Projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago.

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We have studied the projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago senegalensis by using the retrograde transport of horseradish peroxidase (HRP). Injections of various parts of the pulvinar complex, both the inferior and superior divisions, both the tectorecipient zone and the nontectorecipient zone as defined by Glendenning et. al. ('75), produce labeled cells in the lower tier of stratum griseum superficiale. The distribution of labeled cells in the superior colliculus varies with the locus of the injection, indicating a retinotopic projection system from the entire superior colliculus to all sectors of the pulvinar complex. These experiments also provide an opportunity to study the distribution and laminar origin of neurons giving rise to cortical descending projections. The entire visual cortex projects onto the pulvinar complex. The cells or origin can be divided into two populations--one located in layer V and the other in layer VI. In seven of the nine cases reported, the layer V population is restricted entirely or mainly to the striate area. In the two exceptional cases, the layer V population is located in the adjacent extrastriate cortex, areas 18 and 19. The difference in the layer of origin of the cortical descending fibers reflects a difference in the layer of termination of the reciprocal ascending projection. These findings identify the entire visual field as primary visual cortex. The importance of this conclusion is underscored by the fact that the visual field comprises as much as one-half of the whole neocortex.