Binocularity in the little owl, Athene noctua. I. Anatomical investigation of the thalamo-Wulst pathway.

The efferent projections from the visual thalamus to the Wulst were studied in the little owl, Athene noctua. Cells of origin were identified by retrograde labeling after injections of wheat-germ-agglutinin-conjugated horseradish peroxidase into the Wulst. The labeled-cell distribution was correlated with the distribution of anterogradely labeled retinal terminals as revealed by intraocular injections of horseradish peroxidase. The results demonstrated a bilateral thalamofugal visual pathway ascending from the nucleus dorsolateralis anterior thalami (DLA). Projections from the visual thalamus to the Wulst showed a specific and orderly pattern, and retrogradely labeled cells of DLA were distributed equally, in a complementary fashion, on both sides of the brain, with a small region of overlap. Retinal termination fields in DLA largely overlapped relay neurons projecting to either Wulst, although the overlap was not complete. Despite differences in the organization of avian and mammalian visual pathways, which reflect their separate evolutionary origins, similarities between the DLA in the little owl and the lateral geniculate nucleus in mammals suggest that, during evolution similar selective pressures for binocularity led to a similar morphological organization.