Accommodating constraints for modeling of biosonar processing by big brown bats

Abstract

Constraints on modeling of wideband FM biosonar are acoustic (small targets return discrete replicas of the incident sound from individual component parts), auditory (inner-ear transduction segments broadcasts and echoes into numerous, parallel bandpass channels with integration-times of ~350 μs), neural (processing retains parallel frequency bands throughout the auditory pathway that use single spikes to mark frequencies), and perceptual (bats nevertheless perceive true delays of individual glint reflections, not just spectral coloration). The bat’s target model is geometric, registering the glints binaurally along the range axis. However, because closely-spaced glints are represented by their mutual interference, not by separate acoustic reflections, the ripples have to be transposed from an interference pattern back into glint delay estimates. Auditory cortical neurons selective for specific patterns of ripple come to register instead the underlying time separation of the glint reflections. The developing SCAT model of wideband FM biosonar incorporates a network for glint delay transformation that also offers a route for clutter suppression by contrasting dispersed, unfocused clutter images with tightly focused target images in range and azimuth space. The feedforward inhibition of this network preserves real-time operation for target reconstruction as well as for guidance in clutter. [Work supported by ONR.]