An aberrant crossed visual corticotectal pathway in albino rats

This study investigates the dynamic nature of the developing corticotectal pathway arising in the visual cortex. Special attention is given to the interaction occurring between the corticotectal pathways of each side of the brain and between corticotectal and retinotectal terminations. Normally the visual cortex of rats projects only to the ipsilateral superior colliculus. If one visual cortex is removed at birth, the remaining visual cortex subsequently shows a bilateral projection to the superior colliculus. The aberrant corticotectal pathway is heavier if the cortical ablation is accompanied by eye removal at birth but eye enucleation alone is not a sufficient stimulus for production of a crossed corticotectal projection. The aberrant crossed pathway shows a topographic order which appears to correspond to that of the normal ipsilateral corticotectal pathway. The pathway differs from the aberrant projections from the retina in that lesions done as late as 20 days postnatal still result in an aberrant crossed corticotectal pathway. This is similar to the aberrant crossed cortical projections from sensorimotor cortex. The pathway would appear to arise as a result of lack of competition from corticotectal axons normally present contralaterally or from attraction of denervated corticotectal sites. While denervated retinotectal sites stimulate sprouting of the corticotectal axons once in the deafferented colliculus, they do not stimulate crossing of the corticotectal projection.