The visual cortex of the opossum: the retrograde transport of horseradish peroxidase to the lateral geniculate and lateral posterior nuclei.

The visual cortex of opossum was studied by injecting horseradish peroxidase into the cortex and identifying labeled neurons in the thalamus. The results show that the lateral geniculate nucleus projects to area 17 in a topographical manner: the rostral lateral geniculate is represented in caudal striate cortex, and the dorsal extremity of the lateral geniculate, which probably corresponds to the zero vertical meridian, is represented along the border of area 18. Small injections in area 17 produced restricted bands of labeled neurons across the medial-lateral extent of the lateral geniculate, suggesting a greater precision in the topography than previously shown by retrograde degeneration studies. Following injections into area 17, labeled cells were also found in the lateral posterior nucleus. Injections of peristriate cortex produced labeled cells in the lateral posterior nucleus, as well as the lateral intermediate, posterior and intralaminar nuclei. Since the lateral posterior nucleus receives visual projections from the superior colliculus, the results show two visual pathways: the geniculo-striate path projecting just to core area or area 17, and a more diffuse parallel path that projects to both the core and belt. Whether or not this overlap is characteristics of the mammalian prototype it seems to be present in widely separated species.