Acoustic flow in echo amplitudes and spectra: a viable concept for obstacle avoidance in CF-bats?

Abstract

Bats are capable of obstacle avoidance based solely on echolocation information. When doing so, species being classified as CF (constant frequency) bats allocate most signal energy to rather long, narrow-band portions of their echolocation pulses. These signal components may subserve obstacle avoidance by means of information conveyed by proportional changes in amplitude (due to geometric attenuation, absorption, compound directivity of emitter and receiver) and frequency content (due to Doppler shifts) of the echoes along a trajectory. It can be demonstrated that a combination of these two putative sensory variables allows, in principal, for a metrical reconstruction of target position relative to the animal’s flight vector within a hemifield. Thus the contained information seemingly suffices for mediating appropriate avoidance maneuvers. CF bats’ unique narrow-band signal design is matched by a likewise peculiar layout of their auditory system. In particular, the existence of an auditory fovea featuring extremely high filter qualities may be hypothesized to aid estimation of proportional changes in frequency. In order to establish the extent to which the informational content of the signal parameters under consideration may be accessible in the auditory system’s primal sketch, the properties of a gammatone filter bank fitted to physiologically determined filter qualities are explored.