BRAINSTEM PROCESSING OF BINAURAL INFORMATION

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This research program is focused on the neuronal mechanisms through which information derived from the two ears is processed to enhance directional hearing and the formation of auditory images. Two ears provide the brain with binaural information that plays an important role in sound localization, enhances listening sensitivity and aids in discrimination of signals against background noise. How does the auditory system utilize binaural cues in typical acoustic environments where the signals at each ear fluctuate constantly? Does the dynamic nature of common acoustic stimuli degrade the neural processing of binaural cues, or do the dynamic features provide additional information? Is our perception of auditory motion mediated through mechanisms that repetitively sample the time- varying signal and calculate the instantaneous interaural time and level difference cues to assemble an image of motion, or are there specialized neural mechanisms that process the temporal variations in the interaural cues? These issues will be addressed with extracellular single neuron recording in anesthetized cats and gerbils. The experiments will document the responses of single neurons to stimuli containing binaural features of apparent motion: ongoing variation in interaural amplitude and phase disparities. Previous literature has emphasized the sharp interaural disparity tuning of the central auditory system as evidence of neural selectivity for sound source direction. However, our data (some published, some preliminary) suggest that under dynamic binaural conditions the higher auditory system may compromise its absolute directional sensitivity in favor of enhanced sensitivity to change in interaural disparity. This behavior might arise through hierarchical transformation of a code based on discrete directional selectivity at the first site of binaural convergence (the superior olivary complex), or might indicate a failure of established theory to account for interaural disparity processing under conditions where the disparities are not constant. The proposed experiments are designed to resolve this conflict. This research program will help to specify not only the features of acoustic signals that are most salient in the neural integration of binaural information, but also some of the dynamic processing constraints: issues of relevance both for healthy listeners and for those suffering unilateral deafness.