Target shape perception and clutter rejection use the same mechanism in bat sonar

Big brown bats (Eptesicus) emit multiple-harmonic FM sounds (FM1, FM2) and exploit the relative weakening of higher harmonics in lowpass echoes from the surrounding scene to suppress clutter by defocusing of wideband images. Only echoes from a frontally located targets arrive as unfiltered, focused images. Experiments using electronically generated echoes show that lowpass filtering of masking echoes causes clutter masking to disappear. Lowpass filtering induces amplitude-latency trading, which retards response times at higher frequencies in clutter echoes relative to lower frequencies. Introducing countervailing changes in presentation-times of higher frequencies in electronically generated clutter echoes restores masking. In the big brown bat's inferior colliculus, FM sounds mimicking broadcasts and echoes evoke ~1 spike per sound at each neuron's best frequency; however, amplitude tuning is very broad. To exploit their high acuity for detecting coherence or non-coherence of echo responses, bats work in the latency domain instead, removing background objects through deliberate imposition of response de-synchronization and concomitant inattention on undesired clutter. Overall, the results indicate that big brown bats use neuronal response timing for virtually all auditory computations of echo delay, including clutter rejection. This use of active perceptual processes in biosonar instead of conventional sonar processes opens a new view toward biomimetic design.