Alterations of retinal inputs following striate cortex removal in adult monkey.

The morphology of the retina and central retino-recipient nuclei was studied in two monkeys that had undergone total bilateral striate cortex removal as adults. These animals had been behaviorally tested for two years after lesioning and had demonstrated significant recovery of pattern vision. Light and electron microscopy and autoradiography were done on the central retino-recipient nuclei following a monocular intravitreal injection of 3H-proline. Light microscopic analysis of retinal ganglion cell number showed a 30% loss in the parafoveal retina due to retrograde trans-synaptic degeneration. The most striking central change in retinal axon distribution was in the dorsal lateral geniculate nucleus (dLGN) where the parvocellular but not the magnocellular region showed a marked reduction in retinal input. Despite the loss of almost all dLGN neurons through retrograde degeneration, at the EM level both parvocellular and magnocellular regions contained islands of neuropil made up of retinal and several other types of synaptic terminals as well as small dendrites and pale unidentified processes. Approximately equal numbers of retinal terminals were identified by EM autoradiography in both regions of dLGN, which did not explain the apparent differences in labeling between the two regions seen in the light microscope. A second change in central retinal pathways was found in the olivary pretectal nucleus where a significant loss of retinal input also occurred. A third change, but in the opposite direction, was found in the pregeniculate nucleus (PGN) where the area of retinal terminals appeared enlarged. The remaining central retino-recipient nuclei had the same distribution of retinal input as the control animals.