Encoding phase information is critical for high resolution spatial imaging in biosonar

Abstract

The auditory system is responsible for translating acoustic information into a robust neural representation. In echolocating mammals, precise timing of neural onset-responses is critical to reconstruct complex acoustic scenes. Phase is often ignored in transmit and receive beam patterns, but it holds significance when considering broadband signals. A beam pattern's phase alternates outside of the main lobe, which leads to a frequency-dependent structure that is useful for spatial localization. For imaging in azimuth, binaural spectral patterns and time delay between the ears encode angular position. Imaging in elevation relies principally on specific spectral patterns encoded by each ear. The additional phase information decorrelates broadband echoes arriving from off-axis. This decorrelation only occurs on the order of a single wave period; however, the pattern of dispersion across frequency is sufficient to defocus echoes arriving from off-axis in the peripheral region, while accepting echoes arriving from the focal area of attention. We propose that acoustic spectral pattern matching by echolocating animals includes both magnitude and phase components of beams in the form of timing the onset response. Computational modeling results are presented showing how encoding phase information leads to high-resolution images despite dynamic environments and variability in the target strength of objects.