Abstract The effects of early, unilateral cerebral hemi- spherectomy on retinogeniculate projections were stud- ied in the vervet monkey (Cercopithecus aethiops sabeus). Hemispherectomy eliminates all geniculocorti- cal pathways and thus removes cortical factors involved in the survival of retinogeniculate projections. Complete removal of the left cerebral cortex was performed in two monkeys at 6 months and 8 months of age. After a post- surgical survival period of 50 months (SHG3) and 45 months (SHG4), both animals and a normal adult monkey received intraocular injections of [ 3 H]proline (5 mCi) in the left eye and WGA-HRP (100 μl, 5%) in the right eye. The dorsal lateral geniculate nucleuseuron (LGNd) ipsilateral to the hemispherectomy was on aver- age 73% smaller than the contralateral LGNd. The mag- no- and parvocellular layers ipsilateral to the cortical ab- lation in both hemispherectomized subjects received a layered, eye-specific pattern of retinal input. This sug- gests that retinogeniculate projections could be sustained in the absence of geniculate relay cells. Keywords Visual cortex lesion · Residual neurons · Primate · Cell death · Retina · Monkey Introduction Lesions of the striate cortex in primates are followed by primary retrograde degeneration of dorsal lateral genicu- late neurons (LGNd; Matthews 1964; Mihailovic et al. 1971; Wong-Riley 1972) and lead to transneuronal retro- grade degeneration of retinal ganglion cells (RGC). It has been shown that Pβ-like cells are selectively affected by visual cortex lesions in cats (Tong et al. 1982; Payne et al. 1984) and monkeys (Cowey et al. 1989), whereas other RGC types escape retrograde degeneration. The se- lective death of Pβ neurons has been explained by the sustaining collateral hypothesis. This hypothesis propos- es that the survival of Pα and Pγ cell types depends on projections to the superior colliculus (SC) or to other tar- gets (Weller et al. 1979; Weller and Kaas 1984, 1989), whereas Pβ cells degenerate, since they project exclu- sively to the parvocellular layers of the LGNd (Leventhal et al. 1981; Rodieck and Watanabe 1993) Lesions made in infant macaques result in almost complete loss of retinal terminals in parvocellular layers, whereas the density and distribution of labeled terminals found in magnocellular layers are much less affected (Weller and Kaas 1989). The greater density of retinal afferents to magno- compared with parvocellular layers could be explained in part by a greater resistance of Pα- type cells to transneuronal degeneration imparted by the sustaining collaterals they send to targets other than the geniculate nucleus. Retinal projections found in parvo- cellular and magnocellular layers of striate cortex-lesion- ed monkeys suggest they might find in those laminae some available and appropriate postsynaptic sites. In- deed, some surviving neurons in the LGNd of striate cor- tex-ablated monkeys have been found and were inter- preted by some authors as being local interneurons (Pasik et al. 1973). There is, however, evidence for some remaining parvocellular and magnocellular neurons pro- jecting to extrastriate cortices (Benevento and Yoshida 1981; Yukie and Iwai 1981; Hendrickson and Dineen 1982; Cowey and Stoerig 1989). If retinal projections to LGNd in destriate monkeys are maintained through tro- phic support conveyed by geniculoextrastriate pathways, the question then arises as to whether retinal projections to LGNd could be maintained in the absence of genicu- late projection neurons. Several studies report that after hemidecortication the lateral geniculate nucleus undergoes complete degenera- D. Boire · M. Herbin · C. Casanova · M. Ptito ( ✉ ) Ecole d’optométrie, Université de Montréal, CP 6128, succ. Centre-Ville, Montréal, Québec, Canada H3C 3J7 e-mail: ptito@psy.umontreal.ca Tel.: +1-514-3436052, Fax: +1-514-3432382 H. Théoret · M. Herbin · C. Casanova · M. Ptito Groupe de Recherche en Neuropsychologie Expérimentale and Centre de Recherches en Sciences Neurologiques, Université de Montréal, CP 6128, Montréal, PQ, Canada H3C 3J7 Exp Brain Res (2000) 135:373–381 DOI 10.1007/s002210000527 RESEARCH ARTICLE D. Boire · H. Théoret · M. Herbin · C. Casanova M. Ptito Retinogeniculate projections following early cerebral hemispherectomy in the vervet monkey Received: 13 December 1999 / Accepted: 12 July 2000 / Published online: 30 August 2000 © Springer-Verlag 2000