Acute sensitivity to interaural time differences in the inferior colliculus of a bat that relies on passive sound localization

Abstract

Gleaning bats rely on passive hearing to detect and localize terrestrial prey, and display remarkable accuracy in their passive sound localization. This study examines binaural processing in the inferior colliculus (IC) of the pallid bat (Antrozous pallidus), a gleaner that attends to prey-generated noise transients to locate prey. The primary focus is to determine whether neurons in its lateral IC, a region that appears dedicated to passive localization, possess a level of sensitivity to interaural time difference (ITD) sensitivity sufficient to indicate the use of ITDs in sound localization. Such a sensitivity was suspected because the pallid bat is capable of very accurate passive sound localization at the lower end of its audible range, where interaural intensity differences (IIDs) are small and may not provide sufficient spatial information. Because the pallid bat's audible range is too high for neurons to phase-lock to carrier frequencies, neurons were tested with square-wave, amplitude-modulated tones and noise to determine their sensitivity to ITDs in the sound envelope. Their sensitivity to the bat's behaviorally relevant ITD range of ±70 μs, and their low average interaural time/intensity trading ratios (18 μs/dB) suggest that the pallid bat IC may have the greatest ITD sensitivity reported in a high-frequency mammalian auditory system.