Separation of MiPS/MaPS spectrogram transformations in biosonar

Abstract

Echolocating bats and dolphins differ in signals, sound emission, and reception pathways, and important aspects of the acoustic medium. They do, however, receive broadcasts and echoes through similar parallel bandpass filters in the cochlea, which impose integration-times of several hundred microseconds. Echoes consisting of several overlapping reflections from target glints (insect body-parts, fish swim bladders) interfere upon reception to form complex echoes with spectral interference patterns that characterize the target’s shape. Bats transform these patterns into images that depict the glints themselves along the range axis, a process called Spectrogram Correlation and Transformation (SCAT). Experiments with dolphins suggest two different scales for recognizing shape from echo spectra—macro- vs micro-power spectral features for ST (MaPS for short glint separations of <80 µs; MiPS for longer separations >80 µs). We tested this finding in big brown bats trained to distinguish between 2-glint echoes with long, MiPS-like and short MaPS-like spectral features and found that MiPS covers delay separations of about 25-500 µs from the frequency separation of spectral nulls, which have to fit into FM1 for ST to occur. At a deep level, dolphins and bats appear to share a common processing strategy for forming images. [Work supported by ONR.]