Cognitive perception of biosonar echo delay, phase, and spectrum

FM echolocating big brown bats combine the acoustic delay, phase, and spectrum of echoes into a unitary cognitive attribute of perceived delay. In this talk, we will present psychophysical, neurophysiological, and modeling data showing how this might be accomplished. Delay accuracy measured psychophysically approximates coherent matched-filter accuracy. Psychophysical curves of echo-delay resemble pulse-echo crosscorrelation functions, with phase manifested directly within 20 μs. Computational modeling shows that low-pass time-frequency bandpass filtering at 10 kHz enables the contribution from phase. Neural responses from the auditory midbrain are phase-sensitive for tone-burst offsets up to15 kHz. Echo spectrum (spectral nulls and ripples) is transformed into psychophysical percepts of delay. The external-ear spectral null creates an elevation-dependent peak at about 35 μs for vertical target tracking. Off-axis clutter masking is suppressed by a Gestalt-like normalization process that operates within delay percepts. The organization of perceived delay and the appearance of phase shifts is facilitated by delay processing that begins at the low-frequency tail end of FM sweeps and the linking of 1st and 2nd harmonic frequencies. Surprisingly, this result requires the entire echo to be received before any delay is determined. The implications of this result will be discussed. [Work supported by ONR.]