simulated echoes from normal and time-reversed clicks

Abstract

In matched filter processing, a stored template of the emitted sonar pulse is compared to echoes to locate individual replicas of the emitted pulse embedded in the echo stream. A number of experiments with bats have suggested that bats utilize matched filter processing for target ranging, but not for target detection. For dolphins, the few available data suggest that dolphins do not utilize matched filter processing. In this study, the effect of time-reversing a dolphin's emitted click was investigated. If the dolphin relied upon matched filter processing, time-reversal of the click would be expected to reduce the correlation between the (unaltered) click and the echoes and therefore lower detection performance. Two bottlenose dolphins were trained to perform a phantom echo detection task. On a small percentage of trials ("probe trials"), a dolphin's emitted click was time-reversed before interacting with the phantom echo system. Data from the normal and time-reversed trials were then analyzed and compared. There were no significant differences in detection performance or click emissions between the normal and time-reversed conditions for either subject, suggesting that the dolphins did not utilize matched filter processing for this echo detection task.

Neural representation of the self-heard biosonar click in bottlenose dolphins (Tursiops truncatus)

“…The biosonar systems of dolphins and microchiropteran bats are believed to work along similar basic principles: a replica of the emitted sound pulse is compared to returning echoes, with large-scale echo delay differences revealing overall target range, and fine-scale echo delay differences revealing target shape (Simmons and Gaudette, 2012;Simmons et al, 2014). Knowledge of the emitted click-i.e., the animal's internal, neural representation of the click-is thus critical for biosonar tasks, especially those involving echo ranging or range discrimination (Masters and Jacobs, 1989) [but not necessarily for target detection only (Møhl, 1986;Masters and Jacobs, 1989;Finneran et al, 2013b)]. Biosonar clicks produced by dolphins are sufficiently intense to be heard by the echolocating animal and provide the required click replica (e.g., Supin et al, 2003); however, the temporal and spectral properties of the neural representation of the click would likely not match those of the acoustic click measured in the farfield.…”