PARALLEL AUDITORY PATHWAYS IN THE BRAIN STEM

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The inferior colliculus is the target of several auditory pathways which ascend in the brain stem. The question of how the targets of these pathways are related to one another in the inferior colliculus constitutes a fundamental problem in understanding the structure of the central auditory system. The echolocating bat provides a special opportunity for studying this problem. In the lower brain stem of the bat the origins of some of these pathways are hypertrophied or specialized so that their connections can be readily examined. The bat is unique in that its production of an easily characterized sonar signal provides information about the sound stimuli that are processed by its auditory system. The general goal of this proposal is to answer the question of how different ascending auditory connections from the lower brain stem converge or diverge at the inferior colliculus in echolocating bats. Emphasis will be placed on pathways from the nuclei of the lateral lemniscus, because these nuclei are hypertrophied and highly organized in echolocating bats. Specific experiments are designed to establish the connection of individual layers of cells in the ventral nucleus of the lateral lemniscus. Other experiments are designed to examine banded projections to the inferior colliculus and to discover whether projections from one source overlap or interdigitate with projections from a second source. The methods combine electrophysiology and neuronal tracing techniques using [3H]-leucine and wheat germ agglutinin conjugated to horseradish peroxidase. With the method of using the same micropipette to inject the tracer and to record neural responses, the injection site is characterized according to physiological measures, such as response to frequency or location of sound. The specialized parts of the bat's auditory pathway may serve as models of these parts in other mammals. Information about the structure of the bat's auditory system may provide clues for the study of structure and function of auditory pathways in all mammals, including man. Detailed knowledge about the connections of auditory pathways provides information about possibilities and limitations of the system in processing auditory signals. This information is essential to the diagnosis of pathologies of hearing and to the design of prosthetic devices to artificially stimulate the system in cases of profound deafness in man.