The Ventral Tectal Longitudinal Column: A Midbrain Nucleus for Modulation of Auditory Processing in the Cochlear Nucleus, Superior Olivary Complex, and Inferior Colliculus.

A ventral tectal longitudinal column (TLCv) has been described in rats and is hypothesized to provide multisensory modulation of acoustic processing in the superior olivary complex. The TLCv is a column of cells in the dorsomedial tectum extending rostro-caudally through the inferior and superior colliculi. It receives ascending auditory input and projects to the superior olivary complex. Further insight into TLCv function has been hampered by limited information on its connections. Here, we provide evidence that a TLCv is recognizable in mice and that it has more extensive connections than previously believed. Deposit of retrograde tracer into the superior olivary complex labels cells bilaterally in the TLCv, comparable to results seen in rats. Viral labeling of neuronal projections demonstrates input to the TLCv from the superior olivary complex and from the inferior colliculus. Thus, the TLCv in mice has inputs and outputs similar to those described in rats. Additional experiments with retrograde tracers revealed more extensive outputs from the TLCv. Neurons in the TLCv are labeled after the deposition of retrograde tracers into the cochlear nucleus or into the inferior colliculus. The projections from the TLCv to these targets, like those to the superior olivary complex, are bilateral. These projections are much broader than those described previously. The results suggest that the TLCv could exert modulation over a wide expanse of the auditory brainstem, from the cochlear nucleus through the inferior colliculus.