Two fundamental concepts unite the proposed projects. The first is that the topographic (tonotopic) organization characteristic of auditory nuclei reflects their basic functional organization. The second is that the auditory system is made up of multiple parallel pathways arising from different neuronal types. The proposed research is designed to relate the axonal arborization patterns of specific cell types to the three- dimensional frame of reference afforded by isofrequency contours. Eleven experiments are proposed to achieve four specific aims: 1) To characterize the organization of the bushy cell pathway from the cochlear nucleus to the superior olivary complex and from the superior olivary complex to the inferior colliculus. 2) To characterize the multipolar cell path way from the ventral cochlear nucleus to the inferior colliculus. 3) To analyze the periolivary cell groups of the superior olivary complex in terms of specific cell types. 4) To examine the organization of the isofrequency contours in the inferior colliculus in terms of the arborization patterns of lemniscal inputs from the bushy and multi-polar cell pathways and from selected periolivary nuclei. The fourth specific aim provides the framework for interpretation of the results of the other studies. The hypothesis is that, although a number of lower brain stem auditory pathways converge on one isofrequency contour in the inferior colliculus, their terminal arborizations occupy different domains within that contour. Methods to be used include: l) Tracing techniques based on the anterograde and/or retrograde transport of biocytin, Phaseolus vulgaris-leucoagglutin, and a number of different fluorescent dyes, and 2) A fixed slice preparation in which the dendritic arborization patterns of neurons that project to known sources can be examined. Intrinsic plexuses of axons within the inferior colliculus appear to coincide in position and extent with the isofrequency contours defined physiologically and will serve to represent those contours anatomically. Our long-term objective is to understand the organization of the brainstem auditory pathways in terms of connections among the multiple, different neuronal types that form them. Neuroanatomical studies provide information essential for reaching an understanding of the mechanisms of auditory processing. The neuronal activity that leads to functional expression is dependent on the precise and specific connectivities among hundreds of specialized neuronal types. Detailed explications of these connectivities will not only help to explain available functional data, it will provide a basis for developing rational hypotheses about auditory function that can be tested further.