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Schuchmann, Maike (2007): Spatial echo suppression and echo-acoustic object normalization in echolocating bats. Dissertation, LMU München: Faculty of Biology
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Abstract

The processing of acoustic cues is critical for all animals in a wide range of behaviours including orientation, predator-prey interactions and social communication. The auditory system can process these sound information with amazing precision. Echolocating bats have developed an extraordinary ability to deal with acoustic cues. Their echo-imaging system has enabled them to detect, pursue and capture tiny prey like insects, to avoid obstacles and to interact with their environment, often in total darkness. Bats heavily rely on the evaluation of echoes for orientation and hunting. The evaluation of external, echolocation- independent sounds also plays an important role for bats, e.g. while localizing prey via prey-generated noise or for social purposes. The current thesis addresses two different aspects of the very complex echo-acoustic situation these extraordinary animals are confronted with in their daily life. The first approach of this thesis is concerned with the question how bats deal with misleading spatial information of echoes. Acoustic orientation most often takes place in echoic environments. Accurate sound localization in natural, echoic environments is a vital task of the auditory system. Many behavioral studies have shown that for accurate sound localization, the auditory system relies only on the spatial information provided by the first wave front and that spatial information of the (delayed) echoes is suppressed (‘precedence effect’). For a bat, this approach is also useful when localizing external, echolocation-independent sound sources, but it is in conflict with the processing of the echoes of self-generated sounds in an echolocation context. In a two-alternative, forced choice paradigm, it is investigated whether and to what extend the echolocating bats Megaderma lyra and Phyllostomus discolor spontaneously suppress the spatial information of either a second echo of their sonar emission or echoes of different external, echolocation-independent sounds. In general, M. lyra and P. discolor did not suppress the spatial information of a second echo independent of the delay. Only one M. lyra showed significant echo suppression. However, this suppression could not be confirmed in an exact repetition of the experiment. Furthermore, it is shown that in the bat M. lyra, spatial echo suppression is restricted to an external sound which carries semantic meaning for the bat, in this case, a typal contact call. Abstract sounds like an acoustic impulse, a time-inverted contact call, or only the first syllable of the contact call do not induce spontaneous echo suppression. The current data indicate that while bats may be able to suppress the spatial information of echoes, this is not their default mode of auditory processing. The reason for this exceptional absence of spatial echo suppression may lie in the shorter time constants of cochlear processing in the ultrasonic frequency range and the strong influence of cognitive components associated with the precedence effect. This study emphasises the contribution of high-level semantic auditory processing to echo suppression. The aim of the second approach was to characterize how echolocating Phyllostomus discolor deals with size-induced variations in echoes due to different-sized ensonified objects. Echolocating bats can identify three-dimensional objects exclusively through the analysis of acoustic echoes of their ultrasonic emissions. However, objects of the same structure can differ in size and the auditory system must achieve a size-invariant, normalized object representation for reliable object recognition. This study describes the behavioral classification of echoes of complex virtual objects that vary in object size. In a phantom-target playback experiment, it is shown that the bat P. discolor spontaneously classified most scaled versions of objects according to trained standards. This psychophysical performance is reflected in electrophysiological responses of a population of cortical units received from a cooperated study, which showed an object-size invariant response. The current results indicate that echolocating bats have indeed a concept of auditory object normalization.