Changes in the retino-fugal pathways following cortical and tectal lesions in neonatal and adult rats

Summary Several months after unilateral removal of the striate cortex or superior colliculus, or both, in infant and adult rats the retinal projections were studied autoradiographically. The retinal projection areas in adult-operated animals were not different from those of unoperated controls, but aberrant pathways were found in the infant group. Following removal of striate cortex there was a small aberrant pathway to the lateral posterior nucleus of the thalamus (LP) and possibly to the pretectum. After removal of the superior colliculus there was a conspicuous aberrant projection to LP, which was even more prominent after combined removal of striate cortex and superior colliculus. The results support the proposal that when the normal field of termination is damaged, either directly by a tectal lesion or indirectly by a cortical lesion, axons grow and innervate LP, which has been partly deafferented by the lesion and which consequently possesses vacant synaptic space. Although the different consequences of early and late lesions may indicate that only infantile damaged terminals can redistribute themselves an alternative is that in infants many axons have not yet reached their normal terminal sites at the time of operation and that only those axons have the ability to continue growing and to form an aberrant pathways. The role of the aberrant pathway in vision is unknown.