AUDITORY BRAIN-STEM PROJECTIONS TO THE FERRET SUPERIOR COLLICULUS - ANATOMICAL CONTRIBUTION TO THE NEURAL CODING OF SOUND AZIMUTH

The mammalian superior colliculus (SC) contains a neural map of audito ry space. It is not known whether this topographic representation emer ges at the level of the SC or is relayed there from other auditory are as. We have used retrograde labelling techniques in ferrets to examine the sources and pattern of innervation from auditory brainstem nuclei . After multiple injections of wheat germ agglutinin conjugated to hor seradish peroxidase (WGA-HRP) into the SC, the heaviest concentrations of labelled cells were found in the nucleus of the brachium (BIN) and external nucleus of the inferior colliculus, with much weaker labelli ng in the nucleus sagulum, dorsal, intermediate and ventral nuclei of the lateral lemniscus, paralemniscal regions, and periolivary nuclei, The projections were predominantly ipsilateral, although labelled cell s were found on both sides of the brainstem. Single injections of WGA- HRP or discrete injections of red and green latex microspheres reveale d that the caudal and lateral regions of the SC receive the heaviest p rojections, although the majority of the retrogradely labelled neurons in the contralateral BIN project to rostral SC, On the ipsilateral si de, neurons in rostral and caudal regions of the BIN were labelled pri marily by the tracer injected into rostral and caudal regions of the S C, respectively. However, no clear segregation was apparent in the BIN after injections into the medial and lateral regions or in any of the other nuclei after either injection paradigm. These data suggest that converging inputs from several auditory brainstem nuclei contribute t o the construction of the auditory space map in the SC, although infor mation about sound azimuth may be conveyed to this nucleus tia a spati ally ordered projection from the ipsilateral BIN. (C) 1998 Wiley-Liss, Inc.