Topography of developing thalamic and cortical pathways in the visual system of the cat

Adult patterns of connectivity could emerge during development by a process of selective elimination from an earlier, more widespread, connectivity. We have addressed this issue by examining the topography of developing projections to area 17 in the cat. At different postnatal ages, paired injections of the retrograde tracers diamidino yellow and fast blue were made in area 17. Interinjection separations were carefully controlled and the spatial distribution of the two populations of labelled neurones investigated. Projections to the striate cortex from the lateral geniculate nucleus, area 18, as well as connections intrinsic to area 17 were analysed quantitatively with a graphic method that uses a two-dimensional model of the projection. This allows two parameters of the projection to be calculated: the divergence (the spatial extent of area 17 contacted by an infinitely small region of an afferent structure) and the convergence (the extent of an afferent structure that projects to an infinitely small region of area 17). During postnatal development, the bulk of the connections making up the geniculostriate and corticocortical pathways showed no variation either in their convergence and divergence. However, the projection of area 18 to area 17 and the intrinsic area 17 connections (but not the geniculostriate projection) in the 3-15-day-old kittens were each found to contain a small subpopulation of widely scattered neurones with widespread axonal trajectories. These results, showing that many initially formed connections display a high degree of topographical order, are discussed in terms of the control mechanisms specifying axonal trajectories during development.