Evidence that the lateral geniculate nucleus regulates the normal development of visual corticocortical projections in the cat.

The aim of this work was to examine the influence of subcortical afferents on the development of corticocortical projections in the cat's visual cortex. In the adult, corticocortical axons project with precision to link retinotopically corresponding points in visual areas 17 and 18. In the newborn kitten, an excess of corticocortical connections is generated, leading to a degree of imprecision in the early pathways. During the first postnatal month, the loss of some of these early connections lowers their densities and increases the accuracy with which they project. These processes occur in an environment already influenced by afferents from the lateral geniculate nucleus and we tested the extent to which these existing inputs are required for corticocortical development. We lesioned the lateral geniculate nucleus with ibotenic acid in newborn kittens and studied connections from area 17 to area 18, and vice versa, after 1 month. In lesioned kittens, there were fewer corticocortical projections than normal in these reciprocal pathways and those that were present retained an immature, widespread pattern of projection. These results suggest that geniculate afferents are crucial for generating sufficient numbers of corticocortical projections and for creating the precision in their mapping.