Reconstruction of Target Images in the Sonar of Bats

Abstract

The echolocating bat, Eptesicus fuscus emits frequency-modulated (FM) sonar signals and perceives target range from echo delay. Reconstruction of the bat’s image of a point-target from behavioral data yields approximately the ambiguity (crosscorrelation) function of echoes with additional sharpening of the function’ s central peak according to the signal-to-noise ratio of echoes. Bats perceive a shift in the phase of this function even though phase conventionally is thought to be discarded during auditory encoding of sounds in the ultrasonic frequency band used by the bat (20 to 100 kHz). The perceived range of target images undergoes amplitude-latency trading indicating that the timing of neural discharges encodes the delay of echoes including echo phase information. The bat’s auditory code resembles a spectrogram of the transmitted and the received signals in which the instantaneous frequency of FM sweeps is represented by the timing of neural discharges to each frequency in succession. A computational model of echo processing using physiologically identified peripheral auditory mechanisms indeed can produce the crosscorrelation function of echoes from a spectrogram representation. The model correlates transmission and echo spectrograms to determine target range and as a parallel process it also can transform the echo spectrum back into the time domain to estimate target shape. The combined algorithms carry out the operations of Spectrogram Correlation and Transformation (SCAT). The critical feature of auditory coding for retention of phase information in the spectrograms is half-wave-rectification of excitation at the receptor-nerve interface.