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First published online May 30, 2008
Journal of Experimental Biology 211, 1850-1858 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017715
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Comparing passive and active hearing: spectral analysis of transient sounds in bats

Holger R. Goerlitz*, Mathias Hübner and Lutz Wiegrebe

Department Biologie II, Neurobiologie, Ludwig-Maximilians-Universität München, Großhadernerstrasse 2, 82152 Martinsried, Germany

* Author for correspondence at present address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK (e-mail: holger.goerlitz{at}bristol.ac.uk)

Accepted 27 March 2008

In vision, colour constancy allows the evaluation of the colour of objects independent of the spectral composition of a light source. In the auditory system, comparable mechanisms have been described that allows the evaluation of the spectral shape of sounds independent of the spectral composition of ambient background sounds. For echolocating bats, the evaluation of spectral shape is vitally important both for the analysis of external sounds and the analysis of the echoes of self-generated sonar emissions. Here, we investigated how the echolocating bat Phyllostomus discolor evaluates the spectral shape of transient sounds both in passive hearing and in echolocation as a specialized mode of active hearing. Bats were trained to classify transients of different spectral shape as low- or highpass. We then assessed how the spectral shape of an ambient background noise influenced the spontaneous classification of the transients. In the passive-hearing condition, the bats spontaneously changed their classification boundary depending on the spectral shape of the background. In the echo-acoustic condition, the classification boundary did not change although the background- and spectral-shape manipulations were identical in the two conditions. These data show that auditory processing differs between passive and active hearing: echolocation represents an independent mode of active hearing with its own rules of auditory spectral analysis.

Key words: background noise, call analysis, hearing, spectral shape, perceptual constancy


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