Single axon analysis of pulvinocortical connections to several visual areas in the Macaque

The pulvinar nucleus is a major source of input to visual cortical areas, but many important facts are still unknown concerning the organization of pulvinocortical (PC) connections and their possible interactions with other connectional systems. In order to address some of these questions, we labeled PC connections by extracellular injections of biotinylated dextran amine into the lateral pulvinar of two monkeys, and analyzed 25 individual axons in several extrastriate areas by serial section reconstruction. This approach yielded four results: (1) in all extrastriate areas examined (V2, V3, V4, and middle temporal area [MT]/V5), PC axons consistently have 2–6 multiple, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbindin‐positive giant projection neurons in the pulvinar; (3) as previously reported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasionally, layer 1) as collaterals of a single axon; in addition, (4) the size of individual arbors and of the terminal field as a whole varies with cortical area. In areas V2 and V3, there is typically a single principal arbor (0.25–0.50 mm in diameter) and several smaller arbors. In area V4, the principal arbor is larger (2.0‐ to 2.5‐mm‐wide), but in area MT/V5, the arbors tend to be smaller (0.15 mm in diameter). Size differences might result from specializations of the target areas, or may be more related to the particular injection site and how this projects to individual cortical areas.