SOURCES OF SUBCORTICAL PROJECTIONS TO THE SUPERIOR COLLICULUS IN THE FERRET

We have identified brainstem and other subcortical afferents to the su perior colliculus (SC) in the ferret, by examining the pattern of retr ograde labelling that resulted from unilateral injections of red and g reen fluorescent latex microspheres or of wheat germ agglutinin conjug ated to horseradish peroxidase into different regions of this midbrain nucleus. Labelled neurons were found in many structures, including so matosensory, visual and auditory nuclei. These subcortical inputs are almost all bilateral, although most are primarily ipsilateral. Despite some differences in organization, the subcortical projections to the ferret SC broadly resemble those described in other species. Following injections of red and green microspheres in either the rostral and ca udal or the medial and lateral regions of the SC, double-labelled cell s usually accounted for less than 10% of the total number of retrograd ely labelled cells, indicating that most of the subcortical neurons pr oject to discrete regions of the SC. Many of these afferents show some topographic organization, which is much more evident along the rostro caudal axis of the SC than in the medial-lateral dimension. The interc ollicular projection is restricted mainly to the rostral SC and demons trates a point-to-point organization. Most of the subcortical structur es caudal to the central region of the SC were labelled mainly by the tracers injected in caudal SC, while those rostral to this region, inc luding the zona incerta and the pretectal nuclei, were labelled largel y by injections in rostral SC. These findings suggest that projections originating from nuclei posterior to the central region of the SC ter minate principally in caudal SC whereas afferents from structures ante rior to the central region project largely to rostral SC.