The long-term goal of this project is to understand the neural mechanisms for the localization of sound sources in mammals. Sound localization is difficult to study comprehensively because, in natural situations, there are many interacting cues to the location of sound sources whose relative importance depends on stimulus conditions. We propose to generate stimuli that simulate the complex combination of cues found in free-field conditions using closed acoustic systems, and study the response of single units in the inferior colliculus of anesthetized cats to these """"""""virtual- space stimuli"""""""". This technique provides a bridge between free-field and dichotic studies of sound localization, and will make it possible to examine the relative importance of different acoustic cues to sound localization for neural processing for realistic stimulus conditions. Specifically, we will measure the transformations of sound pressure from the free field to the eardrum in cats for sounds originating from different directions, then use these measurements to construct stimuli that, when presented through closed acoustic systems, generate the same pressure at both eardrums as free-field stimuli. We will then compare the responses of inferior-colliculus neurons to virtual-space stimuli derived from measurements in the same cat with responses to stimuli derived from measurements from another cat in order to determine if neurons show special sensitivity to characteristics of individual ears. We will also modify virtual-space stimuli by selectively eliminating certain acoustic cues to sound location (such as interaural time and intensity differences) in order to determine whether the sensitivity of specific inferior-colliculus neurons is primarily due to one of these cues, or to a combination of cues.
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