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Using the monkey as a model for human hemispherectomy, the effects of early removal of a whole cerebral hemisphere on the cytoarchitecture and cytochrome oxidase histochemistry of the superior colliculus (SC) were evaluated. Results show that the SC ipsilateral to the cortical lesion suffers a 29.9% average volume reduction and a 32.7% total loss of neurons compared to the contralateral SC. Neuronal densities and metabolic activity are similar in normal and hemispherectomized monkeys. Furthermore, the ipsi- and contralesional SC receive retinal inputs as revealed with intraocular injections of tritiated proline. These data suggest that the superior colliculus retains functional capabilities following hemispherectomy in monkey.

The assessment of the anatomical consequences of cortical lesions on subcortical visual relays is necessary to further understand residual visual capacities. Unbiased stereological techniques were used to evaluate cell numbers in the substantia nigra (SN), a structure involved in the control of saccadic eye movements. Cell numbers were very similar in the ipsi- and contralateral SN of the hemispherectomized animal (329,926 vs. 310,248). These numbers are close to what was observed in the normal monkey (300,130 and 320, 859). In one case, part of the striatum was lesioned in addition to the cerebral hemisphere. Noticeable effects were observed in the SN ipsilateral to the cortical lesion: volume was reduced by 30.5% while the number of neurons, compared to the contralateral side, dropped by 43.2% (186,644 vs. 328,757). These results suggest that due to its anatomical sparing following hemispherectomy the SN, in addition to other subcortical structures, is in a prime position to modulate the spared saccadic behaviors seen after massive cortical injuries.

We have assessed the extent of transneuronal retrograde degeneration of retinal ganglion cells (RGCs) following the removal of a whole cerebral hemisphere at postnatal age 16 and 25 days. In the P16 animal, the nasal retina contralateral to the lesion suffered a 41% cell loss, whereas cell loss in the temporal retina ipsilateral to the lesion was 33%. Cell loss was greater in nasal retina and mainly included medium sized cells (200–600 μm2). In the P25 animal overall there was no evidence for ganglion cell loss.