Projections of three subcortical visual centers to marmoset lateral geniculate nucleus.

The dorsal lateral geniculate nucleus receives projections from visuotopically organized subcortical nuclei, in addition to inputs from the retina, visual cortices, and the thalamic reticular nucleus. Here, we study subcortical projections to the geniculate from the superior colliculus (SC) and parabigeminal nucleus (PBG) in the midbrain, and the nucleus of the optic tract (NOT) in the pretectum of marmosets. Marmosets are New World diurnal foveate monkeys, and are an increasingly popular model for studying the primate visual system. Furthermore, the koniocellular geniculate layers in marmosets, unlike those in the geniculate of commonly studied diurnal Old World monkeys, are well differentiated from the parvocellular and magnocellular layers. Thus, in the present study, we have made small iontophoretic injections of the retrograde tracer microruby, targeted to the koniocellular layers in the geniculates of four marmosets. We found direct projections from the ipsilateral SC, PBG, and NOT to the koniocellular geniculate layers. The distribution of retrogradely labeled cells in the superficial, visual layers of SC is consistent with the idea that projections from the SC to the koniocellular layers are visuotopically organized. A little over 20 years ago, Vivien Casagrande () introduced the idea that koniocellular geniculate layers (rather than the parvocellular and magnocellular layers) are principal targets of visuotopically organized subcortical nuclei. Our results add to subsequent evidence assembled by Casagrande and others in favor of this hypothesis.