Topographic projection from the optic tectum to the auditory space map in the inferior colliculus of the barn owl

In the barn owl (Tyto alba), the external nucleus of the inferior colliculu s (ICX) contains a map of auditory space that is calibrated by visual exper ience. The source of the visually based instructive signal to the ICX is un known. Injections of biotinylated dextran amine and Fluoro-Gold in the ICX retrogradely labelled neurons in layers 8-15 of the ipsilateral optic tectu m (OT) that could carry this instructive signal. This projection was point- to-point and in register with the feed-forward, auditory projection from th e ICX to the OT. Most labelled neurons were in layers 10-11, and most were bipolar. Tripolar, multipolar, and unipolar neurons were also observed. Mul tipolar neurons had dendrites that were oriented parallel to the tectal lam inae. In contrast, most labelled bipolar and tripolar neurons had dendrites oriented perpendicular to the tectal laminae, extending superficially into the retino-recipient laminae and deep into the auditory recipient laminae. Therefore, these neurons were positioned to receive both visual and audito ry information from particular locations in space. Biocytin injected into t he superficial layers of the OT labelled bouton-laden axons in the ICX. The se axons were generally finer than, but had similar bouton densities as, fe ed-forward auditory fibers in the ICX, labelled by injections of biocytin i nto the central nucleus of the inferior colliculus (ICC). These data demons trate a point-to-point projection from the OT to the ICX that could provide a spatial template for calibrating the auditory space map in the ICX. (C) 2000 Wiley-Liss, Inc.