NEURAL MECHANISMS OF BINAURAL HEARING

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The difference in the time of arrival of a signal at each ear is a major cue for localizing sounds along the azimuth. Interaural time cues are conveyed by the fine structure for low frequency signals and by the envelope for high frequency, Complex signals. The comparison of interaural time differences (ITDs) begins in the superior olivary complex (SOC) and further processing then occurs in the midbrain, thalamus and cortex. We have not yet examined ITD sensitivity in the last nucleus in the ascending auditory pathway, the auditory cortex. For this proposed project, we intend to fill that gap. These studies will be conducted in the unanesthetized rabbit using extracellular, single-unit recording techniques. Another aim is to determine the circuitry underlying ITD sensitivity from the SOC to the cortex. This will be accomplished by making small injections of retrograde and anterograde tracers into physiologically characterized regions of ITD sensitivity in the inferior colliculus (IC) and cortex. In our studies of ITD sensitivity, we have noted that the tuning to ITDs, or azimuthal receptive field size, becomes progressively sharper from the SOC to the auditory thalamus. A major question we will address is if there is a further sharpening at the auditory cortex. Sharpening of ITD appears to come in two forms: either symmetrically such that the peaks of ITD sensitivity becomes narrower, or asymmetrically such that one slope of the ITD.vs.-response curve becomes very steep, so that a sharp "edge" in ITD sensitivity is present. Asymmetrical, "edge-detector" neurons often show intermediate characteristic phases. The proportion of such neurons progressively increases at higher levels of the auditory pathway. These two forms of sharpening of ITD may subserve different functions useful for localizing sources of sound and their numbers may increase in the cortex. Populations of assymetrical, "edge-detector" neurons would be most useful for achieving accurate localization of a single sound source location, while neurons with narrow, symmetrical ITD-tuning would be most useful for resolving the locations of multiple sound sources. We suggest that sharpening of ITD tuning seen in the IC is, at least in part, created by the convergence of inputs from the ipsilateral, media and lateral superior olive. This possibility will be examined by making tracer injections into specific ITD sensitive regions of the IC, e.g., areas that show predominantly peak-type, trough-type, or intermediate-type responses. We will also identify regions of segregation and convergence among the three types of ITD responses as the information ascends from the IC to cortex. Segregation is expected in regions such as the ventral nucleus of the MGB, where information is relayed with considerable faithfulness to the auditory cortex, while convergence may be found in the media and dorsal divisions, where neurons that extract particular information from combinations of different inputs may exist. These experiments along with our previous studies will reveal the neural mechanisms and neural substrates for ITD processing at each level of the auditory pathway.