The goal is to understand the mechanisms that neurons use to process ongoing interaural time differences (ITD's) in the auditory pathway. The ITD between the signals at the two ears is used for the localization of low-frequency sounds on the azimuth. With earphones, humans can lateralize ITD's that are much smaller than those suggested by responses of neurons in the superior olivary complex (SOC). However, our data in the inferior colliculus (IC) and medial geniculate body (MGB) of the unanesthetized rabbit indicate that this discrepancy is reduced at levels above the SOC. The range of ITD sensitivity of IC neurons is about 50% of that we estimate for neurons in the medial superior olive (MSO); in the MGB the range is about 30% of that in the IC. If we view the range of ITD sensitivity as an estimate of the neuron's spatial receptive field on the azimuth, then, it appears that one consequence of ITD processing in the binaural pathway is to reduce receptive field size. We will test this hypothesis, by making unit recordings from the MSO and lateral superior olive (LSO) of the unanesthetized rabbit so their receptive field estimates may be compared to our data in the IC and MGB. These stuides also would provide the first opportunity to examine ITD processing, free from anesthetic effects, at the primary sites of binaural interaction. We will also test the hypothesis that in the MGB, inhibitory mechanisms are involved in the reduction of receptive field size. There is strong evidence that inhibitory mechanisms play a role in ITD processing by MGB neurons. We will assess directly the role of inhibitory events in ITD processing by recording intracellularly from MGB neurons in the unanesthetized rabbit. Finally, we will test the hypothesis that segregated ITD regions in the MGB received distinct patterns of input from the midbrain. We propose, through unit recordings, to map the distribution of ITD regions in the MGB. These studies should elucidate the differences in ITD processing between subdivisions of the MGB. We will then inject retrograde tracers into these regions. The anatomical studies should clarify whether ITD processing in the MGB involves particular cell types and whether they involve converging or parallel pathways.
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