Abstract:

When simulated echoes have a bandwidth between 20 and 80 kHz, bats (Eptesicus fuscus) can discriminate between pulse-to-pulse delay jitter and constant echo delay with a sensitivity of 10 ns [Simmons et al., J. Comp. Physiol. A 167, 589--616 (1990)]. To investigate this amazing acuity, high-pass and low-pass filters were inserted into the channel between the simulated target and the bat. The effect of such filters on jitter acuity was measured. The data are explained by a receiver model that attempts to equalize the channel filter. The equalizer uses a smeared version of the actual channel transfer function, where spectral smearing induces relatively slow cutoff. Spectral smearing occurs in spectrogram (short-time Fourier transform) analysis. Smearing varies inversely with the impulse response duration of the analysis filters. This impulse response duration corresponds to the critical interval, which is frequency dependent for proportional bandwidth filtering. Bat jitter discrimination data imply channel equalization with a smeared version of the actual channel transfer function. This smearing is commensurate with critical intervals that are obtained from other experiments [Simmons et al., J. Acoust. Soc. Am. 86, 1318--1332 (1989)]. Bats apparently use time-frequency echo representations to perform environmentally adaptive filtering.