Retinotopic order appears before ocular separation in developing visual pathways.

In mammals, the major subcortical visual structures receive projections from both eyes, with the uncrossed projection being smaller than the crossed. Each projection is arranged as a separate orderly map of one hemiretina. Although these hemiretinal maps are separate in the nuclei, they are aligned so that the representations of points in the visual field are in register, thus there is a continuity of visual field representation between them. During the early development of the binocular pathways, terminals from the two eyes overlap almost entirely. As development proceeds, terminals arising from each eye segregate to form the adult pattern. In the present study, local retinal lesions were made in ferrets at various stages in development before the separation of the projections from the two eyes. A neuronal tracer was then injected into the damaged eye, defining the pattern of projection from that eye. As reported here, the lesion resulted in a limited interruption in the pattern of terminal label on both sides of the brain, demonstrating that terminals from each eye are arranged in an orderly retinotopic manner at this stage. hence, during later development, as one projection is reduced relative to the other, the two maps must slide in relation to each other.