Projection of the mammalian superior colliculus upon the dorsal lateral geniculate nucleus: organization of tectogeniculate pathways in nineteen species.

Anterograde and retrograde transport methods have been used to analyze the projection of the superior colliculus upon the dorsal lateral geniculate nucleus in 19 mammalian species. Our retrograde findings reveal that tectogeniculate neurons are relatively small, and lie dorsally within the superficial gray. These small tectogeniculate neurons are spatially related to a dense tier of W-cell retinal input. Our anterograde tracing results show that tectogeniculate axons are visuotopically distributed to small-celled regions of the lateral geniculate in all nineteen species. In the majority of these species, the small-celled, tectally innervated regions of the lateral geniculate lie adjacent to the optic tract and contain W-cell-like neurons. Our findings suggest that neuroanatomical demonstration of the tectogeniculate projection is a relatively simple and straightforward way of revealing regions of the lateral geniculate which contain W-cells. This is true even in species in which the lateral geniculate lacks obvious cellular laminae, and in regions of the lateral geniculate where W-cells are few in number. The present data are especially interesting in light of the cortical projections of tectally innervated, small-celled regions of the lateral geniculate to the patches or puffs within layer III of area 17. Since these regions of small-celled geniculocortical axons are co-extensive with zones ("blobs") rich in cytochrome oxidase, it might be that information carried over the tectogeniculate circuitry plays an important role in the functions of the blob system.