Cortical computational maps for auditory imaging

Abstract The mustached bat emits complex biosonar signals (pulses) and listens to echoes for orientation and hunting flying insects. Different types of biosonar information are conveyed by different parameters characterizing pulse-echo pairs. For example, range information is conveyed by echo delay, while velocity information is carried by Doppler shift. At the auditory periphery, frequency is expressed by the anatomical location along the basilar membrane and also along the array of ganglion cells, while amplitude and time (duration of signals and interval between signals) are not expressed by anatomical locations, but by discharge rate and the temporal pattern of nerve discharges, respectively. In the auditory cortex, however, not only frequency but also other information-bearing parameters (IBPs) such as echo delay and Doppler shift are systematically expressed by anatomical locations. That is, the IBPs are mapped. These computational maps greatly depend upon subcortical signal processing. The subcortical auditory nuclei create delay lines and multipliers (or AND gates) for processing range (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 or CF/CF combination-sensitive neurons, respectively. Signal processing in the auditory system is parallel-hierarchical. The neurophysiological studies of the bat's auditory system provide an excellent data base for computational models.