Defocusing of echo-delay images for clutter rejection in echolocating bats.

Abstract

Echolocating big brown bats emit ultrasonic FM sounds containing two prominent harmonics (FM1 and FM2) and perceive target distance from echo delay. Each broadcast and its echoes are processed by time‐domain mechanisms incorporating neuronal delay‐lines for spectrogram correlation. Pulse‐to‐pulse variability requires the bat to store a template of the FM sweeps in each broadcast to recognize its exact echoes. Ordinarily, gradations of mismatch, or decorrelation, between echoes and broadcasts would lead to a correspondingly graded decline in delay accuracy. However, when the relation between FM1 and FM2 is gradually disrupted, bats abruptly lose delay acuity through a process that amounts to deliberate defocusing of the image. Bats actually have two computational pathways for perception of delay—one involving overall delay to determine target range, and the other involving delay differences between glint reflections to determine shape, extracted from the interference spectrum between the overlapping reflections. Defocusing occurs in the shape, or fine delay part of the image, possibly due to failure of the spectrally based transformation process to converge upon a simple solution that would depict a manageably small number of glints from a regular pattern of interference notches. [Work supported by ONR and NIMH.]