Spatial echo suppression and echo-acoustic object normalization in echolocating bats

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.