The topographic relationship between shifting binocular maps in the developing dorsal lateral geniculate nucleus.

The major mammalian subcortical visual structures receive topographically ordered projections from both eyes. In the adult dorsal lateral geniculate nucleus (dLGN) each projection terminates in separate restricted regions of the nucleus. This pattern is different during development. Initially in ferrets the projections from each eye to the dLGN overlap throughout this structure. Although the projections do not occupy regions that are appropriate given the adult pattern, they are both retinotopically organised. Consequently, the formation of the adult pattern requires that the two retinotopic projections shift in relation to one another. The experiments undertaken here on the newborn ferret demonstrate the relationship between the two unsegregated projections in terms of their retinal origin and relative pattern of projection to the dLGN. By establishing the relationship between the projections at this stage of development it is possible to determine the relative changes that must be made between them in order to bring about the adult pattern of registration. By mapping the two unsegregated projections with a combination of retinal lesions and anterograde tracing methods it is demonstrated that at birth the ipsilateral projection arises from the temporal retina, and the contralateral projection arises from the entire retina. Because of the significant contralateral projection from the temporal retina the relatively sharp nasotemporal division found in the adult is not present at this stage. This element of the contralateral projection maps in continuity with the rest of this projection and terminates at the caudal pole of the nucleus. However, it is probably lost before the adult pattern has clearly started to develop.(ABSTRACT TRUNCATED AT 250 WORDS)