Target ranging and the role of time-frequency structure of synthetic echoes in big brown bats, Eptesicus fuscus

Abstract

Echolocating bats judge the distance to a target on basis of the delay between the emitted cry and the returning echo. In a phantom echo set-up it was investigated how changes in the time-frequency structure of synthetic echoes affect ranging accuracy of big brown bats, Eptesicus fuscus.A one channel phantom target simulator and a Y/N paradigm was used. Five Eptesicusfuscus were trained to discriminate between phantom targets with different virtual distances (delays). The phantom echo was stored in a memory and broadcast from a loudspeaker after a certain delay following the bat's triggering of the system via a trigger microphone. The ranging accuracy was compared using 5 different signals with equal energy as phantom echoes: a standard cry (a natural bat cry), two kinds of noise signals, a high pass, and a low pass filtered version of the standard cry.The standard cry was recorded from one of the bats while judging the distance to a real target. The duration was 1.1 ms, the first harmonic swept down from 55 to 25 kHz and there was energy also in the second and third harmonic. Both noise signals had the same duration, power spectrum, and energy as the standard cry. One noise signal was stored in a memory and hence was exactly the same each time the bat triggered the system. The other variable noise signal was produced by storing the envelope of the standard cry and multiplying on-line with band pass filtered noise. The time-frequency structure (e.g. rise time) of this noise signal changed from triggering to triggering. The filtered signals were produced by either 40 kHz high pass or 40 kHz low pass filtering of the standard cry.The range difference thresholds for the 5 bats were around 1-2 cm (51-119 gs) using the standard cry as echo. The range difference threshold with both noise Abbreviations: FM frequency modulated; CF constant frequency; peSPL peak equivalent sound pressure level; SD standard deviation; SE standard error of mean; EPROM erasable programmable read only memory; FFT fast Fourier transform; SIN signal-to-noise ratio signals was 7-8 cm (around 450 gs delay difference). The 40 kHz high pass filtered cry increased the threshold to approximately twice the threshold with the standard cry. With the 40 kHz low pass filtered cry the threshold was increased 2.5-3 times relative to the threshold with the standard cry. A single bat was tested with a signal filtered with a 55 kHz low pass filter leaving the whole first harmonic. The threshold was the same as that with the standard signal.The reduced ranging accuracy with the filtered signals indicates that the full band width of the first harmonic is utilised for ranging by the bats. The substantial reduction in accuracy with the noise signals indicates that not only the full band width but also the orderly timefrequency structure (the FM sweep) of the cry is important for ranging in echolocating bats.

Temporal Masking Reveals Properties of Sound-Evoked Inhibition in Duration-Tuned Neurons of the Inferior Colliculus

“…Figure 5A shows that the BEFs of most duration-tuned neurons were in the frequency range from 25 to 50 kHz. This frequency band is important for target ranging in E. fuscus (Surlykke, 1992) and closely matches the frequencies present in the fundamental FM sweep of the echolocation call (Simmons et al, 1995;Surlykke and Moss, 2000). A behavioral audiogram of E. fuscus ( Fig.…”

Foraging Blainville's beaked whales (Mesoplodon densirostris)produce distinct click types matched to different phases of echolocation

“…Nonetheless, there is compelling evidence (Simmons, 1971;Simmons, 1993;Simmons et al, 1979;Simmons et al, 1990;Masters and Jacobs, 1989;Surlykke, 1992) that the combination of FM signals and some specialized auditory processing allows bats to achieve high enough ranging accuracy and resolution to home in on small targets despite ensonification with pulses of several milliseconds duration, corresponding to pulse lengths in air of a meter or more (Fig.·7). The short broadband transients produced by dolphins are so different from the signals of FM bats that bioacousticians have proposed a different receiver strategy.…”

Echo SPL influences the ranging performance of the big brown bat, Eptesicus fuscus

“…They were presented simultanously or sequentially. By decreasing the range difference between the two targets the bats' range difference thresholds were determined (Simmons and Vernon 1971;Simmons 1971Simmons , 1973Simmons and Lavender 1976;Masters and Jacobs 1989;Miller 1991;Surlykke 1992)). In Eptesicus fuscus thresholds between 6 mm and 20 mm were measured, which correspond to a time delay difference between 36 ~ts and 119 Its.…”