The superior colliculus of the ferret: cortical afferents and efferent connections to dorsal thalamus.

By injecting both biotinylated and fluorescent dextran amines into the superior colliculus of the ferret we traced its afferent connections from the cerebral cortex and its projections to nuclei of the dorsal thalamus. All visual and auditory cortical areas had retrogradely labelled layer 5 pyramidal neurons projecting to the superior colliculus, with the highest density in areas 18 and 21. Secondary somatosensory and motor cortical regions also projected to the superior colliculus. No retrograde labelling was observed in primary somatosensory, primary motor or prefrontal cortex. All visual dorsal thalamic nuclei received connections from the superior colliculus. Within the LGN, lamina C and the medial interlaminar nucleus (MIN) received dense afferents terminating with large labelled boutons. Within the lateral posterior nucleus (LP) a distinct band of moderately dense small, labelled boutons was observed, and within the pulvinar a broader and less dense region of small bouton labelling was observed. For the most part these connections are similar to those seen in the cat and other mammals, however, specific comparisons appear to delineate potential evolutionary trends related to complexity of the visual system.