Ascending projections to the inferior colliculus.

Cells that send ascending projections to the inferior colliculus were identified following injections of horseradish peroxidase into the colliculus. Labelled cells were found in all subcollicular auditory nuclei. Virtually all cells of the ipsilateral ventral nucleus of the lateral lemniscus and medial superior olive appear to project to the colliculus. Very few cells in these nuclei were labelled on the contralateral side. Heavy labelling on the contralateral side was found in the dorsal nucleus of the lateral lemniscus and cochlear nucleus, with less labelling being found ipsilaterally in these nuclei. The lateral superior olive was approximately evenly labelled on the two sides, with about half the cells from each side projecting to each colliculus. Cells in all periolivary cell groups were labelled, with most being found adjacent to the medial superior olive. An effort was made to identify individual cell types that were labelled and some 24 cell types were identified. In the cochlear nucleus there were marked differences between cell types in the extent of their labelling. Topographic projections matched previously described tonotopic organization of the colliculus and all major subcollicular nuclei except the ventral nucleus of the lateral lemniscus. A description of the cells in the nucleus is provided.