Dolphin Biosonar

Echolocation is a highly specialized skill that enables dolphins to explore their environment and search out prey in conditions were vision is limited and sometimes of little use. As sound travels four and a half times faster in water and air, the dolphin's brain must be extremely well adapted in order to make a rapid analysis of the complicated information provided by the echoes.

Dolphins echolocate by emitting short, high-intensity clicks and processing the echoes reflected from objects. These clicks are emitted at very high frequencies, with the bottlenose dolphin's range reaching up to 160 khz - well above the limit of human hearing. The ability of dolphins to detect and discriminate among objects via echolocation is well documented.

Current Research Areas

Cross-Modal Matching

At Ocean Park Hong Kong, researchers from ARL have successfully trained a dolphin in a task known as cross-modal matching. This is a complex task that requires the animal to match one of a set of complex shapes that are of similar appearance, construction and surface area, where the sample is positioned either behind an opaque screen (for the echolocation sense) or in-air (for the visual sense) and the alternatives are presented to the alternate sensory modality. This is the only dolphin in the world to do so at present.

The ability of this dolphin to perform this task with little or no errors demonstrates that dolphins have a complex cognitive imaging capability similar to that thought to exist in humans. Research has shown that during the cross-modal task, when the dolphin is inspecting the complex objects through its echolocation, it sends out a rapid train of clicks with inter-click-intervals as low as 1.2 milliseconds. Such information and results from the experiments demonstrates that the information contained in the reflections must be sufficient to reconstruct the shape in question. However, this still does not explain the high-resolution capability thought necessary to correctly image the object holistically.

The problem is that no one has ever recorded the back-scattered acoustic field of a dolphin interrogating a complex object. This project aims to investigate the underlying properties of both the forward- and back-scattered acoustic field that enable the dolphin to image the ensonified object.

Rotational Invariance in Cross-Modal Matching

This experiment is investigating how rotation of either the sample or the alternative object affects the animal's performance during the cross-modal matching. Is the animal able to recognize the shape of an object independent of its orientation? What type of rotation would be difficult for the dolphin?

During these experiments, 3-dimensional objects will be rotated around all axes with varying degrees. By looking at the dolphins success rate in matching these rotations we can better understand the role of aspect and rotation in the dolphin's ability to recognize objects through echolocation.

Synthetic Aperture

One of our current hypothesis on dolphin echolocation that we are investigating is that the animal uses a synthetic aperture approach to obtain better angular and range resolution of the object. Testing this hypothesis involves using objects that the dolphin previously could discriminate and training this dolphin involved in cross-modal matching to remain in a fixed position whilst shown the sample object. If synthetic aperture is one of the essential features of the dolphin's ability to resolve shape, then the dolphin should have increased difficulties in resolving the cross-modal matching as a result of not being able to interrogate different aspects of the object. Such findings would support evidence that synthetic aperture is one of the key features used by dolphins to resolve shapes and features below the angular resolution assumed through the wavelength of the emitted signal.

Vertical Angular Resolution

To further investigate echolocation and understand the sensory channels involved ARL researchers will conduct a vertical angular resolution test. Previous research has demonstrated a horizontal angular resolution in an echolocating dolphin (Branstetter et al. 2003). Expanding on this research, if a dolphin is using horizontally aligned arrays of sensors, i.e. fatty channels or teeth in the lower jaw, we hypothesize there should be a clear difference in angular resolution between the horizontal and the vertical orientation. Results from this investigation will help us understand the limitations of using a sparse array and also able us to design signal processing algorithms that could compensate for the frequency dependent range and angular resolution.

Relevant Publications

[1]	M. Hoffmann-Kuhnt, P. J. Seekings, and M. A. Chitre, “ASPOD - acoustic source position overlay device - real-time visual and acoustic localization of marine mammals,” in 18th Biennial Conference on the Biology of Marine Mammals, Quebec, Canada, 2009.
[2]	M. Hoffmann-Kuhnt and M. A. Chitre, “Do dolphins use synthetic aperture during echolocation shape perception?,” in 5th Animal Sonar Symposium, Kyoto, Japan, 2009. Invited paper.
[3]	M. Hoffmann-Kuhnt, M. Chitre, P. Seekings, and G. Abel, “Acoustics of shape recognition by a dolphin in a cross-modal matching-to-sample paradigm,” in ASA-EAA Joint Conference Acoustics'08, Paris, France, vol. 123, p. 3361, 2008. Invited paper.
[4]	M. Hoffmann-Kuhnt, M. A. Chitre, J. R. Potter, and C. Cai, “What do dolphins see in sound? - a simulation of the backscattered sound field from an object insonified by an echolocation click,” in 17th Biennial conference on the biology of marine mammals, Cape Town, South Africa (P. Best and M. Bester, eds.), 2007.
[5]	P. Seekings, J. Tan, J. Potter, M. Hoffman-Kuhnt, A. Pack, and L. Herman, “Denoising dolphin click series in the presence of tonals, using singular spectrum analysis and higher order statistics,” in OCEANS 2006 - Asia Pacific, pp. 1-6, May 2006. [ DOI ]
[6]	M. Hoffmann-Kuhnt, S. Duncan, R. Kinoshita, G. Abel, and G. Wong, “Matching across the senses - how to train dolphins to integrate vision and echolocation?,” in International Marine Animal Trainers Association, Honolulu, USA, 2006.
[7]	M. Hoffmann-Kuhnt, M. A. Chitre, J. R. Potter, and W. J. Lee, “Dolphin echolocation - seeing through the clutter,” in 16th Biennial Conference of the Biology of Marine Mammals, San Diego, USA, 2005.
[8]	M. Hoffmann-Kuhnt, M. Chitre, and J. Potter, “"Ghosts in the image" - aliasing problems with incoherent synthetic aperture using a sparse array,” in OCEANS '04. MTTS/IEEE TECHNO-OCEAN '04, vol. 2, pp. 621-625, November 2004. [ DOI \| .pdf ]
[9]	A. A. Pack, L. M. Herman, and M. Hoffmann-Kuhnt, “Dolphin echolocation shape perception: from sound to object,” University of Chicago Press; Chicago & London, pp. 298-308, 2003.
[10]	A. A. Pack, L. M. Herman, M. Hoffmann-Kuhnt, and B. K. Branstetter, “The object behind the echo: Dolphins (Tursiops truncatus) perceive object shape globally through echolocation,” Behavioural Processes, vol. 58, pp. 1-26, 2002. [ .pdf ]
[11]	J. R. Potter and E. Taylor, “On novel reception models for Bottlenose dolphin echolocation,” in Proceedings of the Institute of Acoustics, Biosonar, Loughborough, UK, vol. 24, 2001. [ .pdf ]
[12]	L. M. Herman, A. A. Pack, and M. Hoffmann-Kuhnt, “Seeing through sound: Dolphins perceive the spatial structure of objects through echolocation,” Journal Of Comparative Psychology, vol. 112, pp. 292-305, September 1998.
[13]	J. R. Potter, “Ambient noise imaging potential for intelligent terrestrial animals,” in Singapore Acoustical Society Annual Conference, Singapore, 1998.
[14]	E. Taylor, J. R. Potter, and M. A. Chitre, “Ambient noise imaging potential of marine mammals,” in Bioacoustics Conference, Loughborough (UK), 1997.
[15]	J. R. Potter, E. Taylor, and M. A. Chitre, “Could Marine Mammals use Ambient Noise Imaging techniques ?,” in 134th Acoustical Society of America Meeting, San Diego, 1997.