Evidence for an analytic perception of multiharmonic sounds in the bat, Megaderma lyra, and its possible role for echo spectral analysis

For echolocation, the gleaning bat Megaderma lyra relies on short and broad band calls consisting of multiple harmonic components, each of which is dow nward frequency modulated. The harmonic components in M. lyra's calls have a relatively small frequency excursion and do not overlap spectrally. Broad band calls of other bat species, on the other hand, often consist of only a few harmonics which are modulated over broad and sometimes overlapping fre quency ranges. A call consisting of narrow and nonoverlapping harmonic comp onents may provide a less complete representation of target structure than a call which consists of broadly modulated components. However, a multiharm onic call may help the bats to perceive local spectral changes in the echo from shifts in the peak frequencies of single harmonics, and thereby to ext ract additional information about the target. To assess this hypothesis, th e accuracy with which M. lyra can analyze frequency shifts of single partia ls in multiharmonic complex tones was investigated. A two-alternative, forc ed-choice behavioral task was used to measure M. lyra's frequency discrimin ation threshold for the third partial in complex tones whose spectral compo sition resembled that of the bat's sonar calls. The discrimination threshol d for the third partial in a 21.5-kHz harmonic tone amounted to about 2% an d was similar to the bat's pure-tone discrimination threshold at 64.5 kHz. Discrimination performance was essentially unaffected by random frequency c hanges of the other partials and by reducing stimulus duration from 50.5 to 1.5 ms. Both findings are in accordance with predictions made on the basis of the shape of M. lyra's cochlear filters. The comparison between the obs erved frequency discrimination performance and a computational estimate of the expected frequency shift in the third harmonic of an echo reflected by a simple, two-front target showed that M. lyra's frequency resolution is su fficient for analyzing the target-specific information conveyed by shifts i n the peak frequency of single echo components. (C) 2001 Acoustical Society of America.