Is rejection of clutter achieved by disrupting perception of delay in bat sonar?

Abstract

Big brown bats (Eptesicus fuscus) emit frequency‐modulated (FM) biosonar sounds containing two prominent harmonics, FM1 (55–22 kHz) and FM2 (105–145 kHz), and perceive target distance from echo delay. Ordinarily, echoes from objects arrive with both harmonics at the same delay, although FM2 becomes progressively more attenuated than FM1 with increasing target distance and off‐axis direction. Off‐axis or long‐range echoes naturally undergo lowpass filtering, which causes significant loss of acuity. Misalignment of FM2 with respect to FM1 or selective removal of low‐end frequencies disproportionately affects the accuracy of delay perception. Delay acuity is sharp for echoes containing both harmonics, less sharp for echoes containing only FM1, and very poor for echoes containing only FM2. Acoustically unnatural highpass filtering to remove low‐end frequencies from FM1 causes acuity to collapse. Attenuation of FM2 relative to FM1 by 3 dB decreases delay acuity, but shortening of the delay for FM2 by 48 μs counteracts amplitude‐latency trading and restores delay acuity. Bats may have a spatial perceptual “fovea” covering a narrow zone in front of them for high‐acuity perception surrounded by a zone of much lower acuity that suppresses the perceptual salience of background clutter. [Work supported by NASA RI Space Grant and ONR.]