The long term objective of this project is to describe the representation of sound direction in the auditory forebrain of cats and monkeys. This will be accomplished through the study of responses of neurons to sound stimulation, functional maps of neuron response properties and patterns of neural connectivity. This project focuses on the joint representation of sound direction and level in the forebrain pathways leading to and including AI. Single units will be studied in anesthetized cats and awake rhesus monkeys performing a behavioral task. Unit discharge rates, temporal response patterns and response latencies will be quantitatively related to sounds which vary in azimuth, elevation, level, frequency and spectral bandwidth. These data will be used to answer several specific questions. What patterns of directional, level and spectral selectivity are displayed by different units? What is the effect of changes in spectral bandwidth on a unit's level and directional selectivities? Are a single unit's spectral, directional and level selectivities interrelated? What are the similarities and differences among unit response properties located in different thalamic nuclei and in AI?; in monkeys and cats? Other experiments focus on description of the functional organization of sound direction and level in AI and thalamic nuclei which project to AI in barbituate anesthetized cats and monkeys. Data gathered in these single unit and mapping experiments will provide information about the range of directional and level selectivities exhibited by units with similar best frequencies. A major goal of this project is to develop a behavioral paradigm which allows for detailed study of single unit response properties in unanesthetized rhesus monkeys. Becaue of close phylogenetic relations, observations obtained in the monkey can be more easily extrapolated to humans than observations obtained in most other mammals. Relatively little is known about the organization of thalamocortical and corticocortical connectivity of AI in the monkey. Injections of two different tracers into AI using best frequency maps as guides will provide information on the topographic organization of thalamic and cortical connections of AI. Detailed best-frequency maps of the auditory thalamus will be obtained in the same animals receiving tracer injections to facilitate relating patterns of labeling to the physiological organization of the thalamus. These studies will provide a basis for identifying thalamic nuclei which project to AI, areas in which we will quantitatively study single unit repsonse properties.
