Harmonic beamforming: Categorical perception segregates targets from clutter in bat sonar.

Abstract

Beaming for FM biosonar transmissions of big brown bats is broad and frequency-dependent (70 deg at 25 kHz to 30 deg at 80 kHz). In flight, sound reception is directional and frequency-dependent, too, being oriented to the front and centered on-axis. A broadcast beam defined by the harmonic ratio of FM2 to FM1 depicts the frontal zone for flat-spectrum ensonification surrounded by increasingly lowpass ensonification for objects located off to the sides and farther away. The width and flat front of the central lobe of this harmonic-ratio beam predict the bat’s acoustic behavior during flights in obstacle arrays of different densities [Petrites et al., JCP-A (2009)] and the spatial unmasking of target detection by relocating clutter to progressively larger horizontal separations from the target [Sumer et al., JCP-A (2009)]. Amplitude-latency trading for responses to FM2 relative to FM1 in lowpass echoes causes echo-delay acuity to defocus, which also prevents masking from clutter. Only focused delay images can mask other focused images, which effectively excludes clutter echoes from causing masking. [Work supported by the ONR and the NIMH.]