Significance of terminal frequency in FM sweeps for bat echolocation

Abstract

For echolocation of CF-FM bats, the CF component is expected to detect wing-beat of flying insects. However, it also plays an important role to serve as a stable signal carrier from a distant target. Then, once the carrier frequency is removed, echo signal can be easily detected. For this purpose, Doppler-shift compensation and echo amplitude compensation must be conducted to stabilize the signal carrier. What about the FM bats? FM bats also appear to utilize a relatively long pseudo-CF component to detect wing-beats of flying insects. They might also utilize pseudo-CF component for an echo signal carrier. However, Doppler-shift compensation in FM bats has not been reported. In the inferior colliculus, majority of neurons are tuned to the pseudo-CF frequency, 20 kHz for Eptesicus fuscus while 40 kHz for Pipistrellus abramus, which are corresponding to the terminal frequencies of downward FM sweep. Overrepresentation of a particular frequency may enhance frequency discrimination, detection of insect wing-beat. Therefore, we may expect majority of cochlear nerve fibers are also tuned to the pseudo-CF frequency. However, overrepresentation of hair cells for a particular frequency does not appear to be shown. [Research supported by MEXT, Japan and Shandong University, China.]