The long-term objective is to understand the organization of the brainstem auditory pathways in terms of connections among the multiple neuronal types that form them. The objective of the proposed studies is an analysis of the termination patterns of specific cell types projecting into and out of the inferior colliculus (IC) of the gerbil. Nine experiments are proposed to achieve three specific aims: 1) To further characterize a) the multipolar cell pathway(s) from the ventral cochlear nucleus to the IC and b) the fusiform and giant cell pathways from the dorsal cochlear nucleus to the IC; 2) To determine the extent to which the descending projections from the auditory cortex to the IC overlap with ascending projections from the superior olivary complex versus those from the cochlear nuclei; 3) To determine whether different collicular domains give rise to projections to different parts of the medial geniculate nucleus. The working hypothesis that unites the specific aims is that zones in the inferior colliculus can be identified which are the targets of specific subsets of the ascending pathways and, further, that these zones project differentially into the forebrain. Neuroanatomical methods to be employed include techniques based on the anterograde and/or retrograde axonal transport of various tracer molecules; immunocytochemistry, and histochemistry. Neuroanatomical studies such as the ones proposed 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 connections among the hundreds of specialized neuronal types which collectively form the auditory system. Detailed understanding of these connections will not only help to explain available functional data, they will also provide a basis for developing rational hypotheses about auditory function that can be tested further.
