Abstract
Echolocating bats (Eptesicus fuscus) detect displacements in the arrival times of sonar echoes as small as 500 ns. They can thus detect differences in target range or fluttering motions of prey resulting in displacements in range as small as 100–200 μm. The range-axis acoustic image of a planar target corresponds to the half-wave rectified crosscorrelation function between the bat's broadband, FM sonar transmissions and echoes. The bat perceives an intersignal periodicity histogram for the sounds. The FM sweep is rapid enough that the signal frequency is within the tuning curve of any given primary auditory neuron for a few periods at most. Thus, to the auditory system of Eptesicus, the sonar sound is a set of impulses with center frequencies distributed across the echolocation bandwidth. Only the first or “on” discharge of each primary neuron needs to be locked to individual stimulus waves for the neural representation of the signal to preserve transmission-echo phase or period. These bats model sonar targets as acoustic “glints” in the nervous system, and they discriminate among target features with a time-domain sonar receiver.
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