Echolocating Bats Perceive Natural-Size Targets as a Unitary Class Using Micro-Spectral Ripples in Echoes

Echolocating big brown bats emit frequency-modulated (FM) sounds covering ultrasonic frequencies in two harmonic sweeps (FM1 from 50-60 kHz sweeping down to 20-25 kHz, FM2 from 100-110 kHz sweeping down to 45 kHz). Using a complex interplay of acoustic cues, the bats perceive object distance from echo delay and object shape from echo spectra. Typical natural targets-flying insects-return discrete reflections, called glints, from prominent body parts (e.g., head, wings). Insect sizes are mostly 0.5 cm to about 3.5 cm, corresponding to reflected time separations of 30 to 210 mu s. When closely spaced reflections overlap, they interfere to create a characteristic echo spectrum containing repetitive peaks and nulls. Time spacing of these glints (Delta t) is transposed into spectral ripples at frequency intervals (Delta f = 1/Delta t) from about 30 kHz down to 5 kHz. The bat's perception of rippled echoes as a distinct class was tested in two-alternative forced-choice discrimination experiments with a standard stimulus of 100 mu s glint separation (10 kHz ripples), in the middle of natural ripple separations (30 kHz to 5 kHz). Stimuli were electronically generated virtual targets with simulated 2-glint sizes from smaller than 0.5 cm to larger than 3.5 cm. Bats perceived insect-sized virtual objects (glint delays of 36-300 mu s) as similar to the 100-mu s standard, while smaller or larger virtual objects were perceived as not similar. Insect-sized echoes contained a pattern of microscale spectral ripples that stood apart from the coarser, macroscale spectral features of smaller objects or the temporally separate reflections of larger objects.