Connections of higher order visual relays in the thalamus: a study of corticothalamic pathways in cats.

Axonal markers injected into layers 5 and 6 of cortical areas 17, 18, or 19 labeled axons going to the lateral geniculate nucleus (LGN), the lateral part of the lateralis posterior nucleus (LPl), and pulvinar (P). Area 19 sends fine axons (type 1, Guillery [1966] J Comp Neurol 128:21-50) to LGN, LPl, and P, and thicker, type 2 axons to LPl and P. Areas 17 and 18 send type 1 axons to LGN, and a few type 1, but mainly type 2 axons to LPl and P. Type 1 and 2 axons from a single small cortical locus distribute to distinct, generally nonoverlapping parts of LP and P; type 1 axons have a broader distribution than type 2 axons. Type 2 axons, putative drivers of thalamic relay cells (Sherman and Guillery [1998] Proc Natl Acad Sci USA 95:7121-7126; Sherman and Guillery [2001] Exploring the thalamus. San Diego: Academic Press), supply small terminal arbors (100- to 200-microm diameter) in LPl and P, and then continue into the midbrain. Each thalamic type 2 arbor contains two terminal types. One, at the center of the arbor, is complex and multilobulated; the other, with a more peripheral distribution, is simpler and may contribute to adjacent arbors. Type 2 arbors from a single injection are scattered around and along "isocortical columns" in LPl, (i.e., columns that represent cells having connections to a common cortical locus). Evidence is presented that the connections and consequently the functional properties of cells in LP change along these isocortical columns. Type 2 driver afferents from a single cortical locus can, thus, be seen as representing functionally distinct, parallel pathways from cortex to thalamus.