PARALLEL AUDITORY PATHWAYS IN THE BRAIN STEM

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The general theme of the proposed research is to understand the operations of the inferior colliculus (IC) in terms of the behavior of the animal. The behavior chosen here is echolocation in the bat. The experimental strategies are to examine sensory processing of behaviorally relevant stimuli in the IC and to examine interactions of the IC with motor systems. Three specific aims are proposed. The first aim is to test the hypothesis that interactions of excitatory and inhibitory inputs, offset from one another in time, produce tuning to time varying sounds. Three kinds of tuning that are important in the animal's behavior will be considered: Tuning to sound duration, tuning to frequency modulated sweeps and tuning to rate of repetitive frequency modulations. Electrophysiological studies, including whole-cell patch- clamp observations, will be used to examine the sequence of excitatory and inhibitory synaptic events that are elicited by these sounds. The second aim is to test the hypothesis that processing in the IC is influenced by motor activities. For example, does input from substantia nigra modulate sensory processing in the IC? In these studies, methods will be used to stimulate or to inactivate motor-related input from the substantia nigra to the IC while recording the response of IC neurons to sound. The third aim is to determine whether the IC sends the same class of information to the motor system as it sends to the thalamocortical system. These studies will utilize double retrograde marking techniques to identify whether or not IC neurons project to two different targets. For example, do the same neurons in the IC project to the thalamus and to the pontine grey? The findings of the proposed research will be significant for understanding the mechanisms underlying the nervous system's ability to select biologically important sounds and for understanding how the selection process is transformed into behavior.