Topographic organization of the optic radiation of the cat.

Pairs of injections of different neuroanatomical tracers--peroxidase-conjugated wheat-germ agglutinin (WGA) and [3H]proline--were made into the lateral geniculate nucleus (LGN) of the cat, and the course of the labeled fibers in the optic radiation was reconstructed. When the two injections were widely separated in the rostrocaudal dimension of the LGN (i.e., one in the representation of the lower quadrant of the visual field and one in the upper quadrant), the two sets of labeled fibers also remained separated in the long (roughly rostrocaudal) axis of the optic radiation. When the injections were widely separated in the mediolateral dimension of the LGN (i.e., one at the representation of the area centralis and one on the horizontal meridian in the far periphery of the field), the two sets of labeled fibers were separated in the short (mediolateral) dimension of the radiation. Shortly before reaching area 17, however, the medially and laterally placed fibers exchanged positions. This crossing is the basis of the topological inversion in the optic radiation deduced previously by Connolly and Van Essen (J. Comp. Neurol. 226:544-564, '84). The retinotopic organization of fibers in the radiation is less precise (in either dimension) than that of their terminal arborizations in visual cortex, but even injections as close as 1 mm to each other gave rise to spatially distinct fiber distributions. The WGA injections also labeled the corticogeniculate fibers by retrograde transport; these fibers traveled in a separate pathway medial to the optic radiation.