Beamforming as a mechanism for azimuthal localization of targets by FM-echolocating big brown bats.

Abstract

Echolocating big brown bats broadcast downward-sweeping, frequency-modulated (FM) biosonar sounds and process the returning echoes to track and capture flying insects. Neurons in the inferior colliculus are tuned to different frequencies along the FM sweep and form a representation of target azimuth largely on the basis of sensitivity to interaural differences in intensity, with canonical interaural time difference cues by themselves less reliable. Here, we reanalyze neurophysiological data on binaural sensitivity of inferior colliculus neurons consistent with a signal-processing model used in engineered sonar for generating directional beams. Model inputs are the times-of-occurrence of phasic-on responses to FM sweeps presented at a range of interaural intensities and azimuths. Tuning curves for interaural intensity were computed, and the slopes of these tuning curves plotted against neuronal best frequency and sound azimuth. The resulting aggregate plots suggest that the bat forms two distinct beams, one encompassing the entire FM sweep for a narrow zone of azimuths straight ahead and the other restricted to lower frequencies and offside to the left and the right. When interpreted with respect to psychophysical data, these analyses suggest that the forward beam allows for acute target perception, while the offside beams allow for perception of multiple objects that constitute clutter. The distribution of azimuths further suggests the formation of numerous detailed offside beams pointing in different directions to accommodate the width of the spatial arrangement of the clutter.