PARALLEL AUDITORY PATHWAYS IN THE BRAIN STEM

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The central nucleus of the inferior colliculus (ICc) is the site of convergence of multiple specialized parallel pathways from the lower brainstem. Thus, the ICc is the first stage at which there is major integration of the inputs of pathways specialized for binaural comparison, spectral analysis and temporal pattern analysis. A large proportion of input to the ICc appears to be either GABAergic or glycinergic, suggesting that inhibition may play a major role in the integrative process. The aim of the proposed studies is to investigate the hypotheses: 1) that inhibitory inputs play a crucial role in setting up sound-evoked windows of increased or decreased sensitivity to a subsequent sound, and 2) that inhibitory inputs to single neurons in the ICc shape their spectral sensitivity. Echolocating bats are used because we know from their biosonar signals which temporal and spectral features of sound are biologically important. However, the basic neural mechanisms for sharpening spectral sensitivity and for creating filters to specific temporal patterns of sound are likely to be relevant for neural processing of all complex sounds including speech. The specific aims are as follows: 1) to determine the origins of sound-evoked temporal windows of increased or decreased responsiveness to subsequent sounds and test whether temporal windows form the basis for a neuron's selectivity for specific sound patterns such as rate of amplitude modulation; 2) to determine whether sound-evoked windows of sensitivity are initiated or controlled by inputs from the monaural timing pathways of the nuclei of the lateral lemniscus; 3) to determine whether spectral sensitivity is sharpened by inhibitory mechanisms at the ICc; 4) to determine whether inhibitory receptor distribution is related to functional properties such as temporal and spectral selectivity. A variety of methods will be used to address these questions. These include: extracellular electrophysiological recording of single neurons, anatomical tracing methods, immunocytochemistry; receptor autoradiography, neuropharmacology of single neurons, in vivo whole-cell patch-clamp techniques. The data obtained should provide new insights into the neural mechanisms for analyzing complex patterns of sound. For example, the data should explain fundamental phenomena, such as how phase-locked responses to amplitude modulation at the brainstem level are transformed, at the midbrain, to temporal filters that are selective for narrow rates of modulation.