The long-term objective of this project is a thorough understanding of the behavioral and neural mechanisms of sound localization. Previous studies of sound localization have chiefly centered on two areas: human and animal psychophysical work has established the important cues for localization while animal physiological work has shown the neural mechanisms by which the auditory system encodes these cues. This application is an effort to link these two approaches by combining animal psychophysics with physiology. This application proposes to extend our present behavioral preparation for testing sound localization in casts by freeing the head of the cat so that it can orient to the sound with unrestrained head and/or eye movements. In addition we will continue our physiological studies of sound localization by recording from cells during this behavior. There are two general specific aims: one directed to behavior and the other to physiology.
Specific aim I will develop the head-free preparation by monitoring eye, head and ear movement using the search coil technique and standard operant conditioning. We will compare the accuracy of localization in the head-fixed and head-free conditions, study the effect spectral cues on localization by narrowband pass filtering noise stimuli, and examine the role of pinna muscles by studying the movements of the pinna as well as the effect on localization ability of paralyzing the pinna muscles.
Specific aim II is aimed at physiological recordings in these animals while they are actively localizing sounds. We will continue our examination of the motor error hypothesis by recording in the superior colliculus and studying the effect of eye position on auditory responses. Then we propose to move to the auditory cortex where we expect to find cells whose response properties are correlated with the behavioral localization of the stimulus. By progressively narrowband pass filtering the noise stimulus, we expect he cat to gradually mislocalize the sound. At some point the cat will be near threshold, looking half the time at the proper location and the other half to some phantom location. At this point we can deliver the identical stimulus with two different behaviors and correlate the neural activity to one behavior or another. Spatial hearing and sound localization are important basic functions of the auditory system: defects in binaural function in human patients can lead to considerable difficulty in detecting signals embedded in noise, such as understanding conversations in a crowded room, which is perhaps the most common complaint of the hearing-impaired and can lead to severe social withdrawal.
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