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, b ut many important facts are still unknown concerning the organization of pu lvinocortical (PC) connections and their possible interactions with other c onnectional systems. In order to address some of these questions, we labele d 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 approa ch yielded four results: (1) in all extrastriate areas examined (V2, V3, V4 , and middle temporal area [MT]/V5), PC axons consistently have 2-6 multipl e, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbi ndin-positive giant projection neurons in the pulvinar; (3) as previously r eported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasio nally, 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 corti cal 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 princ ipal arbor is larger (2.0- to 2.5-mm-wide), but in area MT/V5, the arbors t end 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 particu lar injection site and how this projects to individual cortical areas. Feedforward cortical axons, except in area V2, have multiple arbors, but th ese do not show any obvious size progression. Thus, in areas V2, V3, and es pecially V4, PC fields are larger than those of cortical axons, but in MT/V 5 they are smaller. Terminal specializations of PC connections tend to be l arger than those of corticocortical, but the projection foci are less dense . Further work is necessary to determine the differential interactions with in and between systems, and how these might result in the complex patterns of suppression and enhancement, postulated as gating mechanisms in cortical attentional effects, or in different states of arousal. J. Comp. Neurol. 4 06:221-250, 1999. Published 1999 Wiley-Liss, Inc.