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First published online December 14, 2007
Journal of Experimental Biology 211, 9-14 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012823

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Time-variant spectral peak and notch detection in echolocation-call sequences in bats

Daria Genzel^* and Lutz Wiegrebe

Department of Biologie II, Universität München, Großhadernerstr. 2 82152 Planegg-Martinsried, Germany

^* Author for correspondence (e-mail: genzel{at}zi.biologie.uni-muenchen.de)

Accepted 17 October 2007

Bats are able to recognize and discriminate three-dimensional objects in complete darkness by analyzing the echoes of their ultrasonic emissions. Bats typically ensonify objects from different aspects to gain an internal representation of the three-dimensional object shape. Previous work suggests that, as a result, bats rely on the echo-acoustic analysis of spectral peaks and notches. Dependent on the aspect of ensonification, this spectral interference pattern changes over time in an object-specific manner. The speed with which the bats' auditory system can follow time-variant spectral interference patterns is unknown.

Here, we measured the detection thresholds for temporal variations in the spectral content of synthesized echolocation calls in the echolocating bat, Megaderma lyra. In a two-alternative, forced-choice procedure, bats were trained to discriminate synthesized echolocation-call sequences with time-variant spectral peaks or notches from echolocation-call sequences with invariant peaks or notches. Detection thresholds of the spectral modulations were measured by varying the modulation depth of the time-variant echolocation-call sequences for modulation rates ranging from 2 to 16 Hz. Both for spectral peaks and notches, modulation-detection thresholds were at a modulation depth of 11% of the centre frequency. Interestingly, thresholds were relatively independent of modulation rate. Acknowledging reservations about direct comparisons of active-acoustic and passive-acoustic auditory processing, the effectual sensitivity and modulation-rate independency of the obtained results indicate that the bats are well capable of tracking changes in the spectral composition of echoes reflected by complex objects from different angles.

Key words: echolocation, object discrimination, frequency modulation, bat, Megaderma lyra