Signal Processing Systems for Echolocation in the Bat's Brain

Abstract

The neural base of the bat's biosonar system is discussed here. The bat emits complex biosonar sounds (pulses) and listens to echoes for orientation and hunting flying insects. Different types of biosonar information are carried by different parameters characterizing pulse-echo pairs. For example, distance information is conveyed by echo delay, while velocity information is carried by Doppler shift. In the auditory cortex of the bat, not only frequency but also other information baring parameters such as echo delay and Doppler shift are systematically mapped as subareas. These computational maps greatly depend on subcortical signal processing. The subcortical auditory nuclei create delay lines and multipliers (or AND gates) for processing distance (echo delay) information, and also create level-tolerant frequency tuning and multipliers (or AND gates) for processing velocity (Doppler shift) information. These multipliers are called FM-FM and CF/CF combination sensitive neurons, respectively. The neurophysiological investigations of the bat's biosonar system provide an excellent database for neural computational models and sonar systems. Application of an agonist of inhibitory neurotransmitter to DSCF or FM-FM area behaviorally revealed the functions of these auditory subareas. Findings made by an on-board telemetry microphone from flying bats confirmed Doppler-shift compensation.