Maurice Ptito, MD, Ph

In this series of studies, we used a monkey model of human hemispherectomy in order to characterize behavioral recovery and anatomical reorganization of the visual system. Monkeys were tested neurobehaviorally on a variety of tests every 6 months for a period of 30 months. Thereafter, the organization of the retinofugal pathways was studied by means of histological techniques. At the behavioral level, there was a striking recovery of sensory and motor functions that rendered lesioned baby monkeys undistinguishable from controls. Anatomically, hemidecordication in baby monkeys produced a transneuronal retrograde degeneration of the retinal ganglion cells (RGCs) that affected mainly the foveal rim. Although the density of RGCs in this region was drastically diminished, the soma sizes of the surviving cells remained normal. The lateral geniculate nucleus (dLGN) ipsilateral to the removed cortex was dramatically reduced in size, although it still showed normal layering. There was a marked reduction in the number of neurons in both the parvocellular and magnocellular layers and a heavy gliosis. By contrast, the superior colliculus ipsilateral to the lesion was remarkably well preserved: although slightly reduced in volume, it showed little gliosis and a metabolic activity, as revealed by cytochrome oxidase histochemistry, similar to the superior colliculus contralateral to the lesion. Behavioral perimetry indicated a partial sparing of vision up to 45 degrees in the 'blind' hemifield. We argue that the preservation of the retino-tectal pathway mediates most of the residual visual functions found in the 'blind field' of hemispherectomized human subjects.