Auditory brainstem 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 auditory space. It is not known whether this topographic representation emerges at the level of the SC or is relayed there from other auditory areas. 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 horseradish 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 labelling 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 cells were found on both sides of the brainstem. Single injections of WGA-HRP or discrete injections of red and green latex microspheres revealed that the caudal and lateral regions of the SC receive the heaviest projections, although the majority of the retrogradely labelled neurons in the contralateral BIN project to rostral SC. On the ipsilateral side, neurons in rostral and caudal regions of the BIN were labelled primarily by the tracer injected into rostral and caudal regions of the SC, 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 to the construction of the auditory space map in the SC, although information about sound azimuth may be conveyed to this nucleus via a spatially ordered projection from the ipsilateral BIN.