The natural acoustical environment contains many sound sources that are active at the same time. The ears receive the sum of all of these sounds and from this complex waveform, the auditory system manages to determine the location of each source. With age, the ability to assign locations to sound sources deteriorates, and with it, the ability to attend selectively to a single source in a noisy environment. This project will examine how the brain is able to establish the location of multiple, simultaneous, sound sources. The model system is the barn owl, a nocturnal predator renowned for its ability to localize sounds and the fact that its brain contains a retina-like map of auditory space. Like cells of the visual system, these space-mapping cells respond only when stimuli emanate from a discrete location in space and are arrayed so that the spatial relationships of the stimuli are preserved. In this project, neurophysiological experiments will first explore the ability of the space- map to resolve two simultaneously-active sound sources as the similarity between the speaker's sounds and the distance separating the speakers are manipulated. Further neurophysiological tests will explore the possible mechanisms that underlie the ability of the space-map to represent concurrent sound sources. Particular attention is focused on the situation in which two sound sources concomitantly emit broadband noises, an acoustical signal often produced by the owl's prey. Finally, the ability of the barn owl to resolve simultaneously-active sound sources will be tested behaviorally so that neural and behavioral capabilities can be compared.
