Abstract
The bat’s inner ear and auditory brainstem register successive frequencies in FM sweeps of broadcasts and echoes with single spikes. Using the neural spectrogram of each broadcast as reference, spike latencies trace the FM sweeps for imaging echo delay by spectrogram correlation. If mutually-interfering glint reflections are present, the echo’s neural spectrogram has regularly-spaced ripples caused by amplitude-latency trading. Glint-delay estimates are generated by transforming the ripple pattern from frequency to time in real time using feedforward inhibition. Several glint delay estimates can coexist in focused target images before accumulation of nulls generates too many glint estimates and blurs the image. Echo lowpass filtering, characteristic of clutter, affects a wide swath of frequencies and changes the slope of the neural spectrogram, which evokes multiple nulls and causes image blurring that suppresses the clutter. Echo Doppler shifts also change the slope of the FM sweep, and the same mechanism is capable of blurring Doppler-shifted images so that troublesome ambiguity is suppressed. By internally adjusting the neural FM sweep of the broadcast reference, the images of echoes can be refocused onto any desired location not only in range, azimuth, and elevation, but also on the range-Doppler plane. [Work supported by ONR.]
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