Shifting retinal maps in the development of the lateral geniculate nucleus.

During development, the bilateral projections from each eye to subcortical visual structures in the mammal initially overlap throughout the majority of the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) before retracting to their separate territories. It has been shown in the ferret that during this period the larger contralateral retinal projection to both the dLGN and SC is retinotopically organised. By making small retinal lesions, and then anterogradely labelling the remaining retinofugal pathway from one eye, this study demonstrates that on the day of birth there is a superficial region of the dLGN in which the retinotopic map cannot be demonstrated. This region may be the presumptive C laminae. Further, by making small lesions in the temporal retina it has been shown that the smaller ipsilateral projection is also retinotopically organised before it retracts. Large lesions confined to the nasal retina had no effect on the pattern of label in the ipsilateral dLGN. Consequently, the ipsilateral projection which fills the nucleus at this stage must arise from the temporal retina. Because of this, the process of segregation requires that the retinotopic maps from each eye shift in relation to one another, and the borders of the nucleus to form the adult pattern.