Auditory processing in the mustache bat's inferior colliculus

Abstract

The mustache bat's biosonar calls are dominated by a long 60 kHz constant frequency (CF) component. These animals adjust their calls to compensate for velocity-induced Doppler shifts in their echoes, and thereby hold the echo CF within a very narrow frequency band close to 60 kHz. The representation of 60 kHz is greatly enlarged in the central nucleus of the inferior colliculus, and occupies approximately a third of its volume. The neurons representing 60 kHz all have very sharp tuning curves, being an order of magnitude sharper than the tuning curves of other neurons. The sharp tuning enables these neurons to discharge in temporal registry with the modulation pattern imposed by an insect's wingbeats upon the echo CF component, thereby providing the neural code for recognition and identification of prey. In addition, the reception of 60 kHz at the two ears creates substantial interaural intensity disparities, whose value is a function of the spatial position of the target. The inferior colliculus has a large population of binaural neurons sensitive to interaural intensity disparities. Here we consider the neuronal machinery these animals possess for processing echoes from insects, the relation of these mechanisms to Doppler compensation, and how the ‘acoustic signature’ of an insect and its location in space are represented in the enlarged 60 kHz region of the bat's inferior colliculus.